High-efficiency conversion of trimethylsilyl ethers to their corresponding ethers using carbon-based solid acid as a new catalyst in heterogeneous mixtures

Article abstract of DOI:10.1080/00397910701767080

Carbon-based solid acid was used as a new catalyst for conversion of trimethylsilyl ethers to their corresponding ethers in heterogeneous mixtures. The experiments were done moderately at room temperature, and high yields in suitable times were obtained under these conditions. Copyright Taylor & Francis Group, LLC.

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High‐Efficiency Conversion of Trimethylsilyl
Ethers to Their Corresponding Ethers using
Carbon‐Based Solid Acid as a New Catalyst in
Heterogeneous Mixtures
Arash Shokrollahi a , Abbas Zali a & Hamid Reza Pouretedal a
a Malek‐Ashtar University of Technology, Department of Chemistry ,
Shahin‐shahr, Iran
Published online: 28 Jan 2008.
To cite this article: Arash Shokrollahi , Abbas Zali & Hamid Reza Pouretedal (2008) High‐Efficiency Conversion
of Trimethylsilyl Ethers to Their Corresponding Ethers using Carbon‐Based Solid Acid as a New Catalyst in
Heterogeneous Mixtures, Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 38:3, 371-375, DOI: 10.1080/00397910701767080
To link to this article: http://dx.doi.org/10.1080/00397910701767080
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Synthetic Communicationsw, 38: 371–375, 2008
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701767080
High-Efficiency Conversion
of Trimethylsilyl Ethers to Their
Corresponding Ethers using Carbon-Based
Solid Acid as a New Catalyst in
Heterogeneous Mixtures
Arash Shokrollahi, Abbas Zali, and Hamid Reza Pouretedal
Malek-Ashtar University of Technology, Department of Chemistry,
Shahin-shahr, Iran
Abstract: Carbon-based solid acid was used as a new catalyst for conversion of tri-
methylsilyl ethers to their corresponding ethers in heterogeneous mixtures. The exper-
iments were done moderately at room temperature, and high yields in suitable times
were obtained under these conditions.
Keywords: carbon-based solid acid, heterogeneous mixtures, trimethylsilyl ether
INTRODUCTION
Sulfuric acid is an essential catalyst for the production of industrial chemicals.
However, such liquid-acid catalyst requires special processing in the form of
neutralization, which involves costly and inefficient separation of catalyst
from product and results in an unrecyclable sulfate waste.
According the principal of green chemistry, the production method
should be refined so as to minimize adverse effect on the environment and
human health.[1] The migration to strong acids, which are recyclable and
nontoxic, from liquid acids such as sulfuric acid is therefore a desirable
Received in the U.K. March 28, 2007
Address correspondence to Arash Shokrollahi, Malek-Ashtar University of
TechnologyDepartment of Chemistry, Shahin-shahr, Iran. E-mail: ar_shokrolahi@
mut-es.ac.ir
371

372
A. Shokrollahi, A. Zali, and H. R. Pouretedal
goal.[2,3] Solid acids are conventional materials that have wide applications
in chemical production, separation/purification, and polymer–electrolyte
fuel-cell (PEFC) technologies, and the chemical industry is currently
searching for a highly active and stable solid acid to improve the environ-
mental safety of the production of chemicals and energy. An ideal solid
material for the applications considered here should have high stability
and numerous strong protonic acid sites.[4] Michikazu Hara and
coworkers, reported the synthesis of a carbon-based solid acid with a high
density of sulfonic acid groups (SO3H). Such carbon-based solid acids can
be readily prepared by heating aromatic compounds such as naphthalene
in sulfuric acid at 473–573 K.[5] In this synthesis, the sulfonation of the
aromatic compounds is the first stage of the reaction. The resulting
sulfonated aromatic compounds are incompletely carbonized, which
results in the formation of a solid with a nominal sample composition of
CH0.35 O0.35S0.14. The resulting black powder is insoluble in solvents such
as water, methanol, ethanol, benzene, and hexane, even at boiling
temperatures.
Symmetrical and unsymmetrical alkyl/aryl ether formation is an
important reaction in organic synthesis and has generated significant interest
in recent years.[6–10] The classic Williamson procedure, the Mitsunobu
reaction, and the Ullmann condensation method can be use for this transform-
ation. Ion-exchange resin and heteroploy acid–catalyzed etherification have
also been reported.[11–14] Rearrangement and elimination of alkyl alcohols
that are sterically hindered is the major disadvantage of the older procedures
for ether synthesis. This problem is amplified when the alcohols are immobi-
lized on solid supports and the activated species becomes the limiting
reagent.[15] The alcohol–sulfuric acid reaction is most often used for the con-
version of simple primary alcohols into symmetrical ethers. Secondary and
tertiary alcohols predominantly undergo dehydration when subjected to
these conditions. Occasionally a small amount of the symmetrical ether is
formed as a by-product in the case of secondary alcohols.[16] In 1987,
Sassaman’s group reported a facile procedure for the synthesis of symmetrical
and unsymmetrical ethers via reductive coupling of carbonyl compounds
under mild acid-free conditions.[17]
The goal of this work is conversion of trimethylsilyl ether to the corre-
sponding ether in the presence of a carbon-based solid acid. In the proposed
procedure, this reagent was used for ether synthesis from primary and
secondary trimethylsilyl ether (Scheme 1).
Scheme 1.

