Reaction of 2,3-dihydro-1H-1,3,2-diazaboroles and diphenylketene: a novel synthesis of 1,3,2-oxazaborolidines

Article abstract of DOI:10.1021/om000757z

Reaction of equimolar amounts of diphenylketene with a series of 1,3-di-tert-butyl-2,3-dihydro-1H-1,3,2-diazaboroles tBuqqX [X = Br (1a), F (1b), NH₂ (1c), NMe₂ (1d), Me (1e), SnMe₃ (1f), CHqqC(SnMe₃)C₆H₄-4-Cl (1g)] regioselectivity afforded good yields of the 1,3,2-oxazaborolidines tBuqqX (2a-g). The X-ray structure analysis of 2d revealed an essentially planar five-membered heterocycle with a long B-O bond and a strong exocyclic BN-π bond.

Full text of DOI:10.1021/om000757z

Organometallics 2000, 19, 5791-5794
5791
Rea ction of 2,3-Dih yd r o-1H-1,3,2-d ia za bor oles a n d
Dip h en ylk eten e: A Novel Syn th esis of
1,3,2-Oxa za bor olid in es
Lothar Weber,* Markus Schnieder, T. Claudete Maciel, Henning B. Wartig, and
Michaela Schimmel
Fakulta¨t fu¨r Chemie der Universita¨t Bielefeld, Universita¨tsstrasse 25,
D-33615 Bielefeld, Germany
Roland Boese and Dieter Bla¨ser
Institut fu¨r Anorganische Chemie der Universita¨t Essen GH, Universita¨tsstrasse 5-7,
D-45141 Essen, Germany
Received August 30, 2000
Summary: Reaction of equimolar amounts of diphen-
ylketene with a series of 1,3-di-tert-butyl-2,3-dihydro-1H-
chiral secondary alcohols3,8,9 and the enantioselective
addition of diethylzinc to aldehydes to afford secondary
alcohols.5,6
1,3,2-diazaboroles tBuNCHdCH-N(tBu)BX [X ) Br
(1a ), F (1b), NH2 (1c), NMe2 (1d ), Me (1e), SnMe3 (1f),
CHdC(SnMe3)C6H4-4-Cl (1g)] regioselectively afforded
A series of 1,3,2-oxazaborolidin-5-ones were obtained
by the borylation of (S)-alanine, (S)-valine, (S)-leucine,
(S)-isoleucine, and (S,R)-tert-leucine with (tert-butyl-
imino)-(2,2,6,6-tetramethylpiperidino)borane.10 Amino
acid derived oxazaborolidin-5-ones are excellent cata-
lysts for highly enantioselective Diels-Alder reac-
tions,11-13 the Mukaiyama aldol reaction of aldehydes
and silyl enol ethers,14,15 and asymmetric aldol reactions
of silyl ketene acetals.16,17
good yields of the 1,3,2-oxazaborolidines tBuN-CH-
(CHdNtBu)C(dCPh2)OBX (2a -g). The X-ray structure
analysis of 2d revealed an essentially planar five-
membered heterocycle with a long B-O bond and a
strong exocyclic BN-π bond.
In a program on functionalized 2,3-dihydro-1H-1,3,2-
diazaboroles it was demonstrated that the 2-haloderiva-
tives18,19 can easily be converted into 2-cyano-,19 2-iso-
cyanato-,19 2-isothiocyanato-,19 2-hydro-,20 2-alkyl-,20
2-alkynyl-,20 2-amino-,21 and 2-stannyl-2,3-dihydro-1H-
1,3,2-diazaboroles20 by halide substitution with the
respective nucleophile. Various alkynes were inserted
into the B-Sn bond of the latter compound to afford
highly functionalized 2-alkenyl-2,3-dihydro-1H-1,3,2-
In tr od u ction
The chemistry of 1,3,2-oxazaborolidines originated in
the 1970s with the first studies by Cragg.1 Interest in
these heterocycles increased markedly in the past
decade with the recognition that compounds of this type
are efficient catalysts in a series of chemical transfor-
mations. Usually oxazaborolidines were prepared from
boranes RBX2 (X ) leaving group) and 1,2-amino
alcohols.1,2 The employment of chiral amino alcohols
such as (S)-(-)-2-(diphenylhydroxymethyl)pyrrolidine,3,4
ephedrine,4-7 and pseudoephedrine8a or others8b fur-
nished chiral oxazaborolidines8 which enantioselectively
catalyzed borane reduction of prochiral ketones to give
(9) Review: Deloux, L.; Srebnik, M. Chem. Rev. 1993, 93, 763-784.
