A practical and eco-friendly method for conversion of epoxides to thiiranes with immobilized thiourea on CaCO3

Article abstract of DOI:10.1080/10426507.2011.583963

Solvent-free conversion of various epoxides to the corresponding thiiranes was carried out efficiently with immobilized thiourea on CaCO₃. The reactions were completed within 1-12 min under oil bath (60 °C-70 °C) conditions to afford thiiranes in 88%-98% yields. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426507.2011.583963

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Phosphorus, Sulfur, and Silicon and the
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A Practical and Eco-Friendly Method for
Conversion of Epoxides to Thiiranes with
Immobilized Thiourea on CaCO3
Behzad Zeynizadeh a , Mohammad Mehdi Baradarani a & Ronak Eisavi
a
a Department of Chemistry, Faculty of Science, Urmia University,
Urmia, Iran
Published online: 31 Oct 2011.
To cite this article: Behzad Zeynizadeh , Mohammad Mehdi Baradarani & Ronak Eisavi (2011): A
Practical and Eco-Friendly Method for Conversion of Epoxides to Thiiranes with Immobilized Thiourea
on CaCO3 , Phosphorus, Sulfur, and Silicon and the Related Elements, 186:11, 2208-2215
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Phosphorus, Sulfur, and Silicon, 186:2208–2215, 2011
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2011.583963
A PRACTICAL AND ECO-FRIENDLY METHOD
FOR CONVERSION OF EPOXIDES TO THIIRANES
WITH IMMOBILIZED THIOUREA ON CaCO3
Behzad Zeynizadeh, Mohammad Mehdi Baradarani,
and Ronak Eisavi
Department of Chemistry, Faculty of Science, Urmia University, Urmia, Iran
GRAPHICAL ABSTRACT
R
R'
R
R'
CaCO3/thiourea
Oil bath (60 oC−70 oC)
1−12 min, 88%−98%
O
S
Abstract Solvent-free conversion of various epoxides to the corresponding thiiranes was
carried out efficiently with immobilized thiourea on CaCO3. The reactions were completed
within 1–12 min under oil bath (60 ◦C–70 ◦C) conditions to afford thiiranes in 88%–98%
yields.
[Supplemental materials are available for this article. Go to the publisher’s online edition of
Phosphorus, Sulfur, and Silicon and the Related Elements for the following free supplemental
resource: Figures S1–S3.]
Keywords Epoxide; CaCO3; thiirane; thiourea; solvent free
INTRODUCTION
Synthetic chemists continue to explore new methods to carry out chemical transfor-
mations. One of these new methods is to run reactions on the surface of solids. Surfaces
have properties that are not duplicated in the solution or gas phase; hence entirely new
chemistry may occur.1 Even in the absence of new chemistry, a surface reaction may be
more desirable than a solution counterpart, because the reaction can be more convenient to
run or a higher yield of the product is attained. For these reasons, surface synthetic organic
chemistry is a rapidly growing field of study. In organic syntheses, increasing attention is
being focused on green chemistry,2 using environmentally benign reagents and conditions,
particularly solvent-free procedures,3 which often lead to clean, eco-friendly, and highly
efficient procedures involving simplified workups. The absence of solvent reduces the risk
of hazardous explosion when the reaction takes place in a closed vessel. Moreover, aprotic
Received 30 December 2010; accepted 21 April 2011.
The financial support of this work was gratefully acknowledged by the Research Council of Urmia University.
Address correspondence to B. Zeynizadeh, Department of Chemistry, Faculty of Science, Urmia University,
Urmia 57159-165, Iran. E-mail: bzeynizadeh@gmail.com
2208