Conversion of Trimethylsilyl Ethers
EXPERIMENTAL
General
373
Chemicals were purchased from Fluka, Merck, and Aldrich chemical
companies. The products were characterized by comparison of their spectral
1
(IR, H NMR), thin-layer chromatographic (TLC), and physical data with
the authentic samples. All silyl ethers were synthesized according to the
reported procedure.[18]
Typical Experimental Procedure
A mixture of the substrate 1 (4 mmol), n-hexane (20 mL), and carbon-based
solid acid (0.8 g) was stirred at room temperature for the specified time.
The trimethylsilyl compounds, the corresponding ether products, time of
reaction, and percentage of yield of reactions are collected in Table 1.
Table 1. Direct conversion of primary and secondary trimethylsilyl ethers to corre-
sponding ethers on the carbon-based solid acid as catalyst
Time Yield
(%)
Entry
Substrates
Productsa
(min)
1
2
2-CH3OC6H4CH2OTMS
3-CH3OC6H4CH2OTMS
4-CH3OC6H4CH2OTMS
4-(CH3)2CHC6H4CH2OTMS
4-(CH3)3CC6H4CH2OTMS
2-ClC6H4CH2OTMS
(2-CH3OC6H4CH2)2O
(3-CH3OC6H4CH2)2O
(4-CH3OC6H4CH2)2O
(4-(CH3)2CH C6H4CH2)2O
(4-(CH3)3C C6H4CH2)2O
(2-ClC6H4CH2)2O
7
7
85
90
90
90
90
85
85
80
85
85
80
90
85
80
80
80
80
85
85
80
3
7
4
7
5
7
6
20
21
28
28
20
27
28
8
7
4-ClC6H4CH2OTMS
2,4-Cl2C6H3CH2OTMS
2-BrC6H4CH2OTMS
(4-ClC6H4CH2)2O
(2,4-Cl2C6H3CH2)2O
(2-BrC6H4CH2)2O
8
9
10
11
12
13
14
15
16
17
18
19
20
2-O2NC6H4CH2OTMS
3-O2NC6H4CH2OTMS
4-O2NC6H4CH2OTMS
C6H5CHCH3CH2OTMS
C6H5CH55CHCH2OTMS
4-C6H5CH2OC6H4CH2OTMS (4-C6H5CH2OC6H4CH2)2O
n-C7H15OTMS
n-C8H17OTMS
(2-O2NC6H4CH2)2O
(3-O2NC6H4CH2)2O
(4-O2NC6H4CH2)2O
(C6H5CHCH3CH2)2O
(C6H5CH55CHCH2)2O
7
7
(n-C7H15)2O
(n-C8H17)2O
7
8
(C6H5)2CHOTMS
4-ClC6H4CHCH3OTMS
C6H5CH3CHOTMS
((C6H5)2CH)2O
(4-ClC6H4CHCH3)2O
(C6H5CH3CH)2O
20
32
14
aAll of the products are known and their spectral and other data match those reported
in the literature.

374
A. Shokrollahi, A. Zali, and H. R. Pouretedal
All of the products are known, and their spectral and physical data match those
reported in the literature. The percentages of yield are isolated yields.
The reaction was monitored by TLC. After completion of the reaction, the
mixture was filtered and the solid residue was washed with n-hexane
solvent. Evaporation of the solvent was followed by column chromatography
on silica gel, and pure ether was obtained in excellent yields. In conclusion,
carbon-based solid acid can serve as an efficient catalyst for the direct con-
version of primary and secondary trimethylsilyl ethers into the corresponding
ethers under mild and heterogeneous conditions in a short period of time. The
yields are excellent, and the procedure is simple and convenient. Rearrange-
ment, elimination, and ether cleavage reactions were not observed.
Moreover, the carbon-based solid acid was simply recovered by decantation
and recycled for future reaction. The stability and reusability of the
proposed catalyst was studied with four replications of conversion of 2-nitro-
benzyl alcohol TMS ether to corresponding ether in the same conditions. After
completion of the reaction, the catalyst was separated by filtration and used as
such for subsequent experiments after adding fresh substrate and n-hexane
solvent under similar reaction conditions. The yields of reaction were 85,
83, 85, and 84% for four catalytic cycles, which shows stability of the catalyst.
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