(10) Geisberger, G.; No¨th, H. Chem. Ber. 1990, 123, 953-961.
(11) (a) Corey, E. J .; Loh, T.-P. J . Am. Chem. Soc. 1991, 113, 8966-
8967. (b) Corey, E. J .; Loh, T.-P.; Roper, T. D.; Azimiora, M. D.; Noe,
M. C. J . Am. Chem. Soc. 1992, 114, 8290-8292. (c) Corey, E. J .; Loh,
T.-P. Tetrahedron Lett. 1993, 34, 3979-3982. (d) Corey, E. J .; Guzman-
Perez, A.; Loh. T.-P. J . Am. Chem. Soc. 1994, 116, 3611-3612.
(12) Takasu, M.; Yamamoto, H. Synlett 1990, 194-196.
(13) (a) Sartor, D.; Saffrich, J .; Helmchen, G. Synlett 1990, 197-
198. (b) Sartor, D.; Saffrich, J .; Helmchen, G. Richards, C. J .; Lambert,
H. Tetrahedron Asymmetry 1991, 2, 639-642.
(1) Cragg, R. H.; Weston, A. F. J . Chem. Soc., Dalton Trans. 1975,
93-95.
(2) Maringgele, W.; Meller, A. J . Organomet. Chem. 1980, 188, 401-
425.
(3) (a) Corey, E. J .; Bakshi, R. K.; Shibata, S. J . Am. Chem. Soc.
1987, 109, 5551-5553. (b) Corey, E. J .; Bakshi, R. K.; Shibata, S.;
Chen, C.-P.; Singh, V. K. J . Am. Chem. Soc. 1987, 109, 7925-7926.
(c) Corey, E. J .; Shibata, S.; Bakshi, R. K. J . Org. Chem. 1988, 53,
2861-2863.
(4) Chavant, Y. Y.; Vaultier, M. J . Organomet. Chem. 1993, 455,
37-46.
(5) J oshi, N. N.; Srebnik, M.; Brown, H. C. Tetrahedron Lett. 1989,
30, 5551-5554.
(6) El Moualij, N.; Caze, C. Eur. Polym. J . 1995, 31, 193-198.
(7) (a) Brown, J . M.; Lloyd-J ones, G. C. Tetrahedron Asymmetry
1990, 1, 869. (b) Brown, J . M.; Leppard, S. W.; Lloyd-J ones, G. C.
Tetrahedron Asymmetry 1992, 3, 261-266. (c) Brown, J . M.; Lloyd-
J ones, G. C. J . Chem. Soc., Chem. Commun. 1992, 710-712. (d) Brown,
J . M.; Lloyd-J ones, G. C. J . Am. Chem. Soc. 1994, 116, 866-878.
(8) (a) Berenguer, R.; Garcia, J .; Gonza`lez, M.; Vilarrasa, J . Tetra-
hedron Asymmetry 1993, 4, 13-16. (b) Le Toumelin, J .-B.; Baboule`ne,
M. Tetrahedron Asymmetry 1997, 8, 1259-1265.
(14) Corey, E. J .; Cywin, C. L.; Roper, T. D. Tetrahedron Lett. 1992,
33, 6907-6910.
(15) Ishihara, K.; Kondo, S.; Yamamoto, H. Synlett 1999, 1283-
1285.
