synthesis and characterization of [n]cumulenes
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Synthesis and Characterization of [n]Cumulenes
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Synthese und Charakterisierung von [n]Cumulenen
Der Naturwissenschaftlichen Fakultät der Friedrich-Alexander-Universität
Erlangen-Nürnberg
zur Erlangung des Doktorgrades Dr. rer. nat.
vorgelegt von
Johanna Agnes Januszewski
aus Bytów
Als Dissertation genehmigt
von der Naturwissenschaftlichen Fakultät
der Friedrich-Alexander-Universität Erlangen-Nürnberg
Tag der mündlichen Prüfung: 2.12.2014
Vorsitzender des Promotionsorgans: Prof. Dr. Jörn Wilms
Gutachter: Prof. Rik R. Tykwinski PhD
Prof. Dr. Jürgen Schatz
Prof. Dr. Peter R. Schreiner
Die vorliegende Arbeit entstand in der Zeit von Juni 2010 bis August 2014 am Institut
für Organische Chemie (Lehrstuhl I) der Friedrich-Alexander-Universität (FAU)
Erlangen-Nürnberg. Meinem Doktorvater Prof. Rik R. Tykwinski PhD gilt besonderer
Dank für das interessante Dissertationsthema und seine Unterstützung.
Für meine Familie
(v.a. Tata Jan, Mama Krystyna, Kasia,
Wiesia, Darek, Babcia Aniela & Ciocia Ula)
Kocham was
Dla człowieka,
podobnie jak dla ptaka,
świat ma wiele miejsc,
gdzie można odpocząć,
ale gniazdo tylko jedno.
Oliver Wendell Holmes, Sr.
Kurzzusammenfassung
Die vorliegende Arbeit beschreibt die Synthese von zwei ungeradzahligen Serien von
[n]Cumulenen (n = 3, 5, 7, 9, 11 und 13) mit 3,5-Di-t-butylphenyl- und Mesitylendgruppen.
Zahlreiche Charakterisierungsmethoden werden präsentiert, darunter spektroskopische
(NMR- und UV/vis-Spektroskopie), elektronische (Cyclovoltammetrie) sowie strukturelle
Analysen (Röntgenstrukturanalyse). Die erhaltenen Ergebnisse werden zusätzlich durch
theoretische Berechnungen bestärkt. Die hergestellten Cumulene werden untereinander als
auch mit einigen bereits literaturbekannten Cumulenen verglichen und ihre optischen,
elektronischen und strukturellen Eigenschaften untersucht.
Die Synthese von kürzeren [n]Cumulenen (n < 9) war erfolgreich; ab n ≥ 9 zeigten die
Cumulene jedoch eine Stabilitätsgrenze auf. Dennoch wurden Synthesewege für [9]-, [11]-
und [13]Cumulene durchgeführt, die jedoch keinen eindeutigen Nachweis für die Bildung der
längeren [n]Cumulene, mit n = 11 und 13, ergaben. Um die vorhandene Stabilitätsgrenze zu
umgehen, wurden verschiedene Optimierungsansätze vorgenommen. Des Weiteren wurden
Makrozyklen zur Bildung von Cumulenrotaxanen eingesetzt. Die daraus folgende bessere
Abschirmung der instabilen Cumulenkette zeigte bereits beim [9]Cumulenrotaxan, im
Vergleich zu seinem analogen „nackten“ Vertreter, eine erhöhte Stabilität auf. Somit konnten
weitere mögliche Oligomerisierungs- bzw. Zerfallsreaktionen vermieden werden.
Die UV/vis-Spektroskopie von Cumulenen zeigt mit Verlängerung der Cumulenkette,
d.h. mit größerem n, eine Rotverschiebung der kleinsten Energieabsorption λmax und somit
eine Verkleinerung der optischen Bandlücke. Diese Beobachtungen stimmen mit den
Cyclovoltammetrieergebnissen überein, in denen die elektronische Bandlücke mit der
Verlängerung der Cumulenkette ebenfalls verkleinert wird und ähnliche Energiewerte besitzt
wie die optische Bandlücke. Die Röntgenstrukturanalyse bestätigt die bereits für
Polyinsysteme bekannte Bindungslängenalternanz (BLA) und zeigt eine Senkung der BLA
mit Verlängerung der Kumulenkette als auch eine langsame Annäherung an einen Plateauwert
auf.
Abgesehen von der Synthese und Charakterisierung von [n]Cumulenen wurde auch
ihre Reaktivität mittels verschiedener Reaktionen untersucht. Eine davon basierte auf
Additionsreaktionen des Elektronenakzeptors Tetracyanoethylens (TCNE) an die
Kumulenkette verschiedener [5]Cumulene als auch eines [7]Cumulens. Die Reaktion
zwischen dem [5]Cumulen mit 3,5-Di-t-butylphenylgruppen und TCNE ergab sehr
interessante Produkte, u.a. Cyclobutanderivate, Radialene und Dendralene. Die Produkte
konnten durch Additionsreaktionen mit Br2 und ROH funktionalisiert werden. In einem
weiteren Projekt wurde ein funktionalisiertes [5]Cumulen als HCl-Addukt durch die Reaktion
des [9]Cumulens mit 3,5-Di-t-butylphenylgruppen und SnCl2 sowie HCl in CH2Cl2
hergestellt. Zuletzt ergab eine thermische Dimerisierungsreaktion eines [5]Cumulens ein auf
einem Cyclobutanring basierendes Dimer mit vier exocyclischen Alleneinheiten, das durch
Röntgenstrukturanalyse nachgewiesen werden konnte.
Abstract
This thesis deals with the synthesis of two odd-numbered series of [n]cumulenes with
n = 3, 5, 7, 9, 11, and 13 containing 3,5-di-t-butylphenyl and mesityl endgroups. Several
characterization methods have been performed including spectroscopic (NMR- and UV/vis
spectroscopy), electronic (cyclic voltammetry), and structural analysis (X-ray
crystallography). The findings are additionally supported and discussed by the use of
theoretical calculations. The synthesized cumulenes are compared to each other as well as to
several literature known cumulenes, and their optical, electronic, and structural properties
have been investigated.
The synthesis of lower [n]cumulenes (n < 9) was successful; higher cumulenes,
however, showed a stability limit at n ≥ 9. Nevertheless, synthetic approaches to [9]-, [11]-
and [13]cumulene were performed giving no definite confirmation for the formation of
[n]cumulenes with n = 11 and 13. To overcome the stability limitations, several reaction
optimizations, as well as incorporation of macrocycles in order to form cumulene rotaxanes,
were applied. As a result, [9]cumulene rotaxanes already showed a higher stability than their
„naked“ representatives leading to a better shielding of the unstable cumulene chain through
the macrocycle and thus preventing further oligomerization reactions and decomposition.
UV/vis spectroscopy of cumulenes reveals a red-shift of the lowest energy absorption
λmax and thus a reduction of the optical band gap energy by increasing chain length n. These
results are consistent with the findings in cyclic voltammetry measurements, in which the
electronic band gap also decreases with increasing chain length having similar energy values
as the optical band gap. X-ray crystallographic analysis confirms bond length alternation
(BLA), which is already well-known for polyynes, and reveals a decreasing BLA with
increasing cumulene chain length n, tentatively approaching a plateau value.
Aside from the synthesis and characterization of [n]cumulenes, reactivity was
investigated via several approaches. The first one included addition reactions of the electron
accepting tetracyanoethylene (TCNE) molecule to the cumulenic chain of several
[5]cumulenes and one [7]cumulene. The reaction of a [5]cumulene with 3,5-di-t-butylphenyl
endgroups and TCNE resulted in very interesting products including cyclobutane derivatives,
as well as radialenes and dendralenes. These products could be further functionalized by Br2
and ROH addition reactions. In another project, a functionalized [5]cumulene was obtained by
treatment of a [9]cumulene containing 3,5-di-t-butylphenyl groups with SnCl2 and HCl in
CH2Cl2 leading to the formation of a HCl adduct. Finally, thermal dimerization reactions of a
[5]cumulene resulted in the formation of a cyclobutane based dimer with four exocyclic allene
units, which was confirmed via X-ray crystallographic analysis.
Table of Contents
1. Chapter I. Introduction to cumulenes ............................................................................. 1
1.1 Definition of the one-dimensional carbon allotrope carbyne ....................................... 1
1.2 Cumulenes as one possible isomer of the carbon allotrope carbyne ............................ 2
1.2.1 Cumulenes in history and nature .......................................................................... 3
1.2.2 Cumulenes in organometallic chemistry .............................................................. 4
1.2.3 Structural differences of cumulenes ..................................................................... 6
1.3 Synthesis of [n]cumulenes ........................................................................................... 7
1.3.1 General cumulene synthesis ................................................................................. 7
1.3.2 [3]Cumulenes ....................................................................................................... 8
1.3.3 [4]Cumulenes ..................................................................................................... 10
1.3.4 [5]Cumulenes ..................................................................................................... 12
1.3.5 [6]Cumulenes ..................................................................................................... 14
1.3.6 [7]Cumulenes ..................................................................................................... 15
1.3.7 [9]Cumulenes ..................................................................................................... 16
1.4 Reactions of [n]cumulenes with n ≥ 5........................................................................ 17
1.4.1 Miscellaneous reactions ..................................................................................... 18
1.4.2 Cycloaddition and oligomerization reactions ..................................................... 21
1.5 Motivation and goals of the doctoral thesis ............................................................... 26
1.6 References .................................................................................................................. 27
2. Chapter II. Cumulenes – Synthesis of tetraarylcumulenes [n]tBuPh and [n]Mes .... 36
2.1 Synthesis and structure of tetraarylcumulenes [n]tBuPh and [n]Mes....................... 36
2.1.1 General aspects and motivation .......................................................................... 36
2.1.2 Synthesis of the [n]tBuPh cumulene series ....................................................... 38
2.1.2.1 Synthesis of the bis-(3,5-di-t-butylphenyl)methanone endgroup ................... 38
2.1.2.2 Synthesis of [3]cumulene [3]tBuPh ............................................................... 39
2.1.2.3 Synthesis of [5]cumulene [5]tBuPh ............................................................... 40
2.1.2.4 Synthesis of [7]cumulene [7]tBuPh ............................................................... 41
2.1.2.5 Synthesis of [9]cumulene [9]tBuPh ............................................................... 47
2.1.2.6 Synthetic approaches to [11]cumulene [11]tBuPh and [13]cumulene
[13]tBuPh ...................................................................................................................... 53
2.1.3 Synthesis of the [n]Mes cumulene series ........................................................... 56
2.1.3.1 Limitations of “common” synthetic pathways: Toward the synthesis of
precursors to [n]Mes ..................................................................................................... 56
2.1.3.2 Synthesis of [9]cumulene [9]Mes ................................................................... 57
2.1.3.3 Synthesis of [7]cumulene [7]Mes ................................................................... 58
2.1.3.4 Synthesis of [5]cumulene [5]Mes ................................................................... 62
2.1.3.5 Synthesis of [3]cumulene [3]Mes ................................................................... 64
2.2 Summary and conclusion regarding the stability of [n]cumulenes ............................ 64
2.3 Experimental part ....................................................................................................... 66
2.3.1 General procedures and methods ....................................................................... 66
2.3.2 Experimental data and compound characterization............................................ 67
2.4 References .................................................................................................................. 88
3. Chapter III. Cumulene rotaxanes – Synthesis and stability of [n]tBuPh rotaxanes . 91
3.1 General introduction to rotaxanes .............................................................................. 91
3.2 Polyyne rotaxanes as motivation for cumulene rotaxane formation .......................... 93
3.3 Introduction to cumulene rotaxanes: Motivation and target ...................................... 95
3.4 Synthesis of rotaxane precursors and the appropriate cumulene rotaxanes ([9]tBuPh
rotaxanes) using three different macrocycles ....................................................................... 96
3.5 Stability of [9]cumulene rotaxanes and comparison to [9]tBuPh ........................... 100
3.6 Synthetic approach to higher [n]cumulene rotaxanes (n > 9) .................................. 103
3.6.1 Synthetic approach to [11]cumulene rotaxane ................................................. 103
3.6.2 Synthetic approach to [13]cumulene rotaxanes including UV/vis spectroscopy
studies .......................................................................................................................... 106
3.7 Summary and conclusion ......................................................................................... 111
3.8 Experimental part ..................................................................................................... 112
3.8.1 General procedures and methods ..................................................................... 112
3.8.2 Experimental data and compound characterization.......................................... 113
3.9 References ................................................................................................................ 116
4. Chapter IV. Characterization of [3]-, [5]-, [7]-, and [9]tBuPh including [9]tBuPh
rotaxanes and comparison to different series of [n]cumulenes ........................................ 118
4.1 UV/vis spectroscopy ................................................................................................ 118
4.1.1 Introduction ...................................................................................................... 118
4.1.2 UV/vis spectroscopy of [3]-, [5]-, [7]-, and [9]tBuPh ..................................... 120
4.1.2.1 General observations .................................................................................... 120
4.1.2.2 Influence of cumulene chain length .............................................................. 121
4.1.2.3 Influence of endgroups ................................................................................. 122
4.1.2.4 Conclusion including comparison of the band gap of cumulenes ................ 124
4.1.3 UV/vis spectroscopy of [9]cumulene rotaxanes and comparison to [9]tBuPh 124
4.2 X-ray crystallography of [n]cumulenes and discussion of bond length alternation
(BLA) .................................................................................................................................. 127
4.2.1 Introduction ...................................................................................................... 127
4.2.2 General observations ........................................................................................ 128
4.2.3 Bond angles ...................................................................................................... 130
4.2.4 Bond lengths ..................................................................................................... 131
4.2.5 Torsional angles ............................................................................................... 133
4.2.6 Bond length alternation .................................................................................... 134
4.3 Theoretical studies including comparison to UV/vis spectroscopy and BLA analysis .
.................................................................................................................................. 138
4.3.1 Influence of twist angles on BLA and electronic absorption energy ............... 138
4.3.2 UV/vis spectroscopy – Theory and experiment ............................................... 142
4.4 Electrochemistry (cyclic voltammetry) including comparison of the electronic band
gap (Eele) to the optical band gap (Eopt) .............................................................................. 144
4.4.1 Introduction ...................................................................................................... 144
4.4.2 Cyclic voltammetry of [3]tBuPh, [5]tBuPh, and [7]tBuPh ............................ 144
4.4.3 Comparison to known cumulene systems ........................................................ 146
4.4.4 Cyclic voltammetry of a [9]cumulene rotaxane ............................................... 149
4.4.5 Electronic and optical band gap of [n]tBuPh .................................................. 150
4.4.6 Comparison of electrochemical properties of cumulenes and polyynes .......... 152
4.5 NMR spectroscopy of [n]cumulenes ........................................................................ 153
4.5.1 Introduction ...................................................................................................... 153
4.5.2 13C NMR spectroscopy of [9]cumulene rotaxanes and their precursors .......... 154
4.5.3 13C- and correlation NMR spectroscopy of [n]tBuPh (n = 3, 5, and 7) and
[9]cumulene rotaxanes .................................................................................................... 156
4.5.4 Discussion and comparison .............................................................................. 162
4.6 Summary and conclusion ......................................................................................... 164
4.7 References ................................................................................................................ 165
5. Chapter V. Reactions of [n]cumulenes ........................................................................ 169
5.1 Addition reaction of [5]tBuPh with tetracyanoethylene (TCNE) ............................ 169
5.1.1 Motivation and objective .................................................................................. 169
5.1.2 Target, synthetic pathway, and test reactions ................................................... 174
5.1.3 Mechanistic studies and characterization by UV/vis spectroscopy and X-ray
crystallography ................................................................................................................ 182
5.2 Addition reactions of other cumulenes with TCNE ................................................. 189
5.2.1 [7]tBuPh cumulene .......................................................................................... 189
5.2.2 [5]MeOPh cumulene ....................................................................................... 196
5.2.3 [5]oTol cumulene ............................................................................................. 201
5.3 Addition reaction of a [9]cumulene with HCl.......................................................... 202
5.3.1 Synthesis of [5]cumulene 5.31 ......................................................................... 202
5.3.2 Characterization of [5]cumulene 5.31 via UV/vis spectroscopy and X-ray
crystallography ................................................................................................................ 203
5.4 Dimerization of [5]tBuPh ........................................................................................ 206
5.4.1 Synthesis of the dimer of [5]tBuPh ................................................................. 208
5.4.2 Characterization of the dimer of [5]tBuPh ...................................................... 211
5.4.3 X-ray crystallographic data: Discussion and comparison ................................ 212
5.5 Conclusion and summary ......................................................................................... 213
5.6 Experimental part ..................................................................................................... 215
5.6.1 General procedures and methods ..................................................................... 215
5.6.2 Experimental data and compound characterization.......................................... 216
5.7 References ................................................................................................................ 225
List of Figures
Figure 1.1 Schematic depiction of carbyne and homologous series of polyynes and cumulenes
as model compounds for carbyne. .............................................................................................. 2
Figure 1.2 Naturally occurring [3]cumulenes. .......................................................................... 4
Figure 1.3 Three common forms of organometallic cumulenes................................................ 5
Figure 1.4 Triferrocenyl[n]cumulenium salts (n = 2, 4, 6, 8) including mesomeric polyynic
structures. ................................................................................................................................... 6
Figure 1.5 Axial chirality and cis-trans isomerism of cumulenes. ........................................... 6
Figure 1.6 Schematic depiction of major structural classes of [n]cumulenes discussed in this
thesis, where n is the number of cumulated double bonds in a chain constructed of n + 1
carbon atoms. ............................................................................................................................. 8
Figure 1.7 Selected cumulene complexes with diverse coordination patterns. ....................... 20
Figure 1.8 Three possible structures for a [3]H iron carbonyl complex (cumulenic bonds that
are relevant for coordination are marked red). ......................................................................... 21
Figure 1.9 Diradical mesomeric structures as suggested by Hartzler for reactions of a
[5]cumulene. ............................................................................................................................. 22
Figure 1.10 Dimerization products of [5]cumulenes. .............................................................. 24
Figure 2.1 Structures of selected [n]cumulenes. ..................................................................... 37
Figure 2.2 Comparison of hybridization of the outermost carbon atom in a polyyne (left) and
cumulene (right) chain. ............................................................................................................ 38
Figure 2.3 UV/vis spectra taken during attempted conversion of precursor 2.31 to [13]tBuPh
(in Et2O). .................................................................................................................................. 56
Figure 3.1 Definition of a [2]rotaxane. .................................................................................... 91
Figure 3.2 Three common methods for rotaxane formation: a) capping, b) clipping, and c)
slipping. .................................................................................................................................... 92
Figure 3.3 Active template method for rotaxane formation. ................................................... 93
Figure 3.4 Polyyne rotaxanes reported by a) Gladysz and b) Anderson/Tykwinski. .............. 94
Figure 3.5 Three macrocycles, compounds 3.1, 3.2, and 3.3, used in the synthesis of
cumulene rotaxanes. ................................................................................................................. 96
Figure 3.6 DSC scan of [9]cumulene rotaxane 3.9. .............................................................. 102
Figure 3.7 DSC scan of [7]tBuPh. ........................................................................................ 103
Figure 3.8 UV/vis spectra taken during attempted conversion of precursor 3.14 to
[13]cumulene rotaxane 3.13 (in Et2O). .................................................................................. 108
Figure 3.9 UV/vis spectra taken during attempted conversion of precursor 3.16 to
[13]cumulene rotaxane 3.17 (in Et2O). .................................................................................. 111
Figure 4.1 Electronic effects based on odd- and even-numbered, as well as alkyl- and aryl
endcapped [n]cumulenes, demonstrated schematically with canonical structures for [4]- and
[5]cumulenes. ......................................................................................................................... 119
Figure 4.2 UV/vis spectra of [n]cumulenes: UV/vis spectra of a) [n]tBuPh and b) [n]Mes.
Both sets of spectra were measured in Et2O and normalized to the most intense low energy
absorption. Spectra of c) [n]Ph (in benzene) and d) [n]Cy (in Et2O). Spectra of [n]Ph and
[n]Cy were adapted with permission from reference 1. Copyright 1964 John Wiley & Sons.
................................................................................................................................................ 121
Figure 4.3 Qualitative UV/vis spectra (in Et2O) of the [9]cumulene rotaxanes 3.8, 3.9, and
3.10 as well as the “naked” [9]cumulene [9]tBuPh (a quantitative spectrum was recorded for
3.9, see right axis). ................................................................................................................. 125
Figure 4.4 Quantitative UV/vis spectra of [n]tBuPh (n = 3, 5, 9, in CHCl3) and [9]cumulene
rotaxane 3.9 (in Et2O). ............................................................................................................ 127
Figure 4.5 Description of bond lengths and twist angles using [9]Mes (ORTEP drawings with
20% probability level): a) structure of [9]Mes including carbon labeling, b) planes defining
the twist angle (front view), and c) planes defining the twist angle (side view). Carbons C11–
C16 define the grey-colored plane, while carbons C11, C21, and C1–C5, as well as the
appropriate symmetric atoms define the blue-colored plane.................................................. 129
Figure 4.6 Bond angles of the cumulene chain in [n]tBuPh and [n]Mes with n = 3, 5, 7 and
n = 5, 7, 9, respectively. ......................................................................................................... 130
Figure 4.7 Illustration of possible intramolecular C–H/π-interactions of [3]tBuPh. ............ 131
Figure 4.8 X-ray crystallographic structures of [3]tBuPh, [5]tBuPh, [7]tBuPh, [5]Mes,
[7]Mes, and [9]Mes (ORTEP drawings with 20% probability level). ................................... 132
Figure 4.9 Bond lengths (in Å) of the cumulene chain of the series [n]tBuPh, [n]Mes, [n]Ph,
and [n]Cy. .............................................................................................................................. 133
Figure 4.10 BLA values (inset) versus chain length n (lines are only a guide for the eye). . 137
Figure 4.11 Optimized geometries of [9]tBuPh and [9]MePh cumulenes. .......................... 138
Figure 4.12 BLA calculation of [9]Ph versus the aryl twist angle. ....................................... 139
Figure 4.13 Left: [9]tBuPh cumulene: B3LYP-MOs for equilibrium geometry (aryl twist
angle of 32°). Right: [9]Mes cumulene: B3LYP-MOs for SCS-MP2/def2-TZVPP equilibrium
geometry (aryl twist angle of 49°). ......................................................................................... 140
Figure 4.14 Hartree-Fock-MOs for [9]Ph with an aryl twist angle of 90°. .......................... 141
Figure 4.15 UV/vis spectra of one possible conformer of [9]Ph (D2 symmetry) in dependence
of the phenyl twist angle calculated at CC2/def2-TZVPP//SCS-MP2/def2-TZVPP level. ... 142
Figure 4.16 Calculated and experimental UV/vis spectra of [9]MePh/[9]tBuPh (top) and
[9]Mes (bottom) cumulenes. The twist angles are 31° for [9]MePh, as well as 49° for [9]Mes.
All theoretical UV/vis spectra have been computed at the CC2/def2-TZVPP//SCS-MP2/def2-
TZVPP level of theory. .......................................................................................................... 143
Figure 4.17 Cyclic voltammogram of [3]tBuPh. 0.1 M electrolyte (Bu4NPF6) in CH2Cl2.
Ferrocene (Fc) has been used as the internal standard. Scan rate: 150 mV/s......................... 145
Figure 4.18 Cyclic voltammogram of [5]tBuPh. 0.1 M electrolyte (Bu4NPF6) in CH2Cl2.
Ferrocene (Fc) has been used as the internal standard. Scan rate: 150 mV/s......................... 145
Figure 4.19 Cyclic voltammogram of [7]tBuPh. 0.1 M electrolyte (Bu4NPF6) in CH2Cl2.
Ferrocene (Fc) has been used as the internal standard. Scan rate: 150 mV/s......................... 146
Figure 4.20 Cyclic voltammogram of 0.001 M [5]Ph. 0.2 M electrolyte (Bu4NPF6) in
CH2Cl2, referenced to SCE. Scan rate 200 mV/s. The graphic is adapted from ref[20]. ....... 147
Figure 4.21 Cyclic voltammogram of rotaxane 3.9. 0.1 M electrolyte (Bu4NPF6) in CH2Cl2.
Ferrocene (Fc) has been used as the internal standard. Scan rate: 150 mV/s......................... 150
Figure 4.22 Plots of electronic (left) and optical (right) band gaps (Eele and Eopt, respectively)
versus 1/n for the cumulenes [3]tBuPh, [5]tBuPh, [7]tBuPh, and 3.9 (band gap data taken
from Table 4.7). ...................................................................................................................... 152
Figure 4.23 Carbon atom labeling of precursors and [9]cumulene rotaxanes for NMR
spectroscopic discussion. ....................................................................................................... 154
Figure 4.24 Comparison of 13C NMR spectra of [9]cumulene rotaxane 3.9 and precursor 3.6
(dotted lines highlight the assignment of signals in the spectrum of [9]cumulene rotaxane 3.9
that result from precursor 3.6, which is present as an impurity). ........................................... 155
Figure 4.25 13C NMR spectra (165–100 ppm region) of [9]cumulene rotaxanes 3.9 (top) and
3.8 (bottom). ........................................................................................................................... 156
Figure 4.26 Decoupled and coupled 13C NMR spectra of [3]tBuPh including the
corresponding HMBC NMR spectrum (aryl region). ............................................................ 158
Figure 4.27 Decoupled and coupled 13C NMR spectra of [5]tBuPh including the
corresponding HMBC NMR spectrum (aryl region). ............................................................ 159
Figure 4.28 Decoupled and coupled 13C NMR spectra of [7]tBuPh including the
corresponding HMBC NMR spectrum (aryl region). ............................................................ 160
Figure 4.29 Expansion of the HMBC spectrum of [9]cumulene rotaxane 3.9 (inset: relevant
correlation signals between H2 and C1). ............................................................................... 161
Figure 4.30 Expansion of the HMBC spectrum of [9]cumulene rotaxane 3.8 (inset: relevant
correlation signals between H2 and C1). ............................................................................... 162
Figure 4.31 Plot of 13C NMR carbon chemical shifts versus the number of double bonds n for
[n]tBuPh (n = 3, 5, 7) and [9]cumulene rotaxanes 3.8 and 3.9. ............................................ 163
Figure 5.1 HOMO and LUMO of TCNE and [5]tBuPh. ...................................................... 173
Figure 5.2 Performed reaction of [5]tBuPh with TCNE, monitored by TLC analysis. ........ 175
Figure 5.3 1H NMR spectrum of product A (inset: expansion of the aryl region). ............... 176
Figure 5.4 13C NMR spectrum of product A, allene signal highlighted; * = Et2O. ............... 176
Figure 5.5 Top: Overlaying of product A (in CH2Cl2) with EtOH and MeOH giving adducts
5.12 and 5.13, respectively. Bottom: ORTEP drawings (20% probability level) for compounds
5.12 and 5.13. ......................................................................................................................... 177
Figure 5.6 Identification of product A as cyclic [3]dendralene 5.14..................................... 178
Figure 5.7 Left: Identification of product B as [4]radialene 5.15. Right: ORTEP drawing
(20% probability level) for compound 5.15. .......................................................................... 178
Figure 5.8 Conversion of product C to A (compound 5.14) and B (compound 5.15), and the
associated TLC analysis. ........................................................................................................ 179
Figure 5.9 Conversion of product C with Br2 affording [4]dendralene 5.17; ORTEP drawing
(20% probability level) for compound 5.17. .......................................................................... 180
Figure 5.10 Identification of product C as cyclobutane 5.11. ............................................... 182
Figure 5.11 Computed relative energies (kcal/mol) of the products 5.11 (product C), 5.14
(product A), and 5.15 (product B), as well as the hypothesized product 5.10a, in comparison
to the reactants TCNE and [5]tBuPh. Calculations based on DFT including (red), and without
(blue) dispersion interaction corrections. ............................................................................... 186
Figure 5.12 Quantitative UV/vis spectrum of radialene 5.15 (in CHCl3); Inset: λmax values for
5.15 as a function of solvent. .................................................................................................. 187
Figure 5.13 Reaction of [7]tBuPh with TCNE, and the associated TLC analysis. ............... 190
Figure 5.14 Qualitative UV/vis spectrum of product A measured in hexanes/CH2Cl2 (eluent
from column chromatography). .............................................................................................. 191
Figure 5.15 IR spectrum of product A. ................................................................................. 192
Figure 5.16 1H NMR spectrum (recorded in CD2Cl2) of product C after addition of HCl. .. 193
Figure 5.17 Qualitative UV/vis spectrum of product C before (black curve) and after (red
curve) addition of HCl measured in hexanes/CH2Cl2 (eluent from column chromatography).
................................................................................................................................................ 194
Figure 5.18 Qualitative UV/vis spectrum of product D measured in hexanes/CH2Cl2 (eluent
from column chromatography). .............................................................................................. 195
Figure 5.19 Reaction of [5]MeOPh (containing pink and baseline spot) with TCNE, and the
associated TLC analysis. ........................................................................................................ 197
Figure 5.20 Products B–D with appropriate ORTEP drawings (20% probability level). ..... 198
Figure 5.21 Second and third reactions of [5]MeOPh with TCNE, and the associated TLC
analysis, in comparison to the first reaction. The precursor contained a) only the pink spot and
b) the pink and baseline spot. ................................................................................................. 199
Figure 5.22 Reaction of [5]oTol with TCNE, and the associated TLC analysis. Proposed
product structures are shown as A, B, and D. ........................................................................ 201
Figure 5.23 Qualitative UV/vis spectrum of [5]cumulene 5.31 (in Et2O). ........................... 204
Figure 5.24 Two canonical structures of [5]cumulene 5.31. ................................................. 205
Figure 5.25 Scale-up of [5]tBuPh. ........................................................................................ 207
Figure 5.26 Thermal reaction of [5]tBuPh in toluene, and associated TLC analysis. .......... 209
Figure 5.27 [4]Radialene 5.32 as the desired product from the thermal reaction of [5]tBuPh
in toluene. Additionally, the possible head-to-tail and head-to-head dimers are shown. ...... 209
Figure 5.28 Qualitative UV/vis spectrum of the pink reaction mixture (A) as presented in
Figure 5.26 (in toluene). ......................................................................................................... 211
List of Schemes
Scheme 1.1 Synthesis of [3]cumulenes based on the carbene/carbenoid route. ........................ 9
Scheme 1.2 Synthesis of [3]cumulenes based on reductive elimination of trihaloalkanes. ...... 9
Scheme 1.3 Synthesis of [3]cumulenes based on acetylenic diol derivatives. ........................ 10
Scheme 1.4 Synthesis of [4]Fc based on a diol derivative. ..................................................... 10
Scheme 1.5 Synthesis of [4]cumulenes from diesters. ............................................................ 11
Scheme 1.6 Synthesis of [4]cumulenes from [3]cumulenes. ................................................... 11
Scheme 1.7 Synthesis of a [4]cumulene via carbene trapping with an olefin. ........................ 12
Scheme 1.8 Synthesis of [5]cumulenes based on carbenes/carbenoids. .................................. 13
Scheme 1.9 Synthesis of unsymmetrical substituted [5]cumulenes based on carbenoid
precursors. ................................................................................................................................ 13
Scheme 1.10 Synthesis of [5]Me from [3]Me. ........................................................................ 14
Scheme 1.11 Synthesis of [5]cumulenes based on acetylenic diol derivatives. ...................... 14
Scheme 1.12 Synthesis of [6]Fc. ............................................................................................. 15
Scheme 1.13 Synthesis of [7]cumulenes based on acetylenic diol derivatives. ...................... 16
Scheme 1.14 Synthesis of the bis[7]cumulene [7]pPh. ........................................................... 16
Scheme 1.15 Synthesis of [9]cumulenes based on acetylenic diol derivatives. ...................... 17
Scheme 1.16 Oxidation products of [5]tBu. ............................................................................ 18
Scheme 1.17 Examples of cycloaddition and dimerization reactions reported for
[5]cumulenes. ........................................................................................................................... 22
Scheme 1.18 Trimerization reaction of a [5]cumulene. ........................................................... 25
Scheme 1.19 Trimerization reaction of [5]Ph. ........................................................................ 26
Scheme 2.1 Synthesis of ketone 2.2. ....................................................................................... 39
Scheme 2.2 Synthesis of terminal acetylene 2.4. ..................................................................... 39
Scheme 2.3 Synthesis of precursor 2.5 and reductive elimination to [3]tBuPh. ..................... 40
Scheme 2.4 Synthesis of [5]tBuPh. ......................................................................................... 41
Scheme 2.5 Synthetic approaches to precursors to [7]tBuPh, compounds 2.10 and 2.11
(lithiation route). ....................................................................................................................... 42
Scheme 2.6 Synthetic approach to the precursor (2.13) to [7]tBuPh (“mixed” homocoupling
route). ....................................................................................................................................... 43
Scheme 2.7 Synthetic approach to the precursor (2.18) to [7]tBuPh (FBW route). ............... 44
Scheme 2.8 Rearrangement reaction of ketone 2.16 to triyne 2.18, followed by a reductive
elimination to [7]tBuPh. .......................................................................................................... 45
Scheme 2.9 Synthetic approaches to triynes 2.18 and 2.22 (precursors to [7]tBuPh) using
bistrimethylsilyltriyne 2.19. ..................................................................................................... 47
Scheme 2.10 Synthesis of [9]tBuPh. ....................................................................................... 48
Scheme 2.11 Synthesis of tetrayne 2.14. ................................................................................. 49
Scheme 2.12 Synthetic approach to [11]tBuPh. ...................................................................... 53
Scheme 2.13 Synthetic approach to [13]tBuPh. ...................................................................... 54
Scheme 2.14 Unsuccessful approach to mesityl acetylene 2.33. ............................................. 57
Scheme 2.15 Synthesis of [9]Mes. .......................................................................................... 58
Scheme 2.16 Synthesis of [7]Mes. .......................................................................................... 59
Scheme 2.17 Reaction of terminal acetylene 2.42 with EtMgBr and ethyl formate. .............. 62
Scheme 2.18 Synthesis of [5]Mes. .......................................................................................... 63
Scheme 2.19 Pd-catalyzed homocoupling reactions giving precursor 2.49. ........................... 64
Scheme 3.1 Synthesis of polyyne rotaxanes. ........................................................................... 95
Scheme 3.2 Formation of rotaxanes 3.4, 3.5, and 3.6 via an active metal templated
homocoupling reaction. ............................................................................................................ 97
Scheme 3.3 Formation of rotaxanes 3.4 and 3.6 via an active metal templated Cadiot-
Chodkiewicz heterocoupling reaction. ..................................................................................... 98
Scheme 3.4 Synthesis of cumulene rotaxanes 3.8, 3.9, and 3.10............................................. 99
Scheme 3.5 Synthetic approach to [11]cumulene rotaxane 3.11. .......................................... 105
Scheme 3.6 Synthetic approach to [13]cumulene rotaxane 3.13. .......................................... 106
Scheme 3.7 Synthetic approach to [13]cumulene rotaxane 3.17. .......................................... 109
Scheme 5.1 Reaction of [3]cumulenes with TCNE in CH2Cl2 at rt....................................... 170
Scheme 5.2 Conversion of cyclobutane 5.3 to 5.7 and 5.8. ................................................... 170
Scheme 5.3 TCNE addition to [5]Fc giving cycloadduct 5.9. ............................................... 171
Scheme 5.4 General cyclization/cycloreversion method giving donor-substituted TCBD
derivatives. ............................................................................................................................. 171
Scheme 5.5 Expected cyclization/cycloreversion reaction of a [5]cumulene with TCNE to the
unsymmetrically substituted [3]cumulene 5.10. .................................................................... 172
Scheme 5.6 Reaction of a push-pull [3]cumulene with EtOH and Br2, respectively. ........... 179
Scheme 5.7 Conversion of product B (compound 5.15) to [4]dendralenes 5.19 and 5.20;
ORTEP drawing (20% probability level) for compound 5.19. .............................................. 181
Scheme 5.8 Overview of compounds identified from the reaction of [5]tBuPh with TCNE
including further addition reactions. ...................................................................................... 183
Scheme 5.9 Summary of a mechanism of the reaction of a [3]cumulene with TCNE as
suggested by Kawamura and coworkers. ............................................................................... 183
Scheme 5.10 Proposed mechanism for the conversion of 5.11 to 5.14 and 5.15. ................. 184
Scheme 5.11 Proposed concerted mechanism for the addition reaction of bromine to 5.11 and
the stepwise addition of ROH to 5.14. ................................................................................... 185
Scheme 5.12 Synthetic approach to [5]cumulene [5]MeOPh. .............................................. 200
Scheme 5.13 Synthesis of [5]cumulene 5.31 via reductive elimination of tetrayne 2.14 in
CH2Cl2, and the associated TLC analysis. ............................................................................. 203
Scheme 5.14 Thermal dimerization of [5]cumulenes reported by Hartzler and Iyoda. ......... 207
Scheme 5.15 Thermal trimerization of [5]Ph reported by Kawamura. ................................. 208
List of Tables
Table 2.1 Reductive elimination of 2.25 under various conditions. ........................................ 52
Table 2.2 Methylation reaction of 2.42 under various conditions. .......................................... 60
Table 3.1 Comparison of the qualitative stability of [9]tBuPh and [9]cumulene rotaxane 3.9
when kept under an argon atmosphere at rt. ........................................................................... 101
Table 4.1 Lowest energy absorption λmax (in nm) and energy values Eg (in eV) of [n]tBuPh,
[n]Mes, [n]Ph, and [n]Cy (in Et2O). ..................................................................................... 122
Table 4.2 UV/vis spectroscopic data (λmax in nm) of selected [5]cumulenes with different
endgroups. .............................................................................................................................. 123
Table 4.3 UV/vis spectroscopic data (absorption wavelengths in nm) of [9]tBuPh and
[9]cumulene rotaxanes 3.9, 3.10, and 3.8 (in Et2O). .............................................................. 126
Table 4.4. Aryl twist angles of aromatic ring relative to cumulenic framework. .................. 134
Table 4.5 Selected bond lengths (Å) for cumulene series [n]Ph, [n]Cy, [n]tBuPh and
[n]Mes, as well as theoretically calculated values for [n]H including BLA data. ................ 136
Table 4.6 CV data of [5]cumulenes [5]tBu, [5]tBu/Ph, [5]EtPh, and [5]tBuPh. ................ 149
Table 4.7 Selected UV/vis spectroscopic and electrochemical details including optical (Eopt)
and electronic (Eele) band gap values for [3]tBuPh, [5]tBuPh, [7]tBuPh, and 3.9. .............. 151
Table 5.1 Natural population analysis (NPA) charges of the cumulenic C-atoms in
[5]cumulene [5]tBuPh. .......................................................................................................... 173
Table 5.2 Bond lengths (Å) of radialene 5.15 and selected radialenes known from literature.
................................................................................................................................................ 189
Table 5.3 Bond lengths (Å) and bond angles (°) of [5]cumulene 5.31. ................................. 206
Table 5.4 Bond lengths (Å) and bond angles (°) of [4]radialene 5.32 (left) and Iyoda’s
[4]radialene 5.33 (right). ........................................................................................................ 213
List of Abbreviations and Symbols
Å Angström
Ac acetyl
ACN acetonitrile
Ad adamantyl
An 10,10-dimethyl-9,10-dihydroanthracenyl
anal analytical
anhydr anhydrous
APPI atmospheric pressure photoionization
aq aqueous
Ar aryl
ATR attenuated total reflection
BLA bond length alternation
br broad
Bu butyl
calcd calculated
CCDC Cambridge Crystallographic Data Centre
cm centimeter(s)
cmpd compound
CSD Cambridge Structural Database
CuAAC copper(I)-catalyzed azide-alkyne cycloaddition
CV cyclic voltammetry
Cy 2,2,6,6-tetramethylcyclohexyl
d doublet
d day(s)
deg degree(s)
decomp decomposition
DWCNT double-wall carbon nanotube
δ chemical shift
∆ heat
DCTB trans-2-(3-(4-t-butylphenyl)-2-methyl-2-propenylidene)malononitrile
DFT density functional theory
DSC differential scanning calorimetry
E potential
ε molar extinction coefficient
EI MS electron impact mass spectroscopy
ESD estimated standard deviation
ESI electrospray ionization
Et ethyl
EtOAc ethyl acetate
equiv equivalent(s)
eV electron volt(s)
FBW Fritsch-Buttenberg-Wiechell
Fc ferrocenyl
Fl fluorenyl
FT-ICR Fourier transform ion cyclotron resonance mass spectrometry
g gram(s)
h hour(s)
HMBC heteronuclear multiple bond correlation
HOMO highest occupied molecular orbital
HRMS high resolution mass spectrometry
Hz Hertz
i iso
IR infrared
irrev irreversible
J coupling constant
kcal kilocalorie
λ absorption wavelength
λmax lowest energy absorption wavelength
L liter(s)
LUMO lowest unoccupied molecular orbital
m multiplet (NMR)
m medium (IR)
µ micro
m meta
m-CPBA meta-chloroperoxybenzoic acid
M formula weight
M molar
MALDI matrix-assisted laser desorption ionization
Me methyl
Mes mesityl
mg milligram(s)
MHz megaHertz
mL milliliter(s)
mmol millimole(s)
mol mole(s)
Mp melting point
MS mass spectroscopy
mV millivolt(s)
m/z mass-to-charge ratio
n-BuLi n-butyllithium
NBS N-bromosuccinimide
NIR near-infrared
nm nanometer(s)
NMR nuclear magnetic resonance
o ortho
ORTEP Oak Ridge thermal ellipsoid plot
p para
PCC pyridinium chlorochromate
Ph phenyl
ppm parts per million
Pr propyl
q quartet
quant quantitative
qurev quasireversible
Rf retention factor
ref reference
rt room temperature
s singlet (NMR)
s strong (IR)
s second(s)
sev several
Sub suberyl
supertrityl tris(3,5-di-t-butylphenyl)methyl
t triplet (NMR)
t tertiary
TBAF tetrabutyl ammonium fluoride
TCBD tetracyanobutadiene
TCNE tetracyanoethylene
temp temperature
TFE trifluoroethylene
THF tetrahydrofuran
THP tetrahydropuran
TIPS triisopropylsilyl
TLC thin layer chromatography
TMEDA N,N,Nʹ′,Nʹ′-tetramethylethylenediamine
TMS trimethylsilyl
TOF time-of-flight
Tol tolyl
UV ultraviolet
UV/vis ultraviolet-visible
vw very weak (IR)
w weak (IR)
1
1. Chapter I. Introduction to cumulenes†
1.1 Definition of the one-dimensional carbon allotrope carbyne
The three- and two-dimensional carbon allotropes represent the insulating sp3-
hybridized carbon allotrope diamond and the conducting sp2-hybridized carbon allotropes
graphite and graphene (as well as the recently discovered fullerenes and carbon nanotubes),
respectively. Both allotrope classes are well-investigated and show interesting electronic,
physical, and optical properties. Resulting applications are used in all fields of chemistry
ranging from molecular wires over thin film transistors to solar cells, etc.1–6 Aside from these
carbon allotropes, one example, the one-dimensional sp-hybridized carbon allotrope carbyne,
is missing. In contrast to the three- and two-dimensional allotropes, the one-dimensional
allotrope carbyne has rarely been investigated and characterized. The existence of carbyne has
been a topic of much and sometimes controversial7,8 discussion over the years.6,9–15 The
natural existence of this one-dimensional allotrope has been proposed in, for example,
meteorites,16,17 interstellar dust,18 and shock-compressed graphite,19 as well as terrestrial plant,
fungal, and marine sources (in the case of polyynes).20–22 Carbyne is synthetically not readily
available or isolable like other carbon allotropes. Evidence of carbyne formation has been
provided by a variety of processes, e.g., by solution-phase synthesis,13,23,24 laser irradiation of
graphite,25 or gas-phase deposition methods.26 Whether naturally occurring or produced in a
laboratory, carbyne remains rather poorly characterized as a material, and specific properties
of carbyne are thus often difficult to define. There are, nevertheless, quite a number of
fascinating properties and resulting applications predicted for carbyne based on unusual
electrical and optical nature of sp-hybridized carbon,27,28 such as nanoelectronic or spintronic
devices,29 nonlinear optical materials,30–33 and molecular wires.34–36 Recent theoretical
calculations have also suggested that under tension, carbyne could be twice as stiff as the
stiffest known materials, such as carbon nanotubes, graphene, and diamond.29,37 Furthermore,
† A version of this chapter has been published: J. A. Januszewski, R. R. Tykwinski, Chem. Soc. Rev. 2014, 43,
3184–3203, see http://dx.doi.org/10.1039/C4CS00022F - Reproduced by permission of The Royal Society of
Chemistry; Portions of this chapter have been published: J. A. Januszewski, F. Hampel, C. Neiss, A. Görling, R.
R. Tykwinski, Angew. Chem. Int. Ed. 2014, 53, 3743–3747.
2
even though carbyne might be extremely stiff against strain, it might also allow for
deformation through bending without influence on properties.38
1.2 Cumulenes as one possible isomer of the carbon allotrope carbyne
In principle, two possible forms of carbyne exist. On one hand, carbyne can be built up
from sp-hybridized carbon atoms with alternating triple and single bonds forming a series of
semiconducting molecules called polyynes. On the other hand, cumulenes, composed of
consecutive double bonds, define the alternative form of carbyne and are assumed to possess
metallic behavior (Figure 1.1).39,40 Which form is preferred remains a so far unanswered
question that still needs to be investigated.
Figure 1.1 Schematic depiction of carbyne and homologous series of polyynes and cumulenes
as model compounds for carbyne.
Considering the synthetical point-of-view, polyynes have been studied intensively
over the past decades.41–48 The longest polyyne that has been synthesized contains 44 carbon
atoms in a chain constructed of 22 acetylene units.41 Due to the fact that instability increases
with increasing chain length, this polyyne is endcapped with bulky endgroups shielding the
most reactive outer acetylene units of the chain. The experimental results illustrate that this
compound has not yet achieved a carbyne-like “status” when comparing the UV/vis
spectroscopic data, which show that no saturation of the optical band gap has been reached.
Extrapolation from the spectroscopic data predicts saturation of properties at the point of 48
acetylene units in order to form a carbyne-like compound, i.e., where elongation through
additional acetylene units has no effect. In contrast, the investigation of [n]cumulenes (where
n is the number of cumulated double bonds in a chain constructed of n + 1 carbon atoms) as
model compounds for carbyne has barely been discussed in the literature. To date, the longest
3
cumulenes to be synthesized and studied are [9]cumulenes, i.e., molecules with nine
consecutive double bonds in a chain of 10 carbons.49–52 On the basis of structure, a
[9]cumulene is only approximately equal to the length of a rather short polyyne (a tetrayne).
Unlike polyynes of this length, however, [9]cumulenes show dramatic instability under
ambient conditions.
In conclusion, regarding the practical results achieved in laboratory, polyynes seem to
be more accessible compared to cumulenes. Recent theoretical predictions, however, show
that polyynes and cumulenes are linked to each other, more than initially expected. Yakobson
and coworkers,29 for example, predict a transition from cumulenic to acetylenic geometry as
carbyne is stretched. This effect goes along with an increase in bond length alternation (BLA),
energy of Peierls distortion, and band gap. Thus, a transition from a metallic to an insulating
state could be reached, suggesting an interesting property for carbyne-like compounds as
conducting polymers. The reverse process has also been predicted, and studies suggest a
transition from a polyyne structure to a cumulene form, for example, under UV
photoexcitation53 leading to a new photogenerated species that is cumulenic. Another study of
the transition from a polyynic to a cumulenic form is suggested to proceed via a charge
transfer between metal nanoparticles such as silver and gold connected by phenyl-substituted
oligoynes.39 Consequently, the observed rearrangement of the polyynic structure to a
cumulenic-like structure could result in possible applications as tunable electronics. Finally,
theoretical calculations typically predict higher stability for the polyyne form of
carbyne.29,54,55 This prediction is also consistent with practical results. An ultimate assignment
of carbyne to either the polyyne or the cumulene form, however, remains undetermined.
