A facile and efficient synthesis of aryltriethoxysilanes via sonochemical Barbier-type reaction

Article abstract of DOI:10.1016/j.tetlet.2006.07.078

A series of aryltriethoxysilanes was synthesized from the reaction mixture of aryl bromide, Mg powder, 1,2-dibromoethane and tetraethyl orthosilicate in THF under ultrasound. This sonochemical Barbier-type reaction provides a simple and efficient method for preparation of aryltriethoxysilanes. The Hiyama cross-coupling reaction of phenyltriethoxysilane with bromobenzene was investigated under different palladium catalysts and Pd(PhCN)₂Cl₂ was found to be the best choice.

Full text of DOI:10.1016/j.tetlet.2006.07.078

Tetrahedron Letters 47 (2006) 7085–7087
A facile and efficient synthesis of aryltriethoxysilanes via
sonochemical Barbier-type reaction
Adam Shih-Yuan Lee,* Yu-Ting Chang, Shu-Fang Chu and Kuo-Wei Tsao
Department of Chemistry, Tamkang University, Tamsui, Taipei 251, Taiwan
Received 23 June 2006; revised 13 July 2006; accepted 18 July 2006
Available online 14 August 2006
Abstract—A series of aryltriethoxysilanes was synthesized from the reaction mixture of aryl bromide, Mg powder, 1,2-dibromo-
ethane and tetraethyl orthosilicate in THF under ultrasound. This sonochemical Barbier-type reaction provides a simple and
efficient method for preparation of aryltriethoxysilanes. The Hiyama cross-coupling reaction of phenyltriethoxysilane with bromo-
benzene was investigated under different palladium catalysts and Pd(PhCN)2Cl2 was found to be the best choice.
Ó 2006 Elsevier Ltd. All rights reserved.
The transition metal catalyzed Stille1–4 and Suzuki–
Miyaura5–8 cross-coupling reactions are one of the most
extensively employed methods for the synthesis of
biologically active compounds and useful synthetic
intermediates such as unsymmetric biaryl and alkenyl
derivatives. Recently, arylsilane compounds have
increasingly played an important role and emerged as
a powerful reagent for palladium-catalyzed Hiyama9–18
cross-coupling reactions with conventional organo-
halides. The arylsiloxane molecules avoid the inherent
limitations which are associated with traditional meth-
odologies such as organostannanes (Stille) and byprod-
ucts are toxic, and organoboranes (Suzuki) are difficult
to be synthesized and purified. The reported synthetic
methods of arylsiloxane in the literatures fall into two
categories: (1) treatment of organomagnesium (Grig-
nard) reagents or organolithium reagents with silicon
compounds19 and (2) silylation of aryl iodides by trialk-
oxysilane in the presence of palladium catalysts.20,21 The
metalloid reaction method is limited by the formation
difficulties of arylmetalloids and generally inferior yields
of siloxane. The Pd-mediated silylation method is lim-
ited due to the difficult synthesis of aryl iodides and only
electron rich, para-substituted aryl iodides afford good
yield of arylsiloxane. The simple and efficient prepara-
tion of aryltrialkoxysilane is a prerequisite for further
transformation with a view for the application of Pd-
catalyzed Hiyama cross-coupling reactions in organic
synthesis. Recently, DeShong’s laboratory have reported
the synthesis of aryltrialkoxysilanes from aryl Grignard
and organolithium reagents with tetraalkyl orthosili-
cate.19 The synthesis of arylsiloxanes via Grignard
reagent was limited by the formation of diarylated
(Ar2Si(OR)2) and triarylated silanes (Ar3Si(OR)).
DeShong and co-workers found that Grignard silylation
reaction which was proceeded at much lower tempera-
ture of À30 °C gave predominately monoaryl siloxanes.
Our previous studies have shown that Barbier-type reac-
tions were improved and successfully achieved by intro-
ducing ultrasound.22–24 Thus, we investigated the
additional reaction of aryl bromide with tetraethyl
orthosilicate under sonochemical Barbier-type reaction
condition. Herewith, we wish to report a simple and effi-
cient method for synthesis of aryltriethoxysilanes at
room temperature under sonochemical Barbier-type
reaction condition (Scheme 1).
To
a
reaction mixture of magnesium powder
(2.5 mmol), 1,2-dibromoethane (1.0 mmol) and tetra-
ethyl orthosilicate (3.0 mmol) in anhydrous THF was
added dropwise para-methoxyphenyl bromide (1.0
mmol) under ultrasound and the reaction mixture was
continuously sonicated at room temperature for
30 min. The monoarylated product (22%) was obtained
with a mixture of polyarylated silanes (Scheme 2). The
highest yield (65%) was obtained when less amount of
3.0 Mg, 0.5 BrCH2CH2Br, 5.0 Si(OEt)4
Ar Si(OEt)3
Ar Br
THF, , 30 min.
*
Corresponding author. Tel.: +886 2 2621 5656; fax: +886 2 2622
3830; e-mail: adamlee@mail.tku.edu.tw
Scheme 1.
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.07.078

