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Published on the web June 1, 2013
Hydrosilylation of Various Multiple Bonds by a Simple Combined Catalyst
of a Tungstate Monomer and Rhodium Acetate
Shintaro Itagaki, Hanako Sunaba, Keigo Kamata, Kazuya Yamaguchi, and Noritaka Mizuno*
Department of Applied Chemistry, School of Engineering, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656
(Received April 12, 2013; CL-130338; E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp)
In the presence of a simple combined catalyst of a tungstate
C=O, and C=N bonds.6 Although hydrosilylation of relatively
unreactive multiple bonds, e.g., C¸N bonds, has also been
realized using several transition-metal complexes through an
ionic hydrosilylation mechanism,7 room still remains for
improvement.
monomer (TBA2WO4, TBA: tetra-n-butylammonium) and
rhodium acetate (Rh2(OAc)4), hydrosilylation of various types
of substances including ketone, aldehyde, carbon dioxide,
alkene, nitrile, and furan derivatives efficiently proceeded,
affording the corresponding hydrosilylation products in moder-
ate to high yields (²63% yields). In addition, the present system
was also applicable to the one-pot reduction of benzamide to
benzylamine (95% yield).
As above-mentioned, a combined catalyst of TBA2WO4 and
Rh2(OAc)4 can activate hydrosilanes to form silicon electrophiles
and metal hydride species,4 which are desirable active species
for hydrosilylation. In this manuscript, we report for the first
time that hydrosilylation of various types of substances including
ketone, aldehyde, carbon dioxide, alkene, nitrile, and furan
derivatives efficiently proceed in the presence of the combined
catalyst. In addition, an application of the present system to
reduction of benzamide to benzylamine is also demonstrated.
Initially, the hydrosilylation of acetophenone (1a) with
dimethylphenylsilane (2a) was carried out under various
reaction conditions.8 In the previously reported N-silylation
of indole derivatives, we have examined 33 combinations of
bases and metal catalysts and found that the combination of
TBA2WO4 and Rh2(OAc)4 was the best choice (Table S1).4,5
Thus, we herein focused on using TBA2WO4 and Rh2(OAc)4
for the hydrosilylation. Among the solvents examined with
TBA2WO4 (acetonitrile, DMSO, THF, 1,4-dioxane, methanol,
dichloromethane, toluene, and n-hexane), acetonitrile gave the
highest yield of the corresponding hydrosilylation product
3aa. Under the conditions described in Scheme 1, no reaction
proceeded in the absence of the catalysts. TBA2WO4 and
Rh2(OAc)4 gave 3aa in 39% and 20% yields, respectively. No
hydrosilylation proceeded in the presence of a simple soluble
base of TBAOH. Notably, when using a mixture of TBA2WO4
and Rh2(OAc)4, the hydrosilylation was significantly promoted,
and the yield of 3aa increased to 96% under the same
conditions.
Polyoxometalates (POMs) are a large family of anionic
metal-oxygen clusters consisting of the group 5 and 6 metals in
their highest oxidation states and thermally, oxidatively stable
in comparison with commonly utilized organometallic catalysts
and organocatalysts.1 Their chemical and physical properties,
e.g., acidities, basicities, redox potentials, (multi)electron-trans-
fer properties, and solubilities, can finely be tuned by selecting
constituent elements and counter cations, and tailor-made
structures with precisely controlled active sites can be designed.1
In addition to the above-mentioned properties, bare nucleophilic
surfaces resulting from their external oxygen atoms, i.e., M=O
and M-O-M species, are the important feature of POMs and can
act as nucleophilic catalysts to generate and/or stabilize cationic
(electrophilic) intermediates.2-4
Recently, we have reported a series of two manuscripts
describing the development of efficient synthetic procedures for
organosilicon compounds using oxometalate-based catalysts.3,4
First, we have designed rare-earth-metal-containing POMs for
cyanosilylation of carbonyl compounds with trimethylsilyl
cyanide.3 The rare-earth-metal sites and the POM frameworks
(nucleophilic surfaces) can simultaneously activate carbonyl
compounds and trimethylsilyl cyanide, respectively, resulting
in effective promotion of cyanosilylation.3 Second, we have
demonstrated dehydrogenative N-silylation of indole derivatives
(including pyrrole and carbazole) with hydrosilanes in the
presence of a simple combined catalyst of TBA2WO4 and
Rh2(OAc)4.4 The overarching strategy of these works is the use
of oxometalates as nucleophilic catalysts to generate and/or
stabilize silicon electrophiles.3,4 For example, the detailed
mechanistic investigation for the above-mentioned N-silylation
suggests the formation of the silicon electrophiles stabilized on
TBA2WO4 (Figure S1)5 and the metal hydride species from
hydrosilanes, and the N-silylation possibly proceeds through an
“ionic silylation mechanism.”4
Notably, with regard to carbonyl compounds, hydrosilyla-
tion efficiently proceeded in the presence of TBA2WO4 alone
(without additional metal catalysts). In this case, triphenylsilane
(2b) was a good coupling partner for carbonyl compounds. For
example, when the hydrosilylation of 1a with 2b was performed
even under “solvent-free” conditions, the desired hydrosilylation
O
OSiMe Ph
2
catalyst
H
+
PhMe SiH
2
Ph
Ph
1a
2a
3aa
with TBA2WO4: 39% yield, with Rh2(OAc)4: 20% yield
with TBA2WO4 + Rh2(OAc)4: 96% yield
Hydrosilylation is the most useful and powerful reaction for
synthesis of organosilicon compounds and for selective reduc-
tion of various multiple bonds.6 Up to the present, a number of
platinum group as well as group 11 metal-based catalysts have
been reported to be active for hydrosilylation of C=C, C¸C,
Scheme 1. Hydrosilylation of acetophenone (1a) with dimeth-
ylphenylsilane (2a). Reaction conditions: TBA2WO4 (2 mol %
with respect to 1a) and/or Rh2(OAc)4 (1 mol %), 1a (0.5 mmol),
2a (1 mmol), acetonitrile (2 mL), 50 °C, Ar (1 atm), 2 h.
Chem. Lett. 2013, 42, 980-982
© 2013 The Chemical Society of Japan