I

1 Structural studies of organoboron compounds. XXVI.' I 8-Methyl-6,6-diphenyl-5,7-dioxa-8-azonia-6-borata-6,7-dihydro-5H-benzocycloheptene

W. KLIEGEL AND U. LAUTERBACH Institut fiir Pharmazeutische Chernie, der Technischen Universitat Braunschweig, 3300 Braunschweig,

BeethovenstrJ3e 55, Bundesrepublik Deutschland

i A N D

STEVEN J. RETTIG AND JAMES TROTTER Department of Chemistry, University ofBritish Columbia, 2036 Main Mall, Vancouver, B.C. V6T IY6

Received September 4, 1986

W. KLIEGEL, U. LAUTERBACH, STEVEN J. RETTIG, and JAMES TROTTER. Can. J. Chem. 65, 1457 (1987). Crystals of 8-methyl-6,6-diphe~yl-5,7-dioxa-8-azonia-6-borata-6,7-dihydro-5H-be11~0~ycloheptene are triclijc, a = 12.917(1),

b = 13.285(1), c = 10.567(1) A, a = 94.183(8), P = 90.094(5), y = 72.176(6)", Z = 4, space group P 1. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to R = 0.045 and R,,, = 0.042 for 2914 reflections with I 2 2u(I) . The envelope-like conformation adopted by the seven-membered chelate ring places one B-phenyl group above the plane of the fused-ring system facilitating a thermally-induced 1,4-migration of a B-phenyl group to the nitrone carbon atom. Mean bond lengths (corrected for libration) are (N)O-B = 1.559(6), (C)O-B = 1.496(3), C-B = 1.610(4) A.

W. KLIEGEL, U. LAUTERBACH, STEVEN J. RETTIG et JAMES TROTTER. Can. J. Chem. 65, 1457 (1987). Les cristaux du mCthyl-8 diphCnyl-6,6 dioxa-5,7 azpnia-8 borata-6 dihydro-6,7 5H-benzocycloheptkne sont tricliniques,

a = 12,917(1), b = 13,285(1) et c = 10,567(1) A, a = 94,183(8), P = 90,094(5) et y = 72,176(6)", Z = 4 et groupe d'espace PT. On a rCsolu la structure par des mCthodes directes et on l'a affinCe par la mCthode des moindres carrCs (matrice complkte) jusqu'a des valeurs de R = 0,045 et R,,. = 0,042 pour 2914 reflexions avec I 2 2u( I ) . La conformation en forme d'enveloppe qu'adopte le cycle chClatt i sept chainons conduit a placer un groupement B-phtnyle au-dessus du plan du systkme de cycles condensts et ainsi i faciliter la migration-1,4, induite thermiquement, d'un groupement B-phCnyle vers l'atome de carbone de la nitrone. Les longueurs Foyennes des liaisons (corrigies pour la libration) sont : (N)O-B = 1,559(6), (C)O-B = 1,496(3) et C-B = 1,610(4) A.

[Traduit par la revue]

I

Introduction I The diphenylboron chelate of N-methylsalicyclaldonitrone

(8-methyl-6,6-diphenyl-5,7-dioxa-8-azonia-6-borata-6,7-di-

i hydro-5H-benzocycloheptene, 1 ) reacts with phenylboronic , acid upon heating, eliminating water to give the tricyclic

, compound 4. The phenylboronate structure of 4 has been I established by an earlier X-ray crystallographic analysis (1). ' The mechanism of this reaction involves an inter- or intra-

molecular shift of one phenyl group which results in the oxidation of the diphenylborinate group to a phenylboronate

i group and an associated reduction of the nitrone moiety to a I hydroxylamine function. A combined theoretical and experi-

1 mental approach to the question of the reaction mechanism (2)

suggests an intramolecular 1,4-shift of one B-phenyl group occurs. The same type of phenyl group shift has also been observed in the thermal rearrangement of open-chain triphenyl- borane-adducts of nitrones (1, 3). Such a 1,4-migration of a phenyl group in 1 would require a conformation of the seven- membered ring which would bring one of the phenyl groups close enough to the nitrone group to allow a transition state such as 2. This would favor the reaction via the intermediate 3 which has not as yet been isolated but rather reacts immediately with phenylboronic acid (or its anhydride) to give 4. In order to determine the conformation of the seven-membered ring in 1 and, more importantly, the orientation of the B-phenyl groups, an X-ray crystallographic analysis was undertaken.

'For Part XXV, see ref. 26.

