A green protocol for solvent-free conversion of epoxides to thiiranes with Dowex-50WX8-supported thiourea

Article abstract of DOI:10.1080/10426500802157515

Structurally different epoxides were efficiently converted to the corresponding thiiranes by Dowex-50WX8-supported thiourea under solvent-free conditions. The reactions were carried out either in an oil bath or under microwave irradiation to give the thiiranes in 75-98% yields within 30 sec-120 min.

Full text of DOI:10.1080/10426500802157515

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A Green Protocol for
Solvent-Free Conversion of
Epoxides to Thiiranes with
Dowex-50WX8–Supported
Thiourea
Behzad Zeynizadeh a & Samal Yeghaneh a
a Department of Chemistry, Faculty of Sciences ,
Urmia University , Urmia, Iran
Published online: 04 Feb 2009.
To cite this article: Behzad Zeynizadeh & Samal Yeghaneh (2009) A Green Protocol
for Solvent-Free Conversion of Epoxides to Thiiranes with Dowex-50WX8–Supported
Thiourea, Phosphorus, Sulfur, and Silicon and the Related Elements, 184:2, 362-368,
DOI: 10.1080/10426500802157515
To link to this article: http://dx.doi.org/10.1080/10426500802157515
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Phosphorus, Sulfur, and Silicon, 184:362–368, 2009
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500802157515
A Green Protocol for Solvent-Free Conversion of Epoxides
to Thiiranes with Dowex-50WX8–Supported Thiourea
Behzad Zeynizadeh and Samal Yeghaneh
Department of Chemistry, Faculty of Sciences, Urmia University,
Urmia, Iran
Structurally different epoxides were efficiently converted to the corresponding thi-
iranes by Dowex-50WX8–supported thiourea under solvent-free conditions. The re-
actions were carried out either in an oil bath or under microwave irradiation to
give the thiiranes in 75–98% yields within 30 sec–120 min.
Keywords Dowex-50WX8; epoxide; microwave; solvent-free; thiirane; thiourea
INTRODUCTION
Epoxides are small molecules with numerous synthetic applications.
They are able to react with various nucleophiles, and their potential
to undergo regioselective ring opening reactions makes them highly
useful precursors for the synthesis of organic compounds.1 Thiirane is
the simplest heterocyclic ring system carrying a sulfur atom in the ring
and it is found in naturally occurring compounds, herbicides, pesticides,
polymeric compounds, pharmaceuticals, and in many other manmade
chemicals.2 Transformation of epoxides to thiiranes by an oxygen–
sulfur exchange reaction is a useful reaction and has been achieved
with various methods using sulfur transfer agents.3–27 However, some
of these methods often suffer from one or more disadvantages, such as
low yields of the products, long reaction times, difficult workup proce-
dures, use of expensive reagents, formation of polymeric byproducts,
and the need for aqueous reaction conditions.
Currently, increasing attention is being focused on green chemistry
in organic synthesis, using environmentally benign reagents and condi-
tions, particularly solvent-free reactions. Working without solvents has
Received 23 March 2008; accepted 24 April 2008.
We acknowledge the financial support of this work by the research council of Urmia
University.
Address correspondence to Behzad Zeynizadeh, Department of Chemistry, Faculty of
Sciences, Urmia University, Urmia 57159-165, Iran. E-mail: bzeynizadeh@gmail.com
362

Conversion of Epoxides to Thiiranes
363
the advantage of a simpler process and smaller plants, and eliminates
the energy costs of removal, recycling, and eventual disposal of waste
solvents.28
As a part of our research program on the synthesis of thiiranes,
in this article we wish to report the efficient conversion of epoxides
to the corresponding thiiranes by Dowex-50WX8–supported thiourea
under solvent-free conditions either in an oil bath (60–70◦C) or under
microwave irradiation (Scheme 1).
SCHEME 1
RESULTS AND DISCUSSION
In recent years, the use of reagents/catalysts immobilized on solid sup-
ports has received considerable attention, and numerous reviews and
papers have demonstrated their importance. Reagents supported on
organic polymers or on the surface of inorganic materials have distin-
guishing characteristics such as simplicity of purification process, recy-
clability, selectivity, and an increase of the stability of air- or moisture-
sensitive reagents/catalysts.29
Although thiourea has been one of the most widely used reagents
for the conversion of epoxides to thiiranes,30 it suffers from various
disadvantages such as low reactivity, the need for aqueous or alcoholic
conditions, long reaction times, low yields, and in some cases desulfura-
tion of the resulting thiiranes to olefins.5 Among the outlined strategies,
a survey of the literature shows that the transformation of epoxides to
thiiranes with thiourea under solid supported and solvent-free condi-
tions has been reported only for silica-gel–supported thiourea.6 This
method provides a green protocol for the preparation of thiiranes from
epoxides; however, it suffers from low to moderate yields (67–89%).
In the course of our investigations in green protocols for the prepa-
ration of thiiranes from epoxides, we found that thiourea supported on
Dowex-50WX8, an easily available anionic exchange resin, promotes
the conversion of epoxides to the corresponding thiiranes under solvent-
free conditions. The optimized experimental conditions showed that
the reaction of styrene oxide (1 mmol) with thiourea (3 mmol) sup-
ported on Dowex-50WX8 (0.5 g) proceeded efficiently in an oil bath
at 60–70◦C. The epoxide was converted to the corresponding thiirane
in 93% yield within 60 min. The capability of this synthetic method
was further investigated by reacting activated, deactivated, and cyclic

