Supported silver-nanoparticle-catalyzed highly efficient aqueous oxidation of phenylsilanes to silanols

Article abstract of DOI:10.1002/anie.200802761

Bon Apatite! Hydroxyapatite-supported silver nanoparticles act as a highly efficient heterogeneous catalyst for the oxidation of diverse phenylsilanes into silanols in water (see picture; C orange, H red, O blue, R purple, Si green), while suppressing significant condensation to the disiloxanes. The solid silver catalyst is readily reusable without any loss of activity or selectivity. (Figure Presented)

Full text of DOI:10.1002/anie.200802761

Communications
DOI: 10.1002/anie.200802761
Heterogeneous Catalysis
Supported Silver-Nanoparticle-Catalyzed Highly Efficient Aqueous
Oxidation of Phenylsilanes to Silanols**
Takato Mitsudome, Shusuke Arita, Haruhiko Mori, Tomoo Mizugaki, Koichiro Jitsukawa, and
Kiyotomi Kaneda*
Metal nanoparticles have attracted a great deal of attention in
various areas of materials science, such as catalysis, optoelec-
tronics, and drug delivery owing to their unique physical and
of organic solvents. However, reported synthetic methods, in
the absence of organic solvents, are unfortunately precluded
by the main production of disiloxanes.
Herein, we report that Ag nanoparticles supported on
hydroxyapatite (Ag-HAp) exhibit high catalytic activities for
the selective oxidation of silanes into silanols using water as
an oxidant without the use of organic solvents. To our
knowledge, this is the first catalytic system that uses water as a
solvent for the selective oxidation of silanes into the
corresponding silanols.
[
9]
[
1]
chemical properties. Recently, some metal nanoparticles
have been revealed to have unprecedented catalytic perform-
ances that far exceed those of conventional homogeneous
[
2]
catalysts. Of all metal nanoparticles, silver-nanoparticle
catalysts are generally considered to have low activities for
many liquid-phase organic reactions and, consequently,
investigations into their catalytic properties have been
[3]
scarce. We recently discovered that hydrotalcite-supported
Ag nanoparticles exhibit high catalytic activity for the
oxidant-free dehydrogenation of alcohols into carbonyl com-
pounds under liquid-phase conditions, where the combination
of Ag with hydrotalcite plays a crucial role in the dehydrogen-
Hydroxyapatites (HAps), a phosphate mineral species
found in teeth and bones, are of considerable interest owing to
[
11]
[12]
their potential usefulness as biomaterials,
and ion-exchangers.
adsorbents,
[
13]
We have reported the benefit of
utilizing HAp as a support for high-performance heteroge-
[4]
[14]
ation. Further applications of the unique catalytic behavior
obtained by combining Ag nanoparticles with inorganic
supports have been explored.
neous catalysts in various organic syntheses. Ag-HAp was
synthesized as follows: Ca-HAp (Ca (PO ) (OH), Ca/P =
5
4 3
1.68, 2.0 g), was soaked in an aqueous solution of AgNO
3
À3
Silanols are useful as building blocks for silicon-based
(6.7 ” 10 m, 150 mL) and stirred at room temperature for 6 h.
The resultant solid was collected by filtration, washed, and
dried at room temperature in vacuo. Reduction with an
aqueous solution of potassium borohydride under an argon
atmosphere gave HAp-supported Ag nanoparticles (Ag-
HAp).
[
5]
polymeric materials and nucleophilic coupling partners in
[6]
organic synthesis. However, they are generally synthesized
using toxic reagents and their synthesis generates vast
[
7]
amounts of environmentally damaging waste. Some prom-
ising alternative catalytic transformations of silanes into
silanols, using H O as a green oxidant, have been reported
The XRD peak positions of Ag-HAp were similar to those
of the parent HAp. Elemental analysis showed that the Ca/P
ratio of Ag-HAp remained at 1.68 and the Ag loading on Ag-
2
[
8–10]
in the presence of organic solvents.