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1458 CAN. I . CHEM. VOL. 65. 1987

Experimental 8-Methyl-6,6-diphenyl-5,7-dioxa-8-azonia-6-borata-6,7-dihydro-5H-

benzocycloheptene, I Compound 1 was prepared as described previously (4). Crystals

suitable for X-ray analysis were obtained as described below. Dichlo- romethane was added to a suspension of the newly synthesized and purified substance in ethanol until complete dissolution was achieved at room temperature. The solution was slowly evaporated at room temperature until crystallization began after 2 weeks. The compound decomposes at 160- 16 1 OC (4).

X-ray crystallographic analysis A crystal ca. 0.2 X 0.3 x 0.4 mm in size was mounted in a general

orientation. Unit-cell parameters were refined by least-squares on 2 sin 0/A values for 25 reflections (20 = 56-81") measured on a diffractometer with Cu-Ka radiation (A(Ka = 1.540562, A(Ka2) = 1.544390 A). Crystal data at 22OC are:

CzoHlsBNOz fw = 315.2 Triclinic, a = 12.917(1), b = 13.285(1), c = 10.567(1) A, a = 94.183(8), P = 90.094(5), y = 72.176(6)" (cell with c < a < b and two obtuse angles conforms to the conventions of ref. 5; the Buerger reduced cell is the same cell reoriented: 0011 100/010), V = 1721.2(3) A3, Z = 4, pc = 1.216 Mg m-3, F(OO0) = 664, p(Cu-Ka) = 5.8 cm-'. Absent reflections: none, space group from structure analysis.

Intensities were measured with nickel-filtered Cu-Ka radiation on an Enraf-Nonius CAD4-F diffractometer. An 0-20 scan at 1.06- 10.06" min-' over a range of (0.65 + 0.14 tan 0) degrees in o (extended by 25% on both sides for background measurement) was employed. Data were measured to 20 = 150". The intensities of three check reflections, measured every 3600 s throughout the data collection, remained constant to within 4%. After data r e d ~ c t i o n , ~ no absorption correction was made due to irregularity of the crystal surface. Of the 7071 independent reflections measured, 2914 (41.2%) had intensities greater than or equal to 2u( I ) above background where u 2 ( ~ ) = S + 2B + (0.04(S - B ) ) ~ with S = scan count and B = normalized background count.

The structure was solved by direct methods, all non-hydrogen atoms of each of the two crystallographically independent molecules being positioned from an E-map and hydrogen atoms from subsequent difference maps. In the final stages of refinement the non-hydrogen atoms were refined with anisotropic thermal parameters and the hydrogen atoms with isotropic thermal parameters. The scattering factors of ref. 6 were used for non-hydrogen atoms and those of ref. 7 for hydrogen atoms. The weighting scheme w = l / u 2 ( ~ ) , where u'(F) is derived from the previously defined u2(1), gave uniform average values of w(lFoI - over ranges of both IFo/ and sin 0/A and was employed in the final stages of full-matrix refinement of 577 variables. Reflections with I < 2u( I ) were not included in the refinement. Convergence was reached at R = 0.045 and R,, = 0.042 for 2914 reflections with I 2 2u(I) . For all 7071 reflections R = 0.155. The function minimized was Cw(lFoI - IF,^)', R =

CllFoI -IFcII/CIFoI and R,, = (Cw(lFoI - I F ~ I ) ~ / C ~ ~ F ~ I ~ ) " ~ . On the final cycle of refinement the mean and maximum parameter

shifts corresponded to 0.02 and 0.47u, respectively. The mean error in an observation of unit weight was 1.463. A final difference map showed maximum fluctuations of -0.32 to +0.21 e The final positional and thermal parameters appear in Tables 1 and 5,3

I he computer programs used include locally written programs for data processing and locally modified versions of the following: MULTAN 80, multisolution program by P. Main, S. J. Fiske, S. E. Hull, L. Lessinger, G. Germain, J. P. Declercq, andM. M. Woolfson; AGNOST, absorption corrections, by J. A. Ibers; ORFLS, full-matrix least-squares, and ORFFE, function and errors, by W. R. Busing, K. 0. Martin and H. A. Levy; FORDAP, Patterson and Fourier syntheses, by A. Zalkin; ORTEP 11, illustrations, by C. K. Johnson.

TABLE 1. Final positional (fraftional X lo4) and isotropic thermal parameters ( U x lo3 A ~ ) with estimated standard

deviations in parentheses

Atom x Y z Ucs

respectively. Measured and calculated structure factors have been placed in the Depository of Unpublished ~ a t a . ~

The ellipsoids of thermal motion for the non-hydrogen atoms are shown in Fig. 1. The thermal motion has been analysed in terms of the rigid-body modes of translation, libration, and screw motion (8). The rms standard error in the temperature factors uUij (derived from the

3The structure factor table, Table 1 (including H atom coordinates), Table 5 (anisotropic thermal parameters), and other material mentioned in the text may be purchased from the Depository of Unpublished Data, CISTI, National Research Council of Canada, Ottawa, Ont., Canada KIA 0S2.