1.2.1 Cumulenes in history and nature
The history and thus the interest in research of polyynes have started in the end of the
19th century and the beginning of the 20th century represented by pioneers like von Bayer56
and Dupont.57 Later, from 1950 on, Jones,45,58 Walton,43,44,59 and Bohlmann41,60 have played a
huge role in this field. The chemistry of cumulenes, on the other hand, has started with its
most famous pioneer, Richard Kuhn, in the 1930s.61 It is Kuhn, who has lent the cumulene the
name („Kumulen“) that is commonly used to date. Before 1938, however, some evidence of
cumulenes have been observed. Allenes ([2]cumulenes), the smallest cumulenes, containing
two cumulated double bonds, have been investigated starting with the unknowing formation
4
of an allene by Burton and Pechmann in 188762 (the allenic structure was revealed 67 years
later by Jones63). The first [3]cumulene, the tetraphenylbutatriene has been accidentally made
by Brand64 in 1921. The first [5]cumulene, also a tetraphenyl-substituted cumulene has been
formed by Kuhn61 in 1938, and since then, the study of cumulenes has reached their zenith,
mostly investigated by the groups of Kuhn, Cadiot, and Bohlmann, especially in the 1950s
and 1960s.50,51,65–69
Cumulenes, mostly as allenes, have also been observed in and extracted from natural
sources70 from the 1920s to the 1960s by, e.g., Staudinger,71 Celmer,72 and Jones.73–75
[3]Cumulenes are also present in nature, but rarely. The first „natural“ [3]cumulene has been
isolated by Bohlmann et al. from Conyza bonariensis, and its instability allows handling only
in solution or in crystalline form at −70 °C, otherwise it tends to polymerization.76 Three
additional [3]cumulenes, also extracted by Bohlmann, have followed between 1966 and 1971,
and are the last examples discovered to date (Figure 1.2). All four [3]cumulene natural
products show high instability, thus, cumulenes are not only less common in nature compared
to the polyynes, but also less stable.20,77,78
Figure 1.2 Naturally occurring [3]cumulenes.
1.2.2 Cumulenes in organometallic chemistry
Aside from the application of cumulenes in organic chemistry, this class of
compounds has also been extensively used in organometallic chemistry, as metallacumulenes
(metallated cumulenes), i.e., coordinated directly to metal centers, or as cumulenes that
possess organometallic endgroups, e.g., ferrocenyl groups. Three common forms of
organometallic cumulenes are presented in Figure 1.3. Finally, several “unusual” cumulene
complexes exist that show a more exotic coordination of the cumulene chain to the metal
centers, such as multiple coordinations or bimetallic systems.79–83 All organometallic
cumulenes are presented in more detail, regarding the reactivity of these compounds, in
Section 1.3.1.
5
Figure 1.3 Three common forms of organometallic cumulenes.
While the smaller metallacumulenes appear quite numerous in the field of
organometallic chemistry as vinylidene or allenylidene metal complexes84–86 containing a
M=C=C or a M=C=C=C unit, respectively, complexes with higher cumulenes remain rare to
date.87,88 In the case of higher metallacumulenes, pentatetraenylidene complexes82,89–91 are
reported most frequently, while there are only few analogues of the lower butatrienylidene
complexes92 and just one heptahexaenylidene93 complex known although it has not been
isolated. Considering application possibilities, metallacumulenes have gained attention as
potential precursors for optical or electronic materials, such as e.g., molecular wires
(as bimetallic metallacumulenes),83,91 or in synthesis for important catalytic reactions.82,86,88,91
In the case of ferrocenyl-substituted cumulenes, Bildstein94 is the pioneer in this field
after forming a series of ferrocenyl[n]cumulene, ranging from n = 2–6 as well as
ferrocenyl[n]cumulenium salts with n = 2, 4, 6, 8 (Figure 1.4). This class of compounds is
positioned between “metallacumulenes” and typical “organic cumulenes” and shows redox
activity, donor properties, and reasonable stability that can be attributed to the ferrocenyl
endgroups.
6
Figure 1.4 Triferrocenyl[n]cumulenium salts (n = 2, 4, 6, 8) including mesomeric polyynic
structures.
1.2.3 Structural differences of cumulenes
Depending on the number of double bonds n in a cumulenic chain, [n]cumulenes show
different stereochemical properties. Even-numbered [n]cumulenes (n = even) show axial
chirality and thus optical activity in the case of adequate choice of endgroups. In contrast,
odd-numbered [n]cumulenes (n = odd) can possess cis-trans isomerism (Figure 1.5).95 This
difference not only highly influence the properties of these two classes of cumulenes, it also
has a big impact from the synthetic point-of-view. While odd-numbered cumulenes can be
synthesized rather easily using several standard synthetic methods, even-numbered cumulenes
are more difficult to obtain due to synthetic accessibility of the precursors, and no general
synthetic route can be applied.
Figure 1.5 Axial chirality and cis-trans isomerism of cumulenes.
7
1.3 Synthesis of [n]cumulenes
1.3.1 General cumulene synthesis
Since Kuhn’s early work on cumulenes in the 1930s,61 quite a number of synthetic
approaches have been developed to provide [n]cumulenes of various lengths. For the shorter
analogues with n = 3–5, various common methods exist, mainly because of greater stability of
these cumulenes when compared to longer [n]cumulenes (n > 5). Better stability of the
cumulene product also fosters greater functional group tolerance, and many more shorter
cumulenes have been synthesized with n < 5. The synthesis of longer cumulenes has benefited
greatly from the pioneering advances for assembling acetylenic compounds made by the
groups of Bohlmann,41,60 Walton,43,44,59 and Jones,45,58 as well as others.96–98 From this era,
acetylene compounds, and more precisely oligoyne diols,60,99 have emerged as most
convenient synthetic precursors for [n]cumulenes.
The following overview gives a summary of synthetic methods for cumulenes and is
presented based on ascending molecular length, starting from [3]cumulenes and concluding
with the longest [n]cumulenes known to date, the [9]cumulenes.100 Standard synthetic
pathways as well as alternative synthetic approaches are presented giving mainly symmetrical
substituted cumulenes (four identical endgroups), although some exceptions are found. It is
also noteworthy that the synthesis of tetraalkyl-substituted [n]cumulenes is typically more
difficult when compared to tetraaryl[n]cumulene, primarily due to stability of the products.
Finally, the assembly of even-numbered [n]cumulenes (n = 4 and 6) is more complicated than
odd-numbered [n]cumulenes (n = 5, 7, 9) often due to synthetic accessibility of the
precursors. Metallacumulenes are not included in this synthetic summary.88,101–104 The
synthetic routes to [2]cumulenes, i.e., allenes, are not covered in this thesis. In addition, only a
few synthetic methods for [3]cumulenes are depicted, since too many individual approaches
have been reported to date, and this work has been reviewed.105–107
To facilitate discussions in synthesis and reactivity sections in the introduction as well
as the main part of the thesis, Figure 1.6 offers a legend that introduces the endgroups of
cumulenes and the associated nomenclature.
8
R
R R
R
m
t-Bu
t-Bu
[n]Fl [n]An
R
R
=
Ph
Ph
[n]Ph
t-Bu
t-Bu
[n]Cy[n]Ad
[n]Mes[n]tBuPh
[n]Sub
Fe
Fe
[n]Fc
[n]Me[n]tBu
[n]MeOPh
OMe
OMe
[n]cumulenes(n = m + 2)
H
H
[n]H
[n]EtPh [n]pPh
[n]oTol
Fe
[n]Fc/Ph
[n]tBu/Ph
[n]MePh
Figure 1.6 Schematic depiction of major structural classes of [n]cumulenes discussed in this
thesis, where n is the number of cumulated double bonds in a chain constructed of n + 1
carbon atoms.
1.3.2 [3]Cumulenes
One of the common routes to [3]cumulenes is the metal catalyzed dimerization of
carbenes/carbenoids (Scheme 1.1), as summarized in a review by Stang.108 These can be
accomplished with Cu(I) catalyst for aryl or alkynyl substituents as described by
Diederich,109,110 as well as Kunieda and Takizawa.111 On the other hand, Iyoda112,113 has used
a Ni(II) catalyst to form [3]cumulenes. Tetraalkyl[3]cumulenes can be made directly from a
lithium carbenoid as reported independently by, for example, Köbrich,114 Komatsu,115 and
Oda.116 Stang, on the other hand, successfully has formed a variety of alkyl-substituted
[3]cumulene via the carbenoid route using ethynylvinyl triflates as precursors.117,118
9
Scheme 1.1 Synthesis of [3]cumulenes based on the carbene/carbenoid route.
An alternative method to generate carbenoids for the synthesis of [3]cumulenes is
based on elimination of trihaloalkanes (Scheme 1.2). Direct elimination using a Cu/Cu(I)
catalyst system gives [3]cumulene products in moderate yields (ca. 40–60%).111 Brand and
coworkers,119,120 on the other hand, describe a route that proceeds through a dichloroalkene
intermediate formed through reduction of the trihaloalkane leading to higher overall yields
(>85%) than the direct elimination although it requires three distinct synthetic steps.
Scheme 1.2 Synthesis of [3]cumulenes based on reductive elimination of trihaloalkanes.
The most common synthetic route to [3]cumulenes is based on the reduction of an
acetylenic diol or diether derivative (Scheme 1.3). The precursor is usually a
Li- or Mg-acetylide, 49,50,57,61,94,121,122 which is used in an addition reaction with a ketone that
defines the endgroups. In cases where the free alcohol may not be compatible with subsequent
synthetic steps, it can be blocked by formation of, for example, a methyl ether through
trapping with MeI.49 From either the diol or diether, aryl-substituted [3]cumulenes can be
formed directly via reduction with SnCl2,49,94 although P2I4
61 or HI and I2122 have also been
used. Tetraalkyl[3]cumulenes are usually formed via halogenation, followed by reductive
elimination (Scheme 1.3).50,123,124
10
Scheme 1.3 Synthesis of [3]cumulenes based on acetylenic diol derivatives.
1.3.3 [4]Cumulenes
Bildstein and coworkers have reported the formation of [4]Fc based on an adaptation
of the diol approach described above for [3]cumulenes (Scheme 1.4).94,125 A homopropargylic
ether is formed, lithiated, and then added to diferrocenylketone to complete the carbon
skeleton. Treatment with acid (HBF4) results in the stabilized (and isolable) [3]cumulenium
intermediate, which can then be converted to [4]Fc through base induced elimination.
Bildstein’s approach is conceptually similar to that reported earlier by Nakagawa and
coworkers,126 in which two different endgroups have been introduced in order to explore
optical activity and racemization of [4]cumulenes.
Scheme 1.4 Synthesis of [4]Fc based on a diol derivative.
11
Another possibility to form tetraaryl[4]cumulenes has first been described by Kuhn127
and later by Karich and Jochims128 and relies on the intermediate formation of
dibromo-1,4-pentadienes from the appropriate unsubstituted dienes (Scheme 1.5). The diene
is then converted to the [4]cumulene in good yield through base-induced elimination.
Scheme 1.5 Synthesis of [4]cumulenes from diesters.
[3]Cumulenes can be converted to [4]cumulenes via addition of dichlorocarbene to a
[3]cumulene, followed by rearrangement or reductive elimination, depending on the structure
of the precursor (Scheme 1.6). Jochims and coworkers highlight the potential effectiveness of
this route through formation of [4]Ad, where MeLi gives quantitative yield in the reduction
step. Their route also provides rare examples of mixed dialkyl/diaryl-endcapped
[4]cumulenes.128 Irngartinger and Götzmann have developed a slightly modified version of
this general protocol to synthesize [4]Cy, using Zn in the final reductive elimination step
(Scheme 1.6).129
Scheme 1.6 Synthesis of [4]cumulenes from [3]cumulenes.
[4]Cumulenes can be obtained via carbene trapping as demonstrated by le Noble and
coworkers (Scheme 1.7).130,131 The dialkylpentatetraenylidene intermediate is formed and
reacts in situ with tetramethylethylene to give the [4]cumulene, which quickly converts to a
12
radialene through dimerization (in the case of R = Me). The analogous reaction with
2-adamantylpentatetraenylidene, on the other hand, forms the stable [4]cumulene.
Scheme 1.7 Synthesis of a [4]cumulene via carbene trapping with an olefin.
1.3.4 [5]Cumulenes
Of the higher [n]cumulenes (n ≥ 5), [5]cumulenes are by far the most studied, and
there are thus a number of efficient routes that have been developed for their synthesis.100 In
analogy to the syntheses described for [3]cumulenes, dimerization reactions of carbenes have
also been used to give [5]cumulenes (Scheme 1.8).67,121,132–135 Starting with a terminal
acetylene and a leaving group in the propargylic position, reaction with base produces a
carbene intermediate, which leads to the [5]cumulene via dimerization. An alternative
carbenoid route to [5]cumulenes has been described by Kollmar and Fischer,136 in which the
vinylidene carbenoid is generated directly from a haloallene (Scheme 1.8). It is interesting to
note that the influence of endgroups in this reaction is likely enhanced versus the analogous
reaction to give [3]cumulenes, considering the mesomeric stabilization of this intermediate
carbene (Scheme 1.8).
13
Scheme 1.8 Synthesis of [5]cumulenes based on carbenes/carbenoids.
Unsymmetrical [5]cumulenes can be synthesized through trapping of vinylidene
carbenes, as reported by Stang and coworkers (Scheme 1.9).118,137,138 More specifically,
elimination of a diyne vinyl triflate forms the intermediate carbene, which can be trapped by
either addition to electron rich alkenes (i.e., tetramethylethylene and cyclohexene) or M–H
bond insertion using R3MH (M = Si, Ge). In some cases, the resulting [5]cumulene is not
stable and isomerizes or polymerizes during the reaction.
Scheme 1.9 Synthesis of unsymmetrical substituted [5]cumulenes based on carbenoid
precursors.
[5]Cumulenes can be formed based on the reaction of a [3]cumulene with
dibromocarbene as described by Skattebol (Scheme 1.10).139 The addition of dibromocarbene
14
to tetramethyl[3]cumulene gives the bicyclopropylidene product, and the subsequent
rearrangement reaction, induced with MeLi, gives the unstable [5]Me product.
Scheme 1.10 Synthesis of [5]Me from [3]Me.
Assembly of [5]cumulenes based on acetylenic diol precursors is quite common
(Scheme 1.11), in part because the necessary precursor, a diyne diol, can be efficiently formed
via a number of routes. Often mimicking strategies described for [3]cumulenes, acetylenic
diols are thus readily assembled through, for example, the addition of a Li- or Mg-acetylide to
a ketone. Alternatively, oxidative homocoupling of propargyl alcohol derivatives is usually
quite efficient. With the diyne diol in hand, conversion directly to an aryl-substituted
[5]cumulene is accomplished via reduction with P2I4,61 CrCl2,
65 or SnCl2.49,68,140 In the
majority of recent studies, SnCl2 is the reductant of choice and usually gives good yields. In
the case of alkyl substitution, conversion of the diyne diol to the corresponding dihalide with
PI3,50 PBr3,
50,140 HBr,140 or HCl140 is required, which is then followed by reductive elimination
using Zn or n-BuLi.
R
R
OH
R
R
HO2
H
R
R
HO
M M2
R
R R
R
R
R
X
X
R
R
R = aryl
R = alkyl
reductive
elimination
Zn or n-BuLi
X = Br or Cl
reductiveelimination
halogenation
O
R R
M = Li, MgBr
R = alkyl, aryl
homo-
coupling
Scheme 1.11 Synthesis of [5]cumulenes based on acetylenic diol derivatives.
1.3.5 [6]Cumulenes
To date, only one synthesis has been reported for a [6]cumulene, namely [6]Fc
(Scheme 1.12). Bildstein and coworkers141 have shown that a diyne diether can be readily
15
assembled via an acetylenic cross-coupling reaction, and the [6]cumulene is then realized
through an elimination process similar to the synthesis of the [4]Fc. It is worth mentioning
that after the first elimination step, an unusual, air-stable cumulenium salt is obtained, and the
stability is explained by the presence of four ferrocene donor groups and the unsaturated
cumulene chain. Unfortunately, the resulting air-sensitive [6]Fc product cannot be isolated,
but UV/vis spectroscopy confirms the formation of the cumulene framework.
Scheme 1.12 Synthesis of [6]Fc.
1.3.6 [7]Cumulenes
To our knowledge, only seven tetraaryl[7]cumulenes and one tetraalkyl[7]cumulene
have been reported to date49,50,66,69,142 including the [7]cumulenes formed during this thesis
research. All [7]cumulenes have been assembled from the corresponding triyne diol precursor
(Scheme 1.13), which is typically formed as described above for [3]- and [5]cumulene
syntheses. An alternative approach to the requisite diol has recently been developed in this
thesis, using a carbenoid Fritsch-Buttenberg-Wiechell (FBW) rearrangement143–145 in
combination with Colvin’s reagent146–148 to form the triyne framework.49 This synthetic
pathway will be covered in more detail in the main part of the thesis (see Chapter II).
Following the trends established by [3]- and [5]cumulene syntheses, formation of the
tetraalkyl[7]cumulenes requires conversion of the diol to the dibromide, followed by
reductive elimination with Zn. In contrast, the tetraaryl[7]cumulenes can be formed directly
by reduction with either P2I4 or SnCl2. In most cases, the [7]cumulenes tend to decompose
quickly, both in solution and the solid state.
16
Scheme 1.13 Synthesis of [7]cumulenes based on acetylenic diol derivatives.
Perhaps the most structurally interesting [7]cumulene reported to date, [7]pPh, has
been synthesized by Cadiot and coworkers through a variation of the diol approach as
outlined in Scheme 1.14.69 In this case, the acetylenic diol precursor has been assembled using
a Cu-catalyzed heterocoupling between a terminal diacetylene and a bromoacetylene
derivative. While the final product cannot be isolated, UV/vis spectroscopy confirms
formation, with a lowest energy absorption at 700 nm that is red-shifted versus that of all
other [7]cumulenes (see Chapter IV).
Scheme 1.14 Synthesis of the bis[7]cumulene [7]pPh.
1.3.7 [9]Cumulenes
As a result of instability of the final product, very few [9]cumulenes have been
successfully assembled, and there are only five examples to be found in the literature,
including four tetraaryl- ([9]Ph, [9]Sub, [9]tBuPh, [9]Mes)49,51,52 and one
17
tetraalkyl[9]cumulene ([9]Cy).50 From that, two of the [9]cumulenes, [9]tBuPh and [9]Mes
have been formed in the group of Tykwinski.149 The synthetic pathway to [9]cumulenes is,
predictably, analogous to that of [5]cumulenes, except diynes provide the precursors for the
dimerization, rather than terminal alkynes (Scheme 1.15). As per usual, tetraaryl derivatives
are formed directly from the diol via reductive elimination using P2I4 or SnCl2, while the
tetraalkyl[9]cumulene [9]Cy must be synthesized from the dibromide, via reductive
elimination with Zn. Isolated yields have not been reported for any of the [9]cumulenes due to
the inability to isolate the unstable products.
Scheme 1.15 Synthesis of [9]cumulenes based on acetylenic diol derivatives.
1.4 Reactions of [n]cumulenes with n ≥ 5
The reactivity of [n]cumulenes with n = 2 and 3 has been reviewed by Diederich,150
Chauvin,105 and Ma.151 The increasing instability in longer [n]cumulenes (n ≥ 5), however,
has limited the reactions of these molecules. Thus, reactions of longer cumulenes have been
rare and are limited almost exclusively to reactions of [5]cumulenes. Due to its multitude of
double bonds, members of this class of compounds show many attractive positions for
addition and/or cyclization reactions, and products thus vary in symmetry and conjugation
depending on the regiochemistry of the addition leading to unique structures as outlined
below.
18
1.4.1 Miscellaneous reactions
Simple reactions of [5]cumulenes have been reported such as hydrogenations134,152 and
partial hydrogenations153,154 of the cumulene core using H2/Rh/alumina and Al/Hg or the
Lindlar catalyst. The oxidation of a [5]cumulene via epoxidation has been described by
Crandall and coworkers (Scheme 1.16).155 For example, reaction of [5]tBu with m-CPBA
gives a cyclopropanone intermediate, which goes on to give an allenic ester as the product.
Epoxidation with dimethyldioxirane gives the cyclopropanone as a stable product, which can
then be used to form [4]tBu through either thermal or photochemical loss of carbon
monoxide. The [4]cumulene [4]tBu has also been subjected to oxidation with m-CPBA, and
this reaction also gives the cyclopropanone product.
Scheme 1.16 Oxidation products of [5]tBu.
Theoretical predictions regarding the reactivity of cumulenes have been recently
reported, particularly concerning oxygen sensitivity with respect to the carbon allotrope
carbyne.156,157 Using density functional theory calculations, Moseler and coworkers report that
reaction of O2 with the cumulene chain can cause cleavage, followed by repeated shortening
of the chain through additional oxidation and loss of CO2.158
A variety of metal complexes can be formed through the reaction of an electrophilic
metal with the π-rich skeleton of a [5]cumulene (see Section 1.1.3. and Figure 1.7).100 A
number of unusual cumulene complexes with unique structural properties are outlined below
in Figure 1.7. Stang, for example, has reported about rhodium and platinum complexes of [3]-
and [5]cumulenes.159–161 Similar to Stang, Werner and coworkers also have reported about
rhodium complexes including [5]cumulenes.162,163 Complexes of [5]Ph with rhodium prefer
bonding to the β-bond when triphenylphoshine is used as a ligand.159 The analogous system
with (i-Pr)3P ligands shows rhodium bonded to the γ-bond at low temperature (the kinetic
19
product), while complete conversion to thermodynamic product with Rh-complexation at the
β-bond is achieved upon warming162,163 (for description of β- and γ-bonds of cumulenes see
Scheme 1.17). Complexation of (Ph3P)2Pt to [5]Ph reveals similar behaviour, namely an
equilibrium between the kinetic complex at the γ-bond and the thermodynamic complex at the
β-bond.159 Another metal center for cumulene complexes, namely iron, has been used by
Nakamura79,80 and King81 as iron carbonyl compounds. The reaction of [5]tBu with Fe2(CO)9
or Fe3(CO)12 gives a mixture of the mono- and dinuclear iron complexes,81 while Iyoda and
coworkers have shown that under the appropriate conditions using non-sterically demanding
endgroups, the reaction of either [5]H or [5]tBu with Fe3(CO)12 can be forced all the way to
the tetranuclear iron complex.79,80 In contrast to the typically coordinated cumulenes, Suzuki
has presented a new class of cumulene complexes, where the cumulene coordinates to the
metal, i.e., zirconium to form a metallacycle including an endocyclic acetylene unit.164–167
Finally, Suzuki and coworkers have shown that [5]cumulenes can be trapped with the low-
valent zirconocene–bisphosphine complex [Cp2Zr(PMe3)2] to give the very strained
zirconacyclopent-3-yne products.165–167
20
Figure 1.7 Selected cumulene complexes with diverse coordination patterns.
To conclude, cumulene complexes can be obtained in a variety of unique compounds
with interesting coordination behavior. Furthermore, this metal complexation can provide a
higher stability of the cumulene framework compared to the bare cumulene structure.
Nakamura has shown, for example, that the usually unstable unsubstituted [3]cumulene [3]H
can be stabilized through formation of a stable iron carbonyl π-complex.168 Three possible
structures for this [3]cumulene complex have been proposed (Figure 1.8). Due to absence of
an X-ray structure, the correct structure cannot be assigned to the unsubstituted [3]cumulene
iron carbonyl complex.
21
Figure 1.8 Three possible structures for a [3]H iron carbonyl complex (cumulenic bonds that
are relevant for coordination are marked red).
1.4.2 Cycloaddition and oligomerization reactions
The most commonly investigated reactions for [5]cumulenes are cycloadditions,
including cumulene oligomerization and either the addition of alkenes or alkynes. To date, no
general synthetic pathway for reactions of cumulenes has been reported, however, Scheme
1.17 summarizes the basic reactivity of [5]cumulenes concerning cycloaddition reactions.
Regarding the addition of alkenes to cumulenes, on one hand, it has been reported that
tetrafluoroethene (TFE) attacks the central γ-bond of [5]tBu to give the symmetric
cyclobutane derivative 1.1.121,133 On the other hand, Bildstein and coworkers have shown that
cycloaddition reaction of [5]Fc with either tetracyanoethylene (TCNE) or C60 (at a 6,6-ring
junction) occurs at the β-bond, which affords the unsymmetrical cyclobutane derivatives
1.2.132 Noteworthy, all three known examples in Scheme 1.17 utilize electron deficient
alkenes, but nevertheless two different reactivity patterns are clearly operative, i.e., at the
β- or γ-bond. Bildstein suggests that addition to the β-bond is the thermodynamic reaction
pathway, while the alternative, addition to the γ-bond to give the symmetrical adduct, is
described as the kinetic reaction pathway,169 similar to the situation described for metal
complexes in Section 1.3.1.
22
Scheme 1.17 Examples of cycloaddition and dimerization reactions reported for
[5]cumulenes.
Hartzler, on the other hand suggests that the [2 + 2] addition probably occurs by way
of a thermally accessible diradical of the cumulene, and thus cycloaddition reactions at the
central double bond might be expected, especially if the terminal carbon atoms are sterically
hindered by substituents such as t-butyl (Figure 1.9). This is consistent with the experimental
results, which show that addition of the highly reactive reagent tetrafluoroethene occurs at the
γ-bond.
Figure 1.9 Diradical mesomeric structures as suggested by Hartzler for reactions of a
[5]cumulene.121
23
Two examples of alkyne addition to [5]cumulenes have been reported, and both
reactions use highly activated acetylenes to give the symmetrical [2 + 2] cyclobutene adduct
1.3 via reaction at the central γ-bond (Scheme 1.17).121,132 The nature of the cumulene varies
significantly in these two reactions, from R = t-Bu to R = Fc, but both authors suggest that the
addition to the γ-bond is observed because of steric hindrance resulting from endgroups.
The reaction of [5]cumulenes often results in a formal cycloaddition between two
cumulene molecules. Most commonly, such reactions proceed either thermally or via a Ni-
catalyzed reaction (Scheme 1.17). Thermal dimerization is often observed for cumulenes with
bulky alkyl substituents, giving a symmetrical vinylidene-substituted [4]radialene 1.4, i.e., a
cyclobutane ring that possesses four equally substituted exocyclic allene units (Scheme
1.17).170,171 This has been first demonstrated by Hartzler and coworkers for [5]tBu, i.e., when
[5]tBu melts, the resulting liquid resolidifies with the formation of the radialene product.121 In
a later report by Iyoda, it has been suggested that the dimerization reaction occurs at the
central, γ-double bond, due to the crisscross nature of the structure (transition state) that
would be required for a thermal [2 + 2] reaction.140 It is interesting to ponder why the thermal
reactions of tetraaryl[5]cumulenes do not seem to follow a similar pathway in the solid state,
i.e., dimerization to give a radialene. It might be due to molecular structure and stronger BLA
(see Chapter IV), or perhaps steric factors based on the endgroups. Alternatively, it is also
conceivable that favorable intermolecular stacking interactions of the aryl endgroups might
prevent a crisscross orientation that would be necessary for a thermal [2 + 2] reaction.
Aside from solid state reactions, radialenes 1.4 are also formed from alkyl-substituted
cumulenes in solution under Ni-catalysis.140 With slightly less bulky alkyl endgroups,
Ni-catalyzed head-to-tail dimerization at the β-bond results in the formation of [4]radialenes
1.5, while tetraaryl[5]cumulenes afford the deep blue head-to-head dimers 1.6 (Scheme 1.17).
The authors suggest that the bulkiness of the terminal substituents in the [5]cumulenes
controls the course of metal coordination, and leads to the selectivity observed in the
oligomerization reactions.140
The final mode of dimerization reaction for [5]cumulenes has been documented
independently by the works of Stang172 and Scott,134 and later by Hopf and coworkers with
the unsubstituted [5]H.173 In these three cases, the lack of sterically encumbered endgroups
permits reaction at the α-bonds, with concomitant rearrangement of the cumulene framework
to give a butadiyne moiety (Figure 1.10). Scott has reported that the Cu(I)-catalyzed
cyclodimerization of [5]Me leads to the symmetrical 12-membered ring 1.7.134 Stang and
coworkers have shown that no metal catalysis is necessary, and macrocycle 1.8 is isolated in
24
31% yield as the only viable product.172 In the case of 1.8, a radical is suggested, which
avoids the necessity of a symmetry forbidden [6 + 6] thermal cycloaddition. Finally, Hopf and
coworkers report the formation of macrocycle 1.9,173 conceivably through dimerization of the
parent system, [5]H, although the intermediate presence of [5]H has not been established.
Figure 1.10 Dimerization products of [5]cumulenes.
Similar to dimerization, [5]cumulenes also show to formally undergo trimerization
reactions, although in each of the two reported cases, dimerization precedes formation of the
trimer. The first example from Kawase et al. is outlined in Scheme 1.18 and forms a novel
tricyclobutabenzene derivative from a precursor in which a [5]cumulene links two quinone
rings.174 The authors suggest a mechanism in which the [5]cumulene is converted to a radical
intermediate via oxidation. This intermediate then dimerizes to a cyclobutene intermediate,
and subsequent addition of the third [5]cumulene unit gives a dicyclobutene intermediate.
Finally, further oxidation and a final cyclization step give the tricyclobutabenzene product.
25
Scheme 1.18 Trimerization reaction of a [5]cumulene.
The second trimerization example describes the cyclotrimerization of [5]Ph to a
tricyclodecadiene derivative as reported by Kawamura and coworkers (Scheme 1.19).175 The
authors suggest that the reaction initiates with a solution-state dimerization of [5]Ph to give
the symmetrical [4]radialene. A third equivalent of [5]Ph adds to this intermediate and gives
the Diels-Alder adduct, which is ultimately converted to the product via an electrocyclization
reaction. The reaction yield is quite reasonable (up to ca. 70%), as long as the concentration
of [5]Ph is >13 mmol·L–1. The product is rather stable, and subsequent Diels-Alder or
electrocyclic reactions are not observed. This overall reaction is particularly unusual since it
relies on the thermal dimerization of a tetraaryl[5]cumulene at the γ-bond, which is a
reactivity pattern typically reserved for [5]cumulenes terminated with sterically bulky alkyl
groups.
26
Scheme 1.19 Trimerization reaction of [5]Ph.
1.5 Motivation and goals of the doctoral thesis
As described in Section 1.1, the structure of carbyne is on the one hand fundamental to
its properties, and on the other hand not well understood. Thus, investigation of carbyne might
be best approached through the rational synthesis and study of molecules with defined
structure. There is a growing fundamental interest in long [n]cumulenes as model compounds
for the sp-carbon allotrope carbyne, and insight from the study of [n]cumulenes of defined
length should allow researchers to compare and contrast properties of the cumulenic and
polyyne versions of carbyne. The unique π-electron structure of cumulenes provides
distinctive electronic and optical properties that suggest fascinating opportunities in molecular
electronics and materials science. Key to exploiting this potential is the development of more
stable cumulene structures, and the synthetic methods to realize these targets. Regarding the
reactivity of cumulenes, a variety of possible synthetic transformations might exploit the
reactive cumulenic π-system, particularly in the field of cycloaddition reactions giving unique
compounds as candidates for precursors to unprecedented interesting and potentially useful
conjugated structures.
This thesis deals with the design, synthesis, characterization, and reactivity of
cumulenes as model compounds for carbyne. The instability of [n]cumulenes has no doubt
slowed progress on their synthesis and study. Therefore, to enhance the stability of longer
cumulenes, two bulkier endgroups, 1) the 3,5-di-t-butylphenyl group, a modified version of
27
the so called “supertrityl” group, and 2) the mesityl group have been incorporated into the
cumulene system to achieve better shielding of the cumulene chain. These two series of
cumulenes, and consequently their synthesis, characterization, and the resulting properties
will be discussed in Chapter II and IV, respectively. Another alternative to enhance the
stability of longer cumulenes is given in Chapter III and describes the formation of cumulene
rotaxanes that include a phenanthrene-based macrocycle designed to shield the cumulene
framework. Finally, the reactivity of several cumulenes is presented in Chapter V, including
addition reactions and dimerization reactions as well as the appropriate characterization and
discussion of the obtained results.
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36
2. Chapter II. Cumulenes – Synthesis of tetraarylcumulenes [n]tBuPh and
[n]Mes‡
2.1 Synthesis and structure of tetraarylcumulenes [n]tBuPh and [n]Mes
This chapter describes the synthesis of two series of tetraarylcumulenes containing
either 3,5-di-t-butylphenyl or mesityl endgroups, [n]tBuPh and [n]Mes, respectively (Figure
2.1). The main emphasis of this chapter is put on the series of [n]tBuPh cumulenes, whereas
only a short summary and several synthetic features are presented for the [n]Mes cumulene
series.§ The synthetic steps outline the formation of [n]tBuPh and [n]Mes with n = 3, 5, 7,
and 9. Furthermore, synthetic improvements and approaches to higher cumulenes are
presented. Regarding the [n]Mes cumulene series, discussions of synthetic challenges and
thus, deviations from standard synthetic methods, are given. Finally, stability issues of
cumulenes in solution and the solid state are discussed.
2.1.1 General aspects and motivation
As already mentioned in Chapter I, compared to polyynes, the study of cumulenes has
lain essentially dormant since early work1,2 reported by Kuhn3,4 and Bohlmann.5,6 Thus, many
unanswered questions remain about the physical properties of this intriguing class of linear
molecules. To date, UV/vis spectroscopy has been the most useful method for the
characterization of cumulenes,7 and reported analyses of cumulenes document a lowering of
the lowest energy absorption (λmax) as a function of cumulene length, such as that for known
cumulenes [n]Ph and [n]Cy (n = 3, 5, 7, 9, Figure 2.1).4–6 Changes in λmax versus chain length
n are, obviously, intricately dependent on the structure and on the degree of bond length
alternation (BLA, defined as the bond length difference between the two central-most double
‡ Portions of this chapter have been published: J. A. Januszewski, D. Wendinger, C. D. Methfessel, F. Hampel,
R. R. Tykwinski, Angew. Chem. Int. Ed. 2013, 52, 1817–1821. § The chemistry of [n]Mes was introduced during my doctoral thesis and continued by doctoral student Dominik
Wendinger who synthesized the series of [n]Mes. In addition, Christian D. Methfessel performed the synthesis
of [5]Mes (bachelor thesis under my supervision).
37
bonds of the cumulene chain). Recent theoretical studies predict that the BLA for cumulenes
will rapidly approach zero (BLA ≤ 0.01) with increasing length of the cumulene chain,8–10 i.e.,
Peierls distortion is essentially absent.11 Experimentally, X-ray crystallographic analysis
would provide an opportunity to confirm or refute theoretical trends in BLA as a function of
cumulene length. Unfortunately, very few solid-state structures have been reported for
cumulenes, and no crystallographic data for [n]cumulenes with n > 5 are available.12
Figure 2.1 Structures of selected [n]cumulenes.
To better understand the properties of cumulenes, a series of stabilized derivatives has
to be formed that enables the study of physical properties even for longer [n]cumulenes
(n > 5). Therefore, a bulky endgroup, i.e., an analog of the “supertrityl” endgroup has been
applied for the synthesis of cumulenes, since it has been predicted to shield the reactive
cumulene chain as in the case of longer polyyne chains.13 For cumulenes, however, the
supertrityl endgroup is “modified” possessing only two aryl groups instead of usually three
because of the hybridization of the outer carbon atoms in a cumulene chain (Figure 2.2). More
specifically, the outer carbon atoms are sp2-hybridized in the cumulene chain, while the
analogous carbons are sp3-hybridized in the polyyne chain.
38
Figure 2.2 Comparison of hybridization of the outermost carbon atom in a polyyne (left) and
cumulene (right) chain.
The formation of a cumulene typically derives from an oligoyne that possesses a
leaving group in the terminal propargylic position, such as OH or OMe. The final synthetic
step then involves reductive elimination, usually performed with SnCl2 under acidic
conditions. This approach has been followed for the current syntheses.
2.1.2 Synthesis of the [n]tBuPh cumulene series
2.1.2.1 Synthesis of the bis-(3,5-di-t-butylphenyl)methanone endgroup
The synthesis of [3]-,[5]-,[7]-, and [9]tBuPh cumulenes has been adapted from the
known general synthetic approaches that have already been discussed in Chapter I and are
here presented in more detail. The endgroup for this series is based on acetylide addition to a
ketone unit, and thus, the ketone has been the first target.
3,5-Di-t-butylphenylbromide was converted to a Grignard reagent and further treated
with ethyl formate to afford secondary alcohol 2.1 in 89% yield (route a, Scheme 2.1).
Alcohol 2.1 was then oxidized to ketone 2.2 with pyridinium chlorochromate (PCC) in 99%
yield. Route a, however, was not reproducible due to problems concerning the formation of
the Grignard reagent. Alternatively, route b was attempted involving lithiation of the 3,5-di-t-
butylphenylbromide with n-BuLi followed by addition to ethyl formate (Scheme 2.1). This
reaction provided a lower yield of 2.1 (57% versus 89%), but it was much more reproducible.
In addition, ketone 2.2 was formed simultaneously in 14% yield by route b, but the products
could be easily separated via column chromatography.
39
Scheme 2.1 Synthesis of ketone 2.2.
2.1.2.2 Synthesis of [3]cumulene [3]tBuPh
The synthesis of [3]cumulene [3]tBuPh started with the formation of trimethylsilyl-
protected alkyne 2.3 through the reaction of a Li-acetylide with ketone 2.2 followed by
treatment with MeI (Scheme 2.2). It was necessary to form a methyl ether group instead of the
free hydroxy group to allow for further synthetic steps. Compound 2.3 was desilylated with
K2CO3 in MeOH/THF (20:1) to afford terminal acetylene 2.4 (Scheme 2.2).
Scheme 2.2 Synthesis of terminal acetylene 2.4.
With the terminal acetylene 2.4 in hand, the precursor 2.5 was synthesized by
lithiation of 2.4 and addition of the resulting acetylide to ketone 2.2 (Scheme 2.3). Reductive
elimination of 2.5 using anhydrous SnCl2 and HCl in Et2O under an inert atmosphere gave
pure [3]tBuPh as bright yellow solid in 84% yield. In the case of the stable lower
40
[n]cumulenes (n = 3, 5), it was not necessary to use anhydrous and oxygen-free conditions,
but the yield and purity of the products were usually increased by this practice.
Scheme 2.3 Synthesis of precursor 2.5 and reductive elimination to [3]tBuPh.
2.1.2.3 Synthesis of [5]cumulene [5]tBuPh
The synthetic route to [5]cumulene [5]tBuPh was similar to that used for [3]tBuPh
and started with the formation of acetylene 2.6, which was obtained by addition of lithiated
trimethylsilylacetylene to ketone 2.2 (Scheme 2.4). Desilylation of 2.6 gave terminal
acetylene 2.7, and subsequent oxidative homocoupling under Hay conditions
(CuCl and TMEDA) afforded the precursor 2.8 in 98% yield. Diol 2.8 was reduced to the red
[5]cumulene [5]tBuPh in 74% yield using the same conditions (SnCl2 and HCl) as given
above for [3]tBuPh.
41
Scheme 2.4 Synthesis of [5]tBuPh.
2.1.2.4 Synthesis of [7]cumulene [7]tBuPh
The preliminary synthetic steps to achieve the precursor to [7]tBuPh differed
compared to that of cumulenes [3]tBuPh and [5]tBuPh. Several approaches were attempted
and have been summarized below. The first three pathways (Schemes 2.5–2.7) were adapted
from earlier studies,14 but were also examined in greater detail during my doctoral research
period.
The lithiation route in Scheme 2.5 was based on the
triisopropylsilyltrimethylsilyltriyne 2.9 which was selectively lithiated via exchange at the
trimethylsilyl group. The resulting Li-acetylide was added to ketone 2.2 affording triyne 2.10
or 2.11, depending on the absence or presence of a final MeI addition, respectively. Triyne
2.10 could only be obtained from the reaction as an impure mixture, and efforts to purify 2.10
failed. In contrast, triyne 2.11 was isolated pure, but the following desilylation reactions with
CsF failed.
42
Scheme 2.5 Synthetic approaches to precursors to [7]tBuPh, compounds 2.10 and 2.11
(lithiation route).14
The “mixed” homocoupling route in Scheme 2.6 was attempted via the reaction of
monoyne 2.7 and diyne 2.12 under Hay conditions (CuCl and TMEDA). Herein, all three
possible products were formed, i.e., compounds 2.8, 2.13, as well as 2.14, the precursors to
[5]tBuPh, [7]tBuPh, and [9]tBuPh, respectively. Preliminary analysis of the reaction showed
that the desired product 2.13 was produced in much lower yield compared to the
homocoupling products 2.8 and 2.14. Unfortunately, separation of the products via column
chromatography or recrystallization failed. Even the use of different functional groups, i.e.,
OH group in alkyne 2.7 and OMe group in diyne 2.12 did not afford a better separation for the
products.
43
Scheme 2.6 Synthetic approach to the precursor (2.13) to [7]tBuPh (“mixed” homocoupling
route).14
The synthesis described in Scheme 2.7 employed the conventional Fritsch-Buttenberg-
Wiechell (FBW) rearrangement reaction of a dibromoolefin. Initially, secondary alcohol 2.15
was obtained via a Grignard exchange reaction between 2.4 and EtMgBr, following by
addition of ethynyl Grignard to ethyl formate (Scheme 2.7). Oxidation of 2.15 with PCC gave
ketone 2.16 in 67% yield. Ketone 2.16 was treated with CBr4 and PPh3 under Ramirez
conditions in order to give dibromoolefin 2.17. Dibromoolefin 2.17 should then be converted
in a rearrangement reaction to triyne 2.18 using n-BuLi in hexanes (FBW rearrangement
reaction). Unfortunately, compound 2.17 could not be synthesized, not even with additional
amounts of Zn that has normally accelerated reactions of this type.15
44
Scheme 2.7 Synthetic approach to the precursor (2.18) to [7]tBuPh (FBW route).14
It was not possible to synthesize the precursor to [7]tBuPh via lithiation- or coupling
reactions. An alternative route was developed based on ketone 2.16. Triyne 2.18 was formed
from 2.16 using lithiated trimethylsilyldiazomethane (Colvin’s reagent) as a reagent to
generate a carbene/carbenoid intermediate (Scheme 2.8), a version of the FBW
rearrangement.16–18 The use of Colvin’s reagent avoided the synthesis of the dibromoolefin
intermediate as described in Scheme 2.7. Unfortunately, the application of Colvin’s reagent
also had several disadvantages, such as low yields, bad reproducibility, as well as high
toxicity of trimethylsilyldiazomethane. In addition, the purification process was laborious
leading to loss of product and lowered reaction yield. Nevertheless, after reductive
elimination of precursor 2.18 under inert conditions, [7]tBuPh was obtained as stable solid
via careful crystallization by overlaying of a CH2Cl2 solution of [7]tBuPh with MeOH. This
45
procedure reproducibly afforded deep purple needles of [7]tBuPh in a yield as high as 44%.
The [7]cumulene was, however, not stable in solution or as amorphous powder and started to
decompose within days/weeks. In contrast, the crystalline solid of [7]tBuPh was indefinitely
stable.
Scheme 2.8 Rearrangement reaction of ketone 2.16 to triyne 2.18, followed by a reductive
elimination to [7]tBuPh.
During my diploma thesis14 and the beginnings of my doctoral research, the
rearrangement reaction using trimethylsilyldiazomethane proved to be the only method to
form [7]tBuPh. Because of the high toxicity of trimethylsilyldiazomethane and the low
reaction yields, the search for alternatives was continued. Thus, bistrimethylsilyltriyne 2.19
was used as a starting material (Scheme 2.9). This triyne, a stable white solid, was synthesized
using a known procedure.19–21
The reaction of 2.19 with MeLi (>1 equiv) was initiated at –78 °C (route a, Scheme
2.9). The temperature was increased to –5 °C, and ketone 2.2 was added. Finally, quenching
of the reaction via addition of MeI gave triyne 2.18, i.e., the precursor to [7]tBuPh. The
reaction afforded many byproducts and unconverted ketone 2.2, and triyne 2.18 could not be
isolated. It has been noteworthy that low temperatures (<–15 °C) were important for lithiation
reactions of bistrimethylsilyltriyne 2.19 to avoid polymerization of the in situ formed
dilithiated triynes.22
46
Route b was performed at lower temperature: Triyne 2.19 was treated with MeLi
(1 equiv) at –78 °C, and ketone 2.2 was added at –25 °C (Scheme 2.9). After quenching of the
reaction with MeI, the reaction outcome included a mixture of ketone 2.2, the triyne 2.18, and
the terminal triyne 2.20 that could not be separated. At this point, it seemed that all reactions
performed using MeI addition in the final step showed bad reproducibility aside from low
yields and isolation problems.
An analogous reaction without MeI addition was attempted as shown by route c in
Scheme 2.9. Triyne 2.19 was treated with MeLi at –20 °C, followed by addition of ketone 2.2.
Aqueous work-up gave terminal triyne 2.21 as the main product in yields of 30–50%. The
second product, triyne 2.22, the precursor to [7]tBuPh, was obtained in 20% yield. An
interesting observation of this reaction was that no TMS protected triyne was observed in the
product mixture, i.e., the bistrimethylsilyltriyne appeared to be lithiated on both ends using
only one equivalent of MeLi or the remaining trimethylsilyl groups were inadvertently
removed during work-up. To conclude, an alternative non-toxic synthetic pathway to the
precursor to [7]tBuPh, triyne 2.22 (containing OH groups instead of OMe groups), was
developed. This route, however, had disadvantages such as low reaction yields and poor
reproducibility.
47
Scheme 2.9 Synthetic approaches to triynes 2.18 and 2.22 (precursors to [7]tBuPh) using
bistrimethylsilyltriyne 2.19.
2.1.2.5 Synthesis of [9]cumulene [9]tBuPh
The synthesis of [9]tBuPh was straightforward starting with diyne 2.23, which was
formed through addition of lithiated trimethylsilylbutadiyne to ketone 2.2 (Scheme 2.10).
Desilylation of diyne 2.23 with K2CO3 gave terminal diyne 2.24. Oxidative homocoupling of
2.24 under Hay conditions (CuCl and TMEDA) gave tetrayne diol 2.25, the precursor to
[9]tBuPh, in 96% yield. Reductive elimination of 2.25 afforded a blue solution of [9]tBuPh.
Unfortunately, [9]tBuPh could not be obtained in solid form and was unstable in solution
decomposing after a period of hours/days even in deoxygenated solvents and shielded from
light.
48
Scheme 2.10 Synthesis of [9]tBuPh.
Cumulene [9]tBuPh was successfully synthesized and purified through filtration,
although handling was only possible in solution. While [7]tBuPh could reproducibly be
crystallized in form of stable needles, all crystallization attempts to afford [9]tBuPh as a
stable solid failed. The stability of [9]tBuPh was limited to several days when kept under
inert conditions. After several days, or in the presence of O2/hν, decomposition occured,
indicated by a color change from blue to brown or decolorization. It was noted during my
diploma research14 that the color of [9]tBuPh persisted for only several hours or a maximum
of ca. one day. This “time of survival” could be increased by changing several conditions.
The purity of [9]tBuPh in the reaction solution played an important role, and the product
purity could be increased by the use of an argon atmosphere instead of a nitrogen atmosphere
during the reductive elimination step. Additionally, neutral alumina was replaced by basic
alumina in the filtration step to efficiently trap the acidic aqueous residue. Finally, intense
attention was paid to maintain strictly anhydrous conditions, e.g., with dried anhydrous SnCl2
as reductant instead of SnCl2·2H2O. These optimization attempts increased the time before
decolorization of [9]tBuPh occurred, but unfortunately did not enable the isolation of
[9]tBuPh as a solid. Therefore, another precursor to [9]tBuPh was targeted, namely tetrayne
2.14 containing OMe groups instead of OH groups (Scheme 2.11). The idea was that the
replacement of elimination of H2O by an elimination of MeOH during the reductive
elimination step might positively affect the stability of [9]tBuPh. The reaction started with
49
bistrimethylsilyldiyne 2.26, which was lithiated by MeLi and treated with ketone 2.2. Final
addition of MeI gave compound 2.27, which could not be purified and thus, was directly
desilylated with K2CO3 to give diyne 2.12 in an overall yield of 66% (Scheme 2.11).
Compound 2.12 was converted to tetrayne 2.14 in 87% yield via a homocoupling reaction
under Hay conditions (CuCl and TMEDA). Although the 1H NMR- and 13C NMR spectra
confirmed the formation of very pure 2.14, TLC analysis showed a second spot in addition to
the product. The solvent mixture hexanes/CH2Cl2 (1:1) showed very good spot separation on
the TLC plate, however, column chromatography failed using this mixture. Recrystallization
in hexanes led to an off-white crystalline compound, which was also contaminated with the
byproduct. Another solvent mixture, namely hexanes/ethyl acetate (20:1) was used for column
chromatography, and pure 2.14 could be obtained in 27% yield.
Scheme 2.11 Synthesis of tetrayne 2.14.
Conversion of precursor 2.14 to [9]tBuPh was accomplished under inert conditions at
0 °C. After 1.5 h, tetrayne 2.14 was still present in the reaction mixture while a brown residue
had already appeared. After 3 h, TLC analysis showed two additional byproducts in addition
to the baseline fraction, precursor 2.14, and [9]tBuPh. The reaction mixture seemed to
decompose gradually without complete conversion of the starting material. Regarding the
synthesis of [9]tBuPh, tetrayne 2.14 seemed to be less suitable as precursor to [9]tBuPh than
tetrayne 2.25 (Scheme 2.10).
