Structural studies of organoboron compounds LI.' ~-~eth~lh~drox~lamine(O-~)2,5-diphen~l-1 ,3,2-dioxaborolan-4-one2

WOLFGANG KLIEGEL AND UTE SCHUMACHER Itlsritut fur Phartnazeufische Chemie der Technischen Universitat Braunschweig,

Beerhovenstrasse 55, 3300 Braunschweig, Germany

AND

STEVEN J . RE'ITIG AND JAMES TROTTER Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, B.C., Canada V6T 121

Received January 21, 1991'

WOLFGANG KLIEGEL, UTE SCHUMACHER, STEVEN J. RETTIG, and JAMES TROTTER. Can. J . Chem. 70, 1188 (1992). Three synthetic methods for the preparation of N-methylhydroxylamine(0-B)2,5-diphenyl-1,3,2-dioxaborolan-4-one

(2-(1-oxa-2-azoniapropy1)-23diphenyl- l,3-dioxa-2-boratacyclopentan-4-one)~, 9, in good yield are described. Crystals of this material are triclinic, a 10.1073(7), b = 12.159(2), c = 14.1 16(2) A, a = 65.079(8)", P = 99.255(9)", y = 92.69(1)", Z = 4, space group P I . The structure was solved by direct methods and was refined by full-matrix least-squares procedures to R = 0.039 and R,,. = 0.045 for 2603 reflections with 12 3n(I). Compound 9 is the first structurally char- acterized example of an N-alkylhydroxylamine(0-B)boron complex having an open-chain B,N-betaine structure. Av- erage bond lengths involving the tetrahedrally coordinated boron atoms in the two crystallographically independent molecules (corrected for th~rmal libration) are (N)O-B = 1.494, (carboxy1ate)O-B = 1.524, (alcoho1ic)O-B = 1.486, and (ary1)C-B = 1.594 A.

WOLFGANG KLIEGEL, UTE SCHUMACHER, STEVEN J. RETTIG et JAMES TROTTER. Can. J. Chem. 70, 1188 (1992). On dCcrit trois mCthodes de synthitse de la N-mCthylhydroxylamine(0-B)2,5-diphenyl-1,3,2-dioxaborolan-4-one (2-

(1-oxa-2-azoniapropy1)-2,5-diphknyl- l,3-dioxa-2-bo~atacyclopentan-4-one), 9, avec un bon rendement. Les cristaux de cette substance sont tricliniques, groupe d'espace P I , avec a = 10,1073(7), b = 12,159(2) et c = 14,116(2) A, a = 65,079(8)", P = 99,255(9)" et y = 92,69(1)" et Z = 4. On a rCsolu la structure par des mkthodes directes et on I'affinee par la methode des moindres carrCs jusqu'a des valeurs de R = 0,039 et R,,, = 0,045 pour 2603 rkflexions avec 1 2 3u(I). Le composC 9 correspond au premier exemple d'un complexe de N-alkylhydroxylamine(0-B)bore possCdant une struc- ture de bCtaine-B,N en chaine ouverte. Les longeurs moyennes des liaisons impliquant les atomes de bore cordonnCs d'une f a ~ o n tCtraCdrique dans les deux molCcules independantes d'un point de vue cristallographique (corrigCes pour la libra- tion thcrmique) sont : (N)O-B = 1,494, (carboxy1ate)O-B = 1,524, (alcoo1ique)O-B = 1,486 et (ary1)C-B = 1,594 A.

[Traduit par la redaction]

Introduction

N-Methylmandelohydroxamic acid 1 was reacted with phenylboronic acid in ethanolic solution in an attempt to synthesize the six-membered cyclic phenylboronate 2. Sim- ilar condensations leading to cycloboronates of type 2 were successful with various N-alkyl- and N-aryl-substituted diarylglycolohydroxamic acids using absolute ethanol as ~ o l v e n t . ~ The use of the monoarylglycolohydroxamic acid 1 as an educt, however, resulted in a crystalline compound having the elemental composition of 2 containing one mole of water. In addition to the ordinary hydrate structure 2 . H20, an intermediate "half-condensate" structure 3 (which could be stabilized by intramolecular O-+B coordination) had to be considered, but the C=O stretching frequency at 1710 cm-' in the infrared spectrum seemed contradictory since cyclic boronates of type 2 typically absorb at frequen- cies between 1640 and 1670 ~ m - ' . ~ The 'H nmr spectrum would also be consistent with the isomeric structure 4, the hypothetical rearrangement product of an N+O acyl shift, also stabilized by intramolecular coordination (N+B in this case). N+O acylotropy has been observed in thermally in-

