Scholarly article on N-(2-hydroxypropylidene)diphenylmethanamine N-oxide 320780-05-0 from Canadian Journal of Chemistry p. 1325

1325

Nitrones and oximes of bifunctional carbonyl

compounds and their reaction products with

diarylborinic acids. Crystal and molecular

structure of examples of five-, six-, and seven-

membered boron chelates¹

Wolfgang Kliegel, Gottfried Lubkowitz, Jens O. Pokriefke, Steven J. Rettig,

and James Trotter

Abstract: Synthesis has been carried out of diarylboron chelates of 2- and 3-hydroxynitrones, of 2- and 3-

hydroxyoximes, and of 2-carboxynitrones and a 2-carboxyoxime. The structures have been determined from spectro-

scopic data and from X-ray analyses of 5d, 9a, 11b, and 19. Crystals (at 180 K) of 5d are monoclinic, a = 10.543(2),

b = 19.085(4), c = 10.2667(3) Å, β = 90.4978(7)°, Z = 4, space group P2₁/c; those of 9a are orthorhombic, a =

10.9913(5), b = 14.9329(7), c = 10.2460(13) Å, Z = 4, space group P2₁2₁2₁; those of 11b are monoclinic, a =

11.227(2), b = 9.967(2), c = 17.0537(4) Å, β = 105.4179(5)°, Z = 4, space group P2₁/n; those of 19 are monoclinic,

a = 11.1847(15), b = 13.715(3), c = 11.5559(5) Å, β = 104.8730(10)°, Z = 4, space group P2₁/n. The structures were

solved by direct methods and refined by full-matrix least-squares procedures to R(F, I ≥ 3σ(I)) = 0.049, 0.047, 0.042,

and 0.047, respectively, for CCD data for 5d, 9a, 11b, and 19. The four molecules contain five-, seven-, six-, and

five-membered rings, respectively, with O-B-N groups in the 5d, 11b, and 19, and O-B-O in 9a; the rings exhibit

various deviations from planarity, particularly the seven-membered ring.

Key words: diarylboron chelates, hydroxyoximes, hydroxynitrones, carboxyoximes, carboxynitrones, organoboron

compounds, crystal structure.

Resumé : On a réalisé la synthèse de chélates du diarylbore avec de 2- et 3-hydroxynitrones, de 2- et 3-

hydroxyoximes, de 2-carboxynitrones et d’une 2-carboxyoxime. On a déterminé les structures à partir de données

spectroscopiques et d’analyses par diffraction des rayons X des composés 5d, 9a, 11b et 19. Les cristaux du composé

5d sont monocliniques (à 180 K), groupe d’espace P2₁/c avec a = 10,543(2), b = 19,085(4) et c = 10,2667(3) Å,

β = 90,4978(7)° et Z = 4; ceux du composé 9a sont orthorhombiques, groupe d’espace P2₁2₁2₁, avec a = 10,9913(5),

b = 14,9329(7) et c = 10,2460(13) Å et Z = 4; ceux du composé 11b sont monocliniques, groupe d’espace P2₁/n avec

a = 11,227(2), b = 9,967(2) et c = 17,0537(4) Å, β = 105,4179(5)° et Z = 4 et ceux du composé 19 sont

monocliniques, groupe d’espace P2₁/n avec a = 11,1847(15), b = 13,715(3) et c = 11,5559(5) Å, β = 104,8730(10)° et

Z = 4. On a résolu les structures par des méthodes directes et on les a affinées par la méthode des moindres carrés

(matrices entières) jusqu’à des valeurs de R(F, I ≥ 3σ(I )) = 0,049, 0,047, 0,042 et 0,047 respectivement pour les

données pour les molécules 5d, 9a, 11b et 19. Les quatre molécules comportent respectivement des cycles à cinq,

sept, six et cinq chaînons et des groupes O-B-N dans les molécules 5d, 11b et 19 et O-B-O pour la molécule 9a;

les cycles présentent des déviations diverses par rapport à la planéité, en particulier pour le cycle à sept chaînons.

Mots Clés : chélates de dialkylbores, hydroxyoximes, hydroxynitrones, carboxyoximes, carboxynitrones, composés

organiques du bore, structure cristalline.

Kliegel et al. 1344

Received May 26, 2000. Published on the NRC Research Press website on October 11, 2000.

W. Kliegel, G. Lubkowitz, and J.O. Pokriefke. Institut für Pharmazeutische Chemie der Technischen Universität Braunschweig,

Beethovenstrasse 55, 38106 Braunschweig, Germany.

S.J. Rettig and J. Trotter.²Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC V6T 1Z1,

Canada.

¹Structural studies of organoboron compounds LXXII. Previous paper in this series: ref. 38.

²Author to whom correspondence may be addressed. Telephone: (604) 822-4865. Fax: (604) 822-2847.

e-mail: jtrt@xray4.chem.ubc.ca

Can. J. Chem. 78: 1325–1344 (2000)

1326

Can. J. Chem. Vol. 78, 2000

CHNO elemental analyzer 1106; boron and chlorine

titrimetrically after Schöniger-oxidation with Metrohm

Potentiograph E 536/Dosimat 655.

Introduction

In the course of our investigations on cyclic boron nitro-

gen betaines (1) we synthesized a series of five-, six-, and

seven-membered boron chelates containing bidentate nitrone

ligands with an N-(1-hydroxyalkyl) group (2) (ring system

A), an N-(2-hydroxylalkyl) group (3) or N-(1-carboxyalkyl)

group (4) (ring system B), and an N-(3-hydroxylalkyl) group

(5) (ring system C), all having the nitrone C=N bond located

outside of the chelate ring (“exocyclic” or “semicyclic”

(6, see footnote for exocyclic double bonds, p. 954)). As an

extension of our work in this area we investigated compara-

ble chelate ring structures with an “endocyclic” C=N double

bond, as depicted in the ring systems D, E, and F. Five-

membered boron chelates of type D had been obtained pre-

viously from various hydroxamic acid derivatives (7). We

now report, continuing and completing this series, the syn-

thesis and structural characteristics of six- and seven-membered

boron chelates, type E and F, derived from the correspond-

ing bidentate nitrones. In connection with these compounds

we describe also the synthesis and structural features of sev-

eral boron chelates of analogous bifunctional oximes. Unam-

biguous proof of coordination sites and ring geometries is

furnished by X-ray crystallographic analyses in selected

cases.

2-(Methylimino)ethanol N-oxide (2a)

Hydroxyacetaldehyde (1.20 g, 20 mmol) and N-methyl-

hydroxylamine hydrochloride (1.67 g, 20 mmol) are dis-

solved in 20 mL of ethanol (96%) and mixed with a solution

of K₂CO₃(1.38 g, 10 mmol) in 5 mL of water under stirring.

After 48 h, KCl is filtered off, the solvent removed in vacuo,

and the residue taken up in 50 mL of absolute ethanol. After

filtration the solvent is distilled off in vacuo. The remaining

oil is not purified further, and is characterized only with

NMR data. ¹H NMR (90 MHz, CDCl₃–TMS) δ (ppm):

3.65 (s, N-CH₃), 4.15 (s, broad, exchangeable, OH), 4.35 (d,

J = 4 Hz, O-CH₂), 6.92 (t, J = 4 Hz, N=CH).

N-(Benzylimino)ethanol N-oxide (2b)

Hydroxyacetaldehyde (1.20 g, 20 mmol), N-benzyl-

hydroxylamine hydrochloride (3.19 g, 20 mmol), and K₂CO₃

(1.38 g, 10 mmol) are reacted as described above for 2a. The

remaining solid is recrystallized from ethanol–ether. Yield:

2.30 g (70%) of colorless plates, mp 82–84°C. IR (KBr, cm^–1):

3 300 and 3 170 (broad, O-H), 1610 (C=N). ¹H NMR

(90 MHz, CDCl₃–TMS) δ (ppm): 4–5 (s, very broad,

exchangeable, OH), 4.35 (d, J = 4 Hz, O-CH₂), 4.83 (s,

N-CH₂), 6.88 (t, J = 4 Hz, N=CH), 7.37 (s, C₆H₅). Anal.

calcd. for C₉H₁₁NO: C 65.44, H 6.71, N 8.48; found:

C 65.41, H 6.71, N 8.57.

1-(Methylimino)-2-propanol N-oxide (2c)³

This compound was prepared as described previously (9).

1-(tert-Butylimino)-2-propanol N-oxide (2d)

2-Hydroxypropanal (1.48 g, 20 mmol) and N-tert-butyl-

hydroxylamine (1.80 g, 20 mmol) are dissolved in 20 mL of

absolute ethanol and stirred for 48 h at room temperature.

The solvent is distilled off in vacuo. The remaining oily resi-

due is not purified further, and is characterized only with

1

NMR data. H NMR (90 MHz, CDCl₃–TMS) δ (ppm): 1.37

(d, J = 7 Hz, OC-CH₃), 1.51 (s, C(CH₃)₃), 4.68 (m, 1 H ex-

changeable, OH and O-CH), 6.93 (d, J = 5 Hz, N=CH).

1-(Cyclohexylimino)-2-propanol N-oxide (2e)

2-Hydroxypropanal (2.22 g, 30 mmol) and N-cyclo-

hexylhydroxylamine (3.45 g, 30 mmol) are reacted as de-

scribed above for 2d. The remaining solid is recrystallized

from absolute ethanol. Yield: 4.42 g (86%) of colorless nee-

dles, mp 129°C. IR (KBr, cm^–1): 3 210 (broad, O-H), 1 600

Experimental

General procedures

1

(C=N). H NMR (90 MHz, CDCl₃–TMS) δ (ppm): 1.04–

Oxybis(diphenylborane) was prepared according to litera-

ture methods (8). All reactions were carried out under nor-

mal conditions, without protective gas atmosphere. Melting

points (uncorrected): Linström apparatus and Büchi SMP 20.

IR: PYE-Unicam SP 3-200, Philips PU 9800 FT-IR-

Spectrometer, V 3.00, and ATI Mattson Genesis Series FT-

2.20 (m, (CH₂)₅), 1.39 (d, J = 7 Hz, CH₃), 3.64 (m, N-CH),

4.71 (m, O-CH), 5.61 (s, exchangeable, OH), 6.85 (d, J =

5 Hz, N=CH). Anal. calcd. for C₉H₁₇NO₂: C 63.13, H 10.01,

N 8.18; found: C 63.09, H 10.15, N 7.97.

1

IR^TMRev. I. H NMR: Varian EM 390 and Bruker AM 400.

1-(Benzylimino)-2-propanol N-oxide (2f)

This compound was prepared as described previously (9).

¹¹B NMR: Bruker AC 200. Elemental analyses: Carlo Erba

³Alternative nomenclature for the compounds of type 2 (generally): N-(2-hydroxyalkylidene) alkanamine N-oxide, as used in ref. 9.

Kliegel et al.

1327

(decomp.). IR (KBr, cm^–1): 1 653 (C=N). ¹H NMR

(90 MHz, CDCl₃–TMS) δ (ppm): 0.80–2.47 (m, (CH₂)₅),

3.53 (m, N-CH), 4.47 (s, O-CH₂), 6.90 (s, N=CH), 6.97–

7.67 (m, B(C₆H₅)₂). ¹¹B NMR (64 MHz, CDCl₃–Et₂OBF₃)

δ (ppm): 7.1 (w_1/2= 300 Hz). Anal. calcd. for C₂₀H₂₄BNO₂:

C 74.78, H 7.53, B 3.37, N 4.36; found: C 74.84, H 7.84,

B 3.35, N 4.15.

1-(Benzhydrylimino)-2-propanol N-oxide (2g)

2-Hydroxypropanal (1.48 g, 20 mmol) and N-benzhydryl-

hydroxylamine (3.98 g, 20 mmol) are reacted as described

above for 2d. The remaining solid is recrystallized from eth-

anol–ether. Yield: 4.21 g (82%) of colorless crystals, mp

110–111°C. IR (KBr, cm^–1): 3 290 (broad, O-H), 1 590

1

(C=N). H NMR (90 MHz, CDCl₃–TMS) δ (ppm): 1.34 (d,

J = 7 Hz, CH₃), 4.71 (m, O-CH), 5.47 (s, broad, exchange-

able, OH), 6.15 (s, N-CH), 6.82 (d, J = 5 Hz, N=CH), 7.32

(s, C(C₆H₅)₂). Anal. calcd. for C₁₆H₁₇NO₂: C 75.27, H 6.71,

N 5.49; found: C 75.47, H 6.71, N 5.64.

4-Benzyl-2,2-diphenyl-1,3-dioxa-4-azonia-2-borata-4-

cyclohexene (3c)

2b (0.33 g, 2 mmol) and oxybis(diphenylborane) (0.35 g,

1 mmol) are reacted as described above for 3a. Yield: 0.53 g

(80%) of colorless needles⁶, mp 125°C (decomp.). IR

(KBr, cm^–1): 1 655 (C=N). ¹H NMR (90 MHz, CDCl₃–

TMS) δ (ppm): 4.49 (m, O-CH₂), 4.84 (s, N-CH₂), 6.92 (s,

N=CH), 7.02–7.45 (m, 15 aromatic H). Anal. calcd. for

C₂₁H₂₀BNO₂: C 76.62, H 6.12, N 4.26; found: C 76.60,

H 6.15, N 4.19.

3-(Cyclohexylimino)-2-butanol N-oxide (2h)

3-Hydroxy-2-butanone (0.88 g, 10 mmol) and N-cyclo-

hexylhydroxylamine (1.15 g, 10 mmol) are dissolved in

30 mL of benzene and refluxed for 5 h using a Dean–Stark

trap for continuous removal of water. The solution is evapo-

rated down to one-quarter of the initial volume. After addi-

tion of 10 mL of hexane and cooling down to –18°C

crystallization starts after 48 h. The air-sensitive compound

can be recrystallized carefully from chloroform–ether. Yield:

0.90 g (49%) of colorless crystals, mp 118°C. IR (KBr, cm^–1):

3 221 (broad, O-H), 1 587 (C=N). ¹H NMR (90 MHz,

CDCl₃–TMS) δ (ppm): 0.67–2.67 (m, (CH₂)₅), 1.37 (d, J =

6 Hz, OC-CH₃), 2.13 (s, NC-CH₃), 3.93 (m, N-CH), 4.67 (q,

J = 6 Hz, O-CH), 6.53 (s, broad, exchangeable, OH). Anal.

calcd. for C₁₀H₁₉NO₂: C 64.83, H 10.34, N 7.56; found:

C 64.92, H 10.41, N 7.46.

