Table 2 RuHAP-catalysed oxidation of silanes in the presence of
a
2
H
2
O and O
b
b
Entry
Substrate
Time/h
Conv./%
Yield/%
1
2
3
4
5
PhMe
2
SiH
3
3
100
100
100
100
100
89
99
97
Scheme 1 Reaction conditions: dimethylphenylsilane (1 mmol), ethyl
acetate (5 mL), RuHAP (0.05 g, Ru: 0.05 mmol), H
Recycle 1
Recycle 2
Recycle 3
Recycle 4
18
2
O (10 mmol),
3
98
97
ꢀ
8
0 C, O
2
atmosphere, 2h.
3
3
98
89
c
6
t-BuMe
Ph MeSiH
Et SiH
(n-Bu)
(n-C
2
SiH
24
12
6
reaction intermediate. XAFS analysis revealed that no struc-
tural changes around the Ru
3
+
7
8
9
2
95
93
92
92
center were observed after
the oxidation.z In the oxidation of dimethylphenylsilane, the
3
3
SiH
13SiH
12
12
100
92
> 99
90
catalyst was filtered off after ca. 50% conversion at the reaction
ꢀ
1
0
6
H
13
)
temperature. The filtrate was further reacted at 80 C for 3 h
1
1
14
92
92
and no oxidation of dimethylphenylsilane occurred. Ru leach-
ing in the filtrate was not observed by ICP analysis, whose
detection limit is 0.04 ppm. It can be said that this silane oxida-
tion proceeds with heterogeneous Ru species. Furthermore,
the RuHAP catalyst could be reused four times without loss
1
1
2
3
12
9
93
90
d
98
Ph
2
SiH
2
100
d
91
1
4
24
91
of the high catalytic activity and selectivity (entries 2–5).
RuHAP-catalyzed oxidation of 1 with isotopic H O led to
a
18
Reaction conditions: silane (1 mmol), ethyl acetate (5 mL), RuHAP
ꢀ
2
b
atomsphere. De-
18
the selective formation of O-labeled silanol in a quantitative
(0.05 g, Ru: 0.05 mmol), H
termined by GC analysis using an internal standard technique.
2
O (5 mmol), 80 C, O
2
c
H
2
O
yield (Scheme 1). The oxygen atom incorporated into silanol is
not derived from molecular oxygen but from water. In addi-
tion, the molar ratio of O uptake to silanol 2 yield was 1:2.
ꢀ
d
3 mmol), 70 C. Isolated yield.
(
2
A plausible mechanism for this silane oxidation is proposed
as follows. Initially, ligand exchange between a silane and a
surface Cl moiety of RuHAP gives a silyl-metal intermediate,
which undergoes nucleophilic attack by water to produce the
corresponding silanol and a Ru–H species. Reaction of the
hydride species with molecular oxygen affords a Ru–OOH spe-
cies, followed by ligand exchange with the silane to regenerate
in Table 2. In our oxidation system, the corresponding silanols
were obtained without any condensation products. For exam-
ple, even the sterically exposed silane of triethylsilane was
exclusively oxidized to give triethylsilanol in 92% yield
(
entry 8). Generally, silanols containing small substituents
are easily converted to the disiloxane due to heat, acid, or base
instability, which is a crucial drawback in other reported syn-
the silyl-metal intermediate together with formation of O
O.x The inversion of the stereochemistry of silane in the oxi-
dation shows that the nucleophilic attack of water occurs from
2
and
1
,7
H
2
thetic procedures. The RuHAP catalyst was also applicable
to the oxidation of silanes possessing alkynyl and alkenyl
groups to form the corresponding silanols in high yields
2
,10
the backside of a Ru–silicon bond.
In summary, RuHAP was found to offer an efficient hetero-
geneous catalyst system for the oxidation of silanes employing
a combined oxidant of water and molecular oxygen. The
oxidation proceeded selectively with functional group toler-
ance. The spent RuHAP catalyst was recyclable with retention
of the high activity and selectivity. We are continuing to
design functionalized hydroxyapatite catalysts with the aim
of developing environmentally benign chemical processes.
