Structural studies of organoboron compounds. XXIX.' 6,6-Diethyl-2,2-diphenyl- 1,3-dioxa-6-azonia-2-boratacyclooctane monohydrate

WOLFGANG KLIEGEL Institut fur Pharmazeutische Chemie, der Technischen Universitiit Braunschweig, 3300 Braunsch~veig,

BeethovenstraJe 55, Bundesrepublik Deutschland

A N D

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

Received June 30, 1987

WOLFGANG KLIEGEL, STEVEN J. RETTIG, and JAMES TROTTER. Can. J. Chem. 66, 109 1 (1988). Details of the preparation and structure of the title compound are given. Crystals of 6,6-diethyl-2,2-diphen 1-1,3-dioxa-6-

azonia-2-boratacyclooctane monohydrate are orthorhombic, a = 7.0322(2), b = 16.3505(7), c = 16.8164(4) 1, Z = 4, space group P212121. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to R = 0.045 and R,, = 0.050 for 2013 reflections with I 2 3 4 4 . The monocyclic eight-membered B, N-betaine is the first to be structurally characterized. The eight-membered chelate ring has a conformation intermediate between the S4 and boat-boat forms which is probably stabilized by transannular C-H...O interactions. The average libration-corrected 0-B and B-C bond lengths of 1.500 and 1.639 A are, respectively, the shortest and longest yet observed for an 0,O-chelate of diphenylboron.

WOLFGANG KLIEGEL, STEVEN J. RETTIG et JAMES TROTTER. Can. J. Chem. 66, 1091 (1988). On rapporte les dCtails de la priparation et de la ditermination de la structure du compost mentionne dans le titre. Les cristaux

du monohydrate du diCthyl-6,6 diphknyl-2,2 dioxa-1,3 azonia-6 borata-2 cyclooctane sont orthorhombiques avec a = 7,0322(2), b = 16,3505(7) et c = 16,8164(4) A, Z = 4 et groupe d'espace P212121. On a rCsolu la structure par des mkthodes directes et on l'a affinCe par la mCthode des moindres carrCs (matrice entibre) jusqu'a des valeurs de R = 0,045 et R, = 0,050 pour 2013 rkflexions avec I 2 3 4 4 . La B,N-bCtai'ne monocyclique h huit chainons est la premikre a avoir CtC caracttriste du point de vue de sa structure. Le chtlate cyclique a huit chaions possbde une conformation qui est intermtdiaire entre les formes S4 et bateau-bateau et celle-ci est probablement stabilist par des interactions transannulaires C-H..-O. Les longueurs des liaisons 0-B et B-C conigCes pour la libration sont respectivement 1,500 et 1,639 A et ce sont respectivement les-liaisons la plus courte et la plus longue qui aient CtC observtes pour un chtlate-0,O du diphtnylbore.

[Traduit par la revue]

Introduction 1,3-Dioxa-6-aza-2-boracyclooctanes 1 are known to undergo

intramolecular (transannular) N+B coordination to give very stable bicyclo[3.3 .O]octane ring systems 2, confirmed by chemical and physical investigations (1-6). Generally the extent of the N+B interaction depends on the nitrogen and boron substitution as well as on the conformational conditions. The size and the electronic effects of the substituents markedly influence the magnitude of N-B coordination. In accordance with these findings, replacement of the ring nitrogen atom by oxygen or sulphur led to fairly unstable non-crystalline com- pounds without any detectable transannular bonding (7).