4-Benzhydryl-2,2-diphenyl-1,3-dioxa-4-azonia-2-borata-

4-cyclohexene (3d)

Hydroxyacetaldehyde (0.30 g, 5 mmol), N-benzhydryl-

hydroxylamine (1.00 g, 5 mmol), and oxybis (diphenyl-

borane) (0.87 g, 2.5 mmol) are reacted as described for 3b.

Yield: 1.05 g (52%) of colorless crystals⁶, mp 126°C. IR

(KBr, cm^–1): 1 638 (C=N). ¹H NMR (90 MHz, CDCl₃–

TMS) δ (ppm): 4.60 (s, CH₂), 6.30 (s, N-CH), 7.00–7.67 (m,

20 aromatic H and N=CH). ¹¹B NMR (64 MHz, CDCl₃–

Et₂OBF₃) δ (ppm): 6.4 (w_1/2= 257 Hz), 26.0, ratio of peak

areas 20:1.⁷Anal. calcd. for C₂₇H₂₄BNO₂: C 80.00, H 5.97,

B 2.67, N 3.46; found: C 80.08, H 6.04, B 2.68, N 3.34.

4-Methyl-2,2-diphenyl-1,3-dioxa-4-azonia-2-borata-4-

cyclohexene (3a)

2a (0.2 g of the crude product) in 1 mL of ethanol and

oxybis(diphenylborane) (0.35 g, 1 mmol) in 1 mL ethanol

are mixed under stirring. After 5 h at room temperature, the

analytically pure crystals are filtered off and washed with

ether.⁴⁵Yield: 0.31 g (61%) of colorless crystals⁶, mp 104–

4-Cyclohexyl-2,2-di(4-chlorophenyl)-1,3-dioxa-4-azonia-

2-borata-4-cyclohexene (3e)

Hydroxyacetaldehyde (0.12 g, 2 mmol), N-cyclohexylhy-

droxylamine (0.23 g, 2 mmol), and di(4-chlorophenyl)bor-

inic acid (11) (0.50 g, 2 mmol) are reacted as described for

3b. Yield: 0.55 g (71%) of colorless crystals⁶, mp 141°C

(decomp.). IR (KBr, cm^–1): 1 650 (C=N). ¹H NMR

(90 MHz, CDCl₃–TMS) δ (ppm): 0.93–2.43 (m, (CH₂)₅),

3.73 (m, N-CH), 4.60 (s, CH₂), 7.00–7.53 (m, aromatic H

and N=CH). ¹¹B NMR (64 MHz, CDCl₃–Et₂OBF₃) δ (ppm):

6.7 (w_1/2= 300 Hz). Anal. calcd. for C₂₀H₂₂BCl₂NO₂:

C 61.58, H 5.68, B 2.77, N 3.59; found: C 61.58, H 5.71,

B 2.80, N 3.37.

1

105°C (decomp.). IR (KBr, cm^–1): 1 665 (C=N). H NMR

(90 MHz, CDCl₃–TMS) δ (ppm): 3.14 (s, CH₃), 4.23 (m, O-

CH₂), 6.25 (s, N=CH), 6.87–7.40 (m, B(C₆H₅)₂). Anal.

calcd. for C₁₅H₁₆BNO₂: C 71.18, H 6.37, N 5.53; found:

C 71.17, H 6.37, N 5.49. (Boron elemental analyses and

NMR data were obtained only for some of the examples of

compounds of type 3.)

4-Cyclohexyl-2,2-diphenyl-1,3-dioxa-4-azonia-2-borata-4-

cyclohexene (3b)

Hydroxyacetaldehyde (0.12 g, 2 mmol) and N-cyclohexyl-

hydroxylamine (0.23 g, 2 mmol) are dissolved in 5 mL of

ethanol. After addition of oxybis(diphenylborane) (0.35 g,

1 mmol) in 1 mL of ethanol the crystallization commences

at room temperature or after several hours of cooling. The

analytically pure crystals are filtered off and washed with

ether.⁴Yield: 0.43 g (77%) of colorless crystals⁶, mp 131°C

4-Cyclohexyl-2,2-di(1′-naphthyl)-1,3-dioxa-4-azonia-2-

borata-4-cyclohexene (3f)

Hydroxyacetaldehyde (0.12 g, 2 mmol), N-cyclohexylhy-

droxylamine (0.23 g, 2 mmol), and di(1′-naphthyl)borinic

acid (12) (0.56 g, 2 mmol) are reacted as described for 3b.

Yield: 0.66 g (78%) of colorless crystals⁶, mp 153°C

(decomp.). IR (KBr, cm^–1): 1 640 (C=N). ¹H NMR

⁴Recrystallization should be avoided because of a thermally induced rearrangement reaction with an aryl 1,4-shift leading to heterocyclic

arylboronates.⁵Very careful recrystallization without thermal stress or reprecipitation from chloroform with petroleum ether at mild temper-

ature is possible in some cases.

⁵W. Kliegel, G. Lubkowitz, J.O. Pokriefke, S.J. Rettig, and J. Trotter. To be published.

⁶The product gives a blue-colored solution upon the addition of diphenylcarbazone in ethanol, indicating the presence of the chelated

diarylborenium (Ar₂B⁺) ion (10).

⁷The signals with very small peak areas originate from impurities caused by thermally induced rearrangement of the type 3 compounds, re-

sulting in heterocyclic arylboronates 3N (see discussion below).

1328

Can. J. Chem. Vol. 78, 2000

(90 MHz, CDCl₃–TMS) δ (ppm): 0.87–2.27 (m, (CH₂)₅),

3.33 (m, N-CH), 4.17 (s, O-CH₂), 6.63 (s, N=CH), 7.00–

8.13 (m, 12 aromatic H), 8.33–8.70 (m, 2 aromatic H). ¹¹B

NMR (64 MHz, CDCl₃–Et₂OBF₃) δ (ppm): 9.1 (w_1/2= 514 Hz).

Anal. calcd. for C₂₈H₂₈BNO₂: C 79.82, H 6.70, B 2.57,

N 3.32; found: C 79.64, H 6.51, B 2.63, N 3.02.

of slightly yellow crystals⁶, mp 107–108°C (decomp.). IR

(KBr, cm^–1): 1 635 (C=N). ¹H NMR (90 MHz, CDCl₃–

TMS) δ (ppm): 1.38 (d, J = 7 Hz, CH₃), 4.53 (q, J = 7 Hz,

O-CH), 4.74 (s, N-CH₂), 6.82 (s, N=CH), 6.97–7.44 (m, 15

1

aromatic H). H NMR (400 MHz, CDCl₃–TMS) δ (ppm):

1.39 (d, J = 7 Hz, CH₃), 4.59 (q, J = 7 Hz, O-CH), 4.81 (q,

AB system, N-CH₂), 6.88 (d, J = 1 Hz, N=CH), 7.07–7.49

(m, 15 aromatic H). Anal. calcd. for C₂₂H₂₂BNO₂: C 76.99,

H 6.46, N 4.08; found: C 77.09, H 6.45, N 4.19.

4-Cyclohexyl-2,2-di[(2′, 2′′-ethylene)phenyl]-1,3-dioxa-4-

azonia-2-borata-4-cyclohexene (3g)

Hydroxyacetaldehyde (0.24 g, 4 mmol), N-cyclohexyl-

hydroxylamine (0.46 g, 4 mmol), and 5,5′-oxybis(10,11-

dihydro-5H-dibenzo[b,f]borepin) (12) (0.18 g, 2 mmol) are

reacted as described for 3b. Yield: 1.10 g (79%) of colorless

crystals⁶, mp 132°C. IR (KBr, cm^–1): 1 643 (C=N). ¹H NMR

(90 MHz, CDCl₃–TMS) δ (ppm): 0.90–2.33 (m, (CH₂)₅),

3.23 (s, ar-CH₂CH₂-ar), 3.67 (m, N-CH), 4.43 (s, O-CH₂),

6.67–7.67 (m, 8 aromatic H and N=CH). ¹¹B NMR

(64 MHz, CDCl₃–Et₂OBF₃) δ (ppm): 3.6 (w_1/2= 342 Hz).

Anal. calcd. for C₂₂H₂₆BNO₂: C 76.09, H 7.55, B 3.11,

N 4.03; found: C 76.27, H 7.58, B 3.22, N 3.77.

4-Benzhydryl-6-methyl-2,2-diphenyl-1,3-dioxa-4-azonia-

2-borata-4-cyclohexene (3l) (·EtOH)

2g (0.51 g, 2 mmol) and oxybis(diphenylborane) (0.35 g,

1 mmol) are reacted as described for 3a. Yield: 0.85 g (91%)

of slightly yellow plates⁶, mp 86–90°C (decomp.). IR

1

(KBr, cm^–1): 3 390 (broad, O-H), 1 650 (C=N). H NMR

(90 MHz, CDCl₃–TMS) δ (ppm): 1.22 (t, J = 7 Hz, CH₃of

ethanol), 1.48 (d, J = 7 Hz, CH₃), 3.66 (q, J = 7 Hz, O-CH₂

of ethanol), 4.73 (m, O-CH), 6.31 (s, N-CH), 7.06–7.50 (m,

20 aromatic H, N=CH, and OH of ethanol). Anal. calcd. for

C₂₈H₂₆BNO₂+ C₂H₅OH: C 77.42, H 6.93, N 3.01; found:

C 77.30, H 6.94, N 3.22.

4,6-Dimethyl-2,2-diphenyl-1,3-dioxa-4-azonia-2-borata-4-

cyclohexene (3h)

2c (0.41 g of the crude product, about 4 mmol) and

oxybi(diphenylborane) (0.69 g, 2 mmol) are reacted as de-

scribed for 3a. Yield: 0.80 g (75%) of colorless crystals⁶, mp

113–114°C (decomp.). IR (KBr, cm^–1): 1 660 (C=N).

¹H NMR (90 MHz, CDCl₃–TMS) δ (ppm): 1.36 (d, J =

7 Hz, C-CH₃), 3.27 (s, N-CH₃), 4.40 (m, O-CH), 6.51 (s,

N=CH), 6.88–7.40 (m, B(C₆H₅)₂). Anal. calcd. for

C₁₆H₁₈BNO₂: C 71.94, H 6.79, B 4.05, N 5.24; found:

C 71.99, H 6.82, B 4.05, N 5.21.

4-Cyclohexyl-5-methyl-2,2-diphenyl-1,3-dioxa-4-azonia-

2-borata-4-cyclohexene (3m)

Hydroxyacetone (0.37 g,

5 mmol) and oxybis(di-

phenylborane) (0.87 g, 2.5 mmol) are dissolved in 5 mL of

absolute ethanol, if necessary under slight warming. A solu-

tion of N-cyclohexylhydroxylamine (0.58 g, 5 mmol) in

5 mL of ethanol is added without stirring, after precooling of

both solutions. At –18°C, crystallization starts spontaneously

or after rubbing with a glass stick. The crystals are filtered

off and are washed rapidly with a small amount of ethanol

and small portions of ether.⁴Yield: 0.52 g (32%) of colorless

crystals⁶, mp 156°C (decomp.). IR (KBr, cm^–1): 1 647

4-tert-Butyl-6-methyl-2,2-diphenyl-1,3-dioxa-4-azonia-2-

borata-4-cyclohexene (3i)

1

2d (0.58 g of the crude product, about 4 mmol) and

oxybis(diphenylborane) (0.69 g, 2 mmol) are reacted as de-

scribed for 3a. Yield: 1.10 g (89%) of colorless crystals⁶, mp

106–107°C (decomp.). IR (KBr, cm^–1): 1 620 (C=N). ¹H

NMR (90 MHz, CDCl₃–TMS) δ (ppm): 1.38 (d, J = 7 Hz,

OC-CH₃), 1.56 (s, C(CH₃)₃), 4.72 (m, O-CH), 7.00–7.50 (m,

B(C₆H₅)₂and N=CH). Anal. calcd. for C₁₉H₂₄BNO₂: C 73.80,

H 7.82, B 3.50, N 4.53; found: C 73.97, H 7.92, B 3.52,

N 4.50.

(C=N). H NMR (90 MHz, CDCl₃–TMS) δ (ppm): 0.90–

2.67 (m, (CH₂)₅), 1.83 (s, CH₃), 3.90 (m, N-CH), 4.50 (s, O-

CH₂), 6.9 7.73 (m, B(C₆H₅) ₂). ¹¹B NMR (64 MHz, CDCl₃–

Et₂OBF₃) δ (ppm): 6.4 (w_1/2= 257 Hz). Anal. calcd. for

C₂₁H₂₆BNO₂: C 75.24, H 7.82, B 3.22, N 4.18; found:

C 75.24, H 7.96, B 3.22, N 3.98.

4-Benzyl-5-methyl-2,2-diphenyl-1,3-dioxa-4-azonia-2-

borata-4-cyclohexene (3n)

Hydroxyacetone (0.37 g, 5 mmol) and oxybis(diphenyl-

borane) (0.87 g, 2.5 mmol) are dissolved in 5 mL of abso-

lute ethanol, if necessary under slight warming. To a

solution of N-benzylhydroxylamine hydrochloride (0.80 g,

5 mmol), triethylamine (0.51 g, 5 mmol) is added. Both so-

lutions are cooled and mixed without stirring. Crystallization

starts during storage at –18°C spontaneously or upon rub-

bing with a glass stick. The crystals are filtered off and

washed rapidly with a small amount of ethanol and small

portions of ether.⁴Yield: 1.3 g (66%) of colorless crystals⁶,

mp 118°C. IR (KBr, cm^–1): 1 654 (C=N). ¹H NMR

(90 MHz, CDCl₃–TMS) δ (ppm): 1.93 (s, CH₃), 4.53 (s, O-

CH₂), 5.07 (s, N-CH₂), 6.90–7.90 (m, 15 aromatic H). ¹¹B

4-Cyclohexyl-6-methyl-2,2-diphenyl-1,3-dioxa-4-azonia-

2-borata-4-cyclohexene (3j)

2e (0.34 g, 2 mmol) and oxybis(diphenylborane) (0.35 g,

1 mmol) are reacted as described for 3a. Yield: 0.51 g (76%)

of colorless crystals⁶, mp 117–118°C (decomp.). IR

(KBr, cm^–1): 1 625 (C=N). ¹H NMR (90 MHz, CDCl₃–

TMS) δ (ppm): 1.00–2.17 (m, (CH₂)₅), 1.44 (d, J = 7 Hz,

CH₃), 3.57 (m, N-CH), 4.60 (q, J = 7 Hz, O-CH), 6.95

(s, N=CH), 7.00–7.53 (m, B(C₆H₅)₂). Anal. calcd. for

C₂₁H₂₆BNO₂: C 75.24, H 7.82, N 4.18; found: C 75.03,

H 7.87, N 4.32.

