Pd/C-catalyzed cross-coupling reaction of benzyloxysilanes with halosilanes for selective synthesis of unsymmetrical siloxanes

Article abstract of DOI:10.1039/c4ra02126f

A new protocol for the nonhydrolytic synthesis of unsymmetrical siloxanes has been developed. The cross-coupling reaction of benzyloxysilanes with halosilanes catalyzed by Pd/C afforded various unsymmetrical siloxanes with co-production of benzyl halides. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra02126f

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Pd/C-catalyzed cross-coupling reaction of
benzyloxysilanes with halosilanes for selective
Cite this: RSC Adv., 2014, 4, 19099
synthesis of unsymmetrical siloxanes†
Received 11th March 2014
Accepted 11th April 2014
Masayasu Igarashi, Keiko Kubo, Tomohiro Matsumoto, Kazuhiko Sato, Wataru Ando
*
and Shigeru Shimada
DOI: 10.1039/c4ra02126f
www.rsc.org/advances
A new protocol for the nonhydrolytic synthesis of unsymmetrical developed. Piers–Rubinsztajn reaction is a catalytic unsym-
siloxanes has been developed. The cross-coupling reaction of ben- metrical siloxane formation reaction taking place between
zyloxysilanes with halosilanes catalyzed by Pd/C afforded various hydrosilanes and alkoxysilanes in the presence of B(C6F5)3
unsymmetrical siloxanes with co-production of benzyl halides.
catalyst with simultaneous formation of alkanes.6 More
recently, Kuroda and co-workers found that BiCl3 efficiently
catalyzes cross-coupling reaction of alkoxysilanes (t-butylox-
ysilanes and (diphenylmethoxy)silanes) with chlorosilanes to
form siloxanes with simultaneous generation of chloroalkanes.7
Although these reactions have well applied to the synthesis of
variety of siloxanes, a practical problem associated with these
homogeneous catalysts is the difficulty of catalyst separation.
Herein we report a novel cross-coupling siloxane formation
reaction, namely, nonhydrolytic siloxane synthesis by the reac-
tion of benzyloxysilanes with halosilanes using simple hetero-
geneous palladium on carbon (Pd/C) catalysts which can be
easily separated from the reaction mixture.
Very recently, we reported a Pd/C-catalyzed hydrogenolysis of
benzyloxysilanes to the corresponding silanols under non-
hydrolytic conditions.8 By taking advantage of the nonhydrolytic
nature of the reaction, we tried to synthesize unsymmetrical
siloxanes by performing the reaction in the presence of hal-
osilanes. Commercially available Pd/Cs are generally sold as
moistened with water for safety. Thus, to avoid hydrolysis of
benzyloxysilanes and halosilanes, Pd/Cs used in this study were
dried by heating at 120 ꢀC for 48 h under vacuum before use.9 As
expected, the reaction of Ph3SiOBn (Bn ¼ benzyl) (1) in the
presence of 2 equivalents of Me3SiCl and 10 mol% Pd/C
(N. E. CHEMCAT, OH type) under H2 atmosphere proceeded to
give cross-coupled siloxane Ph3SiOSiMe3 (2) in almost quanti-
tative yield (Scheme 1). However, during the course of detailed
study on the reaction conditions, we unexpectedly found that
the cross-coupling reaction of 1 with Me3SiCl does proceed in the
Polysiloxanes (silicones) play important roles in a wide range of
industries as irreplaceable materials due to their excellent
properties such as thermal, light and oxidation stabilities, high
electric insulation properties, high gas permeability, and so on.1
Precise control of Si–O–Si bond formation is essential for the
preparation of structurally well-dened siloxanes. In spite of
siloxanes' importance, their synthetic methods are very
limited.1,2 The most important and widely used method is the
hydrolysis of compounds containing Si–X (X ¼ halogen, alkoxy,
hydrogen, acyloxy, etc.) groups. However, this hydrolytic
method is not suitable for selective synthesis of structurally
well-dened siloxanes, because self-condensation of silanol
intermediates spontaneously takes place under the hydrolytic
reaction conditions and is difficult to control. Therefore, non-
hydrolytic and highly selective reactions for siloxane synthesis
has received signicant attentions recently.
About half a century ago, colloidal Ni-, ZnCl2- and H2PtCl6/
6H2O-catalyzed dehydrocoupling reaction of hydrosilanes with
silanols to afford unsymmetrical siloxanes was discovered.3
Michalska reported that Rh complexes such as Wilkinson's
catalyst were also useful for the dehydrocoupling of hydro-
silanes with silanols.4 This Rh-catalyzed as well as Pd- and Pt-
catalyzed reactions were applied to the synthesis of well-dened
siloxane polymers.5 However, this method can be used only for
stable silanols, such as tertiary silanols. During the last decade,
homogeneous Lewis acid-catalyzed cross-coupling siloxane
formation reactions under nonhydrolytic conditions have been
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba
Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan. E-mail: s-shimada@
aist.go.jp; Fax: +81-298-61-4568; Tel: +81-298-61-6257
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra02126f
Scheme 1 Reaction of 1 in the presence of Me3SiCl.
