A new and efficient method for the synthesis of bromosilanes from hydrosilanes using Br3CCOOEt/PdCl2 as the catalyst

Article abstract of DOI:10.1016/j.tetlet.2011.03.028

Bromosilanes were prepared conveniently and efficiently via the reaction of hydrosilanes and Br₃CCOOEt in the presence of a catalytic amount of PdCl₂ in refluxing THF over 15 min in high yields. The developed methodology was further applied for the one-pot synthesis of silyl ethers and silyl esters in excellent yields.

Full text of DOI:10.1016/j.tetlet.2011.03.028

Tetrahedron Letters 52 (2011) 2505–2507
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A new and efficient method for the synthesis of bromosilanes from
hydrosilanes using Br3CCOOEt/PdCl2 as the catalyst
Phatsupha Srithanakit a, Warinthorn Chavasiri b,
⇑
a Program in Petrochemistry and Polymer Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
b Department of Chemistry, Faculty of Science and Center for Petroleum, Petrochemicals, and Advanced Materials, Chulalongkorn University, Bangkok 10330, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
Bromosilanes were prepared conveniently and efficiently via the reaction of hydrosilanes and Br3CCOOEt
in the presence of a catalytic amount of PdCl2 in refluxing THF over 15 min in high yields. The developed
methodology was further applied for the one-pot synthesis of silyl ethers and silyl esters in excellent
yields.
Received 1 January 2011
Revised 25 February 2011
Accepted 4 March 2011
Available online 12 March 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Bromosilane
Hydrosilane
Ethyl tribromoacetate
Halosilanes can be used for ester and ether cleavage, carbonyl
addition processes, and halosilane-accelerated carbon–carbon
bond-forming reactions.1 Specifically for bromosilanes, depending
on the reaction conditions, they can be used for both the protec-
tion2 and deprotection3 of functional groups. Cleavage of cyclic
orthoesters is also possible.4
Several methods have been reported for the preparation of
bromosilanes, such as reactions of triphenylsilanol with SiBr4,5 tri-
alkylarylsilanes with Br2,6 silane with AgBr,7, and selenosilanes and
Br2.8 Reaction of tBu2SiH–NH–SiPh3 and N-bromosuccinimide also
afforded bromosilane which was isolated as a by-product.9
Bromosilanes can also be prepared directly from hydrosilanes
and bromine,2,10 but bromine is highly toxic. Novel methods for
bromosilane preparation from hydrosilanes have been reported
using PdCl2 or NiCl2,11 or Cu(II)-based reagents,12 however, these
methods still require long reaction times or stoichiometric
amounts of copper.
of the reaction is likely to be similar to that of the chloro
counterpart.13i
Various brominating agents were initially screened for the bro-
mination of triisopropylhydrosilane (TIPS-H) to furnish the corre-
sponding triisopropylbromosilane (TIPS-Br) (Table 1). The yields
of TIPS-Br were quantified by 1H NMR spectroscopy using toluene
as an internal standard.
Table 1
The effect of brominating agents on the conversion of TIPS-H into TIPS-Br
1 mol% PdCl2
THF
(i-Pr)3SiH + brominating agent
(i-Pr)3SiBr
rt, 15 min
Entry Brominating
agent
Equivalents
(mmol)
Yielda (%)
TIPS-Br
Recoverya (%) MBc
TIPS-H
(%)
Our group has studied extensively the reactions of halogenating
agents13 and we have previously reported the transformation of
hydrosilanes into chlorosilanes.13i Bromosilanes are much more
reactive than chlorosilanes because of the weaker Si–Br bond.1,14
However, publications on bromosilanes are not as prominent as
those on chlorosilanes because of their synthetic challenge and
low stability. Hence, we have developed an efficient method for
the preparation of bromosilanes using Br3CCOOEt13g in the pres-
ence of a catalytic amount of PdCl2, and subsequently used them
for the in situ preparation of silyl esters and ethers. The mechanism
1
2
3
None
CBr4
—
0
39
36
44
0
60
67
9
7
6
5
9
100
53
64
55
100
39
29
91
91
92
93
90
100
92
100
99
100
99
96
100
98
98
98
99
0.75
1.00
1.00
1.00
1.25
1.50
1.00
1.50
1.50
3.00
3.00
Br3CCOOH
Br3CCOOEt
Br3CCOOEt
Br3CCOOEt
Br3CCOOEt
CHBr3
Br2CHCOOH
Br2CHCOOEt
CH2@CHCH2Br
CH3CH2Br
4
5b
6
7
8
9
10
11
12
a
b
c
Determined by 1H NMR spectroscopy using toluene as an internal standard.
The reaction was carried out in the absence of the Pd(II) catalyst.
Mass balance.
⇑
Corresponding author. Tel.: +66 2 218 7625; fax: +66 2 218 7598.
E-mail address: warintho@yahoo.com (W. Chavasiri).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.03.028

