Silylation of primary alcohols with recyclable ruthenium catalyst and hydrosilanes

Article abstract of DOI:10.1016/j.tetlet.2010.06.119

The silylation of primary alcohols was achieved using hydrosilanes and a recyclable ruthenium catalyst without additives under mild conditions. Notably, this catalyst system is effective for the silylation of alcohols having haloaryl groups, which were in

Full text of DOI:10.1016/j.tetlet.2010.06.119

Tetrahedron Letters 51 (2010) 4573–4575
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Tetrahedron Letters
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Silylation of primary alcohols with recyclable ruthenium catalyst
and hydrosilanes
*
Sungjin Kim, Min Serk Kwon, Jaiwook Park
Department of Chemistry, Pohang University of Science and Technology (POSTECH), San 31 Hyojadong, Pohang, Kyeongbuk 790-784, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
The silylation of primary alcohols was achieved using hydrosilanes and a recyclable ruthenium catalyst
without additives under mild conditions. Notably, this catalyst system is effective for the silylation of
alcohols having haloaryl groups, which were intact during the silylation.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 18 June 2010
Accepted 25 June 2010
Available online 30 June 2010
The silylation of alcohols is an essential and frequent reaction
for the synthesis of organosilicon compounds and for the protec-
tion of reactive hydroxyl groups in multi-step organic synthesis.1
Usually, the silylation is carried out using chlorosilanes,2 which
requires a stoichiometric amount of a base. Transition metal-cat-
alyzed silylation using hydrosilanes is an attractive alternative
method,3 and several catalysts have been reported such as Rh(II)
carboxylates,4a a chip-type Rh(I) catalyst,4b alumina-supported
hyde, conjugated enones, esters, and carbamates.10a,11,12 However,
most of them are homogeneous catalysts which are not easy to re-
cover. There are few examples of recycling catalysts.4 Recovery
has been demonstrated for dirhodium(II) perfluorocarboxylates
bearing long perfluoroalkyl chains through fluorous separation.4a
A chip-type Rh(I) catalyst attached to phosphane-functionalized
gold surface is a notable example that showed high activity and
reusability in the silylation of primary alcohols, although the reac-
tions were performed in micromol scale.4b The catalytic silane
alcoholysis has been achieved using alumina-supported gold
nanoparticles without additional ligands at 100–110 °C. However,
a recycling test has not been carried out for this heterogeneous
catalyst.5
gold
nanoparticles,5
colloidal
palladium
nanoparticles,6
7
Re2(CO)12
,
Grubbs’ catalyst [Cl2(PCy3)2Ru = CHPh],8 iridium com-
plexes,9 Cu(I) complexes,10 and [RuCl(p-cymene)]2.11 They have
been applied for the silylation of alcohols having various func-
tional groups such as alkenes, alkynes, alkyl halides, ketones, alde-
Table 1
Silylation of 4-chlorobenzyl alcohol with triethylsilanea
OH
OSiEt3
OSiEt3
cat.
+
HSiEt3
+
rt, solvent, Ar
Cl
Cl
1
2
3
4
Entry
Catalyst (mol %)
Solvent
Time (h)
Yieldb (%)
3
4
1c
2c
3
4
5
6
7
8
Pd/C (0.20)
Pd/Al2O3 (0.20)
Ru/C (3.0)
Ru/Al2O3 (3.0)
1 (3.0)
1 (3.0)
1 (3.0)
1 (3.0)
Toluene
Toluene
Toluene
Toluene
Toluene
Hexane
3
10
24
24
24
24
24
24
57
69
21
19
91
80
51
62
38
25
0
0
0
0
0
0
Diethyl ether
CH2Cl2
a
b
c
The reaction was performed on 1.0 mmol of 4-chlorobenzyl alcohol and 1.2 mmol of triethylsilane with a catalyst at 25 °C under Ar atmosphere.
Determined by GC with an internal standard.
1.5 mmol of triethylsilane was used.
* Corresponding author. Tel.: +82 54 279 2117; fax: +82 54 279 3399.
E-mail address: pjw@postech.ac.kr (J. Park).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.06.119

