A green protocol for the easy synthesis of thiiranes from epoxides using thiourea/silica gel in the absence of solvent

Article abstract of DOI:10.1080/104265090889404

The use of thiourea/silica gel provides a green protocol for the easy and high-yielding preparation of thiiranes from different classes of epoxides in the absence of solvent at room temperature. The high stereospecific conversion of (R)(+)-styrene oxide to (S)(+)-styrene sulfide is also reported using this reagent system. Copyright Taylor & Francis Inc.

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Phosphorus, Sulfur, and Silicon
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A Green Protocol for the Easy
Synthesis of Thiiranes from
Epoxides Using Thiourea/Silica
Gel in the Absence of Solvent
Nasser Iranpoor a , Habib Firouzabadi a & Abbas Ali
Jafari a
a Department of Chemistry, College of Sciences,
Shiraz University, Shiraz, Iran
Published online: 18 Aug 2006.
To cite this article: Nasser Iranpoor , Habib Firouzabadi & Abbas Ali Jafari (2005):
A Green Protocol for the Easy Synthesis of Thiiranes from Epoxides Using Thiourea/
Silica Gel in the Absence of Solvent, Phosphorus, Sulfur, and Silicon and the Related
Elements, 180:8, 1809-1814
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Phosphorus, Sulfur, and Silicon, 180:1809–1814, 2005
Copyright © Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/104265090889404
A Green Protocol for the Easy Synthesis of Thiiranes
from Epoxides Using Thiourea/Silica Gel in the
Absence of Solvent
Nasser Iranpoor
Habib Firouzabadi
Abbas Ali Jafari
Department of Chemistry, College of Sciences, Shiraz University,
Shiraz, Iran
The use of thiourea/silica gel provides a green protocol for the easy and high-
yielding preparation of thiiranes from different classes of epoxides in the absence
of solvent at room temperature. The high stereospecific conversion of (R)(+)-styrene
oxide to (S)(+)-styrene sulfide is also reported using this reagent system.
Keywords Epoxide; silica gel; thiirane; thiourea
INTRODUCTION
There are many methods reported in the literature for the preparation
of thiiranes.1,2 The most efficient route reported so far is based on the
conversion of epoxides into the corresponding thiiranes by an oxygen–
sulfur exchange reaction.1−8 Thiourea has also been reported as an
example of one of these sulfur agents that reacts with epoxides to give
thiiranes under wet conditions or in aqueous solvents.9,10 For example,
the reaction of styrene oxide with wet thiourea in the absence of solvent
has been reported to afford styrene episulfide in only 45% yield after
10 h. The reaction of epoxides with thiourea in aqueous ethanol is more
or less similar to the reaction of epoxides with KSCN,10 usually with
longer reaction times and lower yields, especially in the cases of cyclic
epoxides. To overcome the difficulties of long reaction times, low yields
and high temperatures of the reactions and the occurrence of side reac-
tions such as polymerization or desulfurization, new methods including
Received July 8, 2004; accepted September 7, 2004.
The authors appreciate the Research Council of Shiraz University and Iran TWAS
Chapter Based at ISMO for the support of this work.
Address correspondence to Nasser Iranpoor, Department of Chemistry, College of Sci-
ences, Shiraz University, Shiraz 71454, Iran. E-mail: iranpoor@chem.susc.ac.ir
1809

1810
N. Iranpoor et al.
the use of different Lewis acids11 or use of polymeric reagents,12 have
been employed by us and other groups.
Due to the importance of reducing environmental pollution in chem-
ical reactions, green protocols garner great attention in both labora-
tory and industrial synthesis.1 In this respect we now report a green
method for the easy and high-yielding synthesis of thiiranes from epox-
ides using thiourea/silica gel under nonsolvent condition. Due to the
absence of solvent in this method and the possibility of applying disti-
lation to the crude mixture, thiiranes can be obtained with high purity
and yields and leave only SiO2 and urea as nonpolluting residues.
RESULTS AND DISCUSSIONS
The use of silica-gel supported KSCN as a source of sulfur in the reaction
with epoxides to produce thiiranes suffers from high reaction temper-
ature (90◦C) and long reaction times (4.5–18 h).8 However, when we
reacted silica-gel-supported–thiourea with epoxides in the absence of
solvent, a quick reaction occurred (3–5 min) and quantitative conver-
sion to thiiranes were observed. For the isolation of thiiranes, vacuum
distillation techniques using a bulb-to-bulb system were applied to the
crude reaction mixture. For those reactions in which distillation was
not possible (e.g., small scale reactions), we also investigated the pos-
sibility of using this conversion in solvent or doing solvent work up.
Among the solvents used for this study, such as CH2Cl2, CHCl3, diethyl
ether, THF, acetone, and CH2Cl2 was found to be most suitable. In com-
parison with the reactions under non-solvent conditions, the reactions
in CH2Cl2 occur slower (0.5–3 h), but the conversion yields are similar
to those obtained in the absence of solvent (Table I). The only substrate
which produces a higher yield of the product in solution is cyclohexene
oxide. The results obtained for conversion of epoxides to thiiranes with
thiourea/silica-gel in the absence of solvent and in solution are shown
in Table I.
Although the synthesis of thiiranes from starting materials other
than epoxides is extensively studied, reports on the preparation of op-
tically active thiiranes are rare.14 Due to the low yields and low op-
tical purity of the thiiranes obtained by these methods, epoxides have
gained favor as alternative starting materials for the preparation of op-
tically active thiiranes. Enantioselective synthesis of (2S,3S)-(+)-trans-
2,3-epithiosuccinate from its corresponding epoxide in 48% yield has
been reported in the presence of triphenylphosphine sulfide and triflu-
oroacetic acid as a catalyst.6 The conversion of optically active styrene

