Polysilane-supported Pd and Pt nanoparticles as efficient catalysts for organic synthesis

Article abstract of DOI:10.1039/b610241g

Polysilane-supported Pd and Pt catalysts have been prepared for the first time, and used successfully in hydrogenation, Suzuki and Sonogashira reactions, and hydrosilylation respectively: the reactions proceeded in high yields, and the catalysts could be recovered almost quantitatively by simple filtration and reused. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b610241g

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COMMUNICATION
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Polysilane-supported Pd and Pt nanoparticles as efficient catalysts for
organic synthesis{
Hidekazu Oyamada, Ryo Akiyama, Hiroyuki Hagio, Takeshi Naito and Shu¯ Kobayashi*
Received (in Cambridge, UK) 17th July 2006, Accepted 28th July 2006
First published as an Advance Article on the web 5th September 2006
DOI: 10.1039/b610241g
Table 1 Preparation of PSi–Pd 2a–e
Polysilane-supported Pd and Pt catalysts have been prepared
for the first time, and used successfully in hydrogenation,
Suzuki and Sonogashira reactions, and hydrosilylation respec-
tively: the reactions proceeded in high yields, and the catalysts
could be recovered almost quantitatively by simple filtration
and reused.
Transition metal complexes have played important roles as
catalysts for various transformations in organic synthesis.1 While
homogeneous transition metal catalysts are often used due to their
high activity and availability, they are expensive, air-sensitive, and
in many cases cannot be recovered and reused. To address these
issues, the immobilization of homogeneous catalysts onto inor-
ganic or organic supports has been widely investigated.2 However,
tedious procedures are often needed for the preparation of such
heterogeneous catalysts, and the lower activity/selectivity and the
leaching of catalysts from the supports are serious problems. In the
course of our investigations to develop efficient heterogeneous
catalysts, we focused on polysilane as a support of heterogeneous
catalysts. Polysilane has been widely studied due to its interesting
electronic properties, such as high hole mobility, photoconductiv-
ity, and nonlinear optical properties caused by s-conjugation of the
silicon backbone.3–5 In addition, polysilane is used as a starting
material for ceramics, and synthetic methods for mass production
are established.6 However, to the best of our knowledge, there are
no reports on the use of polysilane as a support of heterogeneous
catalysts.7,8 In this paper, we describe the first example of highly
active, recoverable and reusable polysilane-supported Pd and Pt
catalysts, their preparation and application to hydrogenation,
Suzuki and Sonogashira reactions, and hydrosilylation.
Pd
Entry Source
Temp/uC, PSi– Loadinga / Yield of
Time/h
Reductant
Pd mmol g21 Pd (%)
1
2
3
4
5
6
a
Pd(PPh3)4 none
Pd(OAc)2 none
Pd(OAc)2 NaBH4 (5 eq.) 0, 1; rt, 1 2c 0.121
Pd(NO3)2 NaBH4 (5 eq.) rt, 4
PdCl2 H2 (1 atm)
Pd(NO3)2 H2 (1 atm)
rt, 2
2a 0.131
93
96
90
83
79
94
0, 1; rt, 1 2b 0.134
2d 0.0947
2e 0.0934
rt, 4
rt, 4
2f
0.106
Determined by ICP analysis.
H2, PSi–Pd 2c–e with Pd loadings of 0.0934–0.121 mmol g21 were
prepared. Interestingly, Pd(PPh3)4 and Pd(OAc)2 were immobi-
lized without any reductants to form PSi–Pd 2a and 2b with higher
Pd loadings.
We then evaluated PSi–Pd 2a–e in the hydrogenation of ethyl
cinnamate using 0.5 mol% Pd (Fig. 1). In the case of using 2a, 2b,
or 2c, the reaction was completed within 60 min, while the reaction
using 2d or 2e was slower. PSi–Pd 2f showed much less activity.{
Recently, we have developed new methods for immobilizing
metal catalysts onto polymers, microencapsulation,9 and polymer
incarcerated methods,10 which are based on the physical envelop-
ment by the polymers, and on the electronic interaction between
the p electrons of the benzene rings of polystyrene-based polymers
and the vacant orbitals of metals. We first examined several
reaction conditions for the preparation of polysilane-supported
palladium catalysts (PSi–Pd) using the microencapsulation techni-
que (Table 1). Poly(methylphenylsilane) (1, Mw = 3.2 6 104) and
several Pd(II) salts were chosen as the polysilane and Pd(II) sources,
respectively. When the Pd(II) salts were reduced using NaBH4 or
Science of Process Laboratories, The HFRE Division, ERATO, Japan
Science and Technology Agency (JST), Matoba, Kawagoe, Saitama
350-1101, Japan, and Graduate School of Pharmaceutical Sciences, The
University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
E-mail: skobayas@mol.f.u-tokyo.ac.jp; Fax: +81-3-5684-0634
{ Electronic supplementary information (ESI) available: Experimental and
TEM images of 2a, 2b, and 2f. See DOI: 10.1039/b610241g
Fig. 1 Hydrogenation of ethyl cinnamate in the presence of 0.5 mol% of
PSi–Pd 2a–e. Reaction conditions: ethyl cinnamate (1 mmol), PSi–Pd
(0.5 mol%), hexane (3 mL), and H2 (1 atm) at room temperature. The
conversions were determined by GC analysis.