A cyclooctane system without a transannular N 4 B interac- tion should be realized by quaternization of both the tertiary amine and boronate groups of 1. Direct N-alkylation and B-arylation of the known (8) compound 112 seemed disadvanta- geous, so the synthesis of 3 was carried out by N-alkylation of 2-diethylaminoethanol 6 via the addition of oxirane (ethylene oxide) and subsequent trapping of the zwitterionic intermediate 4 as the diphenylboron chelate 3. The symmetrical cyclic B,N-betaine structure 3 has been postulated in a preliminary report (9) on the basis of 'H nrnr data which suggest magnet- ically equivalent ethylene moieties within the ring skeleton, consistent with the formation of 3 by insertion of oxirane into the N-B bond of compound 5 (9).* Compound 3 is structurally

'~art XXVIII: ref. 12. 2Compound 5 has been reported as the diphenylborinic acid ester of

diethylaminoethanol (10) and probably contains an intramolecular N-B bond as shown for the N-unsubstituted derivative (1 1). Different samples of 5, prepared by the methods in ref. 10, contain varying amounts of water.

related to the cyclic six-membered B,N-betaines 7 (12) and 8 (13). Eight-membered ring systems like 3 were also formulated in patents on polymeric (14) and monomeric (15) boric acid esters of aminoalcohols, but to our knowledge no structural proof has yet been presented.

High quality crystals of 3 can be grown from ethanol but ir and 'H nmr data as well as elemental analyses indicate that this compound exists as a monohydrate. This could result from alternative symmetric structures with hydrogen-bonded (A) or covalently bound (B and C) water. Asymmetric structures such as D (from a possible 0-alkylation of the diethylaminoethanol 1 followed by chelate formation with N-(hydroxyethyloxy- ethy1)diethylamine acting as the ligand), E (from hydrolytic ring opening of 3), or F (from chelate formation with ethylene glycol as the ligand) would be unlikely in view of the 'H nmr data. Conductivity measurements and osmometric estimations in DMF solution were unclear. An X-ray crystallographic analysis was undertaken to provide a definitive structural assignment.

Experimental 6,6-Diethyl-2,2-diphenyl-1,3-dioxa-6-azonia-2-b0ratacyc100ctane

monohydrate (3. H20) Methods (a) and (b) Cooled oxirane (ethylene oxide, 0.5 g, 11 mmol) is added to

N,N-diethylaminoethanol (1.17 g, 10 mmol) in 10 rnL ethanol. After dissolving (a) oxybisdiphenylborane (1.73 g, 5 mmol) or (b) triphenyl- boron (2.42 g, 10 mmol), the solution is kept at room temperature for 1 h. Crystallization of the product occurs spontaneously at room tem- perature or upon cooling.

Method (c) N-(2-diphenylboryloxyethy1)diethylamine dihydrate (5.2H20, 3.17

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1092 CAN. 1. CHEM. VOL. 66, 1988

g, 10 mmol) in 15 mL ethanol is mixed with cooled oxirane (0.7 g, 15 mmol) and kept at room temperature for 4 days. Crystallization begins after cooling the reaction mixture.

Yields: (a) 41 %, (b ) 40%, ( c ) 60%. Mp (decomp.) 203-205°C (from ethanol). Lit.3 203°C. Anal. calcd. for3.H20: C 69.98, H 8.81, B 3.15, N 4.08. Found: C 69.38, H 8.98, B 3.42, N 4.13. The substance gives a blue color reaction with diphenylcarbazone in methanol, indicating the presence of the diphenylboron moiety (16). Ir (KBr): 3360 cm-' (br, associated H20). 'H nmr (90 MHz, d6-DMSO/ TMS): S(ppm) = 1.12 (t, J = 7 Hz, 2CH3) 3.2-3.8 (m, 6CHz, HzO), 6.7-7.4 (m, BPh3; (60 MHz, CDC~,/F~CCOOH/TMS): S(ppm) = 1.33 (t, J = 7 Hz, 2CH3), 3.35 (q, J = 7 Hz, 2CH2), 3.60 (m, 2CHZ), 4.65 (m, 2CH2), 7.2-7.7 (m, BPh2), H 2 0 associated with CF3COOH; (60 MHz, d6-acetone/TMS at 70°C): S(ppm) = 1.29 (t, J = 7 Hz, 2CH3), 2.72 (s, 2CH2), 3.57 (q, J = 7 Hz, 2CH2), 3.76 (s, 2CH2), 6.7-7.7 (m, BPh2), H 2 0 not detected. "B nmr (32.1 MHz, DMSO/- Et20BF3): S = 4.9 ppm. Crystals suitable for X-ray analysis were obtained by recrystallization from ethanol.

