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Chemistry Letters Vol.36, No.7 (2007)
Platinum-catalyzed Aromatic C–H Silylation of Arenes
with 1,1,1,3,5,5,5-Heptamethyltrisiloxane
Miki Murata,Ã Naoaki Fukuyama, Jun-ichi Wada, Shinji Watanabe, and Yuzuru Masuda
Department of Materials Science, Kitami Institute of Technology, 165 Koencho, Kitami 090-8507
(Received May 16, 2007; CL-070535; E-mail: muratamk@mail.kitami-it.ac.jp)
The intermolecular dehydrogenative coupling of 1,1,1,3,
Table 1. C–H silylation of benzene (2a) with hydrosilanesa
5,5,5-heptamethyltrisiloxane with arenes proceeded in the pres-
ence of a catalytic amount of platinum complexes prepared
in situ from PtCl2 and hydrotris(pyrazolyl)borate derivatives.
Entry
Hydrosilane
Ligand
Yield/%b
1
2
3
4
5c
6
7
1
1
1
1
none
TpK
BpK
0
80
35
93
91
46d
37e
TpMe2
TpMe2
TpMe2
TpMe2
K
The versatility of arylsilanes in modern organic chemistry
renders them attractive targets for synthesis. For example, the
palladium-catalyzed Hiyama cross-coupling reaction of aryl-
(alkoxy)silanes has become a powerful method for carbon–car-
bon bond formation.1 Aryl(alkoxy)silanes can be easily prepared
by the transition-metal-catalyzed silylation of aryl halides
with hydrosilanes, such as triethoxysilane2 and 1,1,1,3,5,5,5-
heptamethyltrisiloxane (1).3 Clearly, from environmental and
economical points of view, direct silylation of aromatic C–H
bonds of arenes 2 is a more attractive goal. Therefore, much
attention has been focused on the C–H silylation of aromatic hy-
drocarbons 2 using hydrosilanes as a silicon source, and numer-
ous catalyst systems, such as IrCl(CO)(PPh3)2,4 Pt(dba)2,5
TpMe2Pt(Me)2(H) (TpMe2 = hydrotris(3,5-dimethylpyrazolyl)-
borate),6 CpÃRh(H)2(SiEt3)2,7 and Ru3(CO)12,8 have been re-
ported to affect this dehydrogenative coupling reaction. Above
all, the platinum complexes are highly efficient catalysts for
the hydrogen-acceptor-free processes; however, the silylating re-
agents were restricted to triorganosilanes.5,6 Tetraorganosilicon
reagents thus obtained are less reactive for the Hiyama cross-
coupling reaction, mainly because electronegative groups, such
as halogen and alkoxy group, on the silicon atom are required
for the formation of penta-coordinated silicate intermediates.1
Consequently, we have sought an alternative silylating reagent
which provided reactive arylsilanes;9,10 in this paper, wish to
report a platinum-catalyzed C–H silylation of arenes 2 with
1,1,1,3,5,5,5-heptamethyltrisiloxane (1; Eq 1).11
1
K
K
K
(Me3SiO)Me2SiH
Et3SiH
aReaction conditions: 1 (0.5 mmol), 2a (5 mmol), PtCl2
(0.015 mmol), ligand (0.015 mmol), 150 ꢀC, 24 h. bGC yields
of 3a are based on 1. cThe reaction was carried out at 200 ꢀC
using PtCl2 (0.005 mmol) and TpMe2K (0.005 mmol). dGC
e
yield of (Me3SiO)Me2SiPh. GC yield of Et3SiPh.
products was completely suppressed. Although dihydrido-
bis(pyrazolyl)borate (Bp) was less efficient than Tp as support-
ing ligands under the same conditions (Entry 3), the use of
TpMe2K showed improved reactivity, providing a 93% yield of
3a (Entry 4). The present reaction system is similar to but more
mild than the previously reported TpMe2Pt(Me)2(H)-catalyzed
C–H silylation.6 Additionally, it is noteworthy that this silylation
was catalyzed by 1 mol % of the catalyst although the elevated
temperature was required (Entry 5). Under our optimized condi-
tions, the use of 1,1,1,3,3-pentamethyldisiloxane and triethyl-
silane induced a lowering of the reactivity (Entries 6 and 7).
The present study demonstrates that 1 is an efficient silylating
reagent for the aromatic C–H silylation.
The results obtained with representative arenes 2, giving
aryltrisiloxanes 3 similarly as above, are listed in Table 2.12
The yields and product ratios were determined by GC analysis
of crude reaction mixtures. The reaction of mono-substituted
arenes, such as trifluoromethylbenzene (2b), toluene (2c), and
chlorobenzene (2d), resulted in a mixture of meta and para
regioisomers and electronic characteristics of the substituent
on 2 hardly affected the meta/para ratios (Entries 1–3).
Electron-deficient arene 2b were more reactive than 2a.13 In
these cases, the formation of ortho isomers was completely
suppressed, whereas the reported platinum-catalyzed silylation
of 2c and 2d generated ortho isomers.5,6 In the reaction of anisole
(2e), however, the product contained all regioisomers (Entry 4).5
Being consistent with these results, disubstituted arenes 2f–2h
were silylated regioselectively for steric reasons (Entries 5–
7).6,7 In addition, the silylation of 1,4-difluorobenzene (2i) oc-
curred exclusively at the 2-position (Entry 8). Unfortunately,
all attempts at the C–H silylation of aniline and phenol were
unsuccessful.
PtCl2 / TpMe2
K
Si
H
+
H Ar
Si Ar
+
(1)
H
2
150 − 200 °C
1
2 (10 equiv)
3
Me3SiO
Si = Me3SiO Si
Me
An initial screen was performed using platinum precursors
and additional (pyrazolyl)borate ligands for the dehydrogenative
coupling of benzene (2a) and 1. These salts are commercially
available and can be easily handled in air. The results are sum-
marized in Table 1. This coupling work was achieved with the
aid of the Tp ligand; i.e., treatment of 1 (1 equiv.) with 2a (10
equiv.) in the presence of 3 mol % of a metal complex prepared
in situ from PtCl2 and potassium hydrotris(pyrazolyl)borate
(TpK) at 150 ꢀC was found to lead to the corresponding phenyl-
trisiloxane 3a in 80% yield (Entries 1 and 2). Under the condi-
tions using an excess amount of 2a, the formation of disilylated
Finally, the potential versatility of the present silylation was
demonstrated by the one-pot synthesis of unsymmetrical biaryls,
as we have reported that isolated 3-aryl-1,1,1,3,5,5,5-hepta-
Copyright ꢀ 2007 The Chemical Society of Japan