A simple and efficient method for reduction of sulfoxide under solvent-free conditions

Article abstract of DOI:10.1080/10426500902994312

Benzyl DABCO bromide was found to be an efficient and mild reagent for the reduction of sulfoxides to the corresponding sulfide using sulfuric acid adsorbed silica gel under solvent-free conditions at room temperature.

Full text of DOI:10.1080/10426500902994312

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Phosphorus, Sulfur, and Silicon and the
Related Elements
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A Simple and Efficient Method for
Reduction of Sulfoxide Under Solvent-
Free Conditions
a
a
Seied Ali Pourmousavi & Parvin Salehi
a
School of Chemistry , Damghan University of Basic Sciences ,
Damghan, Iran
Published online: 23 Mar 2010.
To cite this article: Seied Ali Pourmousavi & Parvin Salehi (2010) A Simple and Efficient Method for
Reduction of Sulfoxide Under Solvent-Free Conditions, Phosphorus, Sulfur, and Silicon and the Related
Elements, 185:4, 803-807, DOI: 10.1080/10426500902994312
To link to this article: http://dx.doi.org/10.1080/10426500902994312
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Phosphorus, Sulfur, and Silicon, 185:803–807, 2010
Copyright ꢀ
Taylor & Francis Group, LLC
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ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500902994312
A SIMPLE AND EFFICIENT METHOD FOR REDUCTION
OF SULFOXIDE UNDER SOLVENT-FREE CONDITIONS
Seied Ali Pourmousavi and Parvin Salehi
School of Chemistry, Damghan University of Basic Sciences, Damghan, Iran
Benzyl DABCO bromide was found to be an efficient and mild reagent for the reduction
of sulfoxides to the corresponding sulfide using sulfuric acid adsorbed silica gel under
solvent-free conditions at room temperature.
Keywords Reduction; solvent free conditions; sulfoxide; sulfuric acid adsorbed silica gel
INTRODUCTION
Sulfoxides are important reagents in the application of organosulfur compounds in
1
organic synthesis. The reduction (or deoxygenation) of sulfoxides to the corresponding
2,3
sulfides is an important organic and biological reaction. Over the years, several methods
have been developed to reduce sulfoxides.4 However, many of these transformations are
limited by side reactions, low yields, lack of chemoselectivity, or harsh conditions and
involve reagents that are strong nucleophiles or very reactive species such as carbenes.12
Consequently, it is desirable to develop alternative methods for the reduction of sulfoxides
using mild conditions.
–11
Recently, the use of catalysts and reagents supported on solid supports and solvent-
free conditions was developed because such reagents not only simplify the purification
processes but also help to prevent the release of toxic reaction residues into the environ-
1
3–17
ment.
In continuation of our investigations on organic reactions in solventless systems and
1
8,19
discovering new applications of supported acids,
we report in this article an extremely
convenient, mild, and efficient procedure for the reduction of sulfoxides to their corre-
sponding sulfides using benzyl DABCO bromide and sulfuric acid adsorbed silica gel
(
H2SO4-silica) under solvent free conditions.
RESULTS AND DISCUSSION
Reactions under solvent-free conditions have received increasing attention in recent
years. The advantage of these methods over conventional homogenous reactions is that they
Received 3 January 2009; accepted 8 April 2009.
We are thankful to the Damghan University of Basic Sciences for support of this work.
Address correspondence to Dr. Seied Ali Pourmousavi, School of Chemistry, Damghan University of Basic
Sciences, Damghan 36715364, Iran. E-mail: pourmousavi@dubs.ac.ir
8
03

804
S. A. POURMOUSAVI AND P. SALEHI
provide greater selectivity, proceed with enhanced reaction rates, give cleaner products, and
involve simple manipulation.
During the course of our studies on the oxidation of sulfides with benzyl DABCO
tribromide,2 we observed that methyl phenyl sulfoxide was reduced to methyl phenyl
sulfide. This finding and the recent reports on the use of H2SO4-silica in various organic
0
21
reactions have encouraged us to develop a simple method for the reduction of sulfoxides
to their corresponding sulfides using benzyl DABCO bromide and H2SO4-silica. First,
the H2SO4-silica was prepared by creating a mixture of H2SO4 (98%) and silica gel
22
(
0.063–0.2 mm) according to the previously reported method. This reagent is stable and
can be kept at room temperature for months without losing its activity. Then, we attempted
the reduction of methyl phenyl sulfoxide by benzyl DABCO bromide in the presence of
H2SO4-silica. The treatment of 1 mmol of methyl phenyl sulfoxide with 2 mmol of benzyl
DABCO bromide in the presence of H2SO4-silica (4 mmol H2SO4 in 1.18 g H2SO4-
silica) afforded the corresponding sulfide in a short time in almost quantitative yield. In a
Table I Reduction of sulfoxides with benzyl DABCO bromide and H2SO4-silica under solvent free conditions
a,b,c
at RT
Substrate
Product
Time (min)
7
Yield
92
5
95
O
S
7
91
1
0
6
45
93
aYield refers to isolated pure products.
b
Sulfoxide/benzyl DABCO bromide/H2SO4-silica (1:2:4 mmol).
c
1
The products were characterized from their spectral (IR and H NMR) and comparison to authentic samples.