(16) (a) Kiyooka, S.; Kaneko, Y.; Konaura, M.; Matsuo, H.; Nakano,
M. J . Org. Chem. 1991, 56, 2276-2278. (b) Kiyooka, S.; Kaneko, Y.;
Kume, K. Tetrahedron Lett. 1992, 4927-4930.
(17) (a) Parmee, E. R.; Tempkin, O.; Masanume, S.; Abiko, A. J .
Am. Chem. Soc. 1991, 113, 9365-9366. (b) Parmee, E. R.; Hong, Y.;
Tempkin, O.; Masanume, S. Tetrahedron Lett. 1992, 33, 1729-1732.
(18) Weber, L.; Dobbert, E.; Stammler, H.-G.; Neumann, B.; Boese,
R.; Bla¨ser, D. Chem. Ber./ Recl. 1997, 130, 705-710.
(19) Weber, L.; Dobbert, E.; Boese, R.; Kirchner, M. T.; Bla¨ser, D.
Eur. J . Inorg. Chem. 1998, 1145-1152.
(20) Weber, L.; Dobbert, E.; Stammler, H.-G.; Neumann, B.; Boese,
R.; Bla¨ser, D. Eur. J . Inorg. Chem. 1999, 491-497.
(21) Weber, L.; Dobbert, E.; Rausch, A.; Stammler, H.-G.; Neumann,
B. Z. Naturforsch. 1999, 54b, 363-371.
10.1021/om000757z CCC: $19.00 © 2000 American Chemical Society
Publication on Web 11/22/2000

5792 Organometallics, Vol. 19, No. 26, 2000
Notes
3
(m, 9H, Ph + CHdN), 7.62 (d, J HH ) 7.5 Hz, 2H, Ph). 13C-
{1H} NMR: δ 28.9 [s, C(CH3)3], 30.5 [s, C(CH3)3], 51.3 [s,
C(CH3)3], 56.2 [s, C(CH3)3], 65.4 (s, CHring), 119.7 (s, CPh2),
126.7, 127.2, 128.9, 129.0, 131.2, 139.0, 139.6 (Ph), 147.1 (s,
C)CPh2), 154.4 (s, CHdN). 11B{1H} NMR: δ 22.3 s. 19F{1H}
NMR: δ 176.9 s. Anal. Calcd for C24H30BFN2O (392.32): C,
73.48; H, 7.71; N, 7.14. Found: C, 73.56; H, 7.64; N, 7.14.
diazaboroles.22 Almost all transformations occur at the
periphery of the heterocycle, and little information on
processes involving the core of the ring system is
available.23
The aim of the work described herein was to provide
a novel synthesis of 1,3,2-oxazaborolidines from 1,3,2-
diazaboroles by treatment with diphenylketene.
tBu N-CH(CHdNtBu )C(dCP h 2)OBNH2 (2c). Colorless
2c (0.28 g, 64%) precipitated from the reaction mixture of
diphenylketene (0.22 g, 1.13 mmol) and 1c (0.22 g, 1.13 mmol)
in n-hexane (40 mL) at -40 °C. IR (KBr): ν˜ 3511 s (νNH),
Exp er im en ta l Section
All operations were performed under dry, oxygen-free dini-
trogen using standard Schlenk techniques. Solvents were dried
by standard methods and freshly distilled under nitrogen prior
1
3417 s (νNH) cm-1. H NMR: δ 0.91 [s, 9H, C(CH3)3], 1.06 [s,
3
9H, C(CH3)3], 1.81 (s, 2H, NH2), 5.10 (d, J HH ) 7.0 Hz, 1H,
3
to use. H, 11B, 13C,19F, and 119Sn NMR spectra were recorded
1
CHring), 7.00-7.24 (m, 9H, Ph + CHdN), 7.75 (d, J HH ) 7.6
Hz, 2H, Ph). 13C{1H} NMR: δ 29.1 [s, C(CH3)3], 31.0 [s,
C(CH3)3], 50.8 [s, C(CH3)3], 56.3 [s, C(CH3)3], 66.1 (s, CHring),
116.7 (s, CPh2), 126.1, 126.8, 128.0, 128.7, 130.0, 131.5, 140.1,
140.8 (Ph), 150.8 (s, C)CPh2), 155.8 (s, CHdN). 11B{1H} NMR
(d6-DMSO): δ 25.2 s. Anal. Calcd for C24H32BN3O (389.35):
C, 74.04; H, 8.28; N, 10.79. Found: C, 73.86; H, 8.54; N, 10.50.