50
Several other test reactions were employed for the synthesis of [9]tBuPh by variation
of different aspects of the conversion, such as reactants, stoichiometry, temperature, and
solvent (Table 2.1). The first test reaction was performed in CH2Cl2 instead of Et2O under an
argon atmosphere at 0 °C (entry 1, Table 2.1). Tetrayne 2.25 was treated with anhydrous
SnCl2 and HCl (1 M in Et2O). The color of the reaction mixture turned to blue indicating the
formation of [9]tBuPh. After few seconds, the color turned to pink/violet affording certainly a
different product, although the identity of this product was not determined (entry 1, Table
2.1).23
The reaction was performed in CH2Cl2 once again (entry 2), but the amount of HCl
(1 M in Et2O) was reduced to only one drop. A blue solution was observed which turned to
grey after several minutes. TLC analysis showed several byproducts aside from tetrayne 2.25
and [9]tBuPh. After some minutes, an excess of HCl was added, and the solution turned to
pink/violet again. TLC analysis illustrated a pink spot and a violet spot right below indicating
two different products. After one week, decomposition of the products (dissolved in MeOH)
was observed.
The third reaction was done in CH2Cl2 at –78 °C instead of 0 °C (entry 3). After 15
min, a blue solution was observed. After additional 30 min, the blue solution became more
intense, and the TLC analysis confirmed formation of [9]tBuPh aside from unconverted 2.25.
The reaction mixture was stirred for 10 min and filtered through a plug of silica (bottom) and
alumina (top). No single crystals could be obtained by several crystallization methods.
Overlaying or diffusion methods using MeOH led to decomposition, while hexanes showed a
longer “time of survival” with decomposition occurring after 3 d when kept at ca. –20 °C.
Next, the synthesis of [9]tBuPh was tried in hexanes (entry 4) since this solvent
showed the most delayed decomposition of a solution of [9]tBuPh. The reaction was
monitored via TLC analysis, which illustrated several byproducts. After 3 h, the solvent was
reduced by bubbling nitrogen through the reaction mixture. During solvent evaporation, the
solution became black indicating decomposition of [9]tBuPh.
The synthesis of [9]tBuPh was performed at rt using only anhydrous SnCl2 without
additional HCl (entries 5 and 6 for Et2O and CH2Cl2, respectively). After 30 min, the color of
the Et2O solution did not change, while the CH2Cl2 solution became bluish. The TLC analysis
of the CH2Cl2 solution showed the greatest amount of byproducts, turning to one big
decomposition spot overnight. After 1 d, the color of the Et2O solution also darkened, and
after 3 d, a brown precipitate was observed.
51
In conclusion, none of the test reactions mentioned above improved the outcome for
the synthesis of [9]tBuPh. While the reaction in CH2Cl2 showed a new pink product after
initial formation of [9]tBuPh, the reaction done in hexanes showed no improvement, and only
decomposition of [9]tBuPh was observed. In addition, in combination with SnCl2, HCl
appeared to be essential for the synthesis of [9]tBuPh.
52
Table 2.1 Reductive elimination of 2.25 under various conditions.
entry solvent temp. SnCl2 (anhydr) HCl (1 M in Et2O) result
1 CH2Cl2 0 °C 3 equiv 4 equiv initial blue solution
([9]tBuPh) turning to
pink/violet, new product
was not completely stable
in solution
2 CH2Cl2 0 °C 3 equiv one drop;
after some
minutes,
excess
bluish solution turning to
grey; after excess of HCl,
pink/violet solution
showing a pink and a violet
spot in TLC
3 CH2Cl2 –78 °C 3 equiv 4 equiv blue solution after 30 min
(2.25 still present)
4 hexanes 0 °C 3 equiv 4 equiv decomposition during
solvent evaporation by
bubbling N2 into the
reaction mixture
5 CH2Cl2 rt 3 equiv none after 30 min blue solution;
many byproducts
6 Et2O rt 3 equiv none after 1 h dark solution
turning to brown residue
53
2.1.2.6 Synthetic approaches to [11]cumulene [11]tBuPh and [13]cumulene [13]tBuPh
Even though [9]tBuPh showed poor stability and difficult product isolation, the
synthesis of higher [n]tBuPh cumulenes with n = 11 and 13 was targeted. Initially, the
synthetic route to [11]tBuPh was pursued via synthesis of secondary alcohol 2.28 in order to
form ketone 2.29 and finally the pentayne 2.30 through a FBW rearrangement (Scheme 2.12).
Compound 2.12 was treated with EtMgBr, and the mixture was stirred for 20 min. Ethyl
formate was added, and the reaction was monitored by TLC analysis. After 10 min, starting
material and several new spots were observed, while one main spot could be assigned to the
product. Stirring of the reaction mixture was continued for 50 min to achieve full conversion
of the starting material. The presumptive product spot by TLC, however, vanished affording
several (>8) new spots. Thus, it seemed that 2.28 was not stable, and no further efforts were
made following this approach.
Scheme 2.12 Synthetic approach to [11]tBuPh.
54
The synthesis of [13]tBuPh was attempted from terminal triyne 2.21 (Scheme 2.13),
which had been successfully formed during the synthesis of 2.18 and 2.22 as outlined in
Scheme 2.9. Compound 2.21 was converted in a homocoupling reaction under Hay conditions
(CuCl and TMEDA) to hexayne 2.31, the precursor to [13]tBuPh (Scheme 2.13). The
reaction mixture showed many spots, as monitored by TLC analysis, and after aqueous work-
up and purification by column chromatography, one main product could be obtained as a
brownish solid in 31% yield. 1H- and 13C NMR spectra of the brownish solid fit very well to
the structure of hexayne 2.31. After repeated dissolution of 2.31, however, it was observed
that 2.31 was not stable and tended to decompose as confirmed by TLC analysis showing
plenty of new spots while the main product spot had vanished.
Scheme 2.13 Synthetic approach to [13]tBuPh.
Attempts were made to convert 2.31 to the [13]tBuPh without using work-up methods
such as filtration over alumina and solvent evaporation, since during this, relatively speaking,
time-consuming work-up, decomposition occured giving no possibility for subsequent
identification or characterization. The conversion of precursor 2.31 to [13]tBuPh was carried
out in Et2O (Figure 2.3), and reaction progress was monitored directly by UV/vis
spectroscopy. Initially, the UV/vis spectrum of pure 2.31 showed no significant absorption in
the visible region >350 nm (black curve, Figure 2.3). After addition of SnCl2 and HCl,
however, an absorption band at 361 nm appeared (red curve), which broadened during the
course of the reaction forming a shoulder (light and dark blue curves). After several hours and
slight evaporation of solvent via bubbling argon through the reaction mixture, several weakly
55
visible absorption signals between 350 and 450 nm were observed. After stirring the mixture
overnight, the UV/vis spectra showed three distinctive absorption signals at 342, 368, and 398
nm resembling the vibronic fine structure of a polyyne with values of ν = 2066 cm–1 and ν =
2048 cm–1. Additionally, a shoulder absorption at about 500 nm was observed (orange curve,
Figure 2.3)
Regarding the color of the reaction progress, the dissolved precursor was colorless,
while the reaction mixture became more and more orange after SnCl2 was added, from pale
apricot-orange to darker orange over time. Hence, there was no intense color change as
usually observed for longer cumulenes ([7]cumulenes: deep violet, [9]cumulenes: deep blue).
Additionally, no absorption bands were present in the lower energy region (>600 nm) that
would hint to cumulene formation. Consequently, no evidence for successful formation of
[13]tBuPh was observed.
56
Figure 2.3 UV/vis spectra taken during attempted conversion of precursor 2.31 to [13]tBuPh
(in Et2O).
2.1.3 Synthesis of the [n]Mes cumulene series
2.1.3.1 Limitations of “common” synthetic pathways: Toward the synthesis of
precursors to [n]Mes
The synthesis of [n]Mes cumulenes deviated significantly from those previously
described for other [n]cumulenes because of the bulkiness of the mesityl endgroup,
particularly with respect to ketone 2.32 (Scheme 2.14). The formation of the precursors to
[n]Mes required either a terminal monoyne with a structure of 2.33 (with m = 1, for [3]Mes,
57
[5]Mes, and [7]Mes) or an analogous terminal diyne 2.33 (with m = 2, for [9]Mes) as
described in the [n]tBuPh series of cumulenes in Section 2.1.2. It was possible to form the
ketone 2.32, but the addition of acetylides to form the acetylenes 2.33 failed, probably due to
steric hindrance from the ortho-methyl groups of the aryl ring (Scheme 2.14). Thus,
alternative synthetic routes had to be developed to circumvent this problem and have been
presented in the following sections starting with the longest representative of the [n]Mes
series, i.e., the [9]cumulene [9]Mes.
Scheme 2.14 Unsuccessful approach to mesityl acetylene 2.33.
2.1.3.2 Synthesis of [9]cumulene [9]Mes
The synthesis of [9]Mes started with mesityl acid chloride, which was treated with
bistrimethylsilyldiyne 2.26 and AlCl3 to give ketone 2.34 (Scheme 2.15).24–26 Addition of
lithiated mesitylene gave diyne 2.35. Desilylation of 2.35 afforded terminal diyne 2.36.
Compound 2.36 was converted to tetrayne 2.37 in a homocoupling reaction under Hay
conditions (CuCl and TMEDA), which was then subjected to reductive elimination to give
[9]cumulene [9]Mes, i.e., using SnCl2 and HCl (Scheme 2.15).
58
Scheme 2.15 Synthesis of [9]Mes.27
2.1.3.3 Synthesis of [7]cumulene [7]Mes
Formation of [7]Mes started with treatment of mesityl aldehyde with ethynyl
Grignard, followed by conversion to the alkynyl bromide 2.38 with NBS/AgNO3.28–30
Reaction of 2.38 with diyne 2.36 using the Cadiot-Chodkiewicz reaction and further oxidation
afforded ketone 2.39. Addition of lithiated mesitylene gave triyne 2.40, and reductive
elimination with SnCl2 and HCl afforded [7]Mes.
59
Scheme 2.16 Synthesis of [7]Mes.27
The synthesis of [7]Mes via a FBW rearrangement was targeted (see Scheme 2.7 for
synthesis of [7]tBuPh via FBW rearrangement reaction). This reaction required the methyl
ether 2.41 to enable the formation of the Li-acetylide and addition to ethyl formate (Table
2.2). The OH leaving group of terminal acetylene 2.42,31 however, has been sterically
shielded by the methyl groups of the mesityl moiety, leading to synthetic challenges in the
conversion of OH to OMe. The following methylation studies were done toward the synthesis
of 2.41 (Table 2.2).
60
Table 2.2 Methylation reaction of 2.42 under various conditions.
entry base
(1 equiv)
time
before MeI
addition
solvent temperature resulting
product(s)
1 EtMgBr 15 min THF rt 2.42
2 EtMgBr 2 h THF rt 2.42
3 n-BuLi 30 min THF –78 °C 2.32, 2.41
(ca. 20%)
4 n-BuLi 3 min THF –78 °C 2.42, 2.32, 2.41
(ca. 22%)
5 n-BuLi 1 min THF –95 °C 2.32, 2.41
(41%)
6 n-BuLi 1 min hexanes/Et2O –78 °C 2.42, 2.32
7 n-BuLi 1 min toluene –78 °C 2.32, 2.41
(ca. 46%)
8 n-BuLi 1 min toluene – 90 °C 2.42, 2.32
Reaction conditions such as solvent, temperature, and time before addition of MeI
were varied. TLC analysis from these reactions showed several products including starting
material 2.42, dimesitylmethanone 2.32, and the desired product 2.41. The best yields for the
synthesis of 2.41 were achieved using toluene as solvent at –78 °C (entry 7, Table 2.2) or
THF at –95 °C (entry 5, Table 2.2) with 46% and 41%, respectively. Despite the moderate
results achieved for some of the methylation studies, the synthesis of 2.41 was abandoned due
to poor reproducibility.
The FBW rearrangement approach was then targeted using propargylic alcohol 2.42
(Scheme 2.17). The acetylenic proton of 2.42 could not be converted to a Grignard reagent by
the use of EtMgBr and thus, the reaction did not afford the desired alcohol 2.43. Two other
61
products, however, were formed depending on the stoichiometry of the reactants. Addition of
one equivalent of EtMgBr (route a, Scheme 2.17) formed a product that was obtained by
column chromatography as a light yellow solid. The IR band at 1930 cm–1 as well as the
signals at 6.11 ppm and 203.6 ppm observed in the 1H- and 13C NMR spectra, respectively,
indicated an allene. The signal at 6.11 ppm in the 1H NMR spectrum, however, integrated to
only one proton. A crystallographic analysis identified the product as allene 2.44, which
contained a bromine atom instead of one allenic proton. In contrast, the use of two equivalents
of EtMgBr afforded a different product, i.e., the alcohol 2.45 as an off-white solid in 11%
yield (route b, Scheme 2.17). This compound was identified by NMR spectroscopy. From the
mechanistic point-of-view, the reaction was assumed to proceed via the aldehyde intermediate
2.46 that was initially formed through addition of the in situ formed acetylide of 2.42 to ethyl
formate. Due to the excess of EtMgBr in the reaction mixture, EtMgBr could then add to
aldehyde 2.46 to give alcohol 2.45.
62
OH
HO
2.45
HHO
2.42
O
EtO H
1. EtMgBr, THF, rt
2.
OH
HO OH
2.43
Br
H
2.44
O
EtO H
1. 1 equiv EtMgBr, THF, rt
2.
16%
O
EtO H
1. 2 equiv EtMgBr, THF, rt
2.
11%
O
HO
route a)
route b)
2.46
H
3. NH4Cl, H2O
3. NH4Cl, H2O
Scheme 2.17 Reaction of terminal acetylene 2.42 with EtMgBr and ethyl formate.
2.1.3.4 Synthesis of [5]cumulene [5]Mes
The triisopropylsilyl-terminated acetylene 2.47 was obtained through a Grignard
reaction using mesitylbromide and ester 2.48 (Scheme 2.18). Desilylation of 2.47 with TBAF
gave the terminal alkyne 2.42. A homocoupling reaction of compound 2.42 under Hay
conditions (CuCl and TMEDA) gave diyne 2.49, which was converted to [5]cumulene [5]Mes
via reductive elimination reaction using SnCl2 and HCl.
63
Scheme 2.18 Synthesis of [5]Mes.27
The homocoupling reaction of compound 2.42 under Hay conditions
(CuCl and TMEDA) showed limited reproducibility and purity of the final product 2.49. Only
impure product was obtained that could not be further purified by either column
chromatography or recrystallization. The crude product was carried forward to the [5]Mes,
although this led to formation of even more byproducts. To circumvent these problems, two
palladium-catalyzed homocoupling reactions were attempted (route a and b, Scheme 2.19),
using Pd(PPh3)2Cl2 and CuI as catalysts.32,33 Route a was performed with Et3N as base and
ethyl bromoacetate as oxidant affording 2.49 in 12% yield (after recrystallization) along with
slight impurity (<10%) of the starting material. Route b used i-Pr2NH as base and I2 as
oxidant affording pure 2.49 in 26% yield. Both reactions were favored compared to the
standard homocoupling reaction under Hay conditions in spite of the low yields, since
purification and characterization of 2.49 was accomplished more easily.
64
Scheme 2.19 Pd-catalyzed homocoupling reactions giving precursor 2.49.
2.1.3.5 Synthesis of [3]cumulene [3]Mes
The synthesis of [3]Mes was not possible, because the necessary precursors could not
be formed. Specifically, terminal acetylene 2.41 would be used for the synthesis of [3]Mes.
The synthesis of compound 2.41, however, was problematic as described in Section 2.1.3.3
and Table 2.2.
2.2 Summary and conclusion regarding the stability of [n]cumulenes
In summary, the synthesis of two series of [n]cumulenes, [n]tBuPh and [n]Mes was
accomplished. For the lower representatives of [n]tBuPh, i.e., with n = 3 and 5, the developed
synthetic routes were straightforward giving good to very good isolated yields. An alternative
reaction to form [7]cumulene [7]tBuPh, as shown in Scheme 2.9, was developed in order to
avoid the use of the highly toxic trimethylsilyldiazomethane that was initially used to form
[7]tBuPh (Scheme 2.8). Both approaches gave low yields, although the approach using
bistrimethylsilyltriyne was favored because of easier formation of intermediates and
precursors. Regarding the synthesis of [n]Mes, it was not possible to proceed using protocols
that were successful for [n]tBuPh because of either the inability to add acetylides to the
mesityl ketone 2.32 or the unsuccessful formation of building blocks containing methyl ether
units (Section 2.1.3.3). During the course of the doctoral research, methylation reactions
affording methyl ether derivatives of propargylic alcohols could finally be realized.
Unfortunately, the methylation reactions showed low yields and bad reproducibility (Table
2.2) and meanwhile, alternative routes had already been developed to enable the synthesis of
[n]Mes.§
65
[n]Cumulenes with n ≤ 5 that have been studied in this thesis are infinitely stable at
ambient conditions either in solution or as solid. In contrast, longer [n]cumulenes with n = 7
or 9 are not particularly stable in solution under ambient conditions, but some derivatives
show sufficient stability in the solid state so that X-ray crystallographic analysis could be
accomplished for all derivatives except [9]tBuPh (see Chapter IV). This includes the first
crystallographic analysis of [7]- and [9]cumulenes reported to date. Attempts to isolate higher
cumulenes via precipitation result in decomposition. The pure cumulene [7]tBuPh can be
isolated by slow crystallization from a solution of CH2Cl2/MeOH, and the crystalline solid is
indefinitely stable in the absence of O2 (glove box).34 Compound [7]Mes is stable in solution
for weeks when kept at ca. –20 °C in a deoxygenated solution, and crystals of [7]Mes from
CH2Cl2/hexane are stable for at least one week when stored at ca. –20 °C in the absence of
light and oxygen.34 Cumulenes [9]tBuPh and [9]Mes decompose rapidly in solution when
exposed to oxygen, but they can be handled for hours ([9]tBuPh) or days ([9]Mes) when kept
in a cold (–20 °C), deoxygenated solution of Et2O that is shielded from ambient light. Crystals
of [9]Mes grown from a C2D2Cl4 solution at –20 °C are stable for at least one week when
stored in the absence of light and O2.34
Changing several aspects of the synthetic conditions improved the stability of higher
cumulenes and their precursors in terms of handling, purity, persistence, and increased
reaction yields. More specifically, nitrogen atmosphere was changed to argon atmosphere for
all syntheses that were accomplished under inert conditions (intermediates, precursors, and
cumulenes). In addition, attention was paid to exclude water using dried anhydrous reactants.
Furthermore, basic alumina was used for cumulene purification instead of neutral alumina.35
Finally, attempts to synthesize even longer [n]tBuPh cumulenes with n = 11 and 13
have failed mainly due to instability at the stage of the precursor syntheses. Hence, it cannot
be ruled out that higher cumulenes might be sufficiently stable for characterization via e.g.,
UV/vis spectroscopy. It is, however, necessary to search for alternative routes for the
synthesis of the precursors to facilitate the synthesis of longer cumulenes.
66
2.3 Experimental part
2.3.1 General procedures and methods
Reagents were purchased reagent grade from commercial suppliers and used without further
purification. THF and Et2O were distilled from sodium/benzophenone. CH2Cl2 was distilled
from CaH2, and MeOH was distilled from magnesium.
Na2SO4 were used as standard drying reagents after aqueous work-up.
1H and 13C NMR spectra were recorded on a Bruker Avance 300 operating at 300 MHz (1H
NMR) and 75 MHz (13C NMR), a Bruker Avance 400 operating at 400 MHz (1H NMR) and
100 MHz (13C NMR), or a Jeol Alpha 500 operating at 500 MHz (1H NMR) and 126 Hz (13C
NMR). NMR spectra were referenced to the residual solvent signal (1H: CDCl3, 7.24 ppm; 13C: CDCl3, 77.0 ppm; 1H: CD2Cl2, 5.32 ppm; 13C: CD2Cl2, 53.8 ppm) and recorded at
ambient probe temperature. CDCl3 (99.8%, Deutero GmbH) was stored over 4 Å molecular
sieves. CD2Cl2 (99.6%, Deutero GmbH) was used as received.
UV/vis spectroscopic measurements were carried out on a Varian Cary 5000 UV/vis-NIR
spectrophotometer or an Agilent Cary 60 UV/Vis spectrophotometer at rt.
Mass spectra were obtained from a Kratos MS50G (EI), Micromass Zabspec (EI), Bruker
9.4T Apex-Qe FTICR (MALDI, Matrix: DCTB), Agilent Technologies 6220 oaTOF (ESI),
Bruker micro TOF II focus, and Bruker maxis 4G (APPI, ESI, in MeOH/ACN) instruments.
IR spectra were recorded on a Varian 660-IR spectrometer as solids in ATR-mode.
Differential scanning calorimetry (DSC) measurements were made on a Mettler Toledo TGA/
STDA 851e/1100/SF.
Cyclic voltammetry was performed using 0.1 M n-Bu4NPF6 as supporting electrolyte. Pt wire
was used as counter electrode, Ag/AgNO3 was used as reference electrode, and a glassy
carbon disc was used as working electrode. Ferrocene was added to the samples serving as
internal standard.
Melting points were measured with an Electrothermal 9100 instrument.
67
TLC analyses were carried out on TLC plates from Macherey-Nagel (ALUGRAM® SIL
G/UV254) and visualized via UV-light (264/364 nm) or standard coloring reagents. Column
chromatography was performed using Silica Gel 60M (Merck).
Several compounds mentioned herein have already been synthesized during my diploma
thesis. These reactions, however, have been modified, optimized, or improved during my
doctoral research, leading to better yields or higher purity. Furthermore, some of the
characterization data missing in my diploma thesis is provided in following experimental
data.35
2.3.2 Experimental data and compound characterization
Bis-(3,5-di-t-butyl-phenyl)-methanol 2.1.14 To a mixture of Mg (2.27 g, 93.3 mmol) in THF
(15 mL) under a N2 atmosphere was added a small amount (5 mL) of a solution of 3,5-di-t-
butylphenylbromide (22.1 g, 82.1 mmol) in THF (30 mL).36 A small crystal of I2 was added,
and the mixture was heated to promote the Grignard formation. The rest of the solution of 3,5-
di-t-butylphenylbromide in THF was added, and the reaction mixture was stirred for 30 min at
a gentle reflux while slowly diluting with THF (150 mL). The reaction was cooled to rt, a
solution of ethyl formate (2.76 g, 3.00 mL, 37.3 mmol) in dry THF (20 mL) was slowly
added, and the mixture was stirred at rt for 3 d. The reaction was quenched via the addition of
saturated aq NH4Cl (150 mL), and Et2O (50 mL) was added. The layers were separated, the
organic phase washed with saturated aq NH4Cl (100 mL) and saturated aq NaCl (100 mL),
dried (Na2SO4), and filtered. Solvent removal and purification by column chromatography
(silica gel, hexanes/EtOAc 20:1) afforded 2.1 (13.6 g, 89%) as a colorless solid. Mp 138–140
°C. Rf = 0.56 (hexanes/EtOAc 5:1). IR 3571 (m), 3058 (vw), 2954 (s), 2900 (m), 2866 (m),
1594 (m), 1469 (m), 1360 (m), 1063 (m), 730 (m) cm−1; 1H NMR (300 MHz, CDCl3) δ 7.34
(t, J = 1.7 Hz, 2H), 7.27 (d, J = 1.7 Hz, 4H), 5.85 (d, J = 3.4 Hz, 1H), 2.26 (d, J = 3.4 Hz, 1H),
1.31 (s, 36H); 13C NMR (75 MHz, CDCl3) δ 150.7, 143.0, 121.4, 121.0, 77.5, 34.9, 31.5. EI
68
HRMS m/z calcd. for C29H44O (M+) 408.33920, found 408.33867. Anal. calcd. for C29H44O:
C, 85.23; H, 10.85. Found: C, 85.22; H, 10.90.
Bis-(3,5-di-t-butyl-phenyl)-methanol 2.1. To a solution of 3,5-di-t-butylphenylbromide (15.4
g, 0.0572 mol) in THF (145 mL) was added n-BuLi (2.5 M in hexanes, 23 mL, 0.058 mol)
under a N2 atmosphere at –78 °C via a syringe.37 After stirring for 30 min, ethyl formate
(2.1 g, 2.3 mL, 0.028 mol) was slowly added, and the mixture was stirred for 30 min. The
cooling bath was removed, and the reaction was stirred overnight. The reaction was quenched
via the addition of saturated aq NH4Cl (150 mL), and Et2O (100 mL) was added. The layers
were separated, the organic phase washed with saturated aq NH4Cl (150 mL) and saturated aq
NaCl (150 mL), dried (Na2SO4), and filtered. Solvent removal and purification by column
chromatography (silica gel, hexanes/EtOAc 20:1) afforded 2.1 (6.64 g, 57%) as a light yellow
solid. Spectral data are consistent with that described above.
Bis-(3,5-di-t-butyl-phenyl)-methanone 2.2.14,38 To a solution of 2.1 (3.00 g, 7.33 mmol) in
CH2Cl2 (70 mL) was added PCC (2.31 g, 10.7 mmol), celite (2 g), and molecular sieves (4 Å,
2 g). After 3 h, the reaction mixture was passed through a plug of silica gel to remove the
chromium waste. Solvent removal afforded 2.2 (2.96 g, 99%) as a colorless solid. Mp 115–
118 °C. Rf = 0.61 (hexanes/EtOAc 5:1). IR 3063 (vw), 2954 (s), 2903 (m), 2867 (m), 1657
(s), 1592 (m), 1462 (m), 1231 (s) cm−1; 1H NMR (400 MHz, CDCl3) δ 7.65 (d, J = 1.8 Hz,
2H), 7.63 (d, J = 1.8 Hz, 4H), 1.34 (s, 36H); 13C NMR (100 MHz, CDCl3) δ 197.9, 150.7,
69
137.3, 126.3, 124.7, 35.0, 31.4. EI MS m/z 406.3 (M+, 25), 391.3 ([M − CH3]+, 100). EI
HRMS m/z calcd. for C29H42O (M+) 406.32358, found 406.32378.
Bis-(3,5-di-t-butyl-phenyl)-methanone 2.2. To a solution of 3,5-di-t-butylphenylbromide
(15.4 g, 0.0572 mol) in THF (145 mL) was added n-BuLi (2.5 M in hexanes, 23 mL,
0.058 mol) under a N2 atmosphere at –78 °C via a syringe. After stirring for 30 min, ethyl
formate (2.1 g, 2.3 mL, 0.028 mol) was slowly added, and the mixture was stirred for 30 min.
The cooling bath was removed, and the reaction was stirred overnight. The reaction was
quenched via the addition of saturated aq NH4Cl (150 mL), and Et2O (100 mL) was added.
The layers were separated, the organic phase washed with saturated aq NH4Cl (150 mL) and
saturated aq NaCl (150 mL), dried (Na2SO4), and filtered. Solvent removal and purification by
column chromatography (silica gel, hexanes/EtOAc 20:1) afforded 2.2 (1.68 g, 14%) as an
off-white solid. Spectral data are consistent with the above mentioned.
3,3-Bis(3,5-di-t-butyl-phenyl)-3-methoxy-1-(trimethylsilyl)prop-1-yne 2.3. To a solution
of trimethylsilylacetylene (2.2 g, 3.1 mL, 22 mmol) in THF (25 mL) at −78 °C was added n-
BuLi (2.5 M in hexanes, 6.5 mL, 16 mmol) under a N2 atmosphere via a syringe. The reaction
mixture was stirred for 40 min, and a solution of 2.2 (6.0 g, 15 mmol) in THF (60 mL) was
added. The cooling bath was removed, the reaction was stirred for 4 h, and MeI (20 g, 9.0 mL,
0.14 mol) was added. The reaction mixture was stirred for 2 h. The solution was quenched via
70
the addition of saturated aq NH4Cl (80 mL), and Et2O (60 mL) was added. The layers were
separated, the organic phase was washed with saturated aq NaCl (60 mL), dried (Na2SO4), and
filtered. Solvent removal and recrystallization from MeOH afforded 2.3 (6.7 g, 86%) as a
colorless solid. Mp 93–95 °C. Rf = 0.64 (hexanes/CH2Cl2 1:1). IR 3076 (vw), 2955 (m), 2901
(m), 2866 (w), 2827 (w), 2161 (w), 1596 (m), 1086 (s), 843 (s) cm−1; 1H NMR (300 MHz,
CDCl3) δ 7.42 (d, J = 1.8 Hz, 4H), 7.28 (t, J = 1.8 Hz, 2H), 3.42 (s, 3H), 1.27 (s, 36H), 0.25
(s, 9H); 13C NMR (75 MHz, CDCl3) δ 150.1, 141.8, 121.3, 121.2, 105.7, 93.7, 82.6, 52.9,
34.9, 31.4, 0.0. ESI MS m/z 487.4 ([M − MeO]+, 100); ESI HRMS m/z calcd. for
C35H54NaOSi ([M + Na]+) 541.38361, found 541.38246.
3,3-Bis(3,5-di-t-butyl-phenyl)-3-methoxyprop-1-yne 2.4. To a solution of 2.3 (6.7 g,
13 mmol) in MeOH/THF (105 mL, 20:1) was added K2CO3 (2.0 g, 14 mmol). The reaction
mixture was stirred for 3 h and quenched via the addition of saturated aq NH4Cl (60 mL). The
layers were separated, and the aqueous phase was extracted with CH2Cl2 (2 x 50 mL). The
organic phase was washed with saturated aq NH4Cl (60 mL) and saturated aq NaCl (60 mL),
dried (Na2SO4), and filtered. Solvent removal afforded 2.4 (5.6 g, 97%) as a colorless solid.
Mp 106–109 °C. Rf = 0.60 (hexanes/CH2Cl2 1:1). IR 3298 (w), 3272 (w), 3070 (vw), 2954 (s),
2902 (m), 2865 (m), 2828 (w), 2156 (w), 1594 (m), 1084 (s) cm−1; 1H NMR (400 MHz,
CDCl3) δ 7.35 (d, J = 1.8 Hz, 4H), 7.29 (t, J = 1.8 Hz, 2H), 3.41 (s, 3H), 2.84 (s, 1H), 1.26 (s,
36H); 13C NMR (100 MHz, CDCl3) δ 150.2, 141.7, 121.40, 121.36, 84.1, 82.1, 76.8, 52.8,
34.9, 31.4. ESI MS m/z 415.3 ([M − MeO]+, 100); ESI HRMS m/z calcd. for C32H46NaO
([M + Na]+) 469.34409, found 469.34511.
71
1,1,4,4-Tetrakis(3,5-di-t-butylphenyl)-4-methoxybut-2-yn-1-ol 2.5. To a solution of 2.4
(1.27 g, 2.84 mmol) in THF (15 mL) at −78 °C was added n-BuLi (2.5 M in hexanes,
1.14 mL, 2.85 mmol) under a N2 atmosphere via a syringe. The reaction mixture was stirred
for 1 h, and a solution of 2.2 (1.15 g, 2.83 mmol) in THF (15 mL) was added. The cooling
bath was removed, and the reaction was stirred for 3 h. The solution was quenched via the
addition of saturated aq NH4Cl (30 mL), and Et2O (30 mL) was added. The layers were
separated, and the organic phase was washed with saturated aq NaCl (30 mL), dried
(Na2SO4), and filtered. Solvent removal and purification by column chromatography (silica
gel, hexanes/EtOAc 30:1) afforded 2.5 (1.9 g, 78%) as a colorless solid. Mp 117−120 °C.
Rf = 0.36 (hexanes/EtOAc 20:1). IR 3464 (br), 3069 (vw), 2955 (s), 2903 (m), 2866 (m), 1596
(m), 1458 (m), 1066 (m), 716 (s) cm−1; 1H NMR (300 MHz, CDCl3) δ 7.49 (br d, J = 1.3 Hz,
4H), 7.37 (br d, J = 1.1 Hz, 4H), 7.34 (br s, 2H), 7.31 (br s, 2H), 3.49 (s, 3H), 2.84 (s, 1H),
1.27 (s, 36H), 1.25 (s, 36H); 13C NMR (75 MHz, CDCl3) δ 150.3, 150.0, 144.1, 142.1, 121.6,
121.4, 121.3, 120.7, 93.2, 87.5, 82.3, 75.8, 52.9, 34.9, 34.8, 31.5 (one signal coincident or not
observed). ESI HRMS m/z calcd. for C61H88NaO2 ([M + Na]+) 875.6677, found 875.6673.
1,1,4,4-Tetrakis(3,5-di-t-butyl-phenyl)buta-1,2,3-triene ([3]tBuPh). To a solution of 2.5
(1.0 g, 1.2 mmol) in Et2O (15 mL) was added anhydrous SnCl2 (0.67 g, 3.5 mmol) and HCl
(1 M in Et2O, 4.7 mL, 4.7 mmol) at 0 °C under an Ar atmosphere. After stirring for 1 h, the
72
solution was filtered through a plug of basic alumina oxide. Solvent removal afforded
[3]tBuPh (0.79 g, 84%) as a yellow solid. Mp 206–208°C. Rf = 0.26 (hexanes/EtOAc 20:1).
UV/vis (CHCl3) λmax (ε) 240 (12600), 277 (26200), 426 (37900) nm. UV/vis (Et2O) λmax 278,
323, 424 nm; IR 3060 (vw), 2952 (s), 2864 (m), 1587 (m), 1244 (s) cm−1; 1H NMR (400
MHz, CD2Cl2) δ 7.43 (s, 4H), 7.37 (d, J = 1.6 Hz, 8H), 1.31 (s, 72H); 13C NMR (100 MHz,
CD2Cl2) δ 151.9, 151.0, 139.0, 124.1, 124.0, 122.4, 35.2, 31.7. MALDI HRMS m/z calcd. for
C60H84 (M+) 804.65675, found 804.65640.
Crystal data for [3]tBuPh: C60H84, M = 805.27; monoclinic crystal system; space group C2/c,
a = 51.7261(10), b = 10.9049(2), c = 19.0153(4) Å; β = 95.00(3)°; V = 10685.2(4) Å3; Z = 8;
ρcalcd = 1.001 g cm–3; µ(MoKα) = 0.056 mm–1; λ = 0.71073 Å; 173.15 K; 2θ max = 54.98°;
total data collected = 22006; R1 = 0.0873 [6868 observed reflections with F ≥ 4σ(F)]; wR2 =
0.2878 for 541 variables, 12113 unique reflections, and 66 restraints; residual electron density
= 0.76 and –0.53 e Å–3. Several t-butyl groups showed disorder, which have been resolved
and refined to the following occupation factors: C15b/C15e = 40:50%; C13b,c,d/C13e,f,g =
76:24%; C23b,c,d/C23e,f,g = 63:37%; C43b,c,d/C43e,f,g = 68:32%; C45b,c,d/C45e,f,g =
85:15 %. CCDC 903382.
1,1-Bis-(3,5-di-t-butyl-phenyl)-3-trimethylsilyl-prop-2-yn-1-ol 2.6. To a solution of
trimethylsilylacetylene (1.1 g, 1.6 mL, 12 mmol) in THF (25 mL) at −78 °C under a N2
atmosphere was added n-BuLi (2.5 M in hexanes, 3.4 mL, 8.5 mmol) via a syringe. The
reaction mixture was stirred for 1 h, and a solution of 2.2 (3.0 g, 7.4 mmol) in THF (25 mL)
was added. The cooling bath was removed, and the reaction was stirred for 1 h. The solution
was quenched via the addition of saturated aq NH4Cl (30 mL), and Et2O (30 mL) was added.
The layers were separated, the organic phase dried (Na2SO4), and filtered. Solvent removal
afforded 2.6 (3.71 g, quant.) as a colorless solid. Mp 150–152 °C. Rf = 0.67 (hexanes/EtOAc
5:1). IR 3523 (w), 2956 (m), 2901 (m), 2865 (m), 2169 (w), 1596 (m), 843 (s) cm−1; 1H NMR
73
(300 MHz, CDCl3) δ 7.51 (d, J = 1.8 Hz, 4H), 7.30 (t, J = 1.8 Hz, 2H), 2.74 (s, 1H), 1.28 (s,
36H), 0.23 (s, 9H); 13C NMR (100 MHz, CDCl3) δ 150.3, 143.8, 121.4, 120.6, 109.0, 91.2,
75.8, 35.0, 31.4, -0.1. EI MS m/z 504.4 (M+, 7), 57.1 ([t-butyl]+, 100). EI HRMS m/z calcd.
for C34H52SiO (M+) 504.37875, found 504.37688. Anal. calcd for C34H52SiO: C, 80.89; H,
10.38. Found: C, 80.51; H, 10.57.
1,1-Bis-(3,5-di-t-butyl-phenyl)-prop-2-yn-1-ol 2.7. To a solution of 2.6 (3.7 g, 7.3 mmol) in
MeOH (40 mL) was added K2CO3 (1.0 g, 7.2 mmol). After stirring for 5 h, the reaction
mixture was quenched via the addition of saturated aq NH4Cl (30 mL), and CH2Cl2 (30 mL)
was added. The layers were separated, the organic phase was washed with saturated aq NH4Cl
(40 mL) and saturated aq NaCl (40 mL), dried (Na2SO4), and filtered. Solvent removal
afforded 2.7 (2.8 g, 89%) as an off-white solid. Mp 101–104 °C. Rf = 0.43 (hexanes/EtOAc
20:1). IR 3543 (m), 3294 (w), 3065 (vw), 2956 (s), 2902 (m), 2866 (m), 1594 (m), 717 (s)
cm−1; 1H NMR (300 MHz, CDCl3) δ 7.46 (d, J = 1.8 Hz, 4H), 7.34 (t, J = 1.8 Hz, 2H), 2.85 (s,
1H), 2.80 (s, 1H), 1.29 (s, 36H); 13C NMR (75 MHz, CDCl3) δ 150.4, 143.4, 121.6, 120.5,
87.4, 75.4, 74.7, 34.9, 31.4. EI HRMS m/z calcd. for C31H44O (M+) 432.33920, found
432.33958.
74
1,1,6,6-Tetrakis(3,5-di-t-butylphenyl)hexa-2,4-diyne-1,6-diol 2.8. To a solution of 2.7
(2.8 g, 6.5 mmol) in CH2Cl2 (30 mL) was added a solution of Hay catalyst [CuCl (0.64 g,
6.5 mmol) and TMEDA (1.5 g, 1.9 mL, 13 mmol) in CH2Cl2 (10 mL)]. The reaction mixture
was stirred for 1 d, saturated aq NH4Cl (50 mL) was added, and the resulting mixture was
extracted with Et2O (50 mL). The organic phase was washed with saturated aq NH4Cl
(100 mL) and saturated aq NaCl (100 mL), dried (Na2SO4), and filtered. Solvent removal
afforded 2.8 (2.73 g, 98%) as a white solid. Mp 190–191 °C. Rf = 0.31 (hexanes/EtOAc 10:1).
IR 3595 (vw), 3520 (w), 3440 (br), 3070 (vw), 2955 (s), 2903 (m), 2866 (m), 1595 (m), 715
(s) cm−1; 1H NMR (300 MHz, CDCl3) δ 7.42 (d, J = 1.8 Hz, 8H), 7.34 (t, J = 1.8 Hz, 4H),
2.94 (s, 2H), 1.29 (s, 72H); 13C NMR (75 MHz, CDCl3) δ 150.6, 143.1, 121.8, 120.6, 83.1,
76.0, 70.7, 35.0, 31.4. ESI HRMS m/z calcd. for C62H86NaO2 ([M + Na]+) 885.65200, found
885.65186.
Crystal data for 2.8: C64H94O4, M = 927.39, triclinic crystal system; space group P–1, a =
10.1043(2), b = 10.2676(1), c = 16.7900(4) Å, α = 97.703(1)°, β = 102.907(1)°, γ =
112.992(1)°; V = 1514.74(5) Å3, Z = 1; ρcalcd = 1.017 g cm–3; µ(MoKα) = 0.061 mm–1; λ =
0.71073 Å; 173(2) K; 2θ max = 55.06°; total data collected = 12974; R1 = 0.0726 [5614
observed reflections with F ≥ 4σ(F)]; wR2 = 0.2249 for 307 variables, 6905 unique
reflections, and 0 restraints; residual electron density = 0.894 and –0.431 e Å–3. One MeOH
per asymmetric unit.
1,1,6,6-Tetrakis(3,5-di-t-butyl-phenyl)hexa-1,2,3,4,5-pentaene ([5]tBuPh). To a solution of
2.8 (6.9 g, 8.0 mmol) in Et2O (60 mL) was added anhydrous SnCl2 (2.8 g, 15 mmol) and HCl
(1 M in Et2O, 16 mL, 16 mmol) at 0 °C under an Ar atmosphere. After stirring for 1 h, the
solution was filtered through a plug of basic alumina oxide. Solvent removal and purification
by column chromatography (silica gel, hexanes/CH2Cl2 4:1) afforded [5]tBuPh (4.89 g, 74%)
as crystalline red solid. Mp ~ 234 °C. Rf = 0.84 (hexanes/EtOAc 30:1). UV/vis (CHCl3) λmax
75
(ε) 255 (35000), 275 (25500), 296 (25800), 373 (9900), 437 (14900), 510 (66700) nm. UV/vis
(Et2O) λmax 253, 274, 292, 371, 500 nm; IR 3064 (w), 2955 (s), 2903 (m), 2866 (m), 1995
(w), 1585 (m), 1241 (s), 706 (s) cm−1; 1H NMR (300 MHz, CDCl3) δ 7.48 (d, J = 1.7 Hz, 8H),
7.39 (d, J = 1.7 Hz, 4H), 1.33 (s, 72H); 13C NMR (75 MHz, CDCl3) δ 150.7, 148.3, 137.4,
126.3, 125.6, 123.9, 122.6, 35.0, 31.5. MALDI HRMS m/z calcd. for C62H84 (M+) 828.65675,
found 828.65633.
Crystal data for [5]tBuPh: C62H84, M = 829.29, monoclinic crystal system; space group P21/c,
a = 14.1531(2), b = 18.8068(4), c = 11.0114(2) Å, β = 108.030(1)°; V = 2787.03(9) Å3, Z = 2;
ρcalcd = 0.988 g cm–3; µ(MoKα) = 0.055 mm–1; λ = 0.71073 Å; 173.15 K; 2θ max = 55°; total
data collected = 12488; R1 = 0.0661 [4591 observed reflections with F ≥ 4σ(F)]; wR2 =
0.1984 for 293 variables, 6395 unique reflections, and 15 restraints; residual electron density
= 0.29 and –0.33 e Å–3. One t-butyl group showed disorder, which was resolved and refined to
the following occupation factor: C25b,c,d/C25e,f,g = 62:38%. CCDC 903379.
1,1,7,7-Tetrakis(3,5-di-t-butyl-phenyl)-1,7-dimethoxyhepta-2,5-diyn-4-ol 2.15. To a
solution of EtMgBr (1.0 M in THF, 12.1 mL, 12.1 mmol) in THF (10 mL) was added
compound 2.4 (5.42 g, 12.1 mmol) in THF (50 mL) under a N2 atmosphere via a syringe. The
reaction mixture was stirred for 20 min, and ethyl formate (0.45 g, 0.49 mL 6.1 mmol) was
added. The reaction was stirred for 30 min and quenched via the addition of saturated aq
NH4Cl (50 mL), and Et2O (50 mL) was added. The layers were separated, the organic phase
was washed with water (50 mL) and saturated aq NaCl (50 mL), dried (Na2SO4), and filtered.
Solvent removal and purification by column chromatography (silica gel, hexanes/EtOAc 20:1)
afforded 2.15 (4.74 g, 85%) as a colorless solid. Mp 75−80 °C (from EtOAc). Rf = 0.12
(hexanes/EtOAc 20:1). IR 3442 (br), 3071 (vw), 2956 (s), 2903 (m), 2867 (m), 1597 (m),
1072 (s) cm−1; 1H NMR (300 MHz, CDCl3) δ 7.35 (d, J = 1.7 Hz, 4H), 7.33−7.31 (m, 4H),
76
5.53 (d, J = 8.1 Hz, 1H), 3.42 (s, 6H), 2.39 (d, J = 8.1 Hz, 1H), 1.27 (s, 36H), 1.26 (s, 36H); 13C NMR (75 MHz, CDCl3) δ 150.2, 141.5, 141.4, 121.43, 121.40, 85.8, 85.3, 82.1, 53.0,
52.7, 34.9, 31.4. ESI HRMS m/z calcd. for C65H92NaO3 ([M + Na]+) 943.6939 found
943.6928.
1,1,7,7-Tetrakis(3,5-di-t-butyl-phenyl)-1,7-dimethoxyhepta-2,5-diyn-4-one 2.16. To a
solution of 2.15 (4.74 g, 5.15 mmol) in CH2Cl2 (50 mL) were added PCC (1.7 g, 7.9 mmol),
celite (2.0 g), and molecular sieves (4 Å, 2.0 g). The reaction mixture was stirred for 3 h and
passed through a plug of silica gel to remove the chromium waste. Solvent removal and
recrystallization from MeOH afforded 2.16 (3.158 g, 67%) as an off-white solid. Mp 161–164
°C. Rf = 0.62 (hexanes/EtOAc 10:1). IR 3067 (vw), 2954 (s), 2903 (m), 2867 (m), 2208 (w),
1656 (m), 1632 (m), 1594 (m), 1224 (s), 1072 (m), 712 (m) cm−1; 1H NMR (300 MHz,
CDCl3) δ 7.32 (t, J = 1.8 Hz, 4H), 7.26 (d, J = 1.8 Hz, 8H), 3.41 (s, 6H), 1.23 (s, 72H); 13C
NMR (75 MHz, CDCl3) δ 150.6, 140.1, 122.0, 121.4, 92.9, 82.4, 53.4, 34.9, 31.4 (two signals
coincident or not observed). ESI HRMS m/z calcd. for C65H90NaO3 ([M + Na]+) 941.6782,
found 941.6806.
77
1,1,8,8-Tetrakis(3,5-di-t-butyl-phenyl)-1,8-dimethoxyocta-2,4,6-triyne 2.18. To a solution
of trimethylsilyldiazomethane (2 M in Et2O, 3 mL, 6 mmol) in THF (100 mL) was added n-
BuLi (2.5 M in hexanes, 2.4 mL, 5.9 mmol) at –78 °C under a N2 atmosphere. Caution!
Trimethylsilyldiazomethane should be regarded as extremely toxic and should only be
handled by individuals trained in its proper and safe use. All operations must be carried
out in a well-ventilated fume hood and all skin contact should be avoided. The solution
was stirred for 30 min and transferred via a cannula to a solution of ketone 2.16 (2.73 g,
2.97 mmol) in THF (200 mL) cooled to –78 °C under a N2 atmosphere. The reaction was
stirred for 3 h, quenched via the addition of saturated aq NH4Cl (150 mL), and Et2O (100 mL)
was added. The layers were separated, the organic phase washed with saturated aq NaHCO3
(100 mL), water (100 mL), and saturated aq NaCl (100 mL), dried (Na2SO4), and filtered.
Solvent removal and recrystallization from hexanes afforded 2.18 (0.564 g, 21%) as a light
yellow solid. Mp 210 °C. Rf = 0.52 (hexanes/EtOAc 20:1). IR 3068 (vw), 2955 (s), 2903 (m),
2867 (m), 2177 (w), 1593 (m) cm−1; 1H NMR (300 MHz, CDCl3) δ 7.32 (t, J = 1.7 Hz, 4H),
7.26 (d, J = 1.7 Hz, 8H), 3.41 (s, 6H), 1.27 (s, 72H); 13C NMR (75 MHz, CDCl3) δ 150.4,
140.9, 121.8, 121.4, 82.9, 79.5, 73.4, 63.6, 53.3, 34.9, 31.4. ESI MS m/z 883.7 ([M – OMe]+,
100); ESI HRMS m/z calcd. for C66H90NaO2 ([M + Na]+) 937.6833 found 937.6840.
1,1,8,8-Tetrakis(3,5-di-t-butylphenyl)octa-2,4,6-triyne-1,8-diol 2.22. To a solution of 1,6-
bis(trimethylsilyl)hexa-1,3,5-triyne 2.19 (78 mg, 0.36 mmol) in THF (12 mL) was added
MeLi·LiBr complex (1.5 M in Et2O, 0.25 mL, 0.38 mmol) at –20 °C under an Ar atmosphere
via a syringe. The reaction mixture was stirred for 1 h, and a solution of 2.2 (0.16 g, 0.39
mmol) in THF (5 mL) was added. After stirring for 1 h, the cooling bath was removed, and
the reaction was stirred for 2 h at rt. The solution was quenched via the addition of saturated
aq NH4Cl (30 mL), and Et2O (30 mL) was added. The layers were separated, the organic
phase was dried (Na2SO4), and filtered. Solvent removal and purification by column
78
chromatography (silica gel, hexanes/EtOAc 20:1) afforded pure 2.22 (65 mg, 20%) as a
colorless solid. Mp 224–226 °C. Rf = 0.08 (hexanes/EtOAc 20:1). IR 3588 (w), 3440 (bw),
3070 (vw), 2955 (s), 2903 (m), 2866 (m), 1592 (m) cm−1; 1H NMR (300 MHz, CDCl3) δ 7.39
(d, J = 1.7 Hz, 8H), 7.37 (t, J = 1.7 Hz, 4H), 2.87 (s, 2H), 1.31 (s, 72H); 13C NMR (75 MHz,
CDCl3) δ 150.7, 142.7, 122.0, 120.6, 81.7, 76.0, 71.3, 64.1, 35.0, 31.4. ESI HRMS m/z calcd.
for C64H85O ([M – OH]+) 869.6595 found 869.6580.