X-ray crystallographic analysis A crystal bounded by the six faces (followed by the distances in mm

between parallel faces): k(O -1 l ) , 0.46, ?(1 0 l ) , 0.40, (0 1 0),0.59, was mounted in a general orientation. Unit-cell parameters were refined by least-squares on 2 sin 0/A values for 25 reflections (20 = 76-96") measured on a diffractometer with Cu-Ka radiation (A(Ka,)

3 ~ h e melting point of this compound had been mistakenly inter- changed with that of another compound in a table in ref. 9.

TABLE I. Final positional (fractional x lo4) and isotropic thermal parameters ( U X lo3 AZ) with

estimated standard deviations in parentheses*

Atom x Y z @,

*Superscripts refer to site occupancy factors of 0.52,0.48, 0.59, and 0.41, respectively.

= 1.540562, A(Ka2) = 1.544930 A). Crystal data at 22°C are as follows:

CZ0Hz8BN02 .H20 fw = 343.3

Orthorhombic, a = 7.0322(2), b = 16.3505(7), c = 16.8164(4) A, V = 1933.5(1) A3, z = 4, pc = 1.179 Mg mP3, F(OOO) = 744, ~ ( C U - K a ) = 5.76 cm-'. Absentreflections: hOO, h + 2n, OM), k + 2n, and 001, 1 + 2n, uniquely indicate the space group P212121 (D;, NO. 19).

Intensities were measured with nickel-filtered Cu-Ka radiation on an Enraf-Nonius CAD4-F diffractometer. An o-20 scan at 1.2-10.0' min-' over a range of (0.90 + 0.14 tan 0) degrees in w (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 , ~ an absorption correction was applied using the Gaussian integration method (17, 18). Transmis- sion factors ranged from 0.692 to 0.818 for 96 integration points. Of the 2284 independent reflections measured, 2013 (88.1%) had intensi- ties greater than or equal to 3a(I) above background where aZ(I) = S +

4 ~ h e 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.

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KLIEGEL ET AL.

FIG. 1. Stereoscopic view 6,6-diethyl-2,2-diphenyl-l,3-dioxa-6-azonia-2-boratacyclooctane monohydrate; 50% probability thermal ellipsoids are shown for the non-hydrogen atoms. Hydrogen atoms have been assigned arbitrary thermal parameters for the sake of clarity. Broken lines indicate hydrogen bonds.

2B + (0.04(S - B ) ) ~ with S = scan count and B = normalized background count.

The structure was solved by direct methods, the coordinates of the non-hydrogen atoms being determined from an E-map. After refine- ment of the non-hydrogen atoms with anisotropic thermal parameters, anomalous thermal parameters indicated disordering of the N-ethyl groups. A split-atom model was subsequently refined. The site occupancy factors were initially fixed at 0.5 and were subsequently adjusted to give nearly equal average thermal parameters for each group of atoms. In the final stages of refinement the occupancy factors were kept fixed. The ethyl hydrogen atoms were included as fixed atoms (staggered geometry, C-H = 0.98 A, UH Q Ubonded atom) and all other hydrogen atoms were refined with isotropic thermal param- eters. Scattering factors for all atoms and anomalous scattering factors for 0 , N, and C were taken from ref. 19. The weighting scheme w = l /u2(F ), where u2(F ) is derived from the previously defined a2(f), gave uniform average values of w(lFoI - IF,^)^ over ranges of both IFoI and sin O/h and was employed in the final stages of full-matrix refinement of 343 variables. Reflections with I < 3a(I) were not included in the refinement. An isotropic Type I extinction correction (Thornley-Nelmes definition of mosaic anisotropy with a Lorentzian distribution) was applied (20-22). The final value of g was 6.5(2) x lo4. Convergence was reached at R = 0.045 and R, = 0.050 for 201 3 reflections with I ? 3a(I). The mirror-image structure was refined in parallel and rejected at the 99.5% significance level (the R and R,