'Previous paper in this series: ref. 28. 22-(1-Oxa-2-azoniapropyl)-2,5-diphenyl- l,3-dioxa-2-boratacy-

clopentan-4-one. '~ev is ion received October 24, 1991. 4 ~ . Kliegel and U. Schumacher. Unpublished results.

OH Ph Ph

5 6 7

duced rearrangements of N-methyl benzohydroxamic acids (1) as well as in pentanoylation reactions of N-methyl hy- droxylarnine (2), and N+O transacylation products of N-aryl

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KLIEGEL ET AL.: 2 1189

1 a-aminohydroxamic acids have been trapped in situ during amine-catalyzed N+O acyl transfer (3). N-Carbamoylated hydroxylamines are reported to undergo an N+O acyl mi- gration if there is an N-tert-alkyl substituent (4).

The fact that certain hydroxamic acids can serve as li- gands for phenylboronic acid (5) led to the consideration of the five-membered chelate structure 5. Hydroxamate com- plexes of this type, however, show C=O/C=N' absorp- tions between 1610 and 1640 cm-' in their infrared spectra (5). A similar C=O/C=N+ stretching frequency can be expected for the isomeric five-membered chelate 6 having an exocyclic hydroxamic N-OH group. A comparable six- membered 0-B-0 chelate of N-methoxy-N-methylsali- cylamide has a C=O/C=N+ band at 1615 cm-' (6). Thus the C=O/C=N+ bands expected in the infrared spectra of 5 and 6 are quite different from the observed band at 1710 cm-I. The tautomeric structure 7 can be ruled out by the 'H nrnr spectrum, which displays a one-proton singlet at 5.30 ppm, characteristic of a methine proton. In addition, the absence of a color reaction with FeCl, in ethanolic solution suggests that the molecule contains no hydroxamate moiety at all, these being detected by the formation of tris(hydroxamato)iron(III) complexes. An exchange (for- mal) of the N-methylhydroxylamino and B-hydroxy groups leads to the isomeric structures 8 or 9, adducts of N-meth- ylhydroxylamine and the phenylboronate 10 by N+B or O + B coordination, respectively. This is strongly supported by the synthesis via addition of N-methylhydroxylamine to the preformed5 1,3,2-dioxaborolan-4-one 10 or by the one- pot synthesis via three-component condensation of man- delic acid, N-methylhydroxylarnine, and phenylboronic acid. Both of these procedures lead to products identical to that resulting from the reaction of 1 with phenylboronic acid. Yet, the C=O stretching frequency of 1710 cm-I observed for "2 + H2OW is significantly different from the value of 1800 cm-I reported for 10 (7).

out by the spectroscopic data nor by the independent syn- thetic routes studied. In addition, an unambiguous decision between the isomeric structures 8 and 9 could not be made on the basis of the infrared and 'H nmr data. C o m ~ l e x e s of hydroxylamine or N-monosubstituted derivatives thereof with Lewis acid boron compounds are traditionally formulated as N+B adducts (8), but there is also evidence for C h B co- ordination in BF, complexes of hydroxylamine (9). In the transition metal coordination chemistry of hydroxylamine (10) it is known that the neutral ligand can bind to the metal via nitrogen, as H,N-OH, as well as via oxygen in the tauto- meric aminoxide form H,N+-0-. This has been estab- lished by X-ray and neutron diffraction studies, e.g., for Ni(I1)-NH,OH (1 1) and for U(V1)-0N+H3 (12). T o re- solve the ambiguities surrounding the structure of "2 + H20n and the course of the reaction between 1 and phenylboronic acid, an X-ray crystallographic analysis has been carried out.