7086
A. S.-Y. Lee et al. / Tetrahedron Letters 47 (2006) 7085–7087
Table 1. Synthesis of aryltriethoxysilanes
, THF
MeO
Si(OEt)3
MeO
Br
Entry Substrates
Products
Yielda
(%)
2.5 Mg, 1.0 BrCH2CH2Br, 3.0 Si(OEt)4, 30 min
2.5 Mg, 0.5 BrCH2CH2Br, 3.0 Si(OEt)4, 30 min
22%
35%
Br
Si(OEt)3
1
91
89
2.5 Mg, 0.5 BrCH2CH2Br, 4.0 Si(OEt)4, 30 min
2.5 Mg, 0.5 BrCH2CH2Br, 5.0 Si(OEt)4, 30 min
59%
65%
Br
Si(OEt)3
2
MeO
MeO
Cl
Scheme 2.
Cl
Br
Si(OEt)3
Si(OEt)3
3
87
70
60
63
1,2-dibromoethane (0.5 equiv) and more amount of tetra-
ethyl orthosilicate (5.0 equiv) were introduced under the
reaction condition.
Br
4
F3C
F3C
An exothermic phenomenon was observed when 1,2-
dibromoethane was introduced to the reaction mix-
ture.25 Thus, we rearranged the additional order of
1,2-dibromoethane to avoid the suddenly increasing
temperature during reaction. To a reaction mixture of
magnesium powder (3.0 mmol) and tetraethyl orthosili-
cate (5.0 mmol) in THF (4 mL) was added dropwise a
solution of para-methoxyphenyl bromide (1.0 mmol)
and 1,2-dibromoethane (0.5 mmol) in THF (1 mL)
under ultrasound and the reaction mixture was continu-
ously sonicated at room temperature for 30 min
(Scheme 3). A higher yield (76%) of para-methoxy-
phenyltriethoxysilane was achieved under this reaction
condition. We also found that consequently a stepwise
addition of 1,2-dibromoethane and then para-methoxy-
phenyl bromide improve dramatically the formation
yield of para-methoxyphenyltriethoxysilane to 89%
yield. To a reaction mixture of Mg (3.0 equiv) and tetra-
ethyl orthosilicate (5.0 equiv) was added 1,2-dibromoe-
thane (0.5 equiv) and then immediately para-
methoxyphenyl bromide (1.0 equiv) was added dropwise
under ultrasound and the reaction mixture was further
sonicated at room temperature for 30 min. Increasing
the amount of tetraethyl orthosilicate did not improve
the formation of expected monoarylated product.
Si(OEt)3
Si(OEt)3
Br
O
O
O
5
O
Br
6
7
42
59
Si(OEt)3
Br
8
Br
Si(OEt)3
9
49
91
Br
Br
(EtO)3Si
Si(OEt)3
10
Br
Si(OEt)3
S
S
11
12
62
Br
Br
(EtO)3Si
Si(OEt)3
S
S
N.R.b
N
Si(OEt)3
N
Br
a The yields were determined after chromatographic purification.
b No reaction and recovery of starting material.
A series of aryl bromides was investigated under this
sonochemical Barbier-type reaction condition and the
results are shown in Table 1. The typical procedure26
for synthesis of an aryltriethoxysilane is as follows: To
a reaction mixture of magnesium powder (3.0 mmol)
and tetraethyl orthosilicate (5.0 mmol) in dry THF
was added dropwise 1,2-dibromoethane (0.5 mmol)
and then aryl bromide (1.0 mmol) was immediately
added dropwise under ultrasound.27 The reaction mix-
ture was continuously sonicated at room temperature
for 30 min. After the sonication, hexane (10 mL) was
added and then an aqueous NH4Cl (5%) solution was
added to the reaction mixture. The mixture was filtrated
and the organic layer was dried with MgSO4, filtered,
and then the organic solvent was removed under
reduced pressure. Further purification was achieved by
passing through a very short flash chromatographic
column with silica gel and ethyl acetate/hexane (5%)
as eluant.
, THF
MeO
Si(OEt)3
76%
MeO
Br
The experimental results showed that the formation
yield of aryltriethoxysilane by this sonochemical Bar-
bier-type reaction is generally higher than the yield pro-
duced by Grignard addition reaction. The electron-
withdrawing group attached to the aryl bromide usually
gave a lower yield of aryltriethoxysilane (Table 1, entries
3, 4, 9 and 11). Polyaromatic bromides were converted
to their corresponding triethoxysilanes under the reac-
tion condition (Table 1, entries 6–8). It should be noted
3.0 Mg, 5.0 Si(OEt)4; 0.5 BrCH2CH2Br, 30 min
3.0 Mg, 0.5 BrCH2CH2Br, 5.0 Si(OEt)4, 30 min
3.0 Mg, 0.5 BrCH2CH2Br, 6.0 Si(OEt)4, 30 min
3.0 Mg, 0.5 BrCH2CH2Br, 7.0 Si(OEt)4, 30min
3.0 Mg, 0.5 BrCH2CH2Br, 10.0 Si(OEt)4, 30 min
89%
81%
58%
73%
Scheme 3.