By using highly
efficient and reusable heterogeneous catalysts, it should be
possible to find more environmentally benign and practical
processes for synthesizing silanols, that do not require the use
3
HAp was 3.3 wt%. Ag k -weighted K-edge extended X-ray
absorption fine structure (EXAFS) study of Ag-HAp
revealed that a peak around 2.8 Š of the Fourier trans-
formation was assignable to the Ag–Ag shell. UV/Vis analysis
gave a plasmon peak at 414 nm, indicating that the diameter
[*] Dr. T. Mitsudome, S. Arita, H. Mori, Dr. T. Mizugaki,
Prof. Dr. K. Jitsukawa, Prof. Dr. K. Kaneda
Department of Material Engineering Science, Graduate School of
Engineering Science, Osaka University
[15]
of the Ag nanoparticles is 6.0–8.0 nm. Transmission elec-
tron microscopy showed that Ag nanoparticles with a mean
diameter of 7.6 nm, with a narrow size distribution with a
standard deviation of 1.8 nm, were formed on the surface of
1-3, Machikaneyama, Toyonaka, Osaka 560-8531 (Japan)
Fax: (+81)6-6850-6260
E-mail: kaneda@cheng.es.osaka-u.ac.jp
Homepage: http://www.cheng.es.osaka-u.ac.jp/kanedalabo/
index_eng.html
[
16]
the HAp substrate.
When a triphasic mixture of dimethylphenylsilane (1),
water, and Ag-HAp was heated at 808C under an argon
atmosphere for 15 min, oxidation of 1 occurred quantitatively
to afford dimethylphenylsilanol (2) with coproduction of the
Prof. Dr. K. Kaneda
Research Center for Solar Energy Chemistry, Osaka University
[
**] The work was supported by a Grant-in-Aid for Scientific Research
from the Ministry of Education, Culture, Sports, Science, and
Technology of Japan and by a Grant-in-Aid for Scientific Research on
Priority Areas (No. 18065016, “Chemistry of Concerto Catalysis”)
from the Ministry of Education, Culture, Sports, Science and
Technology of Japan. Some of the work was carried out at the
Research Center for Ultrahigh Voltage Electron Microscopy, Osaka
University.
equivalent molar amount of H (Table 1, entry 1). The use of
2
Ag salts, such as AgNO , Ag O, AgPF , and AgOTf, instead
3
2
6
of Ag-HAp resulted in poor yields of 2. A blank experiment
and Ag-free HAp did not provide oxidized products under
[17]
the above conditions (Table 1, Entries 13–16).
Other
immobilized forms of Ag, such as Ag-Al O , Ag-TiO , Ag-
2
3
2
SiO , and Ag-hydrotalcite (Ag-HT), gave moderate yields of
2 (Table 1, Entries 9–12). The combination of Ag with HAp is
Supporting Information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200802761.
2
7938
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Angewandte
Chemie
[
a]
[a]
Table 1: Oxidation of dimethylphenylsilane using Ag-catalysts.
Table 2: Oxidation of various silanes catalyzed by Ag-HAp in water.
[
b]
Entry
Silane
Silanol
t
Conv. Silanol:Disiloxane
[
b]
[min] [%]
1
2
3
4
5
15
180
30
99
>99:1
>99:1
>99:1
>99:1
>99:1
[
b]
[b]
À2 [c]
Entry Catalyst t [min] Conv. [%] Selectivity (2:3) Vm [mLm
]
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
Ag-HAp
reuse1
reuse2
reuse3
reuse4
Ag-HAp
15
15
15
15
15
90
99
99
99
99
99
97
98
99
79
71
29
25
24
27
23
30
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
82:18
0.311
–
–
–
–
–
–
–
0.271
0.246
0.157
–
–
–
–
–
99
97
97
97
[
[
[
d]
e]
f]
Ag-HAp 600
Ag-HAp 15
Ag-Al O3 15
30
2
1
1
1
1
1
1
1
Ag-TiO2
Ag-SiO2
Ag-TC
15
15
15
15
15
15
15
60
AgNO3
Ag O
6
60
5
97
99
>99:1
>99:1
2
AgPF6
AgOTf
88:12
[a] Conditions
unless
otherwise
stated: dimethylphenylsilane
(
[
1.0 mmol), H O (2.0 mL), catalyst (Ag: 0.03 mmol), 808C, Ar flow.