METHOD FOR REDUCTION OF SULFOXIDE
805
N
Br
O
S
N
Ph
S
R1
R2
R
1
R
2
H SO -Silica
2
4
Solvent-Free conditions, RT
R , R =Alkyl and Aryl
1
2
Scheme 1
controlled experiment, we observed that the reaction does not take place in the absence of
H2SO4-silica. Therefore, we employed the above conditions for the conversion of various
sulfoxides to the corresponding sulfides under solvent-free conditions (Scheme 1, Table I).
The possible mechanism of the reaction is shown in Scheme 2; initially H2SO4-silica
reacted with sulfoxide to generate protonated sulfoxide, then the reduction occurs via a
halosulfonium ion subsequently reduced to a sulfide by a second halide ion.
N
H
O
S
N
O
S
Br
H SO -Silica
Ph
2
4
R1
R2
R
1
R
2
H
H
H
O
S
O
S
S
Br
+
H O
H SO -Silica
R1
R2
2
2
4
R1
R2
R
1
R
2
Br
Br
N
N
Br
Ph
S
+
Br2
R1
R2
Scheme 2
The activity of benzyl DABCO bromide/H2SO4-silica seems to be higher than other
known catalysts for the reduction of sulfoxides because of the shorter reaction time and high
yield (Table II). In addition, the simple experimental procedure, mild reaction conditions,
and solvent-free conditions are the advantages of the present protocol.
In conclusion, we have developed an efficient method for the reduction of sulfoxide to
the corresponding sulfides using benzyl DABCO bromide in the presence of H2SO4-silica
with advantages such as the following: a) The reagent is inexpensive and easily handed,
and can be stored on the shelf for months without losing its activity. b) The procedure is

806
S. A. POURMOUSAVI AND P. SALEHI
Table II A comparative study of various systems for reduction of methyl phenyl sulfoxide
Entry
Reagent
Conditions
Time
Yield(Ref)
92
1
2
Benzyl DABCO bromide/H2SO4-silica
Solvent-free /RT
THF/Reflux
THF/RT
THF/Reflux
THF/RT
7 min
2 h
20 h
15 min
16 h
10 min
15 min
a
23
Silane/MoO2Cl2
100
23
66
94
88
93
94
Catecholborane/MoO2Cl2(H2O)2b
BH3 THF/MoO2Cl2c
24
3
24
24
25
4
5
THF/Reflux
CHCl3/RT
d
TABCO/1,3-dithiane
aThe reaction was carried out with 100 mol% of PhSiH3 and with 0.3 mol% of PMHS and 5mol% of MoO2Cl2.
b
The reaction was carried out with 1.0 mmol of sulfoxide, 2 mmol of catecholborane, and 5 mol% of
MoO2Cl2(H2O)2.
c
The reaction was carried out with 1.0 mmol of sulfoxide, 2.0 mmol of BH3-THF, and 5 mol% of MoO2Cl2.
The ratio of sulfoxide:1,3-dithiane:TABCO (2,4,4,6-tetrabromo-2,5-cyclohexadienone) is 1:1.1:0.2.
d
simple and occurs under solvent free conditions. c) The yield of sulfide is good to high, and
the reaction time is short. d) The isolation of product is straightforward. In conclusion, this
method can be a useful alternative to the traditional methods for the reduction of sulfoxides
EXPERIMENTAL
H2SO4-silica,22 benzyl DABCO bromide,26 and all sulfoxides[20] were prepared ac-
cording to the reported methods. Yields refer to isolated pure products. The products were
1
1
characterized by comparison with authentic samples (IR, H NMR spectrum). All H NMR
spectra were recorded at 300 MHz in CDCl3 relative to TMS (0.00 ppm). IR spectra were
recorded with a Perkin Elmer RXI spectrophotometer. Spectra of solids were recorded
using KBr pellets.
General Procedure for Reduction of Sulfoxide
In a mortar, a mixture of sulfoxide (1 mmol), benzyl DABCO bromide (2 mmol), and
H2SO4-silica (4 mmol H2SO4 in 1.18 g H2SO4-silica) was ground with a pestle for the time
specified in Table I. The progress of the reaction was followed by TLC. CH2Cl2 (5 mL) was
added to the reaction mixture, and the solid material was separated by filtration. The filter
cake was washed with CH2Cl2 (2 × 5 mL), and the filtrates were combined. Evaporation
of the solvent resulted in a highly pure product.
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