in C6D6 with Bruker AC 100 (1H, 100.13 MHz, 11B, 32.13 MHz)
and Bruker Avance DRX 500 (1H, 500.13 MHz, 11B, 160.46
MHz, 13C, 125.75 MHz, 19F, 470.60 MHz, 119Sn, 186.51 MHz).
References: SiMe4 (1H, 13C), BF3‚OEt2 (11B), CFCl3 (19F), SnMe4
(
119Sn).
Compounds tBuN-CHdCH-N(tBu)BBr (1a ),18 tBuN-CHd
tBu N-CH (CH dNtBu )C(dCP h 2)OBNMe2 (2d ). Analo-
gously, reaction of 1d (0.66 g, 3.0 mmol) and 0.57 g (3.0 mmol)
of diphenylketene resulted in the formation of yellow micro-
crystalline 2d (0.90 g, 73%). Recrystallization from toluene at
-30 °C afforded crystals suitable for an X-ray structural
analysis. 1H NMR: δ 0.94 [s, 9H, C(CH3)3], 1.23 [s, 9H,
CH-N(tBu)BF (1b),19 tBuN-CHdCH-N(tBu)BNH2 (1c),21
tBuN-CHdCH-N(tBu)B-CH3 (1e),24 tBuN-CHdCH-N(t-
Bu)BSnMe3 (1f),20 tBuN-CHdCH-N(tBu)B-CHdC(SnMe3)-
(C6H4-4-Cl) (1g),22 and Ph2CdCdO25 were synthesized accord-
ing to literature procedures.
3
C(CH3)3], 2.68 (s, 6H, NMe2), 5.18 (d, J HH ) 6.9 Hz, 1H,
3
CHring), 7.06-7.25 (m, 9H, Ph + CHdN), 7.81 (d, J HH ) 6.9
tBu N-CHdCH-N(tBu )BNMe2 (1d ). Gaseous dimethyl-
amine was bubbled into a solution of 1,3-di-tert-butyl-2-bromo-
2,3-dihydro-1H-1,3,2-diazaborole (1a ) (3.00 g, 11.6 mmol) in
60 mL of n-hexane at 20 °C during a period of 15 min. Excess
amine was removed by a flow of argon. It was filtered, and
the filtrate was liberated from solvent and volatile components
in vacuo. Compound 1d was obtained as a yellow oil (1.88 g,
72% yield). 1H NMR: δ 1.33 [s, 18H, C(CH3)3], 2.46 [s, 6H,
N(CH3)2], 6.25 (s, 2H, NCH). 13C{1H} NMR: δ 31.6 [s, C(CH3)3],
41.2 (s, NCH3), 51.8 [s, C(CH3)3], 111.4 (s, NCH). 11B{1H}
NMR: δ 22.8 s. MS/EI: m/z (relative intensity) 223 (63) [M+].