2
b] Determined by GC using internal standard technique. [c] H O
[c]
7
2
monolayer adsorption capacity. [d] Room Temperature. [e] Ethyl acetate
5.0 mL) was added into reaction mixture, À408C. [f] Air atmosphere.
(
8
9
15
99
96
>99:1
>99:1
the most efficient for achieving a high catalytic activity for the
selective oxidation of 1 into 2. The Ag-HAp catalyst was also
found to exhibit extremely high activity in the oxidation of 1
at room temperature (Table 1, entry 6) and moreover, 1 was
converted quantitatively even at À408C (Table 1, entry 7).
Interestingly, Ag-HAp also catalyzed the quantitative for-
mation of 2 and the concerted formation of an equimolar
240
10
(nBu) SiH(
nBu) SiOH1440 trace
tBuMe SiOH1440 trace
–
–
3
3
11
tBuMe SiH
2
2
[
a] Conditions unless otherwise stated: Silane (1.0 mmol), H O (2.0 mL), Ag-
HAp (Ag: 0.03 mmol), 808C, Ar flow. [b] Determined by GC using internal
2
amount of H in an air atmosphere (Table 1, entry 8).
2
To check whether the oxidation takes place on Ag
nanoparticles immobilized by HAp, the reaction mixture
was hot-filtered at 40% conversion of 1. Further stirring of
the filtrate under the above reaction conditions did not yield
any additional product. An inductively coupled plasma
method (detection limit: 0.007 ppm) was used to confirm
that no Ag leached into the filtrate. These results demonstrate
that the present oxidation occurred exclusively on the surface
of Ag-HAp. Solid Ag-HAp could be easily retrieved from the
reaction mixture by simple filtration and was found to be
reusable without any reduction in its activity or selectivity;
the yield of 2 from the oxidation of 1 could be maintained at
over 99% during four recycles of the catalyst (Table 1,
Entries 2–5).
standard technique. [c] Silanes (1.0 mmol), ethyl acetate (5.0 mL), H O
(5.0 mmol), Ag-HAp (Ag: 0.03 mmol), 808C, Ar flow.
2
silanes was oxidized in high yields with over 99% selectivity
for the silanols (Table 2, Entries 1–9). Methylphenylvinylsi-
lane was selectively oxidized to the corresponding unsatu-
rated silanol while retaining the olefinic group (Table 2,
entry 5). Triphenylsilane and 1,4-bis(dimethylsilylbenzene)
are known to be less reactive than the majority of the
investigated phenylsilanes, as a result of their greater steric
[
10]
bulk. However, Ag-HAp smoothly catalyzed the selective
oxidations of these silanes in high yields (Table 2, Entries 7
and 9).
1
00 mmol of 1 was also smoothly converted by Ag-HAp,
to give 2 at a yield of 98% after 9 h, and a turnover number
TON) of up to 1600 was achieved. This value is superior to
those obtained using other catalyst systems, for example,
The interaction between the Ag-HAp surface and silanes
was examined using IR spectroscopy. Following the adsorp-
À1
(
tion of 1 onto Ag-HAp, a band at 1257 cm , assigned to the
À1
aromatic ring vibration, was shifted to 1263 and 1253 cm .
[8a]
[8b]
[
{RuCl (p-cymene)} ] (TON = 455), [IrCl(C H )] (85),
oxorhenium (880), Ru-HAp (TON = 470), and [{RuCl2-
p-cymene)} ]/C (9).
The attempted adsorption of tributylsilane (4) onto Ag-HAp
2
2
9]
8
[10]
12
2
[
gave rise to no corresponding peaks in the IR spectrum,
[
8d]
[16]
(
indicating no adsorption had taken place.
This finding
2
Ag-HAp exhibited high specific activities for the oxida-
tion of various aromatic silanes (Table 2). A range of phenyl-
revealed the strong adsorption of 1 through an aromatic
group interaction onto the surface of Ag-HAp. Measurement
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www.angewandte.org 7939

Communications
of water vapor adsorption onto HAp exhibited the highest
chap. 29; d) E. W. Colvin, Silicon Reagents in Organic Synthesis,
Academic Press, London, 1988, p. 7.
hydrophilicity among tested supports for the oxidations. The
yield of 2 increased with increasing the water adsorption
capacity (V ) of Ag-HAp, Ag-Al O , Ag-TiO , and Ag-SiO
[
6] a) K. Hirabayashi, Y. Nishihara, A. Mori, T. Hiyama, Tetrahe-
dron Lett. 1998, 39, 7893; b) K. Hirabayashi, J. Kawashima, Y.
