Table 1. Synthesis of silyl ether (2) from dimethylphenylsilane (1) with
nBuOH by using various catalysts.[a]
Silyl ethers are of great importance and widely applicable
materials for the synthesis of elastomers,[1a] coatings,[1b] fi-
bers,[1c] and biomedical materials,[1d] besides for reagents[1e]
and protecting groups for alcohols[1f] in organic synthesis.
Generally, silyl ethers have been synthesized through the re-
actions of chlorosilanes[2] or disilazanes[3] with alcohols in
the presence of a base such as pyridine. However, chlorosi-
lanes are often not convenient for handling due to moisture
sensitive and corrosive reagents. In addition, these reactions
produce stoichiometric amounts of salts as a byproduct. As
an alternative method for the synthesis of silyl ethers, cata-
lytic dehydrogenative etherification of hydrosilanes with al-
cohols has been devised because of the only formation of H2
as the co-product. To date, several catalyst systems for the
etherification have appeared.[4] However, these catalysts
have often suffered from substrate limitations, necessity of
high temperature, difficulty in separation and reuse of cata-
lysts, and leaching of active metal species from the supports.
Recently, we disclosed that hydroxyapatite-supported
gold nanoparticles (NPs) can catalyze the dehydrogenative
coupling of hydrosilanes with water, giving the correspond-
ing silanols associated with the formation of H2.[5] The activi-
ty of gold NPs for hydrosilanes was also applicable to the
gold-catalyzed deoxygenation of amides, sulfoxides, and pyr-
idine N-oxides by using hydrosilanes as reductants.[6]
In the course of our study on the interaction between
gold NPs and hydrosilanes, herein, we report that nanohy-
droxyapatite-supported gold NPs (Au/HAPnano) are newly
synthesized, which can act as a highly active and reusable
catalyst for the synthesis of a wide range of silyl ethers
under mild conditions. The Au/HAPnano developed herein
overcomes all of the above problems that reported catalyst
systems have faced. Furthermore, the unique promotional
effect of O2 on the gold-catalyzed reaction was disclosed.
Namely, O2 was found to act not as a stoichiometric oxidiz-
ing reagent but as a no-consumed promoter, significantly
boosting the catalytic activity of AuNPs.
Entry
Catalyst
Atmosphere
t [min]
Yield [%][b]
1
2
3
4
5
6
7
8
Au/HAPnano
Au/HAP
Au/TiO2
Au/SiO2
Au/HT
Au/HAPnano
Au/HAPnano
Au/HAPnano
Au/HAPnano
Pd/HAPnano
Ag/HAPnano
Pt/HAPnano
Ru/HAPnano
Cu/HAPnano
Ar
Ar
Ar
Ar
Ar
air
Ar
air
O2
air
air
air
air
air
20
20
20
20
20
10
5
5
5
5
5
>99
59
29
5
5
>99
35
70
>99
7
9
10
11
12
13
14
5
3
2
2
5
5
5
[a] Reaction conditions: dimethylphenylsilane (2 mmol), nBuOH
(3 mmol), catalyst (M 0.05 mol%), 258C. [b] Determined by GC by using
an internal standard technique.
yapatite as a support (Au/HAPnano) gave the highest activity
to give 2 quantitatively within 20 min accompanied by the
generation of equimolar amounts of H2 (Table 1, entry 1).
AuNPs supported on conventional hydroxyapatite (Au/
HAP) was also found to be effective, but inferior to Au/
HAPnano (entry 1 vs. 2), whereas Au/TiO2, Au/SiO2 and Au/
hydrotalcite (Au/HT) gave low yields of 2 (entries 3–5). In-
terestingly, when using Au/HAPnano under air atmosphere in
place of Ar conditions, the reaction rate was drastically en-
hanced, providing a >99% yield within 10 min (Table 1,
entry 6). Furthermore, the catalytic activity of Au/HAPnano
increased more under pure O2 (1 atm) conditions (Table 1,
entries 7–9). Notably, during the etherification under the
above-described aerobic conditions, equimolar amounts of
H2 were generated associated to the formation of 2, whereas
O2 was not consumed. These phenomena suggest that the
O2 molecule does not act as a stoichiometric oxidizing re-
agent but plays a key role in no-consumed promoter for en-
hancing the activity of Au/HAPnano in this etherification.
Next, various metal NPs supported on HAPnano were instead
employed for the etherification of 1 to 2 under air condi-
tions. They performed very poorly, providing less than 10%
yield of the product (Table 1, entries 10–14). These results
revealed that the use of AuNPs and HAPnano in the presence
of O2 is essential to achieve highly efficient etherification of
1.
Initially, we investigated the catalytic activities of a series
of inorganic metal-oxides-supported AuNPs of approximate-
ly 3 nm in AuNP size in the reaction of dimethyphenylsilane
(1) with nBuOH under solvent-free conditions at 258C
under Ar atmosphere (Table 1, entries 1–5). These AuNP
catalysts promoted the etherification of 1 to the correspond-
ing butoxydimethylphenylsilane (2) with >99% selectivities.
Of the AuNP catalysts tested, the use of nanosized hydrox-
[a] Dr. T. Mitsudome, Y. Yamamoto, A. Noujima, Dr. T. Mizugaki,
Prof. Dr. K. Jitsukawa, Prof. Dr. K. Kaneda
Department of Materials Engineering Science
Graduate School of Engineering Science, Osaka University
1–3 Machikaneyama, Toyonaka, Osaka 560-8531 (Japan)
Fax : (+81)6-6850-6260
The substrate scope of Au/HAPnano for the synthesis of
silyl ethers derived from diverse combinations of silanes
with alcohols is exemplified in Table 2.[7] Au/HAPnano was
significantly effective in the etherification of various silanes,
such as aromatic, aliphatic, and alicyclic silanes with
nBuOH to give excellent yields of the corresponding ethers
(Table 2, entries 1 and 5–11). The present Au/HAPnano cata-
lyst system could be extended to selective synthesis of di-
[b] Prof. Dr. K. Kaneda
Research Center for Solar Energy Chemistry, Osaka University
1–3, Machikaneyama, Toyonaka, Osaka 560-8531 (Japan)
Supporting information for this article is available on the WWW
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and tri-alkoxysilanes from a silane containing three Si H
&
2
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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