J . Org. Chem. 1998, 63, 9421-9424
9421
Cp 2VCl2-Ca ta lyzed Meso-Selective P in a col Cou p lin g Rea ction of
Ald im in es in th e P r esen ce of Ch lor osila n e a n d Zin c Meta l
Bunpei Hatano, Akiya Ogawa, and Toshikazu Hirao*
Department of Applied Chemistry, Faculty of Engineering, Osaka University,
Yamada-oka, Suita, Osaka 565-0871, J apan
Received J uly 17, 1998
A catalytic reductive coupling of aldimines was achieved by using a catalyst Cp2VCl2/PhMe2SiCl/
Zn system. The influence of the catalyst, chlorosilane, co-reductant, solvent, and temperature on
both the yield and diastereoselectivity of the coupling products was investigated in detail. As a
result, the present Cp2VCl2-catalyzed reductive coupling of aldimines in the presence of PhMe2-
SiCl and zinc metal provided the corresponding 1,2-diamines in good yield with high meso selectivity
(up to 92% meso), while the reductive coupling of aldehydes, ketones, and aldimines by the hitherto
known catalytic methods leads to the preferential formation of dl isomers of the coupling products.
In tr od u ction
de).7,8 During the course of our studies on the catalytic
pinacol coupling, we examined a catalytic dimerization
of aldimines using the catalyst Cp2VCl2/PhMe2SiCl/Zn,
which successfully provides the corresponding 1,2-di-
amines in high yield, and more surprisingly, unprec-
edented high meso selectivity was observed (eq 1).
The reductive dimerization of carbonyl compounds and
their imine derivatives with low-valent metal complexes
is an important method for vicinally bifunctionalized
carbon-carbon bond formation.1 Although the dimer-
ization reactions usually require stoichiometric or more
excess amounts of low-valent metal complexes, a catalytic
reductive-coupling procedure has been disclosed recently
by the combination of an early-transition-metal catalyst,
a chlorosilane, and a zerovalent metal as a co-reductant.
For example, Fu¨rstner and co-workers reported the
catalytic McMurry coupling of aldehydes promoted by the
catalyst TiCl3/R3SiCl/Zn,2 and, independently, we achieved
the catalytic pinacol coupling of aldehydes induced by
catalyst Cp2VCl2/Me3SiCl/Zn, which provides 1,3-diox-
olanes as a mixture of diastereoisomers (dl/meso ) 63:
37).3 Furthermore, the catalytic pinacol coupling of
aromatic aldehydes was reported to proceed with high
diastereoselectivity using the catalytic systems of low-
valent titanium complexes.4 Very recently, we have
revealed the highly dl-selective pinacol coupling reaction
of secondary aliphatic aldehydes catalyzed by low-valent
vanadium or titanium complexes.5,6
Resu lts a n d Discu ssion
At first, we examined the reductive dimerization of
N-benzylidenebenzylamine (1a ) with Cp2VCl2 in the
presence or absence of additives such as zinc metal and
Me3SiCl (eq 2, Table 1). When the reaction of 1a with 2
equiv of Cp2VCl2 in refluxing THF was conducted for 24
h, the desired reductive-coupling product was not ob-
(6) For diastereoselective reductive coupling of carbonyl compounds
using stoichiometric amounts of vanadium reagents, see: (a) Handa,
Y.; Inanaga, J . Tetrahedron Lett. 1987, 46, 2717. (b) Freudenberger,
H. J .; Konradi, W. A.; Pederden, F. S. J . Am. Chem. Soc. 1989, 111,
8014. (c) Raw, S. A.; Pedersen, F. J . Org. Chem. 1991, 56, 830. (d)
Konradi, W. A.; Pedersen, F. S. J . Org. Chem. 1992, 57, 28. (e) Konradi,
W. A.; Kemp, J . S.; Pedersen, F. S. J . Am. Chem. Soc. 1994, 116, 1316.
(7) For catalytic reductive coupling of aldimines, see: (a) Tanaka,
H.; Dhimane, H.; Fujita, H.; Ikemoto, Y.; Torii, S. Tetrahedron Lett.
1988, 29, 3811. (b) Liao, P.; Huang, Y.; Zhang, Y. Synth. Commun.
1997, 27, 1483.
As for the catalytic dimerization of aldimines, there
are only a few reports describing that the Cp2TiCl2-
catalyzed reduction of aldimines with Sm metal exhibits
high dl selectivity of the vicinal diamines (up to 80%
(1) For reviews, see: (a) Grame, M. R. In Comprehensive Organic
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563. (b) Fu¨rstner, A., Ed. Active Metals; VCH:
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Am. Chem. Soc. 1987, 109, 3152. (d) Mangeney, P.; Tejero, T.; Alexakis,
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K.; Tsubaki, Y.; Tanaka, S.; Beppu, F.; Fujiwara, Y. Chem. Lett. 1990,
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1990, 20, 981. (h) Imamoto, T.; Nishimura, S. Chem. Lett. 1990, 1141.
(i) Aurrecoechea, J . M.; Ferna´ndez-Acebes, A. Tetrahedron Lett. 1992,
33, 4763. (j) Shono, T.; Kise, N.; Oike, H.; Yoshimoto, M.; Okazaki, E.
Tetrahedron Lett. 1992, 33, 5559. (k) Kalyanam, N.; Rao, G. V.
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Org. Chem. 1998, 63, 2070. For the SmI2-catalyzed reductive coupling
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(5) (a) Hirao, T.; Asahara, M.; Muguruma, Y.; Ogawa, A. J . Org.
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10.1021/jo981396o CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/19/1998