Synthesis of Thiiranes from Epoxides
1811
TABLE I Reaction of Epoxides (1.0 mmol) with Thiourea/Silica-Gela
in the Absence of Solvent, (Method A) and in CH2Cl2 (Method B) at
Room Temperature
Method A
Method B
Entry
1
Epoxide
Time(min)/yield %b Time(min)/yield %c
Product
5/86
80/95
2
3
3/88
3/80
40/92
45/93
4
5
5/90
3/89
120/95
30/97
6
7
8
3/88
5/80
5/67
80/90
135/94
190/92
a3 g of thiourea/silica-gel was used.
bThe conversion yield (gc analysis using internal standard) is quantitative. Isolated
yield after distillation.
cIsolated yield after column chromatography.
oxide to optically active styrene episulfide using NH4SCN in the pres-
ence of Ru(III) or Ce(IV) as catalysts are also reported by our group.11a,d
We observed that using thiourea/silica gel under a solvent-free con-
dition, the conversion of (R)-(+)-styrene oxide into (S)-(+)-styrene sul-
fide occurs with high yield and stereospecificity (Scheme 1). When we
reacted optically active (R)-(+)-styrene oxide with this reagent at 0◦C
in the absence of solvent, (S)-(+)-styrene episulfide was obtained in 96%
yield and 90% optical purity.
SCHEME 1

1812
N. Iranpoor et al.
The optical purity of the product was determined by comparison of
its optical rotation ([α]D = +39.25◦ in n-heptane) with that reported for
styrene sulfide ([α]D = −15.7◦ in n-heptane) which is considered to be
35.8% optically pure with the (R) configuration.15
The reaction of (R)-(+)-styrene oxide with thiourea/silica-gel in
CH2Cl2 at 0◦C produces the optically active (S)-(+)-styrene episulfide
in 93% yield and in only 70% optical purity after 2 h. This result shows
that stereospecificity of the reaction in the absence of solvent is higher.
For comparison, the results obtained for the stereospecific conversion
of optically active styrene oxide to its episulfide with thiourea/silica gel
are compared with other reported methods in the literature (Table II).
In addition to the green nature of this method, the availability of the
reagent, simplicity, and mildness of the method; short reaction times;
easy work-up; and high yields of the products and high stereospecificity
of the reaction are the advantages of this procedure, which makes it very
useful in organic synthesis.
Experimental
The products were characterized by comparison of their b.p, IR, 1H and
13C-NMR with those prepared by known procedures. IR spectra were
recorded on a Perkin Elmer IR-157 G and a Perkin Elmer 781 spec-
trometer. 1H and 13C-NMR spectra were recorded on a Bruker Avance
TABLE II Stereospecific Preparation of Optical Active
(S)-(+)-Styrene Sulfide from (R)-(+)-Styrene Oxide with Different
Reagentsa
Stereochemistry
Entry
Catalyst
Reagent
of Styrene Sulfide % Yieldb % eec
1
2
3
4
5
6
7
RuCl3
Polymer-SCN
NH4SCN11a
NH4SCN11d
(S)
(S)
(S)
(S)
(S)
(S)
(S)
93
90
80
90
93
96
93
78
25
87
25
35
90d
70
(P4VP)-Ce(OTf)4 NH4SCN11d
Ce(IV)
Ti(IV)
Non
NH4SCN11d
NH4SCN11d
Thiourea/Silica Gel
Thiourea/Silica
Gel/CH2Cl2
Non
aOptical active (R)-(+)-styrene oxide, the product of Merck Company, was used
without further purification.
bIsolated yield.
cThe optical purity of the product was determined according to the literature.15
dThe reaction was performed in the absence of solvent. Instead of distillation, the
product was extracted in dichloromethane and purified by chromatography on silica gel
using ethyl acetate as eluent.

Synthesis of Thiiranes from Epoxides
1813
DPX-250. Optical rotation was determined using a Perkin Elmer 241
polarimeter.
Preparation of the Reagent
To a solution of thiourea (0.02 mole, 1.52 g) in acetone, silica gel was
added (28.5 g). The mixture was evaporated on a rotary evaporator and
dried under vacuum to give a homogeneous solid.
General Procedure
Method A
The mixture of epoxide (1 mmol) and 3 g of thiourea/silica gel were
placed in a mortar and were thoroughly mixed for 3–5 min. Monitoring
of the reaction with TLC and GC showed the completion of the reaction
after this period of time. The mixture was transferred to a bulb-to-bulb
distillation system and distilled under reduced pressure. The collecting
flask was kept as cool as possible to avoid the loss of product due to its
vaporization. The pure products were obtained in 80–90% yield. The
reaction mixture can also be worked up by adding solvent, followed by
filtration and chromatography. The products were known compounds
and were identified by comparison of their phycical and spectral data
with those of known samples.11c
Method B
To a solution of epoxides (1.0 mmol) in CH2Cl2 (15 mL), thiourea/
silica gel (3 g) was added and the reaction mixture was stirred at
room temperature for 0.5–3 h. The reaction was monitored with GC
or TLC. After completion of the reaction, the mixture was filtered. The
organic layer was washed with water (20 mL) and dried over anhydrous
Na2SO4. Evaporation of the solvent followed by chromatography on a
short column of silica gel afforded the pure episulfide in 90–97% yield.
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