Structural studies sf organoborsn compounds. XXHV." 5-Methyl-5-nitro-2-pheny1-1 73-dio~a-22 b ~ r a ~ y ~ ~ ~ h e x a n e

W. KLIEGEL AND L. PREU Institut fur Pharmazeutische Chemie, der Technischen Universitat Braunschweig, 3300 Braunschweig,

BeethovenstraJe 55, Bundesrepublik Deutschland

AND

STEVEN J. RETTIG AND JAMES TROTTER Department of Chemistry, Unzversih of British Columbia, 2036 Main Mall, Vancouver, B .C . , Canada V6T 1Y6

Received February 12, 1986

W. KLIEGEL, L. PREU, STEVEN J . RETTIG, and JAMES TROTTER. Can. J. Chem. 64, 1855 (1986). The title compound was prepared according to the literature in order to determine whether it has a bicyclic cage structure

resulting from intramolecular 0 --+ B coordination or a monocyclic boronate structure incorporating a trigonal planar boron atom. Crystals or 5-methyl-5-nitro-2-phenyl-l,3-dioxa-2-boracyclohexane are orthorhombic, a = 17.2358(4), b = 6.5007(2), c = 9.9225(3) A, Z = 4, space group Pnam. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to R = 0.064 and R,. = 0.070 for 798 reflections with 1 r 3u(I). The nlolecule actually has Cl symmetry but, in the solid state, is located at a site of crystallographic C, symmetry. In order to maintain the apparent mirror symmetry the nitro oxygen atoms are disordered over two mirror-related rotational positions around the C(2)-N bond. The molecule was found to have a monocyclic boronate structure, in agreement with earlier predictions. Tile six-membered heterocyclic ring has a "semi-planar" conformation. Bond distances and angles are normal.

W. KLIEGEL, L. PREU, STEVEN J. RETTIG et JAMES TROTTER. Can. J. Chem. 64, 1855 (1986).

Dans le but de dCterminer si le compose mentionnC dans le titre posskde une structure bicyclique en cage resultant d'une coordination intramolCculaire 0 -+ B ou une structure boronate monocyclique incorporant un atome de bore plan trigonal, on a prCparC ce composC en suivant les indications fournies dans la IittCrature. Les cristaux du mCthyl-Sonitro-5 phtnyl-2 dioxa-I ,3 bora-2 cyclohexane sont orthorhombiques, avec a = 17,2358(4), b = 6,5007(2) et c = 9,9225(3) A, Z= 4 et groupe d'espace Pnam. On a rCsolu la structure par des mtthodes directes et on l'a affinCe par la mCthode des moindres carrCs (matrice entikre) jusqu'i des valeurs de R = 0,064 et R, = 0.070 pour 798 rCflexions avec I 2 3u(I). La molCcule posskde une symCtrie Cl ; toutefois, B 1'Ctat solide, elle se situe dans un site cristallographique C,. Dans le but de maintenir une symetrie apparente dans un miroir, les oxygbnes des groupements nitro occupent des positions dCsordonnCes par rapport B deux positions rotationnelles symktriques par rapport a un miroir et resultant d'une rotation autour de la liaison C(2)-N. On a trouvC que la molkcule possbde une structure boronate monocyclique et ce resultat est en accord avec des predictions antCrieures. L'hCtCrocycle a six chainons existe dans une conformation <<semi-planairen. Les distances des liaisons ainsi que les angles sont normaux.

[Traduit par la revue]

Introduction It has been shown by X-ray structure analysis (1) that

phenylboronates of bis(hydroxyalky1)nitrones can have either a bicyclic structure 1 (resulting from intramolecular 0 + B coordination) or a monocyclic structure 3 (stabilized by 0-B p p ( n ) back donation within the trigonal planar boronate system). It was thus of interest to determine whether the phenylboronate of a 2-nitro-1,3-propanediol would possess an analogous bicyclic cage structure 2 or a monocyclic bidentate structure 4. The "semi-planar" form 4, with an "axial" nitro group, has been suggested by Urbanski et al. (2) for the phenylboronates of 2-alkyl-2-nitro-1,3-propanediols on the basis of calculated and experimentally determined dipole moments. It is noteworthy that these nitro derivatives were found to be more stable than other cyclic phenylboronates with respect to hydrolytic agents (3). This finding would also be consistent with intramolecular 0 + B coordination leading to a shielded sp3 boron center. The existence of intramolecular hydrogen bonding in nitroalkanols is well established (4-8). If the 'khelated" protons of a nitroalkanol such as 2-methyl-2- nitro-1,3-propanediol were to be replaced by the (formal) dication P ~ B ~ + (generated from phenylboronic acid), the chelate 2 could be formed. Similarly, a tridentate nitrone ligand led to the chelate 1 (1) and a tridentate N-oxide Iigand gave a bicyclic chelate with the same basic ring skeleton (9). For an

'Part XXIII, ref. 27.

unambiguous decision between the two possible structures, the phenylboron chelate 2 or the phenylboronate 4, an X-ray crystallographic analysis of the compound has been carried out.

