Silica chloride-catalyzed synthesis of thiiranes from oxiranes under solvent-free conditions

Article abstract of DOI:10.1080/10426500903544298

A mild, simple, and efficient method for the synthesis of thiiranes from epoxides using a catalytic amount of silica chloride under solvent-free conditions has been developed. Experimental simplicity, simple work-up procedure, and solvent-free reaction conditions are important features of the present protocol. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426500903544298

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Phosphorus, Sulfur, and Silicon and the
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Silica Chloride–Catalyzed Synthesis of
Thiiranes from Oxiranes Under Solvent-
Free Conditions
Liqiang Wu a , Limin Yang a , Lizhen Fang a , Chunguang Yang a
Fulin Yan a
a School of Pharmacy , Xinxiang Medical University , Xinxiang,
Henan, China
&
Published online: 24 Sep 2010.
To cite this article: Liqiang Wu , Limin Yang , Lizhen Fang , Chunguang Yang & Fulin Yan (2010) Silica
Chloride–Catalyzed Synthesis of Thiiranes from Oxiranes Under Solvent-Free Conditions, Phosphorus,
Sulfur, and Silicon and the Related Elements, 185:10, 2159-2164, DOI: 10.1080/10426500903544298
To link to this article: http://dx.doi.org/10.1080/10426500903544298
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Phosphorus, Sulfur, and Silicon, 185:2159–2164, 2010
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500903544298
SILICA CHLORIDE–CATALYZED SYNTHESIS
OF THIIRANES FROM OXIRANES UNDER
SOLVENT-FREE CONDITIONS
Liqiang Wu, Limin Yang, Lizhen Fang, Chunguang Yang,
and Fulin Yan
School of Pharmacy, Xinxiang Medical University, Xinxiang, Henan, China
A mild, simple, and efficient method for the synthesis of thiiranes from epoxides using
a catalytic amount of silica chloride under solvent-free conditions has been developed.
Experimental simplicity, simple work-up procedure, and solvent-free reaction conditions are
important features of the present protocol.
Keywords Oxirane; silica chloride; solvent-free; thiirane
INTRODUCTION
Thiiranes are sulfur heterocycles that are used in the pharmaceutical, polymer, pes-
ticide, and herbicide industries1 and have played an important role in various synthetic
transformations.2 Various methods have been developed for the preparation of these com-
pounds. The most common and important method is the transformation of oxiranes into
thiiranes with thiourea,3 inorganic thiocyanates,4 Dowex-50WX8–supported thiourea,5
and polymer-supported thiocyanates.6 Along with each sulfurated reagent, a protic acid
such as Mg(HSO4)2,3g oxalic acid,4k or SiO2-HBF4 or a Lewis acid such as NH4Cl,3a
4d
Al(DS)3·3H2O,3b SiO2-AlCl3,3c LiBF4,3h RuCl3,3i SbCl3,3j Montmorillonite K-10,3k I2,4a
LiClO4,4b SnIV(TPP)(BF4)2,4c 2,4,6-trichloro-1,3,5-triazine,4f TiO2,4g SnIV(TPP)(OTf)2,4h
InBr3,4l Bi(TFA)3, Bi(OTf)3,4m BiCl3,4n TiO(CF3CO2)24o, and TiCl3(CF3SO3)4o is em-
4m
ployed. Recently, ionic liquid7 and MW irradiation8 have also been applied to the conversion
of oxiranes into thiiranes. However, long reaction times, high temperature, low yields of the
products, use of organic solvents, use of highly acidic catalyst, difficulties in the separation
of the product from the original reactant and catalyst, formation of polymeric byproducts,
high catalytic loading, and, in most of the cases, the use of expensive and unrecoverable
catalyst are all limitations of these methods.
Silica chloride (SiO2-Cl), which is easily prepared from silica gel and thionyl chloride,
is a good solid acid in terms of convenience, low cost, easy production, and insolubility
in all organic solvents. Due to its insolubility in organic solvents, excellent in situ proton
Received 10 October 2009; accepted 7 December 2009.
We are pleased to acknowledge the financial support from Xinxiang Medical University.
Address correspondence to Liqiang Wu, School of Pharmacy, Xinxiang Medical University, Xinxiang,
Henan 453003, China. E-mail: wliq1974@sohu.com
2159

