1,4-Hydrosilylation of pyridine by ruthenium catalyst: A new reaction and mechanism

Article abstract of DOI:10.1002/anie.201008199

Making introductions: A half-sandwich Ru complex catalyzes the hydrosilylation of pyridine derivatives to produce N-silyl-3-hydropyridine through a 1,4-addition of the Si-H group (see scheme). The reversibility of the hydrosilylation was suggested based on the experimental results.

Full text of DOI:10.1002/anie.201008199

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
À
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.
À
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
À
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-
ꢀ
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
À
À
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

Highlights
[2] a) X. Verdaguer, U. E. W. Lange, M. T. Reding, S. Buchwald, J.
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[9] a) D. V. Gutsulyak, S. F. Vyboishchikov, G. I. Nikonov, J. Am.
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molecule. The bond-forming reactions in the Chalk–Harrod
mechanism and its modified version for olefin hydrosilyla-
tion[13] are the insertion of C C bond into the M H or M Si
bonds and the reductive elimination of product from alkyl-
(silyl)– or alkyl(hydride)–metal intermediates. Reverse pro-
cesses for each of these reactions, b-hydrogen (b-silyl)
=
À
À
À
elimination and cleavage of the Si C bond through oxidative
addition, may occur depending on the metal center and
ligand. Organometallic chemists should be interested in the
mechanistic issues, such as details of the reverse reaction of
the hydrosilylation of pyridine as well as the microscopic
reversibility of the total reaction.
The reaction of DSiMe2Ph with 2 in the presence of the
catalyst yields N-silyl-4-deuteriopyridine (Scheme 1, bottom).
The results imply selective and reversible 1,4-addition in the
hydrosilylation. Tobita et al. studied the properties of the
complex with another Group 8 transition metal (Fe) and a 14-
electron fragment containing Cp and CO ligands.[14] They
found that heating the complex with silyl and pyridine ligands
produces a complex with N-silylpyridine as the p-allylic
ligand, as shown in Equation (2).
[10] D. V. Gutsulyak, A. van der Est, G. I. Nikonov, Angew. Chem.
2011, 123, 1420 – 1423; Angew. Chem. Int. Ed. 2011, 50, 1384 –
1387.
[11] L. Hao, J. F. Harrod, A.-M. Lebuis, Y. Mu, R. Shu, E. Samuel,
H.-G. Woo, Angew. Chem. 1998, 110, 3314 – 3318; Angew. Chem.
Int. Ed. 1998, 37, 3126 – 3129.
[12] An experimental result in the study of the Ti-catalyzed hydro-
silylation of pyridine suggests the reversibility of the reaction,
although it was not stated clearly in the report. The authors
carried out the reaction of PhMeSiD2 and pyridine in the
presence of [Cp2TiMe2] and observed deuteration at not only the
2- but also the 6-position. The substitution of the 3-, 4-, and 5-
positions by SiH(D) hydrogen to a less significant degree may
also be mentioned. See Ref. [11].
[13] a) A. J. Chalk, J. F. Harrod, J. Am. Chem. Soc. 1965, 87, 16 – 21;
b) M. A. Schroeder, M. S. Wrighton, J. Organomet. Chem. 1977,
128, 345 – 358; c) S. Sakaki, M. Ogawa, Y. Musashi, T. Arai, J.
Am. Chem. Soc. 1994, 116, 7258 – 7265.
A similar type of intermediate may be involved in the 1,4-
À
addition of a H Si bond of pyridine in the ruthenium-
catalyzed reaction. Nikonov and co-workers have already
reported coordination of organosilanes to the ruthenium
center of complexes composed of the cationic {Ru(Cp)(PR3)}
or {Ru(Cp*)(PR3)} fragment.[15] The real mechanism of the
catalytic hydrosilylation will be elucidated through a detailed
study of these complexes in the near future.
The Chalk–Harrod-type mechanism has long been dom-
inant in the hydrosilylation of olefins catalyzed by late-
transition-metal complexes. In 2003, Tilley and Glaser
proposed a novel mechanism involving the insertion of an
[14] M. Iwata, M. Okazaki, H. Tobita, Organometallics 2006, 25,
6115 – 6124.
[15] a) D. V. Gutsulyak, A. V. Churakov, L. G. Kuzmina, J. A. K.
Howard, G. I. Nikonov, Organometallics 2009, 28, 2655 – 2657;
b) A. L. Osipov, S. M. Gerdov, L. G. Kuzmina, J. A. K. Howard,
G. I. Nikonov, Organometallics 2005, 24, 587 – 602; c) A. L.
Osipov, S. F. Vyboishchikov, K. Y. Dorogov, L. G. Kuzmina,
J. A. K. Howard, D. A. Lemenovskii, G. I. Nikonov, Chem.
Commun. 2005, 3349 – 3351; D. V. Gutsulyak, A. L. Osipov,
L. G. Kuzmina, J. A. K. Howard, G. I. Nikonov, Dalton Trans.
2008, 6843 – 6850.
À
olefin into the Si H bond of a silylene-coordinated inter-
mediate complex on the basis of results of a detailed study of
the properties and catalytic activity of Ru complexes.[16]
A
new mechanism of the hydrosilylation or a new mode of
catalysis may be revealed from further studies in this field.[17]
Received: December 27, 2010
Published online: February 24, 2011
[16] P. B. Glaser, T. D. Tilley, J. Am. Chem. Soc. 2003, 125, 13640 –
13641.
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À
[17] The insertion of C X (X = C, N, O) bonds into the Si H bond of
the silylene–metal intermediate rather than into the M-Si bond
was also reported. See T. Watanabe, H. Hashimoto, H. Tobita,
Angew. Chem. 2004, 116, 220 – 223; Angew. Chem. Int. Ed. 2004,
43, 218 – 221; T. Watanabe, H. Hashimoto, H. Tobita, J. Am.
Chem. Soc. 2006, 128, 2176 – 2177.
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