Experimental

Method A 2-Hydroxy-N-methyl-2-phenylacetohydroxamic acid 1 (13)

(0.18 g, 1 mmol) and phenylboronic acid (0.12 g, 1 mmol) are dissolved in a small amount of absolute ethanol with slight heat- ing. Upon cooling and addition of petroleum ether colorless crys- tals of high purity are formed.

Method B Mandelic acid (7.61 g, 50 mmol) and phenylboronic acid

(6.10 g, 50 mmol) are suspended in 60 mL of benzene and re- fluxed for 1 h with continuous removal of water in a Dean-Stark trap. The hot solution is filtered and the solvent evaporated off in vacuo to a volume of 15 mL, at which time crystallization of 10 commences (yield: 10.85 g, 91%); mp 123°C (from benzene) (lit. (7) mp 124°C (from benzene/petroleum ether)). 10 (1.19 g, 5 mmol) is dissolved in 20 mL of dichloromethane and is mixed with a solution of N-methylhydroxylamine (5 mm01)~ in absolute ethanol. Crystallization eventually starts at room temperature, af- ter evaporation of the solvents.

Method C Mandelic acid (1.52 g, 10 mmol), phenylboronic acid (1.22 g,

10 mmol), and N-methylhydroxylamine (10 mm01)~ are each dis- solved, separately, in the minimum amount of ethanol (necessary for solution). The three solutions are mixed and kept at room tem- perature. After partial evaporation of the solvent in vacuo, crys- tallization begins.

Yields: 0.20 g (70%, method A), 0.86 g (60%, method B), 2.37 g (83%, method C); mp 145-146°C (from ethanol/petro- leum ether). Infrared (KBr): 3130, 2770, 2540 (N-H), 1710 (C=O) cm-I. IH nmr (90 MHz, d6-DMSOITMS), 6 (ppm): 2.78 (s, CH3), 5.30 (s, 0-CH), 6.97-7.63 (m, B-C6H5, C-C6H5), 9.45- 10.75 (s, broad, exchangeable, NH,). Anal. calcd. for C,5H,6BN04: C 63.19, H 5.66, N 4.91; found: C 63.24, H 5.68, N 4.90. Crys- tals suitable for X-ray analysis were obtained by recrystallization from absolute ethanol/petroleum ether.

X-ray crystallographic analysis of 9 A crystal ca. 0.11 x 0.24 x 0.35 mm in size was mounted on

a glass fiber. Unit-cell parameters were refined by least-squares on setting angles for 25 reflections (20 = 24.0-35.2') measured on a diffracto~eter with Mo-K, radiation (AKol = 0.70930, AK,Z = 0.71359 A). Crystal data at 21°C are: C I S H I ~ B N O ~ fw = 285.11

The formation of one of the isomers 3-6 could not be ruled 6~-~ethylhydroxylamine hydrochlonde is added to the solu- tion of an equimolar amount of KOH in absolute ethanol. After

5 ~ n a modification of the literature method (7), mandelic acid was stirring for 10 min the precipitated KC1 is filtered off, and the so- condensed with phenylboronic acid. lution is used for the reaction.

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1190 CAN. J . CHEM. VOL. 70, 1992

Triclinic, a = 10.1073(7), b = 12.159(2), c = 14.1 16(2) A, %= 65.079(8)", P = 99.255(9)", y = 92.69(1)", V = 1552.4(3) A', Z = 4 (two molecules per asymmetric unit), p,. = 1.220 Mg m-', F(000) = 600,-p(Mo-K,) = 0.82 cm-'. Absent reflections: none, space group P 1 (No. 2) from structure analysis.

Intensities were measured with graphite-monochromated Mo-K, radiation on an Enraf-Nonius CAD4-F diffractometer. An w-20 scan at 1 .O- 10.0' min-' over a range of (0.65 + 0.35 tan 0)" in w (scan extended by 25% on each side for background measure- ment) was employed. Data were measured to 20 = 55'. The inten- sities of three check reflections, measured every hour of X-ray exposure time throughout the data collection, showed only small random variations. The data were processed7 and corrected for Lorentz and polarization effects. Of the 7089 independent reflec- tions measured, 2603 (36.7%) had intensities greater than or equal to 3u(I) above background where u2(1) = [C + 2B + (0.040(C- B))~] with C = scan count, B = normalized total background count.