1,1,6,6-Tetrakis(3,5-di-t-butyl-phenyl)octa-1,2,3,4,5,6,7-heptaene ([7]tBuPh). To a
solution of 2.18 (0.125 g, 0.137 mmol) in Et2O (16 mL) was added anhydrous SnCl2 (78 mg,
0.41 mmol) and HCl (1 M in Et2O, 0.55 mL, 0.55 mmol) at 0 °C under an Ar atmosphere.
After 1 h, the solution was filtered through a plug of basic alumina oxide and eluted with
CH2Cl2 affording the purified [7]tBuPh. Since the cumulene is not stable as amorphous solid,
crystalline [7]tBuPh was obtained as violet/green needles (51 mg, 44%) by overlaying a
CH2Cl2 solution with MeOH. Mp 160–162 °C (decolorization). Rf = 0.61 (hexanes/EtOAc
20:1). UV/vis (CHCl3) λmax (ε) 297 (49800), 315 (66200), 459 (23300), 546 (40200), 573
(65200) nm. UV/vis (Et2O) λmax 294, 313, 453, 542, 565 nm; IR 3066 (vw), 2957 (s), 2904
(s), 2865 (m), 2055 (w) 1586 (m) cm−1; 1H NMR (400 MHz, CDCl3) δ 7.42 (d, J = 1.6 Hz,
8H), 7.40 (t, J = 1.6 Hz, 4H), 1.32 (s, 72H); 13C NMR (100 MHz, CDCl3) δ 150.8, 146.9,
137.2, 126.9, 124.9, 124.1, 123.1, 122.4, 35.0, 31.5. ESI HRMS m/z calcd. for C64H84 (M+)
852.6568 found 852.6535.
Crystal data for [7]tBuPh: C64H84, M = 853.31, triclinic crystal system; space group P–1, a =
13.8993(8), b = 14.0696(9), c = 15.1248(10) Å, α = 81.231(5)°, β = 89.873(5)°, γ =
78.628(5)°, V = 2864.6(3) Å3, Z = 2, ρcalcd = 0.989 g cm–3; µ(CuKα) = 0.406 mm–1, λ =
1.5418 Å; 173.0(8) K; 2θ max = 141.82°; total data collected = 17423; R1 = 0.0698 [7006
observed reflections with F ≥ 4σ(F)]; wR2 = 0.2188 for 627 variables, 10625 unique
79
reflections, and 5 restraints; residual electron density = 0.35 and –0.30 e Å–3. Two t-butyl
group showed disorder, which was resolved and refined to the following occupation factor:
C22,23,24/C22a,23a,24a = 87:13%; C82,83,84/C82a,83a,84a = 61:39%. CCDC 903381.
1,1-Bis(3,5-di-t-butyl-phenyl)-5-(trimethylsilyl)penta-2,4-diyn-1-ol 2.23. To a solution of
1,4-bis(trimethylsilyl)buta-1,3-diyne (2.00 g, 10.3 mmol) in THF (20 mL) was added
MeLi·LiBr complex (1.5 M in Et2O, 7.0 mL, 11 mmol) at 0 °C under a N2 atmosphere via a
syringe. The cooling bath was removed, and the red-brown mixture was stirred for 0.5 h
before it was cooled again to 0 °C. A solution of 2.2 (4.18 g, 10.3 mmol) in THF (40 mL) was
added. The cooling bath was removed, and the reaction was stirred overnight. The solution
was quenched via the addition of saturated aq NH4Cl (70 mL), and Et2O (50 mL) was added.
The layers were separated, the organic phase was dried (Na2SO4), and filtered. Solvent
removal and purification by column chromatography (silica gel, hexanes/EtOAc 20:1)
afforded pure 2.23 (3.87 g, 71%) as a light yellow solid. Mp 114–116 °C. Rf = 0.35
(hexanes/EtOAc 20:1). IR 3547 (w), 3069 (vw), 2956 (m), 2901 (w), 2867 (w), 2218 (w),
2094 (w), 1595 (m), 1248 (m), 845 (s) cm−1; 1H NMR (300 MHz, CDCl3) δ 7.37 (d, J = 1.7
Hz, 4H), 7.33 (t, J = 1.7 Hz, 2H), 2.78 (s, 1H), 1.29 (s, 36H), 0.20 (s, 9H); 13C NMR (75
MHz, CDCl3) δ 150.5, 143.0, 121.8, 120.6, 88.6, 87.5, 80.6, 75.9, 71.4, 34.9, 31.4, –0.4. ESI
MS m/z 511.4 ([M – OH]+, 100); ESI HRMS m/z calcd. for C36H52NaOSi ([M + Na]+)
551.36796, found 551.36687.
80
1,1-Bis(3,5-di-t-butyl-phenyl)penta-2,4-diyn-1-ol 2.24. To a solution of 2.23 (3.35 g,
6.34 mmol) in MeOH (40 mL) was added K2CO3 (1.0 g, 7.2 mmol). The reaction mixture was
stirred overnight and quenched via the addition of saturated aq NH4Cl (40 mL). The layers
were separated, and the aqueous phase was extracted with CH2Cl2 (2 x 20 mL). The organic
phases were combined and washed with saturated aq NH4Cl (30 mL) and saturated aq NaCl
(30 mL), dried (Na2SO4), and filtered. Solvent removal afforded 2.24 (2.73 g, 94%) as an off-
white solid. Mp 120−124 °C. Rf = 0.40 (hexanes/EtOAc 10:1). IR 3591 (m), 3437 (br), 3276
(m), 3068 (vw), 2954 (s), 2902 (m), 2864 (m), 2057 (vw), 2012 (vw), 1597 (m), 1360 (m),
1247 (m), 879 (s), 628 (s) cm−1; 1H NMR (300 MHz, CDCl3) δ 7.38 (d, J = 1.7 Hz, 4H), 7.34
(t, J = 1.7 Hz, 2H), 2.79 (s, 1H), 2.26 (s, 1H), 1.28 (s, 36H); 13C NMR (75 MHz, CDCl3) δ
150.6, 142.8, 121.9, 120.6, 79.2, 75.8, 70.6, 69.4, 67.6, 35.0, 31.4. ESI MS m/z 479.3
([M + Na]+, 62), 439 ([M – OH]+, 100). ESI HRMS m/z calcd. for C33H44NaO ([M + Na]+)
479.32844, found 479.32740.
1,1,10,10-Tetrakis(3,5-di-t-butyl-phenyl)deca-2,4,6,8-tetrayne-1,10-diol 2.25. To a
solution of 2.24 (2.73 g, 5.98 mmol) in CH2Cl2 (30 mL) was added a solution of Hay catalyst
[CuCl (0.600 g, 5.98 mmol) and TMEDA (1.4 g, 1.8 mL, 12 mmol) in CH2Cl2 (20 mL)]. The
reaction mixture was stirred for 1 d, saturated aq NH4Cl (50 mL) was added, and the mixture
was extracted with Et2O (50 mL). The organic phase was washed with saturated aq NH4Cl
(100 mL) and saturated aq NaCl (100 mL), dried (Na2SO4), and filtered. Solvent removal and
81
twofold recrystallization from hexanes afforded 2.25 (2.62 g, 96%) as a light yellow solid. Mp
185 °C. Rf = 0.36 (hexanes/EtOAc 10:1). IR 3588 (vw), 3447 (br), 3070 (vw), 2956 (s), 2904
(m), 2866 (m), 2217 (w), 1595 (m), 715 (s) cm−1; 1H NMR (400 MHz, CDCl3) δ 7.34 (s, 12
H), 2.81 (s, 2H), 1.27 (s, 72H); 13C NMR (100 MHz, CDCl3) δ 150.8, 142.5, 122.1, 120.6,
81.4, 76.1, 71.2, 65.0, 62.3, 35.0, 31.4. ESI MS m/z 893.7 ([M – OH]+, 100); ESI HRMS m/z
calcd. for C66H86NaO2 ([M + Na]+) 933.6520, found 933.6517.
1,1,6,6-Tetrakis(3,5-di-t-butyl-phenyl)deca-1,2,3,4,5,6,7,8,9-nonaene ([9]tBuPh). To a
solution of 2.25 (34 mg, 0.037 mmol) in Et2O (4 mL) was added anhydrous SnCl2 (22 mg,
0.12 mmol) and HCl (1 M in Et2O, 0.15 mL, 0.15 mmol) at 0 °C under an Ar atmosphere.
After 30−60 min, the solution was filtered through a plug of basic alumina oxide and eluted
with Et2O to afford the purified [9]tBuPh as a blue solution in Et2O. The yield could not be
determined due to instability of this compound. Rf = 0.62 (hexanes/EtOAc 20:1). UV/vis
(Et2O) λmax 316, 338, 366, 528, 574, 610, 664 nm.
5,5-Bis(3,5-di-t-butylphenyl)-5-methoxy-1-(trimethylsilyl)penta-1,3-diyne 2.27. To a
solution of 1,4-bis(trimethylsilyl)buta-1,3-diyne 2.26 (1.46 g, 7.51 mmol) in THF (30 mL)
was added MeLi·LiBr complex (1.5 M in Et2O, 5.0 mL, 7.5 mmol) at 0 °C under an Ar
atmosphere via a syringe. The cooling bath was removed, and the red-brown mixture was
82
stirred for 0.5 h before it was cooled again to 0 °C. A solution of 2.2 (3.05 g, 7.50 mmol) in
THF (30 mL) was added. After stirring for 20 min, the cooling bath was removed, and the
reaction mixture was stirred overnight. Saturated aq NH4Cl (70 mL) and Et2O (70 mL) were
added. The layers were separated, the organic phase was washed with saturated aq NH4Cl (70
mL) and saturated aq NaCl (70 mL), dried (Na2SO4), and filtered. Solvent removal afforded
the crude product 2.27 as a yellow solid (3.94 g, 97%) that could not be purified. It was then
carried on to the next step as formed.
5,5-Bis(3,5-di-t-butylphenyl)-5-methoxypenta-1,3-diyne 2.12. To a solution of 2.27 (3.94 g,
7.26 mmol) in MeOH/THF (4:1, 100 mL) was added K2CO3 (1.0 g, 7.2 mmol). The reaction
mixture was stirred for 2 h, and saturated aq NH4Cl (100 mL) was added. The layers were
separated, and the aqueous phase was extracted with CH2Cl2 (2 x 50 mL). The organic phases
were combined and washed with saturated aq NH4Cl (50 mL) and saturated aq NaCl (50 mL),
dried (Na2SO4), and filtered. Solvent removal and purification by column chromatography
(silica gel, hexanes/EtOAc 30:1) afforded pure 2.12 (2.35 g, 66% over two steps based on 2.2)
as a colorless solid. Mp 110−113 °C. Rf = 0.42 (hexanes/CH2Cl2 4:1). IR 3260 (m), 3068
(vw), 2959 (s), 2904 (m), 2868 (m), 2826 (w), 2163 (w), 1592 (m), 1087 (s), 877 (m) cm−1; 1H NMR (300 MHz, CDCl3) δ 7.39 (t, J = 1.7 Hz, 2H), 7.36 (d, J = 1.7 Hz, 4H), 3.49 (s, 3H),
2.24 (s, 1H), 1.35 (s, 36H); 13C NMR (75 MHz, CDCl3) δ 150.4, 141.1, 121.7, 121.4, 82.6,
76.8, 72.7, 68.8, 67.7, 53.2, 34.9, 31.4. ESI HRMS m/z calcd. for C34H46NaO ([M + Na]+)
493.34409, found 493.34459.
MeO OMeMeO H
2.12
CuCl, TMEDA
CH2Cl2
2.14
83
1,1,10,10-Tetrakis(3,5-di-t-butylphenyl)-1,10-dimethoxydeca-2,4,6,8-tetrayne 2.14. To a
solution of 2.12 (1.02 g, 2.17 mmol) in CH2Cl2 (10 mL) was added a solution of Hay catalyst
[CuCl (0.22 g, 2.2 mmol) and TMEDA (0.50 g, 0.65 mL, 4.3 mmol) in CH2Cl2 (5 mL)]. The
reaction mixture was stirred for 1 h, saturated aq NH4Cl (40 mL) was added, and the mixture
was extracted with Et2O (50 mL). The organic phase was washed with saturated aq NH4Cl
(70 mL) and saturated aq NaCl (70 mL), dried (Na2SO4), and filtered. Solvent removal
afforded 2.14 (0.89 g, 87%) as an off-white solid. Mp 78–80 °C. Rf = 0.75 (hexanes/CH2Cl2
1:1). IR 3069 (vw), 2955 (s), 2903 (m), 2867 (m), 2825 (v), 2215 (w), 1594 (m) cm−1; 1H
NMR (400 MHz, CDCl3) δ 7.31 (t, J = 1.8 Hz, 4H), 7.23 (d, J = 1.8 Hz, 8H), 3.39 (s, 6H),
1.25 (s, 72H); 13C NMR (101 MHz, CDCl3) δ 150.5, 140.8, 121.9, 121.4, 82.9, 79.4, 73.3,
64.5, 62.2, 53.3, 34.9, 31.4. APPI HRMS m/z calcd. for C67H87O ([M – OMe]+) 907.67514,
found 907.67536.
1,1-Bis(3,5-di-t-butylphenyl)-7-(trimethylsilyl)hepta-2,4,6-triyn-1-ol S1. To a solution of
1,6-bis(trimethylsilyl)hexa-1,3,5-triyne 2.19 (0.10 g, 0.46 mmol) in THF (30 mL) was added
MeLi·LiBr complex (1.5 M in Et2O, 0.30 mL, 0.45 mmol) at –78 °C under an Ar atmosphere
via a syringe. The reaction mixture was stirred for 1 h at –20 °C, and a solution of 2.2 (0.21 g,
0.52 mmol) in THF (20 mL) was added. The reaction mixture was stirred for 1.5 h, and
saturated aq NH4Cl (70 mL) and Et2O (50 mL) were added. The layers were separated, the
organic phase was washed with saturated aq NH4Cl (70 mL) and saturated aq NaCl (70 mL),
dried (Na2SO4), and filtered. Solvent removal and purification by column chromatography
(silica gel, hexanes/CH2Cl2 1:1) afforded pure S1 (0.104 g, 41%) as a red oil. Rf = 0.38
(hexanes/ CH2Cl2 1:1). 1H NMR (300 MHz, CDCl3) δ 7.36–7.34 (m, 6H), 2.80 (s, 1H), 1.27
(s, 36H), 0.20 (s, 9H); 13C NMR (100 MHz, CDCl3) δ 150.7, 142.7, 122.0, 120.6, 87.9, 80.9,
76.1, 71.3, 64.6, 61.2, 35.0, 31.4, –0.6 (one signal coincident or not observed). MALDI MS
84
m/z 535 ([M − OH]+, 100); ESI HRMS m/z calcd. for C38H52NaOSi ([M + Na]+) 575.3680,
found 575.3677.
1,1-Bis(3,5-di-t-butylphenyl)hepta-2,4,6-triyn-1-ol 2.21. To a solution of 1,6-
bis(trimethylsilyl)hexa-1,3,5-triyne 2.19 (0.10 g, 0.46 mmol) in THF (30 mL) was added
MeLi·LiBr complex (1.5 M in Et2O, 0.30 mL, 0.45 mmol) at –78 °C under a N2 atmosphere
via a syringe. The reaction mixture was stirred for 1 h at –20 °C, and a solution of 2.2 (0.21 g,
0.52 mmol) in THF (20 mL) was added. The reaction mixture was stirred for 1.5 h and
saturated aq NH4Cl (70 mL) and Et2O (50 mL) were added. The layers were separated, the
organic phase was washed with saturated aq NH4Cl (70 mL) and saturated aq NaCl (70 mL),
dried (Na2SO4), and filtered. Solvent removal and purification by column chromatography
(silica gel, hexanes/CH2Cl2 1:1) afforded pure 2.21 (0.029 g, 13%) as a brown solid. Mp >115
°C (decomp.). Rf = 0.31 (hexanes/ CH2Cl2 1:1). IR 3587 (w), 3456 (br), 3230 (m), 3069 (vw),
2957 (s), 2903 (m), 2865 (m), 2038 (vw), 1595 (m) cm−1; 1H NMR (300 MHz, CDCl3) δ 7.35
(s, 6H), 2.80 (s, 1H), 2.16 (s, 1H), 1.27 (s, 36H); 13C NMR (100 MHz, CDCl3) δ 150.8, 142.6,
122.0, 120.6, 80.2, 76.0, 71.0, 68.4, 68.0, 64.3, 60.0, 35.0, 31.4. ESI HRMS m/z calcd. for
C35H44NaO ([M + Na]+) 503.3284, found 503.3288.
Improved synthesis of 2.21:
To a solution of 1,6-bis(trimethylsilyl)hexa-1,3,5-triyne 2.9 (0.655 g, 3.00 mmol) in THF
(100 mL) was added MeLi·LiBr complex (1.5 M in Et2O, 2.0 mL, 3.0 mmol) at –20 °C under
an Ar atmosphere via a syringe. The reaction mixture was stirred for 1.5 h at –20 °C, and a
solution of 2.2 (1.28 g, 3.15 mmol) in THF (20 mL) was added. The reaction mixture was
stirred for 1 h at –20 °C and for further 2 h at rt. Saturated aq NH4Cl (100 mL) and Et2O (100
85
mL) were added. The layers were separated, the organic phase was washed with saturated aq
NH4Cl (100 mL) and saturated aq NaCl (100 mL), dried (Na2SO4), and filtered. Solvent
removal and purification by column chromatography (silica gel, hexanes/EtOAc 20:1)
afforded pure 2.21 (0.689 g, 48%) as an off-white solid.
1,1,14,14-Tetrakis(3,5-di-t-butylphenyl)tetradeca-2,4,6,8,10,12-hexayne-1,14-diol 2.31.
To a solution of 2.21 (0.029 g, 0.060 mmol) in CH2Cl2 (2 mL) was added a solution of Hay
catalyst [CuCl (0.20 g, 2.0 mmol) and TMEDA (0.046 g, 0.060 mL, 0.40 mmol) in CH2Cl2
(1 mL)]. The reaction mixture was stirred for 1.5 h, saturated aq NH4Cl (10 mL) was added,
and the mixture was extracted with Et2O (10 mL). The organic phase was washed with
saturated aq NH4Cl (20 mL) and saturated aq NaCl (20 mL), dried (Na2SO4), and filtered.
Solvent removal and purification by column chromatography (silica gel, hexanes/CH2Cl2 1:1)
afforded pure 2.31 (0.009 g, 31%) as a brown solid. Mp >85 °C (darkening), >95 °C (viscous
black). Rf = 0.22 (hexanes/ CH2Cl2 1:1). IR 3426 (br), 3071 (vw), 2955 (s), 2904 (m), 2866
(m), 2168 (m), 2074 (vw), 1594 (m) cm−1; 1H NMR (400 MHz, CDCl3) δ 7.34 (t, J = 1.8 Hz,
4H), 7.32 (d, J = 1.8 Hz, 8H), 2.82 (s, 2H), 1.27 (s, 72H); 13C NMR (100 MHz, CDCl3) δ
150.8, 142.3, 122.2, 120.6, 81.6, 76.1, 71.1, 65.1, 63.4, 62.9, 61.9, 35.0, 31.4. ESI HRMS m/z
calcd. for C70H86NaO2 ([M + Na]+) 981.6520, found 981.6536.
1,1,6,6-Tetramesitylhexa-2,4-diyne-1,6-diol 2.49. To a solution of 2.42 (50 mg, 0.17 mmol)
in CH2Cl2 (3 mL) was added a solution of Hay catalyst [CuCl (16.9 mg, 0.17 mmol) and
86
TMEDA (0.04 g, 0.05 mL, 0.3 mmol) in CH2Cl2 (1 mL)]. The reaction mixture was stirred for
4 d. Saturated aq NH4Cl (15 mL) and Et2O (15 mL) were added. The organic phase was
washed with saturated aq NH4Cl (30 mL), saturated aq NaCl (30 mL), water (30 mL), dried
(Na2SO4), and filtered. Solvent removal and purification by column chromatography (silica
gel, hexanes/CH2Cl2 4:1) afforded 2.49 as a yellow solid that could not be further purified.
NMR spectra of unpurified material have been included in the Appendix Section.
HO OHHHOPd(PPh3)2Cl2, CuI, Et3N
ethyl bromoacetate, THF
2.42
2.49
1,1,6,6-Tetramesitylhexa-2,4-diyne-1,6-diol 2.49. To a solution of Pd(PPh3)2Cl2 (7.2 mg,
0.010 mmol), CuI (3.9 mg, 0.020 mmol), and Et3N (60 µL) in THF (3 mL) was added 2.42
(50 mg, 0.17 mmol) in THF (3 mL). Finally, ethyl bromoacetate (0.023 g, 0.015 mL, 0.14
mmol) was added, and the reaction mixture was stirred overnight. Saturated aq NH4Cl (15
mL) and Et2O (15 mL) were added. The organic phase was washed with saturated aq NH4Cl
(20 mL), saturated aq NaCl (20 mL), dried (Na2SO4), and filtered. Solvent removal,
purification by column chromatography (silica gel, hexanes/EtOAc 20:1), and further
recrystallization from hexanes afforded 2.49 (6 mg, 12%) as a light brown solid. Rf = 0.21
(hexanes/CH2Cl2 1:4). 1H NMR (400 MHz, CDCl3) δ 6.71 (s, 8H), 2.43 (s, 2H), 2.20 (s, 12H),
2.19 (s, 24H); 13C NMR (100 MHz, CDCl3) δ 139.3, 136.6, 136.1, 131.6, 84.4, 74.7, 23.2,
20.5.
87
1,1,6,6-Tetramesitylhexa-2,4-diyne-1,6-diol 2.49. To a solution of Pd(PPh3)2Cl2 (1.6 mg,
0.0023 mmol), CuI (1.6 mg, 0.0084 mmol), I2 (22 mg, 0.087 mmol), and 2.42 (50 mg,
0.17 mmol) were added i-Pr2NH (2 mL) and THF (2 mL) under a N2 atmosphere. The
reaction mixture was stirred overnight, and saturated aq NH4Cl (10 mL) and Et2O (10 mL)
were added. The organic phase was washed with saturated aq NH4Cl (15 mL), saturated aq
Na2SO3 (15 mL), saturated aq NaCl (15 mL), dried (Na2SO4), and filtered. Solvent removal,
filtration through a plug of silica gel with hexanes/CH2Cl2 = 1:1, and recrystallization from
hexanes afforded 2.49 (13 mg, 26%) as a brownish solid. Spectral data are consistent with that
described above.
(2-Bromo-1,1’-dimesityl)allene 2.44. To a solution of 2.42 (0.298 g, 1.01 mmol) in THF
(5 mL) was added EtMgBr (1 M in THF, 1 mL, 1 mmol) at rt under a N2 atmosphere via a
syringe. The reaction mixture was stirred for 20 min, and ethyl formate (0.039 g, 42 µl, 0.52
mmol) was added. The cooling bath was removed, and the reaction was stirred for 30 min.
Saturated aq NH4Cl (10 mL) and Et2O (10 mL) were added. The layers were separated, the
organic phase washed with brine (10 mL), dried over Na2SO4, and filtered. The solvent was
removed in vacuo to yield the crude product as a viscous oil which was purified by column
chromatography (hexanes/ethyl acetate 20:1) to afford 2.44 (0.05 g, 14%) as a light yellow
solid. Rf = 0.57 (hexanes/EtOAc 20:1). IR 3033 (w), 2956 (m), 2915 (m), 2855 (m), 2730 (w),
1931 (m), 1607 (m), 1442 (s) cm−1; 1H NMR (400 MHz, CDCl3) δ 6.85 (s, 4H), 6.11 (s, 1H),
2.27 (s, 6H), 2.14 (s, 12H); 13C NMR (100 MHz, CDCl3) δ 203.6, 137.4, 137.0, 131.4, 129.7,
112.4, 72.9, 21.2, 20.9.
Crystal data for allene 2.44: C21H23Br, M = 355.30, monoclinic crystal system; space group
P21/n, a = 8.7221(2), b = 8.8492(4), c = 22.8496(8) Å, β = 95.017(2)°, V = 1756.86(11) Å3, Z
= 4, ρcalcd = 1.343 g cm–3; µ(MoKα) = 2.335 mm–1, λ = 0.71073 Å; 173.15 K; 2θ max =
54.94°; total data collected = 5797; R1 = 0.0396 [2905 observed reflections with F ≥ 4σ(F)];
88
wR2 = 0.1143 for 205 variables, 4038 unique reflections, and 0 restraints; residual electron
density = 0.342 and –0.635 e Å–3.
1,1-Dimesitylhex-2-yne-1,4-diol 2.45. To a solution of 2.42 (0.10 g, 0.34 mmol) in THF
(5 mL) was added EtMgBr (1 M in THF, 0.7 mL, 0.7 mmol) at rt under a N2 atmosphere via a
syringe. The reaction mixture was stirred for 30 min, and ethyl formate (0.013 g, 14 µl, 0.18
mmol) was added. The cooling bath was removed, and the reaction was stirred for 40 min.
Saturated aq NH4Cl (10 mL) and Et2O (10 mL) were added. The layers were separated, the
organic phase washed with brine (10 mL), dried over Na2SO4, and filtered. Solvent removal
and recrystallization from hexanes afforded 2.45 (0.013 g, 11%) as an off-white solid. Rf =
0.10 (hexanes/EtOAc 20:1). 1H NMR (300 MHz, CDCl3) δ 6.70 (s, 4H), 4.32 (br s, 1H), 2.47
(s, 1H), 2.21 (s, 12H), 2.20 (s, 6H), 1.72–1.63 (m, 2H), 0.91 (t, J = 7.4 Hz, 3H); 13C NMR (75
MHz, CDCl3) δ 140.22, 140.18, 136.25, 136.23, 135.92, 135.89, 131.6, 131.5, 90.0, 89.1,
76.5, 64.1, 30.4, 23.2, 20.5, 9.4.
2.4 References
1 Early work on cumulenes, see: a) H. Hopf, Classics in Hydrocarbon Chemistry,
Wiley-VCH, Weinheim, 2000, Chapter 9; b) H. Fischer, in The chemistry of alkenes
(Ed.: S. Patai), John Wiley & Sons, New York, 1964, pp. 1025–1159.
2 For two notable exceptions, see: a) Y. Kuwatani, G. Yamamoto, M. Oda, M. Iyoda,
Bull. Chem. Soc. Jpn. 2005, 78, 2188–2208; b) W. Skibar, H. Kopacka, K. Wurst, C.
Salzmann, K.-H. Ongania, F. F. de Biani, P. Zanello, B. Bildstein, Organometallics
2004, 23, 1024–1041.
3 R. Kuhn, H. Krauch, Chem. Ber. 1955, 88, 309–315.
89
4 [n]Ph: a) R. Kuhn, K. Wallenfels, Chem. Ber. 1938, 71, 783–790; b) R. Kuhn, H.
Zahn, Chem. Ber. 1951, 84, 566–570.
5 [n]Ph: F. Bohlmann, K. Kieslich, Abh. Braunsch. Wiss. Ges. 1957, 9, 147–160.
6 [n]Cy: F. Bohlmann, K. Kieslich, Chem. Ber. 1954, 87, 1363–1372.
7 The electronic structure of [n]cumulenes is fundamentally different between two
classes of molecules, i.e., when n is even (n = 2, 4, 6...) or odd (n = 3, 5, 7...). Only for
n = odd, π-conjugation between the endgroups via the cumulene framework is
possible. Only odd cumulenes are considered for the discussion in this chapter.
8 F. Innocenti, A. Milani, C. Castiglioni, J. Raman Spectrosc. 2010, 41, 226–236.
9 M. Weimer, W. Hieringer, F. Della Sala, A. Görling, Chem. Phys. 2005, 309, 77–87.
10 U. Mölder, P. Burk, I. A. Koppel, J. Mol. Struct. THEOCHEM 2004, 712, 81–89.
11 R. Hoffmann, Angew. Chem. Int. Ed. 1987, 26, 846–878; Angew. Chem. 1987, 99,
871–906.
12 Based on a search of WebCSD, see http://webcsd.ccdc.cam.ac.uk/ on 11/07/14, for
equally substituted [n]cumulenes with n = odd and alkyl or aryl endgroups. X-ray
crystallographic structures obtained in this thesis or in publications of the Tykwinski
group have not been considered.
13 W. A. Chalifoux, R. R. Tykwinski, Nature Chem. 2010, 2, 967–971.
14 Diploma thesis “Carbon in One Dimension – Synthesis of [n]Cumulenes”, Johanna A.
Januszewski, Friedrich-Alexander-Universität Erlangen-Nürnberg, May 2010.
15 E. J. Corey, P. L. Fuchs, Tetrahedron Lett. 1972, 36, 3769–3772.
16 S. Eisler, R. R. Tykwinski, in Acetylene Chemistry: Chemistry, Biology, and Material
Science (Eds.: F. Diederich, P. J. Stang, R. R. Tykwinski), Wiley-VCH, Weinheim,
2005, chapter 7.
17 W. A. Chalifoux, R. R. Tykwinski, C. R. Chimie 2009, 12, 341–358.
18 W. A. Chalifoux, R. R. Tykwinski, Chem. Rec. 2006, 6, 169–182.
19 H. Hauptmann, M. Mader, Synthesis 1978, 307–309.
20 J. Anthony, A. M. Boldi, Y. Rubin, M. Hobi, V. Gramlich, C. B. Knobler, P. Seiler, F.
Diederich, Helv. Chim. Acta 1995, 78, 13–45.
21 S. Eisler, N. Chahal, R. McDonald, R. R. Tykwinski, Chem. Eur. J. 2003, 9, 2542–
2550.
22 J. Hlavatý, L. Kavan, K. Okabe, A. Oya, Carbon 2002, 40, 1131–1150.
90
23 This reaction has been studied more intensely, and the resulting outcome is presented
in Section 5.3.1.
24 D. R. M. Walton, F. Waugh, J. Organomet. Chem. 1972, 37, 45–56.
25 A. L. K. Shi Shun, E. T. Chernick, S. Eisler, R. R. Tykwinski, J. Org. Chem. 2003, 68,
1339–1347.
26 T. Luu, E. Elliott, A. D. Slepkov, S. Eisler, R. McDonald, F. A. Hegmann, R. R.
Tykwinski, Org. Lett. 2005, 7, 51–54.
27 The synthesis is adapted from the work of Dominik Wendinger, see also J. A.
Januszewski, D. Wendinger, C. D. Methfessel, F. Hampel, R. R. Tykwinski, Angew.
Chem. Int. Ed. 2013, 52, 1817–1821; Angew. Chem. 2013, 125, 1862–1867.
28 J. P. Mario, H. N. Nguyen, J. Org. Chem. 2002, 67, 6841–6844.
29 L. Schmiech, C. Alayrac, B. Witulski, T. Hofmann, J. Agric. Food Chem. 2009, 57,
11030–11040.
30 T. Lee, H. R. Kang, S. Kim, S. Kim, Tetrahedron 2006, 62, 4081–4085.
31 The synthesis of acetylene 2.42 is presented in Section 2.1.3.4.
32 A. Lei, M. Srivastava, X. Zhang, J. Org. Chem. 2002, 67, 1969–1971.
33 Q. Liu, D. J. Burton, Tetrahedron Lett. 1997, 38, 4371–4374.
34 J. A. Januszewski, D. Wendinger, C. D. Methfessel, F. Hampel, R. R. Tykwinski,
Angew. Chem. Int. Ed. 2013, 52, 1817–1821.
35 Several compounds that have been synthesized during my diploma thesis also appear
in the experimental part of this thesis since the procedures have been repeated several
times under different conditions in order to improve purity and/or yield of the product.
36 a) K. Yamada, Y. Matsumoto, K. B. Selim, Y. Yamamoto, K. Tomioka, Tetrahedron
2012, 68, 4159–4165. b) K. Schreiner, H. Oehling, H. E. Zieger, I. Angres, J. Am.
Chem. Soc. 1977, 99, 2638–2641.
37 a) S. R. Ditto, R. J. Card, P. D. Davis, D. C. Neckers, J. Org. Chem. 1979, 44,
894−896. b) P. D. Bartlett, M. Roha, R. M. Stiles, J. Am. Chem. Soc. 1954, 76, 2349–
2353.
38 A. Ceccon, C. Corvaja, G. Giacometti, A. Venzo, J. Chem. Soc., Perkin Trans. 2
1978, 283–288.
91
3. Chapter III. Cumulene rotaxanes – Synthesis and stability of [n]tBuPh
rotaxanes
3.1 General introduction to rotaxanes
Rotaxanes are compounds that combine two different molecular components. A linear
molecule containing two endgroups, i.e., a dumbbell-shaped molecule, is one part of a
rotaxane and is often called the axle. The axle is surrounded by at least one macrocycle
defining the second component of the rotaxane (Figure 3.1).1 The large endgroups on the
dumbbell, also called stoppers, have to be sufficiently bulky to prevent the loss of the
macrocycle from its position around the linear molecule, the axle. These stoppers are
necessary, since the macrocyle is not bound covalently to the chain, but only via non-covalent
interactions.2 Pseudorotaxanes represents a special subclass of rotaxanes which do not possess
stoppers, and the axle and macrocycle are kept together only by non-covalent interactions.
Rotaxane nomenclature is based on the number of components, which is put in brackets as a
prefix, i.e., one dumbbell-shaped molecule and one macrocycle unit give a [2]rotaxane. In this
thesis, only [2]rotaxanes will be discussed.
Figure 3.1 Definition of a [2]rotaxane.
92
The interesting dynamics of rotaxanes offer potential applications in molecular
electronics, e.g., as sensors, switches, and molecular machines or rotary motors.3,4 Aside from
the above mentioned usage of these interlocked molecules, rotaxanes can also act as
“insulated molecular wires” which have been reviewed by Anderson and Frampton.3 Besides,
rotaxane formation serves for stabilization of the axle compound.5–7 The current chapter
describes the synthesis of rotaxanes with a cumulene as the axle component targeting
stabilization enhancement of the cumulene chain.
Rotaxane formation can be accomplished via several conventional methods including
capping, clipping, and slipping that belong to the passive strategies for template formation of
rotaxanes (Figure 3.2).1 In the case of capping (Figure 3.2a), first the axle and the macrocycle
are connected together via non-covalent interactions forming a pseudorotaxane, which is
finally converted to the rotaxane by a reaction with the stopper molecules. In contrast, the
clipping method (Figure 3.2b) uses an already formed dumbbell-shaped molecule, and the
final addition of an incomplete “open” macrocycle gives the rotaxane via a ring closing
reaction. The slipping method (Figure 3.2c) is based on the kinetic stability of the rotaxane.
Herein, higher temperatures are used to enable the slipping of the macrocycle onto the
dumbbell-shaped chain, where it gets “trapped” by decreasing the temperature.
Figure 3.2 Three common methods for rotaxane formation: a) capping, b) clipping, and c)
slipping.
In contrast to the passive methods described in Figure 3.2, there is an active template
synthesis of rotaxanes (Figure 3.3). This is the most attractive method for rotaxane formation
and has been applied in this thesis. In this active template method, two “half-dumbbell-
shaped” units (e.g., acetylene units containing a bulky endgroup) and the macrocycle are
93
connected via a metal ion to form the rotaxane. This method has been reported by Leigh and
coworkers for the first time8–11 utilizing a Cu(I)-catalyzed reaction. Specifically, a 1,3-
cycloaddition of azides with terminal alkynes, i.e., a CuAAC- or “click” reaction has been
carried out to assemble a [2]rotaxane.8 The idea behind this strategy is that the metal ion has a
dual function acting on one hand as a template for the rotaxane formation and on the other
hand as catalyst that facilitates the bond formation of two half-dumbbell-shaped units. In one
step, the thread is generated, while at the same time the inclusion of the macrocycle via
coordination occurs.
Figure 3.3 Active template method for rotaxane formation.
3.2 Polyyne rotaxanes as motivation for cumulene rotaxane formation
Long polyynes are not particularly stable, upon reaching a certain number of acetylene
units in the chain, polyynes can often only be handled in a dilute solution.12 Recently, several
methods have been developed to increase the stability of longer polyynes or to encapsulate the
polyyne chain. Complexation offers one opportunity for increased stabilization of polyynes.
For example, Taylor and Gabbai13 have described the complexation of the polyyne chain to
tridentate mercury-containing Lewis acids via weak supramolecular interactions.
Encapsulation of polyynes into carbon nanotubes can also shield the sp-hybridized carbon
chain, as demonstrated by Zhao et al.14 Briefly, fusion reactions of linear polyyne units in
double wall carbon nanotubes (DWCNTs) afforded long linear carbon chains. Another
encapsulation method has been described by Ben Shir et al.15 using guest-host complex
formation. Herein, the polyyne forms a molecular rotary motor via insertion of a cucurbituril
host around the carbon chain.
Introduction of macrocycles to form polyyne rotaxanes has been recently reported by
several groups2,4,16 presumably to determine if this strategy can be used for stability
enhancement of a sp-hybridized carbon chain. It is important to also consider that the
macrocycle in a rotaxane does interact strongly with the linear component, i.e., the polyyne
94
chain, and thus the investigation of structural and physical properties should not be hampered
by rotaxane formation. In 2012, Gladysz17 has demonstrated a polyyne rotaxane formation for
a tetrayne with a dimetallic polyynediyl axle and a phenanthroline based macrocycle (Figure
3.4a). Anderson and Tykwinski18 have constructed even longer polyyne rotaxanes with four,
six, and ten acetylene units (Figure 3.4b) using the same phenanthroline-based macrocycle as
Gladysz has done, but with supertrityl endcapped polyynes.
Figure 3.4 Polyyne rotaxanes reported by a) Gladysz17 and b) Anderson/Tykwinski.18
The synthesis of the known polyyne rotaxanes was based on the active metal template
approach and included a complexation as well as a homocoupling reaction. Initially, the
copper-macrocycle complex was formed by treatment of naked macrocycle with CuI in
CH2Cl2/CH3CN at rt.16 Afterwards, addition of the terminal acetylene, K2CO3, and an oxidant
95
in THF gave the desired polyyne rotaxane. Gladysz initially used iodine as oxidant and 55 °C
as reaction temperature resulting in low yields and high impurity, and he switched later to the
use of oxygen as oxidant and decreased the temperature to 50 °C, which gave the rotaxane in
9% yield (Scheme 3.1). Anderson and Tykwinski maintained iodine as oxidant and were able
to achieve rotaxanes in yields of 15–34% using polyynes with supertrityl endgroups (Scheme
3.1). The comparison of the yields for supertrityl endcapped [n]polyynes (34, 32, and 15% for
n = 4, 6, and 10, respectively) demonstrated that the increase of chain length correlated with
the decrease of reaction yields.
Scheme 3.1 Synthesis of polyyne rotaxanes.
3.3 Introduction to cumulene rotaxanes: Motivation and target
During the period of this doctoral thesis, higher [n]cumulenes with n > 5 were
synthesized by the incorporation of bulky endgroups for stabilization. This strategy had been
proven successful for polyyne formation but showed limitations in the synthesis of
cumulenes. Thus far, a [7]cumulene, [7]tBuPh, had been successfully synthesized that has
been infinitely stable as crystalline solid. With respect to the next higher analogue, [9]tBuPh,
the synthesis was successful as well, but the limit was reached at this stage for stabilization of
the cumulene series [n]tBuPh under normal laboratory conditions. Namely, [9]tBuPh was
only stable for several days when kept in Et2O solution at ca. –20 °C under water- and
oxygen-free conditions. Furthermore, it was not possible to obtain this compound as a
crystalline solid in order to probe its stability in the solid state. An alternative strategy to
96
enhance the stability of longer cumulenes had to be found. In the following section, the
formation of cumulene rotaxanes has been presented.
3.4 Synthesis of rotaxane precursors and the appropriate cumulene rotaxanes
([9]tBuPh rotaxanes) using three different macrocycles**
For the synthesis of cumulene rotaxanes, three different macrocycles, 3.1, 3.2, and 3.3,
have been chosen (Figure 3.5). All three macrocycles are based on a phenanthroline unit
which has been reliable for polyyne rotaxane formation as reported by several groups, such as
Saito,2,16 Gladysz,17 and Anderson.3,4,18 The macrocycles possess the same modified
phenanthroline backbone (dotted box in Figure 3.5) with variation of the macrocycle size due
to different linkers. Macrocycle 3.119 shows a relatively rigid structure using a diphenyl ether
group as linker. Compound 3.1 is the smallest homologue, followed by compound 3.220 with
an alkyl linker, i.e., a (CH2)10 unit. The last representative is macrocycle 3.32,16 which uses a
combination of a resorcinol aromatic ring linked via (CH2)6 fragments to the phenanthroline
backbone. Compound 3.3 represents the largest of the macrocycles.
Figure 3.5 Three macrocycles, compounds 3.1, 3.2, and 3.3, used in the synthesis of
cumulene rotaxanes.
** The synthesis of all macrocycles as well as the precursors to rotaxanes (oligoyne diethers) has been performed
by Michael Franz and Levon Movsisyan who are working on polyyne rotaxanes in the Tykwinski and Anderson
groups, respectively.
97
Regarding the synthesis of cumulene rotaxanes, the precursors, i.e., polyyne rotaxanes
3.4, 3.5, and 3.6 had to be synthesized (Scheme 3.2). Thus, using standard methods, two
equivalents of the terminal diyne 2.12 were subjected to rotaxination using K2CO3 as base, I2
as oxidant, and the macrocyclic copper complexes 3.1·CuI, 3.2·CuI, and 3.3·CuI,
respectively.21,22 Unfortunately, this synthetic route was only mildly successful, resulting in
unsatisfying yields of 5% and 6% using macrocycles 3.1 and 3.2, respectively. Macrocycle
3.3, however, showed the highest yield of 68% for the synthesis of 3.6 and thus, 3.6 seemed to
represent the most suitable macrocycle for rotaxane formation of [9]tBuPh cumulenes.
Scheme 3.2 Formation of rotaxanes 3.4, 3.5, and 3.6 via an active metal templated
homocoupling reaction.22
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The yield of polyyne rotaxane 3.6 was further increased via an alternative pathway,
namely a Cadiot-Chodkiewicz heterocoupling (Scheme 3.3).22 Terminal diyne 2.12 and
bromodiyne 3.7 were reacted in the presence of K2CO3 and macrocyclic complex 3.3·CuI in
THF at 60 °C. Additional oxidant was not necessary, and the rotaxane 3.6 could be
synthesized with an increased yield of 74%. This reaction was also performed using complex
3.1·CuI giving 3.4 in a low yield of 5%.
Scheme 3.3 Formation of rotaxanes 3.4 and 3.6 via an active metal templated Cadiot-
Chodkiewicz heterocoupling reaction.22
With precursors 3.4 and 3.6 in hand, further conversion to the cumulene rotaxanes 3.8
and 3.9, respectively, was accomplished via reductive elimination (Scheme 3.4). The
precursors were dissolved in dry Et2O and treated with anhydrous SnCl2 and HCl
(1 M in Et2O) at rt under an argon atmosphere. After ca. 30 min, the cumulene rotaxane
formation was judged complete via TLC analysis, and the reaction mixture was filtered
through a plug of basic alumina. Conventional crystallization attempts by overlaying a
99
CH2Cl2 solution containing the cumulene rotaxanes with MeOH (at ca. –20 °C) afforded
crystalline precipitates of [9]cumulene rotaxanes 3.8 and 3.9 in 29% and 33% yield,
respectively. This was the first time that a 3,5-di-t-butylphenyl substituted [9]cumulene could
be isolated in the solid state. Cumulene rotaxane 3.10 was also successfully synthesized,
however, with much lower yields that appeared already in the formation of the appropriate
precursor 3.5 (Scheme 3.4). The limited amount of precursor 3.5 and cumulene rotaxane 3.10
prevented full characterization and thus, no further comparisons or discussions of its
properties were made.
Scheme 3.4 Synthesis of cumulene rotaxanes 3.8, 3.9, and 3.10.
100
3.5 Stability of [9]cumulene rotaxanes and comparison to [9]tBuPh
The stability of [9]cumulene rotaxane 3.9 was qualitatively compared to the “naked”
[9]cumulene [9]tBuPh by observing optical changes in e.g., color changes. These studies
were performed in Et2O, kept under an argon atmosphere at rt – exposed to light and kept in
the dark. Results have been summarized in Table 3.1. The results showed that a solution of
[9]tBuPh decolorized already after 3 h under ambient light or overnight when kept in the
dark, whereas for 3.9, decolorization was observed after one week under ambient light or after
several weeks when kept in dark. In addition, the color change of the TLC spots of [9]tBuPh
and the cumulene rotaxane 3.9 was compared. The color of the TLC spot changed from blue
to orange/yellow after seconds for [9]tBuPh or within 4 h for 3.9. Regarding [9]cumulenes in
the solid state, the “naked” [9]tBuPh could not be isolated as solid, while crystalline
precipitates of [9]cumulene rotaxanes were obtained reproducibly from solutions of
CH2Cl2/MeOH. All synthesized [9]cumulene rotaxanes were stable for at least weeks to
months in the crystalline solid state.
101
Table 3.1 Comparison of the qualitative stability of [9]tBuPh and [9]cumulene rotaxane 3.9
when kept under an argon atmosphere at rt.
Et2O solution kept at ambient light kept in the dark TLC spot
[9]tBuPh decolorization from
blue to grey/green to
orange within 3
hours
decolorization to
orange overnight
decolorization from
blue to an orange
spot after seconds
and already during
spotting
3.9 decolorization after
one week
decolorization after
several weeks
decolorization from
blue to a light yellow
spot within 4 h
Additional methods for monitoring stability based on decomposition, such as
differential scanning calorimetry (DSC) or melting/decomposition point analysis have been
rare for longer [n]cumulenes with n ≥ 5. For example, Iyoda and coworkers23 reported melting
points of a variety of tetraryl[5]cumulene derivatives that were measured as >250 °C. The
increased stability of the [9]cumulene rotaxanes enabled studies describing more closely the
thermal stability of these rotaxanes. Thus, DSC measurements for [9]cumulene rotaxane 3.9
were performed (Figure 3.6). The results showed that no melting point was observed for
rotaxane 3.9, but rather an onset of decomposition at ca. 170 °C, with a maximum at 176 °C.
102
Figure 3.6 DSC scan of [9]cumulene rotaxane 3.9.
While it was not possible to directly compare the thermal stability of rotaxane 3.9 with
cumulene [9]tBuPh, the DSC analysis of [9]cumulene rotaxane 3.9 was compared to the DSC
scan of [7]cumulene [7]tBuPh (Figure 3.7). Similar to the [9]cumulene rotaxane 3.9,
[7]tBuPh showed no melting point, but instead, an onset of decomposition at ca. 187 °C with
a maximum at 215 °C. Thus, the [7]cumulene [7]tBuPh appeared to possess slightly a higher
thermal stability compared to the [9]cumulene rotaxane 3.9. If cumulenes followed the same
trend as polyynes,24 decomposition of the “naked” [9]cumulene [9]tBuPh should be lower
than 187 °C, the onset of decomposition of [7]tBuPh. If the decomposition of [9]tBuPh
started between 160 and 187 °C, no stabilization of the cumulene via rotaxane formation
would be observed. In contrast, if the decomposition already started below 160 °C, rotaxane
formation would result in stabilization. Consequently, at this point, no conclusive remarks
regarding DSC analysis could be made.
103
Figure 3.7 DSC scan of [7]tBuPh.
Since the [9]cumulene rotaxanes showed an increased stability compared to the
“naked” analogues, especially as microcrystalline solids, extensive characterization was
possible for the first time based on 1D and 2D NMR-, UV/vis-, and IR spectroscopy, as well
as mass spectrometry and cyclic voltammetry. Discussions and comparisons of the
[9]cumulene rotaxanes with the series of [n]tBuPh cumulenes have been described in Chapter
IV.