Nishihara, A. Mori, T. Hiyama, Org. Lett. 1999, 1, 299; c) S. E.
Denmark, D. Wehrli, Org. Lett. 2000, 2, 565; d) S. Chang, S. H.
Yang, P. H. Lee, Tetrahedron Lett. 2001, 42, 4883; e) K.
Hirabayashi, A. Mori, J. Kawashima, M. Suguro, Y. Nishihara,
T. Hiyama, J. Org. Chem. 2000, 65, 5342.
m
2
3
2
2
(
Table 1, Entries 1, 9–11). Moreover, microwave dielectric
[
16]
studies on the dynamics of water
demonstrated the
relaxation peak of water in Ag-HAp appeared at a lower
frequency than that in HAp, suggesting that H O interacts
2
[
7] a) L. H. Sommer, L. A. Ulland, G. A. Parker, J. Am. Chem. Soc.
1972, 94, 3469; b) W. Adam, R. Mello, R. Curci, Angew. Chem.
strongly with the surface of the Ag nanoparticles to generate a
À
[18]
nucleophilic OH species.
1990, 102, 916; Angew. Chem. Int. Ed. Engl. 1990, 29, 890;
The above results indicate that the cooperative action
between Ag nanoparticles and HAp plays an important role
in the specific activity for phenylsilanes in this aqueous
catalytic system. A possible mechanism is as follows: A
c) E. G. Rochow, W. F. Gilliam, J. Am. Chem. Soc. 1941, 63, 798;
d) R. O. Sauer, J. Am. Chem. Soc. 1944, 66, 1707; e) S. M.
Sieburth, W. Mu, J. Org. Chem. 1993, 58, 7584; f) K. Hirabayashi,
A. Mori, T. Hiyama, Tetrahedron Lett. 1997, 38, 461.
8] a) M. Lee, S. Ko, S. Chang, J. Am. Chem. Soc. 2000, 122, 12011;
b) Y. Lee, D. Seomoon, S. Kim, H. Han, S. Chang, P. H. Lee, J.
Org. Chem. 2004, 69, 1741; c) L. H. Sommer, J. E. Lyons, J. Am.
Chem. Soc. 1969, 91, 7061; d) E. Choi, C. Lee, Y. Na, S. Chang,
Org. Lett. 2002, 4, 2369.
9] E. A. Ison, R. A Corbin, M. M. A. Omar, J. Am. Chem. Soc.
2005, 127, 11938.
[10] K. Mori, M. Tano, T. Mizugaki, K. Ebitani, K. Kaneda, New J.
Chem. 2002, 26, 1536.
[
[
phenylsilane dispersed in H O is adsorbed onto the surface of
2
Ag nanoparticles by a strong aryl-ring–Ag interaction. A
À
nucleophilic OH species from H O, activated on Ag sur-
2
[
19]
face, attacks the adsorbed phenylsilane to form the phenyl-
silanol through a pentacoordinate silicon intermediate.
[20]
This hydrophilic environment for the Ag-HAp surface
À
increases the concentration of nucleophiles (OH or H O)
2
and promotes formation of the silanol by suppressing
[
21]
[11] L. L. Guꢀhennec, P. Layrolle, G. Daculsi, Eur. Cells Mater. 2004,
condensation to the disiloxane.
8, 1.
In conclusion, Ag nanoparticles supported on HAp
showed a high catalytic activity for the selective oxidation
of various phenylsilanes into phenylsilanols in water, as a
result of the cooperative behavior between hydrophilic HAp
[
12] a) T. Kawasaki, J. Chromatogr. 1991, 544, 147; b) S. V. Dorozh-
kin, M. Epple, Angew. Chem. 2002, 114, 3260; Angew. Chem. Int.
Ed. 2002, 41, 3130.
