Angewandte
Chemie
First, oxidation of dimethylphenylsilane (1a) to dimethyl-
phenylsilanol (2a) with 30% aqueous H2O2 was carried out
under stoichiometric conditions (substrate/H2O2 = 1:1).
Among the solvents tested, acetonitrile gave the highest
yield (78%) of 2a (Table 1, entry 1). Formation of the
were inactive (Table 1, entries 13 and 15–18). The H2WO4
catalyst showed low selectivity to 2a and significant formation
of 3a (Table 1, entry 14). Divanadium-substituted polyoxo-
tungstate (TBA)4[g-H2SiV2W10O40],[25e] selenium-containing
dinuclear peroxotungstate (TBA)2[SeO4{WO(O2)2}2],[23g] and
phosphorus-containing tetranuclear
peroxotungstate
(THA)3-
Table 1: Catalytic oxidation of dimethylphenylsilane (1a) with H2O2.[a]
[PO4{WO(O2)2}4][23b] showed low
selectivity to 2a, while the reaction
rates were comparable to or higher
than that of I (Table 1, entries 19, 21,
and 22). To investigate the high
selectivity to 2a in the present
system, condensation of 2a to give
3a was carried out in the presence of
various catalysts (Table S1, Support-
ing Information). The H2WO4,
Entry
Catalyst
Solvent
Yield [%]
Selectivity [%]
2a 3a
R0 [mmminÀ1
]
1
2
3
4
5
6
I
I
I
I
I
I
CH3CN
(CH2Cl)2
DMSO
acetone
benzonitrile
DMF
79
59
50
46
45
3
99
97
99
98
98
1
3
1
2
2
8
<1
6
4
1
2.2
1.7
0.9
0.7
1.6
<0.1
2.9
0.2
0.3
2.3
–
(TBA)2[SeO4{WO(O2)2}2],
and
92
7[b]
8[c]
9[d]
10[e]
11[f]
12
13
14
15
16
17
18
19[g]
20
21
22[h]
I
I
I
I
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
80
17
85
75
<1
<1
4
39
2
3
>99
94
(THA)3[PO4{WO(O2)2}4] catalysts
gave 3a in 83, 80, and 43% yield,
respectively. On the other hand,
condensation hardly proceeded in
the presence of I. Thus the low
activity for condensation in the pres-
ent system results in high selectivity
to 2a.
The scope of the catalytic oxida-
tion of organosilanes with H2O2 was
investigated for a range of structur-
ally diverse silanes (Table 2). Various
silanes could efficiently be oxidized
under stoichiometric conditions
(substrate/H2O2 = 1:1), and the cor-
responding silanols were obtained in
high yields and selectivities. Catalytic
oxidation of dimethylphenylsilanes
1a–1d, which contain electron-
donating or electron-withdrawing
para substituents, proceeded selec-
tively to afford the corresponding
96
99
–
–
I
–
–
none
Na2WO4·2H2O
H2WO4
–
>99
27
<1
73
15
<1
<1
–
12
9
47
14
<0.1
0.6
<0.1
<0.1
0.1
–
3.1
1.0
4.0
2.6
(TBA)4[a-SiW12O40]
(TBA)4H4[a-SiW11O39]
(TBA)3H7[a-SiW9O34]
(TBA)4[g-SiW12O40]
(TBA)4[g-H2SiV2W10O40]
(TBA)2[{WO(O2)2}2(m-O)]
(TBA)2[SeO4{WO(O2)2}2]
(THA)3[PO4{WO(O2)2}4]
85
>99
>99
–
4
<1
76
32
>99
94
88
91
53
86
[a] Reaction Conditions: Catalyst (W: 7.3 mol% with respect to 1a and H2O2), 1a (1 mmol), 30%
aqueous H2O2 (1 mmol), solvent (6 mL), 333 K, 2 h, under air (1 atm). Yield was determined by GC
analysis. Yield={[2a (mol)+3a (mol)ꢀ2]/(1a used (mol)}ꢀ100. Selectivity to 2a [%]={(2a (mol)/[2a
(mol)+3a (mol)ꢀ2]}ꢀ100. Selectivity to 3a [%]={3a (mol)ꢀ2/[2a (mol)+3a (mol)ꢀ2]}ꢀ100.
[b] 60% aqueous H2O2 (1 mmol). [c] UHP (1 mmol). [d] 305 K, 24 h. [e] Under Ar (1 atm). [f] H2O
(1 mmol). [g] CH3CN/tBuOH (3/3 mL). [h] THA=[(n-C6H13)4N]+.
corresponding disiloxane (3a) by condensation of 2a was
hardly observed. 1,2-Dichloroethane, DMSO, acetone, and
benzonitrile as solvents gave 2a in 57, 50, 45, and 44% yield,
respectively (Table 1, entries 2–5), while DMF was found to
be a poor solvent (Table 1, entry 6). The reaction rate for the
oxidation of 1a with 60% aqueous H2O2 was slightly higher
than that with 30% aqueous H2O2 (Table 1, entries 1 and 7).
On the other hand, UHP was not an effective oxidant in the
present system (Table 1, entry 8). The reaction proceeded
efficiently even at 305 K without significant changes in yield
and selectivity (Table 1, entries 1 and 9). The yield, selectivity,
and reaction rate for oxidation under argon were almost the
same as those for oxidation under air (Table 1, entries 1 and
10). With water instead of H2O2, oxidation did not proceed at
all (Table 1, entry 11). All these results suggest that partic-
ipation of molecular oxygen in air and water can be excluded.
Oxidation did not proceed in the absence of I (Table 1,
entry 12). The catalyst precursor Na2WO4·2H2O and the TBA
salts of fully occupied and other lacunary polyoxotungstates
silanols 2a–2d in high yields (Table 2, entries 1–4). Not only
aryl silanes 1a–1e but also alkyl silanes 1 f–1j were efficiently
oxidized to the corresponding silanols (Table 2, entries 1–10).
Sterically exposed silanes were smoothly oxidized to silanols
in high yields, and the selectivity to silanols was not sensitive
to the type of substituents. Compound I could be recovered
quantitatively by the addition of an excess of diethyl ether
(precipitation method) to the reaction solution. Recovered I
could be reused at least three times without loss of catalytic
activity and selectivity (see Supporting Information). Reac-
tions of chloro-, alkenyl-, and alkynyl-containing silanes 1k–
1m also proceeded selectively to form the corresponding
silanols 2k–2m (Table 2, entries 11–13). Notably, triethoxysi-
lane (1n) and tri-n-butoxysilane (1o) were also selectively
oxidized to the corresponding silanols 2n and 2o without
significant formation of the hydrative by-products (Table 2,
entries 14 and 15).[26,27] To the best of our knowledge, catalytic
conversion of alkoxy silanes to alkoxy silanols has not been
reported to date.[28,29]
Angew. Chem. Int. Ed. 2009, 48, 8900 –8904
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim