DOI: 10.1002/anie.201008199
Metal-Catalyzed Hydrosilylation
1,4-Hydrosilylation of Pyridine by Ruthenium Catalyst:
A New Reaction and Mechanism**
Kohtaro Osakada*
homogeneous catalysis · hydrosilylation · pyridine ·
ruthenium
T
he hydrosilylation of unsaturated molecules is recognized
as the most common means of introducing a silicon-contain-
ing functionality to organic substrates. Complexes of various
transition metals are employed as homogeneous catalysts for
the hydrosilylation of C C and C O bonds.[1] Reports on the
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hydrosilylation of C N bonds, however, were even much less
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common than those on the reaction of C O bonds until
recently. Over the past 15 years, complexes of various metals,
including early-, late-, and post-transition metals (including
Ti,[2] Re,[3] Ru,[4] Rh,[5] Ir,[6] Cu,[7] and Zn[8]), have been
titanium catalyst (10 mol% of the substrate) and heating
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the reaction mixture at 808C. The 1,2-addition of the Si H
group in the hydrosilylation was confirmed by deuterium-
labeling experiments. The actual products are tetrahydropyr-
idine derivatives which result from the concurrent hydro-
genation of the pyridine ring. Thus, hydrosilylation using the
half-sandwich ruthenium complex as the catalyst differs in
chemoselectivity from the reaction catalyzed by the titanium
complex and proceeds under milder conditions.
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reported to catalyze the addition of Si H groups of primary
or secondary organosilanes or of polymethylhydrosiloxanes
(PMHS) to imines. Since no hydrogenation of imines, giving
the corresponding amines, takes place under mild conditions,
the hydrosilylation of prochiral imines and the subsequent
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hydrogenolysis of the N Si bond formed provide an alter-
native for the conversion of prochiral imines into optically
active amines.
The experimental results in this recent article may
encourage creative applications by the readers who are
interested in the reaction mechanism. Scheme 1 summarizes
Nikonov et al. of Brock University chose a cationic
ruthenium complex, [Cp(iPr3P)Ru(NCMe)2][B(C6F5)4] (1;
Cp = cyclopentadienyl), as the catalyst for the hydrosilylation
of ketones and nitriles in their recent studies.[9] This catalyst
converts aliphatic and aromatic nitriles into the correspond-
ing N-silylimines with high chemoselectivity or into N,N-
disilylamines formed by double hydrosilylation, depending on
the reaction conditions. This Highlights describes the latest
report from this research group on catalysis by the complex in
the hydrosilylation of pyridine.[10]
Scheme 1. Reactions of the hydrosilylation product 2 in the presence
of catalyst 1.
Pyridine and its derivatives having chloro and methyl
substituents at 3- and 5-positions undergo hydrosilylation
with HSiMe2Ph in the presence of catalytic amounts of 1 (2–
5 mol% of the substrate) to yield the N-silyl-4-hydropyridine
derivatives, as shown in Equation (1). The reaction is
complete within several hours at room temperature.
[Cp2TiMe2] catalyzes the hydrosilylation of pyridine,
which is the sole precedent using a homogeneous catalyst.[11]
The reaction, however, requires higher amounts of the
the reactions of N-silyl-4-hydropyridine (2) catalyzed by the
complex 1. PhCN reacts with 2 in the presence of the catalyst
1 leading to the formation of N-silylphenylimine and the
regeneration of pyridine (Scheme 1, top). The formal elimi-
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nation of HSiMe2Ph from 2 and its addition to the C N bond
of PhCN would explain formation of the products. The
addition of 3,5-lutidine to 2 yields an equilibrated mixture of
the 1,4-addition products of pyridine and 3,5-lutidine. These
results suggest the reversibility of this hydrosilylation,
although mechanistic studies of the hydrosilylation reported
thus far did not discuss details of this issue.[12]
[*] Prof. K. Osakada
Chemical Resources Laboratory
Tokyo Institute of Technology
R1-3 4259, Nagatsuta, Midori-ku, Yokohama 226-8503 (Japan)
Fax: (+81)45-924-5224
Hydrosilylation reactions generate two stable chemical
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E-mail: kosakada@res.titech.ac.jp
bonds, H C and X Si (X = C, O, N), and its reverse process
should result in the activation of these bonds in the produced
[**] The author is grateful to Dr. Makoto Tanabe for helpful discussions.
Angew. Chem. Int. Ed. 2011, 50, 3845 – 3846
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3845