Experimental 5-Methyl-5-nitro-2-phen~~1-1,3-dioxa-2-boracyclohexane, 4

The title compound was prepared according to the literature (ref. 3, method A) and was recrystallized from acetone. Mp. 147-14g0C, lit. 148-150°C. Anal. calcd. for ClOHI2BNO4: C 54.34, H 5.47, N 6.34; found: C 53.86, H 5.46, N 6.36. 'H nmr (CDC13/TMS): 6 (ppm) =

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1856 CAN. J. CHEM. VOL. 64, 1986

1.53 (s, CH3), 4.07 and 4.75 (AB system, J = 11 Hz, 2 0CH2), TABLE 1. Final positional (fra2tional X lo4, H X 10') and isotropic 7.15-7.85 (m, C6H5). thermal parameters ( U X 10' A*) with estimated standard deviations

in parentheses X-ray cgvstallographic analysis

A crystal bounded by the seven faces (followed by their distances in mrn from a common origin): i ( l 0 O), 0.08, i ( 0 0 I ) , 0.20. (0 1 I), Atom x Y z Ueq/ Uiso

0.25, (0 1 -I) , 0.22, (0 -1 O), 0.25 was mounted in a general orientation. Unit-cell parameters were refined by least-squares on '('1 8340( 1) 2723( 3) 1301( 2) 69

2 sin 0/X values for 25 reflections (20 = 80-100") measured on a 0(2)t 6651( 6) 2043(14) 1496( 9) 99 O(3)t diffractometer with Cu-Ka radiation (A(Kal) = 1.540562, h(Ka2) = N*

6407( 7) 2822(18) 3453(12) 170

1.544390 A). Crystal data at 22OC are: 6762( 2) 3059( 5) 2500 63 c(1) 7866( 2) 4524( 4) 1250( 3) fw ="221.0

66 C10H12BN04 Orthorhombic, a = 17.2358(4), b = 6.5007(2), c = 9.9225(3) A, V =

c(2)* 7375( 2) 4779( 5) 2500 55 (33)" 6946( 3) 6835( 7) 2500

1 1 1 1 . 7 7 ( 5 ) ~ 3 , Z = 4 , p c = 1.320Mgm-', F(000) =464, ~ ( C U - K a ) 80 (34)" 909 1 ( 2) -39( 5) 2500 = 7.99cm-I. Absent reflections: Okl, k + 1 odd, and h01, h odd, 63 (35) 9327( 2) -938( 5) 1292( 4) 84 space group Pnam (non-standard setting of Pnma, ~ k g , No. 62, (36) 9777( 2) -2697( 6) 1313( 7) 109 equivalent positions: r ( x , y , z ; 6 + x, 5 - y , z; -x, - y , 4 + z ; (37)" 9995( 3) -3575( 9) 2500

- x, + y , 4 + z)) from structure analysis. 118

Intensities were measured with graphite-monochromated Cu-Ka B* 8570( 2) 1922( 6) 2500 55

radiation on an Enraf-Nonius CAD4-F diffractometer. An w-20 scan at H(1a) 758( 2) 448( 5) 32( 3) 100(10)

1.34-10.06" minp' over a range of (0.80 + 0.15 tan 0) degrees in H(lb) 822( 2) 573( 5) 126( 3) 82( 8) H(3a) w (extended by 25% on both sides for background measurement) was H(3b)*

661( 2) 6 8 3 5) 157( 3) 104(11)

employed. Data were measured to 20 = 150". The intensities of three 727( 3) 796( 9) 250 129(21)

H(5) 917( 2) -33( 5) 46( 5) 140(17) check reflections, measured every 3600 s throughout the data collec- tion, remained constant to within 3%. After data reduction,* an H(6) 985(2) -310(8) 4) 146(19)

