Structural studies of organoboron compounds LVIII.' 4-(1'-azoniabicyclo[2.2.2]- octany1)-2,2-diphenyl-2-borata- 1,3-dioxa-l,2,3,4-tetrahydronaphthalene

WOLFGANG KLIEGEL AND KLAUS DRUCKLER Itlstitut fur Pharmazeutische Chemie der Technischen Universitat Bra~mschweig, Beetlzovetlstrasse 55,

3300 Braurzschweig, Germany

AND

STEVEN J . RETTIG AND JAMES TROTTER^ Department of Chernisrry, University of British Columbia, 2036 Main Mall, Vancouver, B.C., Canada V6T I21

Received November 24. 1992

WOLFGANG KLIEGEL, KLAUS DRUCKLER, STEVEN J. RETTIG, and JAMES TROTTER. Can. J. Chem. 71, 919 (1993). Two synthetic routes leading to 4-(l'-azoniabicyclo[2.2.2]octanyl)-2,2-dipheny1-2-borata-1,3-dioxa-1,2,3,4-tetra-

hydronaphthalene, 5, are described. Crystals of the product are orthorhombic, a = 18.099(2), b = 9.729(2), c = 12.113(2) A, Z = 4, space group Pca2,. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to R = 0.034 and R,,. = 0.037 for 1421 reflections with 1 2 3u(F2). Compound 5 represents the first struc- turally characterized crystalline adduct of a trialkylamine to a carbonyl compound in which the newly formed C-N bond is acyclic. The adduct is stabilized by the neighboring Lewis acid diphenylboryloxyaryl moiety. Bond distances involy- ing the boron atom ((ary1)O-B = 1.5 15(4), (alkyl)O-B = 1.508(4), and C(pheny1)-B = 1.6 13(5) and 1.623(5) A) represent relatively strong overall binding of the 0,O-chelating ligand to the diphenylboron moiety.

-

WOLFGANG KLIEGEL, KLAUS DRUCKLER, STEVEN J. RETTIG et JAMES TROTTER. Can. J. Chem. 71, 919 (1993). On dkcrit deux voies de synthese du 4-(1'-azoniabicyclo[2.2.2]octanyl)-2,2-diphknyl-2-borata-1,3-dioxa-1,2,3,4-

tktrahydronaphtalkqe (5) dont les cristaux sont orthorhombiques, groupe d'espace Pca2,, avec a = 18,099(2), b = 9,729(2) et c = 12,113(2) A et Z = 4. On a rksolu la structure par des mkthodes directes et on I'a affinke par la mkthodes des moindres carrks (matrice complete) jusqu'a des valeurs de R = 0,034 et R,, = 0,037 pour 1421 rkflexions avec I 2 3 4 ~ ' ) . Le composk 5 est le premier adduit cristallin d'une trialkylamine et d'un composk carbonyle avec liaison C-N acycli- que a Ctre caractbrisk. L'adduit est stabilisk par la portion acide de Lewis diphknylboryloxyaryle adjacente. Les longueurs (es liaisons impliquant le bore ((ary1)O-B = 1,515(4), (alkyl)O-B = 1,508(4) et C(phkny1)-B = 1,613(5) et 1,623(5) A) correspondent a des liaisons globales relativement fortes du ligand 0,O-coordinant avec la portion diphknylbore.

[Traduit par la rkdaction]

Introduction The reaction product of salicylaldehyde 1 and the anhy-

dride of diphenylborinic acid has been described in the literature (1) as an intramolecular O+B coordinated 2-di- phenylboryloxybenzaldehyde 2. The chelate structure 2 was established by an X-ray crystallographic analysis (2). It was of great interest to us to determine whether a strong nucleo- phile like a tertiary amine could compete with the carbonyl group as a Lewis base, breaking the chelate ring to form an open-chain N+B adduct 3, or if the electron withdrawal from the carbonyl carbon atom would allow the addition of the nucleophile R,N to the carbonyl group to give the zwitter- ionic adduct 5. A similar structure, 6, has been postulated for an intermediate borate complex in a reversible reaction during the boronic acid-catalyzed hydrolysis of an N-sali- cylidene amino acid (3). The importance of zwitterionic in- termediates in the understanding of catalytic reactions in general (4) and enzyme-catalyzed reactions (3, 4) in partic- ular, as well as our interest in the structure and function of stable B,N-betaines (5, 6), has motivated us to establish un- ambiguously the structure 5. The nucleophilic addition of the amine at the carbonyl group to form the 0,N-acetal moiety in 5 resembles the addition of a hydroxylamine at the sali- cylaldehyde carbonyl group leading to the product 7, the structure of which has been established by an X-ray crystal- lographic analysis (7). Moreover, a complex like 5 would represent a stabilized derivative of the labile zwitterionic