CONVERSION OF EPOXIDES TO THIIRANES WITH CaCO3/THIOUREA SYSTEM
2209
dipolar solvents with high boiling points are expensive and are difficult to remove from
reaction mixtures.
Thiiranes are the simplest sulfur heterocycles and occur in nature. They have been
used advantageously in the pharmaceutical, polymer, pesticide, and herbicide industries.4
The general procedure for the synthesis of thiiranes is the conversion of epoxides by
an oxygen–sulfur exchange reaction. Sulfur transferring agents such as thiourea,5 in-
organic thiocyanates,6 silica-supported KSCN7 or (NH2)2CS,8 polymer-supported thio-
cyanates,9 and Dowex-50WX8-supported thiourea10 have been used for this purpose.
Conversion of epoxides to thiiranes with thiourea or NH4SCN in the presence of Lewis
acids or other promoters such as RuCl3,11 BiCl3,12 TiO2,13 TiCl3(OTf),14 Mg(HSO4)2,15
LiBF4,16 LiClO4,17 I2,18 montmorillonite K10,19 (NH4)8[CeW10O36]·20H2O,20 CAN,21
Sn(TTP)(OTf)2,22 [bmim]PF6,23 poly(4-vinylpyridine)-supported Ce(OTf)4,24 polystyrene-
supported AlCl3,25 etidronic acid,26 NH4Cl,27 HBF4/SiO2,28 and silica chloride29 have been
also reported. Although, a vast variety of reagents or methods have been reported for the
preparation of thiiranes from epoxides; however, most of these protocols suffer from some
disadvantages.
As a part of our research program to the synthesis of thiiranes, herein, we wish
to report a green and practical method for solvent-free conversion of epoxides to◦the
corresponding thiiranes by CaCO3-supported thiourea under oil bath conditions (60 C–
70 ◦C) (Scheme 1).
R
R'
R
R'
CaCO3/thiourea
Oil bath (60 oC−70 oC)
S
O
R, R': aryl, alkyl, allyl, H
Scheme 1
RESULTS AND DISCUSSION
A literature review shows that solvent-free conversion of epoxides to the correspond-
ing thiiranes has been rarely studied and this goal was achieved with thiourea at 120 ◦C5
and thiourea/SiO2 system.8 Nevertheless, these methods suffered from high reaction tem-
perature and moderate yields. In this context, we have reported Dowex-50WX8-supported
thiourea as an efficient polymeric supporting system for solvent-free conversion of epox-
ides to thiiranes.10 Lack of information related to the application of alkali and alkali metal
carbonates and sulfates in the preparation of thiiranes from epoxides encouraged us to
investigate the capability of some costless and eco-friendly reagents such as Na2CO3,
K2CO3, Cs2CO3, MgCO3, CaCO3, SrCO3, BaCO3, NaHCO3, KHCO3, NaHSO4, KHSO4,
and Na2SO4 for the titled transformation. Reaction of styrene oxide with the immobilized
thiourea on the carbonates and sulfates was carried out under solvent and solvent-free con-
ditions (Table 1). The results showed that among the reagents only CaCO3 presented the
perfect capability in the conversion of styrene oxide to styrene episulfide. In addition, the
rate enhancement and efficiency under solvent-free conditions was higher than the solution
phase.
In order to clarify the effect of Ca2+ or CO3 ions in the observed promotion, we
2−
also examined the oxygen–sulfur exchange reaction of styrene oxide with the immobilized

2210
B. ZEYNIZADEH ET AL.
Table 1 Optimization experiments for conversion of styrene oxide to styrene episulfide with thiourea under
different conditionsa
Molar
ratio
Time
(min)
Conversion
(%)
Reaction component
Solvent
Conditionb
Epoxide/thiourea/NaHCO3
Epoxide/thiourea/NaHCO3
Epoxide/thiourea/NaHSO4
Epoxide/thiourea/Na2SO4
Epoxide/thiourea/Na2CO3
Epoxide/thiourea/KHCO3
Epoxide/thiourea/KHSO4
Epoxide/thiourea/K2CO3
Epoxide/thiourea/BaCO3
Epoxide/thiourea/BaCO3
Epoxide/thiourea/CaCO3
Epoxide/thiourea/CaCO3
Epoxide/thiourea/CaCO3
Epoxide/thiourea/CaCO3
Epoxide/thiourea/CaCO3
Epoxide/thiourea/CaCO3
Epoxide/thiourea/CaCO3
Epoxide/thiourea/CaO
1:2:2
1:2:2
1:2:2
1:2:2
1:2:2
1:2:2
1:2:2
1:2:2
1:2:2
1:2:2
1:2:2
1:2:2
1:2:2
1:2:2
1:2:2
1:2:3
1:2:5
1:2:3
1:2:3
1:2:2
1:2:2
1:2:2
1:2:2
1:2:2
1:2:2
1:2:2
CH3CN
Reflux
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
Oil bath
Reflux
Reflux
Reflux
Reflux
r.t.
Oil bath
Oil bath
Oil bath
Reflux
r.t.
Reflux
r.t.
r.t.
Reflux
Oil bath
r.t.
60
50
50
50
30
50
60
30
30
120
90
120
35
4
20
30
0
50
0
25
5
0
40
70
95
96
100
100
100
100
100
30
20
5
10
40
40
55
20
15
Solvent free
Solvent free
Solvent free
Solvent free
Solvent free
Solvent free
Solvent free
Solvent free
CH3CN
CH3CN
THF
EtOH
Solvent free
Solvent free
Solvent free
solvent free
CH3CN
Solvent free
CH3CN
Solvent free
Solvent free
CH3CN
1
1
0.5
90
30
60
60
60
60
60
60
60
Epoxide/thiourea/CaO
Epoxide/thiourea/CaCl2
Epoxide/thiourea/CaCl2
Epoxide/thiourea/MgSO4
Epoxide/thiourea/MgSO4
Epoxide/thiourea/MgCO3
Epoxide/thiourea/Cs2CO3
Epoxide/thiourea/SrCO3
Solvent free
Solvent free
Solvent free
r.t.
aAll reactions were carried out with 1 mmol of epoxide. bTemperature of oil bath was 60 ◦C–70 ◦C.
thiourea on CaO and CaCl2 (Table 1). The experiments showed that calcium oxide and
chloride did not show any efficiency in solution or even under solvent-free conditions. On
the basis of these observations, we therefore concluded that the integrated form of Ca2+
2−
and CO3 ions as CaCO3 synergistically promoted the conversion of styrene oxide to
styrene episulfide. Moreover, the immobilization of thiourea (2 mmol) on CaCO3 (2 mmol)
was the requirement for the complete reaction of styrene oxide (1 mmol) within 1–4 min.
In addition, performing the reaction in an oil bath (60 ◦C–70 ◦C) exhibited the more rate
enhancement than room temperature conditions. The capability of thiourea/CaCO3 system
was further investigated by the reaction of activated, deactivated, and cyclic epoxides under
the optimized conditions. Table 2 shows the general trend and versatility of this synthetic
method. Generally, activated epoxides showed higher-rate enhancement than deactivated
◦
◦
ones; however, all reactions were carried out successfully in an oil bath (60 C–70 C)
at solvent-free conditions, and subsequently the corresponding thiiranes were obtained in
high to excellent yields within 1–12 min.
The benefits of the immobilized thiourea on CaCO3 was highlighted by comparison
◦
of our results with those of reported for thiourea/120 C,5 silica-gel-supported thiourea,8
Dowex-50WX8-supported thiourea,10 and thiourea/NH4Cl27 protocols (Table 3). A case
study shows that in viewpoints of availability and costless of the reagents, mild reaction