Anal. Calcd for C12H26BN3 (223.17): C, 64.58; H, 11.74; N,
18.83. Found: C, 64.37; H, 12.08; N, 18.52.
Hz, 2H, Ph). 13C{1H} NMR: δ 28.9 [s, C(CH3)3], 31.7 [s,
C(CH3)3], 39.7 (s, NCH3), 50.9 [s, C(CH3)3], 56.1 [s, C(CH3)3],
66.6 (s, CHring), 115.5 (s, CPh2), 125.6, 126.6, 128.1, 128.6,
129.4, 130.0, 131.4, 140.0, 140.4 (Ph), 149.7 (s, C)CPh2), 155.4
(s, CHdN). 11B{1H} NMR: δ 24.8 s. Anal. Calcd for C26H36
-
BN3O (417.39): C, 74.82; H, 8.69; N, 10.07. Found: C, 74.76;
H, 8.76; N, 9.89.
tBu N-CH(CHdNtBu )C(dCP h 2)OBMe (2e). Analogously,
reaction of 0.55 g (2.8 mmol) of 1e with 0.54 g (2.8 mmol) of
diphenylketene in 50 mL of n-hexane afforded 0.76 g (70%) of
1
colorless microcrystalline 2e. H NMR: δ 0.56 (s, 3H, BCH3),
3
0.92 [s, 9H, C(CH3)3], 1.10 [s, 9H, C(CH3)3], 5.14 (d, J HH
)
6.9 Hz, 1H, CHring), 7.02-7.22 (m, 9H, Ph + CHdN), 7.77 (d,
3J HH ) 6.9 Hz, 2H, Ph). 13C{1H} NMR: δ 29.0 [s, C(CH3)3],
31.4 [s, C(CH3)3], 51.2 [s, C(CH3)3], 56.5 [s, C(CH3)3], 66.6 (s,
CHring), 117.3 (s, CPh2), 126.3, 126.9, 128.3, 128.8, 129.9, 131.4,
140.0, 140.4 (Ph), 151.6 (s, CdCPh2), 155.2 (s, CHdN). 11B-
{1H} NMR: δ 34.5 s. Anal. Calcd for C25H33BN2O (388.35): C,
77.32; H, 8.56; N, 7.21. Found: C, 77.19; H, 8.46; N, 7.28.
tBu N-CH(CHdNtBu )C(dCP h 2)OBBr (2a ). A solution of
diphenylketene (0.62 g, 3.2 mmol) in 10 mL of n-hexane was
added dropwise to a chilled solution (-20 °C) of 1a (0.83 g,
3.2 mmol) in n-hexane (40 mL). The mixture was warmed to
ambient temperature. After 2 h of stirring it was filtered, and
the light yellow filtrate was concentrated in vacuo until it
became cloudy. After storing for 24 h at -30 °C 2a precipitated
1
as a yellow solid (yield: 0.97 g, 67%). H NMR: δ 0.87 [s, 9H,
tBu N-CH(CHdNtBu )C(dCP h 2)OBSn Me3 (2f). A sample
of diphenylketene (0.33 g, 1.7 mmol) was slowly added at room
temperature to the solution of 1f (0.55 g, 1.6 mmol) in 5 mL of
benzene. After stirring for 3 h solvent was removed in vacuo,
and the residue was dissolved in 2 mL of benzene. Product 2f
3
C(CH3)3], 1.21 [s, 9H, C(CH3)3], 5.13 (d, J HH ) 6.6 Hz, 1H,
3
CHring), 7.00-7.13 (m, 9H, Ph + CHdN), 7.66 (d, J HH ) 7.4
Hz, 2H, Ph). 13C{1H} NMR: δ 28.9 [s, C(CH3)3], 31.0 [s,
C(CH3)3], 52.6 [s, C(CH3)3], 56.7 [s, C(CH3)3], 67.3 (s, CHring),
119.4 (s, CPh2), 126.6, 127.3, 128.9, 129.8, 131.1, 139.0, 139.1
(Ph), 149.1 (s, CdCPh2), 153.9 (s, CHdN). 11B{1H} NMR: δ
26.0 s. Anal. Calcd for C24H30BBrN2O (453.22): C, 63.60; H,
6.67; N, 6.18. Found: C, 63.48; H, 6.82; N, 6.08.