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Table 1 Cross-coupling reaction of 1 with Me3SiCl using various types of Pd/Csa
Entry
Pd/C
Conv.b (%)
Yieldc (%)
Selectivityd (%)
1
2
3
ASCA-2 (Pd 4.5 wt%, Pt 0.5 wt%)
NX type (Pd 5 wt%)
OH type (Pd 10 wt%)
96
16
100
61
2
96
64
13
96
a
Reaction conditions: 1 (0.550 mmol), Me3SiCl (1.10 mmol) and Pd/C (10 mol% Pd metal) in EtOAc (6 mL) were stirred at room temperature for 9 h
under Ar. b Conversion of compound 1. c 1H NMR yield by using hexamethylbenzene as an internal standard. d Selectivity ¼ yield/conversion (%).
absence of H2. Actually the reaction under Ar atmosphere imply that Pd/C is indispensable for the siloxane formation in
quantitatively afforded PhCH2Cl but not PhCH3 as a co-product. the case of Me3SiCl and Me3SiBr.
Table 1 shows the comparison of catalytic activity of several
Although Me3SiBr and Me3SiI also led to acceptable yields of
Pd/Cs purchased from N. E. CHEMCAT toward the cross- product 2, further study was conducted by using Me3SiCl,
coupling reaction of 1 with Me3SiCl (2 equivalents) in the because it is more stable, cheaper and readily available than
absence of H2. Initial attempts by using ASCA-2 type Pd/C, Me3SiBr and Me3SiI. As shown in Table 3, scope and limitation
which showed the best result for the deprotection of benzylox- of this reaction were examined by using various benzylox-
ysilanes in the previous studies, proved that it was moderately ysilanes. Introduction of a bulkier substituent, t-butyl group, to
selective catalyst for this reaction (Table 1, Entry 1).8 NX type 1 considerably decreased the yield even aer longer reaction
was much less effective (Table 1, Entry 2), while OH type time (72 h, 91% conversion and 65% yield, Table 3, Entry 2). On
(Pd(OH)2 on carbon), so-called Pearlman's catalyst, almost the other hand, replacement of one or two Ph groups of 1 with
quantitatively afforded the cross-coupling product 2 (Table 1, Me groups considerably affected the selectivities partly because
Entry 3). Simultaneous generation of approximately equimolar of the easiness of homo-coupling reaction following the
amounts of PhCH2Cl (96%) was conrmed by gas chromato- substituent exchange reaction between benzyloxysilanes and
graphic analysis of the reaction mixture.
Me3SiCl (Table 3, Entries 3 and 4). This reaction is well appli-
In order to clarify the difference in reactivity depending of cable to substrates having two to four benzyloxy groups. High
the difference of halogens, we examined the reaction of 1 with selectivity (92%) was observed in the reaction of Ph2Si(OBn)2 to
Me3SiX (X ¼ Cl, Br and I) (Table 2). Reaction times required for produce Ph2Si(OSiMe3)2 (Table 3, Entry 5). In the case of
complete consumption of 1 were determined by monitoring the Me2Si(OBn)2, substituent exchange reaction giving Me2Si-
reaction progress by gas chromatography. As expected, the Cl(OBn), Me2SiCl2 and Me3SiOBn considerably affected the
reactivity is in the order Me3SiCl < Me3SiBr < Me3SiI, while selectivity and afforded the desired product Me2Si(OSiMe3)2 in
Me3SiI afforded 2 in lower yield (76%) than Me3SiBr and lower yield (54%, Table 3, Entry 6). Although the reaction of
Me3SiCl (Table 2, Entries 1–3). Generation of the corresponding tribenzyloxysilanes were expected to be more complicated,
benzyl halides was also conrmed.
corresponding cross-coupled siloxanes were obtained in good
In this reaction, substituent exchange reaction between the
two reactants can take place as a competitive reaction as
observed in the related Lewis acid-catalyzed reactions.7,10 Such
exchange reaction did occur in the reaction of 1 with Me3SiI to
give Ph3SiI and Me3SiOBn, which cause the formation of homo-
coupling by-products, Ph3SiOSiPh3 and Me3SiOSiMe3 (Table 2,
Entry 3). Thus, increasing the amount of Me3SiI from 2 to 3
equivalents improved the yield of 1 to 95% (Table 2, Entry 4).