3.6 Synthetic approach to higher [n]cumulene rotaxanes (n > 9)
3.6.1 Synthetic approach to [11]cumulene rotaxane
Since an enhancement of stability of longer cumulenes via rotaxane formation was
observed through the formation of 3.9, the synthesis of the [11]cumulene rotaxane 3.11 was
attempted (Scheme 3.5). Precursor 3.12 was obtained in a very low yield via a heterocoupling
reaction of terminal triyne 2.21 and bromodiyne 3.7.22 Nevertheless, with a small amount of
104
compound 3.12 in hand, reductive elimination was conducted at rt. TLC analysis confirmed
the formation of [11]cumulene rotaxane 3.11, indicated by a blue-greenish spot, which
showed behavior typical for cumulenes. The blue-greenish reaction mixture, however, started
to decompose within minutes as observed by a color change of the solution to brown and
definitely decomposed when filtered over a plug of basic alumina. Nonetheless, this was the
first time where a [11]cumulene could be synthesized, although, disappointingly, no concrete
confirmation of this statement could be obtained by traditional characterization methods.
105
Scheme 3.5 Synthetic approach to [11]cumulene rotaxane 3.11.
106
3.6.2 Synthetic approach to [13]cumulene rotaxanes including UV/vis spectroscopy
studies
With the terminal triyne 2.21 in hand, the synthesis of the [13]cumulene rotaxane 3.13
was also attempted (Scheme 3.6). Terminal triyne 2.21 was homocoupled in the presence of
3.3·CuI to afford the hexayne rotaxane 3.14 in 17% yield.22 This precursor for the
[13]cumulene rotaxane was further converted in a reductive elimination reaction using
anhydrous SnCl2. The light yellow Et2O solution of 3.14 turned to a brighter yellow color
after SnCl2 and HCl addition before a darkening to orange occurred after several minutes.
TLC analysis, however, showed only a baseline spot aside from the starting material spot.
After additional SnCl2 and HCl were added, more side products were observed without any
evidence for rotaxane formation. In conclusion, TLC analysis and unsuccessful crystallization
attempts indicated that either rotaxane 3.13 was not formed or it was too unstable and
decomposed already in the reaction mixture.
Scheme 3.6 Synthetic approach to [13]cumulene rotaxane 3.13.
Further attempts to convert 3.14 to the rotaxane 3.13 were carried out without using
work-up methods such as filtration over alumina and further solvent evaporation as described
107
for the synthesis of [13]tBuPh in Section 2.1.2.6. The reaction of precursor 3.14 to form
[13]cumulene rotaxane 3.13 was monitored via UV/vis spectroscopy (Figure 3.8). Precursor
3.14 showed a weak shoulder absorption at ca. 360 nm in the UV/vis spectrum (dark blue
curve). After addition of SnCl2 and HCl, this shoulder disappeared, and a new absorption
band at 364 nm was observed (yellow curve, Figure 3.8) similar to the UV/vis spectrum of the
synthesis of [13]tBuPh (red curve, Figure 2.3). After stirring and evaporation via bubbling
argon through the reaction mixture, three signals (e.g., pink curve) were formed at 345, 371,
and 400 nm, analogous to UV/vis spectra of [13]tBuPh in Figure 2.3 (see Section 2.1.2.6).
The values of vibrations were slightly lower with 2032 and 1954 cm–1. After second addition
of reactants (in order to potentially accelerate the reaction) and several hours, only one
absorption band, at 363 nm, remained (green curve), which persisted after stirring overnight
(black curve). The color of the reaction mixture appeared also to be apricot-orange but
slightly brighter than in the case of conversion to the “naked” [13]cumulene [13]tBuPh.
Again, however, no hint for cumulene formation could be observed based on lack of
absorption bands in the lower energy region.
108
Figure 3.8 UV/vis spectra taken during attempted conversion of precursor 3.14 to
[13]cumulene rotaxane 3.13 (in Et2O).
Finally, a new and potentially promising macrocycle, compound 3.15, has been
synthesized (Scheme 3.7).25 This macrocycle possesses a similar structure to macrocycle 3.3,
however, the resorcinol aromatic ring contains two additional t-butyl groups in the ortho
position to the oxygen substituents. Molecular modeling26 of 3,5-di-t-butylphenyl-substituted
[n]cumulene rotaxanes (n = 9 and 11) with macrocycles 3.3 and 3.15 has suggested a better
shielding of the cumulene chain in the case of macrocycle 3.15. Thus, direct synthesis of the
109
[13]cumulene rotaxane has been carried out, starting with the formation of 3.16 in 12%
yield.22
Scheme 3.7 Synthetic approach to [13]cumulene rotaxane 3.17.
110
With compound 3.16 in hand, the synthesis of [13]cumulene rotaxane 3.17 was
attempted. The reaction was monitored via UV/vis absorption spectroscopy before and after
addition of the reactants. Standard reaction methods, such as TLC analysis and work-up via
filtration were not carried out to circumvent potential decomposition. Furthermore, in the case
of decomposition, UV/vis measurements that were recorded directly from the reaction
mixture could show at least potential evidence for the successful formation of the rotaxane
before complete decomposition. The conversion of 3.16 to [13]cumulene rotaxane 3.17 was
performed in Et2O at rt (Figure 3.9). Pure precursor 3.16 showed a shoulder absorption
between 350 and 360 nm in the UV/vis spectrum (black curve). After addition of SnCl2, the
frequently observed absorption band at 366 nm occurred (red curve). Stirring the reaction
mixture for ca. 1 h revealed the formation of three vague signals, rather shoulders, between
350 and 450 nm (green curve). The interesting feature was that by time passing, the intensity
of these absorption decreased, in contrast to both synthetic approaches to [13]tBuPh and
[13]cumulene rotaxane 3.13 in Figure 2.3 and Figure 3.8, respectively, where the intensity of
the three signals was increasing by proceeded time. In addition, two new absorption bands in
the region of 475–550 nm, at 487 and 528 nm were observed (green curve), which showed a
debut compared to the synthetic approaches to [13]tBuPh and [13]cumulene rotaxane 3.13 in
Figure 2.3 and Figure 3.8, respectively, even though two potential absorption bands could be
observed in the case of conversion of precursor 3.14 to rotaxane 3.13 in Figure 3.8. The
UV/vis spectrum that was recorded after stirring overnight revealed again only one absorption
band at ca. 359 nm (violet curve). The two absorption bands in the lower energy region also
disappeared hereby. The color of the reaction mixture was again apricot-orange, and no hint
for cumulene formation by color or by absorption in the low energy region was observed.
111
Figure 3.9 UV/vis spectra taken during attempted conversion of precursor 3.16 to
[13]cumulene rotaxane 3.17 (in Et2O).
3.7 Summary and conclusion
In summary, the strategy for rotaxane formation that has recently been successfully
implemented for polyynes, also succeeded when applied to cumulenes. To the best of my
knowledge, this was the first time that cumulene rotaxanes had been synthesized. The
synthesis of precursors to cumulene rotaxanes (i.e., polyyne rotaxanes) was optimized to the
112
stage that reaction yields reached an outstanding value of 74%. This result was achieved using
macrocycle 3.3, which was the most suitable of the three macrocycles that were tested. The
reaction yields for the synthesis of cumulene rotaxanes 3.8 and 3.9 (29% and
33%, respectively) were also quite good comparing the rather low yields of known polyyne
rotaxane syntheses.17 The introduction of a macrocycle, which ideally has offered protection
to the instable cumulene chain led to the desired stability enhancement of longer cumulenes
and hence, the possibility to afford [9]cumulenes as stable solids. Unfortunately, the limit to
stability enhancement by rotaxane formation appeared to be reached at the stage of a
[13]cumulene as observed by UV/vis spectroscopic studies. More specifically, according to
the similar colors of the reaction outcome in Figures 3.7 and 3.8, and the lack of absorption
bands in the low energy region that have been characteristic for longer cumulenes, no
[13]cumulene rotaxanes, 3.13 and 3.17, were formed in the two synthetic approaches. It
seemed, however, that some new compounds were formed instead, e.g., an acetylene-like
vibronic fine structure was observed for [13]cumulene rotaxane 3.13. In addition, overnight,
the three absorptions signals that were observed in both reactions converted to only one broad
absorption band at ca. 360 nm indicating a potential decomposition of the initially formed yet
unknown and uncharacterized compounds.
As an outlook or future directions to go with this project, it is worth to again approach
the synthesis of a [11]cumulene rotaxane to determine if this cumulene can be stabilized by
encapsulation with a macrocycle, which would represent the first [11]cumulene synthesized to
date. Another optimization that could be done in the case of cumulene rotaxanes is to improve
the reaction yields for the polyyne rotaxanes since further formations of cumulene rotaxanes
appeared much less problematic. Finally, regarding the macrocycles, optimizations, e.g., cycle
size and functionalization issues could be varied to develop the most suitable macrocycle for
cumulene rotaxanes.
3.8 Experimental part
3.8.1 General procedures and methods
The general procedures and methods are analogous to that in Section 2.3.1.
113
3.8.2 Experimental data and compound characterization
1,1,6,6-Tetrakis(3,5-di-t-butylphenyl)deca-1,2,3,4,5,6,7,8,9-nonaene rotaxane 3.8. To a
solution of 3.4 (10 mg, 6.7 µmol) in Et2O (4 mL) was added anhydrous SnCl2 (3.8 mg,
20 µmol) and HCl (1 M in Et2O, 0.03 mL, 30 µmol) at rt under an Ar atmosphere. After 25
min, the solution was filtered through a plug of basic alumina oxide and eluted with CH2Cl2
affording the purified 3.8. Since the cumulene is not stable as amorphous solid and in
solution, “microcrystalline” 3.8 was obtained as blue solid with red shining (2.8 mg, 29%) by
overlaying a CH2Cl2 solution with dry MeOH at ca. –20 °C for ca. 2 days being stable for an
infinite time kept under an Ar atmosphere. Rf = 0.23 (hexanes/EtOAc 10:1). UV/vis (Et2O)
λmax 278, 319, 341, 369, 530, 578, 610, 670 nm; IR 3065 (vw), 2957 (m), 2901 (m), 2862 (w),
1920 (vw) 1584 (m) 1257 (s) cm−1; 1H NMR (400 MHz, CD2Cl2) δ 8.21 (d, J = 8.4 Hz, 2H),
7.94 (d, J = 8.8 Hz, 4H), 7.87 (d, J = 8.4 Hz, 2H), 7.74 (s, 2H), 7.38 (t, J = 1.6 Hz, 4H), 7.28
(d, J = 1.6 Hz, 8H), 7.24−7.17 (m, 8H), 6.88 (d, J = 8.8 Hz, 4H), 5.21 (s, 4H), 1.22 (s, 72H); 13C NMR (100 MHz, CD2Cl2) δ 159.7, 158.3, 158.0, 151.4, 147.0, 144.5, 137.3, 136.4, 134.3,
134.2, 130.0, 127.6, 127.4, 127.3, 125.8, 124.3, 123.7, 123.2, 122.6, 122.2, 122.0, 120.5,
116.5, 70.6, 35.1, 31.5. ESI HRMS m/z calcd. for C104H110N2NaO3 ([M + Na]+) 1457.84087
found 1457.83717.
114
1,1,6,6-Tetrakis(3,5-di-t-butylphenyl)deca-1,2,3,4,5,6,7,8,9-nonaene rotaxane 3.9. To a
solution of 3.5 (12.5 mg, 7.90 µmol) in Et2O (4 mL) was added anhydrous SnCl2 (4.5 mg,
0.024 mmol) and HCl (1 M in Et2O, 0.035 mL, 0.035 mmol) at rt under an Ar atmosphere.
After 20 min, the solution was filtered through a plug of basic alumina oxide and eluted with
CH2Cl2 affording the purified 3.9. Since the cumulene is not stable as amorphous solid and in
solution, “microcrystalline” 3.9 was obtained as blue needles with green shining (4.0 mg,
33%) by overlaying a CH2Cl2 solution with dry MeOH at ca. –20 °C for ca. 2 days being
stable for an infinite time kept under an Ar atmosphere. The isolated product was
contaminated with ca. 5% precursor 3.6. Rf = 0.46 (CH2Cl2). UV/vis (Et2O) λmax (ε) 287
(78200), 319 (65600), 341 (102700), 368 (158700), 530 (56700), 576 (50000), 615 (74500),
666 (38600) nm; IR 3069 (vw), 3036 (vw), 2955 (s), 2902 (m), 2863 (m), 2168 (vw), 2055
(w), 2038 (w), 1919 (w), 1590 (s), 1241 (s) cm−1; 1H NMR (300 MHz, CD2Cl2) δ 8.46 (d, J =
8.9 Hz, 4H), 8.26 (d, J = 8.5 Hz, 2H), 8.08 (d, J = 8.5 Hz, 2H), 7.74 (s, 2H), 7.37 (t, J = 1.6
Hz, 4H), 7.32 (d, J = 1.6 Hz, 8H), 7.16 (d, J = 8.9 Hz, 4H), 7.00 (t, J = 8.2 Hz, 1H), 6.61 (t, J
= 2.3 Hz, 1H), 6.37 (dd, J = 8.2 Hz, J = 2.3 Hz, 2H), 4.08 (t, J = 7.3 Hz, 4H), 3.96 (t, J = 6.6
Hz, 4H), 1.82 (t, J = 6.7 Hz, 4H), 1.75 (t, J = 6.5 Hz, 4H), 1.54 (s, 8H), 1.20 (s, 72H); 13C
NMR (75 MHz, CD2Cl2) δ 160.92, 160.87, 156.1, 151.4, 146.4, 144.8, 137.4, 136.7, 131.9,
129.7, 129.1, 128.7, 127.7, 125.7, 124.5, 124.1, 123.2, 120.3, 119.8, 119.0, 115.2, 107.8,
100.0, 68.4, 68.0, 35.1, 31.4, 29.8, 29.4, 26.13, 26.08. ESI HRMS m/z calcd. for
C108H126N2NaO4 ([M + Na]+) 1537.96098, found 1537.95928, for C108H127N2O4 ([M + H]+)
1515.97904, found 1515.97866.
115
1,1,6,6-Tetrakis(3,5-di-t-butylphenyl)deca-1,2,3,4,5,6,7,8,9-nonaene rotaxane 3.10. To a
solution of 3.5 (10 mg, 6.9 µmol) in Et2O (4 mL) was added anhydrous SnCl2 (9.0 mg, 0.047
mmol) and HCl (1 M in Et2O, 0.1 mL, 0.1 mmol) at 0 °C under an Ar atmosphere. After 15
min, the solution was filtered through a plug of basic alumina oxide and eluted with CH2Cl2
affording the purified 3.10. Since the cumulene is not stable as amorphous solid and in
solution, “microcrystalline” 3.10 was obtained as blue precipitate by overlaying a CH2Cl2
solution with dry MeOH at ca. –20 °C for ca. 2 days being stable for an infinite time kept
under an Ar atmosphere. Due to the small amount of 3.10 that could be isolated, no yield was
determined. Rf = 0.12 (CH2Cl2). UV/vis (Et2O) λmax 286, 320, 341, 369, 532, 578, 616, 668
nm; IR 3069 (vw), 2953 (s), 2920 (s), 2852 (s), 2050 (vw), 1929 (w), 1585 (m) cm−1; 1H
NMR (400 MHz, CDCl3) δ 8.47 (d, J = 8.9 Hz, 4H), 8.17 (d, J = 8.4 Hz, 2H), 7.99 (d, J = 8.4
Hz, 2H), 7.67 (s, 2H), 7.31–7.30 (m, 4H), 7.27 (d, J = 1.6 Hz, 8H), 7.18 (d, J = 8.9 Hz, 4H),
4.18–4.14 (m, 4H), 1.91–1.89 (m, 4H), 1.46 (br s, 12H), 1.17 (s, 72H). Insufficient material
was available to obtain a meaningful 13C spectrum. ESI HRMS m/z calcd. for C100H119N2O2
([M + H]+) 1379.92661 found 1379.92568.
5,5'-(5-Bromo-1-methoxypenta-2,4-diyne-1,1-diyl)bis(1,3-di-t-butylbenzene) 3.7. To a
solution of 2.12 (0.243 g, 0.516 mmol) in acetone (5 mL) was added N-bromosuccinimide
(NBS) (0.138 g, 0.775 mmol) and AgNO3 (8.8 mg, 0.052 mmol) at rt. The reaction mixture
was covered with alumina foil and stirred overnight. n-Pentane (15 mL) and saturated aq
116
Na2S2O5 (15 mL) were added. The layers were separated, and the organic phase was washed
with brine (20 mL), dried over Na2SO4, and filtered. Solvent removal afforded 3.7 (0.28 g,
99%) as a white solid. Mp 48−52 °C. Rf = 0.60 (hexanes/EtOAc 10:1). IR 3070 (vw), 2955
(s), 2903 (m), 2866 (m), 2824 (w), 1594 (m) cm−1; 1H NMR (300 MHz, CDCl3) δ 7.33 (t, J =
1.8 Hz, 2H), 7.29 (d, J = 1.8 Hz, 4H), 3.42 (s, 3H), 1.29 (s, 36H); 13C NMR (75 MHz, CDCl3)
δ 150.4, 141.1, 121.7, 121.4, 82.6, 75.6, 73.4, 65.1, 53.1, 42.0, 34.9, 31.4. ESI HRMS m/z
calcd. for C34H4579BrNaO ([M + Na]+) 571.25460, found 571.25464; for C33H42
79Br ([M –
OMe]+) 517.24644, found 517.24587.
3.9 References
1 F. Aricó, J. D. Badjic, S. J. Cantrill, A. H. Flood, K. C.-F. Leung, Y. Liu, J. F.
Stoddart, Top. Curr. Chem. 2005, 249, 203–259.
2 S. Saito, K. Nakazono, E. Takahashi, J. Org. Chem. 2006, 71, 7477–7480.
3 M. J. Frampton, H. L. Anderson, Angew. Chem. Int. Ed. 2007, 46, 1028–1064; Angew.
Chem. 2007, 119, 1046–1083.
4 M. J. Langton, J. D. Matichak, A. L. Thompson, H. L. Anderson, Chem. Sci. 2011, 2,
1897–1901.
5 Y. Kohsaka, K. Nakazono, Y. Koyama, S. Asai, T. Takata, Angew. Chem. Int. Ed.
2011, 50, 4872–4875.
6 R. Eelkerna, K. Maeda, B. Odell, H. L. Anderson, J. Am. Chem. Soc. 2007, 129,
12384–12385.
7 G. Wenz, B.-H. Han, A. Müller, Chem. Rev. 2006, 106, 782–817.
8 V. Aucagne, K. D. Hänni, D. A. Leigh, P. J. Lusby, D. B. Walker, J. Am. Chem. Soc.
2006, 128, 2186–2187.
9 J. D. Crowley, S. M. Goldup, A.-L. Lee, D. A. Leigh, R. T. McBurney, Chem. Soc.
Rev. 2009, 38, 1530–1541.
10 V. Aucagne, J. Berná, J. D. Crowley, S. M. Goldup, K. D. Hänni, D. A. Leigh, P. J.
Lusby, V. E. Ronaldson, A. M. Slawin, A. Viterisi, D. B. Walker, J. Am. Chem. Soc.
2007, 129, 11950–11963.
11 The active template method using Cu(I) has already been applied to the formation of
catenanes, see: C. O. Dietrich-Buchecker, J. P. Sauvage, J. P. Kintzinger, Tetrahedron
Lett. 1983, 24, 5095–5098; J.-P. Sauvage, Acc. Chem. Res. 1990, 23, 319–327.
117
12 R. Eastmond, T. R. Johnson, D. R. M. Walton, Tetrahedron 1972, 28, 4601–4616.
13 T. J. Taylor, F. P. Gabbai, Organometallics 2006, 25, 2143–2147.
14 C. Zhao, R. Kitaura, H. Hara, S. Irle, H. Shinohara, J. Phys. Chem. C 2011, 115,
13166–13170.
15 I. Ben Shir, S. Sasmal, T. Mejuch, M. K. Sinha, M. Kapon, E. Keinan, J. Org. Chem.
2008, 73, 8772–8779.
16 S. Saito, E. Takahashi, K. Nakazono, Org. Lett. 2006, 8, 5133–5136.
17 N. Weisbach, Z. Baranová, S. Gauthier, J. H. Reibenspies, J. A. Gladysz, Chem.
Commun. 2012, 48, 7562–7564.
18 L. D. Movsisyan, D. V. Kondratuk, M. Franz, A. L. Thompson, R. R. Tykwinski, H.
L. Anderson, Org. Lett. 2012, 14, 3424–3426.
19 A. M. Blanco-Rodríguez, M. Towrie, J.-P. Collin, S. Záliš, A. Vlček Jr, Dalton Trans.
2009, 3941–3949.
20 F. Eggers, U. Lüning, Eur. J. Org. Chem. 2009, 2328–2341.
21 The leaving group of the terminal acetylene plays a huge role in this reaction
manifesting in higher reaction yields for OMe leaving groups than for OH groups. As
an example, the precursor rotaxane formation have been accomplished using 3.2 as
macrocycle and the terminal diyne 2.24. The yields have been lousy, and this reaction
type has not continued.
22 The reaction was performed by Michael Franz from the Tykwinski group.
23 Y. Kuwatani, G. Yamamoto, M. Oda, M. Iyoda, Bull. Chem. Soc. Jpn. 2005, 78,
2188–2208.
24 W. A. Chalifoux, R. R. Tykwinski, Nature Chem. 2010, 2, 967–971.
25 Compound 3.15 was developed and synthesized by Michael Franz.
26 3D models have been calculated by Michael Franz using the software “Chem3D” and
“MM2 geometry optimization” as method.
118
4. Chapter IV. Characterization of [3]-, [5]-, [7]-, and [9]tBuPh including
[9]tBuPh rotaxanes and comparison to different series of [n]cumulenes††
4.1 UV/vis spectroscopy
4.1.1 Introduction
UV/vis spectroscopy has been the most common characterization method for long
[n]cumulenes and in early studies it has been the essential tool to confirm formation of [7]-
and [9]cumulenes.1 Using homologous series of [n]cumulenes, UV/vis spectroscopy allows
analysis of electronic trends, i.e., optical band gap, as a function of cumulene length and
substitution. Similar to that demonstrated for polyynes,2 UV/vis spectroscopic analysis versus
molecular length might also allow extrapolation to infinite chain length, which would offer a
prediction of the band gap of the cumulenic version of carbyne. To date, however, this has not
been possible due to the limited number of model compounds that are currently available.
In principle, three major factors govern trends typically observed in the UV/vis spectra
of [n]cumulenes, including (1) structure of the [n]cumulene (odd- versus even-numbered
[n]cumulenes), (2) molecular length, and (3) the nature of the terminal substitution, i.e.,
endgroup effects (aryl versus alkyl and mesomeric versus inductive effects, respectively).
The influence of odd- versus even-numbered [n]cumulenes is delineated schematically
in Figure 4.1, demonstrating the potential mesomeric or inductive contribution from the
endgroups. For even-numbered cumulenes, there are two π-systems that are degenerate and
spatially orthogonal (Figure 4.1a). In the case of aryl endcapping groups, each orthogonal π-
system can conjugate with substituents at one end of the cumulenic framework, but not both.
The situation is distinctly different for odd-numbered cumulenes (Figure 4.1b), where the two
π-systems of the sp-carbon framework are no longer degenerate. In this case, one π-system
spans the length of the cumulene skeleton and can conjugate with both sets of endgroups
(Figure 4.1b, in red). The other π-system (in blue) does not communicate directly with the
endgroups (i.e., via resonance) and is thus considerably shortened, although hyperconjugation
†† Portions of this chapter have been published: J. A. Januszewski, R. R. Tykwinski, Chem. Soc. Rev. 2014, 43, 3184–3203, see http://dx.doi.org/10.1039/C4CS00022F - Reproduced by permission of The Royal Society of Chemistry;
119
with terminal groups is easily envisioned. It is worth noting that the influence of odd- versus
even-numbered [n]cumulene structure should also be observed in the bond length alternation
(BLA) of cumulenes. As shown by the mesomeric structures in Figure 4.1b, increased BLA is
expected for odd [n]cumulenes and should be further enhanced by groups able to conjugate to
the cumulene core.
Figure 4.1 Electronic effects based on odd- and even-numbered, as well as alkyl- and aryl
endcapped [n]cumulenes, demonstrated schematically with canonical structures for [4]- and
[5]cumulenes.
Within this section, characterization of all representatives of the cumulene series of
[n]tBuPh including [9]cumulene rotaxanes 3.8–3.10 based on qualitative and quantitative
UV/vis spectroscopy has been performed. Initially, the UV/vis data of [n]tBuPh and [n]Mes
will be presented. Comparisons of both series to each other as well as to other cumulene
120
derivatives will be discussed. Finally, the spectroscopic results of [9]cumulene rotaxanes and
comparison to the “naked” [9]cumulene [9]tBuPh as well as to the series of [n]tBuPh will be
given. A discussion of the band gap of cumulenes and a following conclusion will complete
the UV/vis spectroscopic section.
4.1.2 UV/vis spectroscopy of [3]-, [5]-, [7]-, and [9]tBuPh
4.1.2.1 General observations
Figures 4.2a and 4.2b present the measured UV/vis spectra of [n]tBuPh (n = 3, 5, 7, 9)
and [n]Mes (n = 5, 7, 9), respectively. A distinct signal pattern is immediately observed,
showing two regions of absorption bands for both series of tetraaryl[n]cumulenes with the
most intense absorptions mainly found at higher energy in the UV region (<400 nm).
Noteworthy is the observation that the fine structure of the absorption in the UV region
becomes more distinct as a function of molecular length, i.e., when conjugation is extended.
The second set of absorption bands is found in the visible region from 400–700 nm. With
increasing conjugation in the cumulene chain, the lowest absorption λmax values become red-
shifted to lower energy values. In the following sections, the spectroscopic results of
[n]tBuPh and [n]Mes will be compared to two [n]cumulene series, that are known from
literature, [n]Ph and [n]Cy with n = 3, 5, 7, and 9 (Figures 4.2c and 4.2d, respectively).1
These two series offer the most complete analyses reported to date (aside from [n]tBuPh and
[n]Mes). The [n]Ph shows a very similar signal pattern compared to [n]tBuPh and [n]Mes.
The alkyl-substituted [n]Cy also possesses two regions of absorption bands, however, with
the absorption intensities being much more pronounced in the high energy region (<400 nm)
than in the visible region.
121
Figure 4.2 UV/vis spectra of [n]cumulenes: UV/vis spectra of a) [n]tBuPh and b) [n]Mes.
Both sets of spectra were measured in Et2O and normalized to the most intense low energy
absorption. Spectra of c) [n]Ph (in benzene) and d) [n]Cy (in Et2O). Spectra of [n]Ph and
[n]Cy were adapted with permission from reference 1. Copyright 1964 John Wiley & Sons.
4.1.2.2 Influence of cumulene chain length
The most obvious consequence of π-electron conjugation is observed in the lowest
energy absorption values, λmax, as a function of cumulene length chain, as demonstrated by
values of the two [n]cumulene series, [n]tBuPh and [n]Mes that are listed in Table 4.1. A
monotonic red-shift in λmax is clearly visible as cumulene length is increased within each
series, indicating a decreasing HOMO-LUMO energy gap. In the case of [n]tBuPh, the
smallest cumulene homologue, [3]tBuPh, shows a λmax value of 424 nm reaching a value of
664 nm for [9]tBuPh. Similar effects can be observed for [n]Mes with n increasing from 5 to
9 giving 460 and 666 nm, respectively.
As expected, the comparison of tetraaryl[n]cumulenes with tetraalkyl[n]cumulene [n]Cy
showed that λmax values of tetraaryl[n]cumulenes are found at much lower energy than those
122
of the tetraalkyl[n]cumulenes, e.g., 664 nm versus 465 nm for [9]tBuPh and [9]Cy,
respectively (Table 4.1).8 This is explained by the decreased conjugation in the case of the
alkyl endgroups in contrast to tetraaryl[n]cumulenes that possess aryl endgroups that are
contributing to the π-conjugation of the cumulene (Figure 4.1).
Table 4.1 Lowest energy absorption λmax (in nm) and energy values Eg (in eV)[a] of [n]tBuPh,
[n]Mes, [n]Ph, and [n]Cy (in Et2O).
[n]cumulene [3] [5] [7] [9] refs
[n]tBuPh 424 (2.92) 500 (2.48) 564 (2.20) 664 (1.87)
[n]Mes - 460 (2.70) 560 (2.21) 666 (1.86)
[n]Ph 420 (2.95)[b] 489 (2.54)[b] 557 (2.23)[b] 663 (1.87) 3–7
[n]Cy 272 (4.56) 339 (3.66) 401 (3.09) 465 (2.67) 8
[a] Determined via common conversion tools from http://halas.rice.edu/conversions (nm to
eV). [b] Measured in benzene.
4.1.2.3 Influence of endgroups
While λmax values of the tetraaryl[5]cumulenes [5]tBuPh and [5]Mes (500 and 460
nm, respectively), vary rather significantly (Table 4.1), the λmax values of the longer
analogues [7]tBuPh/[7]Mes and [9]tBuPh/[9]Mes are nearly identical, with values of about
560 nm and 665 nm, respectively. Thus, lower [n]cumulenes, e.g., [5]cumulenes have a
pronounced endgroup effect compared to longer [n]cumulenes. Furthermore, the experimental
data shows that the influence of the endgroups decreases rapidly as a function of cumulene
length. In comparison with [5]Ph, which is electronically more or less neutral (i.e., no
endgroup effects are present), the endgroups of [5]cumulenes [5]tBuPh and [5]Mes are
expected to provide positive inductive effects due to the alkyl substituents (t-butyl and
methyl, respectively). The λmax absorption of [5]tBuPh is red-shifted to 500 nm compared to
λmax = 489 nm for [5]Ph4,5 (Table 4.1), while the λmax value of 460 nm for [5]Mes is
decreased compared to [5]Ph (λmax = 489 nm). This correlates well with the diminished
conjugation between the mesityl endgroups and the cumulene chain displayed by the
increased aryl twist angle relative to [5]tBuPh. This effect will be explained below in the X-
ray crystallography section (see Section 4.2).
123
Obviously, the endgroups also affect the λmax values of [n]cumulenes, aside from the
influence of chain length (increase of conjugation) as mentioned above. Therefore, a
comparison of endgroup effects of several [5]cumulenes has been done, and the appropriate
λmax values are summarized below in Table 4.2. The biggest shift of λmax values to lower
energy is observed for cumulenes in which the pendent aryl rings are forced to be coplanar to
the cumulene framework, namely [5]An with λmax = 555 nm9 and [5]Fl with λmax = 540 nm5
(for the structure of the cumulenes, see Figure 1.6). Thus, planarity of the aryl endgroups
appeared to be a dominant factor.
p-Methoxyphenyl-substituted [5]cumulene [5]MeOPh shows red-shifted λmax values
of 517 nm10 as reported by Cadiot relative to [5]Ph (λmax = 488 nm).5 This feature also occurs
in the case of [5]tBuPh and is readily explained by mesomeric and inductive effects of the
OMe endgroups, which donate electron density to the electron deficient sp-hybridized carbon
framework yielding a donor-acceptor-donor (D-A-D) type conjugated system.
Finally, the greatest blue-shift of λmax is found for tetraalkyl[n]cumulenes, such as
[5]Cy and [5]tBu (λmax = 339 and 337 nm, respectively)8,11 which differ by about 150 nm
compared to [5]Ph (λmax = 488 nm). There is, obviously, no mesomeric contribution from the
alkyl endgroups to the conjugated structure of [5]Cy and [5]tBu, but hyperconjugation with
the four methyl units in the cyclohexyl group or t-butyl groups, respectively, appears to be
present based on the comparison to [5]Me with λmax = 320 nm.12
Table 4.2 UV/vis spectroscopic data (λmax in nm) of selected [5]cumulenes with different
endgroups.
[5]An [5]Fl [5]MeOPh [5]tBuPh [5]Ph [5]Mes [5]Cy [5]tBu [5]Me
λmax 555[a] 540[a]
517[a] 510[a] 500[b]
488[a] 460[b] 339[b] 337[b]
320[c]
refs 9 5 10 4,5 8 11 12
[a] Measured in CHCl3. [b] Measured in Et2O. [c] Measured in EtOH.
124
4.1.2.4 Conclusion including comparison of the band gap of cumulenes
UV/vis results that have been studied for polyynes suggest that at some length, the
lowering of the λmax values achieves saturation and reaches a maximum and constant value.
This limiting value would then represent an estimate of the energy gap (Eg) of the material
“polyyne” carbyne. The same should be true for cumulenes. Comparison of the optical band
gap of cumulenes, i.e., the energy values Eg,13 shows a decrease from ca. 2.95 eV to 1.86 eV
by increasing chain length of aryl-substituted cumulenes (Table 4.1). Thus, with values at ca.
1.86 eV for [9]cumulenes, saturation and hence a “carbyne-like” cumulene is still not reached.
Several methods have been used for polyynes to describe the relationship between Eg, λmax,
and n, which also might be applied to cumulenes, for example an empirical power-law14,15 (Eg
= 1/λmax ~ n–x) or the exponential function proposed by Meier and coworkers.16
Unfortunately, attempts to apply these protocols to the cumulenes reported in Table 4.1
provide inconclusive estimates for Eg, and longer cumulenes (n > 9) are needed to complete
this analysis. Thus, no experimental estimate for the cumulenic form of carbyne is currently
available. At this point, a “semiconducting level” is present rather than a “metallic level” as
predicted by theoretical calculations for (infinite) long cumulenes.
In conclusion, evidence of saturation still needs to be established to make a prediction
of the HOMO-LUMO gap of carbyne. As mentioned before in Section 4.1.1, this estimation
was not possible experimentally due to the limited number of model compounds.
4.1.3 UV/vis spectroscopy of [9]cumulene rotaxanes and comparison to [9]tBuPh
The UV/vis spectra of [9]cumulene rotaxanes 3.8–3.10 show the same basic
features as the spectrum of the “naked” [9]cumulene [9]tBuPh including two absorption
regions, a similar fine structure pattern, as well as almost identical λmax values (Figure 4.3 and
Table 4.3). In detail, the λmax values of all three rotaxanes 3.9, 3.10, and 3.8 are slightly red-
shifted with values of 665, 668, and 670 nm, respectively, compared to λmax of 664 nm for
[9]tBuPh. Cumulene rotaxane 3.9 possesses the biggest macrocycle surrounding the
cumulene chain and the closest value of λmax compared to [9]tBuPh. The remaining rotaxanes
3.10 and 3.8 have smaller macrocycles that probably interact slightly with the cumulene
framework, resulting in red-shifted values. While the macrocycle of rotaxane 3.10 possesses
alkyl linkers, that of 3.8 contains an aryl ether that could also contribute to even greater
125
interactions with the cumulene chain, and thus resulting in a higher λmax value. The absorption
values in the high-energy region are similar for all three rotaxanes (Figure 4.3 and Table 4.3).
Compared to the “naked" [9]cumulene [9]tBuPh, the signals of the rotaxanes are slightly red-
shifted about ca. 3 nm.
Figure 4.3 Qualitative UV/vis spectra (in Et2O) of the [9]cumulene rotaxanes 3.8, 3.9, and
3.10 as well as the “naked” [9]cumulene [9]tBuPh (a quantitative spectrum was recorded for
3.9, see right axis).
Due to the increased stability of the cumulene rotaxanes, molar absorptivities of a
[9]cumulene have been measured and are discussed for the first time. A quantitative UV/vis
spectrum of cumulene rotaxane 3.9 measured in Et2O shows intense absorptions at both high
and low energy, e.g., 368 nm (ε = 159,000 M–1cm–1) and 615 nm (ε = 75,000 M–1cm–1). The
ε values at high energy resemble those of polyynes, which are well-known to show very
intense absorptions, such as, for example, precursor to rotaxane 3.9, tetrayne 3.6 (245 nm,
ε = 125,000 M–1cm–1, hexanes),17 supertrityl-substituted tetrayne (268 nm, ε = 149,000 M–
1cm–1, hexanes),18 and t-butyl-substituted tetrayne (λmax = 240 nm, ε = 277,000 M–1cm–1,
hexanes).19
126
Table 4.3 UV/vis spectroscopic data (absorption wavelengths in nm) of [9]tBuPh and
[9]cumulene rotaxanes 3.9, 3.10, and 3.8 (in Et2O).
compound [9]tBuPh 3.9[a] 3.10 3.8
Absorption
bands
316
338
366
528
574
610
664
321 (319)
341
368
531 (530)
577 (576)
615
665 (666)
320
341
369
532
578
616
668
319
341
369
530
578
610
670
[a] Absorption bands for qualitative and quantitative UV/vis spectra. If qualitative and
quantitative data differ, the quantitative values are given in parentheses.
The comparison of molar absorptivity ε values of [n]tBuPh (n = 3, 5, 7) measured in
CHCl3 and [9]cumulene rotaxane 3.9 (measured in Et2O) shows that ε (mainly in the case of
absorptions of the fine structure in the high energy region) increases as the chain length of the
cumulene increases (Figure 4.4). This feature mirrors the effect already known for
polyynes2,18 as well as several reported cumulene series, such as [n]Ph and [n]Cy (Figure
4.2c and Figure 4.2d, respectively),1 as well as the [n]Fc series with n = 1, 3, and 5.20
127
Figure 4.4 Quantitative UV/vis spectra of [n]tBuPh (n = 3, 5, 9, in CHCl3) and [9]cumulene
rotaxane 3.9 (in Et2O).
4.2 X-ray crystallography of [n]cumulenes and discussion of bond length
alternation (BLA)
4.2.1 Introduction
X-ray crystallographic analysis is relatively uncommon for [n]cumulenes (n ≥ 5) due
to the instability of longer cumulenes under ambient conditions and limited synthetic
accessibility. X-ray crystallography, however, offers profound insight into both the physical
and electronic structure of cumulenes, especially via the analysis of bond length alternation as
a function of molecular length (BLA, defined as the bond length difference between the two
central-most double bonds of the cumulene chain). As discussed above, UV/vis spectroscopy
nicely shows that there is a relationship between the optical HOMO-LUMO gap and the
length of the cumulene chain. In principle, this change in HOMO-LUMO gap should coincide
with structural changes of the cumulene framework. More specifically, BLA should diminish
for longer cumulenes and eventually reach a constant value. Several theoretical calculations
128
for cumulenes suggest that BLA cumulenes should approach a value of nearly zero,21–23
although at the time of these reports experimental validation of these predictions does not
exist.
4.2.2 General observations
Regarding reports of X-ray crystallography of cumulenes to date, analysis of
[3]cumulenes is common (>15 structures), while data for [5]cumulenes is rare (three
structures), and no data exists for longer cumulenes to the best of my knowledge.24
In the following section, new X-ray crystallographic structures, [3]tBuPh, [5]tBuPh,
[7]tBuPh, [5]Mes, [7]Mes, and [9]Mes are presented.25 All single crystals have been obtained
in this research group26 with [7]tBuPh, [7]Mes, and [9]Mes representing the longest
[n]cumulenes yet known to be analyzed via X-ray crystallography. These series of molecules
enables a detailed discussion of results and structural trends and of the BLA analysis that is
required to answer a key question concerning cumulenes as possible model compounds for
carbyne: Do cumulenes show experimental evidence of reduced BLA as a function of
increasing length and can thus reach a “carbyne-like” status? Initially, general trends
regarding bond angles and bond lengths will be presented. The effects of twist angles (also
named as torsional angles) that are derived from aryl endgroups and the cumulene core are
discussed. Finally, a discussion of BLA of the cumulenes will complete this section including
a comparison to cumulenes known from literature. More specifically, only series of
cumulenes with at least three reported analogous X-ray structures are considered for
comparison, i.e., the two cumulene series, [n]Ph27–29 and [n]Cy27,30 with n = 3, 4, 5. As well,
X-ray crystallographic analysis is included for even-numbered cumulenes in addition to the
odd-numbered cumulenes that are the main focus in this thesis. For a better understanding of
following discussions, Figure 4.5 shows a [9]cumulene including the carbon atom labeling of
the cumulene chain as well as the origin of the twist angle calculations via intersection of
planes formed from the aryl rings (C11–C16, grey-colored plane) and the cumulene skeleton
(constructed from C11, C21, and C1–C5, as well as the appropriate symmetric atoms, blue-
colored plane).
129
Figure 4.5 Description of bond lengths and twist angles using [9]Mes (ORTEP
drawings with 20% probability level): a) structure of [9]Mes including carbon labeling, b)
planes defining the twist angle (front view), and c) planes defining the twist angle (side view).
Carbons C11–C16 define the grey-colored plane, while carbons C11, C21, and C1–C5, as
well as the appropriate symmetric atoms define the blue-colored plane.
130
4.2.3 Bond angles
Figure 4.6 shows the bond angles of the cumulenic chain of [3]tBuPh, [5]tBuPh,
[7]tBuPh, [5]Mes, [7]Mes, and [9]Mes. While X-ray crystallographic structures of polyynes
regularly show a bending (“bow” or “S” shape) of the sp-carbon framework in the solid
state,31,32 cumulenes appear to maintain a more linear structure. An examination of the bond
angles of the two series [n]tBuPh and [n]Mes shows that they rarely vary by more than a few
degrees from the ideal of 180°. This feature fits to other known cumulenes reported in
literature.33
Figure 4.6 Bond angles of the cumulene chain in [n]tBuPh and [n]Mes with n = 3, 5, 7 and
n = 5, 7, 9, respectively.
The bond angles of the cumulene chain in [n]tBuPh and [n]Mes show values between
177.1° and 179.8° (Figure 4.6). An exception to the almost linear cumulene chains described
so far is the structure of [3]tBuPh with cumulenic angles of 169.4° (C1–C2–C3) and 168.4°
(C2–C3–C4). In addition to the likely influence of undefined crystal-packing forces, the bent
shape of the [3]tBuPh might arise from favorable, intramolecular C–H/π-interactions between
hydrogen atoms of a t-butyl unit of the aryl group with the aromatic ring at the opposite
terminus of the cumulene chain (Figure 4.7).25 This premise is supported by short C–H/π
distances of 2.81–3.27 Å,34 and these interactions are reminiscent of a recent study by
Grimme and Schreiner highlighting dispersive forces in hexaphenylethane derivatives.25,35
131
Figure 4.7 Illustration of possible intramolecular C–H/π-interactions of [3]tBuPh.
In conclusion, the bond angle values of [n]tBuPh and [n]Mes show no indication for
trends, such as increase or decrease of linearity by increasing the chain length. In contrast,
comparison of the bond angles in the series of [n]Ph27–29 and [n]Cy27,30 shows that by
increasing the chain length, the bond angles resemble more and more 180° with values
reaching almost 179° for both [5]cumulenes, [5]Ph and [5]Cy starting with values of ca. 176°
for [3]Ph and [3]Cy. Comparison of the bond angles regarding the difference of substitution
(aryl versus alkyl) in [n]Ph and [n]Cy shows no significant change. Consequently, the
substitution pattern does not appear to influence the linearity of [n]cumulenes.
4.2.4 Bond lengths
Figure 4.8 shows the crystallographic structures of [3]tBuPh, [5]tBuPh, [7]tBuPh,
[5]Mes, [7]Mes, and [9]Mes including the labeling of carbon atoms of the cumulene chain.
Appropriate data for further discussion of the bond lengths of the cumulene chain can be
gathered from Figure 4.9 and Table 4.5.
The terminal double bonds, C1-C2 (α-bonds) of the cumulene chain are always the
longest at 1.33–1.35 Å. The second outermost double bonds, C2-C3 (β-bonds) of cumulenes
are the shortest with values of 1.25–1.26 Å resembling almost acetylenic bonds (Figure 4.9).
Herein, one exception needs to be mentioned, i.e., in the case of the [7]cumulenes, the central
γ-bonds with 1.252 and 1.257 Å are slightly shorter than the β-bonds lengths of 1.254 and
132
1.260 Å for [7]tBuPh and [7]Mes, respectively. Finally, the central double bonds (γ-bonds)
are intermediate to those of α- and β-double bonds (Figure 4.9). Hence, a significant
alternation in bond length is maintaining in the cumulene chain and will be discussed in
Section 4.2.6.
Figure 4.8 X-ray crystallographic structures of [3]tBuPh, [5]tBuPh, [7]tBuPh, [5]Mes,
[7]Mes, and [9]Mes (ORTEP drawings with 20% probability level).
A comparison between even-numbered and odd-numbered [n]cumulenes shows that
the α-bonds in even-numbered cumulenes are much shorter than in the odd-numbered analogs
with a decrease of almost 0.02 Å and 0.015 Å for [4]Ph and [4]Cy, respectively, compared to
the α bonds of [n]Ph and [n]Cy with n = 3, 5 (Figure 4.9). This feature can be explained by
the mesomeric structures of odd-numbered [n]cumulenes that differ to the even-numbered
[n]cumulenes as already described in Section 4.1 (Figure 4.1). The α-bonds in even-numbered
cumulenes show more double bond character in the canonical structures than in odd-
numbered cumulenes that contain more single bond character. While even-numbered
cumulenes show shorter α-bonds than odd-numbered cumulenes, the β-double bonds show
the opposite trend, namely they are longer in even-numbered- than in odd-numbered
[n]cumulenes, however, in lower extent. The difference between β-bond lengths and α-bond
lengths is lower for alkyl-substituted [n]cumulenes, with ca. 0.03–0.07 Å for [n]Cy compared
to the difference in bond lengths of aryl-substituted [n]cumulenes giving values of 0.06–0.10
Å for [n]tBuPh, [n]Mes, and [n]Ph.
133
Interestingly, the comparison of [5]cumulenes [5]tBuPh, [5]Mes, [5]Ph, and [5]Cy
shows that the bond lengths of [5]tBuPh resemble those of [5]Ph, while the bond lengths of
[5]Mes are similar to those of the alkyl-substituted [5]Cy (Figure 4.9). Consequently, it
seemed that the structures of mesityl-substituted cumulenes more closely resemble those of
alkyl-substituted cumulenes than aryl-substituted cumulenes (when comparing to [n]Ph). This
fact might be explained by the significant torsional angle of the endgroups of [n]Mes, which
prevents communication of these endgroups with the cumulene chain as it is the case for
alkyl-substituted cumulenes.
Figure 4.9 Bond lengths (in Å) of the cumulene chain of the series [n]tBuPh, [n]Mes,
[n]Ph,27–29 and [n]Cy.27,30
4.2.5 Torsional angles
For a cumulene chain to conjugate efficiently with the endgroups, these endgroups
should be coplanar to the π-orbitals of the chain (red π-system in Figure 4.1b). In both
cumulene series [n]tBuPh and [n]Mes, however, a twisting of the endgroups to the chain is
134
observed (Figure 4.5, Figure 4.8, and Table 4.4). [n]tBuPh cumulenes show two pairs of
twisted endgroups having different torsional angles and being centrosymmetric to each other.
One pair of endgroups is more coplanar to the red π-system of the cumulene chain (see Figure
4.1b) with torsional angles between 14° and 21° (Table 4.4, aryl rings A and C).36 The other
pair, however, is twisted to a much higher extent with values of 44° to 55° (Table 4.4, aryl
rings B and D). In contrast, in the [n]Mes series, all four endgroups are twisted significantly
out of the plane of the cumulene framework, in a range of 45° to 52° (Table 4.4). In
conclusion, the resulting steric congestion prevents coplanarity between the endgroups and
the cumulene chain for both cumulenes, however, in a lower extent for the [n]tBuPh series
since one pair of endgroups is in a position to enable conjugation with the sp-hybridized
carbon chain. Nevertheless, this feature affects the BLA of cumulenes that will be covered in
the following section.
Table 4.4. Aryl twist angles of aromatic ring relative to cumulenic framework.[a]
cumulene ring[b] [3]tBuPh [5]tBuPh [7]tBuPh [5]Mes [7]Mes [9]Mes
aryl twist angle (°)
A 30.9 14.4 16.6 46.2 45.4 48.8
B 43.1 47.9 54.9 51.4 52.1 54.4
C 26.6 20.6
D 40.4 43.6
[a] Aryl twist angles have been calculated as the difference between planes generated from (i)
the six carbons of the aryl ring and (ii) the carbons of the cumulene skeleton, along with the
four ipso-carbons of the aryl rings (see Figure 4.5). [b] See Figure 4.8 for labeling of aryl
rings.
4.2.6 Bond length alternation
The values for bond lengths in a cumulene chain confirm definitely BLA in the
cumulene chain. Table 4.5 summarizes all relevant bond lengths and the appropriate BLA
values of the cumulene chain for four series of cumulenes, [n]Ph,27–29 [n]Cy,27,30 [n]tBuPh,
and [n]Mes, as well as selected [n]H21–23 cumulenes from theoretical studies. The BLA values
for all cumulenes decrease as a function of length, e.g., from 0.086 Å ([3]tBuPh) to 0.052 Å
([7]tBuPh, Table 4.5, entries 7–9) or from 0.048 Å ([5]Mes) to 0.038 Å ([9]Mes, Table 4.5,
entries 10–12). An interesting fact is observed when BLA values have been compared
135
between even- and odd-numbered [n]cumulenes. More specifically, BLA values for the [4]-
and [5]cumulenes in both series, [n]Ph and [n]Cy, are approximately identical and vary only
by 0.001–0.002 Å (Table 4.5, entries 2/3 and 5/6, respectively).
