An ionic liquid-mediated expeditious route to the syntheses of diaryl sulfoxides

Article abstract of DOI:10.1016/S0040-4039(02)02786-7

A fast and efficient protocol is proposed for the synthesis of diaryl sulfoxides in the ionic liquid 1-butyl-3-methylimidazolium chloroaluminate, [bmim]Cl·AlCl₃, N=0.67, by employing arenes and thionyl chloride. The ionic liquid plays a dual role of Lewis acid catalyst and solvent, under ambient conditions, offering good yields of the product. The influence of the Lewis acidity of the ionic liquid on the extent of conversion is studied.

Full text of DOI:10.1016/S0040-4039(02)02786-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 1255–1258
An ionic liquid-mediated expeditious route to the syntheses of
diaryl sulfoxides
Swapnil S. Mohile, Mahesh K. Potdar and Manikrao M. Salunkhe*
Department of Chemistry, The Institute of Science, 15 Madam Cama Road, Mumbai 400 032, India
Received 18 October 2002; revised 29 November 2002; accepted 6 December 2002
Abstract—A fast and efficient protocol is proposed for the synthesis of diaryl sulfoxides in the ionic liquid 1-butyl-3-methylimida-
zolium chloroaluminate, [bmim]Cl·AlCl3, N=0.67, by employing arenes and thionyl chloride. The ionic liquid plays a dual role
of Lewis acid catalyst and solvent, under ambient conditions, offering good yields of the product. The influence of the Lewis
acidity of the ionic liquid on the extent of conversion is studied. © 2003 Elsevier Science Ltd. All rights reserved.
Sulfoxides and sulfones have fascinated organic
chemists for a long time owing to their varied reactivity
as a functional group for transformation into a variety
of organo sulfur compounds. These transformations are
useful for the synthesis of drugs and sulfur-substituted
natural products.1,2 Optically active3 sulfoxides con-
tinue to deserve much attention as important chiral
auxiliaries in asymmetric synthesis4 and in CꢀC bond
forming reactions.5 Syntheses of diaryl sulfoxides also
provide a convenient route to triarylsulfonium salts
which are used as photoactive cationic initiators6 and
for the photogeneration of protonic acids in the litho-
graphic resist field.7
The discovery of ionic liquids has widened the horizons
for newer synthetic strategies in the area of organic
chemistry. These innovative fluids are organic salts,
whose cation, anion and the alkyl chain attached to
organic cation can be varied virtually at will, to change
their chemical and physical properties as per the needs
of the chemical process. A key feature of these liquids is
their intrinsic ability to solvate a wide array of organic
and inorganic substrates. They are blessed with quali-
ties such as incredibly large liquidus ranges, negligible
vapour pressure and recyclability.16 In short, these ionic
liquids have enlightened the way to complete environ-
mentally benign procedures.17 Due to their intriguing
properties, these liquids have fascinated us. We have
studied the applicability of ionic liquids in various
synthetically useful transformations.18 Chloroaluminate
ionic liquids remain important.19,20 In continuation of
our research to explore new reactions in ionic liquids,
we thought it would be worthwhile to investigate the
direct synthesis of diaryl sulfoxides in these liquids. We
report herein, for the first time, syntheses of diaryl
sulfoxides by the reaction of thionyl chloride with
arenes in the Lewis acidic 1-butyl-3-methylimidazolium
chloroaluminate, [bmim]Cl·AlCl3, 0.5<N50.67, ionic
liquids (Scheme 1).
Methodologies for the direct synthesis of sulfoxides are
rarely observed in the literature. Usually the synthesis
of sulfoxides is accomplished by the oxidation of
sulfides,8 indirect reduction of sulfones9 and by the
reaction of organometallic reagents with sulfinic acid
esters, mixed anhydrides or sulfines. A direct method
for the synthesis of diaryl sulfoxides is Friedel–Crafts
10
sulfinylation of arenes using a catalyst such as AlCl3
or trifluoromethane sulfonic acid.11 Sulfoxide synthesis
is also achieved by dehydration of arenes and SO2
mediated by magic acid (FSO3H/SbF5).12 The most
recent reports on sulfoxide synthesis include their for-
mation from thionyl chloride and arenes employing
trifluoromethane sulfonic acid13 or scandium triflate14
as the catalyst. The direct preparation of sulfoxides
often suffers from the formation of mixtures of prod-
ucts containing sulfonium salts and chlorinated by-
products along with the desired sulfoxides.15
Scheme 1. Synthesis of diaryl sulfoxides from arenes and
thionyl chloride in the [bmim]Cl·AlCl3, 0.5<N50.67, ionic
liquid.
* Corresponding author. Tel./fax: (91-22) 22816750; e-mail:
mmsalunkhe@hotmail.com
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02786-7

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S. S. Mohile et al. / Tetrahedron Letters 44 (2003) 1255–1258
Table 1. Syntheses of diaryl sulfoxides in the ionic liquid, [bmim]Cl·AlCl3, N=0.67
Initially, we investigated the synthesis of sulfoxides in
basic, neutral and acidic chloroaluminate ionic liquids
by changing the apparent mole fraction of AlCl3 in the
liquid, N, over the range 0–0.5, 0.5 and 0.5–0.67,
respectively. No reaction was observed in liquids corre-
sponding to N=0–0.5, even after 3 h at 100°C. Only

S. S. Mohile et al. / Tetrahedron Letters 44 (2003) 1255–1258
1257
in Table 1. To check the effect of substituents on the
isomer distribution of products, we carried out reac-
tions with toluene, o-xylene, m-xylene and o-Cl-anisole
under the same reaction conditions.21 The results are
depicted in Table 2. As evident from the results, in all
cases the major product obtained was the p,p%-disubsti-
tuted sulfoxide, with small amounts of o,p- and o,o%-
products.
In conclusion, the ionic liquid [bmim]Cl·AlCl3, N=
0.67, furnished a homogeneous Lewis acidic medium
which catalyses the syntheses of sulfoxides employing
arenes and thionyl chloride. The experimental proce-
dure is simple and time saving. Further investigations
on the synthesis of asymmetric diaryl sulfoxides using
aryl sulfinyl chloride and arenes in these liquids are
underway. As a part of our studies of mechanistic
details of the Friedel–Crafts reactions in these ionic
liquids,18b we also intend to exploit 33S and 27Al NMR
spectroscopy for rationalizing the mechanism of these
reactions.
Figure 1. Effect of the mole fraction of AlCl3 (N), in
[bmim]Cl·AlCl3, 0.5<N50.67, on the % conversion of toluene
to ditoluyl sulfoxide.
Table 2. The effect of the position of substituent(s) in
mono- and di-substituted benzenes on the isomer distribu-
tion of the products in [bmim]Cl·AlCl3, N=0.67 mediated
reactions of thionyl chloride with arenes
Acknowledgements
Substrate
Isomer distributiona (%)
Authors are thankful to the Lady Tata Memorial Trust,
Mumbai, for providing scholarships to S.S.M. and
M.K.P.
p,p%
o,p
o,o%
Toluene
79
87
85
86
13
2.5
4
2
0.5
2
o-Xylene
m-Xylene
o-Cl-anisole
2.0
0.5
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a The % of p,p%-, o,p- and o,o%-diaryl sulfoxides was determined by
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