NMR (64 MHz, CDCl₃–Et₂OBF₃) δ (ppm): 6.9 (w_1/2

=

4-Benzyl-6-methyl-2,2-diphenyl-1,3-dioxa-4-azonia-2-

borata-4-cyclohexene (3k)

2f (0.36 g, 2 mmol) and oxybis(diphenylborane) (0.35 g,

1 mmol) are reacted as described for 3a. Yield: 0.59 g (86%)

257 Hz), 27.0, ratio of peak areas 10:1.⁷Anal. calcd. for

C₂₂H₂₂BNO₂: C 76.99, H 6.46, B 3.15, N 4.08; found:

C 76.48, H 6.31, B 3.07, N 3.88.

Kliegel et al.

1329

C₁₉H₂₄BNO₂: C 73.80, H 7.82, B 3.50, N 4.53; found:

C 73.84, H 7.99, B 3.53, N 4.40.

5-Methyl-4-(4′-methylphenyl)-2,2-diphenyl-1,3-dioxa-4-

azonia-2-borata-4-cyclohexene (3o)

Hydroxyacetone (0.37 g, 5 mmol), oxybis(diphenylbor-

ane) (0.87 g, 2.5 mmol), and N-(4-methylphenyl)hydroxyl-

amine (0.62 g, 5 mmol) are reacted as described for 3m.

Yield: 0.85 g (50%) of colorless crystals⁶, mp 113°C. IR

(KBr, cm^–1): 1 636 (C=N). ¹H NMR (90 MHz, CDCl₃–

TMS) δ (ppm): 1.70 (s, N=C-CH₃), 2.43 (s, aryl-CH₃), 4.73

(s, O-CH₂), 6.77–8.03 (m, 14 aromatic H)⁸. ¹¹B NMR

(64 MHz, CDCl₃–Et₂OBF₃) δ (ppm): 7.6 (w_1/2= 257 Hz),

29.2, ratio of peak areas 20:1.⁷Anal. calcd. for

C₂₂H₂₂BNO₂: C 76.99, H 6.46, B 3.15, N 4.08; found:

C 77.12, H 6.63, B 3.28, N 3.93.

4-Isopropyl-5,6-dimethyl-2,2-diphenyl-1,3-dioxa-4-

azonia-2-borata-4-cyclohexene (3s)

3-Hydroxy-2-butanone (0.44 g, 5 mmol), oxybis(diphenyl-

borane) (0.87 g, 2.5 mmol), N-isopropylhydroxylamine hy-

drochloride (0.56 g, 5 mmol), and triethylamine (0.51 g,

5 mmol) are reacted as described for 3n. Yield: 0.99 g (64%)

of colorless crystals⁶, mp 133°C. IR (KBr, cm^–1): 1 625

1

(C=N). H NMR (90 MHz, CDCl₃–TMS) δ (ppm): 1.30–

1.67 (m, N-C(CH₃)₂and O-C-CH₃), 1.83 (s, N=C-CH₃),

4.20 (sept, J = 7 Hz, N-CH), 4.36 (q, J = 7 Hz, O-CH),

6.80–7.70 (m, B(C₆H₅) )¹⁰. ¹¹B NMR (64 MHz, CDCl₃–

2

Et₂OBF₃) δ (ppm): 6.2 (w_1/2= 171 Hz). Anal. calcd. for

C₁₉H₂₄BNO₂: C 73.80, H 7.82, B 3.50, N 4.53; found:

C 73.86, H 7.98, B 3.49, N 4.80.

4-Benzhydryl-5-ethyl-2,2-diphenyl-1,3-dioxa-4-azonia-2-

borata-4-cyclohexene (3p)

1-Hydroxy-2-butanone (0.44 g, 5 mmol), oxybis(di-

phenylborane) (0.87 g, 2.5 mmol), and N-benzhydryl-

hydroxylamine (1.00 g, 5 mmol) are reacted as described for

3m. Yield: 1.23 g (57%) of colorless crystals⁶, mp 120°C. IR

(KBr, cm^–1): 1 626 (C=N). ¹H NMR (90 MHz, CDCl₃–

TMS) δ (ppm): 0.97 (t, J = 7 Hz, CH₃), 2.33 (q, J = 7 Hz,

N=C-CH₂), 4.67 (s, O-CH₂), 6.50 (d, N-CH), 6.83–7.90

(m, 20 aromatic H). ¹¹B NMR (64 MHz, CDCl₃–Et₂OBF₃)

δ (ppm): 7.4 (w_1/2= 257 Hz). Anal. calcd. for C₂₉H₂₈BNO₂:

C 80.38, H 6.51, B 2.49, N 3.23; found: C 80.58, H 6.57,

B 2.55, N 2.96.

4-Cyclohexyl-5,6-dimethyl-2,2-diphenyl-1,3-dioxa-4-

azonia-2-borata-4-cyclohexene (3t)

Method A

2h (0.19 g, 1 mmol) and oxybis(diphenylborane) (0.17 g,

0.5 mmol) are reacted as described for 3a. Yield: 0.14 g

(40%).

Method B

3-Hydroxy-2-butanone (0.44 g, 5 mmol), oxybis(diphenyl-

borane) (0.87 g, 2.5 mmol), and N-cyclohexylhydroxylamine

(0.55 g, 5 mmol) are reacted as described for 3m. Yield:

1.10 g (63%) of colorless crystals⁶, mp 132°C. IR (KBr, cm^–1):

1 614 (C=N). ¹H NMR 90 MHz, CDCl₃–TMS) δ (ppm):

0.93–2.57 (m, (CH₂)₅), 1.53 (d, J = 7 Hz, O-C-CH₃), 1.93 (s,

N=C-CH₃), 3.87 (m, N-CH), 4.63 (q, J = 7 Hz, O-CH),

6.90–7.70 (m, B(C₆H₅)₂). ¹¹B NMR (64 MHz, CDCl₃–

Et₂OBF₃) δ (ppm): 6.4 (w_1/2= 257 Hz). Anal. calcd. for

C₂₂H₂₈BNO₂: C 75.65, H 8.08, B 3.10, N 4.01; found:

C 75.64, H 8.16, B 3.05, N 3.84.

4,5,6-Trimethyl-2,2-diphenyl-1,3-dioxa-4-azonia-2-

borata-4-cyclohexene (3q)

3-Hydroxy-2-butanone (0.44 g, 5 mmol), oxybis(diphenyl-

borane) (0.87 g, 2.5 mmol), N-methylhydroxylamine hydro-

chloride (0.42 g, 5 mmol), and triethylamine (0.51 g,

5 mmol) are reacted as described for 3n. Yield: 0.79 g (56%)

of colorless crystals⁶, mp 120°C. IR (KBr, cm^–1): 1 655

1

(C=N). H NMR (90 MHz, CDCl₃–TMS) δ (ppm): 1.57 (t,

J = 7 Hz, O-C-CH₃), 1.67 (s, N=C-CH₃), 3.47 (s, N-CH₃),

4.53 (q, J = 7 Hz, O-CH), 6.90–8.03 (m, B(C₆H₅)₂)⁹. ¹¹B

NMR (64 MHz, CDCl₃–Et₂OBF₃) δ (ppm): 6.6 (w_1/2= 214 Hz),

27.0, ratio of peak areas 20:1.⁷Anal. calcd. for

C₁₇H₂₀BNO₂: C 72.62, H 7.17, B 3.85, N 4.98; found:

C 72.65, H 7.28, B 3.86, N 4.84.

4-Benzyl-5,6-dimethyl-2,2-diphenyl-1,3-dioxa-4-azonia-2-

borata-4-cyclohexene (3u)

3-Hydroxy-2-butanone (0.44 g, 5 mmol), oxybis(diphenyl-

borane) (0.87 g, 2.5 mmol), N-benzylhydroxylamine hydro-

chloride (0.80 g, 5 mmol), and triethylamine (0.51 g, 5 mmol)

are reacted as described for 3n. Yield: 1.21 g (68%) of col-

orless crystals⁶, mp 120°C. IR (KBr, cm^–1): 1 630 (C=N). ¹H

NMR (90 MHz, CDCl₃–TMS) δ (ppm): 1.50 (d, J = 7 Hz,

O-C-CH₃), 1.83 (s, N=C-CH₃), 4.40–4.70 (m, O-CH),

5,6-Dimethyl-2,2-diphenyl-4-propyl-1,3-dioxa-4-azonia-2-

borata-4-cyclohexene (3r)

3-Hydroxy-2-butanone (0.44 g, 5 mmol), oxybis(diphenyl-

borane) (0.87 g, 2.5 mmol), N-propylhydroxylamine hydro-

chloride (0.56 g, 5 mmol), and triethylamine (0.51 g,

5 mmol) are reacted as described for 3n. Yield: 0.79 g (51%)

of colorless crystals⁶, mp 115°C. IR (KBr, cm^–1): 1 637

4.87/5.07 (d/d, AB system, J = 15 Hz, N-CH ), 7.00–7.90

2

1

(m, 15 aromatic H)¹¹. H NMR (400 MHz, CD₃Cl₂–TMS,

–17°C) δ (ppm): 1.53 (d, J = 7 Hz, O-C-CH₃), 1.93 (s, N=C-

CH₃), 4.65 (q, J = 7 Hz, O-CH), 5.02/5.12 (d/d, AB sys-

tem, J = 15 Hz, N-CH₂), 7.03–7.96 (m, 15 aromatic H).

1

(C=N). H NMR (90 MHz, CDCl₃–TMS) δ (ppm): 0.97 (t,

J = 7 Hz, N-C-C-CH₃), 1.57 (d, J = 7 Hz, O-C-CH₃), 1.63–

2.30 (m, N-C-CH₂-C), 1.83 (s, N=C-CH₃), 3.73 (t, J = 7 Hz,

N-CH₂), 4.33–4.60 (m, O-CH), 6.80–8.03 (m, B(C₆H₅)₂)¹⁰.

¹¹B NMR (64 MHz, CDCl₃–Et₂OBF₃) δ (ppm): 7.1 (w_1/2

=

214 Hz), 27.8, ratio of peak areas 5:1. Anal. calcd. for

C₂₃H₂₄BNO₂: C 77.33, H 6.77, B 3.03, N 3.92; found:

C 77.48, H 6.91, B 2.99, N 3.72.

¹¹B NMR (64 MHz, CDCl₃–Et₂OBF₃) δ (ppm): 7.0 (w_1/2

=

214 Hz), 27.0, ratio of peak areas 20:1.⁷Anal. calcd. for

⁸Additional small peaks at: 1.47 (s, N-C-CH₃), 2.77 (s, aryl-CH₃).⁷

⁹Additional small peaks at: 1.03 (d, J = 7 Hz, O-C-CH₃), 1.43 (s, N-C-CH₃), 2.50 (s, N-CH₃).⁷

¹⁰Additional small peaks at: 0.83 (t?, J = 7 Hz, N-C-C-CH₃), 1.40 (s, N-C-CH₃).⁷

¹¹Additional small peaks at: 1.00 (d, J = 7 Hz, O-C-CH₃), 1.53 (s, N-C-CH₃), 3.63 (s, N- CH₂).⁷

1330

Can. J. Chem. Vol. 78, 2000

4-Benzhydryl-5,6-dimethyl-2,2-diphenyl-1,3-dioxa-4-

azonia-2-borata-4-cyclohexene (3v)

4-Cyclohexyl-5,6-tetramethylene-2,2-diphenyl-1,3-dioxa-

4-azonia-2-borata-4-cyclohexene (3z)

3-Hydroxy-2-butanone (0.44 g, 5 mmol), oxybis(diphenyl-

borane) (0.87 g, 2.5 mmol), and N-benzhydrylhydroxyl-

amine (1.00 g, 5 mmol) are reacted as described for 3m.

Yield: 1.05 g (49%) of colorless crystals⁶, mp 105°C (rearr.).

MS (CI, NH₃, pos. mode, 90°C) m/z: 433 (26%, M⁺), 434

2-Hydroxycyclohexanone (0.57 g, 5 mmol) and oxy-

bis(diphenylborane) (0.87 g, 2.5 mmol) are dissolved in

10 mL of absolute ethanol under slight heating and stirring

for a further 10 min. After cooling down to room tempera-

ture, a solution of N-cyclohexylhydroxylamine (0.58 g,

5 mmol) in 5 mL of absolute ethanol is added, and the mix-

ture stored at room temperature. Crystallization starts spon-

taneously or upon cooling. The usually analytically pure

crystals are filtered off after 1 h and can be recrystallized

from ethanol. Yield: 1.20 g (64%) of colorless crystals⁶, mp

(100%, M⁺+ H). IR (KBr, cm^–1): 1 607 (C=N/C=C). H

1

NMR (90 MHz, CDCl₃–TMS) δ (ppm): 1.57 (d, J = 7 Hz,

O-C-CH₃), 1.97 (s, N=C-CH₃), 4.37 (q, J = 7 Hz, O-CH),

6.40 (s, N-CH), 6.67–7.97 (m, B(C₆H₅)₂)¹². ¹¹B NMR

(64 MHz, CDCl₃–Et₂OBF₃) δ (ppm): 6.4 (w_1/2= 257 Hz),

27.0, ratio of peak areas 10:1. Anal. calcd. for C₂₉H₂₈BNO₂:

C 80.38, H 6.51, B 2.49, N 3.23; found: C 80.07, H 6.74,

B 2.47, N 3.02.

1

151°C. IR (KBr, cm^–1): 1 619 (C=N). H NMR (90 MHz,

CDCl₃–TMS) δ (ppm): 1.00–2.90 (m, (CH₂)₄and (CH₂)₅),

4.10 (m, N-CH), 4.47 (dd, J = 12 Hz, J = 7 Hz, O-CH),

6.83–7.67 (m, B(C₆H₅)₂). ¹¹B NMR (64 MHz, CDCl₃–

Et₂OBF₃) δ (ppm): 6.1 (w_1/2= 214 Hz). Anal. calcd. for

C₂₄H₃₀BNO₂: C 76.81, H 8.06, B 2.88, N 3.73; found:

C 76.87, H 8.24, B 2.87, N 3.56.