2506
P. Srithanakit, W. Chavasiri / Tetrahedron Letters 52 (2011) 2505–2507
Table 1 reveals that using CBr4, Br3CCOOH and Br3CCOOEt pro-
duced TIPS-Br in moderate yields (entries 2–4) while other bromi-
nating agents yielded minimal amounts of TIPS-Br (entries 8–12).
Due to the toxicity of CBr4 and the low solubility of Br3CCOOH in
the reaction medium, Br3CCOOEt was the best choice for further
study. Addition of a Pd(II) catalyst was also found to be essential
for this reaction (entry 5). Increasing the amount of Br3CCOOEt
from 1 to 1.5 equiv improved the yield to 67% at ambient
temperature.
Table 2
The conversion of hydrosilanes into bromosilanes using
Br3CCOOEt
a
Pd(II) catalyst and
1 mol% PdCl2, THF
The reaction conditions were further investigated by varying
the solvent, catalyst, reaction temperature, and amount of
Br3CCOOEt, and the optimized conditions are as follows: TIPS-H/
PdCl2/Br3CCOOEt = 1:0.01:1.25 at reflux in THF for 15 min which
produced TIPS-Br in quantitative yield. The results indicated that
THF and CH3CN were suitable solvents for this reaction giving
the product in high yield. When the reaction temperature was
raised to reflux (THF), the number of equivalents of Br3CCOOEt
could be reduced from 1.50 to 1.25.
This method was successfully applied to various hydrosilanes
(Table 2). Surprisingly, increasing the amount of Br3CCOOEt only
improved the yield of Ph3SiH, but the amounts of products were al-
most the same for other substrates.
Hydrosilane
Br3CCOOEt
bromosilane
+
15 min, reflux
Entry Hydrosilane Equivalents
(mmol)
Yielda (%)
Si–Br
% Recoverya (%) MB
Si–H
(%)
1
2
3
4
5
6
7
8
(i-Pr)3SiH
Ph3SiH
Ph3SiH
1.25
1.25
1.50
1.75
1.25
1.25
1.25
100
65
75
90
82
51
60
61
0
35
24
9
18
47
37
35
100
100
99
99
100
98
Ph3SiH
Ph2ClSiH
PhMe2SiH
Et3SiH
97
96
t-BuMe2SiH 1.25
Determined by 1H NMR spectroscopy.
a
Table 3
The one-pot synthesis of silyl ethers
Step I
Step II
1 mol% PdCl2, Br3CCOOEt
ROH (1.0 mmol)
(i-Pr)3SiH
ROSi(i-Pr)3
THF, reflux, 15 min
imidazole (1.5 mmol),
DMAP (0.5 mmol),
1 h, reflux, THF (0.25 ml)
Entry
1
Alcohol
Yield (%)
OH
86a
OH
2
100b
100b
HO
3
a
Separated on an alumina column.
b
Determined by 1H NMR spectroscopy.
Table 4
The one-pot synthesis of silyl esters
Step I
Step II
1 mol% PdCl2, Br3CCOOEt
RCOOH (1.0 mmol)
RCOOSi(i-Pr)3
(i-Pr)3SiH
THF, reflux, 15 min
imidazole (1.5 mmol),
DMAP (0.5 mmol),
1 h, reflux, THF (0.25 ml)
Entry
1
Carboxylic acid
Yielda (%)
100
OH
O
O
2
100
100
OH
O
MeO
3
OH
a
Determined by 1H NMR spectroscopy.

P. Srithanakit, W. Chavasiri / Tetrahedron Letters 52 (2011) 2505–2507
2507
In addition, this protocol could be further applied for the one-
pot synthesis of silyl ethers and esters by reacting the in situ gen-
erated bromosilane with the appropriate alcohol or carboxylic acid
(Tables 3 and 4). The target silyl ethers and esters were obtained in
high yields and the reaction times were shorter than when using
chlorosilane.15
Chemistry, Faculty of Science, Chulalongkorn University for provid-
ing chemicals and laboratory facilities. Financial support from the
90th Anniversary of Chulalongkorn University Fund (Ratchadaph-
iseksomphot Endowment Fund) and the Graduate School, Chul-
alongkorn University is also acknowledged.
In conclusion, Br3CCOOEt coupled with a catalytic amount of
PdCl2 has been utilized as an efficient brominating system for the
bromination of hydrosilane. The method developed could be suc-
cessfully applied to other hydrosilanes and further employed in a
one-pot procedure for the synthesis of silyl ethers and esters in
high yields.
References and notes
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A typical procedure for the preparation of bromosilanes from
hydrosilanes: To a stirred solution of PdCl2 (1 mol %, 1.8 mg) and
Br3CCOOEt (1.25 equiv, 1.25 mmol, 185
added TIPS-H (1 equiv, 1.0 mmol, 205
l
L) in THF (0.25 mL) was
l
L) and the mixture was
heated at reflux in THF under an N2 atmosphere for 15 min. The
crude mixture was analyzed by 1H NMR spectroscopy with the
addition of toluene (1 equiv, 1.0 mmol, 106 lL) as an internal stan-
dard. In a typical case, TIPS-Br (229 mg, 96% yield) was isolated by
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Chavasiri, W.; Jang, D. O. Tetrahedron Lett. 2006, 47, 5693–5696; (d)
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filtration and vacuum distillation.
For the one-pot synthesis of silyl ethers and esters, after stirring
the mixture described above for 15 min, the alcohol or acid
(1 mmol), DMAP (0.5 mmol), and imidazole (1.5 mmol) were
added and the mixture was stirred at reflux for 1 h. After cooling,
the yield of product was either determined by 1H NMR spectros-
copy using toluene (1 equiv, 1.0 mmol, 106 lL) as an internal stan-
dard or was isolated by column chromatography on alumina.
Acknowledgments
14. Olmsted III, J.; Williams, G. M. Chemistry: The Molecular Science; C.V. Mosby,
1996.
15. Pongkittiphan, V. One-pot Synthesis of Halosilanes and Application. M.Sc. Thesis;
Chulalongkorn University: Thailand, 2008.
We are grateful to the Natural Products Research Unit, Center
for Petroleum, Petrochemicals and Advanced Materials and Pro-
gram in Petrochemistry and Polymer Science, Department of