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KLIEGEL ET AL

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FIG. 1. Stereoscopic views of the two independent 8-methyl-6,6-dipheny1-5,7-dioxa-8-azonia-6-borata-6,7-dihydro-5H-benzocycloheptene 1 molecules; 50% probability thermal ellipsoids are shown for the non-hydrogen atoms. Hydrogen atoms have been assigned arbitrary thermal

/ parameters for the sake of clarity. The mirror image of the primed molecule is shown for ease of comparison.

least-squares analysis) is 0.0028 A'. The subunits PhB and "methyl- benzocycloheptene" of each of the two independent n~olecules were analysed separately (rms AUij = 0.0025-0.0065 A'). The appropriate bond distances have been corrected for libration (8, 9), using shape parameters q 2 of 0.08 for all atoms involved. Corrected bond lengths appear in Table 2 along with the uncorrected values; corrected bond angles do not differ by more than 1 u from the uncorrected values given in Table 3. Intra-annular torsion angles defining the conformations of the seven-membered chelate rings are listed in Table 4. Bond lengths and angles involving hydrogen and a complete listing of torsion angles (Tables 6-8) are included as supplementary material.

Results and discussion The crystal structure of 8-methyl-6,6-diphenyl-5,7-dioxa-8-

azonia-6- borata-6,7-dihydro-5H- benzocycloheptene, 1, consists of discrete molecules separated by normal van der Waals

distances. The shortest intermolecular $ontact between non- hydrogen atoms is C-. .C = 3.336(5) A. The two crystallo- graphically independent molecules in the asymmetric unit are virtually identical (Fig. 1, Tables 2-4) and are related to one another in the crystal lattice by a pseudo-glide plane. The coordinates of the primed molecule are related to those of the unprimed molecule by the approximate operation: - 1 / 2 + x , y , 1.45 - Z.

The X-ray analysis confirms the seven-membered chelate structure of 1. Only a few stable seven-membered chelate rings involving bidentate ligands have been reported in the literature (4). This is the first crystallographic example of a boron chelate with such a ring system although the structures of compounds with seven-membered rings containing 4-coordinate (10) and 3-coordinate (11, 12) boron have been reported. A similar

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1460 CAN. J. CHEM. VOL. 65, 1987

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

Length Length

Bond Uncorr. Corr. Bond Uncorr. Corr.

O(l)-c(l) 1.341( 3) 1.345 0(l1)-C(1 ' ) 1.327(3) 1.331 O(1)-B 1.494(4) 1.499 0(l1)-B' 1.489(4) 1.493 O(2)-N 1.358( 3) 1.363 O(2')-N' 1.361(3) 1.365 O(2)-B 1.545( 4) 1.553 O(2')-B' 1.557(4) 1.564 N-C(7) 1.287( 4) 1.291 N'-C(7 ' ) 1.284(4) 1.288 N-C(8) 1.469( 4) 1.472 N'-C(8') 1.462(4) 1.465 c ( 1 )--c(2) 1.406( 4) 1.414 C(lf)-C(2') 1.416(4) 1.421 c ( 1)-c(6) 1.396( 4) 1.401 C(I1)-C(6') 1.395(4) 1.400 C(2)-C(3) 1.409( 4) 1.414 C(2')-C(3') 1.415(4) 1.419 C(2)-C(7) 1.436( 4) 1.442 C(2')-C(7 ' ) 1.440(4) 1.445 c(3)-c(4) 1.360( 5) 1.365 C(3 ')-C(4') 1.362(5) 1.366 c(4)-c(5) 1.376( 5) 1.382 C(4')-C(5') 1.375(5) 1.381 C(5)-C(6) 1.369(5) 1.372 C(5')-C(6') 1.366(5) 1.370 C(9)-C(10) 1.390( 4) 1.398 C(9')-C(10') 1.397(4) 1.405 C(9)-C(14) 1.392( 4) 1.401 C(9')-C(14') 1.393(4) 1.406 C(9)-B 1.607( 4) 1.613 C(9')-B' 1.605(4) 1.612 C(I0)-C(1 1) 1.389( 5) 1.396 C ( 0 ) - ( I 1 1.392(5) 1.399 C(l1)-C(12) 1.363( 6) 1.371 1 ) - ( 1 1.357(7) 1.370 C(12)-C(13) 1.357( 6) 1.365 C(12')-C(13') 1.377(7) 1.385 C(13)-C(14) 1.393( 5) 1.399 C(13')-C(14') 1.388(5) 1.395 C(15)-C(16) 1.385( 5) 1.403 C(15')-C(16') 1.393(4) 1.407 C(15)-C(20) 1.398( 5) 1.410 C(15')-C(20') 1.399(4) 1.406 C(15)-B 1.595( 5) 1.605 C(15')-B' 1 .603(4) 1 ,609 C(16)-C(17) 1.387(10) 1.398 C(16')-C(17') 1.389(5) 1.394 C(17)-C(18) 1.342(11) 1.354 C(17')-C(18') 1.378(6) 1.385 C(18)-C(19) 1.380(11) 1.397 C(18')-C(19') 1.364(5) 1.378 C(19)-C(20) 1.389( 6) 1.401 C(19 ')-C(20') 1.385(5) 1.390

