ORGANIC
LETTERS
2000
Vol. 2, No. 24
3921-3923
B(C F ) -Catalyzed Hydrosilation of
Imines via Silyliminium Intermediates
6 5 3
James M. Blackwell, Eric R. Sonmor, Tiziana Scoccitti, and Warren E. Piers*
Department of Chemistry, UniVersity of Calgary, 2500 UniVersity DriVe N.W.,
Calgary, Alberta, T2N 1N4 Canada
Received October 5, 2000
ABSTRACT
A broad range of benzaldimines and ketimines can be hydrosilated efficiently, employing B(C F5)3 as a catalyst in conjunction with PhMe2SiH.
6
Spectral evidence supports the intermediacy of a silyliminium cation with a hydridoborate counterion formed via abstraction of a hydride from
PhMe2SiH by B(C F5)3 in the presence of imines.
6
Although used primarily as an olefin polymerization cocata-
lyst,1 applications of the commercially available borane,
B(C6F5)3, in organic synthesis are growing.2 While it is
proposed to operate in a typical carbonyl-activating capacity
in aldol and Diels-Alder type reactions,2a in the hydrosilation
and allylstannation reactions it mediates, mechanistic studies
suggest its role is to activate the group 14 reagent rather
than the carbonyl or alcohol substrate.3 For silanes, the silicon
center assumes silylium character as the borane abstracts the
silane hydride; substrate then displaces [HB(C6F5)3]-, pro-
ducing an ion pair which collapses to the observed products.
In these reactions, silylium intermediates have not been
directly observed.4
The reduction of imines to amines is an important
transformation in organic chemistry.5 Most methods in-
volve borohydride reagents or transition metal hydrogena-
tion catalysts; few general methods employing main group
Lewis acid catalysts have appeared.5 In this Letter, we re-
port the use of B(C6F5)3 as a mild, effective catalyst for
the hydrosilation of imine functions. In addition to demon-
strating the scope of B(C6F5)3-mediated imine reduction, we
present convincing spectral evidence for the intermediacy
of a silyliminium6,7/hydridoborate ion pair,8 supporting
the mechanistic proposals advanced for carbonyl hydrosila-
tion.3a,b
(5) For recent reviews on imine reductions, see (a) Kobayashi, S.; Ishitani,
H. Chem. ReV. 1999, 99, 1069. (b) Hutchins, R. O.; Hutchins, M. K.
ComprehensiVe Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon
Press: New York, 1991; Vol. 8, p 25.
(6) For proposed intermediacy of silyliminium cations, see: (a) Jahangir;
MacLean, D. B.; Brook, M. A.; Holland, H. L. J. Chem. Soc., Chem.
Commun. 1986, 1608. (b) Johannsen, M.; Jorgensen, K. A.; Helmchen, G.
J. Am. Chem. Soc. 1998, 120, 7637. For a proposed stannyliminium
intermediate, see: Suwa, T.; Shibata, I.; Nishino, K.; Baba, A. Org. Lett.
1999, 1, 1579.
(1) Chen, Y.-X. E.; Marks, T. J. Chem. ReV. 2000, 100, 1391.
(2) For example, see: (a) Ishihara, K.; Yamamoto, H. Eur. J. Org. Chem.
1999, 527. (b) Ooi, T.; Uraguchi, D.; Kagoshima, N.; Maruoka, K. J. Am.
Chem. Soc. 1998, 120, 5327. (c) Gevorgyan, V.; Rubin, M.; Benson, S.;
Liu, J.-X.; Yamamoto, Y. J. Org. Chem. 2000, 65, 6179. (d) Gevorgyan,
V.; Liu, J.-X.; Yamamoto, Y. J. Chem. Soc., Chem. Commun. 1997, 37.
(e) Ishihara, K.; Hanaki, N.; Funahasi, M.; Miyata, M.; Yamamoto, H. Bull.
Chem. Soc. Jpn. 1995, 68, 1721.
(3) (a) Parks, D. J.; Piers, W. E. J. Am. Chem. Soc. 1996, 118, 9440. (b)
Park, D. J.; Blackwell, J. M.; Piers, W. E. J. Org. Chem. 2000, 65, 3090.
(c) Blackwell, J. M.; Foster, K. L.; Beck, V. H.; Piers, W. E. J. Org. Chem.
1999, 64, 4887. (d) Blackwell, J. M.; Piers, W. E.; Parvez, M. Org. Lett.
2000, 2, 695.
(4) Stannyloxonium intermediates have, however, been observed by NMR
spectroscopy, see ref 3d.
(7) Silylnitrilium, silylpyridinium, and silylimidazolium cations have been
characterized, see: Lambert, J. B.; Kania, L.; Zhang, S. Chem. ReV. 1995,
95, 1191 and references therein.
(8) Abstraction of hydride from stannanes by B(C6F5)3 has been proposed,
see: Lambert, J. B.; Kuhlmann, B. J. Chem. Soc., Chem. Commun.
1992, 931. However, no direct spectral evidence has been provided yet for
silanes.
10.1021/ol006695q CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/10/2000