CONVERSION OF EPOXIDES TO THIIRANES WITH CaCO3/THIOUREA SYSTEM
2211
Table 2 Solvent-free conversion of epoxides to thiiranes with CaCO immobilized thioureaa
Molar ratio
epoxide/thiourea/
CaCO3
Time
(min)
Yield
(%)b
Epoxide
Thiirane
Reference
27
1:2:2
1:2:2
1
2
96
98
26
27
23
23
23
1:2:2
1:2:2
1:2:2
1:2:2
10
12
8
95
96
95
97
12
1:2:2
1:2:2
1
5
98
97
27
27
1:2:2
1:2:2
4
3
92
94
23
23
1:2:2
1:2:2
7
3
97
98
27
27
1:2:2
1:2:2
6
3
88
90
23
27
1:2:2
1:2:2
8
91
98
27
28
10
(Continued on next page)

2212
B. ZEYNIZADEH ET AL.
Table 2 Solvent-free conversion of epoxides to thiiranes with CaCO immobilized thioureaa (Continued)
Molar ratio
epoxide/thiourea/
CaCO3
Time
(min)
Yield
(%)b
Epoxide
Thiirane
Reference
28
1:2:2
2
97
1:2:2
1:2:2
2
3
90
95
28
28
aAll reactions were carried out in oil bath (60 ◦C–70 ◦C) at solvent-free conditions. bIsolated yields.
conditions, and the yield of thiiranes, the present method shows the more or comparable
efficiency than the other methods.
In conclusion, we have shown that various epoxides are easily and efficiently con-
verted to the corresponding thiiranes with thiourea immobilized on CaCO3 in oil bath
and under solvent-free conditions. The cheapness and easy preparation of thiourea/CaCO3
system, mild reaction conditions, high yield of thiiranes, simple workup procedure as well
as the benefits of solvent-free conditions make this method a useful addition to the present
methodologies.
EXPERIMENTAL
General
All reagents and substrates were purchased from commercial sources with the best
1
quality and were used without further purification. IR and H/13C NMR spectra were
recorded on Thermo Nicolet Nexus 670 FT-IR and 300 MHz Bruker Avance spectrometers,
respectively. The products were characterized by their spectra data and comparison with
the reported data in literature. TLC was applied for the purity determination of substrates,
products, and reaction monitoring over silica gel 60 F254 aluminum sheet.
Preparation of Immobilized Thiourea on CaCO3
A mixture of thiourea (0.152 g, 2 mmol) and CaCO3 (0.2 g, 2 mmol) were placed in
a mortar and then were thoroughly mixed for 2 min at room temperature to give CaCO3
immobilized thiourea (0.35 g, 76% w/w). FT-IR (νmax/cm−1, KBr): 3808, 3388, 3285, 3180,
2692, 2519, 2365, 1787, 1613, 1508, 1085, 858, 729, 633, 497. FT-IR spectrum of thiourea,
CaCO3, and CaCO3/thiourea system are provided as supplementary data.
General Procedure for Solvent-Free Conversion of Epoxides
to Thiiranes with Thiourea Immobilized on CaCO3
In an experimental tube equipped with a magnetic stirrer, the epoxide (1 mmol) and
CaCO3 immobilized thiourea (0.35 g, 76% w/w) were well mixed. The reaction mixture was

2213

2214
B. ZEYNIZADEH ET AL.
stirred and heated in an oil bath (60 ◦C–70 ◦C) for the appropriate time mentioned in Table 2.
The progress of the reaction was monitored by TLC. After completion of the reaction, simply
washing of the reaction mixture with an organic solvent and then evaporation of the filtrate
under reduced pressure afforded the pure thiirane in 88%–98% yield.
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