364
B. Zeynizadeh and S. Yeghaneh
TABLE I Conversion of Epoxides to Thiiranes with (NH2)2CS/Dowex-
50WX8 Systema
Oil bathb
Microwavec
Time
(min)
Yield
(%)d
Time
(sec)
Yield
(%)d
Epoxide
Thiirane
(1)
(2)
(3)
(4)
60
93
85
91
95
90
96
90
93
95
60
20
20
90
90
30
(5)
(6)
30
60
96
93
30
60
98
96
(7)
(8)
15
94
75
60
94
78
120
120
(9)
50
97
60
98
aAll reactions were carried out with 1 mmol of epoxide and Dowex-50WX8 (0.5 g)
supported thiourea (3 mmol) at solvent-free conditions.
b Temperature of oil bath was 60–70◦C.
c
Microwave irradiation was carried out with 40% power amplitude (400 W).
d Yields refer to isolated pure products.
epoxides with Dowex-50WX8–supported thiourea under the optimized
conditions. Table I shows the general trend and the versatility of this
synthetic method. All reactions were carried out efficiently to give the
thiiranes in 75–98% yields within 15–120 min.
Microwave heating and its application in organic synthesis has been
developed successfully, and in the past few years there has been a
tremendous amount of interest in this area. Remarkable decrease in
reaction times and, in most cases, clean reaction and better yields
have been reported after applying microwave irradiation.31 We in-
vestigated the solvent-free conversion of epoxides to thiiranes with
Dowex-50WX8–supported thiourea under microwave irradiation. The

Conversion of Epoxides to Thiiranes
365
experiments showed that the irradiation of a mixture of styrene oxide
(1 mmol) and Dowex-50WX8 (0.5 g)–supported thiourea (3 mmol) with
microwave (40% power amplitude, 400 W) resulted in a complete reac-
tion within 90 sec with 96% yield. This result encouraged us to perform
the same reaction with the examined epoxides at the optimized con-
ditions. The results of this investigation are summarized in Table I.
As becomes evident from Table I, there is a distinct rate enhancement
under microwave conditions with better yields (78–98%): the reactions
are completed within 30–120 sec as compared to 15–120 min under
conventional heating.
In conclusion, we have shown that structurally different epoxides
are easily and efficiently converted to the corresponding thiiranes
with Dowex-50WX8–supported thiourea under solvent-free conditions.
The reactions can be carried out under conventional heating or mi-
crowave irradiation. We believe that the present protocols offer a mild,
simple, and efficient method for the preparation of thiiranes from
epoxides. In addition, availability of the reagents, high yields of thi-
iranes, and short reaction times, as well as the advantages of solvent-
free conditions make these methods useful additions to the present
methodologies.
EXPERIMENTAL
All reagents and substrates were purchased from commercial sources
with the best quality and were used without further purification. Ir-
radiation with microwave was carried out with Yousch domestic mi-
1
crowave oven (overall power 1000 W). IR and H NMR spectra were
recorded with a Thermo Nicolet Nexus 670 FT-IR spectrophotometer
and a 300 MHz Bruker spectrometer, respectively. The products were
characterized by their 1H NMR and IR spectra and by comparison
with the reported data in the literature. All yields refer to isolated
pure products. TLC (silica gel 60 F254 aluminum sheet) was applied for
the purity determination of substrates and products and for reaction
monitoring.
Preparation of Dowex-50WX8–Supported Thiourea
To a round-bottomed flask charged with finely grinded Dowex-50WX8
(0.5 g), thiourea (0.228 g, 3 mmol) and water (2 mL) were added. The
mixture was stirred for 5 min at room temperature. Evaporation of the
solvent under microwave irradiation (1000 W) gave Dowex-50WX8–
supported thiourea in quantitative yield.