(
entries 11 and 12). In the cases of diphenylsilane and 1,4-
bis(dimethylsilylbenzene), the silanediols were obtained in
almost quantitative yields (entries 13 and 14). Sterically bulky
silanes such as triphenylsilane and triisopropylsilane were
hardly oxidized under the present system. The RuHAP cataly-
tic system represents a highly suitable method for large-scale
operations; a 100 mmol scale oxidation of 1 was completed
within 6 h to provide 94% of 2. Growing interest has been
shown in the synthesis of optically active silanols as useful syn-
8
thetic intermediates for a variety of bioactive compounds.
Experimental
More significantly, the RuHAP-catalyzed oxidation of optic-
ally active silanes proceeded exclusively with inversion of the
silicon configuration; the oxidation of (+)-methylethylphenyl-
silane in the presence of RuHAP, followed by reduction with
LiAlH
Similar stereospecific oxidation has also been achieved with
the [RuCl (p-cymene)] complex, but the selectivity with
RuHAP was higher than that reported for the homogeneous
Ru complex. Such a prominent performance of RuHAP
might be due to the structurally robust monomeric active site
on a solid surface, which provides strict steric control of the
Procedure for the RuHAP-catalyzed silane oxidation
Into a reaction vessel equipped with a reflux condenser were
successively placed RuHAP (0.2 g, Ru : 0.2 mol %), ethyl
3
+
4
, afforded (ꢂ)-methylethylphenylsilane in 97% ee.y
acetate (50 mL), dimethylphenylsilane (100 mmol), and water
ꢀ
(
200 mmol). The reaction mixture was stirred at 80 C under
2
2
2
atmospheric O pressure. After 6 h, RuHAP was separated
2
by filtration and the organic layer was distilled to afford pure
dimethylphenylsilanol (94% isolated yield). The product was
identified by mass spectrometry and its H NMR spectrum.
1
z The Ru K-edge XANES spectrum of the recovered RuHAP catalyst
was similar to that of the fresh one and the EXAFS analysis showed no
Ru–Ru bond. These results support notion that the Ru species exists as
y Reaction of (+)-methylethylphenylsilane: the optically active (+)-
methylethylphenylsilane (ee 98%) was obtained from racemic methyl-
ethylphenylsilane by preparative HPLC (Daicel Chiralcel OJ-H ꢁ 2,
3+
a Ru monomer even in the used RuHAP. No chlorine was confirmed
by XPS analysis of the used RuHAP, whereas the atomic ratio of Ru
to Cl was 1:1 in the fresh RuHAP. Hence, we think that the Cl ligand
ꢂ1
ꢀ
n-hexane, 1.0 mL min , 0 C, 254 nm, t
R
s ¼ 13.1 min). Into a reaction
ꢂ
vessel equipped with a reflux condenser were successively placed
RuHAP (0.01 g), 1,4-dioxane (4 mL), 0.1 mol L of (+)-methylethyl-
ꢂ1
ꢂ
of RuHAP was exchanged with OH during the silane oxidation.
phenylsilane in n-hexane solution (1 mL), and water (1 mmol). After
ꢀ
x Hydrolytic oxidation of silanes using the homogeneous [RuCl
2
(p-
the reaction mixture was stirred at 80 C under O
2
atmosphere for 24
h, RuHAP was separated by filtration. To the filtrate was added
cymene)] catalyst is moderately promoted under an oxygen atmo-
2
2
sphere. The above oxidation proceeds via the oxidative addition of
silane to ruthenium, while the catalytic pathway with RuHAP is
initiated by the ligand exchange with silane. We think that this unique
activation of the Si–H bond using RuHAP is attributed to robust
LiAlH
ꢀ
4
(0.3 mmol) solution in 1,4-dioxane (2 mL), followed by stirring
at 90 C for 24 h to afford (ꢂ)-methylethylphenylsilane (ee 97%, deter-
mined by HPLC, t
silanols using LiAlH
R
s ¼ 14.2 min). It is known that the reduction of
4
9
3+
proceeds with a retention of stereochemistry.
monomeric Ru species generated on solid surfaces.
New J. Chem., 2002, 26, 1536–1538
1537
Mori, Kohsuke