4574
S. Kim et al. / Tetrahedron Letters 51 (2010) 4573–4575
Table 3
Here, we report the silylation of alcohols without additives un-
Recycling test for 1a
der mild conditions using hydrosilanes and a recyclable ruthenium
catalyst (Ru/AlO(OH) (1)). Notably, C–X bonds of haloaryl groups
survive under the conditions of the silylation,13 which are gener-
ally labile toward oxidative addition in transition metal-catalyzed
reactions. The ruthenium catalyst 1 is composed of ruthenium
nanoparticles entrapped in aluminum oxyhydroxide, and can be
easily prepared from readily available reagents through a simple
sol–gel process.14a We have shown its high activity in the dehydro-
1 (3.0 mol %)
OSiEt3
OH
Ph
+
HSiEt3
Ph
rt, toluene, Ar
Run
1
95
2
3
4
93
5
94
Yieldb (%)
93
93
a
The reaction mixture was filtered through a glass filter. The recovered catalyst
was washed with acetone, dried under vacuum for 3 h, and reused.
b
Determined by GC with an internal standard.
genation of alcohols and in the synthesis of a,b-unsaturated esters
through one-pot alcohol oxidation–Wittig reaction.14
The characteristic activity of 1 was observed in the silylation of
4-chlorobenzyl alcohol with triethylsilane (Table 1). Although
commercial Pd catalysts showed high activities in silylation,
dechlorinated products were competitively formed in substantial
amounts (entries 1 and 2). Commercial Ru catalysts showed che-
moselectivities for the silylation, but the activities were much low-
er than that of 1 (entries 3 and 4). The silylation using 1 was tested
in various solvents such as toluene, hexane, diethyl ether, and
dichloromethane (entries 5–8). Toluene was the best among the
solvents tested; (4-chlorobenzyloxy)triethylsilane was obtained
selectively in 91% yield using 3.0 mol % of Ru after 24 h at room
temperature under an argon atmosphere. The activity of 1 in hex-
ane was comparable with that in toluene, while those in diethyl
ether and dichloromethane were significantly lower.
Various alcohols were subjected in the catalytic silylation under
the conditions of the entry 5 of Table 1 (Table 2). The catalytic sily-
lation was effective not only for C–Cl bonds but also for C–Br, C–I,
and C–F bonds of haloaryl groups (entries 1–6). Aliphatic primary
alcohols were also successfully silylated (entries 9, 10, and 12).
The silylations with dimethylphenylsilane were faster that those
with triethylsilane (entries 11 and 12). However, our catalyst system
was not effective for the silylation using t-BuMe2SiH (entry 13).
Meanwhile, a secondary alcohol, 1-phenylethanol, was silylated
only in 30% after 24 h (entry 14). In a competitive reaction using ben-
zyl alcohol, 1-phenylethanol, and dimethylphenylsilane in 1:1:1.2
Table 2
Silylation of various alcohols using 1a
catalyst 1
+
HSiR'3
R-OSiR'3
Yieldb (%)
ROH
rt, toluene, Ar
Entry
Silane
HSiEt3
Product
Time (h)
24
OSiEt3
OSiEt3
1
2
91c,d
Cl
HSiEt3
24
88d,e
Cl
OSiEt3
OSiEt3
OSiEt3
OSiEt3
3
4
5
HSiEt3
HSiEt3
HSiEt3
24
24
24
87d,e
95c,d
87d,e
Cl
Br
I
ratio,
a
78:22 mixture of (benzyloxy)dimethylphenylsilane
90d,e
87
and 1-(dimethylphenylsiloxy)-1-phenylethane was obtained. tert-
Butanol and an electron-deficient benzyl alcohol, (4-nitro-
phenyl)methanol, were practically inert toward our catalyst system
(entries 15 and 16).
We tested the recyclability of 1 in the silylation of 2-phenyleth-
anol with triethylsilane. The solid catalyst was recovered by filtra-
tion. Even in the fifth run, the catalytic activity is almost the same
as that in the first use (Table 3).
In conclusion, we have demonstrated the silylation of various
primary alcohols with hydrosilanes and a recyclable ruthenium
catalyst under mild conditions without additives. The catalyst sys-
tem is effective for the silylation of alcohols having haloaryl
groups, and the C–X bonds survive during the silylation.
6
7
8
HSiEt3
HSiEt3
HSiEt3
24
24
24
F
OSiEt3
OSiEt3
85
MeO
Ph
9
HSiEt3
HSiEt3
24
24
95c
90
OSiEt3
10
C7H15
OSiEt3
OSiMe2Ph
11
HSiMe2Ph
18
95c
C7H15
Ph
OSiMe2Ph
12
13
HSiMe2Ph
18
24
87
Tracec
Acknowledgments
HSiMe2(t-Bu)
OSiMe2(t-Bu)
We are grateful for the financial supports from POSCO through
the STSC program and the Korean Ministry of Education through
the BK21 project for our graduate program.
14
HSiMe2Ph
OSiMe2Ph
24
30c
15
16
HSiMe2Ph
HSiEt3
24
24
Tracec
Tracec
OSiMe2Ph
Supplementary data
OSiEt3
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.06.119.
O2N
a
The reaction was performed on 1.0 mmol of alcohol and 1.2 mmol of hydrosi-
lane in 3.0 mL of dry toluene with 1 (3.0 mol % of Ru) at 25 °C under Ar atmosphere.
References and notes
b
Isolation yield.
Determined by GC with an internal standard.
Dehalogenated products were not observed.
c
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d
e
5.0 mol % of Ru was used.

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4575
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