The structure analysis was initiated in the centrosymmetric space group P I on the basis of the E-statistics, this choice being con- finped by the subsequent successful solution and refinement of the strbcture. The structure was solved by direct methods, the coor- dinates of all non-hydrogen atoms of two crystallographically independent molecules being determined from an E-map. The non-hydrogen atoms were refined with anisotropic thermal param- eters and the hydrogen atoms were fixed in idealized positions (N-H = 0.92, C(sp2)-H = 0.97, C(sp3)-H = 0.98 A, UH =

1.2 Ubondeda,om). Scattering factors for all atoms and anomalous dispersion corrections for the non-hydrogen atoms were taken from ref. 14. The weighting scheme w = l /u2(F) gave uniform aver- age values of w(l~,I - IF,I)' over ranges of both IF,I and sin 0/h and was employed in the final stages of full-matrix least-squares refinement of 379 variables. Reflections with I < 3u(I) were not included in the refinement. Convergence was reached at R = 0.039 and R,,. = 0.045 for 2603 reflections with I ? 3u(I). The function minimized was Cw((FoI - R = xIIFo/ - IFcll/xl~,I, R,v = (xw(lFol - I F ~ ( ) ~ / ~ W I F , ~ ~ ) ' " . On the final cycle of refinement the maximum parameter shift corresponded to 0 . 1 4 ~ . The mean error in an observation of unit weight was 1.36. The final difference map showed maximum fluctuations of k0.16 e

The thermal motion has been analyzed in terms of the TLS model (15). The rms error in the temperature factors Uv (derived from the least-squares analysis) is 0.0022 A'. Three structural subunits of each of the two independent molecules were separately analyzed (rms AUv = 0.0028-0.0045 A'). The bond lengths have been cor- rected for libration (15, 16) using shape parameters q2 of 0.08 for all atoms. The final positional and (equivalent) isotropic thermal parameters for the non-hydrogen atoms appear in Table 1. Bond lengths (corrected and uncorrected) and angles are given in Tables 2 and 3 and intra-annular torsion angles in Table 4. Hydrogen atom parameters, anisotropic thermal parameters, general torsion an- gles, and structure factors have been deposited.'

7 ~ o m p u t e r programs used include locally written programs for data processing and locally modified versions of the following: MULTAN80, multisolution program by P. Main, S. J. Fiske, S . E. Hull, L. Lessinger, G. Germain, J. P. Declercq, and M. M. Woolfson; ORFLS, full-matrix least squares, and ORFFE, func- tion 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. ' Supplementary material mentioned in the text may be pur-

chased from the Depository of Unpublished Data, Document De- livery, CISTI, National Research Council Canada, Ottawa, Canada K I A 0R6.

The table of calculated hydrogen atom coordinates has also been deposited with the Cambridge Crystallographic Data Centre and can be obtained on request from The Director, Cambridge Crystallo- graphic Data Centre, University Chemical Laboratory, Lensfield Road, Cambridge CB2 IEW U.K.

TABLE 1. Final positional (fractional X 10") and isotropic thermal parameters (U X 10%') with estimated standard deviations in pa-

rentheses

Atom x y z ucq

Results and discussion The X-ray analysis establishes the structure 9 for the re-

action product. The two crystallographically independent molecules of 9 (Fig. 1) have bond lengths that agree to within experimental error, the most obvious difference between the two being in the orientation of the phenyl rings. There are two chiral centres in the molecule, at C(2) and B. The two independent molecules have the same relative configura- tions at the chiral centres (Fig. 1 shows the 2S,5R enan- tiomer (chemical numbering; B and C(2) in Fig. 1)) and equal numbers of both enantiomers are present in the centrosym- metric crystals. The crystal structure (Fig. 2) consists of in- finite chains of molecules, double-linked by N-H- . -0 hydrogen bonds. The asymmetric unit comprises two inde- pendent molecules that form a pseudo-centrosymmetri~ di- mer linked by N-H. . -0 hydrogen bonds involving the carbonyl oxygen atom (N-H(N 1). . -0(3 ' ) and N'-H(N12) . .0(3) , H. .O = 1.86 and 1.96 A, N. - -0 =

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KLIEGEL ET AL.: 2

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

Bond Uncorr. Corr. Bond Uncorr. Corr.

