K.-i. Shimizu et al.
the catalyst, 100 mmol of 1a was quantitatively converted to
2a, corresponding to a TON of >99000 (Table 2, entry 3).
The TOF (based on total Pd content) for the oxidation of
1a (20000 hÀ1) is more than two orders of magnitude higher
than those
of
[{RuCl
(pcymene)}2]
(285 hÀ1),[5b]
A
(C8H12)}2] (80 hÀ1),[5a] AuNPore (100 hÀ1),[3a] AuHAP
(60 hÀ1),[2a] AgHAP (<1 hÀ1),[2b] and Pt nanoclusters
(125 hÀ1),[5d] and even slightly higher than that of the state-
of-the-art AuCNT catalyst (18000 hÀ1).[3b] Oxidation of deac-
tivated triphenylsilane at 358C (Table 2, entry 11) afforded
triphenylsilanol in excellent yield, corresponding to a higher
TOF (790 hÀ1) than those of AuCNT (327 hÀ1),[3b] AuNPore
(20 hÀ1),[3a] AgHAP (396 hÀ1),[2b] Pt nanoclusters (37 hÀ1),[5d]
and [{RuCl2-(p-cymene)}2] (98 hÀ1).[5b] Even in the case of
water as the solvent, triphenylsilane (Table 2, entry 10) and
alkyl silanes (entries 2, 4, 6, 8) were smoothly oxidized to
the corresponding silanols in high yield. Notably, the reac-
tion of sterically hindered silanes (triisopropylsilane and
tert-butyldimethylsilane) proceeded smoothly and selectively
in water.
Scheme 1. A proposed mechanism of Pd/C-catalyzed hydration of silanes.
moted by Os,[11–13] it is highly probable that Os plays an im-
portant role in the dissociative activation of H2O as a critical
step of the present reaction. The result in Table S3 (in the
Supporting Information) supports this hypothesis. For the
Pd/C-500H catalyst containing a negligible amount of Os,
the kH/kD value (4.9) for the oxidation of 1a with H2O and
D2O was larger than that for Pd/C-100Hox (2.2). Recent
theoretical work showed that dissociative adsorption of
water on the clean Pd surfaces was an energetically unfavor-
able process at low temperature.[11] However, the reaction
To propose a possible reaction pathway, kinetic experi-
ments were carried out for the oxidation of 1a with Pd/C-
100Hox under the conditions in Table 1. As shown in Fig-
ure S6 in the Supporting Information, the reaction rate in
the absence of O2 (under flowing N2) was nearly the same
as those in the presence of O2, and a nearly zero-order de-
pendence with respect to O2 was observed. When the cata-
lyst was added to the reaction mixture the formation of bub-
bles was observed. A separate catalytic test combined with
GC analysis of the gas-phase products confirmed a stoichio-
metric amount of H2 formation with respect to silanol (see
the Experimental Section in the Supporting Information).
These results indicate that O2 is not necessary in the remov-
al of hydride from Pd-H species, and the regeneration of the
Pd site is not a rate-limiting step. The rate increased with
the H2O concentration up to 1.4m and leveled off at higher
concentrations. The reaction order with respect to H2O was
0.76. Whereas, the rate dependence on the Et3SiH (1a) con-
centration showed a nearly zero-order dependence. Com-
bined with the kinetic isotope effect (kH/kD) of 2.2 observed
for the oxidation of 1a with H2O and D2O at 358C
(Table S3 in the Supporting Information), these results indi-
barrier for the water dissociation on the Pd
contaminated with Os (0.34 eV) was significantly lower than
that on the clean Pd(111) surface (0.92 eV) owing to the
ACHTUNGTREN(NUNG 111) surface
AHCTUNGTRENNUNG
bonding interaction between Os and a hydrogen atom in
H2O at the transition state (OsdÀ···Hd+···OH). This model
was consistent with previous results of the surface science
experiments for water dissociation on clean and oxygen pre-
covered Pd surfaces, in which OH groups were detected
only on the latter surface.[13] In the present reaction, the Os
site acts as a Brønsted base to abstract a proton from H2O,
resulting in the formation of Hd+ on the Os site and HOdÀ
on the Pd site. In the final step of the cycle, Pd-HdÀ and
OsdÀ···Hd+ species should react to produce H2, resulting in
the regeneration of the catalyst surface. The Os-enhanced
reactivity of precious-metal surfaces has been recently con-
firmed by surface science experiments. For example, Shavor-
skiy et al. reported that a small amount of oxygen induced
dissociation of water on the surface of five metals (Ru, Rh,
Pd, Ir, and Pt).[12a] Recent progress in surface science estab-
À
cate that the O H bond cleavage of H2O is relatively slow,
whereas the activation of Et3SiH is very fast. The relatively
low kH/kD value (2.2) rules out a consecutive pathway via a
free OHÀ intermediate formed by water dissociation. This
suggests that water dissociation and a nucleophilic attack of
OHdÀ species occurs simultaneously through a concerted
mechanism. Considering the literature on the metal-cata-
lyzed hydrolytic oxidation of silanes, a mechanism that is
consistent with the above results is given in Scheme 1. The
lished that an oxygen atom on AuACTHNUGTRNENUG(111) or IrACHTUGNTREN(NUGN 111) surfaces
acts as a basic co-catalyst that promotes various organic re-
actions at low temperatures.[9] Cooperation of metal NPs as
redox sites and the surface oxygen as a basic co-catalyst
may open new possibilities for the tailoring of metal NP cat-
alysts for green organic reactions.
In conclusion, we have demonstrated a surface-science
driven strategy of catalyst design for green organic synthesis.
Based on the fact that oxygen-adsorbed Pd surfaces show
higher reactivity for water dissociation than clean Pd surfa-
ces, we have found that carbon-supported Pd NPs with sur-
face oxygen atoms show higher activity for hydrolytic oxida-
tion of silanes than previously reported catalysts. The syn-
À
reaction begins with the insertion of the Si H bond into the
surface Pd0 species to generate a silyl–metal hydride inter-
mediate. The silyl–metal hydride intermediate is subse-
quently attacked by a nucleophile derived from water to
generate the silanol. Knowing the well-established fact that
water dissociation on clean Pd surfaces is significantly pro-
2228
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 2226 – 2229