ratios both = 1.006). The function minimized was Cw(lFoI - I F , I ) ~ , R = ~IIFoI - IFcII/CIFoI and R, = (Zw(lFoI - I F , I ) ~ / ~ W ~ F ~ I ~ ) " ~ .

On the final cycle of refinement the mean and maximum parameter shifts corresponded to 0.02 and 0.3 l a , respectively. The mean error in an observation of unit weight was 1.158. The largest peak on the final difference map was 0.18 e Ap3. The final positional and thermal parameters appear in Tables 1 and 5,5 respectively. Measured and calculated structure factors have been laced in the Devositow 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 (23). The rms standard error in the temperature factors uUij (derived from the least-squares analysis) is 0.0030 A2. The structural subunits PhB , the eight-membered ring, and (-CH2CH2)2N(CH2CH3)2 were separately analysed (rms AUij = 0.0018-0.0059 A2). The appropriate bond distances have been corrected for libration (23, 24), 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

he structure factor table, 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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CAN. J . CHEM. VOL. 66, 1988

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

Angle Angle Bond (deg) Bonds (deg)

in Table 3. Intra-annular torsion angles defining the conformation of eight-membered ring 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 materiaL5

Results and discussion The crystal structure consists of molecules having the

originally suggested structure 3A (9), bridged by hydrogen- bonded water molecules (Fig. 1) to form ribbons extending along the a-axis [0(3)-H(03a). . .0(1) and O(3)-H(p3b) .0(2) (1 + x , y, z), H a - - 0 = 2.06(6) and 1.77(8) A, 0-..0 = 2.775(3) and 2.761(3) A, 0-H.e.0 = 158(5) and 153(6)", re- spectively). These ribbons are reinforced (C(2)-H(2a). . .0(3),

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

Atoms Torsion angle (deg)

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3L ET AL. 1095

3 boat-boat 3 boat-boat (bow-stem substitution) (flank substitution)

3 boat-chair 3 crown

H.a .0 = 2.41(3) A, C . . - O = 3.344(4) A, C-H.-.O = 161(3)") and cross-linked (C(2)-H(2b). . .0(3) ( x - 112, 112 - y , 1 - z), H.a .0 = 2.34(3) A, C . . .O = 3.247(4) A, C-H...O = 141(2)") by probable C-H.9.O hydrogen bonds. The water molecule is thus fully utilized as both a donor and an acceptor of hydrogen bonds.

Compound 3 is the first monocyclic eight-membered B,N-be- taine to be structurally characterized. Other boracyclooctanes having trivalent boron and nitrogen atoms in the 1- and 5-positions are prone to transannular N+B coordination, forming bicyclo[3.3.0]octanes as shown for several different types of boron heterocycles (3,25,26). Structural stabilization, weak in eight-membered cyclic boronates with little or no transannular coordination (5,7), is achieved in 3 by quaterniza- tion of both boron and nitrogen resulting in a zwitterionic salt structure. Dissociative and reassociative processes in solutions of cyclic boronates of bis(2-hydroxyethy1)amines (9 o 10) have been monitored by dynamic nmr spectroscopy (5). Opening of the bicyclic structure 10 proceeds via an intermediate cyclo- octane system 9 having a boat-boat (saddle) conformation. The boat-boat conformation of cyclooctane itself is one of five conformations within 2 kcal/mol of the minimum energy boat-chair conformation, the former being destabilized with respect to the latter primarily by short transannular contacts between inward-pointing hydrogen atoms (27, 28). In the case of 3 these inward-pointing hydrogen atoms are opposite the ring oxygen atoms and cannot cause an energetically unfavorable interaction. Weak attractive transannular C-H . . 0 interac- tions (C(2)--H(2a)-.-O(l) and C(3)-H(3b) - . -0(2) , H . . -O = 2.41(3) and 2.31(3) A, C . s . 0 = 2.843(3) and 2.791(4) A, C-H.a.0 = 106(2) and 115(2)") probably contribute to conformational stability. Both the boat-boat and the closely related but less symmetrical S4 conformations have two types of ring positions: "bow/stern" (inner) and "flank" (outer). Disub- stitution of a pair of opposite "bow/stern" positions is sterically unfavorable, thus the substituted N and B atoms of 3 occupy "flank" positions. The conformation actually observed for 3 is intermediate between the boat-boat (torsion angles of +52.5",