1
separated as colorless needles (yield: 0.59 g, 69%). H NMR:
δ 0.31 [s, 9H, 2J SnH ) 49.9 Hz, Sn(CH3)3], 0.87 [s, 9H, C(CH3)3],
1.17 [s, 9H, C(CH3)3], 5.20 (d, 3J HH ) 7.0 Hz, 1H, CHring), 7.04-
3
7.23 (m, 9H, Ph + CHdN), 7.22 (d, J HH ) 7.4 Hz, 2H, Ph).
1
13C{1H} NMR: δ -10.5 [s, J SnC ) 289.2 Hz, Sn(CH3)3], 28.9
tBu N-CH(CHdNtBu )C(dCP h 2)OBF (2b). Analogously,
a sample of diphenylketene (0.37 g, 1.90 mmol) was reacted
with 1b (0.38 g, 1.90 mmol) to afford 0.47 g (64%) of 2b as a
light yellow solid. 1H NMR: δ 0.87 [s, 9H, C(CH3)3], 1.07 [s,
[s, C(CH3)3], 32.4 [s, C(CH3)3], 52.3 [s, C(CH3)3], 56.9 [s,
C(CH3)3], 66.0 (s, CHring), 118.2 (s, CPh2), 126.5, 127.0, 128.9,
3
129.9, 131.2, 139.7, 140.0 (Ph), 152.9 (s, J SnC ) 49.4 Hz,
C)CPh2), 154.7 (s, CHdN). 11B{1H} NMR: δ 38.2 s. 119Sn-
{1H} NMR: δ -110.7. MS/EI m/z (relative intensity): 538 (75)
[M+]. Anal. Calcd for C27H39BN2OSn (537.14): C, 60.37; H,
7.32; N, 5.22. Found: C, 60.45; H, 7.40; N, 5.47.
3
9H, C(CH3)3], 5.10 (d, J HH ) 6.9 Hz, 1H, CHring), 7.02-7.19
(22) Weber, L.; Wartig, H. B.; Stammler, H.-G.; Stammler, A.;
Neumann, B. Organometallics 2000, 19, 2891.
(23) For earlier work on 2,3-dihydro-1H-1,3,2-diazaboroles see:
Schmid, G.; Polk, M.; Boese, R. Inorg. Chem. 1990, 29, 4421-4429,
and references therein.
(24) Schmid, G.; Schulze, J . Chem. Ber. 1977, 110, 2744-2750.
(25) Taylor, E. C.; McKillop, A.; Hawks, G. H. Org. Synth. 1972,
52, 36.
tBu N-CH(CHdNtBu )C(dCP h 2)OB-CHdC(Sn Me3)C6H4-
4-Cl (2g). Analogously reaction of 1g (0.56 g, 1.2 mmol) and
0.25 g of diphenylketene (1.3 mmol) in 5 mL of benzene for 6
h afforded colorless crystalline 2g (yield: 0.49 g, 62%) 1H

Notes
Organometallics, Vol. 19, No. 26, 2000 5793
Sch em e 1
Sch em e 2
doublet for the proton at the ring carbon atom in 2 was
observed at δ 5.10-5.38 (3J HH ) 6.3-7.0 Hz). In the
precursors 1 the two equivalent ring protons gave rise
to singlets at δ 5.99-6.50. The second CH group in 1
was converted into the exocyclic methane imino func-
tionality in 2, the proton signal of which is obscured by
the phenyl hydrogens. The 13C carbon atom of the CHd
N group gives rise to singlets at δ 153.9-155.8 in the
13C{1H} NMR spectra of the products, whereas the ring
carbon atom is assigned to a singlet at δ 65.4-67.3. A
singlet in the range δ 147.1-152.9 is due to the sp2-
hybridized ring carbon atom introduced by the ketene
building block. The exocyclic carbon atom of the double
bond in 2 was observed as a singlet at δ 115.5-119.7.
