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Chemistry Letters Vol.38, No.6 (2009)
A Novel Method for Preparing Silanols from Silylmethanols
Daisuke Takeda, Ryo Oyama, and Shozo YamadaÃ
Chemical Technology Laboratory, CMC Center, Taiho Pharmaceutical Co., Ltd.,
200-22 Motohara, Kamikawa-machi, Kodama-gun, Saitama 367-0241
(Received February 26, 2009; CL-090200; E-mail: sho-yamada@taiho.co.jp)
Various types of silylmethanols were converted into their
Table 1. Study of oxidation conditions
corresponding silanols in good to excellent yield under mild ox-
idation conditions using TEMPO (2,2,6,6-tetramethyl-1-piperi-
dinyloxyl).
MeO
MeO
Me
Si OH
Me
Si
OH
Me
Me
1a
2a
Conditions Yielda/%
Today, silanol compounds are attracting increased attention
because they are valuable building blocks in Pd-catalyzed cross-
coupling reactions.1 Many methods, including hydrolysis of
chlorosilanes,2 oxidation of organosilanes using stoichiometric
amounts of oxidants,3 and reaction of lithium reagents with
cyclotrisiloxanes4 have been developed for preparing silanols.
However, the yields obtained in most of the known methods
are often low because of formation of siloxanes.2a,5 Further,
some of these methods generate large amounts of environmen-
tally hazardous wastes. Recently, environmentally benign reac-
tions for the oxidation of silanes into silanols have been reported
by several groups,6 these reactions use water as a green oxidant
in the presence of metal catalysts. To our best knowledge, how-
ever, none of the above-mentioned reactions have been used for
the preparation of silanols bearing ester groups or amide groups
or carboxylic acids. The only method that can be applied to
ester-bearing substrates is Pd-catalyzed silylation of aryl bro-
mides with 1,2-diethoxy-1,1,2,2-tetramethyldisilane and subse-
quent hydrolysis of the obtained ethyl ethers.7 Although this
method is effective, it requires expensive reagents. Herein, we
wish to report a novel method for the preparation of silanols
without using expensive or environmentally hazardous reagents.
Since our method requires mild conditions, it can be applied to a
variety of substrates.
Initially, we found that silylmethanol compounds could be
converted into their corresponding silanols under oxidation con-
ditions (Table 1). We investigated the oxidation of [(4-methoxy-
phenyl)dimethylsilyl]methanol (1a) under different conditions.
Use of tetrapropylammonium perruthenate (TPAP) with N-
methylmorpholine N-oxide (NMO), the Swern oxidation, and
the Dess–Martin reagent resulted in poor yields (Entries 1–3).
With sulfer-trioxide pyridine complex (Entry 4) and 2,2,6,6-
tetramethyl-1-piperidinyloxyl (TEMPO) (Entry 5) we could ob-
tain high yields, and further, only traces of disiloxanes were
formed. The synthetic route of 1a is shown in Supporting Infor-
mation.8
Entry
1
Reagents
Solvent
CH2Cl2
TPAP (0.1 equiv)
NMO (2.5 equiv)
MS4AP
rt, 1.5 h
62
2
(COCl)2 (2.0 equiv)
Et3N (4.0 equiv)
DMSO (6.0 equiv)
CH2Cl2
À78 ꢀC, 1 h
46
3
4
5
Dess–Martin
reagent (2.0 equiv)
CH2Cl2
DMSO
rt, 1 h
31
81
.
SO3 Py (3.0 equiv)
Et3N (6.5 equiv)
rt, 30 min
TEMPO (0.1 equiv)
NaOCl (2.0 equiv)
KBr (0.1 equiv)
Acetone
aq NaHCO3
rt, 1 h
82
aIsolated yield after silica-gel column chromatography.
yield (Entry 7). Dimethyl(2-thienyl)silylmethanol was also suc-
cessfully converted into its corresponding silanol (Entry 8).
However, (3-furyldimethylsilyl)methanol decomposed under
the present conditions (Entry 9); this might be due to the low
stability of the furan ring under oxidation conditions.9 A general
experimental procedure and spectroscopic data of silanols are
shown in Supporting Information.8
NaClO
NaCl
Me Me
Si
OH
R
KBr
KBrO
Me Me
Si
O
N
O
N
O
H
R
OH
We extended the silylmethanol–silanol conversion reaction
to several substrates (Table 2). Arylsilylmethanols containing
both electron-rich and electron-deficient groups were converted
into their corresponding silanols in good to excellent yields
(Entries 1–3). In those reactions, ester groups were not affected.
Halogen-bearing substrates were also successfully converted
into corresponding silanols (Entries 4 and 5). Bissilylmethanol
was oxidized into its corresponding silanol in excellent yield
(Entry 6). A substrate possessing a carboxylic acid group was
converted into its corresponding silanol in almost quantitative
Cl
O
Me Me
Si OH
R
Me Me
Me Me
Si
R
Si
R
O
O
Cl
Cl O
Scheme 1. Plausible mechanism for the silylmethanol–silanol
conversion.
Copyright Ó 2009 The Chemical Society of Japan
Takeda, Daisuke