2,2-Di(4′-chlorophenyl)-4-cyclohexyl-5,6-dimethyl-1,3-

dioxa-4-azonia-2-borata-4-cyclohexene (3w)

3-Hydroxy-2-butanone (0.18 g, 2 mmol), di(4-chloro-

phenyl)borinic acid (11) (0.5 g, 2 mmol), and N-cyclohexyl-

hydroxylamine (0.23 g, 2 mmol) are reacted as described for

3m. Yield: 0.39 g (47%) of colorless crystals⁶, mp 150°C. IR

4-Benzyl-5,6-tetramethylene-2,2-diphenyl-1,3-dioxa-4-

azonia-2-borata-4-cyclohexene (3aa)

1

(KBr, cm^–1): 1 579 (C=N/C=C). H NMR (90 MHz, CDCl₃–

2-Hydroxycyclohexanone (0.57 g, 5 mmol) and oxy-

bis(diphenylborane) (0.87 g, 2.5 mmol) in 10 mL of ethanol

are reacted with N-benzylhydroxylamine hydrochloride

(0.80 g, 5 mmol) and triethylamine (0.51 g, 5 mmol) in

5 mL of ethanol as described for 3z. Yield: 1.50 g (78%) of

colorless crystals⁶, mp 138°C. IR (KBr, cm^–1): 1 632 (C=N).

¹H NMR (90 MHz, CDCl₃–TMS) δ (ppm): 0.87–2.83 (m,

(CH₂)₄), 4.40 (dd, J = 12 Hz, J = 6 Hz, O-CH), 5.03/5.17

(d/d, AB system, J = 15 Hz, N-CH₂), 6.87–7.87 (m, 15 aro-

matic H). ¹¹B NMR (64 MHz, CDCl₃–Et₂OBF₃) δ (ppm):

6.1 (w_1/2= 257 Hz). Anal. calcd. for C₂₅H₂₆BNO₂: C 78.34,

H 6.84, B 2.82, N 3.65; found: C 78.31, H 6.88, B 2.88,

N 3.48.

TMS) δ (ppm): 0.90–2.50 (m, (CH₂)₅), 1.50 (d, J = 7 Hz,

O-C-CH₃), 1.97 (s, N=C-CH₃), 3.90 (m, N-CH), 4.57 (q, J =

7 Hz, O-C-CH₃), 7.00–7.50 (m, 8 aromatic H). ¹¹B NMR

(64 MHz, CDCl₃–Et₂OBF₃) δ (ppm): 5.7 (w_1/2= 258 Hz).

Anal. calcd. for C₂₂H₂₆BCl₂NO₂: C 63.19, H 6.27, B 2.59,

N 3.35; found: C 63.21, H 7.03, B 2.64, N 3.14.

4-Benzhydryl-2,2-di(4-chlorophenyl)-5,6-dimethyl-1,3-

dioxa-4-azonia-2-borata-4-cyclohexene (3x)

3-Hydroxy-2-butanone (0.88 g, 10 mmol), di(4-chloro-

phenyl)borinic acid (11) (2.50 g, 10 mmol), and N-benzhy-

drylhydroxylamine (1.99 g, 10 mmol) are reacted as

described for 3m. Yield: 2.42 g (48%) of colorless crystals⁶,

mp 130°C (rearr.). MS (CI, NH₃, pos. mode, 100°C) m/z:

4-Benzhydryl-5,6-tetramethylene-2,2-diphenyl-1,3-dioxa-

4-azonia-2-borata-4-cyclohexene (3bb)

1

502 (2%, M⁺). IR (KBr, cm^–1): 1 578 (C=N/C=C). H NMR

(90 MHz, CDCl₃–TMS) δ (ppm): 1.57 (d, J = 7 Hz, O-C-

CH₃), 2.10 (s, N=C-CH₃), 4.67 (q, J = 7 Hz, O-C-CH), 6.43

(s, N-CH), 6.63–7.90 (m, 18 aromatic H). ¹¹B NMR

(64 MHz, CDCl₃–Et₂OBF₃) δ (ppm): 6.9 (w_1/2= 342 Hz).

Anal. calcd. for C₂₉H₂₆BCl₂NO₂: C 69.35, H 5.22, B 2.15,

Cl 14.12, N 2.79; found: C 69.29, H 5.20, B 2.28, Cl 14.22,

N 2.71.

2-Hydroxycyclohexanone (0.57 g, 5 mmol), oxybis(di-

phenylborane) (0.87 g, 2.5 mmol), and N-benzhydrylhy-

droxylamine (1.00 g, 5 mmol) are reacted as described for

3z. Yield: 1.50 g (65%) of colorless crystals⁶, mp 144°C. IR

(KBr, cm^–1): 1 610 (C=N/C=C). ¹H NMR (400 MHz,

CDCl₃–TMS) δ (ppm): 0.94–2.91 (m, (CH₂)₄), 4.57 (dd, J =

12 Hz, J = 6 Hz, O-CH), 6.66 (s, N-CH), 6.90–7.82 (m, 20

aromatic H). ¹¹B NMR (64 MHz, CDCl₃–Et₂OBF₃) δ (ppm):

6.9 (w_1/2= 257 Hz). Anal. calcd. for C₃₁H₃₀BNO₂: C 81.05,

H 6.58, B 2.35, N 3.05; found: C 81.08, H 6.63, B 2.39,

N 2.79.

4-Cyclohexyl-5,6-dimethyl-2,2-di(1′-naphthyl)-1,3-dioxa-

4-azonia-2-borata-4-cyclohexene (3y)

3-Hydroxy-2-butanone (0.18 g, 2 mmol), di(1-naphthyl)bor-

inic acid (11) (0.56 g, 2 mmol), and N-cyclohexyl-

hydroxylamine (0.23 g, 2 mmol) are reacted as described for

3m. Yield: 0.51 g (57%) of colorless crystals⁶, mp 157°C. IR

(KBr, cm^–1): 1 625 (C=N). ¹H NMR (90 MHz, CDCl₃–

TMS) δ (ppm): 0.87–2.42 (m, (CH₂)₅), 1.47 (d, J = 7 Hz, O-

C-CH₃), 1.87 (s, N=C-CH₃), 3.93 (m, N-CH), 4.37 (q, J =

7 Hz, O-CH), 7.00–8.00 (m, 12 aromatic H), 8.43–8.87 (m,

2 aromatic H). ¹¹B NMR (64 MHz, CDCl₃–Et₂OBF₃) δ (ppm):

9.2 (w_1/2= 342 Hz). Anal. calcd. for C₃₀H₃₂BNO₂: C 80.18,

H 7.18, B 2.41, N 3.12; found: C 80.21, H 7.25, B 2.45, N 3.00.

5,5-Tetramethylene-4-(4 -methylphenyl)-2,2-diphenyl-1,3-

′

dioxa-4-azonia-2-borata-4-cyclohexene (3cc)

2-Hydroxycyclohexanone (0.57 g, 5 mmol), oxybis(diphenyl-

borane) (0.87 g, 2.5 mmol), and N-(4-methylphenyl)hydroxyl-

amine (0.62 g, 5 mmol) are reacted as described for 3z.

Yield: 1.28 g (67%) of colorless crystals⁶, mp 140°C. IR

1

(KBr, cm^–1): 1 657 (C=N), 1625 (C=C). H NMR (90 MHz,

CDCl₃–TMS) δ (ppm): 1.00–2.90 (m, (CH₂)₄), 2.40 (s, CH₃),

4.63 (dd, J = 12 Hz, J = 6 Hz, O-CH), 6.90–7.80 (m, 14 aromatic

¹²Additional small peaks at: 0.90 (d, J = 7 Hz, O-C-CH₃), 0.97 (s, N-C-CH₃), 4.97 (s, N-CH)⁷.

Kliegel et al.

1331

H). ¹¹B NMR (64 MHz, CDCl₃–Et₂OBF₃) δ (ppm): 6.3

(w_1/2= 299 Hz). Anal. calcd. for C₂₅H₂₆BNO₂: C 78.34, H

6.84, B 2.82, N 3.65; found: C 78.87, H 7.01, B 2.89, N

3.48.

C₂₆H₂₂BNO₂: C 79.81, H 5.67, B 2.76, N 3.58; found:

C 97.91, H 5.70, B 2.89, N 3.42. Single crystals suitable for

X-ray crystallographic analysis were obtained by very slow

crystallization from absolute ethanol.

4,5-Dimethyl-2,2-diphenyl-1-oxa-3-azonia-2-borata-3-

cyclopenten-3-ol (5a)

3-(Methylimino)-2,2-dimethylpropanol N-oxide (8a)

3-Hydroxy-2,2-dimethylpropanal (16) (1.02 g, 10 mmol),

N-methylhydroxylamine hydrochloride (0.84 g, 10 mmol),

and K₂CO₃(0.7 g) are suspended in 10 mol of water and

stirred for 10 min. After addition of 20 mL of benzene the

mixture is refluxed using a Dean–Stark trap for continuous

removal of water. After 5 h, the precipitated KCl is filtered

off and the solvents removed in vacuo. The oily residue is

purified by destillation in vacuo. Yield: 0.61 g (47%) of

slightly orange oil, bp 135°C/5 torr (1 torr = 133 Pa).

MS (CI, NH₃, pos. mode) m/z: 132 (100%, M⁺+ H). IR

(NaCl, film, cm^–1): 3 323 (broad, O-H), 1 606 (C=N). ¹¹H

NMR (CDCl₃–TMS) δ (ppm): 1.23 (s, C(CH₃)₂), 3.63 (s, O-

CH₂), 3.70 (s, N-CH₃), 4.57 (s, broad, exchangeable, OH),

6.53 (s, N=CH). Anal. calcd. for C₆H₁₃NO₂: C 54.94,

H 9.99, N 10.86; found: C 54.92, H 9.84, N 10.10.

3-Hydroxy-2-butanonoxime (13) (0.52 g, 5 mmol) and

oxybis(diphenylborane) (0.87 g, 2.5 mmol) are dissolved

separately in 5 mL of absolute ethanol each. The solutions

are mixed at room temperature under continuous stirring.

Crystallization commences after a few minutes, if necessary

after slight cooling or addition of petroleum ether. The usu-

ally analytically pure substance can be recrystallized from

ethanol. Yield: 1.02 g (76%) of colorless crystals⁶, mp

170°C. IR (KBr, cm^–1): 2 598 (broad, O-H), 1 664 (C=N).

¹H NMR (90 MHz, DMSO-d₆–TMS) δ (ppm): 1.40 (d, J =

7 Hz, O-C-CH₃), 2.10 (s, N=C-CH₃), 4.83 (q, J = 7 Hz,

O-CH), 6.80–7.80 (m, B(C₆H₅)₂), 11.93 (s, exchangeable,

OH). ¹¹B NMR (64 MHz, DMSO-d₆–Et₂OBF₃) δ (ppm): 9.7

(w_1/2= 385 Hz). Anal. calcd. for C₁₆H₁₈BNO₂: C 71.94,

H 6.79, B 4.05, N 5.24; found: C 71.92, H 6.80, B 4.02,

N 5.22.

3-(Propylimino)-2,2-dimethylpropanol N-oxide (8b)

3-Hydroxy-2,2-dimethylpropanol (1.02 g, 10 mmol),

N-propylhydroxylamine hydrochloride (0.12 g, 10 mmol),

and K₂CO₃(0.7 g) are reacted as described for 8a. Yield:

0.75 g (47%) of a slightly yellow oil, bp 142°C/10 torr

(1 torr = 133 Pa). MS (CI, NH₃, pos. mode) m/z: 160 (100%,

M⁺+ H). IR (NaCl, film, cm^–1): 3 338 (broad, O-H), 1 595

(C=N). ¹¹H NMR (CDCl₃–TMS) δ (ppm): 0.97 (t, J = 6 Hz,

N-C-C-CH₃), 1.27 (s, C(CH₃)₂), 1.67–2.20 (m, N-C-CH₂),

3.57 (s, O-CH₂), 3.73 (t, J = 6 Hz, N-CH₂), 6.00 (s,

broad, exchangeable, OH), 6.60 (s, N=CH). Anal. calcd. for

C₈H₁₇NO₂: C 60.35, H 10.76, N 8.80; found: C 60.42, H

10.26, N 8.49.

4-Ethyl-2,2-diphenyl-1-oxa-3-azonia-2-borata-3-

cyclopenten-3-ol (5b)

1-Hydroxy-2-butanonoxime (14) (0.52 g, 5 mmol) and

oxybis(diphenylborane) (0.87 g, 2.5 mmol) are reacted as

described for 5a. Yield: 0.35 g (26%) of colorless crystals⁶,

mp 145°C. IR (KBr, cm^–1): 2 775 (broad, O-H), 1 668

1

(C=N). H NMR (90 MHz, DMSO-d₆–TMS) δ (ppm): 1.10

(t, J = 7 Hz, CH₃), 2.57 (q, J = 7 Hz, N=C-CH₂), 4.73 (s,

O-CH₂), 7.00–8.00 (m, B(C₆H₅)₂), 11.9 (s, exchangeable,

OH). ¹¹B NMR (64 MHz, DMSO-d₆–Et₂OBF₃) δ (ppm):

10.1 (w_1/2= 470 Hz). Anal. calcd. for C₁₆H₁₈BNO₂: C 71.94,

H 6.79, B 4.05, N 5.24; found: C 71.98, H 6.90, B 3.99,

N 5.20.

3-(Cyclohexylimino)-2,2-dimethylpropanol N-oxide (8c)

3-Hydroxy-2,2-dimethylpropanal (1.02 g, 10 mmol) and

N-cyclohexylhydroxylamine are refluxed in 20 mL of ben-

zene using a Dean–Stark trap for continuous removal of wa-

ter. After 5 h, the solvent is distilled off and the remaining

orange-colored oil is dissolved in a small amount of ethanol.

Crystallization is achieved upon cooling and addition of n-

hexane. Yield: 1.30 g (66%) of colorless crystals, mp 72°C

(from EtOH–hexane). IR (KBr, cm^–1): 3 284 (broad, O-H),

4,5-Tetramethylene-2,2-diphenyl-1-oxa-3-azonia-2-

borata-3-cyclopenten-3-ol (5c)

2-Hydroxycyclohexanonoxime (15) (0.39 g, 3 mmol) and

oxybis(diphenylborane) (0.52 g, 1.5 mmol) are reacted as

described for 5a. Yield: 0.71 g (81%) of colorless crystals⁶,

mp 180°C. IR (KBr, cm^–1): 2 472 (broad, O-H), 1 674

1

(C=N). H NMR (90 MHz, DMSO-d₆–TMS) δ (ppm): 1.00–

1

3.23 (m, (CH₂)₄), 4.60 (m, O-CH), 7.00–7.83 (m, B(C₆H₅)₂),

11.83 (s, exchangeable, OH). ¹¹B NMR (64 MHz, DMSO-

d₆–Et₂OBF₃) δ (ppm): 10.2 (w_1/2= 470 Hz). Anal. calcd. for

C₁₈H₂₀BNO₂: C 73.75, H 6.88, B 3.69, N 4.78; found:

C 73.67, H 6.99, B 3.69, N 4.74.