'~revious paper in this series: ref. 19. '~uthor to whom correspondence may be addressed.

primary adduct 4 of an amine to salicylaldehyde. Such an adduct, 4, corresponds directly to the proposed (8) mode of interaction between (tertiary) amines and the en01 tautomers of P-dicarbonyl compounds, the latter being functionally related to salicylaldehyde.

Quinuclidine (1-azabicyclo[2.2.2]octane) was chosen as a suitable trialkylamine having a relatively low degree of steric hindrance and was reacted both directly with the diphenylboron chelate 2 and in an alternative three-com- ponent condensation with salicylaldehyde 1 and oxybis- (diphenylborane). The latter reaction could involve either 2 or 4 as an intermediate. Both reactions produced colorless crystals having the same melting point and identical in- frared spectra. The absence of a C=O absorption in the solid state (KBr pellet) infrared spectrum and the elemental com- position strongly support the expected structure 5. The 'H nmr spectrum (DMSO-d, solution), however, shows two sepa- rate signals for the aldehyde methine proton, one at 6 = 5.72 ppm (typical for an N-CH-0 acetal grouping) and an- other one at 10.25 ppm (typical for an aldehyde (CH=O group), the two peaks comprising 88% and 12%, respec- tively, of the total peak area. A rearrangement by intramo- lecular exchange of the Lewis bases at the diphenylboryloxy moiety (5 e 3) does not seem very likely since the 'H nmr spectrum also displays a peak corresponding to a phenolic OH proton at 10.94 ppm with 12% of the total peak area for one proton. This would indicate a partial hydrolysis (by sol- vent water) resulting in the liberation of salicylaldehyde 1. An X-ray crystallographic analysis was undertaken in order to establish the structure of the crystalline addition product.

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

FLO

RID

A S

TA

TE

UN

IVE

RSI

TY

on

11/1

3/14

For

pers

onal

use

onl

y.

CAN. J. CHEM. VOL. 71, 1993

TABLE 1. Final atomic coordinates (fractional) and B,, (A')"

Atom

"B,, = (8/3).rr2XX~,,a,*uI*(n,~ a,).

Experimental 4-(1'-Azotziabicyclo[2.2.2]octanyl)-2,2-diphenyl-2-borata-l,3-

dioxa-1,2,3,4-tetrahydronaphthalene, 5 (R3N =

quinuclidine)

Method A 2-~i~hen~lborylox~benzaldehyde~ (1) (0.57 g, 2 mmol) is dis-

solved in 150 mL of dry diethyl ether, heating slightly. After the addition of a solution of quinuclidine (0.22 g, 2 mmol) in 10 mL

TABLE 2. Bond lengths (A) with estimated standard deviations

Atom Atom Distance Atom Atom Distance

(41) c(1) 1.355(4) C(10) C(14) 1.500(6) 0(1) B(1) 1.5 15(4) C(I1) C(12) 1.530(5) o(2) c(7) 1.362(4) C(13) C(14) 1.540(5) 0(2) B(1) 1.508(4) C(15) C(16) 1.385(5) N(1) C(1) 1.590(4) C(15) C(20) 1.393(5) N(1) C(8) 1.488(4) C(15) B(1) 1.623(5) N(1) C(l2) 1.501(4) C(16) C(17) 1.394(6) N(1) C(l3) 1.495(5) C(17) C(18) 1.366(8) (31) c(2) 1.395(5) C(18) C(19) 1.380(8) c(1) (36) 1.385(6) C(19) C(20) 1.374(6) c(2) (33) 1.390(6) C(21) C(22) 1.387(5) c(3) c(4) 1.380(7) C(21) C(26) 1.384(5) (34) c(5) 1.364(7) C(21) B(1) 1.613(5) C(5) C(6) 1.399(5) C(22) C(23) 1.380(5) C(6) C(7) 1.504(5) C(23) C(24) 1.357(6) c(8) c(9) 1.525(5) C(24) C(25) 1.366(5) C(9) C(l0) 1.505(7) C(25) C(26) 1.377(5) C(10) C(11) 1.509(6)

of diethyl ether, crystallization starts spontaneously. One hour later the crystalline precipitate is filtered off and washed with diethyl ether. Yield: 0.67 g (85%). The same procedure, using absolute benzene (50 mL and 5 mL, respectively) as reaction medium yields 0.61 g (77%).

