Communications
DOI: 10.1002/anie.201104948
Synthetic Methods
Practical Conversion of Chlorosilanes into Alkoxysilanes without
Generating HCl**
Ryutaro Wakabayashi, Yasushi Sugiura, Toshimichi Shibue, and Kazuyuki Kuroda*
Alkoxysilanes are used as key starting materials in inorganic,
devoted to exploring reagents other than alcohols for the
alkoxylation of chlorosilanes. Such reagents are alkali-metal
[1]
organic, and materials chemistry for sol-gel processes,
[
2]
[6]
surface modification (silane coupling agent), protection of
alkoxides (MOR: M = Li, Na, K; R = Me, Et) and trialkyl
[
3]
[7]
hydroxy groups (silyl ether), and cross-coupling reactions
orthoformates (HC(OR) ), for example. However, they
3
[
4]
(
Hiyama coupling). Various alkoxysilanes are industrially
themselves are unstable to moisture, and the latter generates
several by-products, including alkyl formates (HCOOR) and
alkyl chlorides (RCl), in alkoxylation reactions. Thus, these
reagents have rarely been used in experimental and industrial
processes to date, therefore making a truly practical synthetic
procedure that does not involve hazardous substances desir-
able.
Herein we report the practical synthesis of alkoxysilanes
directly from chlorosilanes (Scheme 1). Our method uses
unsymmetrical ethers instead of alcohols. Unsymmetrical
produced for diverse purposes.
The industrial and classical methods for synthesizing
alkoxysilanes are based on the reaction of a chlorosilane and
[
5]
an alcohol; a reaction used since 1845. However, the
generation of HCl as a by-product has the potential to
cause serious environmental and health problems. Gaseous
HCl is not only very toxic but also decomposes alcohols,
thereby lowering the yield of the reaction. A simple way to
avoid the generation of HCl is to use a Lewis base as an acid
scavenger. Lewis bases, such as pyridine and 2,6-lutidine,
easily form their HCl salts and make the reaction system
neutral. However, the required amount of such an acid
scavenger is equimolar to that of generated HCl, thus making
this approach not feasible for industrial processes. Moreover,
filtration is required to remove these HCl salts. These steps
are laborious because alkoxysilanes are moisture sensitive.
HCl generation can be avoided through a two-step reaction;
reduction of the chlorosilane to the hydrosilane and subse-
quent alkoxylation with alcohol. However, this process may
cause hazards arising from highly reactive reducing agents
Scheme 1. Synthesis of alkoxysilanes and acetoxysilanes from chlorosi-
lanes.
1
2
1
(
LiAlH etc.) and the release of H . Much effort has been
ethers (R OR ), such as MTBE (methyl tert-butyl ether: R =
4
2
2
1
tBu, R = Me) and ETBE (ethyl tert-butyl ether: R = tBu,
R = Et), can be used as the source of methoxy and ethoxy
2
[*] R. Wakabayashi, Prof. K. Kuroda
groups, respectively. They are relatively safe, stable, and easy
Department of Applied Chemistry, Waseda University
Ohkubo-3, Shinjuku-ku, Tokyo 169-8555 (Japan)
and
Kagami Memorial Research Institute for Materials Science and
Technology, Waseda University
Nishiwaseda-2, Shinjuku-ku, Tokyo, 169-0051 (Japan)
E-mail: kuroda@waseda.jp
Homepage: http://www.waseda.jp/sem-kuroda_lab/
to handle relative to other agents (MOR or HC(OR) ) used
3
for this purpose, and this is assured by their extensive usage as
an additive for gasoline. Although the representative by-
1
product, tert-butyl chloride (R = tBu) has to be captured and
should not be released into the environment, it will be less
problematic because the by-product is easily distilled (b.p.
5
28C) and has no serious toxicity. In addition, alkyl chlorides
Y. Sugiura
1
(R Cl) are useful as reactants for various organic reactions,
Dow Corning Toray Co., Ltd.
Chigusa-Kaigan-2, Ichihara-shi, Chiba 299-0108 (Japan)
including Grignard reactions.
The use of unsymmetrical ethers in the present reaction
scheme has led to the synthesis of alkoxysilane monomers
through selective cleavage of CÀO bond. This idea has been
Dr. T. Shibue
Materials Characterization Central Laboratory, Waseda University
Ohkubo-3, Shinjuku-ku, Tokyo 169-8555 (Japan)
[
**] The authors are grateful to Dr. S. Sueki, Mr. K. Kawahara, Mr. H.
Tachibana, and Ms. M. Tamai (Waseda University) for helpful
discussions. This work was supported in part by a Grant-in-Aid for
Scientific Research (No. 23245044) and the Global COE program
inspired by previous studies on nonhydrolytic sol-gel process
[
8]
which uses symmetrical ethers. Symmetrical ethers are
known to react with chlorosilanes in the presence of Lewis
acids to form silica. The cleavage of the CÀO bond strongly
“
Practical Chemical Wisdom” from the MEXT (Japan). K.K. also
depends upon the stability of the resulting carbocations in the
acknowledges the support by Elements Science and Technology
Project “Functional Designs of Silicon-Oxygen-Based Compounds
by Precise Synthetic Strategies” from the MEXT (Japan).
[
8]
[8–10]
case of symmetrical ethers and alkoxysilanes.
Nonhy-
drolytic sol-gel processes are applicable not only to silica but
also to other inorganic oxides. Though alkoxysilanes are
formed in situ, they are intermediates and alkoxysilane
1
0708
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 10708 –10711