1 595 (C=N). H NMR (90 MHz, CDCl₃–TMS) δ (ppm):

0.57–2.33 (m, (CH₂)₅), 1.23 (s, C(CH₃)₂), 3.40–3.87 (m,

N-CH), 3.60 (d, J = 6 Hz, O-CH₂), 6.07 (t, J = 6 Hz, broad,

exchangeable, OH), 6.53 (s, N=CH). Anal. calcd. for

C₁₁H₂₁NO₂: C 66.30, H 10.62, N 7.03; found: C 66.29,

H 10.61, N 6.82.

2,2,4,5-Tetraphenyl-1-oxa-3-azonia-2-borata-3-

cyclopenten-3-ol (5d)

3-(Benzylimino)-2,2-dimethylpropanol N-oxide (8d)

3-Hydroxy-2,2-dimethylpropanal (2.04 g, 20 mmol),

N-benzylhydroxylamine hydrochloride (3.19 g, 20 mmol),

and K₂CO₃(1.40 g) are reacted as described for 8a. After

the refluxing, about two-thirds of the benzene is distilled off

and crystallization is achieved by addition of ether. Yield:

1.77 g (43%) of colorless crystals, mp 98°C (from benzene–

ether). IR (KBr, cm^–1): 3 243 (broad, O-H), 1 600

(C=N/C=C). ¹H NMR (90 MHz, CDCl₃–TMS) δ (ppm): 1.23

α-Benzoinoxime (commercial product “Cupron”) (0.23 g,

1 mmol) and oxybis(diphenylborane) (0.17 g, 0.5 mmol) are

reacted as described for 5a. Yield: 0.32 g (82%) of colorless

crystals⁶, mp 170°C. IR (KBr, cm^–1): 2 734 (broad, O-H),

1

1 637 (C=N), 1596 (C=C). H NMR (90 MHz, DMSO-d₆–

TMS) δ (ppm): 6.53 (s, O-CH), 6.93–7.90 (m, 20 aromatic

H), 12.77 (s, exchangeable, OH). ¹¹B NMR (64 MHz,

DMSO-d₆–Et₂OBF₃)

δ

(ppm): 8.0. Anal. calcd. for

1332

Can. J. Chem. Vol. 78, 2000

1

(s, C(CH₃)₂), 3.63 (d, J = 6 Hz, O-CH₂), 4.87 (s, N-CH₂),

5.70 (t, J = 6 Hz, exchangeable, OH), 6.53 (s, N=CH), 7.43

(s, C₆H₅). Anal. calcd. for C₁₂H₁₇NO₂: C 69.54, H 8.27,

N 6.76; found: C 69.52, H 8.27, N 6.67.

(C=N). H NMR (90 MHz, CDCl₃–TMS) δ (ppm): 0.67–

2.27 (m, (CH₂)₅), 1.30 (s, C(CH₃)₂), 2.97 (m, N-CH), 3.77

(s, O-CH₂), 6.93 (s, N=CH), 7.07–8.00 (m, B(C₆H₅)₂). ¹¹B

NMR (64 MHz, CDCl₃–Et₂OBF₃) δ (ppm): 10.6 (w_1/2

=

342 Hz), 46.7, ratio of peak areas 10:1.¹³Anal. calcd. for

C₂₃H₃₀BNO₂: C 76.06, H 8.32, B 2.98, N 3.86; found: C

76.22, H 8.36, B 3.05, N 3.66.

3-(Benzhydrylimino)-2,2-dimethylpropanol N-oxide (8e)

3-Hydroxy-2,2-dimethylpropanal (1.02 g, 10 mmol) and

N-benzhydrylhydroxylamine (1.99 g, 10 mmol) are reacted

as described for 8c. After the refluxing about two-thirds of

the benzene is distilled off and crystallization is achieved

upon cooling and addition of petroleum ether. Yield: 1.84 g

(65%) of colorless crystals, mp 123°C (from abs ethanol–pe-

troleum ether). IR (KBr, cm^–1): 3 400 (broad, O-H), 1 587

(C=N/C=C). ¹H NMR (90 MHz, CDCl₃-TMS), δ (ppm):

1.20 (s, C(CH₃)₂), 3.63 (d, J = 6 Hz, O-CH₂), 5.67 (t, J =

6 Hz, exchangeable, OH), 6.20 (s, N-CH), 6.57 (s, N=CH),

7.37 (s, C(C₆H₅)₂). Anal. calcd. for C₁₈H₂₁NO₂: C 76.30,

H 7.47, N 4.94; found: C 76.31, H 7.46, N 4.71.

4-Benzyl-6,6-dimethyl-2,2-diphenyl-1,3-dioxa-4-azonia-2-

borata-4-cycloheptene (9d)

8d (0.42 g, 2 mmol) and oxybis(diphenylborane) (0.35 g,

1 mmol) are reacted as described for 9a. Yield: 0.73 g (98%)

of colorless crystals⁶, mp 174°C. IR (KBr, cm^–1): 1 652

1

(C=N), 1 605 (C=C). H NMR (90 MHz, CDCl₃–TMS)

δ (ppm): 1.23 (s, C(CH₃)₂), 3.77 (s, O-CH₂), 4.27 (s,

N-CH₂), 6.73 (s, N=CH), 6.93–8.00 (m, 15 aromatic H). ¹¹B

NMR (64 MHz, CDCl₃–Et₂OBF₃) δ (ppm): 10.7 (w_1/2

=

258 Hz), 47.3, ratio of peak areas 10:1.¹³Anal. calcd. for

C₂₄H₂₆BNO₂: C 77.64, H 7.06, B 2.91, N 3.77; found:

C 77.75, H 7.21, B 3.01, N 3.69.

4,6,6-Trimethyl-2,2-diphenyl-1,3-dioxa-4-azonia-2-

borata-4-cycloheptene (9a)

Solutions of 8a (0.13 g, 1 mmol) in 1 mL of ethanol and

of oxybis(diphenylborane) (0.17 g, 0.5 mmol) in 1 mL of

ethanol are mixed under stirring at room temperature. Crys-

tallization commences usually after a few minutes or upon

cooling and rubbing with a glass stick. If necessary the sub-

stance can be recrystallized from absolute ethanol. Yield:

0.21 g (71%) of colorless crystals⁶, mp 176°C. IR (KBr, cm^–1):

4-Benzhydryl-6,6-dimethyl-2,2-diphenyl-1,3-dioxa-4-

azonia-2-borata-4-cycloheptene (9e)

8e (0.57 g, 2 mmol) and oxybis(diphenylborane) (0.35 g,

1 mmol) are reacted as described for 9a. Yield: 0.76 g (85%)

of colorless crystals⁶, mp 168°C. IR (KBr, cm^–1): 1 664

(C=N), 1 586 (C=C). ¹H NMR (90 MHz, CDCl₃–TMS)

δ (ppm): 1.30 (s, C(CH₃)₂), 3.83 (s, O-CH₂), 5.60 (s, N-CH),

6.67–8.00 (m, 20 aromatic H and N=CH). ¹¹B NMR

(64 MHz, CDCl₃–Et₂OBF₃) δ (ppm): 10.5 (w_1/2= 342 Hz),

46.0, ratio of peak areas 10:1.¹³Anal. calcd. for

C₃₀H₃₀BNO₂: C 80.54, H 6.76, B 2.42, N 3.13; found:

C 80.37, H 6.80, B 2.49, N 2.91.

1

1 664 (C=N). H NMR (90 MHz, CDCl₃–TMS) δ (ppm):

1.27 (s, C(CH₃)₂), 3.03 (s, N-CH₃), 3.70 (s, O-CH₂), 6.80

(s, N=CH), 7.00–7.97 (m, B(C₆H₅)₂). ¹¹B NMR (64 MHz,

CDCl₃–Et₂OBF₃) δ (ppm): 10.6 (w_1/2= 257 Hz). Anal. calcd.

for C₁₈H₂₂BNO₂: C 73.24, H 7.51, B 3.66, N 4.57; found:

C 73.37, H 7.53, B 3.77, N 4.67. Single crystals suitable for

X-ray crystallographic analysis were obtained by very slow

crystallization from ethanol.

5,5-Dimethyl-2,2-diphenyl-1-oxa-3-azonia-2-borata-3-

cyclohexen-3-ol (11a)

3-Hydroxy-2,2-dimethylpropionaldoxime (16) (0.12 g,

1 mmol) and oxybis(diphenylborane) (0.17 g, 0.5 mmol) are

reacted as described for 5a. Yield: 0.25 g (89%) of colorless

crystals⁶, mp 165°C (from ethanol). IR (KBr, cm^–1): 2 566

(broad, O-H), 1 638 (C=N). ¹H NMR (90 MHz, CDCl₃–

TMS) δ (ppm): 1.30 (s, C(CH₃)₂), 3.57 (s, O-CH₂), 7.13–

7.67 (m, 1 H exchangeable, OH and B(C₆H₅)₂, and N=CH).

¹H NMR (400 MHz, CDCl₃–TMS) δ (ppm): 1.30 (s,

C(CH₃)₂), 3.53 (s, O-CH₂), 7.13 (m, exchangeable, OH),

7.23–7.57 (m, B(C₆H₅)₂), 7.60 (s, N=CH). ¹¹B NMR

(64 MHz, CDCl₃–Et₂OBF₃) δ (ppm): 4.00 (w_1/2= 257 Hz).

Anal. calcd. for C₁₇H₂₀BNO₂: C 72.62, H 7.17, B 3.84,

N 4.98; found: C 72.67, H 7.22, B 3.98, N 4.84.

6,6-Dimethyl-2,2-diphenyl-4-propyl-1,3-dioxa-4-azonia-2-

borata-4-cycloheptene (9b)

8b (0.16 g, 1 mmol) and oxybis(diphenylborane) (0.17 g,

0.5 mmol) are reacted as described for 9a. Yield: 0.22 g

(68%) of colorless crystals⁶, mp 170°C. IR (KBr, cm^–1):

1

1 655 (C=N). H NMR (90 MHz, CDCl₃–TMS) δ (ppm):

0.77 (t, J = 6 Hz, N-C-C₃), 1.27 (s, C(CH₃)₂), 1.47–1.97 (m,

N-C-CH₂), 3.10 (t, J = 6 Hz, N-CH₂), 3.73 (s, O-CH), 6.90

(s, N=CH), 7.00–7.90 (m, B(C₆H₅)₂). ¹¹B NMR (64 MHz,

CDCl₃–Et₂OBF₃) δ (ppm): 10.3 (w_1/2= 257 Hz), 44.6, ratio

of peak areas 10:1.¹³Anal. calcd. for C₂₀H₂₆BNO₂: C 74.32,

H 8.11, B 3.34, N 4.33; found: C 74.20, H 8.10, B 3.40,

N 4.17.

2,2-Di(4′-methoxyphenyl)-5,5-dimethyl-1-oxa-3-azonia-2-

borata-3-cyclohexen-3-ol (11b)

3-Hydroxy-2,2-dimethylpropionaldoxime (16) (0.23 g,

2 mmol) and di(4-methoxyphenyl)-borinic acid (obtained

from the 2-aminoethyl ester (19) by hydrolysis (11)) (0.48 g,

2 mmol) are reacted as described for 5a. Yield: 0.33 g (49%)

4-Cyclohexyl-6,6-dimethyl-2,2-diphenyl-1,3-dioxa-4-

azonia-2-borata-4-cycloheptene (9c)

8c (0.20 g, 2 mmol) and oxybis(diphenylborane) (0.35 g,

1 mmol) are reacted as described for 9a. Yield: 0.64 g (88%)

of colorless crystals⁶, mp 178°C. IR (KBr, cm^–1): 1 636

¹³Side signals with a small peak area at about 46 ppm have also been observed in the ¹¹B NMR spectra of diphenylboron chelates from other

nitrone ligands (17) indicating a trivalent (sp²) boron atom of a Ph₂B-O-R species (18). It is not yet clear whether the signal originates from

an open-chain diphenylborinate (scission of the nitrone O-B coordination in 9), or from free diphenylborinic acid caused by partial

solvolysis.

Kliegel et al.

1333

of colorless crystals⁶, mp 128°C (from ethanol). IR

(KBr, cm^–1): 2 517 (broad, O-H), 1 640 (C=N), 1 599

(C=C). H NMR (90 MHz, CDCl₃–TMS) δ (ppm): 1.27 (s,

C(CH₃)₂), 3.60 (s, O-CH₂-C), 6.67–8.00 (m, 1 H exchange-

able, OH and N=CH, and 8 aromatic H). ¹¹B NMR

(64 MHz, CDCl₃–Et₂OBF₃) δ (ppm): 9.9 (w_1/2= 342 Hz).

Anal. calcd. for C₁₉H₂₄BNO₄: C 66.88, H 7.09, B 3.17,

N 4.11; found: C 66.81, H 7.15, B 3.28, N 3.95. Single crys-

tals suitable for X-ray crystallography were obtained by very

slow crystallization from absolute ethanol.

Method B

Large quantities (up to 40 mmol) can be prepared advan-

tageously also by the procedure described in the literature

(21) for 16d (see below), which gives better yields, but re-

quires more time.

1

(tert-Butylimino)acetic acid N-oxide (16b)

Method A, as described above for 16a, is applied for

N-tert-butylhydroxylamine (0.45 g, 5 mmol), without tri-

ethylamine in n-hexane as the solvent. Yield: 0.58 g (68%)

of colorless crystals, mp 62°C (from hexane). IR (KBr, cm^–1):

2 600–2 170 (curtain, O-H assoc.), 1 703 (C=O), C=N band

4,5-Dimethyl-2,2-diphenyl-1-oxa-3-azonia-2-borata-3-

cyclohexen-3-ol (11c)

1

not identified. H NMR (90 MHz, CDCl₃–TMS) δ (ppm):

1.60 (s, C(CH₃)₃), 7.47 (s, N=CH), 14.63 (s, broad, ex-

changeable, OH). Anal. calcd. for C₆H₁₁NO₃: C 49.65,

H 7.64, N 9.65; found: C 49.42, H 7.73, N 9.42.