In 2b (X ) F), 2c (X ) NH2), and 2d (X ) NMe2), where
the boron atoms are linked to π-donating groups X the
11B NMR resonances (δ 22.3-25.2) are deshielded by
only δ 2.0-3.1 on going from 1 to 2. For comparison
1,3,2-oxazaborolidin-5-ones derived from R-amino acids
and an amino-iminoborane display 11B resonances at δ
27.0-27.4.10 In the remaining products 2a (X ) Br, δ
26.0), 2e (X ) Me, δ 34.5), 2f (X ) SnMe3, δ 38.2), and
2g (X ) alkenyl, δ 30.9) a more pronounced deshielding
(∆δ 8.3-12.4) of the boron nuclei relative to the corre-
sponding 1,3,2-diazaborole precursors is observed.
2
NMR: δ 0.13 [s, 9H, J SnH ) 55.3 Hz, Sn(CH3)3], 0.93 [s, 9H,
3
C(CH3)3], 1.22 [s, 9H, C(CH3)3], 5.38 (d, J HH ) 6.3 Hz, 1H,
CHring), 6.82 (s, 1H, BCH), 6.93-7.24 (m, 13H, Ph + CHdN +
p-ClC6H4), 7.54 (d, J HH ) 7.2 Hz, 2H, Ph). 13C{1H} NMR: δ
3
-7.3 [s, 1J SnC ) 347.1 Hz, Sn(CH3)3], 29.1 [s, C(CH3)3], 31.9 [s,
C(CH3)3], 52.2 [s, C(CH3)3], 56.5 [s, C(CH3)3], 66.1 (s, CHring),
119.2 (s, CPh2), 127.6, 128.3, 128.7, 128.8, 129.8, 130.3, 131.0,
132.4, 140.4, 148.0 (Ph), 138.5 (s, br, BC), 151.2 (s, CdCPh2),
154.7 (s, CHdN), 163.8 (s, dC-Sn). 11B{1H} NMR: δ 30.9 s
br. 119Sn{1H} NMR: δ -38.8 s. MS/EI m/z (relative inten-
sity): 674 (6) [M+], 559 (33), 167 (100). Anal. Calcd for C35H44
-
BClN2OSn (673.72): C, 62.40; H, 6.58; N, 4.16. Found: C,
62.58; H, 6.69; N, 4.14.
Resu lts a n d Discu ssion s
Reaction of diphenylketene and equimolar amounts
of the 2,3-dihydro-1H-1,3,2-diazaboroles 1a -e in n-
hexane in the temperature range between -20 and +20
°C led to the formation of the 1,3,2-oxazaborolidines
2a -e in 64-70% yield. Similarly, the 1,3,2-diazaboroles
1f and 1g were converted into the corresponding 1,3,2-
oxazaborolidines 2f and 2g by treatment with Ph2Cd
CdO in benzene at room temperature. The progress of
this transformation was monitored by 11B NMR spec-
troscopy, and products 2a -e were obtained as light
yellow to colorless crystals from n-pentane. The deriva-
tives 2f and 2g were isolated as colorless deliquescent
crystals from benzene (Scheme 1).
In the 19F{1H} NMR spectrum of 2b a singlet was
registered at δ 176.9. This resonance compares well with
the one in the nonaromatic heterocycle MeNCH2CH2N-
(Me)BF (δ 168)26 but is markedly deshielded with
respect to precursor 1b (δ 56.57).
In line with Zweifel’s27 and Herberich’s28 results on
the reaction of 3-borolenes with aldehydes, ketones, and
ketenes to give 1,2-oxaborolane derivatives it is conceiv-
able that the formation of 2 is initiated by a nucleophilic
attack of the ketene oxygen atom at the boron atom of
the ring to give zwitterion I. Attack of the electron-
deficient ketene carbon center at the CdC bond of the
ring leads to II. Fission of the BN bond of II eventually
afforded 2 as a pair of enantiomers. The reaction
between a 2,3-dihydro-1H-1,3,2-diazaborole and a ketene
constitutes a novel synthetic approach to oxazaboro-
lidines with a stereogenic center at the ring carbon
atom. Experiments focusing on the asymmetric synthe-
sis of such heterocycles are underway.
