An efficient catalyst-free synthesis of thiiranes from oxiranes using polyethylene glycol as the reaction medium

Article abstract of DOI:10.1016/j.tetlet.2006.09.153

A catalyst-free, high-yielding conversion of oxiranes into thiiranes has efficiently been carried out by treatment with KSCN or thiourea at room temperature using polyethylene glycol (PEG-400) as the reaction medium.

Full text of DOI:10.1016/j.tetlet.2006.09.153

Tetrahedron Letters 47 (2006) 8471–8473
An efficient catalyst-free synthesis of thiiranes from oxiranes
using polyethylene glycol as the reaction mediumI
Biswanath Das,* V. Saidi Reddy and M. Krishnaiah
Organic Chemistry Division-I, Indian Institute of Chemical Technology, Hyderabad 500 007, India
Received 26 August 2006; revised 20 September 2006; accepted 28 September 2006
Abstract—A catalyst-free, high-yielding conversion of oxiranes into thiiranes has efficiently been carried out by treatment with
KSCN or thiourea at room temperature using polyethylene glycol (PEG-400) as the reaction medium.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Thiiranes are frequently used in pharmaceutical, pesti-
cide and polymer industries.1 Various methods have
been developed for the preparation of these compounds2
with the most general being the conversion of oxiranes
into thiiranes. For this purpose, several sulfur-introduc-
ing agents including inorganic thiocyanates,3a–e 3-meth-
ylbenzothiozole-2-thione,3f dimethylthioformamide,3g
thiourea3h,i and phosphine sulfide3j have been utilized.
Along with each sulfurated reagent, a protic acid such
as TFA3f,g,j or a Lewis acid such as RuCl3,3b BiCl3,3c
A series of thiiranes were prepared from several oxiranes
(Table 1) by following the above method. No additional
catalyst was required and the conversion was complete
within 45 min. Different types of oxiranes such as sty-
rene oxides and aliphatic and bicyclic oxiranes under-
went the conversion smoothly. A chiral oxirane (Table
1, entry 1) was also converted into the corresponding
thiirane with inversion of configuration. The products
were formed in excellent yields and no side products
were detected. In the absence of PEG, the conversion
of oxiranes under neat reaction conditions was very slow
and the yields were also very low (12–16% in 24 h). In
the present study, both KSCN and (NH2)2CS showed
a similar activity as the reaction times were the same
(45 min) and the yields were very similar. The structures
of the products were established from their spectral (1H
NMR and MS) data.
3e
TiO(CF3COO)2,3d TiCl3(CF3SO3)3d or InBr3 is em-
ployed. Recently, an ionic liquid has also been applied
to the conversion of oxiranes into thiiranes.4
In continuation of our work5 on the development of use-
ful synthetic methodologies, we have recently observed
that oxiranes can conveniently be converted into thiir-
anes by treatment with KSCN or (NH2)2CS in polyeth-
ylene glycol (PEG-400) at room temperature (Scheme 1).
Polyethylene glycol (PEG-400)6 has been applied here
as an efficient reaction medium for the preparation of
thiiranes. It is a biologically acceptable inexpensive
polymer and is eco-friendly. Its applications as a reac-
tion medium in organic syntheses have not yet been
fully explored. In the present conversion, the role of
PEG is possibly to activate the oxiranes through
hydrogen bonding. The PEG was recovered from
the reaction mixture and recycled without loss of
activity.
O
S
KSCN or (NH2)2CS
PEG
r. t., 45 min
R
R
2
1
89-95%
Scheme 1.
In conclusion, we have developed a simple and efficient
protocol for the synthesis of thiiranes from oxiranes by
treatment with KSCN or (NH2)2CS using PEG at room
temperature. The mildness and eco-friendly nature of
the synthesis, catalyst-free conversion, short reaction
Keywords: Oxirane; Thiirane; KSCN; Thiourea; PEG-400.
q Part 112 in the series, ‘Studies on novel synthetic methodologies’.
IICT Communication No. 060918.
*
Corresponding author. Tel./fax: +91 40 7160512; e-mail:
biswanathdas@yahoo.com
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.09.153

8472
B. Das et al. / Tetrahedron Letters 47 (2006) 8471–8473
Table 1. Preparation of thiiranes from oxiranes using KSCN and PEGa
Entry
a
Oxirane 1
Thiirane 2
Isolated yield (%)
95
O
S
O
S
b
c
d
e
f
90
92
95
91
89
Cl
Cl
O
S
O
S
MeO
Cl
MeO
Cl
O
O
S
O
O
O
S
O
MeO
MeO
S
O
O
O
g
92
94
O
S
O
O
h
MeO
MeO
O
S
i
90
89
92
S
O
O
O
j
O2N
O2N
O
S
k
O
O
O
l
91
S
S
O
m
n
94
93
S
O
a The structures of the thiiranes were determined from their spectral (1H NMR and MS) data.
time and excellent yields are notable advantages of the
protocol.
(NH2)2CS (1.2 mmol) was added and the mixture was
stirred at room temperature for 45 min (TLC showed
the completion of the reaction). The mixture was poured
onto water (10 mL) and extracted with EtOAc
(3 · 10 mL). The combined extract was concentrated
General experimental procedure: To a stirred suspension
of an oxirane (1 mmol) in PEG-400 (2 g), KSCN or

B. Das et al. / Tetrahedron Letters 47 (2006) 8471–8473
8473
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tography (silica gel, hexane–EtOAc, 9:1) to afford pure
thiirane.
Acknowledgements
The authors thank CSIR, New Delhi, for financial
assistance.
References and notes
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