Conversion of oxiranes to thiiranes catalyzed with silica gel-supported aluminium chloride

Article abstract of DOI:10.1080/00397910500214102

Silica gel-supported aluminium chloride, SiO₂-AlCl₃, catalysizes the efficient conversion of different oxiranes to their corresponding thiiranes in the presence of thiourea under nonaqueous conditions. Copyright Taylor & Francis, Inc.

Full text of DOI:10.1080/00397910500214102

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Conversion of Oxiranes to
Thiiranes Catalyzed with Silica
Gel–Supported Aluminium
Chloride
K. Parvanak Borujeni a
a College of Sciences, Shahrekord University,
Shahrekord, Iran
Published online: 18 Aug 2006.
To cite this article: K. Parvanak Borujeni (2005): Conversion of Oxiranes to Thiiranes
Catalyzed with Silica Gel–Supported Aluminium Chloride, Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry,
35:19, 2575-2579
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Synthetic Communicationsw, 35: 2575–2579, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500214102
Conversion of Oxiranes to Thiiranes
Catalyzed with Silica Gel–Supported
Aluminium Chloride
K. Parvanak Borujeni
College of Sciences, Shahrekord University, Shahrekord, Iran
Abstract: Silica gel–supported aluminium chloride, SiO2-AlCl3, catalysizes the
efficient conversion of different oxiranes to their corresponding thiiranes in the
presence of thiourea under nonaqueous conditions.
Keywords: Aluminium chloride, oxirane, silica gel, thiirane
INTRODUCTION
Various procedures have been reported for the preparation of thiiranes,[1] and
the most general one has been the conversion of oxiranes to thiiranes through
their reactions with thiourea[2,3] or inorganic thiocyanates[4,5] in water or other
aqueous solvents. Also, several different Lewis acids, such as Ce(IV) as ceric
ammonium nitrate (CAN),[6] Ru(III) as ruthenium trichloride (RuCl3),[7]
BiCl3,[8] TiO(CF3CO2)2,[9] and TiCl3(CF3SO3),[9] have been used as
catalysts for the conversion of epoxides to thiiranes. However, some of
these methods suffer from drawbacks such as tedious workup, aqueous
and/or alcoholic media, low yields, rapid increase of pH during the
reaction, the presence of trifluroacetic acid, formation of the polymeric
by-products, and in, some cases, expensive and unreusable catalysts. Thus,
it seems that convenient and general methods for this purpose, especially by
heterogeneous catalysts, are still in demand.
Received in Poland May 13, 2005
Address correspondence to K. Parvanak Borujeni, College of Sciences, Shahrekord
University, Shahrekord, Iran. Fax: 0098-381-4424419; E-mail: parvanak-ka@sci.sku.
ac.ir
2575

2576
K. P. Borujeni
In continuation of our recent work on the use of heterogeneous
catalysts,[10–13] we described the preparation and application of silica
gel–supported aluminium chloride, (SiO2-AlCl3), as an efficient and chemo-
selective heterogeneous catalyst for dithioacetalization.[12] Along this line,
in this article a simple and efficient method for conversion of oxiranes to
their corresponding thiiranes is described by reaction with thiourea in the
presence of a catalytic amount of SiO2-AlCl3 is described (Scheme 1).
RESULTS AND DISCUSSION
SiO2-AlCl3 was prepared by reacting silica gel with aluminium chloride in
refluxing carbon tetrachloride. Using this solid acid catalyst, different kinds
of epoxides were converted to their corresponding thiiranes with thiourea in
acetonitrile (Table 1). The effects of other solvents such as acetone and
tert-butanol were also studied, but in comparison with CH3CN the reaction
times were longer and the yields were found to be considerably lower. The
reactions proceed in a clean manner and the conditions employed are both
simple and convenient. The optimum molar ratio of catalyst to epoxide was
found to be 0.2 : 1.
The present method has the following advantages: the reagents are readily
available, the procedure is simple, the reaction times are short, it can be
performed with a wide range of epoxides under mild conditions, and the
yield of thiiranes is high. Most important, the workup is reduced to mere
filtration and evaporation of the solvent. The performance of reactions in
non-aqueous media can be ideal for substrates containing water-sensitive
groups. The catalyst can be prepared easily with cheap starting materials
and is reusable.
EXPERIMENTAL
Material and Techniques
Starting materials were obtained from Fluka Company. Products were charac-
terized by comparison of their physical and spectral data with those of known
samples. Gas chromatography was recorded on Shimadzu GC 14-A. IR
spectra were recorded on a Perkin-Elmer 781 spectrometer. 1H and
Scheme 1.

Oxirane to Thiirane Conversion
2577
a
Table 1. Conversion of oxiranes to thiiranes with thiourea catalyzed by SiO2-AlCl3
Bp (8C)/Torr
Time
(hr)
Yield
(%)b
Entry
1
Thiirane
Found
Reported
1.5
95
103.5–104.5/7
103–104/7[14]
2
3
1.0
1.8
94
93
79–81/5
81–82/5[14]
78–79/8[14]
78.5–80/8
4
1.9
95
48–50/1
49–51/1[6]
5
6
2.0
1.4
92
85
59–60/30
86–89/5
60–62/30[8]
86/5[14]
7
8
1.1
1.3
91
89
71.5–70.5/70
54–56.5/7
72–73/70[6]
55–56/7[8]
aThe molar ratio of epoxide/thiourea/cat. is 1 : 2 : 0.2.
bYields refer to pure isolated products. All thiiranes are known compounds and were
characterized by comparison of their physical and spectral data with those of the
authentic samples.
13C NMR spectra were recorded on a Bruker Advance DPX FT 250 MHz
instrument.
Preparation of SiO2-AlCl3
This catalyst was prepared as reported in the literature.[13,15] The loading of
aluminium chloride determined was 1.3 mmol/gr.[16]
Conversion of Epoxides to Thiiranes: General Procedure
SiO2-AlCl3 (0.2 mmol) was added to a mixture of epoxide (1 mmol) and
thiourea (2 mmol) in acetonitrile (10 ml), and the reaction mixture was
stirred magnetically at 458C. Progress of the reaction was followed by TLC
and GC. Upon completion of the reaction, the solvent was evaporated and

2578
K. P. Borujeni
diethyl ether (2 Â 20 ml) was added to the residue and filtered. The filtrate was
washed with water (2 Â 20 ml) and the organic layer dried with anhydrous
Na2SO4 to obtain the pure products in 85–95% yields after evaporation of
the solvent. The spent catalyst from different experiments was washed with
diethyl ether and used again without further drying.
ACKNOWLEDGMENT
The author is thankful to Shahrekord University Research Council for partial
support of this work.
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Oxirane to Thiirane Conversion
2579
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