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

Angle Angle Bonds (deg) Bonds (deg)

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1192 CAN. J . CHEM. VOL. 70. 1992

FIG. 1. Stereoview of one of the two crystallographically independent molecules of 9; 33% probability thermal ellipsoids are shown for the non-hydrogen atoms. The 2S,5R enantiomer is shown. The primed molecule has a slightly different orientation of the B-phenyl group.

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

Value Atoms ( d m

2.751(3) and 2.817(3) A, N-H...O = 163" and 155"). Each of the two independent molecules is also hydrogen-bonded about a crystallographic centre of symmetry (at 1,1/2,1 and 1 /2,1/2,1/2 for the unprimed and primed molecules, re- spectively) by interactions involving a ring oxygen atom (N-H(N2). . .0(2)(2-x, 1-y,2-z) and N'-H(Nf 1). . .0(2') (1-x, 1-y, 1-z), He . .O = 1.96 and 1.86 A, N. . .O = 2.838(3) and 2.75 l(3) A, N-H . .O = 159" and 161°), thus form- ing the infinite chain.

The reaction of 1 with phenylboronic acid in ethanol appears to involve a solvolytic scission of the hydroxamic acid releasing N-methylhydroxylamine which then adds to the Lewis acid cyclic phenylboronate 10, a condensation product easily formed in ethanolic s o l ~ t i o n . ~ N-Methylhy- droxylamine coordinates through the oxygen atom to the electron-deficient boron atom of the 1,3,2-dioxaborolan-4-one ring of 10, resulting in the zwitterionic addition compound

9 ~ e have subsequently been able to obtain the six-membered cycloboronate 2 by carrying out the condensation reaction of 1 and phenylboronic acid in benzene solvent under water-eliminating conditions (see footnote 4).

9 having a (rehybridized) tetravalent borate anion and an ammonium cation. These findings confirm the assumption that N-methylhydroxylamine coordinates to Lewis acid boron compounds via the oxygen atom of the tautomeric N-oxide form rather than via the nitrogen atom of the hydroxyl- amine form, supporting the formulations postulated for boron complexes of unsubstituted hydroxylamine (9). To the best of our knowledge, compound 9 is the first structurally char- acterized example of an N-alkylhydroxylamine(0-B)boron complex having an open-chain B, N-betaine structure ( 17).

The geometry about the boron atom in 9 is very similar to that reported for compound 11, an addition product be- tween a 1,3,2-dioxaborolan-4,5-dione and a cyclic nitrone (7). The mean (N)O-B, (C)O-B, and B-C(pheny1) distances1' of 1.494, 1.505, and 1.594 A for 9 can be compar~d to corresponding values of 1.502, 1.503, and 1.576 A for 11 (7). The mean bond angles about the dis- torted tetrahedral boron atom in 9: O(1)-B-O(2) (within the five-membered chelate ring) = 102. lo, (pheny1)C- B-O(N-oxide) = 106. lo, O(1)-B-O(4) = 109.2", O(2)- B-O(4) = 11 1.5", C(10)-B-0(1) = 112.5", and C(10)- B-O(2) = 116.0" are similar to the corresponding values of 102.9", 105.6", 109.5", 109.S0, 113.7", and 115.6" re- ported for 11. The molecular structures of two other phen- ylboronate N-oxide complexes with P~-B-(O-C),(O-N)+ 1,3-betaine moieties have been determined. Both of these compounds, 12 (18) and 13 (19), are cyclic B,N-betaines and represent examples of the intramolecular coordination of N- oxides to the phenylboronate function. The sums of bond lengths about the boron atom (one B-C(phenyl), one B-O(N), and two B-O(C)) are 6.099 (corrected) for 12, 6.101 (corrected) for 13, 6.100 for 9, and 6.083 A for 11. The B-O(N) bonds in the a~ycl ic compounds 9 and 11 (mean 1.494(2) and 1.502(8) A, respectively) are distinctly shorter than the corresponding bonds in tbe cyclic deriva- tives 12 and 13 (1.539(2) and 1.604(7) A, respectively),

10 Libration-corrected bond lengths for 9 are employed through- out the discussion. Values reported for other compounds are un- corrected unless otherwise indicated.