structure, 5, was suggested by Balaban et al. (13) for the catecholatoborate complex of salicylaldoxime but has been ruled out by the X-ray analysis of the diphenylboron chelate of salicylaldoxime, 6 (14), and by chemical findings (15). The seven-membered ring in 1 has an "envelope-like" conformativn in which the boron atom is displaced (by 0.751(4), 0.740(4) A) from the mean plane formed by the remaining ring atoms (maximum deviations Lrom these six-atom mean planes are 0.091(3) and 0.123(3) A for the two molecules). The O-C- C-C-N-0 portion of the seven-membered ring actually con- sists of a pair of planar 4-atom groups, O(1)-C(1)-C(2)-C(7) and C(2)-C(7)-N-0(2), which are rotated about the C(2)-C(7) bond with respect to one another (by 11.8(6) and

15.8(6)"). The ring conformation places one B-phenyl group distinctly above the mean plane of the fused-ring system, the mean4 distance between the boron-bound phenyl carbon a t o p C(16) and the nitrone carbon atom C(7) being 3.190(4) A. This distance, which corresponds to a van der Waals contact, is probably sufficiently short to facilitate the 1,4-migration of the phenyl group via a transition state 2. The aromatic rings in 1 are all planar to within experimental error, their geometry being consistent with normal substituent effects (16).

The C=N and N-0 bond distances in :he nitrone group of 1 (mean values of 1.290(2) and 1.364(1) A, respectively) are comparable to those reported for the boro! chelate 7 containing a nitrone ligand5 (1.286(6) and 1.355(5) A) (17). In both cases the relatively short C=N and the long N-0 bond distances indicate an increased iminium salt character of the nitrone moiety within a B ,N-betaine complex. The C=N bond lengths in free (uncomplexed) nitrones are somewhat longer in most cases while the N-0 bonds are generally shorter (17-22).

As in other diphenylboron chelates with an N-oxide ligand (18,23-25), the 0-B bond betw~en the nitrone oxygen atom and boron in 1 (mean 1.559(6) A) iso longer than the 0-B bond to the phenolate (mean 1.496(3) A) or, in other examples,

Me alcoholate ligand. The sum of bond lengths involving the boron

ph-'.8&, k8,/Me 4Here and elsewhere in this report mean values refer to weighted ' 0 o\o,o. o\a, 0 B

means with rms deviations from the mean in parentheses. B '

Ph' 'Ph 5~ibration corrected bond lengths (esd's assumed equal to those of

1 the uncorrected values) are employed in the discussion of the geometry Ph of the boron atom and are compared with similarly treated (8)

7 8 distances unless otherwise stated.

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

Bonds Angle (deg) Bonds Angle (deg)

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

Atoms Value (deg)

atom in 0,O-chelates of diphenylboron is essentially coGstant for a given chelate ring size: 6.328(7) and 6.28(1) A for five-and six-membered rings, respectively (26, and references therein). In 1, with a sev~n-membered chelate ring, the corresponding sum is 6.276 A. The ayerage 0-B and C-B bond lengths in 1 (1.528 and 1.610 A), when compared with those in similar compounds having six-membered chelate rings, indicate relatively strong overall binding of the 0,O-chelate to the PhzB moiety (26). Compound 1 may be regarded formally as a ring-enlarged analog (by o-phenyl insertion) of the N-methylacethydroximate 8 which shows comparable bond distances in the ring system and in the diphenylboron moiety (27).

Acknowledgements We thank the Natural Sciences and Engineering Research

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

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1462 CAN. J. CHEM. VOL. 65. 1987

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(1980). 4. W. KLIEGEL and D. NANNINGA. J. Organomet. Chem. 243, 373

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p. 2. Natl. Bur. Stand., Washington. 1973. 6. D. T. CROMER and J. B. MANN. Acta Crystallogr. Sect. A, 24,

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16. A. DOMENICANO, P. MURRAY-RUST, and A. VACIAGO. Acta Crystallogr. Sect. B, 39, 457 (1983) and references therein.

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590 (1987). 27. S. J. RETTIG, J. TROTTER, W. KLIEGEL, and D. NANNINGA.

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