Method B Oxybis(dipheny1borane) (0.87 g, 2.5 mmol) is dissolved in

30 mL of absolute benzene and mixed with a solution of quinucli- dine (0.56 g, 5 mmol) in 10 mL of absolute benzene with stirring. Turbidity develops, but disappears after the addition of salicyl- aldehyde (0.61 g, 5 mmol). After 1 h crystallization commences and is complete after about 6 h at room temperature. The crystals are filtered off and washed with diethyl ether. The colorless needles are analytically pure. Yield: 1.73 g (87%); mp 184-1 85°C (dec.).

Infrared (KBr): 1605, 1580 cm-I. 'H nmr (DMSO-$/TMS) 8 (ppm): 1.51-1.98 (m, H-C(CH2),), 2.963.23 (m, N(CH,),), 5.72 (s, 88% of IH, N-CH-0), 6.62-7.70 (m, 14 aromatic H), 10.25 (s, 12% of 1 H, CH=O), 10.94 (s, 12% of IH, OH). Anal. calcd. for C26Hz8BN02: C 78.60, H 7.10, B 2.72, N 3.53; found: C 78.56, H 7.16, B 2.75, N 3.47%. Crystals suitable for X-ray analysis were selected from the batch of crystals obtained from the benzene so- lution method B).

'~lternative nomenclature: (salicylaldehydato)diphenylboron (2); X-ray crystallographic analysis of 5 (R,N = quinuclidine) or systematically: 2,2-diphenyl-2-borata-1,3-dioxa- l,2-dihydro- A crystal ca. 0.05 x 0.10 x 0.35 mm is size was mounted on a naphthalene. glass fiber. Unit-cell parameters were refined by least squares on

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

FLO

RID

A S

TA

TE

UN

IVE

RSI

TY

on

11/1

3/14

For

pers

onal

use

onl

y.

KLIEGEL ET AL.

TABLE 3. Bond angles (deg) with estimated standard deviations

Atom

C(1) C(7) C(7) C(7) C(7) C(8) C(8) C(12) O(1) O(1) C(2) C(1) C(2) C(3) C(4) C(1) C(1) C(5) O(2) O(2) N(1) N(1) C(8) C(9) C(9) C(11)

Atom Atom Angle Atom Atom Atom Angle

TABLE 4. Intra-annular torsion angles (deg) with estimated standard deviations in parentheses

Atoms Value (deg)

O(2)-B(1)-O(1)-C(1) 58.5(3) B(1)-O(1)-C(1)-C(6) -36.3(4) O(1)-C(1)-C(6)-C(7) -5.9(5) C(1)-C(6)-C(7)-O(2) 24.7(4) C(6)-C(7)-O(2)-B(l) 1.6(4) C(7)-O(2)-B(1)-O(1) -40.9(4)

setting angles for 25 reflections (28 = 37.2-64.7°)ameasured on a diffractometer with Cu-K, radiation (A = 1.54178 A). Crystal data at 21°C are as follows: C?6HzsBN02 f.w. = 397.32 Orthorhombic, a. = 18.099(2), b = 9.729(2), c = 12.1 13(2) A, V = 2132.7(5) A ~ , Z = 4 , p, = 1.237 Mg m-', F(000) = 848, ~(CU-K,) = 5.62 cm-I. Absent reflections: Okl, 1 odd, and h01, h odd, space group Pca2, (No. 29) from structure analysis.