BLA values also reflect endgroup effects, as shown for [5]cumulenes [5]Cy, [5]Mes,
[5]tBuPh, and [5]Ph with 0.028/0.040, 0.048, 0.054, and 0.058 Å, respectively (Table 4.5,
entries 6, 10, 8, and 3, respectively). In contrast to aryl-substituted cumulenes, alkyl-
substituted cumulenes, e.g., [n]Cy, show the lowest BLA values so far known for cumulenes.
This might be explained by the absence of a significant endgroup influence on the cumulene
chain via conjugation (see Figure 4.1). The aryl-substituted cumulenes show similar BLA
values although the [n]Mes series shows slight deviations, probably due to the limited
conjugation between the endgroups and the sp-hybridized cumulene chain as a result of the
twisted mesityl endgroups.
Theoretical studies of BLA in long cumulenes that have been reported in literature
suggest that BLA values converge quite rapidly to a value of nearly zero (Table 4.5).21–23
Castiglioni and coworkers, for example, have calculated the BLA for a series of [n]H,
showing already very low BLA values with 0.0136 and 0.010 Å for [7]H and [9]H,
respectively (Table 4.5, entries 14 and 16). Calculations for even longer [n]H cumulenes with
n = 29 result in a BLA value of 0.004 Å (Table 4.5, entry 19). In addition, Shakibazadeh and
coworkers have reported that BLA values for [7]H and [9]H are almost the same, 0.014 and
0.010 Å, respectively (Table 4.5, entries 13 and 15). Furthermore, Görling and coworkers
have calculated BLA values of 0.009 Å and 0.006 Å for [11]H and [n]H (n = 19–39),
respectively, (Table 4.5, entries 17 and 18) approaching BLA = 0, i.e., the central double
bonds in the cumulene chain would be equal length.
136
Table 4.5 Selected bond lengths (Å) for cumulene series [n]Ph, [n]Cy, [n]tBuPh and
[n]Mes, as well as theoretically calculated values for [n]H including BLA data.[a]
Entry Cumulene C1−C2 C2−C3 C3−C4 C4−C5 C5−C5’ BLA[b] Ref
1 [3]Ph 1.344(3)[c]
1.346(2)[d]
1.246(3)[c]
1.260(2)[d]
1.345(3)[c]
1.349(2)[d] – –
0.099
0.088 28
2 [4]Ph[e] 1.327 1.270 1.271 1.326 – 0.056 27
3 [5]Ph 1.3453(17) 1.2503(18) 1.3091(19) 1.2515(18) 1.3456(17) 0.058 29
4 [3]Cy[e] 1.328[g]
1.332
1.256[g,h]
1.261[h] – – –
0.072
0.071 27,30
5 [4]Cy[e] 1.317 1.273 1.279 1.313 – 0.039 27
6 [5]Cy[e] 1.329[i]
1.332[d]
1.260[i]
1.267[d]
1.300[f,i]
1.295[d,f]
–
–
0.040
0.028 30
7 [3]tBuPh 1.334(3)
1.336(3)[j]
1.249(3)
– – 0.086 25
8 [5]tBuPh 1.342(2) 1.255(2) 1.309(3)[f] - – 0.054 25
9 [7]tBuPh 1.345(3)
1.347(3)[k]
1.254(3)
1.252(3)[l]
1.302(3)
1.306(3)[m] 1.252(3) - 0.052 25
10 [5]Mes 1.339(2) 1.255(2) 1.303(3)[f] – – 0.048 25
11 [7]Mes 1.334(3) 1.260(3) 1.299(3) 1.257(4)[n] – 0.042 25
12 [9]Mes 1.330(3) 1.255(3) 1.298(3) 1.260(4) 1.298(5) 0.038 25
13[o] [7]H 1.319 1.274 1.289 1.275 - 0.014 22
14[p] [7]H 1.310342 1.266802 1.281530 1.267928 - 0.0136 21
15[o] [9]H 1.319 1.274 1.289 1.277 1.287 0.010 22
16[p] [9]H 1.310424 1.267176 1.281012 1.268959 1.279270 0.010 21
17[q] [11]H - - - - - 0.009[r] 23
18[q] [19]H–[39]H - - - - - 0.006[s] 23
19[p] [29]H - - - - - 0.004[t] 21
[a] See Figure 4.5 for atomic numbering scheme. [b] Calculated as difference in bond length
between the two central-most bonds. For non-centrosymmetric structures, BLA has been
calculated using the average of positionally equivalent bonds. [c] Structure determination at
20 °C. [d] Structure determination at –160 °C. [e] ESDs not reported in refs[27,30]. [f]
C3−C3’. [g] Averaged in case of multiple determination (see ref[27]). [h] C2−C2’. [i]
Structure determination at 22 °C. [j] C3−C4. [k] C7−C8. [l] C6−C7. [m] C5−C6. [n] C4−C4’.
[o] Geometry optimization at B3LYP/6-31G* level. [p] Geometry optimization at
PBE1PBE/cc-pVTZ level. [q] Geometry optimization at B3LYP/TZVPP level. [r] Bond
length values of 1.280 Å and 1.271 Å. [s] Bond length values of 1.278 Å and 1.272 Å. [t]
Estimated (bond lengths are not given for this value in ref[21]).
137
A plot of cumulene BLA values versus the number of double bonds n is depicted in
Figure 4.10 for [n]Cy, [n]Ph, [n]Mes, and [n]tBuPh. The graph suggests a limiting BLA
value of 0.03–0.05 Å for [n]Mes and [n]tBuPh (the inserted lines are only a guide for the
eye). For [n]Cy and [n]Ph, only two data points for each are available hindering extrapolation
to afford an suggested asymptotic limit. Noticeable, however, is that the BLA values of the
[n]Ph series with n = 3, 5 are slightly increased compared to [n]Mes and [n]tBuPh. These
predictions are, however, still relatively higher than that predicted by theory for the “parent”
series [n]H. While computational results can differ depending on the method of analysis, the
trend appears clear that BLA ≤ 0.01 Å by the length of [9]H. The difference between
experiment and theory likely arises from endgroup effects, although confirmation of this
hypothesis is still necessary.
Figure 4.10 BLA values (inset) versus chain length n (lines are only a guide for the eye).
138
4.3 Theoretical studies including comparison to UV/vis spectroscopy and BLA
analysis
In a collaborative project, theoretical studies have been performed for [9]cumulenes by
Görling and coworkers,37 dealing with BLA, twist angles, and electronic absorptions. In the
case of [9]tBuPh, a simplified structure, i.e., [9]MePh is used since [9]tBuPh is too large for
SCS-MP2 and CC2 calculations (Figure 4.11). The substitution change from t-Bu to Me,
however, is expected to have a negligible influence on the molecular structure and low-lying
excitation energies.
Figure 4.11 Optimized geometries of [9]tBuPh and [9]MePh cumulenes.
4.3.1 Influence of twist angles on BLA and electronic absorption energy
The influence of the twist angle between the aryl endgroups and the cumulene chain
on the BLA is depicted in Figure 4.12. The two lines describe two different calculation
methods, that have been performed, i.e., B3LYP (red) and SCS-MP2 (blue). The same trend is
predicted by both methods, namely a decrease of BLA by increasing the twist angle values
reaching a BLA between 0.01 and 0.02 Å for twist angles of 90°. In contrast, twist angle
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values of 10° give a much higher BLA with ca. 0.034 Å using B3LYP and ca. 0.062 Å using
SCS-MP2.38 These results confirm that conjugation between the aryl endgroups and the
cumulene chain does indeed influence the BLA values of cumulenes.
Figure 4.12 BLA calculation of [9]Ph versus the aryl twist angle.
HOMO–1, HOMO, LUMO, and LUMO+1 for [9]tBuPh and [9]Mes, with twist
angles fixed to 32° and 49°, respectively, calculated via the B3LYP method are outlined in
Figure 4.13. The HOMO and LUMO of both [9]cumulenes show that conjugation of aryl
endgroups with the cumulene chain is present. In contrast, HOMO and LUMO of [9]Ph with
a twist angle of exact 90° (Figure 4.14) show that, as expected, the endgroups do not
conjugate to the cumulene core.
140
Figure 4.13 Left: [9]tBuPh cumulene: B3LYP-MOs for equilibrium geometry (aryl twist
angle of 32°). Right: [9]Mes cumulene: B3LYP-MOs for SCS-MP2/def2-TZVPP equilibrium
geometry (aryl twist angle of 49°).
141
Figure 4.14 Hartree-Fock-MOs for [9]Ph with an aryl twist angle of 90°.
The influence of the twist angles also affects the lowest energy absorption λmax for
[n]cumulenes (Figure 4.15). A red-shift of λmax with values ranging from ca. 500 nm � 550
nm � 630 nm � 680 nm39 for twist angle values of 90°, 60°, 30°, and 10°, respectively, is
observed for one possible conformer of [9]Ph (with D2 symmetry).40 In contrast, the shift of
absorption values in the high energy region are not dramatically affected by the magnitude
twist angles. The intensities of the higher energy absorptions, however, seem to increase as a
function of the twist angle value, while the intensities of the λmax values decrease by
increasing twist angle.
142
Figure 4.15 UV/vis spectra of one possible conformer of [9]Ph (D2 symmetry)40 in
dependence of the phenyl twist angle calculated at CC2/def2-TZVPP//SCS-MP2/def2-TZVPP
level.
4.3.2 UV/vis spectroscopy – Theory and experiment
Experimentally obtained UV/vis spectra of [9]tBuPh and [9]Mes, and the
corresponding theoretical spectra (calculated at the CC2/def2-TZVPP//SCS-MP2/def2-
TZVPP level of theory) are depicted in Figure 4.16. Herein, [9]MePh is used instead of
[9]tBuPh for theoretical comparisons (see Figure 4.11). Both, the lower and the higher energy
region of the theoretical spectra fit well to the experimentally measured UV/vis spectra of the
[9]cumulenes. Furthermore, theory suggests that the high-energy absorptions between 300
and 400 nm are dominated by HOMO–1 to LUMO+1 transition of the cumulene chain (see
Figure 4.13). The absorption bands in the lower energy region correspond mainly to HOMO
to LUMO transitions, which are also influenced by the endgroups, as shown in Figure 4.13.
Much stronger coupling of the cumulene π-system to the aryl rings is observed for the
HOMO-LUMO transitions compared to the high energy region, and the resulting fine
structure in the experimental spectrum can be attributed to several cumulene chain coupled
143
vibrations such as, for example, C-C stretch vibrations of the aryl rings coupled to C-H
deformation and C-C stretch vibrations of the cumulene chain.
Figure 4.16 Calculated and experimental UV/vis spectra of [9]MePh/[9]tBuPh (top) and
[9]Mes (bottom) cumulenes. The twist angles are 31° for [9]MePh, as well as 49° for [9]Mes.
All theoretical UV/vis spectra have been computed at the CC2/def2-TZVPP//SCS-MP2/def2-
TZVPP level of theory.
144
4.4 Electrochemistry (cyclic voltammetry) including comparison of the
electronic band gap (Eele) to the optical band gap (Eopt)
4.4.1 Introduction
To date, very little is known about electrochemical analysis of carbyne. While
polyynes have been sporadically analyzed via cyclic voltammetry (CV), e.g., by Tykwinski,41
Gladysz,42–44 and Hirsch,45 relatively little has been reported for cumulenes. To date, several
reports on CV studies on lower [n]cumulenes (n ≤ 3) have been published, for example, by
Kemula and Kornacki46 as well as recently by Zhu and coworkers.47 Furthermore, CV data
have been briefly discussed for six higher [n]cumulenes, i.e., [5]cumulenes with ferrocenyl
([5]Fc),20,48 ferrocenyl/phenyl ([5]Fc/Ph),49 phenyl ([5]Ph),20,50 t-butyl/phenyl endcapping
groups ([5]tBu/Ph),51 [5]tBu,51 and [5]EtPh.51
4.4.2 Cyclic voltammetry of [3]tBuPh, [5]tBuPh, and [7]tBuPh
The synthetic accessibility and stability of the cumulene series [n]tBuPh with n = 3, 5,
and even 7 has facilitated electrochemical studies. The cyclic voltammetry experiments have
been carried out in CH2Cl2 as solvent with 0.1 M Bu4NPF6 as supporting electrolyte using a
standard three-electrode cell. A glassy carbon disc has been used as working electrode, and
ferrocene has been added to the samples serving as internal standard. Figures 4.17–4.20
present the cyclic voltammograms of [3]tBuPh, [5]tBuPh, and [7]tBuPh, respectively, and
the according data for oxidation and reduction potentials is summarized in Table 4.7. For all
three [n]tBuPh cumulenes (n = 3, 5, 7), two separate oxidation steps and at least one
reduction process are observed. For [3]tBuPh, the first oxidation process (E1/2 = 0.49 V) is
reversible52 while the second oxidation process (E1/2 = 0.95 V) shows a quasireversible level
(Figure 4.17). In the case of [5]tBuPh (Figure 4.18), oxidation processes are similar giving a
reversible first oxidation step (E1/2 = 0.43 V) and a quasireversible second oxidation step (E1/2
= 0.80 V). For [7]tBuPh, different results occur, i.e., the first oxidation process (E1/2 = 0.42 V)
is quasireversible (Figure 4.19), while the second oxidation process (E1/2 = 0.68 V) appears to
be reversible.
145
Figure 4.17 Cyclic voltammogram of [3]tBuPh. 0.1 M electrolyte (Bu4NPF6) in CH2Cl2.
Ferrocene (Fc) has been used as the internal standard. Scan rate: 150 mV/s.
Figure 4.18 Cyclic voltammogram of [5]tBuPh. 0.1 M electrolyte (Bu4NPF6) in CH2Cl2.
Ferrocene (Fc) has been used as the internal standard. Scan rate: 150 mV/s.
146
Figure 4.19 Cyclic voltammogram of [7]tBuPh. 0.1 M electrolyte (Bu4NPF6) in CH2Cl2.
Ferrocene (Fc) has been used as the internal standard. Scan rate: 150 mV/s.
A single, quasireversible reduction step (E1/2 = –2.18 V) is found for [3]tBuPh (Figure
4.17). In contrast, [5]tBuPh shows two reduction processes, the first one (E1/2 = –1.72 V) is
reversible and the second reduction step (E1/2 = –2.19 V) is quasireversible step (Figure 4.18).
For [7]tBuPh, two reversible reduction processes (E1/2 = –1.37 and –1.72 V) are observed
(Figure 4.19). For the reduction processes of [n]tBuPh, a trend is observed showing an
increase of reduction steps accompanied by an increase of reversibility with increasing chain
length.
4.4.3 Comparison to known cumulene systems
Comparisons of the electrochemical results to known cumulene systems are limited
due to few and poorly investigated CV studies of cumulenes reported to date. Kemula and
Kornacki, for example have studied the reduction of the [3]Ph cumulene and present CV
voltammograms of the resulting reduction processes at –1.30, –1.63, –2.22, and –2.46 (in
DMF, referred to S.C.E.).46 Furthermore, Hoijtink and van der Meij have also reported about
147
the reduction of [3]Ph as well as [5]Ph, which show two reductions – at 1.20 and 1.03 V, as
well as, 1.55 and 1.30 V, respectively (in 1,2-dimethoxyethane, relative to the reduction
potential of biphenyl).50,53 In both cases, no oxidation processes are discussed. Bildstein and
coworkers have reported the synthesis and characterization of ferrocenyl-substituted
cumulenes [n]Fc also including CV studies.20,49 The redox properties of these cumulenes
depend on the length of the cumulenic chain as well as on the number of double bonds (odd-
versus even-numbered cumulenes). Thus, strong electronic communication between the
electron-donating ferrocenyl groups through the bridging cumulene chain offers interesting
CV characteristics including both oxidation and reduction processes.20,49 The group of
Bildstein20 has performed cyclic voltammetry of [5]Ph that serves as a reference to the
ferrocenyl endcapped cumulenes (Figure 4.20). Herein, two reduction steps, one reversible
two-electron (E1/2 = –1.11 V) and one quasireversible two-electron (E1/2 = –1.43 V) process,
are observed. Furthermore, a partially reversible two-electron oxidation (E1/2 = 1.02 V) is
found, followed by a poorly resolved second oxidation step (Figure 4.20). The cyclic
voltammograms of [5]Ph and [5]tBuPh resemble each other (Figure 4.20 and Figure 4.18,
respectively), both showing two oxidation and reduction events, respectively. The calculated
energy values of the HOMO-LUMO gap of both [5]cumulenes are almost identic with 2.13
eV and 2.15 eV for [5]Ph and [5]tBuPh, respectively. No further comparisons, however, have
been done, since both CV measurements have been recorded under different conditions, such
as concentration, scan rate, temperature, etc.
Figure 4.20 Cyclic voltammogram of 0.001 M [5]Ph. 0.2 M electrolyte (Bu4NPF6) in
CH2Cl2, referenced to SCE. Scan rate 200 mV/s. The graphic is adapted from ref[20].
148
Suzuki and coworkers have reported CV studies on [5]tBu/Ph and compared these
results to the two [5]cumulenes [5]tBu and [5]EtPh (Table 4.6).51 The voltammograms of
[5]tBu show no reduction (in the region reaching –2.2 V) but an irreversible oxidation
process. Replacement of two t-butyl groups by two phenyl groups yielding [5]tBu/Ph leads to
a reversible reduction step as well as two reversible oxidation steps. [5]EtPh containing only
aryl endgroups, shows two reduction events with the first one being reversible and the second
irreversible. Two oxidation steps are observed, with the first one being irreversible and second
reversible. The redox potentials of [5]tBu/Ph are positioned between those of the [5]tBu and
the aryl-substituted [5]EtPh or [5]tBuPh, which is logical due to the respective substituent
effects. A comparison of the [5]cumulenes [5]tBu, [5]tBu/Ph, [5]EtPh, and [5]tBuPh shows
an increased number of reduction events with increasing number of aryl rings as endgroups.
In addition, a facilitated reduction is observed. The increase of alkyl substituents on the aryl
rings results in a more difficult first reduction comparing [5]EtPh and [5]tBuPh (see Table
4.6). A similar behavior is observed when comparing the first oxidation processes of [5]tBu,
[5]tBu/Ph, [5]EtPh, and [5]tBuPh. By increasing the number of aryl endgroups, the oxidation
processes become easier. The alkyl groups on the aryl rings seem to influence the oxidation
events in an opposite way compared to the reduction events. The increase of alkyl substituents
on the aryl rings results in an easier first oxidation resulting in [5]tBuPh as the [5]cumulene
with the lowest oxidation potential (Table 4.6).
149
Table 4.6 CV data of [5]cumulenes [5]tBu, [5]tBu/Ph, [5]EtPh,[a] and [5]tBuPh.[b]
reduction potential (V) oxidation potential (V)
[5]tBu51 n.d. (< –2.2) +0.84irr
[5]tBu/Ph51 –1.93 +0.78, +1.00
[5]EtPh51 –1.92irr, –1.58 +0.51irr, +0.72
[5]tBuPh –2.19qurev, –1.72 +0.43, +0.80 [a] Conditions: CH2Cl2, –20 °C, 0.1 M Et4NPF6, 0.001 M [5]cumulene, scan rate = 100 mV/s,
irr = irreversible (referenced to ferrocene). [b] Conditions: CH2Cl2, rt, 0.1 M Bu4NPF6,
0.002 M [5]cumulene scan rate = 150 mV/s, qurev = quasireversible (referenced to ferrocene).
4.4.4 Cyclic voltammetry of a [9]cumulene rotaxane
A cyclic voltammogram has been recorded for the [9]cumulene rotaxane 3.9 (Figure
4.21). Herein, only an irreversible oxidation (Epeak = 0.43 V) is observed aside from two
reversible reduction steps (E1/2 = –1.20 and –1.56 V). The lack of a reversible oxidation
process for [9]cumulene rotaxane 3.9 might be explained by the presence of the macrocycle,
which on the one hand could primary be oxidized compared to the cumulene framework. On
the other hand, the reason for the irreversibility could rely on a reaction between the
macrocycle with the resulting cation of the cumulene. Due to insufficient amount of 3.9, no
repeated or concentration-varied measurement has been made. Regarding the reduction
behavior, the CV studies of 3.9 show that the [9]cumulene rotaxane fits well to the series of
[3]tBuPh, [5]tBuPh, and [7]tBuPh cumulenes giving the longest cumulene with the lowest
reduction potential, i.e., the reduction of [n]cumulenes becomes facilitated as a function of
cumulene chain length.
150
Figure 4.21 Cyclic voltammogram of rotaxane 3.9. 0.1 M electrolyte (Bu4NPF6) in CH2Cl2.
Ferrocene (Fc) has been used as the internal standard. Scan rate: 150 mV/s.
4.4.5 Electronic and optical band gap of [n]tBuPh
Table 4.7 presents CV data including absorption energies and absorption cutoff (taken
from the UV/vis spectra), the optical (Eopt) and electronic (Eele) band gap values in eV, as well
as the potentials for oxidation and reduction processes of [3]tBuPh, [5]tBuPh, [7]tBuPh, and
[9]cumulene rotaxane 3.9. The comparison of the first oxidation steps reveals that there is a
slightly facilitated first oxidation process with increasing chain length of the cumulenes with
potentials of 0.49, 0.43, and 0.42 V for [3]tBuPh, [5]tBuPh, and [7]tBuPh, respectively. The
second oxidation event, however, appears significantly facilitated when the chain is
lengthened (0.95, 0.80, and 0.68 V for [3]tBuPh, [5]tBuPh, and [7]tBuPh, respectively). The
first reduction process for all four cumulene compounds shows that the cumulene can be
reduced easier when the cumulene chain is elongated, i.e., –2.18, –1.72, –1.37, and –1.20 V
for [3]tBuPh, [5]tBuPh, [7]tBuPh, and [9]cumulene rotaxane 3.9, respectively. This feature
is also common for reported polyyne structures.44,45 Furthermore, the second reduction
151
process for [5]tBuPh, [7]tBuPh, and [9]cumulene rotaxane 3.9 shows the same trend, i.e.,
facilitated reduction with increasing chain length.
Comparison of the optical band gap values (Eopt) of [3]tBuPh, [5]tBuPh, and
[7]tBuPh determined from the UV/vis spectra and the electronic band gap values (Eele) from
CV measurements shows a high accordance (Table 4.7). The biggest difference is observed
for [7]tBuPh with 1.97 eV (Eopt) versus 1.79 eV (Eele).
Table 4.7 Selected UV/vis spectroscopic and electrochemical details including optical (Eopt)
and electronic (Eele) band gap values for [3]tBuPh, [5]tBuPh, [7]tBuPh, and 3.9.
compound [3]tBuPh [5]tBuPh [7]tBuPh 3.9
λmax abs. energies (nm),
(ε in M–1cm–1) 426 (37,900) 510 (66,700) 573 (65,200) 666 (38,600)
λmax abs. cutoff (nm),
(Eopt in eV)[a] 470 (2.64) 550 (2.25) 628 (1.97) 711 (1.74)
Eele (eV)[b] 2.67 2.15 1.79 1.63[c]
E1/2 ox2 0.95 0.80 0.68 -
Epeak ox1 - - - 0.43
E1/2 ox1 0.49 0.43 0.42 -
E1/2 red1 –2.18 –1.72 –1.37 –1.20
E1/2 red2 - - –1.72 –1.56
Epeak red2 - –2.19 - -
[a] As estimated from the intercept of a tangent line to the lowest energy absorption of the
UV/vis spectrum with the x-axis. [b] As estimated from cyclic voltammetry, Eox1 – Ered1. [c]
estimated value using the potentials of Epeak ox1 and E1/2 red1.
Figure 4.22 shows the plot of the electronic (Eele) and optical (Eopt) band gaps versus
1/n of the cumulene series [n]tBuPh with n = 3, 5, and 7 and 3.9. Regarding endgroup
influences in the shorter cumulene, [3]tBuPh, only the higher cumulenes with n = 5, 7 and
[9]cumulene rotaxane 3.9 have been considered for the extrapolation of data in Figure 4.22. In
both cases, a linear correlation between the HOMO-LUMO gaps and 1/n is observed. A linear
dependence is common in previous studies of conjugated oligomers, as described, for
example, by Hirsch,45 Chance,54 Diederich,55,56 and Bäuerle.57 The extrapolation of the data to
infinite cumulene chain length predicts a HOMO-LUMO gap of 0.96 eV and 1.13 eV for the
electronic (Eele) and optical (Eopt) band gaps, respectively, which gives λmax values of 1287
152
and 1097 nm, respectively, for an infinite cumulenic chain. In contrast to the applied
extrapolation method using the plot of E versus 1/n, the exponential function proposed by
Meier and coworkers16 represents a more significant extrapolation method giving a better
estimate for infinite cumulenes. In order to make this estimate, however, data for longer
cumulenes (n > 9) are necessary.
Figure 4.22 Plots of electronic (left) and optical (right) band gaps (Eele and Eopt, respectively)
versus 1/n for the cumulenes [3]tBuPh, [5]tBuPh, [7]tBuPh, and 3.9 (band gap data taken
from Table 4.7).
4.4.6 Comparison of electrochemical properties of cumulenes and polyynes
Regarding the electrochemical behavior of polyynes, no oxidation processes are
expected for aryl-substituted polyynes because of their electron deficient nature. For example,
Hirsch’s aryl-endcapped dendrameric polyynes show only reduction processes without any
hint for oxidation.45 The endcapping groups show that there is a strong influence or
interaction of the endgroups with the polyyne chain and thus its π-system. Tykwinski and
coworkers, hence, have suggested that by the use of alkyl-endcapping groups, CV
measurements of polyynes should give more suitable redox characteristics of the “pure”
polyyne chain due to lack of interactions between the endgroups and the chain framework.41
Thus, polyynes with adamantyl endgroups have been examined by CV including the tetrayne,
hexayne, and octayne.41 The results vary showing no redox event, oxidation and reduction
events, as well as only a reduction event for the tetrayne, hexayne, and octayne, respectively.
Thus, no concrete statements have yet been done concerning the influence of aryl- or alkyl
endgroups on the redox activity of polyynes.
153
Regarding the electrochemical behavior of cumulenes, however, both, oxidation and
reduction events occur contrary to polyynes. A preliminary trend can be drawn for the series
of [n]tBuPh with n = 3, 5, and 7, including [9]cumulene rotaxane 3.9, as well as for various
[5]cumulenes: With increasing chain length and increasing number of aryl endgroups attached
to the cumulene chain, reduction and oxidation processes become facilitated and more
distinct, and also the number of redox events increases.
4.5 NMR spectroscopy of [n]cumulenes
4.5.1 Introduction
With a rigid framework of sp-hybridized carbons that can be unstable for longer
derivatives, 13C NMR spectroscopy of both cumulenes and polyynes can be challenging due
to long T1 relaxation times, insolubility of the sample, and decomposition of the material
during acquisition. This is particularly true for the analysis of cumulenes. Whereas several
very recent studies provide more details for polyynes,32,58–61 no 13C NMR spectroscopic data
are reported for cumulenes beyond the length of a [5]cumulene, and even data describing
[5]cumulenes are scarce. Iyoda has described the characterization of several [5]cumulene
derivatives by 13C NMR spectroscopy, however, comparisons to longer or shorter derivatives
are not available.62,63
During the doctoral thesis research, a 13C NMR spectrum of one longer [n]cumulene
with n > 5, namely [7]tBuPh has been recorded for the first time.25 The next higher odd-
numbered cumulene, the [9]cumulene [9]tBuPh is already too unstable for NMR
characterization. Thus, the stabilization of this [9]cumulene by rotaxane formation to yield 3.8
and 3.9 offers the first opportunity to explore 13C NMR spectroscopy for longer derivatives.
Since the 1H NMR spectrum is not significant for the discussion of the chemical shift of the
sp-hybridized cumulene chain, only the 13C NMR spectra of [9]cumulene rotaxanes 3.8 and
3.9 will be discussed herein.
The second part of this section deals with 13C-1H correlation NMR spectroscopy, e.g.,
HMBC spectra of the series of [n]tBuPh including [9]cumulene rotaxanes. 13C-1H correlation
studies have been done to assign the shifts of several carbon atoms of the cumulenes including
also the shifts of some carbon atoms within the cumulene chain that have been identified via
this method.
154
4.5.2 13C NMR spectroscopy of [9]cumulene rotaxanes and their precursors
Figure 4.23 shows the carbon atom labeling of precursors 3.4 and 3.6 and the
[9]cumulene rotaxanes 3.8 and 3.9 that will be used for the NMR spectroscopic discussion.
Figure 4.24 shows 13C NMR spectra of [9]cumulene rotaxane 3.9 and precursor 3.6 with the
carbon atoms of the sp-hybridized carbon chain (i.e., C1–C5) and the appropriate chemical
shifts marked in green. The signals for the carbon atoms C1–C5 of the cumulene chain of
[9]cumulene rotaxane 3.9 (top, Figure 4.24) are shifted downfield compared to the carbon
atoms C1–C5 of the acetylene units of precursor 3.6 (bottom, Figure 4.24), from 83–63 ppm
for 3.6 (acetylenic region) to 145–120 ppm for 3.9 (cumulenic region). The downfield region
is also common for the cumulenic carbon atoms of lower [n]cumulenes with n ≤ 7.25 The
appearance of the cumulene and polyyne resonances in completely different regions seems
rather surprising, since both carbon chains consists of sp-hybridized carbon atoms.
Figure 4.23 Carbon atom labeling of precursors and [9]cumulene rotaxanes for NMR
spectroscopic discussion.
155
Figure 4.24 Comparison of 13C NMR spectra of [9]cumulene rotaxane 3.9 and precursor 3.6
(dotted lines highlight the assignment of signals in the spectrum of [9]cumulene rotaxane 3.9
that result from precursor 3.6, which is present as an impurity).
Comparison of [9]cumulene rotaxanes 3.9 and 3.8 shows that the chemical shift of
cumulenic carbon atoms C1–C5 reveals similarities as outlined in Figure 4.25. The cumulenic
carbon signals for both rotaxanes can be divided into three different regions. On the one hand,
one set of three carbon atoms can be assigned into the most upfield region of 125–119 ppm,
whereas, on the other hand, the fourth carbon atoms appear shifted downfield at 128.7 ppm
and 127.4 ppm for 3.9 and 3.8, respectively. The last remaining carbon atom signals are
notably deshielded to 144.8 ppm and 144.5 ppm for 3.9 and 3.8, respectively.
156
Figure 4.25 13C NMR spectra (165–100 ppm region) of [9]cumulene rotaxanes 3.9 (top) and
3.8 (bottom).
4.5.3 13C- and correlation NMR spectroscopy of [n]tBuPh (n = 3, 5, and 7) and
[9]cumulene rotaxanes
The following three Figures 4.26–4.28 present decoupled and coupled 13C NMR
spectra of [n]tBuPh (n = 3, 5, and 7) as well as the two-dimensional heteronuclear multiple
bond coherence (HMBC) spectra. The conventional decoupled spectra as well as the coupled
spectra have been performed to assign scalar couplings between H and C atoms in order to
assign the terminal cumulenic carbon atoms. The cumulenic carbon atoms are marked in red,
and the terminal carbon atom of the cumulene chain is labeled as C1. In all three figures, there
is only one carbon atom of the cumulene chain that shows a splitting pattern in the coupled
spectra defining a triplet, while all remaining carbon atoms describe singlets with the same
chemical shift as in the appropriate decoupled spectra. This feature leads to the assumption
that only 3JCH couplings are observed, while 4
J couplings are probably too small to be
detected for such systems. Using this same analysis and logic, carbon atoms C1 of all three
cumulenes, [3]tBuPh, [5]tBuPh, and [7]tBuPh can be assigned to 123.4, 125.6, and 127.5
157
ppm, respectively. The recorded HMBC spectra that are outlined in Figures 4.26–4.28
confirm the chemical shifts of C1 via a correlation to the aryl protons H2 via 3JCH couplings.
158
Figure 4.26 Decoupled and coupled 13C NMR spectra of [3]tBuPh including the
corresponding HMBC NMR spectrum (aryl region).
159
Figure 4.27 Decoupled and coupled 13C NMR spectra of [5]tBuPh including the
corresponding HMBC NMR spectrum (aryl region).
160
Figure 4.28 Decoupled and coupled 13C NMR spectra of [7]tBuPh including the
corresponding HMBC NMR spectrum (aryl region).
161
[9]Cumulene rotaxanes 3.9 and 3.8 have also been examined via HMBC experiments,
and the corresponding spectra are depicted in Figure 4.29 and Figure 4.30, respectively. With
the help of the HMBC spectra, the position of outermost carbon atoms C1 can be identified
and shows chemical shifts of 128.7 and 127.4 ppm for 3.9 and 3.8, respectively. These signal
positions have also been determined through 3JCH couplings between the protons H2 and
carbon C1 as mentioned before. To date, the C1 remains the only carbon atom of the
cumulenic chain that could be assigned using the HMBC experiment.
Figure 4.29 Expansion of the HMBC spectrum of [9]cumulene rotaxane 3.9 (inset: relevant
correlation signals between H2 and C1).
162
Figure 4.30 Expansion of the HMBC spectrum of [9]cumulene rotaxane 3.8 (inset: relevant
correlation signals between H2 and C1).
4.5.4 Discussion and comparison
In the case of polyynes, the chemical shifts of C1 and C2 (first and second sp-
hybridized carbon atoms of the chain, respectively) follow two interesting trends, which have
been reported on both, an experimental and theoretical basis.32,58–60 On the one hand, TIPS-
substituted polyynes, for example, show that the signals for C1 steadily shift downfield with
increasing chain length while the signals for C2 show the exact opposite behaviour, namely
shifting slightly upfield, indicating differently polarized carbon atoms.32 On the other hand,
the opposite trend has been observed in phenyl-58 and t-butyl-,61 as well as donor- and
acceptor-60substituted polyynes, where the signals for C1 steadily shift upfield with increasing
chain length while the signals for C2 remain almost the same shift without significant trend.
In the case of TIPS- and phenyl substitution, the remaining carbon atoms of the chain are
163
positioned in the most upfield region. The described phenomenon, however, is yet not
completely understood, and the trend of this effect seems to depend on the substituents of the
sp-carbon chain.
In Figure 4.31, the 13C-NMR signals of the cumulene chain in the series of [n]tBuPh
cumulenes (n = 3, 5, and 7) as well as [9]cumulene rotaxanes 3.8 and 3.9 are shown. As
discussed before for the rotaxanes 3.8 and 3.9, “naked” cumulenes show also three regions
with cumulenic 13C signals, one, that is the most downfield shifted region (between 145 and
152 ppm, region I), the second one describes a range between 123 and 129 ppm (region II),
and the last one is shifted upfield between 126 and 120 ppm (region III). The 13C signals of
the first two regions seem to converge with increasing length (n) of the cumulene chain
(Figure 4.31).
Figure 4.31 Plot of 13C NMR carbon chemical shifts versus the number of double bonds n for
[n]tBuPh (n = 3, 5, 7) and [9]cumulene rotaxanes 3.8 and 3.9.
The 13C signals of the second region might be assigned to the outermost carbon C1 of
the cumulene chain by HMBC- as well as coupled 13C NMR spectroscopic measurements
(Figure 4.31). It has been assumed that the 13C signals from region I originates from the
second outermost carbon C2, as it is also the case in the 13C NMR study of TIPS-endcapped
164
polyynes showing a similar convergence of carbon signals in which the carbon positions are
determined by 13C labeling.32 The assumption that region I describes C2 resonances is not
random since these two outermost carbons reflects most the effect of endgroups. The
cumulenes, especially longer cumulenes, however, have never been intensely characterized
using 13C NMR spectroscopy so far. Therefore, the origin of these interesting observations
needs to be further investigated.
4.6 Summary and conclusion
In summary, a variety of characterization methods for the series of [n]tBuPh have
been performed, including UV/vis spectroscopy and structural studies via X-ray
crystallographic analysis. In addition, theoretical studies have been done by Görling and
coworkers to help to explain the observed trend in the performed analyses. Furthermore,
cyclic voltammetry and 13C NMR spectroscopy including C-H correlation studies have been
done, to give unprecedented insight into structural and electronic properties of cumulenes.
UV/vis spectroscopy and X-ray crystallography of [n]cumulenes show a correlation
between cumulene chain length and properties, such as HOMO-LUMO gap and BLA, and
both decrease with increasing chain length, i.e., increasing n. An asymptotic or saturated
value for n = 9 in order to achieve a “carbyne-like” molecule, however, has not yet been
reached. By means of some characterization tools, it is confirmed that the structure of
cumulenes (odd-numbered versus even-numbered) as well as the endgroups affect the
properties of cumulenes in predictable trends. Theoretical studies of cumulenes show good
agreement to experimental results, such as UV/vis data (e.g., optical band gap), bond lengths,
and BLA.
Cyclic voltammetry reveals oxidation and reduction events for the cumulene series
[n]tBuPh including rotaxane 3.9. Trends in both, oxidation and reduction values depend on
cumulene chain length. The influence of chain length on the HOMO-LUMO gap, estimated
from cyclic voltammetry, is also observed to be similar to UV/vis spectroscopic studies.
Finally, NMR spectroscopy has been used to determine the chemical shifts of the first
carbon atoms of the cumulene chain of [n]tBuPh and [9]cumulene rotaxanes using the C-H
correlation technique HMBC. A similar trend of carbon chemical shifts versus n as in the case
of polyynes has been determined giving a converging trend of the shifts of the two outermost
carbons in the chain by increasing the chain length.
165
4.7 References
1 H. Fischer, in The Chemistry of Alkenes (Ed.: S. Patai), John Wiley & Sons, New
York, 1964, pp. 1025–1159.
2 R. R. Tykwinski, W. Chalifoux, S. Eisler, A. Lucotti, M. Tommasini, D. Fazzi, M. Del
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5 R. Kuhn, K. Wallenfels, Chem. Ber. 1938, 71, 783–790.
6 K. Brand, A. Busse-Sundermann, Chem. Ber. 1950, 83, 119–128.
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8 F. Bohlmann, K. Kieslich, Chem. Ber. 1954, 87, 1363–1372.
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10 P. Cadiot, Ann. Chim. [Paris] 1956, 13, 214–272.
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Jpn. 1974, 47, 2398–2405.
12 L. T. Scott, G. J. DeCicco, Tetrahedron Lett. 1976, 31, 2663–2666.
13 Determined via common conversion tools from http://halas.rice.edu/conversions (nm
to eV).
14 C. Bubeck, in Electronic Materials: The Oligomer Approach (Eds.: K. Müllen, G.
Wegner), Wiley-VCH, Weinheim, 1998, Chapter 8.
15 G. N. Lewis, M. Calvin, Chem. Rev. 1939, 25, 273–328.
16 H. Meier, U. Stalmach, H. Kolshorn, Acta Polym. 1997, 48, 379–384.
17 Determined by doctoral student Michael Franz from the Tykwinski group.
18 W. A. Chalifoux, R. R. Tykwinski, Nat. Chem. 2010, 2, 967–971.
19 W. A. Chalifoux, R. McDonald, M. J. Ferguson, R. R. Tykwinski, Angew. Chem. Int.
Ed. 2009, 48, 7915–7919; Angew. Chem. 2009, 121, 8056–8060.
20 B. Bildstein, M. Schweiger, H. Angleitner, H. Kopacka, K. Wurst, K.-H. Ongania, M.
Fontani, P. Zanello, Organometallics 1999, 18, 4286–4295.
21 F. Innocenti, A. Milani, C. Castiglioni, J. Raman Spectrosc. 2010, 41, 226–236.
22 D. Nori-Shargh, F. Deyhimi, J. E. Boggs, S. Jameh-Bozorghi, R. Shakibazadeh, J.
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23 M. Weimer, W. Hieringer, F. Della Sala, A. Görling, Chem. Phys. 2005, 309, 77–87.
166
24 Based on a search of WebCSD, see http://webcsd.ccdc.cam.ac.uk/ on 11/07/14, for
equally substituted [n]cumulenes with n = odd and alkyl or aryl endgroups. Results do
not include [5]Ph (for the structure of [5]Ph, see M. M. Woolfson, Acta. Cryst. 1953,
6, 838–841.)
25 J. A. Januszewski, D. Wendinger, C. D. Methfessel, F. Hampel, R. R. Tykwinski,
Angew. Chem. Int. Ed. 2013, 52, 1817–1821; Angew. Chem. 2013, 125, 1862–1867.
26 [3]tBuPh, [5]tBuPh, [7]tBuPh, and [5]Mes have been prepared by myself or under
my supervision (as in the case of [5]Mes) while [7]Mes and [9]Mes have been
prepared by Dominik Wendinger and used in this thesis for better comprehension,
discussion, and comparison.
27 H. Irngartinger, W. Götzmann, Angew. Chem. Int. Ed. Engl. 1986, 25, 340–342;
Angew. Chem. 1986, 98, 359–361.
28 Z. Berkovitch-Yellin, L. Leiserowitz, Acta Crystallogr. Sect. B 1977, 33, 3657–3669.
29 The structure of [5]Ph has been reported without bond length or angle values see: M.
M. Woolfson, Acta. Cryst. 1953, 6, 838–841. Bond lengths and angles from J. A.
Januszewski, F. Hampel, R. R. Tykwinski, unpublished, CCDC 817302.
30 H. Irngartinger, H.-U. Jäger, Angew. Chem. Int. Ed. Engl. 1976, 15, 562–563; Angew.
Chem. 1976, 88, 615–616.
31 S. Szafert, J. A. Gladysz, Chem. Rev. 2006, 106, PR1–PR33.
32 S. Eisler, A. D. Slepkov, E. Elliott, T. Luu, R. McDonald, F. A. Hegmann, R. R.
Tykwinski, J. Am. Chem. Soc. 2005, 127, 2666–2676.
33 Exceptions include: a) T. Kawase, N. Nishigaki, H. Kurata, M. Oda, Eur. J. Org.
Chem. 2004, 3090–3096. b) Y. Kuwatani, G. Yamamoto, M. Iyoda, Org. Lett. 2003, 5,
3371–3374.
34 Calculated as the distance from H-atom to a plane generated from the six atoms of the
aromatic ring.
35 S. Grimme, P. R. Schreiner, Angew. Chem. Int. Ed. 2011, 50, 12639–12642.
36 Herein, the twist angles of [3]tBuPh are not included due to the specific bending of
the cumulene chain that might be caused by intramolecular C–H/π-interactions.
37 Unpublished results by A. Görling and coworkers in collaboration with R. R.
Tykwinski and coworkers.
38 Values have been estimated from the graph in Figure 4.12.
39 Values have been estimated from the graph in Figure 4.15.
167
40 Two conformers for [9]Ph (D2 and C2h symmetry, respectively) that differ in the
orientation of the phenyl groups have been used for calculations performed by Görling
and coworkers. Both conformers show a similar minimum potential energy (difference
of only 0.13 kJ/mol). The theoretical results show no difference between properties of
the two conformers.
41 W. A. Chalifoux, M. J. Ferguson, R. McDonald, F. Melin, L. Echegoyen, R. R.
Tykwinski, J. Phys. Org. Chem. 2012, 25, 69–76.
42 R. Dembinski, T. Bartik, B. Bartik, M. Jaeger, J. A. Gladysz, J. Am. Chem. Soc. 2002,
122, 810–822.
43 W. Mohr, J. Stahl, F. Hampel, J. A. Gladysz, Chem. Eur. J. 2003, 9, 3324–3340.
44 Q. Zheng, J. C. Bohling, T. B. Peters, A. C. Frisch, F. Hampel, J. A. Gladysz, Chem.
Eur. J. 2006, 12, 6486–6505.
45 T. Gibtner, F. Hampel, J.-P. Gisselbrecht, A. Hirsch, Chem. Eur. J. 2002, 8, 408–432.
46 W. Kemula, J. Kornacki, Rocz. Chem.: Ann. Soc. Chim. Pol. 1962, 36, 1835–1874.
47 Y. Li, K. Chandra Mondal, P. P. Samuel, H. Zhu, C. M. Orben, S. Panneerselvam, B.
Dittrich, B. Schwederski, W. Kaim, T. Mondal, D. Koley, H. W. Roesky, Angew.
Chem. Int. Ed. 2014, 53, 4168−4172; Angew. Chem. 2014, 126, 4252−4256.
48 B. Bildstein, Coord. Chem. Rev. 2000, 206–207, 369−394.
49 W. Skibar, H. Kopacka, K. Wurst, C. Salzmann, K.-H. Ongania, F. Fabrizi de Biani, P.
Zanello, B. Bildstein, Organometallics 2004, 23, 1024−1041.
50 G. J. Hoijtink, P. H. van der Meij, Z. physik. Chem. 1959, 20, 1−14.
51 N. Suzuki, N. Ohara, K. Nishimura, Y. Sakaguchi, S. Nanbu, S. Fukui, H. Nagao, Y.
Masuyama, Organometallics 2011, 30, 3544–3548.
52 Reversibility has been investigated using equation ipa/ipc ≈ 1 for reversible systems.
Redox processes with values of ipa/ipc in a range of 0.7–1.2 and peak widths similar to
that of Fc+/Fc (110−120 mV) have been considered reversible. Otherwise, redox
processes have been considered quasireversible or irreversible (only the oxidation
process of compound 3.9 has been considered irreversible due to the missing cathodic
current peak). This is a simplified determination, since it has not yet been assigned if
the redox processes are one- or multielectron processes.
53 Reduction potentials have not been determined by cyclic voltammetry but by another
method using a glass apparatus for preparation of negative ions under high vacuum as
reported by van der Meij in ref[50].
168
54 J. L. Brédas, R. Silbey, D. S. Boudreaux, R. R. Chance, J. Am. Chem. Soc. 1983, 105,
6555–6559.
55 J. Anthony, C. Boudon, F. Diederich, J.-P. Gisselbrecht, V. Gramlich, M. Gross, M.
Hobi, P. Seiler, Angew. Chem. Int. Ed. Engl. 1994, 33, 763–766; Angew. Chem. 1994,
106, 794–798.
56 R. E. Martin, U. Gubler, C. Boudon, V. Gramlich, C. Bosshard, J.-P. Gisselbrecht, P.
Günter, M. Gross, F. Diederich, Chem. Eur. J. 1997, 3, 1505–1512.
57 P. Bäuerle, Adv. Mater. 1992, 4, 102–107.
58 R. R. Tykwinski, T. Luu, Synthesis 2012, 44, 1915–1922.
59 M. Haque, L. Yin, A. R. T. Nugraha, R. Saito, Carbon 2011, 49, 3340–3345.
60 M. Štefko, M. D. Tzirakis, B. Breiten, M.-O. Ebert, O. Dumele, W. B. Schweizer, J.-P.
Gisselbrecht, C. Boudon, M. T. Beels, I. Biaggio, F. Diederich, Chem. Eur. J. 2013,
19, 12693–12704.
61 F. Bohlmann, M. Brehm, Chem. Ber. 1979, 112, 1071–1073.
62 Y. Kuwatani, G. Yamamoto, M. Oda, M. Iyoda, Bull. Chem. Soc. Jpn. 2005, 78,
2188–2208.
63 This paragraph has been adapted from: S. Frankenberger, J. A. Januszewski, R. R.
Tykwinski, in Fullerenes and Other Carbon-Rich Nanostructures (Ed.: J.-F.
Nierengarten), Springer, Berlin, 2014, vol. 159, 219–256.
169
5. Chapter V. Reactions of [n]cumulenes‡‡
5.1 Addition reaction of [5]tBuPh with tetracyanoethylene (TCNE)
5.1.1 Motivation and objective
Cumulenes undergo reactions such as photo- or thermal cycloadditions, either as
dimerizations or with other reactants, such as alkynes and alkenes, e.g., tetrafluoroethylene or
tetracyanoethylene (TCNE). The reactions of shorter [n]cumulenes (n = 2, 3) with TCNE
have been explored by Kawamura and coworkers giving several novel cyclic compounds with
diverse structures.1,2 Recently, the group of Kawamura has reported results on the reaction of
several [3]cumulenes 5.1 with TCNE (5.2) showing the interesting reactivity of a cumulene
chain at consecutive double bonds (Scheme 5.1).3 The reaction gives two different
cycloadducts, 5.3 and 5.4. Cyclic compound 5.3 is the main product and is assumed to be
formed from intermediate 5.5 that derives from a [2 + 2]-cycloaddition reaction at the central
C=C bond supported by π-π-stacking of the aryl rings. Following bond cleavage of the
(NC)2C-C(CN)2 bond, rotation, and rearrangement forms the product 5.3. In addition, a
[4 + 2]-cycloaddition product, compound 5.6, has been suggested as possible intermediate.
‡‡ Portions of this chapter have been published: J. A. Januszewski, F. Hampel, C. Neiss, A. Görling, R. R.
Tykwinski, Angew. Chem. Int. Ed. 2014, 53, 3743–3747.
170
Scheme 5.1 Reaction of [3]cumulenes with TCNE in CH2Cl2 at rt.