bond angles of 1 18 and 1 19") and S4 (torsion angles alternately 264.9" and + 37.6", bond angles of 1 17 and 1 18") conforma- tions (28), but nearer to the latter (mean Aw and A8 = 5. l and 2.2" relative to S4, 9.7 and 2.7" relative to boat-boat). Calculations for cyclooctane show that the S4 conformation is slightly less strained than the boat-boat (28). The structure of 3 provides an example of the S4 conformation for an eight- membered ring, stabilized by the special arrangement of ring heteroatoms and substitutents. Similar situations have been reported for [NPR2J4, R = Me (29) and NMe2 (30), and [OSe02J4 (31) which also have S4 conformations with the substituted atoms occupying "flank" positions.

As observed earlier for compounds of the type 7 (12), the largest deviations from "ideal" intracyclic bond angles occur at the boron and nitrogen atoms. If the eight-membered ring in 3 had an undistorted boat-boat conformation the molecule would possess a C2 axis passing through the N and B atoms. For the S4 conformation, deviation from C2 symmetry causes a small energy difference between the "flank position substituent sites. Although the difference is not statistically significant, the pseudo-axial B-C bond is 0.008(6) A longer than its pseudo- equatorial counterpart. Significant differences between axial and equatorial B-C bonds have been noted for 7 (12), 8 (13), and a phosphorus analog of 7 (32). The average 0-B and B-C distances6 of 1.500 and 1.639 A are, respectively, the shortest and longest yet observed for an 0,O-chelate of "Ph2B+" (1 2, 33-35), indicating that the monoanion derived from 4 is among the strongest known donors for the "Ph2B+" cation. Relatively strong 0-B binding was also observed for7 (R = (1 12) (CH2)3) and Me) ( 12) where average OTB = 1 .5 10 and 1.5 18 1 and average B-C = 1.625 and 1.623 A. The sum of bond lengths involving the boron atoms in 0-chelates of diphenylboron has been found to be 6.33 l(9) A for compounds containing five-membered chelate rings (?3, 34) and is essea- tially constant f o ~ larger rings; 6.277(9) A (12, 33), 6.276 A ( 3 3 , and 6.278 A for six-, seven-, and eight-membered rings, respectively.

Unlike the short intraannular C-0 distances (average 1.376 A) observed in compounds 7 (12), the values in 3 (1.410(3) and 1.414(3) )) are close to the normal C-0 single bond distance of 1.43 A and are in good agreement with those observed in other boron chelates with hydroxyalkyl ligands (3, 11, 13, 25, 36). Some bond lengths and angles involving the disordered N-ethyl groups deviate significantly from the expected values. This is probably a result of imperfect modelling of the disorder and is commonly observed in such situations. Both phenyl rings are planar within experimental error and show normal substi- tuent-induced geometric distortions (37).

Acknowledgments 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. We are also grateful to Prof. H. Noth, University of Munich, for running the l lB nmr spectra.

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-6~ibration corrected bond lengths (esd's assumed equal to those of the uncorrected values) are employed in the discussion and are compared with similarly treated (23) distances unless otherwise stated.

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