H(7)* 1030( 5) -451(11) 250 169(27) absorption correction was applied using the Gaussian integration method (10, 11). Transmission factors ranged from 0.742 to 0.880 for *Multiplicity factor 0.5. 168 integration points. Of the 12 13 independent reflections measured, +Occupancy factor 0.5. 798 (65.8%) had intensities greater than or equal to 3u(1) above background where a 2 ( 0 = S + 2 8 + (0.04(S - B!)' with S = scan in an observation of unit weight was 3.044. A final {ifkrence map count and B = normalized background count. showed maximum fluctuations of -0.35 to +0.22 e A-'. The final

he systematic absences allow space groups Pna21 or Pnam, the positional and thermal parameters appear in Tables 1 and 5,3 latter being indicated by the E-statistics. The structure was solved by respectively, Measured and calculated structure factors have been direct methods, all non-hydrogen atoms of the molecule (which has placed in the Depository of Unpublished crystallographically imposed mirror symmetry) being positioned from The of thermal motion for the non-hydrogen atoms are an E - m a ~ . were positioned a subsequent shown in Fig, 1. The thermal motion has been analysed in terms of the difference map. 1n the final Stages of refinement the non-hydr0gen rigid-body modes of translation, libration, and screw motion (17). The atoms were refined with anisotropic, and the hydrogen atoms with rms standard error in the temperature factors UU, (derived from the isotropic, thermal parameters. The R value at this point was 0.088. least-squares aVa]ysis) is 0.0032 and, excluding the nitro oxygen The large U22 for the Oxygen atom the nitro group atoms, 0,0017 A', The structural subunits PhB02 and C(1-3), N, and (O(2)) suggested possible disorder or that the actual space group may O(1) (along with their mirror-related countefparts) were analysed be ~ n a 2 1 . ~ t t e m ~ t s to refine the structure in the lower symmetry separately (ms AU,. = 0.0042 and 0.0017 A ~ , respectively). The 'pace group did this problem and were thwarted by high appropriate bond distances have been corrected for libration (17, 18), correlation coefficients. Refinement of the structure was completed using shape parameters q2 of 0.08 for all atoms involved, corrected with dk~rdered nitro oxygen atoms, 0(2) and 0(3) , being refined with bond lengths appear in Table 2 along with the uncorrected values; anisotropic thermal parameters and occupancy factors fixed at 0.5. The corrected bond angles do not differ by more than from the scattering factors of ref. 12 were used for nOn-h~drOgen and values given in Table 3. Intra-annular torsion angles those of ref. 13 for hydrogen atoms. The weighting scheme tz/ = defining the conformation of heterocyclic ring are listed in Table 4. l / a 2 ( F ) , where u2(F) is derived from the previously defined u 2 ( ~ ) , Bond lengths and angles involving hydrogen and a complete listing of gave uniform average values of w( 1 F, I - I F c 1) over ranges of both torsion angles (Tables 6-8) are included as supplementary material. I F,I and sin 0/h and was employed in the final stages of full-matrix refinement of variables. Reflections with 1 < 3u(1) were not included Results and discussion in the refinement. An isotropic Type I extinction correction (Thomley- Nelmes definition of mosaic anisotropy with a Lorentzian distribution) The crysta1 structure of 5-meth~1-5-nitro-2-~hen~1-1 ,3-di0xa-

was applied (14-16). ~h~ final value of was 0,49(23) x 104. 2-boracyclohexane consists of discrete molecules, all inter- Convergence was reached at R = 0.064 and R,, = 0.070 for 798 molecular distances being greater than the sums of van der reflections with I 2 3u(I). For all 1213 reflections R = 0.093. Waals radii. The molecule actually has C1 symmetry but, in the The function minimized was Zw(l Fol - 1 Fc I)*, R = 2 1 1 F,I - solid state, is located at a site of crystallographic C, symmetry. lFClI /~1F,I and R, = (CW(IF , I - I F ~ I ) ~ / C W I F , I ~ ) ~ ! ~ . In order to maintain the apparent mirror symmetry the nitro

On the final cycle of refi~~ement the mean and maximum Parameter oxygen atoms are disordered over two mirror-related rotational shifts corresponded to 0.02 and 0.130, respectively. The mean error positions around the C(2)-N bond.

The observed structure 4 (see Fig. 1) corresponds to that he computer programs used include locally written programs deduced by Urbanski el al. ( 2 ) as being the most probable one

for data processing and locally modified versions of the following: with a calculated dipole moment of [1. = 4.35 D in good MULTAN 80, multisolution program by P. Main, S. J . Fiske, S. E. agreement with the experimental value of 4.25 D. The intra- Hull, L. Lessinger, G . Germain, J. P. Declercq, and M. M. Woolfson; ORFLS, full-matrix least-squares, and ORFFE, function and errors, 3 ~ h e structure factor table, Table 5 (anisotropic thermal parameters) by W. R. Busing, K. 0 . Martin, and H. A. Levy; FORDAP, and other material mentioned in the text are available, at a nominal Patterson andFouriersyntheses, by A. Zalkin;ORTEPII, illustrations, charge, from the Depository of Unpublished Data, CISTI, National by C. K. Johnson. Research Council of Canada, Ottawa, Ont., Canada KIA 0S2.