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Table 2 PSi–Pd 2b-Catalyzed hydrogenation
Entry
1
Substrate
Time/h
1
Product
Yield (%)a
Leaching (%)c
.99b
,0.08
2
2
97
,0.08
3
4
5
2
3
2
.99
.99
.99b
,0.08
,0.08
,0.08
6
1.5
6
.99b
2.28
7d
97
,0.08
8d
9
84(13)
,0.08
9d
2
96
,0.08
10d
11d
10
10
.99
0.40
5.41
93
12d
36
86(9)
0.13
a
b
c
Determined by 1H NMR analysis with reference to an internal standard (IS = durene). Determined by GC analysis. Determined by ICP
analysis. Ethanol was used as a solvent.
d
The substrate scope of the hydrogenation was then examined
using 0.5 mol% PSi–Pd 2b in hexane or ethanol under hydrogen
gas bubbling conditions at room temperature (Table 2). Alkenes
(entries 1, 2), a diene (entry 3), alkynes (entries 4, 9), an
a,b-unsaturated ketone (entry 5), and an a,b-unsaturated
carboxylic acid (entry 7) were reduced smoothly to afford the
desired products in high yields without leaching of Pd. Chalcone
also reacted smoothly to give the corresponding ketone and
alcohol in 84% and 13% yields, respectively (entry 8). While
nitrobenzene was reduced smoothly to afford aniline quantita-
tively, a small amount of Pd leaching was observed (entry 6).
Furthermore, deprotection of a benzyl ether and the benzyloxy-
carbonyl group of an amino group also proceeded smoothly with
small amounts of Pd leaching (entries 10 and 11). It should be
noted that the catalyst was recovered quantitatively by simple
filtration and reused at least five times without leaching of Pd and
loss of the catalytic activity (Table 3).
Next, we compared catalytic activity of PSi–Pd 2b with that of
Pd–C and leaching of Pd to the reaction mixture (Table 4). It is
noteworthy that the activity of PSi–Pd 2b was comparable to that
of 5% Pd–C, and that leaching of Pd using PSi–Pd was much less
than that using 5% Pd–C.
We also conducted Suzuki and Sonogashira reactions using
PSi–Pd 2b (Scheme 1). In both cases, the reactions proceeded
smoothly to afford the desired adducts in good yields, and no
leaching of Pd was detected.
We then prepared polysilane-supported platinum catalysts (PSi–
Pt). After several investigations of platinum sources and
4298 | Chem. Commun., 2006, 4297–4299
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Table 3 Reuse of PSi–Pd 2b
Entry
Run
Conversion (%)a 10 min, 30 min
1
2
3
4
5
a
1st
59, .99b
74, .99
77, .99
76, .99
85, .99
2nd
3rd
4th
5th
Scheme 3 Hydrosilylation using PSi–Pt 3. aA small amount of Pt leach-
ing (0.31%) was detected by ICP analysis. Recovery of PSi–Pt 3: ca. 90%.
Determined by GC analysis. b Pd leaching ,0.031% by ICP analysis.
Polysilane-supported Pt catalysts have been prepared by the same
method and showed high activity in hydrosilylation. Further
investigations to develop other reactions using PSi–Pd and PSi–Pt
as well as to prepare other immobilized metal catalysts based on
polysilane is now in progress.
Table 4 Comparison between PSi–Pd and Pd–Ca
Entry
Pd catalyst
Conversion (%)
Leaching of Pd (%)b
1
2
a
PSi–Pd 2b
5% Pd–C
.99
.99
0.023
0.17
Reaction conditions: ethyl cinnamate (10 mmol), Pd catalyst
(0.5 mol%), MeOH (30 mL) and H2 (1 atm), rt, 2 h. The conversions
were determined by GC analysis. Determined by ICP analysis.
Notes and references
b
1 Reviews: (a) G. W. Parshall and S. D. Ittel, Homogeneous Catalysis,
Wiley, New York, 1992; (b) Applied Homogeneous Catalysis with
Organometallic Compounds, ed. B. Cornils and W. A. Herrmann, Wiley-
VCH, Weinheim, 2002.
2 Reviews: (a) C. A. McNamara, M. J. Dixon and M. Bradley, Chem.
Rev., 2002, 102, 3275; (b) Polymeric Materials in Organic Synthesis and
Catalysis, ed. M. R. Buchmeiser, Wiley-VCH, Weinheim, Germany,
2003.