A. S.-Y. Lee et al. / Tetrahedron Letters 47 (2006) 7085–7087
7087
Table 2. Cross-coupling reaction of phenyltriethoxysilane with bromo-
benzene
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Pd(OAc)2, toluene
PdCl2, toluene
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Pd(dba)2, toluene
Pd2(dba)3, toluene
Pd(dppf)2Cl2, toluene
20%
45%
68%
42%
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66%
61%
63%
90%
82%
Pd(PhCN)2Cl2, toluene
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protons which were easily deprotonated by Grignard
reagent (Table 1, entry 8). The dibromides such as 1,
4-dibromobenzene and 2,5-dibromothiophene were
successfully transformed to their bis(triethoxysilane)
compounds which were the important synthetic interme-
diates for material science (Table 1, entries 9 and 11).
2-Bromopyridine was resistant under the reaction condi-
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without further purification.
The previous studies in the literatures28–35 have shown
that aryltrialkoxysilanes undergo palladium-catalyzed,
fluoride-mediated coupling reactions with various aro-
matic compounds. Among these reported reaction pro-
cedures, aryltrimethoxysilane is generally selected and
investigated for Hiyama cross-coupling reaction. There-
fore, we further investigated the coupling reaction of
bromobenzene with phenyltriethoxysilane under differ-
ent palladium catalysts (Table 2). A reaction mixture
of 3 mol % palladium catalyst, tetrabutylammonium
fluoride (TBAF), phenyltriethoxysilane and bromobenz-
ene in toluene was refluxed for 23 h. The results showed
that the Pd(PhCN)2Cl2 was the best choice of catalyst
for the coupling reaction.
27. The ultrasonic cleaning bath (Elma-T490DH, 50 kHz)
should be filled with water containing some 3–5% deter-
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much more even cavitation in bath water.
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In conclusion, this reaction condition provides a simple
method for the preparation of aryltriethoxysilane which
is the important synthetic intermediate for biologically
active compounds and organic materials. The synthetic
application of aryltriethoxysilane for Hiyama cross-cou-
pling reaction was studied and the proper Pd-catalyzed
reaction condition was established.
Acknowledgements
We thank the National Science Council in Taiwan (NSC
94-2113-M-032-004) and Tamkang University for finan-
cial support.