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KLIEGEL ET AL.: 2

FIG. 2 . Stereoview of the unit-cell of 9 ; 50% probability thermal ellipsoids are shown for the non-hydrogen atoms. Hydrogen bonds are represented by dashed lines. Principal ellipses distinguish the unprimed molecule.

which are subject to some steric strain in the bicyclic ring systems formed by the intramolecular coordination.

The N-0 bond length in the adduct 9 (mean 1.424(2) P\) is considerably shorter than the value of 1.477(2) P\ re- ported for an electron diffraction study of free N-methylhy- droxylamine (20) as a result of the influence of the Lewis acid. This effect appears to be more pronounced than that induced by protonation of N-methylhydroxylamine: N-0 in MeNH,OH+Cl- = 1.447 P\ (21), 1.45 A (22). Similar differences have been reported for the N-0 bonds of un- substituted hydroxylamine where the N-0 bond length in the free base is 1.476(30) A in the solid state (23) and 1.453(2) A in the vapor phase (by microwave spectros-

10 to 9 (1800 * 1710 cm-') can now be interpreted as a consequence of the rehybridization of the boron atom, which can interact with O(2) by pp(.rr) back-donation in 10. This interaction is inhibited by tetracoordination of the boron atom in 9. The high frequency of the C=O stretching vibration in 10 is similar to that reported for P,y-unsaturated five- membered cyclic lactones, showing C=O absorptions around 1800 cm-' that are shifted to lower frequencies if no .rr-bond is present in the P,y-position of the lactone (27). The five- membered BOCCO chelate rings in 9 are planar to within 0.025(3) and 0.044(4) A for the unprimed and primed mol- ecules, respectively, and have flattened C(2)- and B'-enve- lope conformations (see Table 4).

copy) (24), while the N-0 distance is 1.41 l(2) P\ in the Acknowledgements protonated form NH,OH+Cl- (25) and 1.42(1) A in the H,N+-O-U(IV) moiety of a uranium complex (12). ~h~ We thank the Natural Sciences and Engineering Research

N - ~ bond of N-met~ylhydroxylam~ne shows the opposite Council of Canada and the Fonds der Chemischen Indus-

tendency, that in the free base (1.420(2) P\ by electron dif- trie9 Frankfurt am for

fraction) (20) being shorter than those in the ~rotonated form MeNH,OH+Cl- (1.455 A (21) and 1.46 A (22)) and the Lewis acid adduct 9 (mean 1.478(2) A). This shortening of the N-0 bond and lengthening of the N-C bond may be indicative of a respective decrease/increase of repulsion between the nitrogen and the oxygen/methyl carbon atom as the electron-withdrawing proton or Lewis acid interacts with the free base. Steric crowding is not involved in this case.

The difference between the O(2)-B bond involving the alcoholate ligand in 9 (mean 1.486(3) A) and the O(1)-B bond involving the carboxylate ligand (mean 1.524(2) A) probably arises from the higher binding strength of the al- coholate oxygen atom O(2). The reduced donor quality of

, the carboxylate ligand through involvement of O(1) in the 1 mesomeric carboxylate system is a l s ~ obvious from the short

O(1)-C(l) bond (mean 1.3 14(4) A). In comparable y-lac- tone ring systems the average (O=)C-0 bond is known to be 1.350(12) A in length (T6). Correspondingly, the C=O bond in 9 (mean 1.219(5) A) is ?lightly longer than those in the y-lactones (mean 1.201(9) A) (26). The enormous shift of the C=O frequency in the infrared spectrum going from

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1194 CAN. J . CHEM. VOL. 70, 1992

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