3 (a) R. D. Miller and J. Michl, Chem. Rev., 1989, 89, 1359; (b)
R. G. Kepler, J. M. Zeigler, L. A. Harrah and S. R. Kurtz, Phys. Rev.
B: Condens. Matter, 1987, 35, 2818; (c) M. Abkowitz, F. E. Knier,
H. J. Yuh, R. J. Weagley and M. Stolka, Solid State Commun., 1987, 62,
547; (d) R. West, J. Organomet. Chem., 1986, 300, 327; (e) H. Suzuki,
H. Meyer, J. Simmerer, J. Yang and D. Haarer, Adv. Mater., 1993, 5,
743.
4 (a) H. Suzuki, H. Meyer and S. Hoshino, J. Appl. Phys., 1995, 78, 2684;
(b) S. Hoshino and H. Suzuki, Appl. Phys. Lett., 1996, 69, 224.
5 S. Hayase, Prog. Polym. Sci., 2003, 28, 359.
Scheme 1 Suzuki and Sonogashira reactions using PSi–Pd 2b.
6 D. Seyferth, T. G. Wood, H. J. Tracy and J. L. Robinson, J. Am.
Ceram. Soc., 1992, 75, 1300.
7 Recently, Sanji et al. reported an application of poly[1,1-dimethyl-2,2-
dihexyldisilene-b-poly(methacrylic acid)] to immobilize metal nanopar-
ticles. In this method, however, polysilane is not used as a support for
immobilization of metals, but used as a reducing agent of metal ions.
T. Sanji, Y. Ogawa, Y. Nakatsuka, M. Tanaka and H. Sakurai, Chem.
Lett., 2003, 32, 980.
8 It was reported that transition metal atoms such as Ti, V, Cr, and Mo
could be trapped as bis(arene)metal complexes in fluid cyclic oligomers
of phenyl-containing polysilane. G. A. Ozin, M. P. Andrews and
R. West, Inorg. Chem., 1986, 25, 580.
9 Review: (a) S. Kobayashi and R. Akiayam, Chem. Commun., 2003, 449,
see also: (b) T. Ishida, R. Akiyama and S. Kobayashi, Adv. Synth.
Catal., 2003, 345, 576; (c) T. Ishida, R. Akiyama and S. Kobayashi,
Adv. Synth. Catal., 2005, 347, 1189.
Scheme 2 Preparation of PSi–Pt 3.
reductants, it was revealed that PSi–Pt 3 prepared from hydrogen
hexachloroplatinate(IV)
hexahydrate
and
triethoxysilane
(Scheme 2) was highly active for hydrosilylation. In the presence
of 3.6 mol% of PSi–Pt 3, 4,4-diphenyl-1-butene (4) reacted with
triethoxysilane smoothly to afford (4,4-diphenylbutyl)triethoxysi-
lane (5) in good yield (Scheme 3).
10 (a) R. Akiyama and S. Kobayashi, J. Am. Chem. Soc., 2003, 125, 3412;
(b) K. Okamoto, R. Akiyama and S. Kobayashi, J. Org. Chem., 2004,
69, 2871; (c) K. Okamoto, R. Akiyama and S. Kobayashi, Org. Lett.,
2004, 6, 1987; (d) R. Akiyama, T. Sagae, M. Sugiura and S. Kobayashi,
J. Organomet. Chem., 2004, 689, 3806; (e) K. Okamoto, R. Akiyama,
H. Yoshida, T. Yoshida and S. Kobayashi, J. Am. Chem. Soc., 2005,
127, 2125; (f) H. Hagio, M. Sugiura and S. Kobayashi, Synlett, 2005,
813; (g) S. Kobayashi, H. Miyamura, R. Akiyama and T. Ishida, J. Am.
Chem. Soc., 2005, 127, 9251; (h) M. Takeuchi, R. Akiyama and
S. Kobayashi, J. Am. Chem. Soc., 2005, 127, 13096; (i) R. Nishio,
M. Sugiura and S. Kobayashi, Org. Lett., 2005, 7, 4831; (j)
H. Miyamura, R. Akiyama, T. Ishida, R. Matsubara, M. Takeuchi
and S. Kobayashi, Tetrahedron, 2005, 61, 12177; (k) H. Hagio,
M. Sugiura and S. Kobayashi, Org. Lett., 2006, 8, 375; (l) R. Nishio,
M. Sugiura and S. Kobayashi, Org. Biomol. Chem., 2006, 4, 992.
In summary, we have synthesized immobilized Pd and Pt
catalysts based on polysilane for the first time. Polysilane-
supported Pd catalysts have been successfully used in hydrogena-
tion. The reactions proceeded in high yields, and the catalysts
could be recovered almost quantitatively by simple filtration and
reused. Furthermore, no leaching, or a very small amount of
leaching of Pd was confirmed by ICP analysis. The Pd catalyst
was also used in the Suzuki and Sonogashra reactions.
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 4297–4299 | 4299