Intensities were measured with graphite-monochromated Cu-K, radiation on a Rigaku AFC6S diffractometer. An *28 scan at 32" min-' over a range of (0.94 + 0.20 tan 8)" in w (with up to 8 rescans, background/scan time ratio = 0.5) was employed. Data were measured to 28 = 155". The intensities of three check reflec- tions, measured every 200 reflections throughout the data collec- tion, showed only small random variations. After data reduction,' decay and empirical absorption corrections (based on azimuthal

4 ~ ~ ~ ~ ~ ~ / ~ ~ ~ ~ ~ ~ structure analysis package, which includes versions of the following: MITHRIL, integrated direct methods, by C.J. Gilmore; DIRDIF, direct methods for difference structures, by P.T. Beurskens; ORFLS, full-matrix least squares, and ORFFE, func- tion and errors, by W.R. Busing, K.O. Martin, and H.A. Levy, ORTEP 11, illustrations, by C.K. Johnson.

scans for three reflections) were applied. Transmission factors range from 0.96 to 1.00. Intensities were measured for 2305 indepen- dent reflections of which 1421 had intensities greater than or equal to 3u(F2) above background where u2(F2) = [s'(c + 4B) + (O.O~F~) ' ] /L~ ' (S = scan speed, C = scan count, B = total back- ground count, and Lp = ~orentz-polarization factor) and were employed in the solution and refinement of the structure.

The systematic absences are consistent with the space groups Pcam and Pca2,. The structure was solved by direct methods, the analysis being initiated (and successfully completed) in the non- centrosymmetric space group Pca2, on the basis of the E-statistics and structural considerations. With four molecules in the unit cell, an ordered structure in the centrosymmetric space group Pcam is ruled out since the molecule cannot possess mirror symmetry. The non-hydrogen atoms were refined with anisotropic thermal pa- rameters and the hydrogen atoms were fixed in calculated posi- tions (C-H = 0.98 A, B, = 1.2B ,,,, ,, .,,, ). A correction for secondary extinction was applied, the final value of the extinction coefficient being 2.42 x Scattering factors for all atoms and anomalous dispersion corrections for the non-hydrog~n atoms were taken from ref. 9. The weighting scheme w = ~F, - /U~(F ,~) gave uniform average values of W((F,~ - ( ~ ~ 1 ) ' over ranges of both (F,l and sin 8/A and was employed in the final stages of full-matrix least- squares refinement of 271 variables. Reflections with I < 3u(F2) were not included in the refinement. Convergence was reached at R = 0.034 and R,, = 0.037 for 1421 reflections with I ? 3u(F2). Possible minor librational disorder of the C(15)-C(20) phenyl ring and minor rotational disordering of the quinuclidine moiety about the C(7)-N(l) bond or conformational flipping of the N(1)-C-C-C(1O) fragment is noted. No attempt was made to model this minor disorder, but metrical parameters involving these regions of the molecule may be affected. A parallel refinement of the structure having the opposite polarity resulted in slightly higher residuals, the R and R,, ratios being 1.006 and 1.003, respec- tively. The function minimized was zw(lF,I - IF,^)^, R = zIIF,( - l~cll/zl~ol~ R,,, = (zw(l~,I - IF~I)~/~wIF,I~) ' '~ .

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

FLO

RID

A S

TA

TE

UN

IVE

RSI

TY

on

11/1

3/14

For

pers

onal

use

onl

y.

CAN. J . CHEM. VOL. 71, 1993

FIG. 1. Stereoview of 5 (R3N = quinuclidine); 33% probability thermal ellipsoids are shown for the non-hydrogen atoms.

On the final cycle of refinement the maximum parameter shift corresponded to 0 . 0 1 ~ . The mean error in an observation of unit weight was 1.55. The final differtnce map showed maximum fluctuations of -0.10 to +O. 11 e A - ~ . The final positional and (equivalent) isotropic thermal parameters for the non-hydrogen atoms appear in Table 1. Bond lengths, bond angles, and intra-an- nular torsion angles are given in Tables 2 4 , respectively. Hydro- gen atom parameters, anisotropic thermal parameters, torsion angles, intermolecular contacts, least-squares planes, and struc- ture factors have been deposited.'

Results and discussion The X-ray analysis confirms structure 5 for the crystal-

line compou~d. his is, to the best of our knowledge, the first structurally characterized crystalline open-chain6 adduct of a trialkylamine to a carbonyl compound, stabilized by the neighboring Lewis acid diphenylboryloxyaryl moiety. Open- chain adducts of pyridine to carbonyl compounds, stabi- lized by trifluoroborane, have been predicted and calculated (by semi-empirical MNDO procedures) but could not be isolated (10). An SbC1,-stabilized adduct of 4-tert-butyl- pyridine and 4-methoxybenzaldehyde, however, could be crystallized and has been characterized by an X-ray crystal- lographic analysis (1 0).