The 1,3,2-oxazaborolidines 2a -g are less soluble than
the starting materials 1a -g. It is obvious that the ring
transformation reaction tolerates halide substituents,
amino groups, the Me3Sn unit, and alkenyl groups at
the boron atom. No reactions were observed with the
2-hydro-, 2-cyano-, and 2-diphenylketimino derivatives
and 1,3,2-diazaboroles bearing 2,6-dimethylphenyl sub-
stituents at both nitrogen atoms.
The IR spectra of the products confirmed the absence
of a ν(CdO) vibration, which implies a reaction involv-
1
ing the carbonyl unit of the ketene. From the H and
13C NMR spectra of the products 2 it is obvious that
the vertical mirror plane of the precursor molecules 1
is no longer present. Thus the singlet for the 18 protons
of the tert-butyl groups, ranging from δ 1.24 to 1.43, in
1 is replaced by two singlet resonances at δ 0.87-0.94
and δ 1.06-1.23, which are attributed to the chemically
and nonequivalent tert-butyl groups at the ring nitrogen
atom and at the exocyclic methane imino group in 2. A
(26) Fussstetter, H.; No¨th, H.; Wrackmeyer, B. Chem. Ber. 1977,
110, 3172-3182.
(27) Zweifel, G.; Shoup, T. M. J . Am. Chem. Soc. 1988, 110, 5578.
(28) Herberich, G. E.; Englert, U.; Wang, S. Chem. Ber. 1993, 126,
297.

5794 Organometallics, Vol. 19, No. 26, 2000
Notes
parable. The BO bond is markedly elongated when
compared with the BO distances in {tBuNCHdCHN-
(tBu)B}2O (III) [1.365(4), 1.354(4) Å], whereas the BN
bond lengths in III are similar [1.439(4), 1.434(4),
1.442(3) Å]. The C2N plane of the dimethylamino
substituent and the plane of the oxazaborolidine ring
possess an interplanar angle of 22°, which allows for a
relatively strong exocyclic BN-π-bond of 1.405(2) Å. The
bond length C(4)-C(13) of 1.334(2) Å is that of a
localized double bond, as it is observed for the CdN bond
[1.251(2) Å] of the exocyclic N-tert-butylmethane imino
group. Due to steric congestion the exocyclic angles
B(2)-N(1)-C(6) [133.13(12)°] and N(1)-B(2)-N(10)
[134.88(14)°] are markedly widened, when compared
with angles C(6)-N(1)-C(5) [118.96(11)°] and O(3)-
B(2)-N(10) [116.06(13)°]. The same strain is obvious
from the angles at the planar dimethylamino group
[B(2)-N(10)-C(11) 127.40(14)° and B(2)-N(10)-C(12)
119.91(13)°].
F igu r e 1. Molecular structure of 2d in the crystal. The
thermal ellipsoids correspond to 50% probability.
Ack n ow led gm en t. The present work was finan-
cially supported by the Deutsche Forschungsgemein-
schaft (Bonn, Germany) and the Fonds der Chemischen
Industrie (Frankfurt/M., Germany), which is gratefully
acknowledged.
X-r a y Str u ctu r a l An a lysis of 2d . The structure of
2d was confirmed by an X-ray analysis (Figure 1)29
Single crystals of the compound were grown from
toluene at -30 °C. An essential structural feature is an
almost planar five-membered heterocycle (the largest
deviation from the best plane is 0.065 Å). The sum of
the endocyclic angles is 538.68°; which is close to the
theoretical value of 540°. The endocyclic bonds N(1)-
B(2) [1.429(2) Å] and B(2)-O(3) [1.411(2) Å] are com-
Su p p or tin g In for m a tion Ava ila ble: Table of X-ray data,
atomic coordinates, thermal parameters, and complete bond
distances and angles and thermal ellipsoid plots for compound
2d . This material is available free of charge via the Internet
at http://pubs.acs.org.
(29) The crystallographic data for 2d (atomic coordinates and
bonding parameters) have been placed in the Supporting Information.
OM000757Z