Efficient and Facile Ar-Si Bond Cleavage by
Montmorillonite KSF: Synthetic and Mechanistic
Aspects of Solvent-Free Protodesilylation Studied
by Solution and Solid-State MAS NMR
applications in organic synthesis since these reactions always
occur at the ipso position. In conventional applications,
trialkylsilyl groups were used to block reactivity at specific sites
4
,5
or to direct ipso substitutions of arylsilanes for the regiospecific
introduction of electrophilic functional groups.6 A further
7
creative application was discovered independently by Ellman
Yossi Zafrani,* Eytan Gershonov, and Ishay Columbus
8
and Veber in 1995, who developed a silicon-based traceless
Department of Organic Chemistry, Israel Institute for Biological
Research, Ness-Ziona 74100, Israel
linker for solid-phase synthesis, utilizing the well-known
protodesilylation of the Ar-Si bond.
The protodesilylation of some arylsilanes, in which the aryl
moiety was functionalized with electron-donating groups, can
be achieved in comparatively mild acidic conditions such as
ReceiVed April 1, 2007
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TFA, boron trifluoride-acetic acid complex (2/3) and chlo-
1
0
11
roacetic acid, 1,1,1,3,3,3-hexafluoroisopropyl alcohol, or in
the presence of trimethylsilyl chloride and potassium iodide.12
However, for most cleavages studied in the past, a strong acid
was required (aqueous methanolic perchloric acid or sulfuric
4
,5
acid in acetic acid). Since such harsh conditions may cause
decomposition of the substrate or products, there is a consider-
able interest in developing milder and more environmentally
benign methods for performing this process. For instance,
recently, it has been published by Itami et al. that a C-Si
(
including Ar-Si) bond could be rapidly and efficiently cleaved
by supercritical water.
1
3
Herein we report the facile and efficient room temperature
Ar-Si bond cleavage, using mont KSF clay as a solid support.
The facility and efficiency of the process encouraged us to study
the mechanism of this solvent-free protodesilylation (dry media),
using both solution and solid-state magic-angle spinning (SS
A facile and efficient method for the cleavage of the Ar-
Si bond of various aryl trimethyl silanes is described. When
adsorbed on montmorillonite KSF (mont KSF), these aryl-
silanes readily undergo a solvent-free protodesilylation to
the corresponding arenes at room temperature in excellent
yields. This approach seems to be superior to the traditional
mild methods (i.e., desilylation by TFA, TBAF, CsF), in
terms of reaction yield, rate, and environmentally benign
conditions. Some mechanistic studies using both solution and
1
MAS) H NMR techniques. A mechanistic study of such
solvent-free reactions may provide valuable information on the
nature of the active site within the inorganic matrix and on to
the role the diffusion plays in their kinetics. These mechanistic
insights may lead to improved synthetic tools useful for
implementation in the so-called “green chemistry”. Real time
1
solid-state magic-angle spinning (SS MAS) H NMR are also
presented.
(
(
(
4) Eaborn, C. Pure Appl. Chem. 1969, 375.
5) Chan, T. H.; Felming, I. Synthesis 1979, 761.
6) (a) Semmelhack, M. F.; Bisaha, J.; Czarney, M. J. Am. Chem. Soc.
The use of clays as inorganic solid catalysts or as reaction
media in various organic transformations has become wide-
spread in the last two decades.1-3 Such environmentally benign
heterogeneous reactions often involve minimal waste, efficient
catalyst recycling, increased yield and/or selectivity, and easier
setup and workup procedures compared to conventional liquid-
phase homogeneous reactions. In addition, mineral clays benefit
from practical features such as stability, ease of handling, lack
of corrosiveness, and low cost. For example, acid-treated
montmorillonite (commercially known as mont KSF) shows
high activity as solid support in many organic reactions. The
catalytic activity is mainly attributed to the Brønsted acidic sites,
generated by the interaction of water molecules with a metal
ion in the interlamellar space.
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(
(
(
1
(
Protodesilylation and electrophilic aromatic substitution of
arylsilanes discovered a century ago have found extensive
Pilkenton, S.; Hwang, S.-J.; Raftery, D. J. Phys. Chem. B 1999, 103, 11152
and references cited therein. (f) Wagner, G. W.; Bartram, P. W. Langmuir
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10.1021/jo0706170 CCC: $37.00 © 2007 American Chemical Society
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J. Org. Chem. 2007, 72, 7014-7017
Published on Web 08/04/2007