When dissolved in MeOH or CH3CN, 5.3 is converted to the bi- and tricyclic
compounds 5.7 and 5.8, respectively (Scheme 5.2). In both conversions, one aryl group takes
part in the reaction. The synthetic route to 5.7 is assumed to run via a rearrangement reaction,
while the formation of 5.8 is part of an equilibrium that becomes irreversible during a final
hydrolysis step.
Scheme 5.2 Conversion of cyclobutane 5.3 to 5.7 and 5.8.
171
Another [2 + 2] reaction of a cumulene with TCNE (5.2) has been reported by
Bildstein and coworkers.4 [5]Cumulene [5]Fc has been treated with TCNE in benzene
showing addition at the β-bond of the cumulene chain to give cycloadduct 5.9 in yields of ca.
41% (Scheme 5.3).
Scheme 5.3 TCNE addition to [5]Fc giving cycloadduct 5.9.
Reactions of TCNE to carbon rich compounds, such as polyynes, have been
investigated in the last decades giving products with interesting optical properties, e.g., push-
pull chromophores described as potential candidates for nonlinear optics.5–8 Reactions of
acetylenes with TCNE have been developed in the group of Diederich5–7,9 and are based on a
sequence of cyclization and cycloreversion reaction steps (Scheme 5.4).10 Herein, a general
example is presented that uses an acetylenic compound containing an electron-donating group
(this group is required for the realization of the reaction), which is treated with TCNE. As
intermediate, a four-membered cyclobutene derivative is formed via cycloaddition, which is
transformed to donor-substituted tetracyanobutadiene (TCBD) derivatives via electrocyclic
ring opening.
Scheme 5.4 General cyclization/cycloreversion method giving donor-substituted TCBD
derivatives.
Diederich and coworkers,5–7 and others8 describe the [2 + 2]-cycloaddition reaction of
polyynes with TCNE as a very efficient “click reaction” to form new chromophores with
outstanding stability and unique optical properties. This successful work in the subject of
polyyne chemistry gives rise to the question whether a [5]cumulene might be converted to a
[3]cumulene (5.10) via the cyclization and cycloreversion method using TCNE (Scheme 5.5).
172
Scheme 5.5 Expected cyclization/cycloreversion reaction of a [5]cumulene with TCNE to the
unsymmetrically substituted [3]cumulene 5.10.
[3]Cumulenes, such as 5.10, containing donor and acceptor endgroups, are push-pull
systems and are assumed to possess outstanding properties.7,11 In addition, they are predicted
to constitute more potent nonlinear optical chromophores than the appropriate oligoenes and
oligoynes.12,13 To date, several push-pull [3]cumulenes have been synthesized in order to
mainly examine whether these compounds exist in a covalent form (cumulene form) or a
dipolar form (acetylene form). Once formed, the dipolar character has been examined by, e.g.,
X-ray crystallography and NMR spectroscopy studies.7,11,14–18 Furthermore, cumulenes such
as 5.10 show a “chameleonic”, rather acetylene-like reactivity including cycloaddition
reactions as well as polar additions of electrophilic and nucleophilic reactants.7,15
Reported studies of cumulene reactivity offer little guidance as to whether the
regiochemistry of a cycloaddition reaction of a [5]cumulene with TCNE would be governed
by steric or electronic factors. Thus, density-functional calculations have been performed
based on [5]cumulene [5]tBuPh.19 These calculations show that the two central-most carbons
(C3) bear a slight negative charge, while the neighboring carbons (C2) are slightly positively
charged (Table 5.1). This charge distribution may guide the highly electrophilic TCNE toward
the central γ-bond (for bond labeling see Scheme 5.5). The bulky aryl groups at termini are
expected to hinder approach of TCNE to the β-bond.
173
Table 5.1 Natural population analysis (NPA) charges of the cumulenic C-atoms in
[5]cumulene [5]tBuPh.
• • • •Ar
Ar Ar
Ar
[5]tBuPh
C1
C2
C3
C1
C2
C3
C-atom PBE PBE-D3 B3LYP B3LYP-D3
C1 –0.068 –0.068 –0.058 –0.059
C2 +0.053 +0.055 +0.048 +0.050
C3 –0.066 –0.068 –0.064 –0.066
Examination of the frontier molecular orbitals of TCNE and [5]tBuPh shows,
however, that a concerted [2 + 2] addition of TCNE at the γ-bond is forbidden by orbital
symmetry, as expected based on the Woodward-Hoffmann rules for a 4-electron system
(Figure 5.1).20 We have hypothesized that the addition of TCNE to [5]cumulene [5]tBuPh
might occur via a sterically directed stepwise mechanism toward the γ-bond of a [5]cumulene
(Scheme 5.5). A subsequent retro-[2 + 2] reaction from the cyclic intermediate might then
give an unsymmetrically substituted [3]cumulene 5.10. This overall sequence could provide a
useful metathesis reaction to form polarized cumulenes.21
Figure 5.1 HOMO and LUMO of TCNE and [5]tBuPh.
174
5.1.2 Target, synthetic pathway, and test reactions
The reaction of [5]tBuPh and TCNE in CH2Cl2 at rt was explored to form the
unsymmetrically substituted [3]cumulene 5.10a (Figure 5.2). This compound should be
obtained via the cyclobutane derivative 5.11, which has been expected to be formed via
[2 + 2]-cyclization reaction of TCNE and the central double bond of [5]tBuPh, followed by
cycloreversion.
[5]Cumulene [5]tBuPh was treated with TCNE in CH2Cl2 at rt under an argon
atmosphere (Figure 5.2). The reaction mixture was additionally covered with alumina foil,
and it was monitored by TLC analysis resulting in a “virtual rainbow” of different colored
spots. The concentration of several products, however, seemed to be negligible and three
predominant products A, B, and C were observed, which have been discussed in this section
(Figure 5.2). The orange colored spot A seemed to be the main product in this reaction, spot B
was initially orange/brown and turned to green after some minutes. Spot C was initially only
visible under UV light, converting to orange over time. Purification by chromatography
(silica gel, CH2Cl2/hexanes 1:1 or CHCl3) allowed for the isolation of all three fractions A, B,
and C.
175
Figure 5.2 Performed reaction of [5]tBuPh with TCNE, monitored by TLC analysis.
The identity of product A was tackled first. High resolution ESI MS analysis (positive
mode) offered a signal at m/z 979.6557, which was consistent with [[5]tBuPh + TCNE + Na]+
(C68H84N4Na). The aryl region of the 1H NMR spectrum showed triplets at 7.61, 7.53, and
7.49 ppm (integration ratio 1:1:2) and doublets at 7.29, 7.19, and 7.12 ppm (integration ratio
4:2:2, see Figure 5.3). These signals were consistent with four 3,5-di-t-butylphenyl groups,
but also confirmed a loss of symmetry during the course of the reaction. In the 13C NMR
spectrum, there was a noteworthy resonance at 203.4 ppm (Figure 5.4), while the IR spectrum
revealed a weak signal at 1920 cm–1; both suggestive of an allene moiety.
176
Figure 5.3 1H NMR spectrum of product A (inset: expansion of the aryl region).
Figure 5.4 13C NMR spectrum of product A, allene signal highlighted; * = Et2O.
177
Finally, overlaying a solution of A in CH2Cl2 with either EtOH or MeOH afforded
crystals, and structural analysis (Figure 5.5) showed the ethyl and methyl enol ethers 5.12 and
5.13, respectively. Thus, product A could be confidently assigned as cyclic [3]dendralene 5.14
(Figure 5.6).22
MeOH
NC CN
NC CN
O
EtOH
NC CN
NC CN
O
product A
5.12 5.13
Figure 5.5 Top: Overlaying of product A (in CH2Cl2) with EtOH and MeOH giving adducts
5.12 and 5.13, respectively. Bottom: ORTEP drawings (20% probability level) for compounds
5.12 and 5.13.
178
A
C
B
•
NC CN
CNNC
5.14
CH2Cl2/hexanes
1/1
Figure 5.6 Identification of product A as cyclic [3]dendralene 5.14.
Secondly, the identity of product B was investigated. ESI MS analysis showed a
parent peak at m/z 979.6576 ([[5]tBuPh + TCNE + Na]+), analogous to that observed for
product A (i.e., compound 5.14). The aryl region of 1H NMR spectrum for product B,
however, showed two sets of signals, i.e., two triplets and two broad singlets, integrating to 2
and 4, respectively. This corresponded to a structure with only two unique aryl groups and a
two-fold symmetry. Furthermore, no resonance, which was consistent with an allenic sp-
carbon was found in the 13C NMR spectrum. Ultimately, X-ray crystallography identified the
structure of product B as radialene 5.15 (Figure 5.7).
Ar
Ar
Ar
Ar
NCCN
CNNC
Ar =
A
C
B
5.15
CH2Cl2/hexanes
1/1
Figure 5.7 Left: Identification of product B as [4]radialene 5.15. Right: ORTEP drawing
(20% probability level) for compound 5.15.
179
The identification of 5.14 and 5.15 (products A and B) was, in principle, consistent
with intermediate 5.10a during the reaction of [5]tBuPh with TCNE. As the study progressed,
it became clear that product C (Figure 5.8) was not stable, and it transformed to products A
(compound 5.14) and B (compound 5.15) in a ratio of ca. 9:1 over time, by warming, or via
concentration of the reaction mixture. Thus, attention turned to identification of product C.
Figure 5.8 Conversion of product C to A (compound 5.14) and B (compound 5.15), and the
associated TLC analysis.
The work of Viehe and coworkers15 has shown that polarized [3]cumulenes could be
efficiently trapped via reaction with EtOH or Br2 (Scheme 5.6). The resulting butadiene
adducts are obtained through addition of EtOH and Br2 to the central double bond of the
cumulene chain in the push-pull [3]cumulene.
N
N CN
CNEtOH
CN
CN
Br
BrN
N
CN
CN
EtO
HN
N
Br2
Scheme 5.6 Reaction of a push-pull [3]cumulene with EtOH and Br2, respectively.15
In order to test if product C was consistent with the desired polarized [3]cumulene
5.10a (Figure 5.2), product C was isolated at low temperature and treated with EtOH and Br2,
180
respectively, at 0 °C in a solution of CHCl3 (Figure 5.9). The addition of EtOH showed no
productive results, and the butadiene derivative 5.16 was not observed. In contrast, the
outcome of the reaction with Br2 showed two new products D and E by TLC analysis. Product
E was unstable and decomposed. In contrast, the less polar brown product D could be
separated via column chromatography and was identified as the symmetrical and stable
[4]dendralene 5.17, as established by X-ray crystallography (Figure 5.9). Compound 5.17
was, unfortunately, not a product easily linked to the presence of the [3]cumulene 5.10a
during the reaction of [5]tBuPh with TCNE. Besides, no hint for formation of expected
butadiene 5.18 was observed in this reaction.
Figure 5.9 Conversion of product C with Br2 affording [4]dendralene 5.17; ORTEP drawing
(20% probability level) for compound 5.17.
It was hypothesized that dendralene 5.17 could be formed from the reaction of Br2
with radialene 5.15, which might be produced in situ from product C. Therefore, a test
reaction was carried out in which radialene 5.15 was treated directly with an excess of Br2
(Scheme 5.7). This reaction did not give the dibromoadduct 5.17, but rather, [4]dendralene
5.19, an isomeric analog of 5.17 (bromination of one aryl ring is observed). The structure of
5.19 was confirmed by X-ray crystallography. The use of less Br2 also gave a single product
with almost identical Rf value as for 5.19 (as observed via TLC analysis) but differently
181
colored than compound 5.19 (i.e., brown instead of red-brown). Unfortunately, this structure
could not yet be confirmed by X-ray crystallographic analysis but was identified tentatively as
[4]dendralene 5.20 in which no aromatic substitution occured.23
Scheme 5.7 Conversion of product B (compound 5.15) to [4]dendralenes 5.19 and 5.20;
ORTEP drawing (20% probability level) for compound 5.19.
With the identity of product C not yet confirmed, direct isolation and characterization
was attempted. The reaction of [5]tBuPh and TCNE was conducted over 24 h at –25 °C in
CH2Cl2. The desired product C was purified by column chromatography with CDCl3 with the
column temperature maintained between –20 to 0 °C using a jacketed column. The fractions
containing C were stored at low temperature (i.e., on dry ice) to prevent conversion to A and
B. The combined CDCl3 fractions (ca. 250 mL) were reduced to less than 1 mL under
vacuum, and NMR spectra were acquired.
The 1H NMR spectrum showed only one set of signals representative of the 3,5-di-t-
butylphenyl group, namely a broad singlet (suggesting a triplet) at 7.39 ppm and a doublet at
7.16 ppm. This indicated a highly symmetrical structure for product C. More significantly, the 13C NMR spectrum showed the signal of an allene at 203.6 ppm, as well as that for a sp3-
hybridized carbon atom at 42.9 ppm. Combined with ESI MS analysis, which showed a signal
at m/z 979.6575 ([[5]tBuPh + TCNE + Na]+), these data supported that product C could be
assigned as compound 5.11 based on a cyclobutane derivative with two exocyclic allene units
(Figure 5.10).24
182
Figure 5.10 Identification of product C as cyclobutane 5.11.
5.1.3 Mechanistic studies and characterization by UV/vis spectroscopy and X-ray
crystallography
As described in Section 5.1.2, the reaction of [5]tBuPh with TCNE, as well as further
addition reactions using EtOH, MeOH, and Br2, afford a variety of interesting compounds
with potentially unusual properties. This section of the thesis presents the mechanistic studies
that have been performed with the help of theoretical calculations by Görling and coworkers.
It is still necessary to find a fundamental basis of cycloaddition reactions of cumulenes since
to date, no general guidelines concerning the reactivity pattern exist. Thus, we have hoped to
get further insight into the reaction behavior of cumulenes through the help of theoretical
investigations. Furthermore, the results of UV/vis spectroscopic measurements of some
products obtained during the TCNE reaction will be discussed. Based on X-ray
crystallographic analysis, the structural properties of the formed cyclobutanes, radialenes, and
dendralenes will be outlined and compared to known structures. For simplification concerning
further discussions, Scheme 5.8 shows an overview of products identified from the reaction of
[5]tBuPh with TCNE and further addition reactions.
183
Scheme 5.8 Overview of compounds identified from the reaction of [5]tBuPh with TCNE
including further addition reactions.
Kawamura and coworkers have described a possible mechanism for the reaction of
[3]cumulenes with TCNE affording compound 5.3 as mentioned in Section 5.1.1 (see Scheme
5.1).1,3 In their studies, cyclobutane 5.5 (with two exocyclic ethylene units) has been
described as an intermediate that resembles the cyclobutane 5.11 (see Scheme 5.8)
synthesized herein. Kawamura has assumed that the single C-C-bond connecting four cyano
groups of 5.5 is broken and results in compound 5.3 via bond rotation and cyclization
(Scheme 5.9).
Scheme 5.9 Summary of a mechanism of the reaction of a [3]cumulene with TCNE as
suggested by Kawamura and coworkers.1,3
184
Based on Kawamuras’s work, we have postulated a mechanism for the reaction of
[5]tBuPh with TCNE (Scheme 5.10). Homolytic cleavage of the central (NC)2C-C(CN)2 bond
of 5.11 gives intermediate 5.21, in which the resulting radicals are stabilized by allylic
delocalization. Cyclization of rotamer 5.21’ then affords [3]dendralene 5.14. Radialene 5.15
can also be formed from 5.11 through a stepwise mechanism via intermediate 5.21.
Alternatively, neither a concerted reaction that converts 5.11 directly to 5.15 nor a two-step
4e-electrocyclic ring closing/opening via 5.21 can be ruled out.25
NCNC CN
CN
Ar
Ar Ar
Ar
Ar
Ar
NCNC
CN
NC
Ar
Ar
Ar
Ar
Ar
Ar
CNCN
CNNCNCNC CN
CN
Ar
Ar Ar
Ar
NCNC CN
CN
Ar
Ar Ar
Ar
NCNC
CN
CNAr
Ar
Ar
Ar
•
•
5.145.15
5.11
5.11
• •
5.21
concerted
stepwise
5.21'
Ar
Ar Ar
ArNC
NC CN
CN
[5]tBuPh
Ar =
Scheme 5.10 Proposed mechanism for the conversion of 5.11 to 5.14 and 5.15.
Scheme 5.11 shows a possible concerted mechanism for the addition reaction that
have been applied to 5.11 (i.e., a product from the reaction of [5]tBuPh and TCNE) using Br2
as well as a two-step mechanism for addition reactions applied to 5.14 using ROH.
Compound 5.11, for example, adds Br2 onto the allenic carbon atoms. A simultaneous bond
cleavage of the single bond containing the cyano groups would be necessary to obtain
[4]dendralene 5.17. Aside from the concerted mechanism, also a radical containing
transformation can be considered based on the biradical intermediate 5.21 in Scheme 5.10.
Finally, additions of ROH to cyclobutane 5.14 take place on the allenic bond giving adducts
5.12 and 5.13 for R = Et and Me, respectively.
185
Scheme 5.11 Proposed concerted mechanism for the addition reaction of bromine to 5.11 and
the stepwise addition of ROH to 5.14.
DFT calculations show that the reaction of [5]tBuPh with TCNE to 5.11 followed by a
rearrangement reaction to 5.14 and 5.15 is in excellent agreement with predictions based on
stability (Figure 5.11), i.e., compounds 5.14 and 5.15 are clearly thermodynamically more
stable than 5.11. Interestingly, the inclusion of van-der-Waals corrections to the DFT
calculations gives enhanced stabilization of products 5.11, 5.14, and 5.15 when compared to
calculations without this correction. This observation is easily rationalized by the fact that the
products are stabilized by intramolecular dispersive interactions between the aryl and alkyl
groups. The computed energies also explain why 5.10a is not observed, given that metathesis
reaction from 5.11 to 5.10a is significantly endothermic when dispersive interactions are
included. In the absence of dispersion corrections, the formation of 5.14, and 5.15 is still
preferred over 5.10a, albeit the differences are less pronounced.
186
Figure 5.11 Computed relative energies (kcal/mol) of the products 5.11 (product C), 5.14
(product A), and 5.15 (product B), as well as the hypothesized product 5.10a, in comparison
to the reactants TCNE and [5]tBuPh. Calculations based on DFT including (red), and without
(blue) dispersion interaction corrections.
Regarding the properties of all synthesized cyclobutane, radialene, and dendralene
derivatives, radialene 5.15 shows the most interesting features. The most obvious
characteristic of 5.15 is its color, which changes from orange, when adsorbed on silica gel to
green in the solid state.26 Compound 5.15 also shows solvatochromism by UV/vis
spectroscopy. Specifically, the UV/vis spectrum shows a broad low energy absorption with
λmax > 700 nm (Figure 5.12) that ranges from λmax = 720 nm (cyclohexane) to λmax = 771 nm
(CHCl3), which is characteristic for an intramolecular charge transfer absorption.27
187
Figure 5.12 Quantitative UV/vis spectrum of radialene 5.15 (in CHCl3); Inset: λmax values for
5.15 as a function of solvent.
The rectangular structure of 5.1528,29 suggests a donor-acceptor (push-pull) interaction
between the dicyanovinyl acceptor and the electron-rich dialkylaryl groups, with shortened
C1–C2 bond lengths of 1.469(2) Å and longer
C1–C1’ and C2–C2’ bonds (1.494(3) and
1.504(3) Å, respectively). This is concurrent
with elongation of the alkylidene bonds C1–C4
and C2–C3 (1.36–1.37 Å) compared to several
equally substituted symmetrical [4]radialenes
as described in the following paragraph.
Table 5.2 presents several known
[4]radialenes 5.22–5.26 that are substituted by
a variety of endgroups giving symmetrical and unsymmetrical compounds.7,29–32 The bond
lengths of the cyclobutane unit (a–d in red, Table 5.2), as well as the exocyclic double bonds
(e–h in green, Table 5.2) are compared to radialene 5.15. Radialenes 5.22 and 5.23 show
cyclobutane bond lengths a–d of 1.502(6)–1.507(6) Å and 1.484(2)–1.492(2) Å,
respectively.30,31 The exocyclic double bonds e–h show values of 1.343(6)–1.351(6) Å and ca.
188
1.326(2) Å for 5.22 and 5.23, respectively. The bond lengths a and b of radialene 5.15
(1.469(2) Å) are shorter compared to a and b for symmetrical radialenes 5.22
(1.502(6)/1.507(6) Å) and 5.23 (1.484(2) Å) probably caused by push-pull effects in 5.15. In
contrast, the bond lengths e–h of the exocyclic double bonds in 5.15 are slightly longer than
those of radialenes 5.22 and 5.23. This might be derived from the increased single bond
character due to the push-pull effect of the endgroups. Radialene 5.24, reported by Diederich,
possesses similar push-pull endgroups as radialene 5.15, namely dimethylaniline and cyano
endgroups.7 Unlike 5.15, however, compound 5.24 is centrosymmetric showing no effect on
bond lengths of the cyclobutane ring having similar bond lengths a–d of 1.473(3)–1.474(3) Å.
A minor difference in bond length of the exocyclic double bonds can be observed giving
1.383(3) Å for e and h and 1.402(3) Å for f and g, probably due to endgroup effects.
Radialene 5.25 shows similar effects in bond lengths as radialene 5.15 with shortened a and b
and elongated c and d bonds (compared to 5.22 and 5.23), as well as similar values for e–h
(Table 5.2).32 The last example is shown by radialene 5.26 describing similar structural
properties as radialenes 5.25 and 5.15.29 In conclusion, endgroup effects show a significant
influence on the bond lengths of radialenes resulting in shortening and elongation of bond
lengths, thus, enabling the possibility of tuning the properties of radialenes.
189
Table 5.2 Bond lengths (Å) of radialene 5.15 and selected radialenes known from literature.
radialene 5.15 5.22 5.23 5.24 5.25 5.26
a 1.469(2) 1.502(6) 1.484(2) 1.474(3) 1.4279(15) 1.440(3)
b 1.469(2) 1.507(6) 1.484(2) 1.474(3) 1.4275(15) 1.440(3)
c 1.494(3) 1.504(6) -[a] 1.473(3) 1.5156(15) 1.491(3)
d 1.504(3) 1.502(6) 1.492(2) 1.473(3) 1.5157(16) 1.488(3)
e 1.370(2) -[a] 1.326(2) 1.383(3) 1.3747(15) 1.361(3)
f 1.370(2) 1.343(6) 1.326(2) 1.402(3) 1.3722(15) 1.361(3)
g 1.362(3) 1.351(6) 1.326(1) 1.402(3) 1.3735(16) 1.343(3)
h 1.362(3) -[a] 1.326(1) 1.383(3) 1.3753(16) 1.345(3)
ref 30 31 7 32 29
[a] Value not given in literature.
5.2 Addition reactions of other cumulenes with TCNE
5.2.1 [7]tBuPh cumulene
A reaction of TCNE with the [7]cumulene [7]tBuPh was performed (Figure 5.13). A
purified solution of the [7]cumulene [7]tBuPh in CH2Cl2 was treated with TCNE at rt. After
stirring overnight, the color of the reaction mixture turned from violet to bluish. The TLC
analysis showed a wide variety of colorful spots similar to the reaction of [5]tBuPh with
TCNE, and five major products, A–E, were formed:
190
1. The first product A was initially blue and changed on the TLC plate to violet after
few seconds, and finally, complete decolorization occurred. By spotting the
purified product A on the TLC plate and putting it immediately into the TLC
chamber, only the blue spot was visible. Leaving the plate with product A for some
seconds on air before starting TLC analysis, however, resulted in an additional
pink baseline spot.
2. On the TLC plate, product B was initially only visible under UV light changing
into pale pink after some seconds under exposure to UV light.
3. The color of the purified product C was pale yellow, and after spotting on a silica
gel TLC plate, it turned to an intense orange. Using alumina TLC plates, no color
change was observed upon spotting. Based on the observations of the TLC
analysis using silica gel TLC plates, that could be considered as slightly acidic,
addition of HCl to a solution of C was performed also resulting in a color change
from pale yellow to orange.
4. Product D and baseline product E remained yellow and orange, respectively.
Figure 5.13 Reaction of [7]tBuPh with TCNE, and the associated TLC analysis.
After separation of products A–E via column chromatography (hexanes/CH2Cl2 1:1,
silica gel), several characterization methods were applied. A qualitative UV/vis spectrum was
recorded for product A as shown in Figure 5.14. The value for the lowest energy absorption
λmax was at 566 nm, while a shoulder absorption band could be observed at 609 nm. It was
interesting to note, that both values were shifted into the low-energy absorption region
191
compared to [7]tBuPh as well as [5]tBuPh. Hence, addition of TCNE to the cumulene system
affording product A could not be excluded, and the resulting red-shift of λmax could be
explained by the strong electron-accepting cyano groups.
Figure 5.14 Qualitative UV/vis spectrum of product A measured in hexanes/CH2Cl2 (eluent
from column chromatography).
The IR spectrum of product A given in Figure 5.15 showed evidence for the presence
of a cumulene group observed by an IR signal at 2038 cm–1. In general, the region between
1900 and 2050 cm–1 has been common for cumulenic vibrations. Unfortunately, no further
spectroscopic characterization of product A was possible due to decomposition.
192
Figure 5.15 IR spectrum of product A.
Product C (Figure 5.13) showed a color change from almost colorless to orange under
acidic conditions. Unfortunately, this product could not yet been identified, but it was possible
to record a 1H NMR spectrum of the compound in the orange solution, i.e., of product C after
addition of HCl (Figure 5.16). Based on this analysis, the signature of a single unsymmetrical
compound could be observed showing sharp and distinct signals. The upfield region showed
four singlets, each integrated to ca. 18 protons. The downfield region showed four triplets,
each integrated to one proton and three doublets with an integration ratio of 4:2:2.
Additionally, one singlet with an integration of one proton was found. This singlet was
probably caused by an additional proton that was derived from the HCl, which could add to
the structure of C.
193
Figure 5.16 1H NMR spectrum (recorded in CD2Cl2) of product C after addition of HCl.
The color change of product C under acidic conditions was also monitored via UV/vis
spectroscopic measurements. Two UV/vis absorption curves describing fraction C before
(black curve) and after (red curve) HCl addition have been shown in Figure 5.17. The high
energy region showed in both spectra three absorption bands and a shoulder. The values of
C + HCl were shifted slightly to higher energy with e.g., 273 and ca. 303 nm compared to 278
and 307 nm for product C. Noteworthy, a broad absorption band at 478 nm was present after
HCl addition.
194
Figure 5.17 Qualitative UV/vis spectrum of product C before (black curve) and after (red
curve) addition of HCl measured in hexanes/CH2Cl2 (eluent from column chromatography).
Regarding the yellow product D (Figure 5.13), a UV/vis spectrum was recorded for
characterization (Figure 5.18). The absorption region was set to 200–800 nm (default
parameters), and several absorption bands in the higher energy region (250–500 nm) were
observed. In addition, an intense absorption band appeared at 800 nm that extended into the
near-IR region. The low-energy absorption band at ca. 800 nm was reminiscent of
[4]radialene 5.15 that was formed in the reaction of [5]tBuPh and TCNE and showed a λmax
value of 771 nm in CHCl3 (Figure 5.7 and Figure 5.12).
195
Figure 5.18 Qualitative UV/vis spectrum of product D measured in hexanes/CH2Cl2 (eluent
from column chromatography).
In conclusion, initial results from the reaction of [7]tBuPh with TCNE show several
interesting products that have been briefly investigated. Three of the products seem likely to
have cumulenic or radialenic structure. The obtained products, however, show lower stability
than the products from the reaction of [5]tBuPh with TCNE, preventing more complete
characterization under the time limits of this thesis. Unfortunately, the reaction of [7]tBuPh
with TCNE has also not yet been repeated using crystalline (i.e., pure) [7]tBuPh due to time
restrictions. This reaction is certainly suited to further investigations based on the initial
results in order to gain more information about the reactivity of even higher [n]cumulenes.
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5.2.2 [5]MeOPh cumulene
It is of interest to investigate to what extent the endgroups of [n]cumulenes influence
the outcome of the reaction with TCNE and the stability of the products. Thus, the use of
[5]cumulene [5]MeOPh containing p-methoxyphenyl endgroups has been explored in the
reaction with TCNE. Contrary to the 3,5-di-t-butylphenyl group, the p-methoxyphenyl
endgroup, in fact, shows higher potential for the formation of donor-acceptor molecules due
to its more distinctive electron-donating effect. The group of Kawamura has already been
successful in investigation of the reaction between the lower cumulene representative
[3]MeOPh and TCNE,3 which has encouraged us to proceed using the analogous
[5]cumulene [5]MeOPh.
A sample of [5]cumulene [5]MeOPh that was synthesized during my diploma thesis
was used for the reaction with TCNE.33,34 Before using [5]MeOPh for the reaction with
TCNE, TLC analysis was performed and showed a pink (Rf value of ca. 0.6 in CH2Cl2) and a
baseline spot, which have yet not been identified. By filtration through a plug of silica gel in
CH2Cl2, the baseline fraction remained at top of the plug, and the pink fraction could be
obtained pure. Concentration of the pink fraction, however, gave very little product, and only
a poorly resolved 1H NMR spectrum could be recorded. TLC analysis confirmed that the
baseline spot also contained the pink spot (repeated elution of TLC plate after rotation by 90°)
illustrating that there were likely solubility problems.
In the first reaction, the sample of [5]MeOPh (containing the pink and the baseline
spot) was treated with TCNE at rt under inert conditions, and the reaction mixture was stirred
for several days (Figure 5.19). Several products B–E were formed according to TLC analysis,
aside from unconverted starting material (A). Products B and E did not change over time
remaining yellow and reddish on the TLC plate, respectively. Product C was initially only
visible under UV light but turned to green-brownish over time, while product D was initially a
violet spot that turned to yellow.
197
Figure 5.19 Reaction of [5]MeOPh (containing pink and baseline spot) with TCNE, and the
associated TLC analysis.
Three of four compounds, i.e., products B–D could be identified by X-ray
crystallographic analysis (Figure 5.20), and each structure consisted of an HCl or Cl2 adduct.
This led to three possible explanations:
1. The reaction or crystallization solvent (CH2Cl2) reacted with the starting material or
the reaction intermediates.
2. SnCl2 or HCl could be still present in the sample of [5]MeOPh (derived from the
reductive elimination reaction leading to [5]MeOPh) and could react with the starting
material or reaction intermediates that were formed during the reaction of [5]MeOPh
with TCNE.
3. [5]MeOPh that was used as starting material was already contaminated with
impurities or in the worst case, never possessed the structure of the desired
[5]cumulene [5]MeOPh.34
198
Figure 5.20 Products B–D with appropriate ORTEP drawings (20% probability level).
The reaction of [5]MeOPh with TCNE as shown in Figure 5.19 was repeated twice in
order to compare the results with those of Figure 5.19. Each reaction (the “second” and
“third” reactions, respectively) used a different sample as starting material,35 and both
samples showed the same pink and baseline spots in the TLC analysis.
First, the pink and the baseline spots of [5]MeOPh were separated via filtration
through a plug of silica gel using CH2Cl2 as solvent, and the separated pink fraction was used
for the “second” reaction with TCNE (Figure 5.21a). This reaction was monitored via TLC
analysis and showed a slightly different spot splitting compared to the TLC analysis of the
reaction in Figure 5.19. A violet spot, product A, turning to yellow as in the first reaction (see
199
product D in Figure 5.19) was observed (Figure 5.21a). Since the concentration was too low
for separation, the reaction was not further examined.
A “third” reaction of [5]MeOPh with TCNE was done, but with the starting material
(i.e., [5]MeOPh) containing both, the pink fraction and the baseline material (Figure 5.21b).
The results were almost the same as in the second reaction (i.e., when only the pink fraction
was used, Figure 5.21a). The baseline spot, however, was slightly darker, possible due to the
baseline material that already was present before the reaction. Crystallization attempts,
however, failed giving no confirmation if the violet spot, i.e., product A from the second and
third reactions (Figure 5.21a and b, respectively) was equal to the one describing product D
from the prior reaction in Figure 5.19.
Figure 5.21 Second and third reactions of [5]MeOPh with TCNE, and the associated TLC
analysis, in comparison to the first reaction. The precursor contained a) only the pink spot and
b) the pink and baseline spot.
To try to resolve the issues described above, the synthesis of [5]MeOPh was repeated
several times during my doctoral thesis in order to obtain material for final identification and
further reactions with TCNE. The precursor to [5]MeOPh, compound 5.27, was synthesized
200
by a standard synthetic route36 starting with ketone 5.28, followed by an addition of a
trimethylsilyl-substituted acetylide giving compound 5.29 (Scheme 5.12). Desilylation under
basic conditions afforded terminal alkyne 5.30, and final homocoupling reaction under Hay
conditions (CuCl and TMEDA) gave 5.27 which was purified by column chromatography and
carried on toward [5]MeOPh using standard conditions (SnCl2 and HCl). This reaction,
however, did not provide [5]MeOPh because too many byproducts were formed, and
decomposition of the precipitated product [5]MeOPh was observed. Taking a sample of
dissolved [5]MeOPh during the reaction progress using a capillary and spotting it on a TLC
plate already led to a color change from red to dark violet (in the capillary tube and on the
plate). Nevertheless, the formed reddish precipitate from the reaction mixture that was
assumed as [5]MeOPh was filtered, washed with water, EtOH, and Et2O, and dried. 1H NMR
spectroscopy showed two pairs of aryl groups and two signals for MeO groups aside from
impurities consisting of water, grease, and solvent. [5]MeOPh, however, should show only
one pair of aryl groups and one signal defining the OMe groups in the 1H NMR spectrum.
Unfortunately, the sample amount was not sufficient for 13C NMR spectroscopy. For
unknown reasons, it was not possible to reproduce the synthesis of [5]MeOPh in order to
provide adequate amounts for the reaction with TCNE. Consequently, no further studies on
this reaction could be performed, and also no reproducible results could be obtained.
Scheme 5.12 Synthetic approach to [5]cumulene [5]MeOPh.
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5.2.3 [5]oTol cumulene
The readily accessible [5]cumulene [5]oTol37 containing tolyl endgroups was treated
with TCNE (Figure 5.22). TLC analysis showed a yellow spot (A), a UV spot (B) that turned
to green after time, a weak yellow spot (C), a big grey-blue spot (D), and a bright yellow
baseline spot (E). A similar spot pattern was also obtained in the reaction of [5]tBuPh with
TCNE (Figure 5.2). All fractions were separated by column chromatography in CH2Cl2. The
UV product B showed a conversion to products A and D. The same behavior was observed in
the TCNE chemistry of the [5]cumulene [5]tBuPh (Figure 5.8). Consequently, it was assumed
that product B described the cyclobutane derivative consisting of two exocyclic allene groups
(analogous to compound 5.11 in Section 5.1.2) converting to A with the assumed structure of
an unsymmetrical cyclic [3]dendralene (analogous to 5.14) and D that was expected to
possess a radialene structure analogous to compound 5.15 in Section 5.1.2 (Figure 5.22).
Figure 5.22 Reaction of [5]oTol with TCNE, and the associated TLC analysis. Proposed
product structures are shown as A, B, and D.
202
Crystallization attempts for the products from the reaction of [5]oTol with TCNE were
not successful. Thus, no product from each of the fractions could be characterized by X-ray
crystallography. Product D was not infinitely stable as observed during the crystallization
attempts, and after several days, decomposition occurred in methanol and toluene as
confirmed via TLC analysis. In hexanes, D appeared to be more stable. The product D was
assumed to be the analog of radialene 5.15 in Section 5.1.2 and was treated with Br2 to
observe if the same behavior as for 5.15 (Figure 5.8) was present (see Scheme 5.7). The spot
of product D disappeared when a huge excess of Br2 was used, but no products could be
isolated or characterized. Using less Br2 did not significantly change the outcome of the
reaction, although even after heating the reaction mixture to reflux, the spot of the starting
material was still present. The TCNE reaction with [5]oTol was repeated once again in order
to establish the radialenic structure for D. Product D was isolated, and a 1H NMR spectrum
was recorded but too many and broadened signals were observed that hindered assignment to
the assumed radialenic structure.
5.3 Addition reaction of a [9]cumulene with HCl
5.3.1 Synthesis of [5]cumulene 5.31
Several optimization reactions for the synthesis of [9]tBuPh were attempted and are
outlined in Section 2.1.2.5. One reaction was the synthesis of [9]tBuPh performed in CH2Cl2
instead of Et2O to facilitate the following crystallization attempts. Hence, the reaction was
conducted under standard conditions, i.e., argon atmosphere, low temperature, and using
anhydrous SnCl2 and HCl (Scheme 5.13). After addition of HCl to the reaction mixture, the
solution color turned to blue indicating the formation of [9]cumulene [9]tBuPh, however, this
color immediately turned violet/pink. The reaction mixture then changed from violet/pink to
orange brown after several days. Initial TLC analysis of the reaction mixture indicated two
spots, a pink spot (A) and a violet spot (B) right below (Scheme 5.13). The stability of the
pink product A appeared to be much higher than the violet product B. Crystallization attempts
in CH2Cl2/MeOH were made for both products, but only in the case of the pink product A,
single crystals could be obtained. The structure of product A was assigned as 5.31 via X-ray
crystallographic analysis (Scheme 5.13 and Table 5.3). Two equivalents of HCl were added to
both β-bonds of the [9]cumulene [9]tBuPh during the reaction. Influence of solvent was
203
certainly one possible reason for the different outcome in CH2Cl2 compared to Et2O. It was,
however, not clear, how the solvent affected the formation of 5.31. Stabilization effects of the
chlorinated solvent could be one possibility.
Scheme 5.13 Synthesis of [5]cumulene 5.31 via reductive elimination of tetrayne 2.14 in
CH2Cl2, and the associated TLC analysis.
The synthesis of [5]cumulene 5.31 was reproducible as well as the crystallization with
CH2Cl2 and MeOH affording stable deep pink crystals of compound 5.31. In addition, if the
same reaction was repeated at –78 °C instead of 0 °C, HCl addition could be suppressed
giving only a blue solution indicating the [9]cumulene [9]tBuPh showing no following color
change to pink.
5.3.2 Characterization of [5]cumulene 5.31 via UV/vis spectroscopy and X-ray
crystallography
Figure 5.23 presents the qualitative UV/vis spectrum of [5]cumulene 5.31 recorded in
Et2O. The absorption pattern is very similar to other [5]cumulenes showing the most intense
204
absorption in the low energy region (541 nm) followed by a less intense band in the high
energy region (301 nm) and finally a weak absorption in between at 434 nm.
Figure 5.23 Qualitative UV/vis spectrum of [5]cumulene 5.31 (in Et2O).
The value of the lowest energy absorption λmax is 541 nm and red-shifted compared to
[5]tBuPh (500 nm in Et2O), [5]Mes (460 nm in Et2O) as well as [5]Ph (488 nm in CHCl3; for
a complete description of [5]cumulene absorption characteristics, see Section 4.1.2.3). This
red-shift is probably explained by the increased conjugation of the molecule including the
cumulene chain, the ethylene units, and the aryl groups (Figure 5.24).
205
Figure 5.24 Two canonical structures of [5]cumulene 5.31.
The X-ray structure of 5.31 is shown below in Table 5.3 including relevant carbon
atom labels. The comparison of bond angles of the cumulene chain shows that the values
deviate slightly more from the ideal value of 180° compared to other [5]cumulenes. Known
[5]cumulenes show bond angles in a range from 178.8° to 179.7° (Section 4.2). The bond
angles of 5.31 are 175.5° (C3-C4-C5) and 178.2° (C4-C5-C5‘).
The bond lengths in the cumulene chain of 5.31 are consistent with the bond lengths of
other [5]cumulenes that are discussed in Section 4.2 (Table 5.3). The terminal α-bond length
(C3-C4) is the longest with a value of 1.340(3) Å while the β-double bond (C4-C5) is the
shortest bond and resembles more a triple bond with 1.251(3) Å. The value for the central γ-
double bond (C5-C5‘) at 1.308(5) Å lies between the α- and β-bond lengths, resulting in a
BLA value of 0.057, which resembles the values of [5]Ph (0.058) and [5]tBuPh (0.054) the
most.
The bond length value of the double bond C1-C2 is 1.348(3) Å and slightly longer
than an isolated double bond (1.34 Å) while the single bond length of C2-C3 is 1.445(3) Å
and thus slightly shorter than the bond length of an isolated C-C bond that is attached to sp2-
hybridized carbons, i.e., 1.47 Å. This can probably be explained by the canonical structures
(Figure 5.24), i.e., the C1-C2 bond possesses more single bond character while the C2-C3
bond possesses more double bond character.
206
Table 5.3 Bond lengths (Å) and bond angles (°) of [5]cumulene 5.31.[a]
Cmpd C1-C2 C2-C3 C3-C4 C4-C5 C5-C5’ C3-C4-C5 C4-C5-C5’ BLA[b]
5.31 1.348(3) 1.445(3) 1.340(3) 1.251(3) 1.308(5) 175.5(3) 178.2(3) 0.057
[a] ORTEP drawing (20% probability level) of 5.31. [b] Calculated as difference in bond
length between the two central-most bonds.
5.4 Dimerization of [5]tBuPh
During my doctoral thesis, it was possible to prepare [5]tBuPh on a large scale (>5 g)
due to the straightforward synthetic pathway and the good stability of that compound (Figure
5.25). Thus, several studies could be pursued in order to examine the reactivity of [5]tBuPh.
207
Figure 5.25 Scale-up of [5]tBuPh.
Aside from addition reactions of TCNE to [5]tBuPh (see Section 5.1), the thermal
dimerization reaction was explored with [5]tBuPh. Hartzler38 and Iyoda39 reported the
dimerization of [5]cumulenes affording [4]radialenes with four exocyclic allene units
(Scheme 5.14). The [5]cumulenes they have used were substituted by alkyl groups and were
converted to the [4]radialenes by a thermal reaction in the solid state.
• •
R
R••
R
R ••
••
R
R R
R
R
R R
R
thermal
dimerization
α β γ
R
R
=
R = t-Bu
Scheme 5.14 Thermal dimerization of [5]cumulenes reported by Hartzler38 and Iyoda.39
In contrast, Kawamura40 reported about oligomerization of the phenyl-substituted
[5]cumulene [5]Ph, which was heated to reflux in toluene affording a trimer that was assumed
to be synthesized via a [4]radialene intermediate with four exocyclic allene units (Scheme
5.15). Based on these results, similar dimerization routes were investigated with [5]tBuPh.
208
Scheme 5.15 Thermal trimerization of [5]Ph reported by Kawamura.40
5.4.1 Synthesis of the dimer of [5]tBuPh
The [5]cumulene [5]tBuPh was heated to reflux in toluene (Figure 5.26). After 5 days,
new compounds were formed as indicated by TLC analysis. The orange red spot (C) of
[5]tBuPh was converted into a new pink spot (product A) with lower polarity compared to
[5]tBuPh. Product A appeared to be unstable and tended to decompose, forming an additional
weak grey baseline spot. Product A was assumed to contain the expected dimer, i.e.,
[4]radialene 5.32 since no other significant spot was observed (Figure 5.27). In addition to A,
a green product spot B and a baseline spot D were also observed, but showed much lower
intensity. The main product A could initially not be purified by flash chromatography using
hexanes and silica gel and remained at the top of the column. Filtration with ethyl acetate led
to decomposition observed by a color change to orange brown. Using a very short plug of
silica gel, it was finally possible to purify the pink fraction by filtration with hexanes.
209
Figure 5.26 Thermal reaction of [5]tBuPh in toluene, and associated TLC analysis.
A 1H NMR spectrum of product A was recorded giving several signals in the aryl
region and the alkyl region indicating the formation of either an unsymmetrical compound or
a mixture of several compounds. 13C NMR spectroscopy of product A also showed several
signals, e.g., three signals at ca. 200 ppm were observed that were consistent with an allenic
species. Mass spectroscopy showed a signal value which fitted to [4]radialene 5.32 (center-to-
center dimer, see Figure 5.27) but also its isomers, i.e., head-to-tail-, head-to-head dimers, etc.
Figure 5.27 [4]Radialene 5.32 as the desired product from the thermal reaction of [5]tBuPh
in toluene. Additionally, the possible head-to-tail and head-to-head dimers are shown.
Crystallization attempts of product A from CH2Cl2/hexanes afforded square-like single
crystals that were investigated by X-ray crystallographic analysis confirming the structure of
[4]radialene 5.32 (Figure 5.27). The crystals were, however, colorless while the reaction
mixture was deeply pink. This result as well as the results from the NMR spectroscopy
suggested the presence of several products during the thermal dimerization of [5]tBuPh that
were either overlayed or not observed (i.e., not UV active) by TLC analysis. The thermal
210
dimerization reaction was repeated several times and showed completely different outcomes
than in the first attempt with far more byproducts. The reaction was not reproducible and
probably concentration-dependent based on the variety of resulting products. The only feature
that was at least partially reproducible in each reaction were the crystallization attempts which
afforded colorless square crystals that resembled crystals of 5.32. Unfortunately, all square
crystals obtained during these subsequent thermal reactions of [5]tBuPh were not suitable for
X-ray crystallographic analysis. Isolation and purification of the square crystals from the
reaction mixture were attempted. The crystals could not be purified by washing with different
solvents due to dissolution. Purification via column chromatography was tried using toluene
as solvent. Compound 5.32, however, could not be accurately assigned to a certain spot on the
TLC plate that significantly complicated separation. Developing agents, such as iodine or
anisaldehyde, did not show any change regarding TLC analysis. With potassium
permanganate as developing agent, one spot became brighter and more intense. After
separation of this fraction, a 1H NMR spectrum was recorded indicating that the dimer could
be present, but another compound was visible in the spectrum, and unfortunately, no 13C
NMR spectrum could be measured due to insufficient amount of the product. In conclusion,
[4]radialene 5.32 could not be identified via TLC analysis, and it was not possible to achieve
reproducibility for the thermal reaction of [5]tBuPh or the separation and purification of 5.32.
A further drawback of this reaction was the reaction time, i.e., the reaction had to be stirred
for at least one week under reflux to afford complete conversion of the starting material.
The synthesis of [4]radialene 5.32 was also attempted using the microwave under the
same thermal reaction conditions in toluene as mentioned in Figure 5.26. After 7 h, however,
no change was observed. Only starting material remained, and no evidence for formation of
other products was found. Hence, the synthesis of 5.32 using the microwave was abandoned.
Another reaction approach to afford [4]radialene 5.32 was the thermal dimerization
reaction in the solid state according to Hartzler38 and Iyoda.39 [5]Cumulene [5]tBuPh was
thus melted and stirred for 15 min at ca. 230 °C. The liquefied material was cooled to rt and
dissolved in CH2Cl2. TLC analysis showed no starting material ([5]tBuPh) but several
fluorescent spots. Unfortunately, crystallization attempts from CH2Cl2 and hexanes were not
successful. The crude material was put through a short plug of silica and washed with
hexanes. A yellow fraction was collected (filtrate). The remaining fractions were flushed out
using toluene (residue). Both the filtrate and the residue were analyzed by NMR spectroscopy,
but neither showed evidence for formation of [4]radialene 5.32. Surprisingly, it appeared that
211
the residue contained only the bis-3,5-di-t-butylphenylketone 2.2, but characterization of this
product was not further pursued.
5.4.2 Characterization of the dimer of [5]tBuPh
The characterization of [4]radialene 5.32 and pink product A, in which the presence of
5.32 is assumed (Figure 5.26) has been limited due to low substance amount and impossible
product separation, respectively. Nevertheless, a UV/vis spectrum of the crude reaction
mixture (pink fraction A in Figure 5.26) has been recorded in toluene and is shown in Figure
5.28. Two broad absorption bands are observed in the high energy region, at 277 and 321 nm,
while the low energy region shows one absorption band at 527 nm with lower intensity. This
band might be derived from the radialene structure of 5.32 or it could also arise from a
different cumulenic species that has been formed during the thermal reaction.
Figure 5.28 Qualitative UV/vis spectrum of the pink reaction mixture (A) as presented in
Figure 5.26 (in toluene).
212
Besides, the characterization of product A via NMR-, IR-, and mass spectra cannot
definitely confirm that [4]radialene 5.32 is contained in product A (as assumed and mentioned
before), although the mass spectrum reveals a signal that is consistent with [4]radialene 5.32.
This signal, however, can also be derived from possible isomers of 5.32. During the work-up
of the reaction in Figure 5.26 and after crystallization attempts, several fractions have been
obtained via separation and filtration methods, however, with none of them matching to
[4]radialene 5.32.
