CL-150379
Received: April 22, 2015 | Accepted: May 7, 2015 | Web Released: May 14, 2015
O2-enhanced Catalytic Activity of Gold Nanoparticles
in Selective Oxidation of Hydrosilanes to Silanols
Teppei Urayama,1 Takato Mitsudome,1 Zen Maeno,1 Tomoo Mizugaki,1 Koichiro Jitsukawa,1 and Kiyotomi Kaneda*1,2
1Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531
2Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531
(E-mail: kaneda@cheng.es.osaka-u.ac.jp)
O2 acts as a nonconsumed activator for gold nanoparticles
(AuNPs) in the oxidation of hydrosilanes to silanols with water
under O2 atmosphere, providing an acceleration of more than
200 times relative to the reaction rate under Ar atmosphere. The
AuNP catalyst under aerobic conditions exhibits high activity in
the oxidation with high turnover numbers (1230000). Various
hydrosilanes including less-reactive bulky ones can be converted
to the corresponding silanols in excellent yields. Moreover, the
present AuNP catalyst is reusable while maintaining the high
performance.
Table 1. Aqueous oxidation of 1 using Au/HAP under various
conditionsa
Au/HAP
iBu3SiH (1)
H2
H2O
iBu3SiOH (2)
Entry
Atmosphere
Time
Yield of 2/%b
1
2
3
4c
5d
6
7
8
9
10e
11e
Ar
air
O2
O2
O2
CO2
N2
H2
Ar
O2
Ar
8 h
>99
>99
>99
0
0
13
11
11
11
>99
0.5
10 min
10 min
10 min
10 min
10 min
10 min
10 min
10 min
7 min
Silanols are valuable building blocks for silicon-based
polymeric materials1 and are nucleophilic coupling partners in
organic synthesis.2 Previously, silanols have been synthesized
by hydrolysis of chlorosilanes with water in the presence of
bases.3 However, chlorosilanes are unstable in air moisture
and the stoichiometric reaction produces large amounts of salt
by-product. Recently, catalytic oxidation of hydrosilanes to
silanols with water has attracted much attention as a promising
alternative to the traditional reaction of chlorosilanes. To date,
various homogeneous and heterogeneous catalysts have been
reported.4 In this context, we first reported that gold (Au)5
and silver6 nanoparticles (NPs) are capable of promoting the
selective oxidation of hydrosilanes to silanols with water. The
AuNP-catalyzed oxidation of hydrosilanes has since been
explored by other researchers with respect to size and shape of
the AuNPs.7 The developed AuNP catalysts show high activities
for various hydrosilanes. However, the developed AuNP catalyst
systems still suffer from low catalytic performance for bulky
silanes, where poor yields of silanols are often obtained7e or
high loading of AuNPs is required to achieve high yields.7a,7f
Therefore, the development of AuNPs catalyst systems with
high catalytic activity for bulky silanes is still a challenging
issue.
We herein present that molecular oxygen (O2) is found to act
not as a stoichiometric oxidizing reagent but as a nonconsumed
activator for AuNPs, significantly boosting the catalytic activity
of AuNPs in the oxidation of silanes. Diverse hydrosilanes
including bulky hydrosilanes are transformed to the correspond-
ing silanols in high yields with high efficiencies. Moreover, the
AuNP catalyst is recoverable and reusable without the loss of its
activity and selectivity.
In our previous work, we found that hydroxyapatite-
supported AuNPs (Au/HAP) with a mean diameter of 3.0 nm
were effective in the oxidation of hydrosilanes.5 Toward a more
efficient heterogeneous AuNP catalyst system for the oxidation
of silanes, many factors that can affect the AuNP catalysis were
re-investigated.8 We found that the atmospheric condition played
a key role in the oxidation. Table 1 shows the results of Au/
7 min
aReaction conditions: 1 (1 mmol), Au/HAP (0.05 g, Au:
0.4 mol %), water (10 equiv), acetone (4 mL), 30 °C. bDeter-
mined by GC using an internal standard technique. cThe use of
d
e
HAP instead of Au/HAP. In the absence of Au/HAP. Au/
HAP (0.005 g, Au: 0.04 mol %).
HAP activity in the oxidation of tri(iso-butyl)silane (1) as a
model for bulky hydrosilanes in acetone solvent at 30 °C under
various atmospheric conditions. In Ar atmosphere, the oxidation
gave tri(iso-butyl)silanol (2) in 99% yield in 8 h together with
the generation of equimolar amounts of H2 (Table 1, Entry 1).
Notably, under air atmosphere, the catalytic activity of Au/HAP
significantly increased and the quantitative oxidation of 1 was
completed within 10 min (Table 1, Entry 2). To investigate the
effect of air atmosphere, the oxidation was carried out under
various atmospheric conditions such as O2, CO2, N2, and H2
(Table 1, Entries 3 and 6-8). Among the conditions tested, O2
atmosphere was highly effective to afford 2 in quantitative yield
(Table 1, Entry 3), while CO2, N2, and H2 atmosphere did not
influence the catalytic performance of Au/HAP, resulting in
yields of 2 similar to that under Ar for the same duration
(Table 1, Entries 6-8 vs. 9). In addition, neither the use of HAP
nor the absence of Au/HAP catalyzed the oxidation in O2
atmosphere (Table 1, Entries 4 and 5). These results clearly
showed the high efficiency of O2, drastically enhancing the
catalytic activity of AuNPs.
When comparing the catalytic activity of Au/HAP in O2
with Ar atmosphere using the same reaction time with lower
catalyst loading (0.04 mol %), the impact of O2 was more
pronounced. A quantitative yield of 2 was obtained in the
presence of O2, whereas Ar atmosphere gave 2 in 0.5% yield
(Table 1, Entries 10 vs. 11), demonstrating an acceleration of
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