Similar exchange reaction also took place in the reaction of
Me3SiCl and Me3SiBr to some extent, but did not affect the yield
of 2 (Table 2, Entries 1–2).
Table 2 Cross-coupling reaction of 1 with Me3SiX (X ¼ Cl, Br and I) to
produce 2a
Entry
Me3SiX
Time (h)
Me3SiX/1
Yieldc (%)
It is known that Me3SiI can cleave alkoxysilanes to form
iodoalkanes and disiloxanes in the absence of a catalyst.11
Therefore, necessity of the catalyst was checked by performing
the reaction of 1 with Me3SiX under the reaction conditions of
Table 2 without Pd/C (reaction time, 1 day). In the case of
Me3SiI, disiloxane 2 was obtained in 10% yield, while no reac-
tion took place at all for Me3SiCl and Me3SiBr. These results
1
2
Me3SiCl
Me3SiBr
Me3SiI
9
6
1
1
2
2
2
3
96
98
76
95
3
4b
Me3SiI
a
Reaction conditions: 1 (0.550 mmol), Me3SiX (1.10 mmol) and Pd/C
(OH type, 10 mol% Pd metal) in EtOAc were stirred at room
b
c
temperature under Ar. Me3SiI (1.65 mmol) was used. Yields were
determined by 1H NMR analysis by using hexamethylbenzene as an
internal standard.
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Table 3 Syntheses of cross-coupled siloxanes from various benzyloxysilanes, R4ꢁnSi(OBn)n, with Me3SiCla
Entry
Benzyloxysilane
Ph3SiOBn
Time (h)
Conv.b (%)
Yieldc (%)
Selectivityd (%)
1
2
3
4
5
6f
7
8f
9
9
72
9
9
5
9
9
3
6
100
91
Ph3SiOSiMe3 96 (94)e
96
71
74
54
92
54
89
88
93
t
t
Ph2 BuSiOBn
Ph2 BuSiOSiMe3 65
Ph2MeSiOBn
PhMe2SiOBn
Ph2Si(OBn)2
Me2Si(OBn)2
PhSi(OBn)3
MeSi(OBn)3
Si(OBn)4
100
100
100
100
100
100
100
Ph2MeSiOSiMe3 74
PhMe2SiOSiMe3 54
Ph2Si(OSiMe3)2 92
Me2Si(OSiMe3)2 54
PhSi(OSiMe3)3 89
MeSi(OSiMe3)3 88
Si(OSiMe3)4 93
a
Reaction conditions: benzyloxysilane (0.550 mmol), Me3SiCl (2n ꢂ 0.550 mmol) and Pd/C (OH type, n ꢂ 10 mol% Pd metal) in EtOAc were stirred
at room temperature (n ¼ numbers of BnO groups in benzyloxysilanes). Conversion of benzyloxysilanes. c 1H NMR yield by using
b
d
hexamethylbenzene as an internal standard. The value in parentheses shows the yield of isolated product. Selectivity ¼ yield of
e
f
R4ꢁnSi(OSiMe3)n/conversion (%). Ph3SiOBn (10.0 g, 27.3 mmol) was used. Conversion and yield based on benzyloxysilanes were determined
by integral value of 29Si NMR analysis using inverse-gated decoupling pulse sequence with 1,4-bis(trimethylsilyl)benzene as an internal standard.
yields irrespective of the size of substituents; PhSi(OBn)3 and
MeSi(OBn)3 respectively afforded the desired products in 89%
Notes and references
and 88% yields (Table 3, Entries 7 and 8). The reaction of
Si(OBn)4 also proceeded smoothly to give Si(OSiMe3)4 in high
yield (93%, Table 3, Entry 9).
The present method can be carried out on a 10 g scale
(Scheme 2). That is, with 10.0 g of 1 as a starting material,
8.93 g of 2 was obtained by Kugelrohr distillation of the crude
mixture aer the removal of the catalyst by simple ltration
through Celite (94% yield), although longer reaction time
was needed.
In conclusion, we developed a nonhydrolytic Pd/C-catalyzed
cross-coupling siloxane formation reaction. Various cross-
coupled siloxanes were successfully synthesized by the reaction
of benzyloxysilanes with Me3SiCl. Easiness of catalyst removal
from the reaction mixture by centrifugation or ltration is one
of advantageous features of this reaction over the previously
reported cross-coupling type siloxane synthesis catalyzed by
homogeneous catalysts. This feature would be particularly
important in the case of polymeric materials. Application of this
reaction to polymer synthesis as well as mechanistic studies are
underway in this laboratory.
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the reaction mixture under an inert atmosphere and moisten
Pd/Cs with water for safety aer the reaction. Ignoring safety
precautions can lead to re!.
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Scheme 2 A large-scale synthesis of compound 2.
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