5.4.3 X-ray crystallographic data: Discussion and comparison
Table 5.4 shows the X-ray crystallographic structure of the [4]radialene 5.32. One
other [4]radialene with four exocyclic allenes, namely compound 5.33, is known, and has
been reported by Iyoda.39,41 The endgroups of 5.33 are aliphatic based on tetramethyl indane
(Table 5.4). Values of relevant bond lengths and angles of [4]radialenes 5.32 and 5.33 are
given in Table 5.4. While [4]radialene 5.32 is a C4 symmetric compound with the space group
P4/n, [4]radialene 5.33 shows a lower symmetry with the space group P–1. The bond angles
of the exocyclic allene units deviate from the ideal value of 180° with 176.3(4)° for 5.32 and
173.9(3)° and 177.8(3)° for 5.33. The angles of the cyclobutane are rectangular with values of
89.996(1)° for 5.32 as well as 90.2(3)° and 89.8(3)° for 5.33. The bond lengths of both
radialenes are similar except for the bond lengths C1-C2 (or C4-C5) that are longer for the
aryl-substituted radialene 5.32 (1.325(5) Å) compared to the alkyl-substituted radialene 5.33
(1.300(4) and 1.298(4) Å). The exocyclic double bonds C2-C3 and C5-C6 of both radialenes
range from 1.304 to 1.309 Å. The bond lengths of the cyclobutane ring are 1.497(5) Å for
5.32, as well as 1.499(4) and 1.510(4) Å for 5.33, which are slightly longer as the bond
lengths of single C-C bonds attached to sp2-hybridized carbons (1.47 Å), but similar to a
number of other [4]radialenes (Table 5.2).
213
Table 5.4 Bond lengths (Å) and bond angles (°) of [4]radialene 5.32 (left) and Iyoda’s
[4]radialene 5.33 (right).[a]
•
•
•
•
C1
C2C3
C4
C5
C6C7C8
5.33
Compound 5.32 5.33
C1-C2 1.325(5) 1.300(4)
C2-C3 1.308(5) 1.309(4)
C3-C3’ 1.497(5) 1.499(4)[b]
C4-C5 1.298(4)
C5-C6 1.304(4)
C6-C7 1.510(4)
C1-C2-C3 176.3(4) 173.9(3)
C4-C5-C6 177.8(3)
C3-C3’-C3’’ 89.996(1)
C8-C3-C6 90.2(3)
C3-C8-C7 89.8(3)
[a] ORTEP drawing (20% probability level) of 5.32. [b] C3-C6.
5.5 Conclusion and summary
The addition reaction of TCNE to [5]cumulene [5]tBuPh gave several unique products
that were separated and purified. One of these products, radialene 5.15, showed interesting
214
solvatochromism with a lowest energy absorption (λmax value >700 nm) approaching the
near-IR region. The reaction showed a high reproducibility, and it was possible to use the
products for further modifications by e.g., addition reactions (using Br2) to build up
interesting functionalized building blocks which could probably serve as precursors to
unprecedented conjugated structures. In addition, crystallizations of several products, such as
ROH adducts 5.12/5.13, radialene 5.15, and dendralene 5.17 were successful and
reproducible.
TCNE addition reactions to other [n]cumulenes, i.e., cumulenes [7]tBuPh, [5]MeOPh,
or [5]oTol were not successful and showed no reproducibility, high reaction times, as well as
high instability of the resulting products. Thus, characterization and identification of the
outcome of the reactions was limited.
During several optimization reactions of the reductive elimination to [9]tBuPh using
SnCl2 and HCl in CH2Cl2, an addition of two equivalents of HCl to [9]tBuPh showed the
successful and reproducible formation of a two-fold HCl adduct, i.e., [5]cumulene 5.31.
Through variation of the addition reagents and/or the nature of the [n]cumulene, such
reactions could be promising for the synthesis of substituted or functionalized cumulenes.
Finally, thermal dimerization attempts of [5]tBuPh gave one interesting new
compound, i.e., [4]radialene 5.32 with four exocyclic allene units. The thermal reaction was,
however, not reproducible. Furthermore, the reaction times were long, and the outcome
showed too many products that could not be separated, purified, or characterized.
Consequently, the [4]radialene could not be reproduced and no further investigations could be
done.
Many of the results described above show interesting and promising chemistry using
[n]cumulenes as precursors in (cyclo)addition reactions. Unfortunately, the characterization of
some of these reactions and their products have not been completely studied within the time
of this thesis. Thus, there are a number of future directions to be considered that result from
our preliminary studies of the reactivity of [n]cumulenes:
1. The reaction of a [7]cumulene with TCNE affords interesting products, one of
which shows a lowest energy absorption value in the near-IR region as observed by
UV/vis spectroscopy. Since purely organic chromophores that absorb in the IR are
rare, this reaction should be repeated, and the structure of this product should be
determined.
215
2. It is also intriguing to perform TCNE addition reactions with [n]cumulenes
containing different endgroups. Firstly, the variables concerning the reaction of
[5]MeOPh with TCNE should be determined, since there are several outstanding
questions remaining from the three reactions done during this thesis. This is
especially important, since three interesting products have already been confirmed
by X-ray crystallographic analysis.
3. The addition of HCl to the cumulene chain of a [9]tBuPh seems to show selectivity
for the second outermost double bonds, as demonstrated with the synthesis of 5.31.
The reaction conditions for this and perhaps related reactions should be established.
Also, the resulting product, a [5]cumulene with unsymmetrical substitution at the
termini, is thus interesting for the formation of functionalized cumulenes.
4. It has been established that thermal dimerization reactions of cumulenes can afford
radialene-like structures, such as 5.32, but little remains known about such products.
Iyoda and coworkers have already suggested that “such π-extended radialenes are
expected to constitute novel oligocumulene systems with interesting structures”,39,41
and the work from this thesis provides an outstanding starting point to expand on
Iyoda’s suggestion.
5.6 Experimental part
5.6.1 General procedures and methods
The general procedures and methods are analogous to that in Section 2.3.2.
Tetracyanoethylene (TCNE) was stored at low temperature (–25 °C) and weighed in the glove
box (under an argon atmosphere). Ar has been defined as 3,5-di-t-butylphenyl in the Section
5.6.2.
216
5.6.2 Experimental data and compound characterization
Allene 5.14. TCNE (23 mg, 0.18 mmol) was dissolved in dry CH2Cl2 (10 mL) under an Ar
atmosphere, and [5]cumulene [5]tBuPh (0.15 g, 0.18 mmol) in dry CH2Cl2 (15 mL) was
added at rt. After stirring for 6 d, the solvent was removed, and purification by column
chromatography (silica gel, hexanes/CH2Cl2 1:1) afforded product 5.14 as a yellow-orange
solid (0.117 g, 68%). Mp. 112–114 °C. Rf = 0.56 (hexanes/CH2Cl2 1:1), UV/vis (CHCl3) λmax
(ε) 292 (23800), 351 (19270), 457 (15600) nm. IR 3068 (vw), 2963 (s), 2907 (m), 2869 (w),
2221 (w), 1920 (vw, br), 1593 (m) cm−1; 1H NMR (300 MHz, CDCl3) δ 7.61 (t, J = 1.7 Hz,
1H), 7.53 (t, J = 1.7 Hz, 1H), 7.49 (t, J = 1.7 Hz, 2H), 7.29 (d, J = 1.7 Hz, 4H), 7.19 (d, J =
1.7 Hz, 2H), 7.12 (d, J = 1.7 Hz, 2H), 1.310–1.305 (m, 72H); 13C NMR (75 MHz, CDCl3) δ
203.4, 163.4, 162.3, 152.2, 151.9, 151.5, 136.8, 136.4, 131.6, 127.8, 126.8, 125.6, 125.5,
125.0, 124.1, 124.0, 122.4, 112.9, 111.7, 110.4, 104.2, 73.0, 41.2, 35.15, 35.13, 35.0, 31.4,
31.3, 31.2. MALDI MS m/z 957 ([M]+, 100); ESI HRMS m/z calcd. for C68H84N4Na ([M +
Na]+) 979.6588, found 979.6557.
Radialene 5.15. TCNE (31 mg, 0.24 mmol) was dissolved in dry CH2Cl2 (20 mL) under an
Ar atmosphere, and [5]cumulene [5]tBuPh (0.20 g, 0.24 mmol) in dry CH2Cl2 (10 mL) was
added at rt. After stirring for 3 d, the solvent was removed, and purification by column
chromatography (silica gel, hexanes/CH2Cl2 1:1) gave the radialene product 5.15 as a green
solid (17 mg, 7%). Mp. >270 °C (decomp). Rf = 0.10 (hexanes/CH2Cl2 1:1), UV/vis (CHCl3)
λmax (ε) 264 (18500), 331 (19600), 357 (25900), 381 (22500), 457 (11500), 493 (10700), 771
(3900) nm. IR (ATR) 3060 (vw), 3022 (vw), 2958 (s), 2905 (m), 2867 (m), 2217 (w), 2007
217
(vw), 1979 (vw), 1607 (m) cm−1; IR (KBr) 3063 (vw), 2961 (s), 2906 (m), 2870 (w), 2255
(w), 2218 (w), 1611 (w), 1476 (m) cm−1; 1H NMR (400 MHz, CDCl3) δ 7.52 (t, J = 1.6 Hz,
2H), 7.11 (t, J = 1.5 Hz, 2H), 7.04 (s, 4H), 6.91 (s, 4H), 1.29 (s, 36H), 1.10 (s, 36H); 13C
NMR (100 MHz, CDCl3) δ 167.4, 152.3, 151.8, 149.7, 139.4, 138.3, 135.0, 127.7, 126.7,
126.4, 125.2, 113.3, 110.2, 74.0, 34.9, 34.5, 31.33, 31.29. MALDI MS m/z 957 ([M]+, 100);
ESI HRMS m/z calcd. for C68H84N4Na ([M + Na]+) 979.6588, found 979.6576.
Crystal data for 5.15: C68H84N4, M = 957.39; monoclinic crystal system; space group C2/c, a
= 22.9875(9) Å, b = 23.8037(7) Å, c = 13.4123(6) Å, β = 119.248(5)°, V = 6403.4(4) Å3, Z =
4, ρcalcd = 0.993 mg mm–3; µ(CuKα) = 0.429 mm–1; λ = 1.54184 Å; 173.00(10) K; 2θ max =
147.02°; total data collected = 10892; R1 = 0.0498 [4651 observed reflections with
F ≥ 4σ(F)]; wR2 = 0.1460 for 350 variables, 6149 unique reflections, and 3 restraints; residual
electron density = 0.18 and –0.22 e Å–3. One t-butyl group showed disorder, which have been
resolved and refined to the following occupation factors: C18/C19/C20:C18a/C19a/C20a =
86:14%. CCDC 967293.
Ethyl vinyl ether 5.12. Compound 5.14 (8.2 mg, 0.0086 mmol) was dissolved in CH2Cl2
(2 mL) at rt, and EtOH (2 mL) was added. After stirring for 2.5 h, the solvent was removed to
give product 5.12 as a dark yellow solid (8.5 mg, quant). Rf = 0.47 (hexanes/CH2Cl2 1:1); 1H
NMR (400 MHz, CDCl3) δ 7.85 (bs, 1H), 7.61 (t, J = 1.7 Hz, 1H), 7.52 (t, J = 1.8 Hz, 1H),
7.36 (t, J = 1.8 Hz, 1H), 7.27 (t, J = 1.8 Hz, 1H), 7.17 (d, J = 1.7 Hz, 2H), 7.07 (d, J = 1.8 Hz,
2H), 6.95 (d, J = 1.5 Hz, 2H), 6.83 (bs, 1H), 4.57 (s, 1H), 3.84–3.79 (m, 1H), 3.53–3.48 (m,
1H), 1.43 (s, 9H), 1.31 (s, 18H), 1.28 (s, 18H), 1.26 (s, 18H), 1.22 (s, 9H), 1.05 (t, J = 7.0 Hz,
3H); 13C NMR (100 MHz, CDCl3) δ 170.3, 160.1, 153.0, 151.9, 151.1, 150.2, 144.8, 139.6,
138.2, 136.5, 136.1, 134.0, 126.4, 125.7, 125.5, 125.3, 124.8, 124.5, 123.7, 122.2, 121.2,
115.3, 111.0, 110.8, 109.9, 69.7, 61.6, 38.2, 35.5, 35.1, 35.0, 34.8, 34.7, 31.5, 31.38, 31.36,
31.2, 15.3. Note that there appears to be restricted rotation of one of the aryl rings of 5.12,
which leads to non-degeneracy of protons signals (at 7.85 and 6.83 ppm) as well as carbon
218
signals (an “extra” quaternary carbon signal of the t-butyl group at 35.5–34.7 ppm). MALDI
MS m/z 1003 ([M]+, 100); ESI HRMS m/z calcd. for C70H90N4NaO ([M + Na]+) 1025.7007,
found 1025.6985.
Crystal data for 5.12: C70H90N4O, M = 1003.46; triclinic crystal system; space group P–1, a =
13.6915(6) Å, b = 15.8594(10) Å, c = 15.9165(8) Å, α = 82.646(5)°, β = 76.612(4)°, γ =
86.974(5)°, V = 3333.6(3) Å3, Z = 2, ρcalcd = 1.000 mg mm–3; µ(MoKα) = 0.441 mm–1; λ =
1.54184 Å; 173.0 K; 2θ max = 148.74°; total data collected = 23364; R1 = 0.0580 [9670
observed reflections with F ≥ 4σ(F)]; wR2 = 0.1679 for 701 variables, 13030 unique
reflections, and 0 restraints; residual electron density = 0.538 and –0.335 e Å–3. CCDC
967294.
Methyl vinyl ether 5.13. Compound 5.14 (9.0 mg, 0.0094 mmol) was dissolved in CH2Cl2
(2 mL) at rt, and MeOH (2 mL) was added. After stirring for 2.5 h, the solvent was removed
to give product 5.13 as a yellow-orange solid (9.3 mg, quant.). Rf = 0.46 (hexanes/CH2Cl2
1:1); 1H NMR (400 MHz, CDCl3) δ 7.69 (bs, 1H), 7.62 (t, J = 1.7 Hz, 1H), 7.52 (t, J = 1.7 Hz,
1H), 7.38 (t, J = 1.7 Hz, 1H), 7.29 (t, J = 1.7 Hz, 1H), 7.18 (d, J = 1.6 Hz, 2H), 7.13 (d,
J = 1.8 Hz, 2H), 7.00 (d, J = 1.6 Hz, 2H), 6.85 (bs, 1H), 4.58 (s, 1H), 3.53 (s, 3H), 1.40 (s,
9H), 1.32 (s, 18H), 1.30 (s, 18H), 1.27 (s, 18H), 1.24 (s, 9H); 13C NMR (100 MHz, CDCl3) δ
170.1, 161.7, 153.0, 151.8, 151.5, 151.3, 150.3, 145.1, 139.4, 137.8, 137.0, 136.0, 134.0,
126.9, 125.9, 125.5, 125.3, 125.0, 124.6, 124.3, 123.2, 122.3, 121.3, 115.0, 111.5, 111.3,
109.8, 61.4, 61.1, 38.4, 35.3, 35.12, 35.06, 34.84, 34.76, 31.4, 31.2. Note that there appears to
be restricted rotation of one of the aryl rings of 5.13, which leads to non-degeneracy of proton
signals (at 7.69 and 6.85 ppm) as well as carbon signals (an “extra” quaternary carbon signal
of the t-butyl group at 35.3–34.76 ppm). MALDI MS m/z 989 ([M]+, 100); ESI HRMS m/z
calcd. for C69H88N4NaO ([M + Na]+) 1011.6850, found 1011.6853.
Crystal data for 5.13: C69H88N4O, M = 989.43; triclinic crystal system; space group P–1, a =
14.6553(7) Å, b = 15.5967(7) Å, c = 16.2744(9) Å, α = 75.549(4)°, β = 75.159(5)°, γ =
219
72.932(4)°, V = 3375.7(3) Å3, Z = 2, ρcalcd = 0.973 mg mm–3; µ(CuKα) = 0.430 mm–1; λ =
1.54184 Å; 173.1(4) K; 2θ max = 122.86°; total data collected = 15387; R1 = 0.0578 [8483
observed reflections with F ≥ 4σ(F)]; wR2 = 0.1737 for 697 variables, 10028 unique
reflections, and 30 restraints; residual electron density = 0.42 and –0.39 e Å–3. Two t-butyl
groups showed disorder, which have been resolved and refined to the following occupation
factors: C68/C70 = 58:42%; C72/C74 = 67:33%. CCDC 967295.
Cyclobutane 5.11. TCNE (15 mg, 0.12 mmol) was dissolved in dry CH2Cl2 (5 mL) under an
Ar atmosphere, and [5]cumulene [5]tBuPh (0.10 g, 0.12 mmol) in dry CH2Cl2 (10 mL) was
added at –25 °C. After stirring for 1 d at –25 °C, the solvent was removed, and the resulting
solid was purified by column chromatography. In order to prevent conversion of the
compound 5.11 into 5.14 or 5.15, the fraction was kept cold (≤0 °C), and column
chromatography (silica gel, CDCl3) was performed with a jacketed column cooled to –20 to
0 °C through the use of a constant temperature cryo-cool. Collected fractions were kept
cooled on dry ice, and the CDCl3 solvent was removed under vacuum keeping the solution at
<0 °C. This ultimately gave the desired product 5.11 dissolved in a solution of CDCl3. Rf =
0.47 (CHCl3). Rf = 0.40 (hexanes/CH2Cl2 1:1). 1H NMR (500 MHz, CDCl3) δ 7.39 (br s, 4H),
7.16 (d, J = 2.0 Hz, 8H), 1.17 (s, 72H); 13C NMR (126 MHz, CDCl3) δ 203.6, 151.2, 131.2,
128.7, 124.1, 123.7, 109.9, 99.3, 42.9, 34.7, 31.2. ESI HRMS m/z calcd. for C68H84N4Na ([M
+ Na]+) 979.6588, found 979.6575.
Dendralene 5.17. TCNE (31 mg, 0.24 mmol) was dissolved in dry CH2Cl2 (20 mL) under an
Ar atmosphere at –15 °C, and [5]cumulene [5]tBuPh (0.20 g, 0.24 mmol) in dry CH2Cl2
220
(10 mL) was added. After stirring for 1 d, the solvent was removed, and purification by
column chromatography cooled to 0 to (–10 °C) (silica gel, CHCl3) gave compound 5.11 in
CHCl3 (ca. 200 mL). To this solution was added a huge excess of bromine (2.0 mL, 6.2 g,
39 mmol) at –15 °C. The solution was stirred for 1 d, and additional bromine (1.0 mL, 3.1 g,
19 mmol) was added, and the reaction mixture was stirred for 1 d. An aq sodium bisulphate
solution (200 mL) was added. The separated organic phase was washed with saturated aq
NH4Cl (150 mL) and brine (150 mL), dried over Na2SO4, filtered, and the solvent was
removed. Purification by column chromatography (silica gel, CH2Cl2/hexanes = 1:1) afforded
the desired product 5.17 as a brown orange solid (0.070 g, 26%, over two steps). Mp. 214–
218 °C. Rf = 0.64 (hexanes/CH2Cl2 1:1), IR 3063 (vw), 2959 (s), 2906 (m), 2869 (m), 1591
(s), cm−1; 1H NMR (300 MHz, CD2Cl2) δ 7.45–7.44 (m, 4H), 7.35 (d, J = 1.8 Hz, 4H), 7.16
(bd, J = 1.3 Hz, 4H), 1.31 (s, 36H), 1.30 (s, 36H); 13C NMR (100 MHz, CD2Cl2) δ 161.7,
155.3, 151.3, 138.6, 137.3, 124.4, 123.6, 122.8, 111.0, 108.9, 104.0, 95.9, 35.3, 35.2, 31.45,
31.36. ESI HRMS m/z calcd. for C68H8479Br2N4Na ([M + Na]+) 1137.49550, found
1137.49503.
Crystal data for 5.17: C68H84Br2N4, M = 1117.21, monoclinic crystal system; space group
P21/c, a = 17.69599(15) Å, b = 27.7992(2) Å, c = 13.65440(10) Å, β = 102.2304(8)°, V =
6564.60(9) Å3, Z = 4, ρcalcd = 1.130 mg mm–3; µ(CuKα) = 1.856 mm–1; λ = 1.54184 Å;
153.00(10) K; 2θ max = 148.56°; total data collected = 22669; R1 = 0.0389 [11660 observed
reflections with F ≥ 4σ(F)]; wR2 = 0.1047 for 704 variables, 12781 unique reflections, and 3
restraints; residual electron density = 0.73 and –0.44 e Å–3. One t-butyl group showed
disorder, which have been resolved and refined to the following occupation factors:
C72/C73/C74:C72’/C73’/C74’ = 58:42%. CCDC 967296.
Dendralene 5.19. To a solution of compound 5.15 (3 mg, 0.003 mmol) in CHCl3 (ca. 1 mL)
was added a huge excess of bromine (ca. 0.6 g, 0.2 mL, 4 mmol) at 0 °C. After stirring
221
overnight, aq sodium bisulphate solution (5 mL) was added. The organic phase was separated,
washed with saturated aq NH4Cl (5 mL) and brine (5 mL), dried over Na2SO4, and filtered.
The solvent was removed to give the desired product 5.19 as a red solid (quantitative
conversion according to TLC). Mp. 185–195 °C. Rf = 0.31 (hexanes/CH2Cl2 1:1), IR 3073
(vw), 2958 (s), 2868 (m), 2219 (w), 1589 (m), 1461 (s) cm−1; ESI HRMS m/z calcd. for
C68H8379Br2N4 [M – Br]+ 1113.4979, found 1113.4975, for C68H83
79Br3N4Na ([M + Na]+)
1215.4060, found 1215.4063. 1H and 13C NMR spectra were recorded but due to the
unsymmetrical structure of 5.19, neither spectrum offered useful data toward confirming the
structure of 5.19.
Crystal data for 5.19: C68H83Br3N4, M = 1196.11; monoclinic crystal system; space group
P21/c, a = 14.0086(3) Å, b = 20.6382(4) Å, c = 23.1676(4) Å, β = 99.0353(17)°, V =
6615.0(2) Å3, Z = 4, ρcalcd = 1.201 mg mm–3; µ(CuKα) = 2.555 mm–1; λ = 1.54184 Å;
153.00(10) K; 2θ max = 140.84°; total data collected = 21558; R1 = 0.0368 [10421 observed
reflections with F ≥ 4σ(F)]; wR2 = 0.0998 for 698 variables, 12270 unique reflections, and 3
restraints; residual electron density = 0.63 and –0.69 e Å–3. One t-butyl groups showed
disorder, which have been resolved and refined to the following occupation factors:
C88/C89/C90:C88a/C89a/C90a = 87:13%. CCDC 967297.
Dendralene 5.20. To a solution of compound 5.15 (0.010 g, 0.011 mmol) in CHCl3 (5 mL)
was added an excess of bromine (32 mg, 0.010 mL, 0.20 mmol) at 0 °C. After stirring for 3 h,
additional bromine (0.16 g, 0.051 mL, 1.0 mmol) was added over 2 h in 0.010 mL portions.
After stirring overnight, aq sodium bisulphate solution (10 mL) was added. The organic phase
was separated, washed with saturated aq NH4Cl (10 mL) and brine (10 mL), dried over
Na2SO4, and filtered. The solvent was removed, and purification by flash column
chromatography (silica gel, hexanes/CH2Cl2 1:1) gave the desired product 5.20 as a dark
brown solid (quantitative conversion according to TLC). Mp. 186–187 °C. Rf = 0.32
(hexanes/CH2Cl2 1:1), IR 3065 (vw), 2958 (s), 2904 (m), 2867 (m), 2216 (m), 1587 (m), 1463
(s) cm−1; ESI HRMS m/z calcd. for C68H8479Br2N4Na ([M + Na]+) 1137.4955, found
222
1137.4981, for C68H8479BrN4 ([M – Br]+) 1035.5874, found 1035.5878. Note that 1H and 13C
NMR spectra could be obtained for compound 5.20, but signals in the resulting spectra (see
Appendix) were severely broadened and could not be interpreted.
Products B, C, and D from Figure 5.20 (the “first” reaction). TCNE (5.1 mg, 0.040 mmol)
was dissolved in dry CH2Cl2 (10 mL) under an Ar atmosphere, and [5]cumulene [5]MeOPh
(0.02 g, 0.04 mmol)* in dry CH2Cl2 (10 mL) was added at rt. After stirring for 9 d under Ar,
the solvent was removed, and the resulting product mixture separated by column
chromatography (silica gel, CH2Cl2) to afford products B, C, and D. Each product was
crystallized from CH2Cl2/hexanes at rt, which provided crystals suitable for X-ray
crystallographic analyses. Aside from X-ray crystallographic analyses, however, no further
characterization was possible due to limited amount of pure sample.
* The sample of [5]MeOPh used for this reaction was from my diploma thesis and of
unknown purity. Thus, this is an approximate value for the amount of [5]MeOPh used in this
reaction.
Crystal data for product B: C40H29N4O4Cl, M = 665.12; monoclinic crystal system; space
group C2/c, a = 22.9058(3) Å, b = 14.51631(18) Å, c = 21.6431(3) Å, β = 101.9104(12)°, V =
7041.56(15) Å3, Z = 8, ρcalcd = 1.255 mg mm–3; µ(CuKα) = 1.336 mm–1; λ = 1.5418 Å;
173.00(10) K; 2θ max = 142.5°; total data collected = 11744; R1 = 0.0442 [5746 observed
reflections with F ≥ 4σ(F)]; wR2 = 0.1229 for 446 variables, 6512 unique reflections, and 0
restraints; residual electron density = 0.28 and –0.28 e Å–3.
Crystal data for product C: C34H28Cl2O4, M = 571.46; triclinic crystal system; space group P–
1, a = 10.5303(5) Å, b = 13.0141(9) Å, c = 13.0624(8) Å, α = 116.326(6)°, β = 101.833(4)°,
γ = 92.661(5)°, V = 1550.98(15) Å3, Z = 2, ρcalcd = 1.224 mg mm–3; µ(MoKα) = 0.244 mm–1;
λ = 0.7107 Å; 173.00(10) K; 2θ max = 57.6°; total data collected = 9966; R1 = 0.0592 [5358
223
observed reflections with F ≥ 4σ(F)]; wR2 = 0.1913 for 365 variables, 6901 unique
reflections, and 0 restraints; residual electron density = 0.37 and –0.94 e Å–3.
Crystal data for product D: C40H29N4O4Cl, M = 665.12; monoclinic crystal system; space
group P21/c, a = 13.6353(7) Å, b = 15.0902(6) Å, c = 17.7149(8) Å, β = 112.406(6)°, V =
3369.8(3) Å3, Z = 4, ρcalcd = 1.311 mg mm–3; µ(CuKα) = 1.396 mm–1; λ = 1.5418 Å;
173.00(10) K; 2θ max = 141.2°; total data collected = 9933; R1 = 0.0683 [3880 observed
reflections with F ≥ 4σ(F)]; wR2 = 0.1936 for 457 variables, 5971 unique reflections, and 0
restraints; residual electron density = 0.34 and –0.21 e Å–3. The Cl atom showed disorder over
two positions with an occupation of 60:40%.
[5]cumulene 5.31. To a solution of 2.14 (60 mg, 0.066 mmol) in CH2Cl2 (10 mL) was added
anhydrous SnCl2 (40 mg, 0.21 mmol) and HCl (1 M in Et2O, 0.3 mL, 0.3 mmol) at 0 °C under
an Ar atmosphere. After 10 min, the solution was filtered through a plug of basic alumina
oxide to afford the purified 5.31 as a pink solution in CH2Cl2. Since the cumulene is not stable
as amorphous solid, crystalline 5.31 was obtained as pink needles (9.4 mg, 15%) by
overlaying a CH2Cl2 solution with MeOH. Mp. ~ 85 °C (decomp. via loss of shining, 168 °C
melt). Rf = 0.75 (hexanes/EtOAc 20:1). IR 3059 (vw), 2956 (s), 2903 (m), 2866 (m), 2004
(m), 1970 (w), 1590 (m), cm−1; UV/vis (Et2O) λmax 238, 301, 434, 541 nm. 1H NMR (400
MHz, CD2Cl2) δ 7.37 (t, J = 1.7 Hz, 2H), 7.34 (t, J = 1.7 Hz, 2H), 7.16 (d, J = 1.7 Hz, 4H),
6.93 (d, J = 1.7 Hz, 4H), 6.75 (s, 2H), 1.27 (s, 36H), 1.25 (s, 36H); 13C NMR (100 MHz,
CD2Cl2) δ 151.2, 151.1, 150.4, 143.8, 139.8, 138.0, 125.5, 124.0, 123.6, 123.0, 122.2, 113.2,
35.1, 35.0, 31.5, 31.4. ESI HRMS m/z calcd. for C66H8735Cl2 ([M + H]+) 949.61793, found
949.61736.
224
Crystal data for 5.31: C68H90Cl6, M = 1120.10; triclinic crystal system; space group P–1, a =
9.5841(4) Å, b = 12.3984(5) Å, c = 15.5663(7) Å, α = 106.942(2)°, β = 99.0353(17)°, γ =
109.954(2)°, V = 1615.65(12) Å3, Z = 1, ρcalcd = 1.151 mg mm–3; µ(MoKα) = 0.304 mm–1; λ =
0.71073 Å; 173.0 K; 2θ max = 54.84°; total data collected = 13782; R1 = 0.0584 [4589
observed reflections with F ≥ 4σ(F)]; wR2 = 0.1724 for 346 variables, 7333 unique
reflections, and 0 restraints; residual electron density = 0.401 and –0.701 e Å–3. Two
molecules CH2Cl2 per unit cell.
1,2,3,4-Tetrakis(2,2-bis(3,5-di-t-butylphenyl)vinylidene)cyclobutane 5.32. Cumulene
[5]tBuPh (0.030 g, 0.036 mmol) was dissolved in toluene (5 mL) and heated to reflux for 4 d
under air. Evaporation of the solvent afforded a red solid that contained more than one
compound according to TLC analysis. The solid was dissolved in CH2Cl2, and this solution
was overlayed with hexanes. This crystallization gave colorless square crystals, which were
characterized by X-ray crystallographic analysis showing that the sample was compound 5.32.
Unfortunately, the formation of 5.32 was not reproducible and no further, meaningful
characterization could be done.
Crystal data for 5.32: C124H168, M = 1658.58; tetragonal crystal system; space group P4/n, a =
19.3824(4) Å, b = 19.382(4) Å, c = 14.8604(7) Å, α = 90.00°, β = 90.00°, γ = 90.00°, V =
5582.8(3) Å3, Z = 2, ρcalcd = 0.987 mg mm–3; µ(CuKα) = 0.403 mm–1; λ = 1.5418 Å; 172.9(3)
K; 2θ max = 102.76°; total data collected = 5632; R1 = 0.0759 [2509 observed reflections
with F ≥ 4σ(F)]; wR2 = 0.2091 for 294 variables, 2972 unique reflections, and 46 restraints;
residual electron density = 0.41 and –0.32 e Å–3. Three t-butyl groups showed disorder, which
have been resolved and refined to the following occupation factors:
C18/C19/C20:C18a/C19a/C20a = 75:25%, C22/C23/C24:C22a/C23a/C24a = 75:25%,
C38/C39/C40:C38a/C39a/C40a = 75:25%.
225
5.7 References
1 N. Islam, M. Tsukayama, Y. Kawamura, Int. J. Mod. Phys. B 2006, 20, 4619–4624.
2 The work on reactions of allenes and TCNE by Kawamura and coworkers has been
presented in several poster contributions, see: The 8th International Symposium on
Functional pi-Electron Systems, Graz, Jul. 2008; 12th International Symposium on
Novel Aromatic Compounds, p.193, Osaka, Jul. 2007.
3 S. Ueta, K. Hida, M. Nishiuchi, Y. Kawamura, Org. Biomol. Chem. 2014, 12, 2784–
2791.
4 B. Bildstein, M. Schweiger, H. Angleitner, H. Kopacka, K. Wurst, K.-H. Ongania, M.
Fontani, P. Zanello, Organometallics 1999, 18, 4286–4295.
5 M. Kivala, F. Diederich, Acct. Chem. Res. 2009, 42, 235–248; S.-i. Kato, F. Diederich,
Chem. Commun. 2010, 46, 1994–2006.
6 M. Štefko, M. D. Tzirakis, B. Breiten, M.-O. Ebert, O. Dumele, W. B. Schweizer, J.-P.
Gisselbrecht, C. Boudon, M. T. Beels, I. Biaggio, F. Diederich, Chem. Eur. J. 2013,
19, 12693–12704.
7 Y.-L. Wu, F. Tancini, W. B. Schweizer, D. Paunescu, C. Boudon, J.-P. Gisselbrecht,
P. D. Jarowski, E. Dalcanale, F. Diederich, Chem. Asian J. 2012, 7, 1185–1190.
8 For examples, see: Y. Li, M. Ashizawa, S. Uchida, T. Michinobu, Polym. Chem. 2012,
3, 1996–2005; T. Shoji, S. Ito, T. Okujima, N. Morita, Chem. Eur. J. 2013, 19, 5721–
5730.
9 B. Breiten, Y.-L. Wu, P. D. Jarowski, J.-P. Gisselbrecht, C. Boudon, M. Griesser, C.
Onitsch, G. Gescheidt, W. B. Schweizer, N. Langer, C. Lennartz, F. Diederich, Chem.
Sci. 2011, 2, 88–93.
10 T. Michinobu, C. Boudon, J.-P. Gisselbrecht, P. Seiler, B. Frank, N. N. P. Moonen, M.
Gross, F. Diederich, Chem. Eur. J. 2006, 12, 1889–1905.
11 R. Gompper, U. Wolf, Tetrahedron Lett. 1978, 44, 4263–4264.
12 J. O. Morley, J. Phys. Chem. 1995, 99, 10166–10174.
13 W. Zhu, Y. Jiang, Phys. Chem. Chem. Phys. 1999, 1, 4169–4173.
14 B. Tinant, J.-P. Declercq, D. Bouvy, Z. Janousek, H. G. Viehe, J. Chem. Soc., Perkin
Trans. 2 1993, 911–915.
15 D. Bouvy, Z. Janousek, H. G. Viehe, B. Tinant, J.-P. Declercq, Tetrahedron Lett.
1993, 34, 1779–1782.
16 Z. Yoshida, Pure & Appl. Chem. 1982, 54, 1059–1074.
226
17 H. Fischer, H. Fischer, Chem. Ber. 1967, 100, 755–766.
18 M. Iyoda, S. Tanake, K. Nishioka, M. Oda, Tetrahedron Lett. 1983, 24, 2861–2864.
19 DFT calculations have been performed in collaboration with Prof. A. Görling (FAU).
20 R. B. Woodward, R. Hoffmann, J. Am. Chem. Soc. 1965, 87, 395–397.
21 L. Leroyer, V. Maraval, R. Chauvin, Chem. Rev. 2012, 112, 1310–1343.
22 Cyclic [3]dendralenes, trimethylenecyclobutanes, have been studied, but little is
known about this class of compounds, see for examples: J. K. Williams, W. H.
Sharkey, J. Am. Chem. Soc. 1959, 81, 4269–4272. L. Trabert, H. Hopf, D. Schomburg,
Chem. Ber. 1981, 114, 2405–2414. W. V. Dower, K. P. C. Vollhardt, Angew. Chem.
Suppl. 1982, 21, 1545–1555; Angew. Chem. 1982, 94, 712. W. T. Thorstad, N. S.
Mills, D. Q. Buckelew, L. S. Govea, J. Org. Chem. 1989, 54, 713–776.
23 The structural assignment of 5.20 rested primarily on the mass spectrum and the
analogous formation and properties to 5.19. 1H and 13C NMR spectra were acquired,
but showed severely broadened signals that limited their usefulness (spectra have been
supplied in the Appendix).
24 Formation of the head-head-dimer could not be ruled out, but no evidence for
supporting formation of that dimer was observed to date.
25 See, for example: R. Pal, R. J. Clark, M. Manoharan, I. V. Alabugin, J. Org. Chem.
2010, 75, 8689–8692.
26 Spotted onto a TLC plate from CH2Cl2, this fraction was initially orange/brown and
slowly turned to green as the solvent evaporated.
27 F. Bureš, O. Pytela, M. Kivala, F. Diederich, J. Phys. Org. Chem. 2011, 24, 274–281;
B. Strehmel, A. M. Sarker, H. Detert, ChemPhysChem 2003, 4, 249–259.
28 A. E. Learned, A. M. Arif, P. J. Stang, J. Org. Chem. 1988, 53, 3122–3123.
29 P. I. Dosa, G. D. Whitener, K. P. C. Vollhardt, A. D. Bond, S. J. Teat, Org. Lett. 2002,
4, 2075–2078.
30 M. Iyoda, H. Otani, M. Oda, Y. Kai, Y. Baba, N. Kasai, J. Am. Chem. Soc. 1986, 108,
5371–5372.
31 S. Hashmi, K. Polborn, G. Szeimies, Chem. Ber. 1989, 122, 2399–2401.
32 R. Boese, D. Bläser, R. Latz, Acta Cryst. Sec. C 1999, 55, IUC9900067.
227
33 Compound [5]MeOPh has been previously synthesized, although the synthetic
procedure was different, see: P. Cadiot, Ann. Chim. [Paris] 1956, 13, 214–272. Cadiot
reported that [5]MeOPh could be isolated as a moderately stable red solid.
34 During my diploma thesis (see ref[36]), [5]MeOPh was synthesized three times, but
could be characterized only once via 1H NMR spectroscopy; the other samples showed
broadened signals in the 1H NMR spectra or the presence of impurities. No 13C NMR
spectrum was recorded. Based on UV/vis spectroscopy and TLC analysis, however, all
three samples could be assigned to [5]MeOPh, but with different grades of purity.
35 Samples derived from two individual syntheses of [5]MeOPh done during the
doctoral thesis. 1H NMR spectra of these two samples, however, showed impurities
and/or broadened signals. TLC analysis of both samples showed similar results as
found for samples of [5]MeOPh synthesized during my diploma thesis.
36 For experimental details, see: Diploma thesis “Carbon in One Dimension – Synthesis
of [n]Cumulenes”, Johanna A. Januszewski, Friedrich-Alexander-Universität
Erlangen-Nürnberg, May 2010.
37 [5]oTol was synthesized by Dominik Wendinger.
38 H. D. Hartzler, J. Am. Chem. Soc. 1971, 93, 4527–4531.
39 Y. Kuwatani, G. Yamamoto, M. Oda, M. Iyoda, Bull. Chem. Soc. Jpn. 2005, 78,
2188–2208.
40 N. Islam, T. Ooi, T. Iwasawa, M. Nishiuchi, Y. Kawamura, Chem. Commun. 2009,
574–576.
41 M. Iyoda, M. Oda, Y. Kai, N. Kanehisa, N. Kasai, Chem. Lett. 1990, 2149–2152.
228
Appendix
Theoretical calculations
Computational details
All calculations were performed using the Turbomole quantum chemistry suite.1
Geometries were optimized adopting the PBE density-functional2 and the def2-TZVP
Gaussian basis set.3 In some cases additional calculations using the B3LYP4 density
functional were conducted, however, the results remained qualitatively unchanged. All
energies refer to the PBE functional.
Since we expected that intramolecular dispersion interaction between the aryl groups
may influence the relative energies of the investigated molecules, all calculations were re-
done applying the semi-empirical D3 dispersion correction according to Grimme et al.5
Atomic charges were calculated using the natural population analysis (NPA) of Reed,
Weinstock, and Weinhold,6 which provides physically reliable charges without significant
basis set dependence.
Computational results
Computed Energies: Energies of all computed compounds (in Hartree)
compound PBE PBE-D3 B3LYP B3LYP-D3
TCNE –447.1728148 –447.1768399 –447.4510811 –447.45789608
7 –2410.6845575 –2410.7959376 –2412.3441510 –2412.5257118
8 –2857.8748950 –2858.0126355 N.A. N.A.
9 –1428.9389218 –1428.9955777 –1429.9103034 –1430.0033091
11 –2857.9210665 –2858.0562208 N.A. N.A.
12 –2857.9290589 –2858.0831090 –2859.8465573 –2860.0889277
229
References
1 Turbomole 6.5, a development of University of Karlsruhe and Forschungszentrum
Karlsruhe GmbH, 1989-2007, Turbomole GmbH, since 2007; available from
www.turbomole.com.
2 J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77, 3865–3868.
3 F. Weigend, R. Ahlrichs, Phys. Chem. Chem. Phys. 2005, 8, 3297–3305.
4 A. D. Becke, J. Chem. Phys. 1993, 98, 5648–5652.
5 S. Grimme, J. Anthony, S. Ehrlich, H. Krieg, J. Chem. Phys. 2010, 132, 154104–
154119.
6 A. E. Reed, R. B. Weinstock, F. Weinhold, J. Chem. Phys. 1985, 83, 735–746.
230
Spectra Appendix
Figure A1. 1H NMR spectrum of 2.1.
Figure A2. 13C NMR spectrum of 2.1.
231
Figure A3. 1H NMR spectrum of 2.2.
Figure A4. 13C NMR spectrum of 2.2.
232
Figure A5. 1H NMR spectrum of 2.3.
Figure A6. 13C NMR spectrum of 2.3.
233
Figure A7. 1H NMR spectrum of 2.4.
Figure A8. 13C NMR spectrum of 2.4.
234
Figure A9. 1H NMR spectrum of 2.5.
Figure A10. 13C NMR spectrum of 2.5.
235
Figure A11. 1H NMR spectrum of [3]tBuPh.
Figure A12. 13C NMR spectrum of [3]tBuPh.
236
Figure A13. 1H NMR spectrum of 2.6.
Figure A14. 13C NMR spectrum of 2.6.
237
Figure A15. 1H NMR spectrum of 2.7.
Figure A16. 13C NMR spectrum of 2.7.
238
Figure A17. 1H NMR spectrum of 2.8.
Figure A18. 13C NMR spectrum of 2.8.
239
Figure A19. 1H NMR spectrum of [5]tBuPh.
Figure A20. 13C NMR spectrum of [5]tBuPh.
240
Figure A21. 1H NMR spectrum of 2.15.
Figure A22. 13C NMR spectrum of 2.15.
241
Figure A23. 1H NMR spectrum of 2.16.
Figure A24. 13C NMR spectrum of 2.16.
242
Figure A25. 1H NMR spectrum of 2.18.
Figure A26. 13C NMR spectrum of 2.18.
243
Figure A27. 1H NMR spectrum of 2.22.
Figure A28. 13C NMR spectrum of 2.22.
244
Figure A29. 1H NMR spectrum of [7]tBuPh.
Figure A30. 13C NMR spectrum of [7]tBuPh.
245
Figure A31. 1H NMR spectrum of 2.23.
Figure A32. 13C NMR spectrum of 2.23.
246
Figure A33. 1H NMR spectrum of 2.24.
Figure A34. 13C NMR spectrum of 2.24.
247
Figure A35. 1H NMR spectrum of 2.25.
Figure A36. 13C NMR spectrum of 2.25.
248
Figure A37. 1H NMR spectrum of 2.12.
Figure A38. 13C NMR spectrum of 2.12.
249
Figure A39. 1H NMR spectrum of 2.14.
Figure A40. 13C NMR spectrum of 2.14.
250
Figure A41. 1H NMR spectrum of S1.
Figure A42. 13C NMR spectrum of S1.
251
Figure A43. 1H NMR spectrum of 2.21.
Figure A44. 13C NMR spectrum of 2.21.
252
Figure A45. 1H NMR spectrum of 2.31.
Figure A46. 13C NMR spectrum of 2.31.
253
Figure A47. 1H NMR spectrum of 2.49 (Synthesis of 2.49 under Hay conditions).
Figure A48. 13C NMR spectrum of 2.49 (Synthesis of 2.49 under Hay conditions).
254
Figure A49. 1H NMR spectrum of 2.49 (Synthesis of 2.49 using ethyl bromoacetate as
oxidant).
Figure A50. 13C NMR spectrum of 2.49 (Synthesis of 2.49 using ethyl bromoacetate as
oxidant).
255
Figure A51. 1H NMR spectrum of 2.49 (Synthesis of 2.49 using I2 as oxidant).
Figure A52. 13C NMR spectrum of 2.49 (Synthesis of 2.49 using I2 as oxidant).
256
Figure A53. 1H NMR spectrum of 2.44.
Figure A54. 13C NMR spectrum of 2.44.
257
Figure A55. 1H NMR spectrum of 2.45.
Figure A56. 13C NMR spectrum of 2.45.
258
Figure A57. 1H NMR spectrum of 3.8.
Figure A58. 13C NMR spectrum of 3.8.
259
Figure A59. 1H NMR spectrum of 3.9.
Figure A59. 13C NMR spectrum of 3.9.
260
Figure A60. 1H NMR spectrum of 3.10.
Figure A61. 13C NMR spectrum of 3.10.
261
Figure A62. 1H NMR spectrum of 3.7.
Figure A63. 13C NMR spectrum of 3.7.
262
Figure A64. 1H NMR spectrum of 5.14.
Figure A65. 13C NMR spectrum of 5.14.
263
Figure A66. 1H NMR spectrum of 5.15.
Figure A67. 13C NMR spectrum of 5.15.
264
Figure A68. 1H NMR spectrum of 5.12.
Figure A69. 13C NMR spectrum of 5.12.
265
Figure A70. 1H NMR spectrum of 5.13.
Figure A71. 13C NMR spectrum of 5.13.
266
Figure A72. 1H NMR spectrum of 5.11.
Figure A73. 13C NMR spectrum of 5.11.
267
Figure A74. 1H NMR spectrum of 5.17.
Figure A75. 13C NMR spectrum of 5.17.
268
Figure A76. 1H NMR spectrum of 5.19.
Figure A77. 13C NMR spectrum of 5.19.
269
Figure A78. 1H NMR spectrum of 5.20.
Figure A79. 13C NMR spectrum of 5.20.
270
Figure A80. 1H NMR spectrum of 5.31.
Figure A81. 13C NMR spectrum of 5.31.
271
List of Publications
1. Synthesis and properties of long [n]cumulenes (n ≥ 5)
J. A. Januszewski, R. R. Tykwinski
Chem. Soc. Rev. 2014, 43, 3184–3203
DOI: 10.1039/c4cs00022f
2. Unexpected Formation of a [4]Radialene and Dendralenes via Addition of TCNE to a
Tetraaryl[5]cumulene
J. A. Januszewski, F. Hampel, C. Neiss, A. Görling, R. R. Tykwinski
Angew. Chem. Int. Ed. 2014, 53, 3743–3747
DOI: 10.1002/anie.201309355
3. Unerwartete Bildung eines [4]Radialens und mehrerer Dendralene bei der Addition
von Tetracyanoethylen an ein Tetraaryl[5]cumulen
J. A. Januszewski, F. Hampel, C. Neiss, A. Görling, R. R. Tykwinski
Angew. Chem. 2014, 126, 3818–3822
DOI: 10.1002/ange.201309355
4. Synthesis and Structure of Tetraarylcumulenes: Characterization of Bond-Length
Alternation versus Molecule Length
J. A. Januszewski, D. Wendinger, C. D. Methfessel, F. Hampel, R. R. Tykwinski,
Angew. Chem. Int. Ed. 2013, 52, 1817–1821
DOI: 10.1002/anie.201208058
5. Synthese und Struktur von Tetraarylcumulenen: Charakterisierung der
Bindungslängenalternanz in Abhängigkeit der Moleküllänge
J. A. Januszewski, D. Wendinger, C. D. Methfessel, F. Hampel, R. R. Tykwinski,
Angew. Chem. 2013, 125, 1862–1867
DOI: 10.1002/ange.201208058
6. Oligomers from sp-Hybridized Carbon: Cumulenes and Polyynes
S. Frankenberger, J. A. Januszewski, R. R. Tykwinski
Series Title: Structure and Bonding, Vol. 159, Book Title: Fullerenes and Other
Carbon-Rich Nanostructures, J.-F. Nierengarten (Ed.), Springer Berlin Heidelberg,
2014, 219–256
DOI: 10.1007/430_2013_110
272
CURRICULUM VITAE
PERSÖNLICHE DATEN
Name Johanna A. Januszewski
Geburtsdaten 16.01.1985 (Bytów, Polen)
Staatsangehörigkeit Deutsch
Familienstand Ledig
AKADEMISCHE UND SCHULISCHE AUSBILDUNG
06/2010 – 08/2014 Dissertation „Synthesis and Characterization of
[n]Cumulenes“ an der Friedrich-Alexander-Universität
Erlangen-Nürnberg unter der Betreuung von Prof. Rik R.
Tykwinski
09/2004 – 05/2010 Studium der Chemie (Diplom) an der Friedrich-
Alexander-Universität Erlangen-Nürnberg
09/1995 – 06/2004 Apian-Gymnasium Ingolstadt
AUSLANDSERFAHRUNGEN
09/2011 Dipartimento di Chimica, Politecnico di Milano, Milan,
Italy, Gastwissenschaftlerin
07/2011 14th International Symposium on Novel Aromatic
Compounds (ISNA-14), Eugene, Oregon, USA
Posterpräsentation
AUSZEICHNUNGEN
07/2010 Zerweck-Preis für herausragende Abschlussarbeiten
273
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