Stabilized adducts having the amine nitrogen inserted into a ring system along with the carbonyl carbon atom are al- ready known and have been analyzed by single crystal X-ray analysis. Examples of this type of adduct include the phenyl- boronate-stabilized cyclic adduct 7 (7), the diphenylbori- nate-stabilized amine-aldehyde adducts like 8 ( l l ) , and others (6, 12, 13). The crystal structure of an intramolecu- lar tertiary amine-aldehyde adduct 9, stabilized by 3,5-dini- trobenzoic acid, has recently been published (14).

The carbonyl C=O double bond of salicylaldehyde 1 is lengthened to 1.26 l(4) .& in the diphenylborinate 2 and

by an additional 0.1 .& in the quinuclidine adduct 5 (C(7)- O(2) = 1.362(4) A). The C(7)-O(2) bond in 5, however, still retains some double-bond character, the IT-bond order being estimated at about 0 .3 from plots of bond length vs. CO IT-bond order (15). Similar carbonyl C=O bond lengths have been reported for the intramolecularly s!abilized pyri- dine-formaldehyde adduct 8 ( I I) (1.363(4) A) and for the cyclic intramolecular adduct 9 (14) (1.366(3) A). q e C-0 bond in the 0,N-acetal group of 7 (7) (1.378(2) A) is sig- nificantly longer than that in 5.

The N-C bond lengths in 5 and 7 follow the reverse of the trend noted for the C(7)-0 bonds: that in 5 (N( 1)- C(7) = 1.590(4) A) is significant!^ longer than the corre- sponding bond in 7 (7) (1.565(2) A) and marginally longer than the N-C bondoof the 0 , N-acetal (semi-aminal) group of 9 (14) (1.58 l(4) A). All of these N-C distances are un- usually long, especially those in 5 and 9, which exhibit N-C bond distances normally associated with sterically hindered systems such as the bulky substituted 0, N-acetal mqiety of the stabilized cyclic acetone adduct 10 (13) (1.584(3) A). The

Ph Ph 'Supplementary material mentioned in the text may be pur-

chased from: The Depository of Unpublished Data, Document 10 11

Delivery, CISTI, ~aGonal Research Council Canada. Ottawa, relatively weak N(1)-C(7) bond in 5 could explain the sol- Canada KIA 0R6.

Tables of hydrogen atom coordinates and bond lengths and an- volytic lability of the compound which can be observed in

gles involving hydrogen atoms have also been deposited with the the solutions used for the nrnr experiments (see above).

Cambridge Crystallographic Data Centre and can be obtained on The increase of electron density at the carbonyl' oxygen request from The Director, cambridge crystallographic ~~t~ atom, caused by the addition of quinuclidine to salicylalde- Centre, University Chemical Laboratory, 12 Union Road, hyde 1 or to its diphenylboron complex 2, is made evideat Cambridge CB2 lEZ, U.K. by the shor t~ning of the B-O(2) bond in 2 (1.569(4) A)

6 ~ n the sense that the newly formed C-N bond is acyclic. to 1.508(4) A for the corresponding B(1)-O(2) bond in 5.

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

FLO

RID

A S

TA

TE

UN

IVE

RSI

TY

on

11/1

3/14

For

pers

onal

use

onl

y.

KLIEGEL ET AL. 923

The overall binding strength of the 0,O-chelating ligand ular distances correspond to normal van der Waals interac- with respect to the diphenylboron moiety, defined as (mean tions. C-B)/(mean 0-B), ranges from 1.039 to 1.088 (mean 1.058) for "Ph,BO," compounds having six-membered che- Acknowledgements late rings (16). The value calculated for 5, 1.070, repre- We thank the Natural Sciences and Engineering Research sents relatively strong binding, while that for compound 2, Council of Canada and the Fonds der Chemischen Indus- 1.044, indicates weak binding. In general, complexes of the trie, Frankfurt am Main, for financial support. diphenylborenium moiety, which- behaves like a pseudo- metal cation, will undergo ligand exchange reactions if the difference in the overall binding strengths of the ligands is large enough. In view of the substantial difference between the binding parameters for 2 and 5, it is not surprising that the latter can be synthesized directly from the salicylalde- hyde complex.