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FIG. 1. Stereoscopic view of the 5-methyl-5-nitro-2-phenyl-1,3-dioxa-2-boracyclohexane molecule; 50% probability thermal ellipsoids are shown for the non-hydrogen atoms. Hydrogen atoms have been assigned arbitrary thermal parameters for the sake of clarity. Shaded atoms comprise the asymmetric unit.

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

Length Length

Bond Uncorr. Corr. Bond Uncorr . Corr.

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

Bonds Angle (deg) Bonds Angle (deg)

C(1)-O(1)-B 120.8(2) C(1)-C(2)-C(1)' 110.4 ( 3) O(2)-N-O(3) 121.7(4) C(5)-C(4)-B 120.7 ( 2) O(2)-N-C(2) 120.4(5) C(5)-C(4)-C(5)' 118.6 ( 4) O(3)-N-C(2) 117.9(6) C(4)-C(5)-C(6) 119.9 ( 4) O(1)-C(1)-C(2) 112.4(2) C(5)-C(6)-C(7) 120.9 ( 5) N-C(2)-C(1) 107.8(2) C(6)-C(7)-C(6)' 119.8 ( 5) N-C(2)-C(3) 107.6(3) O(1)-B-C(4) 118.80(15) C(1)-C(2)-C(3) 111.5(2) O(1)-B-O(1)' 122.4 ( 3)

*Primed atoms have coordinates related to those in Table 1 by the symmetry operation I, ?'. 4 - z .

molecular contacts between the nitro oxygen atoms and the The six-membered heterocyclic ring adopts a "semi-planar" boron atom are clearly too long for coorainative interaction conformation with C(2) displaced from the approximate plane (B-O(2) = 3.45(1), B-O(3) = 3.89(1) A). The pp(n) back of the other five atoms and the nitro group in a pseudo-axial donation from the two oxygen atoms to boron in the phenylboro- position. This conformation has been predicted on the basis of nate system is energetically favored over intramolecular 0 + B nrnr (19-22) and dipole moment studies (2, 19), and has been coordination involving a nitro group oxygen atom which would verified by X-ray structure analyses (ref. I and references force the cycloboronate ring into a boat conformation (formula therein). The overall geometry of the 1,3-dioxa-2-boracyclo- 2). hexane ring in 4 is very similar to that of compound 3 (1). The

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1858 CAh- J CHEM VOL 64, 1986

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

Atoms Value (deg)

C(1)-O(1)-B-O(1)' 2.8(5) B-Q(1)-C(1 j-C(2) -26.8(4) O(1)-C(1)-C(2)-C(1)' 48.7(4)

"Symmetry-related torslon angles have the opposite sign.

partial T-bond character of the 0-B bqnds is demonstrated by the short 0-B distance of 1.371(2) A. The relatively short B-C bond of 1.567(5) A is indicative of some n-interaction between the sp2 boron atom and the aromatic system. Both 0-B and B-C distances are in good agreement with those observed for 3 and related compounds (ref. 1 and references therein). The C-N bond in 4 (1.552(4) A) is significantly longer than the corresponding distance of 1.528(3) A in 3: whereas th; N-0 bonds of the nitro group (1.217(8) and 1.143(11) A) in 4 are considerably shorter than the N-0 bond of the nitrnne group in 3 (1.29 l (2) A). This reflects the more pronounced (partial) double bond character of the N-0 bond in the nitro compound 4. The 1o;ger of the two N-0 bonds in 4 is in the range 1.190-1.230 A normally observed for nitro groups (23;25). The anomalously short N-O(3) distance of 1.143(11) A is most likely an artifact of thermal motion and/or disorder although an inequality of the two N-0 bond lengths could arise as a result of differences between the two nitro oxygen atoms with respect to intramolecular non-bonded interactions ( 0 ( 2 ) . . - C ( l ) = 2.66(1), 0 ( 2 ) . . . 0 ( 1 ) = 2.95(1), and 0 ( 2 ) . . . C ( 3 ) = 3 .31(1)A vs. 0 ( 3 ) , . . C ( l ) ' = 2.76(1), 0 ( 3 ) . . . 0 ( 1 ) ' = 3.34(1), and 0 ( 3 ) . . . C ( 3 ) = 2 . 9 3 ( 1 ) ~ ) . The geometrical distortion of the phenyl ring is as expected (ref. 26 and references therein). The phenyl ring is planar within experimental error while the boronate and nitro groups are slightly, but significantiy, non-planar (B and N are displaced 0.017(4) and 0.015(3) A , respectively, from the planes of their substituents). The dihedral angle between the normals to the phenyl and boronate mean planes is 1.6(4)".

Acknowledgments We thank the Natural Sciences and Engineering Research

Council of Canada and the Fonds der Chemischen Industrie, Frankfurt a m Main. for financial support and the University of British Columbia Computing Centre for assistance.

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