[13] J. C. Elliott, Structure and Chemistry of the Apatites and Other
Calcium Orthophosphates, Elsevier, Amsterdam, 1994.
14] a) K. Yamaguchi, K. Mori, T. Mizugaki, K. Ebitani, K. Kaneda,
J. Am. Chem. Soc. 2000, 122, 7144; b) K. Mori, K. Yamaguchi, T.
Hara, T. Mizugaki, K. Ebitani, K. Kaneda, J. Am. Chem. Soc.
and Ag nanoparticles that interact strongly with both H O
2
[
and phenylsilanes. Moreover, Ag-HAp was reusable without
any appreciable loss in activity or selectivity.
2
002, 124, 11572; c) K. Mori, T. Mizugaki, K. Ebitani, K. Kaneda,
Received: June 11, 2008
Published online: September 9, 2008
J. Am. Chem. Soc. 2003, 125, 11460; d) K. Mori, T. Mizugaki, K.
Ebitani, K. Kaneda, J. Am. Chem. Soc. 2004, 126, 10657; e) K.
Mori, M. Oshiba, T. Hara, T. Mizugaki, K. Ebitani, K. Kaneda,
New J. Chem. 2006, 30, 44.
Keywords: heterogeneous catalysis · nanoparticles · silanes ·
.
silver · water chemistry
[15] J. P. Wilcoxon, J. E. Martin, P. Provencio, J. Chem. Phys. 2001,
15, 998.
16] See Supporting Information for details.
1
[
[
[
8a]
[8b]
17] Ru-HAp, [{RuCl (p-cymene)} ] and [{IrCl(C H )} ] , which
2
2
8
12
2
[
1] K. Watanabe, D. Menzel, N. Nilius, H.-J. Freund, Chem. Rev.
006, 106, 4301.
2] a) M. Moreno-Maæas, R. Pleixats, Acc. Chem. Res. 2003, 36, 638;
b) A. Roucoux, J. Schulz, H. Patin, Chem. Rev. 2002, 102, 3757;
c) H. Bönnemann, R. M. Richard, Eur. J. Inorg. Chem. 2001,
have been reported to show high catalytic performances for
selective oxidation of silanes into silanols in organic solvents,
were used under solvent-free conditions. The conversions of 1/
selectivities of 2 obtained using the above three catalysts were
20/99, 95/21 and 99/3, respectively.
2
[
2
455.
3] a) Y. Chen, C. Wang, H. Liu, J. Qiu, X. Bao, Chem. Commun.
005, 5298; b) U. Halbes-Letinois, J.-M. Weibel, P. Pale, Chem.
Soc. Rev. 2007, 36, 759; c) J. Sun, S. A. Kozmin, Angew. Chem.
006, 118, 5113; Angew. Chem. Int. Ed. 2006, 45, 4991.
[18] Membrane-bound water acts as a nucleophile for hydration of
lipids. See A. Ghosh, M. Smits, J. Bredenbeck, M. Bonn, J. Am.
Chem. Soc. 2007, 129, 9608.
[
2
À
[19] There are three possibilities from where the nucleophilic OH
2
species might come: from the solution, the support, or the Ag
nanoparticles. The results of the microwave dielectric studies on
[
4] T. Mitsudome, Y. Mikami, H. Funai, T. Mizugaki, K. Jitsukawa,
À
K. Kaneda, Angew. Chem. 2008, 120, 144; Angew. Chem. Int. Ed.
the dynamics of water, suggest that the OH species comes from
2
008, 47, 138.
5] a) R. Corriu, C. Guerin, J. Moreau, Top. Stereochem. 1984, 15,
3; b) P. D. Lickiss, Adv. Inorg. Chem. 1995, 42, 147; c) I. Ojima,
Z. Li, J. Zhu in The Chemistry of Organic Silicon Compounds
Eds.: S. Rappoport, Y. Apeloig), Wiley, New York, 1998,
the surface of Ag nanoparticles in this instance.
[20] C. Eaborn, L. D. Jenkins, J. Organomet. Chem. 1974, 69, 185.
[
4
[21] The biphasic nature of the organosilane/H O system reduced the
2
effective concentration of water in the organosilane phase,
resulting in condensation of silanols into disiloxanes.
[
9]
(
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