4-Hydroxy-3-methyl-2-butanonoxime (15) (0.59 g, 5 mmol)

and oxybis(diphenylborane) (0.87 g, 2.5 mmol) are reacted

as described for 5a. Instead of petroleum ether, diethyl ether

is added to initiate the crystallization. Yield: 0.85 g (58%) of

colorless crystals⁶, mp 151°C (from ethanol–ether). IR

(Cyclohexylimino)acetic acid N-oxide (16c)

Method A, as described above for 16b, is applied for

N-cyclohexylhydroxylamine (0.58 g, 5 mmol). Yield: 0.46 g

(54%) of colorless crystals, mp 87°C (from hexane). IR

(KBr, cm^–1): 2 500–2 340 (curtain, OH assoc.), 1 727/1 699

1

(KBr, cm^–1): 2 691 (broad, O-H), 1 638 (C=N). H NMR

(90 MHz, CDCl₃–TMS) δ (ppm): 1.40 (d, J = 7 Hz, N=C-C-

CH₃), 2.23 (s, N=C-CH₃), 2.40–3.00 (m, X of ABX system,

N=C-CH), 3.47/3.90 (m/m, AB of ABX system, O-CH₂),

7.07 (s, broad, exchangeable, OH), 7.20–7.67 (m, B(C₆H₅)₂).

1

(C=O), 1 609 (C=N). H NMR (90 MHz, CDCl₃–TMS)

δ (ppm): 1.00–2.37 (m, (CH₂)₅), 3.90 (m, N=CH), 7.43 (s,

N=CH), 14.77 (s, broad, exchangeable, OH). Anal. calcd. for

C₈H₁₃NO₃: C 56.13, H 7.65, N 8.18; found: C 56.02,

H 7.80, N 8.05.

¹¹B NMR (64 MHz, CDCl₃–Et₂OBF₃) δ (ppm): 6.6 (w_1/2

=

235 Hz). Anal. calcd. for C₁₇H₂₀BNO₂: C 72.62, H 7.17, B

3.84, N 4.98; found: C 72.66, H 7.16, B 3.87, N 4.85.

4,6,6-Trimethyl-2,2-diphenyl-1-oxa-3-azonia-2-borata-3-

cyclohexen-3-ol (11d)

(Benzylimino)acetic acid N-oxide (16d)

The compound was prepared as described in the literature

(21), using N-benzylhydroxylamine hydrochloride and

triethylamine in equimolar amounts.

4-Hydroxy-4-methyl-2-pentanonoxime (20) (0.66 g,

5 mmol) and oxybis(diphenylborane) (0.87 g, 2.5 mmol) are

reacted as described for 5a. Yield: 0.60 g (41%) of colorless

crystals⁶, mp 124°C (from ethanol). IR (KBr, cm^–1): 2 702

(broad, O-H), 1 650 (C=N). ¹H NMR (90 MHz, CDCl₃–

TMS) δ (ppm): 1.37 (s, C(CH₃)₂), 2.30 (s, N=C-CH₃), 2.50

(s, N=C-CH₂-C), 7.10–7.90 (m, 1 H exchangeable, OH and

B(C₆H₅)₂). ¹¹B NMR (64 MHz, CDCl₃–Et₂OBF₃) δ (ppm):

6.2 (w_1/2= 128 Hz). Anal. calcd. for C₁₈H₂₂BNO₂: C 73.24,

H 7.51, B 3.66, N 4.75; found: C 73.27, H 7.65, B 3.71,

N 4.58.

4-Methyl-2,2-diphenyl-1,3-dioxa-4-azonia-2-borata-4-

cyclohexen-6-one (17a)

Method A

16a (0.21 g, 2 mmol) in 5 mL of absolute ethanol and

oxybis(diphenylborane) (0.35 g, 1 mmol) in 5 mL of abso-

lute ethanol are mixed. Under stirring at room temperature

crystallization commences. After complete precipitation, the

crystals are washed with 3 × 5 mL of warm ether. Yield:

0.40 g (75%) of colorless crystals⁶, mp 114°C.¹⁴IR

(KBr, cm^–1): 1 719/1 705 (C=O), 1 602 (C=N/C=C). ¹H

NMR (90 MHz, DMSO-d₆–TMS) δ (ppm): 3.97 (s, CH₃),

7.00–8.00 (m, B(C₆H₅)₂), 8.53 (s, N=CH). A ¹¹B NMR spec-

trum could not be obtained because of poor solubility. Anal.

calcd. for C₁₅H₁₄BNO₃: C 67.45, H 5.28, B 4.05, N 5.24;

found: C 67.26, H 5.26, B 4.06, N 5.03.

(Methylimino)acetic acid N-oxide (16a)

Method A

Glyoxylic acid monohydrate (0.46 g, 5 mmol), dried

MgSO₄(0.60 g), N-methylhydroxylamine hydrochloride

(0.42 g, 5 mmol), and triethylamine (0.51 g, 5 mmol) are

refluxed in 50 mL of ether for 5 min. The solid residue is fil-

tered off and extracted several times with boiling ether. The

combined ether solutions are partially evaporated and, if

necessary, cooled for crystallization. Yield: 0.12 g (24%) of

colorless crystals, mp 78°C (from ether). IR (KBr, cm^–1):

2 600–2 000 (“curtain,” O-H assoc.), 1 711 (C=O), 1 580

Method B

Glyoxylic acid monohydrate (0.46 g, 5 mmol) and

oxybis(diphenylborane) (0.87 g, 2.5 mmol) are dissolved in

5 mL of absolute ethanol under stirring at room temperature

for 5 min. A solution of N-methylhydroxylamine hydrochlo-

ride (0.42 g, 5 mmol) and triethylamine (0.51 g, 5 mmol)

in 5–10 mL of ethanol is added and stirred briefly for

homogenization. Yield: 1.00 g (75%) of colorless crystals as

described for method A.

1

(C=N). H NMR (90 MHz, CDCl₃–TMS) δ (ppm): 3.93 (s,

CH₃), 7.43 (s, N=CH), 14.67 (s, very broad, exchangeable,

OH). Anal. calcd. for C₃H₅NO₃: C 34.96, H 4.89, N 13.59;

found: C 34.72, H 4.87, N 13.32.

¹⁴The usually analytical pure compound rearranges under thermal stress in solution⁵; therefore, recrystallization was not possible.

1334

Can. J. Chem. Vol. 78, 2000

also available by a three component reaction of glyoxylic

acid, N-benzylhydroxylamine (hydrochloride + triethylamine),

and di(4-chlorophenyl)borinic acid (method B as described

for 17a).

4-tert-Butyl-2,2-diphenyl-1,3-dioxa-4-azonia-2-borata-4-

cyclohexen-6-one (17b)

16b (0.58 g, 4 mmol) and oxybis(diphenylborane) (0.70 g,

2 mmol) are reacted as described for 17a (method A). Yield:

0.95 g, (77%) of colorless crystals⁶, mp 132°C¹⁴. IR

(KBr, cm^–1): 1 703 (C=O), 1 605 (C=N/C=C). ¹H NMR

(90 MHz, CDCl₃–TMS) δ (ppm): 1.47 (s, C(CH₃)₃), 7.00 –

7.60 (m, B(C₆H₅)₂), 7.70 (s, N=CH). ¹¹B NMR (64 MHz,

CDCl₃–Et₂OBF₃) δ (ppm): 10.5 (w_1/2= 300 Hz), 30.0, ratio

of peak areas 40:1.¹⁵Anal. calcd. for C₁₈H₂₀BNO₃: C 69.93,

H 6.52, B 3.50, N 4.53; found: C 69.93, H 6.50, B 3.60,

N 4.36. The compound is also available by a three compo-

nent reaction of equimolar amounts of glyoxylic acid,

N-tert-butylhydroxylamine, and diphenylborinic acid

(method B as described for 17a, omitting triethylamine).

4-Methyl-2,2,5-triphenyl-1,3-dioxa-4-azonia-2-borata-4-

cyclohexen-6-one (17f)

Phenylglyoxylic acid (0.75 g, 5 mmol) and oxybis(di-

phenylborane) (0.87 g, 2.5 mmol) are dissolved in 5 mL of

absolute ethanol under stirring. After addition of a solution

of N-methylhydroxylamine hydrochloride (0.42 g, 5 mmol)

and triethylamine (0.51 g, 5 mmol) in 5 mL of ethanol and

short heating, crystallization of the analytically pure product

starts, at room temperature, eventually after cooling. Yield:

1.52 g, (89%) of colorless crystals⁶, mp 132°C (from etha-

nol). IR (KBr, cm^–1): 1 711 (C=O), 1 594 (C=N/C=C).

¹H NMR (90 MHz, CDCl₃–TMS) δ (ppm): 3.60 (s, CH₃),

6.80–7.83 (m, 15 aromatic H). ¹¹B NMR (64 MHz, CDCl₃–

Et₂OBF₃) δ (ppm): 10.2 (w_1/2= 214 Hz). Anal. calcd. for

C₂₁H₁₈BNO₃: C 73.50, H 5.29, B 3.15, N 4.08; found:

C 73.45, H 5.36, B 3.33, N 3.90.

4-Cyclohexyl-2,2-diphenyl-1,3-dioxa-4-azonia-2-borata-4-

cyclohexen-6-one (17c)

16c (0.34 g, 2 mmol) and oxybis(diphenylborane) (0.35 g,

1 mmol) are reacted as described for 17a (method A). Yield:

0.21 g, (32%) of colorless crystals⁶, mp 121°C¹⁴. IR (KBr, cm^–1):

1 706 (C=O), 1 603 (C=N/C=C). ¹H NMR (90 MHz,

CDCl₃–TMS) δ (ppm): 0.80–2.33 (m, (CH₂)₅), 3.73 (m,

N-CH), 7.00–7.67 (m, B(C₆H₅)₂and N=CH). ¹¹B NMR

(64 MHz, CDCl₃–Et₂OBF₃) δ (ppm): 10.3 (w_1/2= 385 Hz),

30.1, ratio of peak areas 40:1¹⁵. Anal. calcd. for C₂₀H₂₂BNO₃:

C 71.66, H 6.62, B 3.23, N 4.18; found: C 71.69, H 6.64,

B 3.35, N 4.00. The compound is also available by a three

component reaction of equimolar amounts of glyoxylic acid,

N-cyclohexylhydroxylamine, and diphenylborinic acid

(method B as described for 17a, omitting triethylamine).

2,2,5-Triphenyl-4-propyl-1,3-dioxa-4-azonia-2-borata-4-

cyclohexen-6-one (17g)

Phenylglyoxylic acid (0.75 g, 5 mmol), oxybis(di-

phenylborane) (0.87 g, 2.5 mmol), N-propylhydroxylamine

hydrochloride (0.56 g, 5 mmol), and triethylamine (0.51 g,

5 mmol) are reacted as described for 17f. Yield: 0.66 g,

(36%) of colorless crystals⁶, mp 117°C (from ethanol). IR

(KBr, cm^–1): 1 717 (C=O), 1 594 (C=N/C=C). ¹H NMR

(90 MHz, CDCl₃–TMS) δ (ppm): 0.80 (t, J = 7 Hz, CH₃),

1.67–2.17 (m, N-C-CH₂), 3.70 (t, J = 7 Hz, N-CH₂), 6.80–

7.73 (m, 15 aromatic H). ¹¹B NMR (64 MHz, CDCl₃–Et₂OBF₃)

δ (ppm): 11.1 (w_1/2= 257 Hz). Anal. calcd. for C₂₃H₂₂BNO₃:

C 74.41, H 5.97, B 2.91, N 3.77; found: C 74.33, H 5.97,

B 2.96, N 3.80.

4-Benzyl-2,2-diphenyl-1,3-dioxa-4-azonia-2-borata-4-

cyclohexen-6-one (17d)

16d (0.18 g, 1 mmol) and oxybis(diphenylborane) (0.17 g,

0.5 mmol) are reacted analogous as described for 17a

(method A). Yield: 0.16 g, (47%) of colorless crystals⁶, mp

199°C¹³. IR (KBr, cm^–1): 1 719 (C=O), 1 615 (C=N/C=C).

¹H NMR (90 MHz, CDCl₃–TMS) δ (ppm): 4.87 (s, N-CH₂),

6.67–7.73 (m, aromatic H and N=CH). ¹¹B NMR (64 MHz,

CDCl₃–Et₂OBF₃) δ: C 73.50, H 5.29, B 3.15, N 4.08; found:

C 73.20, H 5.39, B 3.16, N 4.12. The compound is also

available by a three component reaction of equimolar

amounts of glyoxylic acid, N-benzylhydroxylamine (hydro-

chloride + triethylamine), and diphenylborinic acid (method

B as described for 17a).

4-Cyclohexyl-2,2,5-triphenyl-1,3-dioxa-4-azonia-2-

borata-4-cyclohexen-6-one (17h)

Phenylglyoxylic acid (0.75 g, 5 mmol), oxybis(diphenyl-

borane) (0.87 g, 2.5 mmol), and N-cyclohexylhydroxylamine

(0.58 g, 5 mmol) are reacted as described for 17f, omitting

triethylamine. Yield: 1.34 g, (65%) of colorless crystals⁶, mp

139°C (from ethanol). IR (KBr, cm^–1): 1 719 (C=O), 1 586

(C=N/C=C). ¹H NMR (90 MHz, CDCl₃–TMS) δ (ppm):

0.83–2.67 (m, (CH₂)₅), 3.73 (m, N-CH), 6.70–7.87 (m, 15

aromatic H). ¹¹B NMR (64 MHz, CDCl₃–Et₂OBF₃) δ (ppm):

9.2 (w_1/2= 342 Hz). Anal. calcd. for C₂₆H₂₆BNO₃: C 75.93,

H 6.37, B 2.63, N 3.41; found: C 75.82, H 6.36, B 2.57,

N 3.27.

4-Benzyl-2,2-di(4′-chlorophenyl)-1,3-dioxa-4-azonia-2-

borata-4-cyclohexen-6-one (17e)

16d (0.36 g, 2 mmol) and di(4-chlorophenyl)borinic acid

(11) (0.50 g, 2 mmol) are reacted as described for 17a

(method A). Yield: 0.45 g, (55%) of colorless crystals⁶, mp

4-Benzyl-2,2,5-triphenyl-1,3-dioxa-4-azonia-2-borata-4-

cyclohexen-6-one (17i)

1

113°C¹⁴. IR (KBr, cm^–1): 1 709 (C=O), 1 616 (C=N). H

NMR (90 MHz, CDCl₃–TMS) δ (ppm): 4.93/5.13 (d/d, J =

15 Hz, AB system, N-CH₂), 6.67–8.00 (m, 13 aromatic H

and N-CH). ¹¹B NMR (64 MHz, CDCl₃–Et₂OBF₃) δ (ppm):

11.6 (w_1/2= 684 Hz), 31.3, ratio of peak areas 50:1.¹⁵Anal.

calcd. for C₂₁H₁₆BCl₂NO₃: C 61.21, H 3.91, B 2.62, N 3.40;

found: C 61.13, H 3.85, B 2.90, N 3.30. The compound is

Phenylglyoxylic acid (0.75 g, 5 mmol), oxybis(diphenyl-

borane) (0.87 g, 2.5 mmol), N-benzyhlhydroxylamine

hydrochloride (0.80 g, 5 mmol), and triethylamine (0.51 g,

5 mmol) are reacted as described for 17f. Yield: 1.31 g,

(63%) of colorless crystals⁶, mp 112°C (from ethanol). IR

(KBr, cm^–1): 1 713 (C=O), 1 596 (C=N/C=C). ¹H NMR

¹⁵The signal with very small peak area originates from impurities caused by a thermally induced rearrangement reaction.⁵

Kliegel et al.