366
B. Zeynizadeh and S. Yeghaneh
Conversion of Epoxides to Thiiranes with Dowex-50WX8–
Supported Thiourea Under Solvent-Free Conditions:
General Procedure
In an experimental tube, the epoxide (1 mmol) and Dowex-50WX8 (0.5
g)–supported thiourea (0.228, 3 mmol) were well mixed. The tube was
heated in an oil bath (60–70◦C) or irradiated with microwave (40%
power amplitude, 400 W) for the appropriate time. The progress of the
reaction was monitored by TLC. After completion of the reaction, the
mixture was washed with ethyl acetate followed by filtration. Evapo-
ration of the solvent and flash column chromatography of the resulting
crude product afforded the pure liquid episulfide in 93–96% yield.
Epithiostyrene (Styrene Episulfide) (1)
1H NMR (CDCl3): δ = 7.36–7.26 (m, 5H), 3.93 (t, J = 6.2 Hz, 1H),
2.90 (dd, J = 1.5, 6.6 Hz, 1H), 2.68 (dd, J = 1.5, 5.7 Hz, 1H).
7-Thia-bicyclo[3.1.0] Heptane (Cyclohexeneepisulfide) (2)
1H NMR (CDCl3): δ = 3.13–3.05 (m, 2H), 2.00–1.85 (m, 2H), 1.85–
1.71 (m, 2H), 1.48–1.31 (m, 2H), 1.28–1.11 (m, 2H).
Phenoxymethyl Thiirane (3-Phenoxypropylene Sulfide) (3)
1H NMR (CDCl3): δ = 7.39–7.22 (m, 2H), 7.05–6.87 (m, 3H), 4.24 (dd,
J = 3.3, 11.1 Hz, 1H), 3.96 (dd, J = 5.7, 11.1 Hz, 1H), 3.40–3.30 (m, 1H),
2.91 (t, J = 4.5 Hz, 1H), 2.76 (dd, J = 2.5, 4.8 Hz, 1H).
Isopropoxymethyl Thiirane (3-Isopropoxypropylene Sulfide)
(4)
1H NMR (CDCl3): δ = 3.72–3.62 (m, 2H), 3.40 (dd, J = 6.9, 10.5 Hz,
1H), 3.12–3.03 (m, 1H), 2.54 (d, J = 5.7 Hz, 1H), 2.23 (dd, J = 1.2, 5.4
Hz, 1H), 1.91 (dd, J = 3.3, 6.0 Hz, 6H).
Allyloxymethyl Thiirane (3-Allyloxypropylene Sulfide) (5)
1H NMR (CDCl3): δ = 6.01–5.84 (m, 1H), 5.29 (ddd, J = 1.5, 3.3,
17.4 Hz, 1H), 5.21 (dd, J = 1.2, 10.2 Hz, 1H), 4.05 (dt, J = 1.2, 5.7 Hz,
2H), 3.65 (dd, J = 5.7, 10.8, Hz, 1H), 3.46 (dd, J = 6.6, 10.5 Hz, 1H),
3.14–3.04 (m, 1H), 2.53 (d, J = 6.3 Hz, 1H), 2.22 (dd, J = 1.2, 5.4 Hz,
1H).

Conversion of Epoxides to Thiiranes
367
Methacrylatomethyl Thiirane (3-Methacrylpropylene Sulfide)
(6)
1H NMR (CDCl3): δ = 6.15 (d, J = 0.9 Hz, 1H), 5.63–5.57 (m, 1H),
4.31–4.14 (m, 2H), 3.22–3.12 (m, 1H), 2.54 (dd, J = 0.6, 6.0 Hz, 1H),
2.30 (dd, J = 1.5, 5.1 Hz, 1H), 1.95 (s, 3H).
Butyl Episulfide (Butyl Thiirane) (7)
1H NMR (CDCl3): δ = 3.13–3.04 (m, 1H), 2.53 (d, J = 6.0, 1H), 2.22
(d, J = 5.1, 1H), 1.67–1.51 (m, 4H), 1.46–1.32 (m, 2H), 0.93 (t, J = 7.3,
3H).
Chloromethyl Thiirane (Epithiochlorohydrine) (8)
1H NMR (CDCl3): δ = 3.67 (dd, J = 5.8, 10.9 Hz, 1H), 3.49 (dd, J =
6.8, 10.7 Hz, 1H), 3.15–3.03 (m, 1H), 2.54 (dd, J = 1.3, 5.7 Hz, 1H), 2.24
(dd, J = 1.4, 5.1, Hz, 1H).
trans-Stilben Episulfide (9)
1H NMR (CDCl3): δ = 7.35 (s, 10H), 3.94 (s, 2H).
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