Communications
[
a]
Table 1: Oxidation of organosilanes.
(1c) and iPr SiH (1d), the corresponding silanols 2c,d were
3
obtained in high yields by increasing the catalyst loading to
3
mol% (entries 2 and 3). On the other hand, the reaction of
sterically hindered Ph SiH proceeded with 1 mol% of the
3
catalyst, and the corresponding silanol 2e was obtained nearly
quantitatively (entry 4). The AuNPore catalyst could also be
used in the oxidations of diphenylsilane 1 f and phenylsilane
1g, and the corresponding oxygenated products, Ph Si(OH)
[
b]
Entry
Catalyst
Yield of 2a [%]
1
2
3
4
5
fresh
100
98
100
99
reuse 1
reuse 2
reuse 3
reuse 4
2
2
(2 f) and PhSi(OH) (2g), were obtained in 90 and 80%
3
yields, respectively (entries 5 and 6). Alkenyl- and alkynyl-
containing silanes 1h,i were suitable substrates for the current
oxidation reaction, and the corresponding silanols 2h,i were
obtained in high yields (entries 7 and 8).
100
[
a] Reactions were performed using 1a (1.0 mmol), H O (0.1 mL), and
AuNPore (1 mol%) in 1.5 mL of acetone at room temperature for 1 h.
b] Yield of isolated product.
2
[
We then examined the reaction to clarify whether the
dissolved gold species in solvents take part in the current
molecular transformation or not. After the catalytic oxidation
of 1a was carried out for 10 min under the standard
conditions, the nanoporous gold was removed from the
ture of the catalyst plays a crucial role for the current
transformation. It is noteworthy that the formation of
disiloxane, 1,1,3,3-tetramethyl-1,3-diphenyldisiloxane, was
not detected at all by GCMS. Generally, the recovery of the
heterogeneous catalyst is carried out by filtration for the
separation of the catalyst from the reaction mixture. In the
present reaction system, we used some small pieces of the
1
reaction vessel. H NMR analysis of the mixture showed
that 2a was produced in 48% yield at this time. While stirring
of the mixture was continued in the absence of the catalyst for
50 min, further consumption of 1a was not detected at all.
AuNPore was then put back into the mixture. The oxidation
reaction restarted immediately and finally 2a was obtained in
99% yield within 50 min. It is also worth mentioning that
leaching of the gold in the reaction of 1a was not detected by
inductively coupled plasma (ICP) analysis (< 0.0005%).
These results clearly indicated that the current transformation
was catalyzed by the AuNPore catalyst.
nanostructured gold foil as a catalyst with a size of around 5 ꢀ
mm . Thus, the catalyst can be recovered easily by picking
2
2
up by tweezers without any cumbersome work-up procedures.
After a simple washing of the catalyst with diethyl ether, it
was reused without further purification. We used the catalyst
repeatedly (five times), but no significant loss of activity was
observed. The product 2a was obtained nearly quantitatively
every time and the turnover number (TON) reached up to
In conclusion, we have discovered that a nanoporous gold
material exhibited a remarkable catalytic activity in the
oxidation of organosilane compounds with water. The catalyst
was easily recoverable and could be used at least five times
without leaching and loss of activity. The observed excellent
durability of the catalyst was also confirmed by SEM images.
Indeed, the nanoporous structure of the catalyst did not
change, even after five uses for the oxidation of dimethyl-
phenylsilane 1a. Further studies to elucidate the mechanism
of this reaction and to extend the scope of synthetic utility are
1
0700 (entries 1–5). Figure 2b is the SEM image of the
recovered catalyst after five uses (entry 5). No significant
changes were observed in comparison with Figure 2a.
The catalytic oxidation reactions with a variety of organo-
silanes were then carried out (Table 2). Not only aromatic
silanes but also trialkylsilanes were oxidized effectively
entries 1–3). The reaction of sterically less-hindered triethyl-
silane 1b proceeded smoothly in 2 h and the desired
triethylsilanol 2b was produced in 94% yield (entry 1).
(
[
13]
Even with sterically hindered trialkylsilanes, such as Bu SiH
in progress in our laboratory.
3
[
a]
Table 2: Scope of the oxidation of organosilanes.
Experimental Section
The preparation of 2a is given as a representative example. Acetone
(
1.5 mL), H O (0.1 mL), and dimethylphenylsilane 1a (136 mg,
2
1
mmol) were added successively to a catalytic amount of nanoporous
gold (2.0 mg, 1 mol%) in a micro reaction vial at room temperature.
The mixture was stirred for 1 h and the catalyst was removed using
tweezers. The reaction mixture was then concentrated under reduced
pressure and the residue was purified by column chromatography on
silica gel using hexane/ether (2:1) as eluent to give 2a (152 mg)
quantitatively. The recovered catalyst was washed with ether and was
reused without further purification.
Entry
1
R( SiHn
AuNPore
mol%]
t [h]
2
Yield
[%]
4Àn)
[
b]
[
1
2
3
4
5
6
7
1b Et SiH
1c Bu SiH
1d iPr SiH
1e Ph SiH
1 f Ph SiH2
1g PhSiH3
1h (H C=CH)MePh-
1
3
3
1
1
5
1
2
3
5
5
9
6
1
2b 94
2c 95
2d 88
2e 99
2 f 90
2g 80
2h 98
3
3
3
3
2
Received: August 17, 2010
Revised: September 11, 2010
Published online: November 29, 2010
2
SiH
8
1i (PhCꢀC)Me SiH
3
1.5 2i 92
2
[
a] Reactions were performed using 1 (1.0 mmol), H O (0.1 mL), and
2
Keywords: gold · heterogeneous catalysis ·
nanoporous materials · organosilanols · oxidation
AuNPore (n mol%) in 1.5 mL of acetone at room temperature. [b] Yield
of isolated product.
.
1
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 10093 –10095