1335

Table 1. Crystallographic data.^a

Compound

5d

9a

11b

19

Formula

fw

Crystal system

Space group

a (Å)

b (Å)

c (Å)

β (°)

V (Å³)

C₂₆H₂₂BNO₂

391.28

Monoclinic

P2₁/c

10.543(2)

19.085(4)

10.2667(3)

90.4978(7)

2065.6(4)

4

C₁₈H₂₂BNO₂

295.19

Orthorhombic

P2₁2₁2₁

10.9913(5)

14.9329(7)

10.2460(13)

90

C₁₉H₂₄BNO₄

341.21

Monoclinic

P2₁/n

11.227(2)

9.967(2)

17.0537(4)

105.4179(5)

1839.7(3)

4

C₂₀H₁₆BNO₃

329.16

Monoclinic

P2₁/n

11.1847(15)

13.715(3)

11.5559(5)

104.8730(10)

1713.2(3)

4

1681.7(2)

4

1.166

Z

ρ_calc(g/cm³)

F(000)

1.258

824

1.232

728

1.276

688

632

Radiation

µ (cm^–1)

Mo

0.78

Mo

0.74

Mo

0.85

Mo

0.85

Size (mm)

0.25 0.3 0.4

0.83–1.00

0–190

–23–18

0.5

462, 80 s

60.1

19617

0.25 0.35 0.4

0.79–1.00

0–190

–23–18

0.5

462, 80 s

60.1

15953

4438 ^c

0.3 0.35 0.4

0.86–1.00

0–190

–22–18

0.5

460, 60 s

60.0

17151

0.35 0.4 0.5

0.84–1.00

0–190

–22–18

0.5

460, 90 s

60.4

15767

Corr. factors^b

φ range (χ = –90°)

ω range (χ = –90°)

Osc. width (deg)

Images, time

2θ_max(deg)

Total reflections

Unique reflections

R_merge

5150

0.069

4331

0.040

3996

0.025

0.069

No. with I ≥ 3σ(I)

No. of variables

R (F) (I ≥ 3σ (I))

R_w(F²) (all data)

gof

3392

359

0.049

0.128

3338

287

0.047

0.111

2807

322

0.042

0.085

2739

287

0.047

0.095

1.27

1.16

2.07

2.72

Max ∆/σ (final cycle)

Residual density (e/Å³)

0.0003

–0.50–0.46

0.006

–0.49–0.46

0.0003

–0.34–0.36

0.0004

–0.29–0.32

^aTemperature 180 K, Rigaku/ADSC CCD diffractometer, Mo K_a(l = 0.71069 Å), graphite monochromator, takeoff angle 6.0°, aperture 94.0 × 94.0 mm

2

at a distance of 39.2 mm from the crystal, σ ²(F²) = (C + B)/Lp²(C = scan count, B = background count), function minimized ∑w(|F_o| – |F_c²|)²where w

2

2 2

2

= 1/σ ²(F_o), R(F) = ∑||F_o| – |F_c||/∑|F_o|, R_w(F²) = (∑w(|F_o| – |F_c²|)²/∑w|F_o| )^1/2, and gof = [∑w(|F_o| – |F_c²|)²/(m – n)]^1/2

^bIncludes crystal decay (if any), absorption, and scaling corrections performed in a single step.

^cIncludes Friedel pairs, 3 144 are unique.

.

(90 MHz, CDCl₃–TMS) δ (ppm): 4.90 (s, N-CH₂), 6.80–7.70

(m, 15 aromatic H). ¹¹B NMR (64 MHz, CDCl₃–Et₂OBF₃)

δ (ppm): 11.5 (w_1/2= 385 Hz). Anal. calcd. for C₂₇H₂₂BNO₃:

C 77.35, H 5.29, B 2.58, N 3.34; found: C 77.36, H 5.33,

B 2.63, N 3.17.

CDCl₃–TMS) δ (ppm): 7.23–7.73 (m, 13 aromatic H), 8.90

(d, J = 7 Hz, 2 aromatic H), signal for OH not detectable.

¹¹B NMR (64 MHz, CDCl₃–Et₂OBF₃) δ (ppm): 9.3 (w_1/2

=

300 Hz). Anal. calcd. for C₂₀H₁₆BNO₃: C 72.98, H 4.90,

B 3.28, N 4.26; found: C 73.03, H 4.89, B 3.21, N 4.25. Sin-

gle crystals suitable for X-ray crystallography were obtained

by very slow crystallization from chloroform–petroleum

ether (bp 30–70°C) using the liquid diffusion method (23).

3-Hydroxy-2,2,4-triphenyl-1-oxa-3-azonia-2-borata-3-

cyclopenten-5-one (19)

2-(Hydroxyimino)phenylacetic acid (22) (0.83 g, 5 mmol)

is dissolved in 50 mL of refluxing CHCl₃. After the addition

of oxybis(diphenylborane) (0.87 g, 2.5 mmol), dissolved in

2 mL of CHCl₃, the mixture is refluxed briefly. The solvent

is distilled off until the volume is reduced to 10 mL. Crystal-

lization is achieved after addition of small amount of petro-

leum ether and cooling to –18°C for 1 h. Yield: 1.16 g

(70%) of colorless crystals⁶, mp 181°C (from CHCl₃–petro-

leum ether). IR (KBr, cm^–1): 3 156–2 737 (curtain, O-H assoc.),

1 725 (C=O), 1 657 (C=N), 1597 (C=C). ¹H NMR (90 MHz,

X-ray crystallographic analyses of 5d, 9a, 11b, 19

Crystallographic data appear in Table 1. The final unit-cell

parameters were obtained by least-squares based on 12 693,

12 078, 8 118, and 9 840 reflections with 2θ = 4–64° for 5d,

9a, 11b, and 19, respectively. The data were processed,¹⁶

corrected for Lorentz and polarization effects, and absorp-

tion (based on analysis of symmetry-equivalent data).

The structures were solved by direct methods, the coordi-

nates of the atoms being determined from E-maps or from

¹⁶teXsan: Crystal structure analysis package. Unix version 1.7. Molecular Structure Corp. The Woodlands, TX, U.S.A. 1995; and d*TREK:

Area Detector Software. Molecular Structure Corp. The Woodlands, TX, U.S.A. 1997.

1336

Can. J. Chem. Vol. 78, 2000

Table 2. Selected bond lengths (Å, σ = 0.002 Å) and angles (deg, σ = 0.1°).

Structure

5d

9a

11b

19

Ring size

5

7

6

5

(A) Bond lengths (Å)

B—N

1.613

—

1.626

1.608

B—O

B—C

N=C

1.503

1.604, 1.612

1.289

1.464, 1.593

1.616, 1.621

1.284

1.491

1.605, 1.612

1.275

1.517

1.601, 1.608

1.294

N—O

O—C

1.380

1.427

1.357 (ring)

1.417

1.394

1.430

1.360

1.311

C—C (ring)

1.518

1.498, 1.536

1.498, 1.528

1.512

(B) Angles (deg)

O-B-N

C-B-C

B-N=C

B-O-C

94.9

107.0 (OBO)

117.0

112.4 (BON)

113.6

102.8

117.6

126.1

113.8

95.3

115.7

114.1

112.0

118.5

115.3

113.3

subsequent difference Fourier syntheses. The non-hydrogen

atoms were refined with anisotropic thermal parameters, and

the hydrogen atoms were refined with isotropic thermal pa-

rameters. No corrections for secondary extinction were nec-

essary. Neutral atom scattering factors for all atoms and

anomalous dispersion corrections for the non-hydrogen at-

oms were taken from the International Tables for X-Ray

Crystallography. For 9a, the absolute configuration for the

crystal used could not be determined from the small anoma-

lous scattering effect. Selected bond lengths and angles are

listed in Table 2. Complete tables of atomic coordinates and

equivalent isotropic thermal parameters, bond lengths and

angles, hydrogen atom parameters, anisotropic thermal pa-

rameters, torsion angles, intermolecular contacts, and least

squares planes are included as supplementary material.¹⁷

Scheme 1.

characterized by the signals for the N=CH methine proton

between 6.76 and 6.93 ppm and signals for an exchangeable

OH proton which is present also in the spectrum of the

ketonitrone 2h.

Nitrone complexes 3

Results and discussion

Reaction of 2 with a diarylborinic acid or anhydride gave

the diarylboron chelates 3 readily and in good yields. How-

ever, the preformation of the nitrone 2, which is quite diffi-

cult with ketones, could be avoided by a three-component

one-pot reaction of 1, a diarylborinic acid, and an N-alkyl-

hydroxylamine, which proved an advantage for the synthesis

of ketonitrone derivatives applying the 2-hydroxyketones 1

(R¹= alkyl). In the latter case, a preceding complex forma-

tion between the 2-hydroxyketone and the Lewis-acid boron

compound (see Scheme 1)¹⁸might facilitate the nucleophilic

attack of the hydroxylamine at the carbonyl carbon atom.

Thus, only one of the ketonitrones (2h) could be isolated in

a satisfying yield and applied as an educt for the

diphenylboron complex (3t). All the other ketonitrone

chelates 3 were prepared by the one-pot method without iso-

lation of an intermediate 2.

Nitrones 2

The N-alkylnitrones 2 which were required as intermedi-

ates for the synthesis of the six-membered boron chelates 3

could be prepared in the case of the aldonitrone species by

condensation of N-alkylhydroxylamines with 2-hydroxycar-

baldehydes 1 (R¹= H) (9). The preparation of ketonitrones

from 2-hydroxyketones (R¹= alkyl) by condensation with

N-alkylhydroxylamines appeared ineffective and led to some

nitrone only in the case of 2h.

Since it has been recognized that the (Z)-configuration of

aldonitrones is generally favored (24), it can be assumed that

the aldonitrones 2 exist entirely in the (Z)-form which

should be stabilized additionally by the intramolecular

O-H···O bond in this case. The ready formation of 3, de-

scribed below, might corroborate this supposition.

Typical for the nitrones 2 are infrared spectra showing a

C=N absorption band between 1 587 and 1 600 cm^–1and a

broadened band of the associated O-H group at about

The infrared spectra of 3 are characterized by the disap-

pearance of the stretching vibration of the O-H group of the

free ligand 2, and display an N=C band between 1 578 and

1 665 cm^–1, generally shifted to higher wave numbers

3 250 cm^–1. The H NMR spectra of the aldonitrones 2 are

1

¹⁷Copies of material on deposit may be purchased from the Depository of Unpublished Data, Document Delivery, CISTI, National Research

Council Canada, Ottawa, Canada K1A 0S2 (http://www.nrc.ca/cisti/irm/unpub_e.shtml for information on ordering electronically).

¹⁸Diarylboron complexes of this type could not be isolated as crystalline products or intermediates in the reactions of the various

2-hydroxycarbonyl compounds used in this investigation.

Kliegel et al.

1337

Scheme 2.

arylboronates 3′.⁵The isomerization can be retarded mostly

by avoiding thermal stress during the preparation,

recrystallization, and spectroscopy of 3. The thermal stabil-

ity of 3 depends on the substitution pattern. Thus the com-

pounds 3f, g, y can be recrystallized from the boiling

solvent, and especially 3z–cc show a high stability at all

temperatures in various solvents.

compared to the N=C bands of the corresponding educts 2.

In the H NMR spectra of the aldonitrone derivatives (3a–l),

1

the N=CH methine proton shows a signal between 6 and

7

ppm. The ¹¹B NMR spectra of

3 indicate the

tetracoordinate boron nucleus with signals between 3.6 and

9.2 ppm. In some of the spectra a second signal with very

small peak area appears between 26.0 and 29.2 ppm, point-

ing to a trivalent boron atom in the trigonal arrangement of

an arylboronate derivative (18). Small additional peaks were

Oxime complexes 5

Several 2-hydroxyketoximes

4

were reacted with

diphenylborinic acid anhydride and gave readily crystalline

compounds which show an elemental analysis consistent

with both of the isomeric structures, 5 and 6. An intact

diphenylboron moiety is proved by the color reaction with

1

also observed in some of the H NMR spectra and originate

from impurities caused by a thermally induced rearrange-

ment reaction (Scheme 2), leading to the isomeric

1338

Can. J. Chem. Vol. 78, 2000

diphenylcarbazone (10). The infrared spectra, displaying a

C=N absorption between 1 637 and 1 674 cm^–1and a broad

band in the region of stretching vibrations of associated

N-H/O-H groups (2 400–2 800 cm^–1), do not allow an unam-

biguous assignment to the five-membered ring system 5

with N-B coordination, or to the six-membered O-B-O che-

late 6, which is an N-unsubstituted derivative of 3. From the

¹H NMR spectra, as well as from ¹¹B NMR spectra, no posi-

tive proof for one of the ring systems can be obtained. One

anyway, because E/Z-isomerization of oximes (inversion and

(or) rotation) can take place easily, especially in the pres-

ence of an acid (27) or Lewis acid like a boron compound

(28). Of the ketoximes 4 used in this work only α-benzo-

inoxime ((E)-4) (R¹= R²= Ph), was stereochemically de-

fined. An X-ray crystallographic analysis of 5d proves the

five-membered chelate ring structure presenting the oxime

ligand in the (E)-conformation. Since the spectroscopic data

of the other derivatives, 5a–c, are very similar, this structure

should be applicable to all of the compounds 5. To the best

of our knowledge, 5d is the first X-ray structurally characterized

boron (or any other element) chelate of α-benzoinoxime.