The overall geometry of the molecule 5 displays a dis- torted "half-boat" arrangement of the six-membered boron chelate ring, with the fused benzene ring coplanar with the approximately planar O(1)-C(1)-C(6)-C(7) ring seg- ment. The quinuclidine and one of the B-phenyl substi- tuents are in axial positions. T$e axial B(1)-C(15) bond to the B-phenyl group (1.623(5) A) is somewhat longer than the equatorial B(1)-C(21) bond (1.613(5) A). Comparable differences between (pseudo-)axial and (pseudo-)equatorial B-C(ary1) bonds have been noted for a number of other diphenylboron chelates (1 7 , 18, and references therein).

A B,N-betaine of type 5 can be considered, in a sense, as being a reduced form of the B,N-betaine 11, both carrying the formal positive charge outside of the boron chelate ring in trialkyammonium and dialkyliminium groups, respec- tively. The structure of 11, a diphenylboron chelate of N,N- diethylsalicylamide, has also been established by an X-ray crystallographic analysis (18). As can be expected from the different functionalities of the N-C-0 moieties in the two compounds, the most drastic geometrical differences are found there: the N-C and C-0 bonds in compound 11 both display partial double bond character and are thus substan- tially shorter than the corresponding bonds in 5. The gen- eral geometry of the benzo-fused boron chelate rings is very similar in the two compounds.

The polar, but not chiral, crystal structure is built up from pairs of enantiomers (generated by the chiral carbon atom C(7) - the S isomer is depicted in Fig. 1). All intermolec-

1. (a) F . Umland and C. Schleyerbach. Angew. Chem. 77, 426 (1965); (b) I . Bally, A. Arsene, M. Bacescu-Roman, and A.T. Balaban. Tetrahedron Lett. 3929 (1965).

2. S.J. Rettig and J. Trotter. Can. J . Chem. 54, 1168 (1976). 3. G. Rao and M. Phillip. J. Org. Chem. 56, 1505 (1991). 4 . W.P. Jencks. Catalysis in chemistry and enzymology.

McGraw-Hill, New York. 1969. 5 . W. Kliegel. Organomet. Chem. Rev. Sect. A: 8, 153 (1972). 6. W . Kliegel, S.J. Rettig, and J. Trotter. Can. J. Chem. 66, 377

(1988). 7 . W. Kliegel, H.-W. Motzkus, K. Driickler, S.J. Rettig, and J.

Trotter. Can. J. Chem. 68, 64 (1990). 8. (a) A. Gazit and Z. Rappoport. J. Chem. Soc. Perkin Trans.

1, 2863 (1984); (b) J. Emsley, N.J. Freeman, R.J. Parker, and R.E. Overill. J. Chem. Soc. Perkin Trans. 2 , 1479 (1986).

9. International tables for X-ray crystallography. Vol. IV. Kynoch Press, Birmingham (present distributor: Kluwer Academic Publishers, Dordrecht, The Netherlands). 1974. pp. 99-102 and 149.

10. E. Anders, F. Markus, H. Meske, J. Tropsch, and G. Maas. Chem. Ber. 120, 735 (1987).

1 1. W. Kliegel, H.-W. Motzkus, D. Nanninga, S. J. Rettig, and J. Trotter. Can. J. Chem. 64, 507 (1986).

12. S.J. Rettig, J. Trotter, and W. Kliegel. Can. J. Chem. 52, 2531 (1974).

13. W. Kliegel, H.-W. Motzkus, S.J. Rettig, and J. Trotter. Can. J. Chem. 62, 838 (1984).

14. J.D. Carroll 11, P.R. Jones, and R.G. Ball. J. Org. Chem. 56, 4208 (1991).

15. G. Hafelinger. Chem. Ber. 103, 2922 (1970). 16. W.R. Cullen, S.J. Rettig, J. Trotter, and E. B. Wickenheiser.

Can. J. Chem. 66, 2007 (1988), and references therein. 17. E. Ebeling, W. Kliegel, S.J. Rettig, and J. Trotter. Can. J.

Chem. 67, 933 (1989). 18. W . Kliegel, M. Tajerbashi, S.J. Rettig, and J. Trotter. Can.

J . Chem. 67, 1636 (1989). 19. E. Ahlenstiel, W. Kliegel, S . J. Rettig, and J: Trotter. Can. J.

Chem. 71, 263 (1993).

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

FLO

RID

A S

TA

TE

UN

IVE

RSI

TY

on

11/1

3/14

For

pers

onal

use

onl

y.