The crystal structure of 5d consists of discrete molecules

(Fig. 1), linked by NO-H···O hydrogen bonds, the acceptor

oxygen atom being the O(2) ring member:

1

exchangeable proton giving a H signal about 12 ppm for

acidic N-H or O-H, and a ¹¹B signal between 8.0 and 10.2 ppm,

indicating the tetracoordinate boron nucleus (18), might

characterize either of the structures 5 and 6. For most of the

metal complexes of 2-hydroxyoximes a five-membered

O-M-N chelate structure is assumed (25), but evidence is

presented also for six-membered O-M-O ring systems (26),

even from the stereochemically defined (E)-benzoinoxime

(α-benzoinoxime) as an educt (26b). The geometrical

isomerism of the educt seems to be of minor significance,

O (1)···O (2) = 2.583(1), O(1)-H = 0.90(2),

H···O(2) = 1.71(2) Å, O(1)-H···O(2) = 162(2)°

Kliegel et al.

1339

Fig. 1. Views of the molecules of (a) 5d, (b) 9a, (c) 11b, and (d) 19; 50% probability thermal ellipsoids are shown for the non-

hydrogen atoms.

The C-C=N-B system in the five-membered ring in 5d

shows some deviation from exact planarity, torsion angle

9.6(2)°, and the ring conformation is best described as a

half-chair, with B and O displaced on opposite sides of the

C-C=N plane. A relatively narrow “bite” of the oxime ligand

leads to a compressed O-B-N angle of 94.9(1)° (Table 2),

thus distorting the tetrahedral grouping around the boron

atom slightly. This angle is even smaller than the O-B-N an-

gle between 98.2 and 99.9° in other five-membered COBNC

rings derived from 2-aminoethanols and diarylborinic acids

(29). The O-B, B-N, and N=C bond lengths are of the same

order as those found in the diphenylboron chelate of

1340

Can. J. Chem. Vol. 78, 2000

salicylaldoxime (cf. formula 13 below), which represents a

six-membered analogue (30):

to lower field by 0.2–0.4 ppm compared to the ligand

nitrone 8. The ¹¹B NMR spectra of 9 show the expected sig-

nals of a tetravalent boron nucleus at about 10.5 ppm. A

thermally induced 1,4-phenyl shift (from the diphenylboron

group to the N=CH methine carbon), as observed from the

spectra of 3, obviously does not occur in 9, even in boiling

ethanol.

5d: 1.503, 1.613, and 1.289(2) Å, respectively.

13: 1.516, 1.609, and 1.284(2) Å, respectively.

The B—C (phenyl) bond lengths (1.604 and 1.612 Å) are

about normal.

To prove the molecular structure, an X-ray crystallo-

graphic analysis of 9a was carried out and confirmed the

seven-membered ring structure derived from the (Z)-form of

the nitrone ligand (Fig. 1). This is a new crystallographic ex-

ample of a boron chelate with a stable seven-membered ring

system. Similar chelate rings (14) containing a bidentate

ligand with an N-oxide and a phenolate donor group have

been described by X-ray crystallographic analyses in earlier

work (17c, 32, 33). Two of these are derivatives of

salicylaldehyde nitrones (14, X = Y = H, R = Me (32); X =

NEt₂, Y = H, R = CH₂-C₆H₄-2-OH (17c)). Unlike the boron

chelate rings of the salicylaldehyde nitrones 14, which pos-

sess a somewhat distorted envelope (or sofa) conformation

with the boron atom on the flap displaced by 0.8–1 Å from

the mean plane formed by the remaining ring atoms, the

seven-membered ring of 9a has a distorted half-chair confor-

mation (Fig. 1). The ring contains a planar segment, O(1)-

N(1) = C(1)-C(2) of the nitrone moiety (torsion angle 0.1°),

with the other three ring atoms considerably displaced from

this plane (torsion angles as large as 95.5(2)°). This ring

conformation places the B-phenyl groups quite far from the

nitrone carbon atom C(1), the distance between the nearest

boron-bound phenyl carbon atom C(7) and the nitrone car-

bon atom C (1) being 3.75 Å (with C(13)···C(1) = 4.40 Å).

These distances are probably too long to facilitate the 1,4-

migration of the phenyl group as observed in the

diphenylboron chelate of N-methylsalicylaldonitrone (32).

The higher thermal stability of the compounds 9 could be

explained at least partially by that structural difference. As

in other diphenylboron chelates with an N-oxide and an

alcoholate or phenolate ligand (ref. 32 and refs. therein), the

O—B bond between the nitrone oxygen atom and boron in

9a (O(1)—B(1) = 1.593(2) Å) is longer than the O—B bond

from the alcoholate (O(2)—B(1) = 1.464(2) Å). The mean

O—B (1.528(2) Å) and C—B (1.619(2) Å) distances are

similar to the average O—B and C—B bonds lengths in 14

(R = Me, X = Y = H) of 1.528 and 1.610 Å, respectively,

(32), and the mean C—B:O—B bond length ratio of 1.059

for 9a is only slightly larger than the value of 1.054 for 14

(R = Me, X = Y = H), indicating a minor increase of overall

binding strength (34) of the ligand to the Ph₂B⁺nucleus in

both of the seven-membered diphenylboron chelates. 9a has

no hydrogen-bond donating functional group, and the mole-

cules in the crystal are linked by van der Waals forces, with

a possibly significant C-H···O interaction: C···O = 3.235(2),

C-H = 0.93(2), H···O = 2.33(2) Å, C-H···O = 153(2)°.

Nitrones 8

The 3-hydroxyaldehyde nitrones 8 were prepared by con-

densation of 3-hydroxy-2,2-dimethylpropanal with various

N-alkylhydroxylamines to obtain bidentate ligands for the

synthesis of the seven-membered boron chelate type 9. De-

pending on the N-alkyl residue the nitrones 8 are high boil-

ing liquids or crystalline substances under normal

conditions. In the IR spectra the most characteristic absorp-

tions are found in the 3 400–3 200 cm^–1region where

stretching modes of hydrogen bonded O-H groups appear,

and between 1 587 and 1 606 cm^–1where the C=N group of

1

the nitrone function absorbs. The H NMR spectra of 8 dis-

play some typical signals, common to all of the samples,

such as the resonance of the N=CH methine proton between

6.53 and 6.60 ppm and a signal for the hydroxyl proton be-

tween 4.57 and 6.07 (in CDCl₃), as well as from the singlet

of the 2,2-dimethyl grouping at about 1.23 ppm. As men-

tioned above for the aldonitrones 2, it can be assumed that 8

exists in the (Z)-form and thus presents a suitable ligand for

the diphenylborenium (Ph₂B⁺) ion to form the chelate 9.

Nitrone complexes 9

From the reaction of 8 with diphenylborinic acid anhy-

dride the diphenylboron chelates 9 resulted as crystalline

compounds in high yields, favored by larger N-alkyl sub-

stituents. The IR spectra of 9 show C=N absorptions be-

Oxime complexes 11

tween 1 636 and 1 664 cm^–1, a shift of 30–60 cm^–1to higher

frequencies compared to the free ligand 8. Similar observa-

tions have been made from the IR spectra of 2 and 3 and of

other boron chelates and their nitrone ligands (31). In the H

NMR spectra of 9 the most significant signal is that of the

The stability of the nitrone chelates 9 led us to investigate

diarylboron complexes of 3-hydroxyoxime which might

have a six-membered ring structure 11 with N-B coordina-

tion or the seven-membered isomeric structure 12 corresponding

to the nitrone O-B-O chelates 9. The reaction of 3-hydroxy-

aldoximes (10, R¹= H) and ketoximes (10, R¹= alkyl)

1

N=CH methine proton between 6.73 and 6.93 ppm, shifted

Kliegel et al.

1341

(which were prepared according to the literature (15, 16, 20)

without regard to geometrical isomerism) with

a

diarylborinic acid or anhydride gave crystalline compounds

in quite good yields. The elemental analyses and the spectro-

scopic data confirm the 1:1 condensation of oxime and

diarylborinic acid. In the IR spectra distinct C=N stretching

modes at about 1 640 cm^–1and broad absorption bands be-

tween 2 700 and 2 400 cm^–1in the N-H/O-H region are

characteristic but do not allow an unambiguous assignment

1

to either 11 or 12. The same holds for the H NMR spectra,

which give no specific indication of one of the two isomeric

structures. The ¹¹B NMR spectra show the expected signals

in the shift region of tetracoordinate boron nuclei between

4.0 and 9.9 ppm. An intact diphenylboron grouping was

proved by the color reaction with diphenylcarbazone (10).

Positive evidence of the molecular structure however, was

furnished by an X-ray crystallographic analysis of 11b

(Fig. 1) which gave the best single crystals. The six-

membered O-B-N chelate structure 11 based upon the (E)-

form of the oxime ligand 10 corresponds to the structure of

the salicylaldoximediphenylboron chelate 13 (30). Like 13

and also like 5d (see above), the crystal structure of 11b

consists of discrete molecules (Fig. 1) linked by strong O-

H···O hydrogen bonds:

broadened band of low intensity (curtain) in the region of

associated O-H groups between 2 700 and 2 000 cm^–1, car-

boxyl-C=O stretching modes at about 1 700, and C=N vibra-

tions at about 1 600 cm^–1, often hidden by aromatic C=C

1

vibrations. In the H NMR spectra, a typical N=CH methine

proton signal between 7.43 and 8.13 ppm and a broad low

field singlet (13–14 ppm) of the carboxyl-OH identify the

nitrone and the carboxylic acid functions of 16. It can be as-

sumed that the aldonitrones 16 exist in the (Z)-form, stabi-

lized additionally by an intramolecular O-H···O hydrogen

bridge, as has been discussed above for the nitrones 2.

Nitrone complexes 17

Reaction of the aldonitrones 16 (R¹= H) with a

diarylborinic acid (or anhydride) resulted in the crystalline

diarylboron chelates 17a–e which can be isolated under

avoidance of thermal stress leading to isomerization similar

to that mentioned above for some type 3 nitrones.⁵For the

synthesis of ketonitrone chelates 17, the difficulties of pre-

paring the nitrone of an α-ketocarbonic acid 16 could be

avoided by a three-component one-pot reaction of 15 (R¹=

Ph), diarylborinic acid, and N-alkylhydroxylamine, with the

advantage of a Lewis acid catalysis of the nitrone formation,

as described above for the ketonitrone complexes 3. The

ketonitrone derivatives 17f–i turned out to be thermally more

stable, which suggests that they are less susceptible to an

isomerization by 1,4-phenyl migration. In the IR spectra of

the chelates 17 there is not much change as compared with

the free ligands 16, except for the absence of the O-H ab-

sorption. The disappearance of the OH signal is also the

O(1)···O(2) = 2.569(1), O (1)-H = 0.98(2),

H···O (2) = 1.59(2) Å; O(1)-H···O = 175(2)°

Compared to 13 and 5d the hydrogen bond in 11b is the

shortest with the least bent angle:

11b

13

5d

O···O

2.569

175

2.737

167

2.583 Å

162°

O-H···O

A half-chair conformation, with a nearly planar B(1)-N(1) =

C(1)-C(2) segment (torsion angle = 0.9(2)°), and O(2) and

C(3) displaced on opposite sides of the six-membered ring,

characterizes the overall structure, as a result of the presence

of the C=N double bond within the otherwise saturated ring

skeleton of 11. The O-B-N angle in 11b (102.8(1)°, Table 2)

is less compressed than the O-B-N angle in 5d (94.9°), al-

lowing a less distorted tetrahedral arrangement around the

boron atom. Compared to 13 and also to 5d, the oxime C=N

bond in 11b (1.275(2) Å) is slightly shorter by about 0.01 Å,

which is in accordance with a lengthened (by 0.01 Å) N—B

bond.

1

most significant change in the H NMR spectra of the che-

lates 17 compared to the nitrone ligand 16. In the ¹¹B NMR

spectra of 17 the typical signals for tetracoordinate boron

appear between 9.2 and 11.6 ppm. The compounds 17 are

functionally related to the six-membered boron chelates of

N-carboxymethylnitrones (4), with the difference of an

endocyclic nitrone-C=N group.

Oxime complex 19

Nitrones 16

To obtain at least one reaction product of an oxime of a 2-

oxocarbonic acid and diphenylborinic acid, the phenyl-

glyoxylic acid oxime 18 (22) was chosen. Because of the

easy isomerization, even at room temperature in aqueous

As described in the literature (21), the aldonitrones 16

(R¹= H) can be obtained by condensation of glyoxylic acid

with an N-alkylhydroxlamine. The IR spectra of 16 show a

1342

Can. J. Chem. Vol. 78, 2000

solution (22a, b), the oxime was applied without separation

of the E/Z-isomers. The high-melting crystalline product is

obtained in good yield and has an elemental analysis corre-

sponding to the isomeric molecular structures 19 and 20.

The IR spectrum shows the expected bands in the N-H/O-H

region between 2 737 and 3 156 cm^–1, as well as the

carboxylate C=O and the oxime C=N absorptions at 1 725

and 1 657 cm^–1, respectively. An ¹¹B NMR signal for

tetracoordinate boron at 9.3 ppm and the blue color reaction

with diphenylcarbazone indicate the diphenylboron–chelate

character of the compound. The unambiguous discrimination

between the five-membered ring 19 with N-B coordination,

and the six-membered O-B-O chelate system 20 represent-

ing an N-unsubstituted derivative of 17, was made possible

by an X-ray crystallographic analysis. The X-ray analysis

confirms the five-membered boron chelate structure 19

incorporating the (E)-form of the ligand 18. The ring dimen

-

sions are similar to that of 5, with the ring conformation

being more near planar (B-N=C-C torsion angle = 2.7(2)°).

As in 5d, the O-B-N angle in 19 is distinctly compressed (to

95.3(1)°). The ring C(2)-O(1) distance in 19 (1.311(2) Å,

Fig. 1, Table 2), part of the carboxylate function, is shorter

than the ring alcoholate-C(2)-O(2) distance in 5d (1.427(2) Å),

according to expectation. The crystal structure of 19 consists

of discrete molecules linked by N-O-H···O=C hydrogen bonds:

O(2)···O(3) = 2.600(1), O(2)-H = 0.96(2),

H···O (3) = 1.76(2) Å; O-H···O = 144(2)°

Kliegel et al.

1343

The large number of compounds of type 3 with various

substituents has been synthesized for a broader investigation

of the thermal rearrangement reaction of these materials, and

for the interest in new boron compounds with potentially

herbicidal, insecticidal, and other biological effects (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. We are grateful to

the Chemische Institute der Technischen Universität

1

Braunschweig for H and ¹¹B NMR and mass spectra.

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Article Doi

Nitrones and oximes of bifunctional carbonyl compounds and their reaction products with diarylborinic acids. Crystal and molecular structure of examples of five-, six-, and seven-membered boron chelates

DOI: 10.1139/v00-137

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