2160
L. WU ET AL.
generation, and its acceptance of different nucleophiles, it can be used for different purposes
in organic chemistry, such as SiO2-Cl–catalyzed protection of carbonyl compounds9a;
acetylation of alcohols9b; synthesis of 14-substituted-14H-dibenzo[a,j]xanthenes,9c 1,8-
dioxo-octahydroxanthenes,9c and 2-aminothiazoles9d; and SiO2-Cl as a starting material
for the preparation of other silica-bonded reagents.9e In this article, we report a simple,
efficient, and environmentally benign synthesis of thiiranes under solvent-free conditions
in the presence of SiO2-Cl (see Scheme 1).
SiO2-Cl
neat, r.t.
O
S
+
NH4SCN
R
R
3
1
2
Scheme 1
RESULTS AND DISCUSSION
Initially, a systematic study was carried out for evaluation for SiO2-Cl as a catalyst
for the reaction of styrene oxide with ammonium thiocyanate under various conditions
(Table I). The reaction was slow in the absence of the catalyst (Table I, entry 1), and low
yields were obtained in presence of only SiO2 as a catalyst (Table I, entries 2–4). Further,
improvement of the yield was observed in the presence of SiO2-Cl as a catalyst (Table I,
entries 5). Next, we optimized the quantity of the catalyst (SiO2-Cl) at room temperature
under solvent-free conditions (Table I, entries 5–10), and it was observed that the use of
just 100 mg/mmol of SiO2-Cl was sufficient to produce an excellent yield of the product
(Table I, entry 7), whereas more than 100 mg/mmol of the catalyst did not improve the
results (Table I, entries 8–10). However, inferior results were obtained in the presence of
solvents (Table I, entries 11–13).
Table I Reaction of styrene oxide with ammonium thiocyanate under various conditionsa
Entry
Solvent
Catalyst (mg/mmol)
Time (min)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
Neat
Neat
Neat
Neat
Neat
Neat
Neat
Neat
Neat
Neat
THF
CHCl3
CH3CN
—
360
240
240
240
60
45
30
30
30
30
45
45
45
< 5
12
19
31
62
76
96
96
95
94
68
56
64
SiO2 (20 mg)
SiO2 (50 mg)
SiO2 (100 mg)
SiO2-Cl (50 mg)
SiO2-Cl (75 mg)
SiO2-Cl (100 mg)
SiO2-Cl (125mg)
SiO2-Cl (150 mg)
SiO2-Cl (175mg)
SiO2-Cl (100 mg)
SiO2-Cl (100 mg)
SiO2-Cl (100 mg)
aReaction conditions: 2-phenyloxirane (1 mmol); ammonium thiocyanate (1.2 mmol); room temperature.
bIsolated yield.

SILICA CHLORIDE–CATALYZED SYNTHESIS OF THIIRANES
2161
Based on the optimized reaction conditions, a range of thiiranes was synthesized
by the reaction of oxiranes (1, 1 mmol) with ammonium thiocyanate (2, 1.2 mmol) under
solvent-free conditions. The reaction proceeded at room temperature within 1 h in excellent
yields after the addition of SiO2-Cl (100 mg/mmol) as catalyst (see Table II). In these
Table II SiO2-Cl catalyzed conversion of epoxides to thiiranes at room temperaturea
Entry
a
Epoxide
Product
Time (min)
45
Yield (%)b
Reference
10
96 (89, 79, 71)c
b
c
45
45
98
92
11
11
d
e
30
30
91
94
4a
4a
f
30
89
g
h
i
30
30
60
92
90
94
4p
4p
10
j
60
60
92
87
10
k
aReaction conditions: epoxide (1 mmol); ammonium thiocyanate (1.2 mmol); SiO2-Cl (100 mg); room tem-
perature; neat.
bIsolated yield.
cYields after three times of catalyst recovery.

2162
L. WU ET AL.
experiments, the catalyst was isolated by filtration and could be run up to three times
(recycling) without significant loss of activity.
The plausible mechanism of the reaction is shown in Scheme 2. The Si-Cl bond of
SiO2-Cl is labile and can give rise to Lewis acid centers on silica, so SiO2-Cl can act as a
catalyst for the activation of epoxide.4a
O
SiO2
SiO2
R
O
NH4SCN
O
S C N
Cl
S
+
SiO2
R
R
N
N
S
O
O
S
R
R
R
Scheme 2
CONCLUSION
1n conclusion, this article clearly demonstrates the synthesis of thiiranes from oxi-
ranes using a catalytic amount of SiO2-Cl (100 mg/mmol). It is a valid alternative to the
existing methods. The important features of the present method are mild and solvent-free re-
action conditions, fast reaction rates, high yields of the products, relatively clean reactions,
and an inexpensive and easily available catalyst with low loading.
EXPERIMENTAL
NMR spectra were determined on Bruker AV-400 spectrometer at room temperature
using TMS as internal standard; coupling constants (J) were measured in Hz. Elemental
analyses were performed by a Vario-III elemental analyzer. Melting points were determined
on a XT-4 binocular microscope and were uncorrected. SiO2-Cl was prepared according
to the literature.9e Commercially available reagents were used throughout without further
purification unless otherwise stated,
Typical Procedure: Preparation of Thiiranes
A mixture of epoxide (1 mmol), ammonium thiocyanate (1.2 mmol), and SiO2-Cl
(100 mg) was stirred at room temperature for the specified time (Table I). After completion
of the reaction (TLC), diethyl ether (2 × 10 mL) was added in the reaction mixture, and
the solid catalyst was filtered off. The solvent was evaporated, and the crude product was
purified by column chromatography (petroleum ether:ethyl acetate = 9:1) to afford the pure
1
product 3. All products were characterized by comparison of their physical data and H
NMR, 13C NMR data with those of authentic samples.4a,4p,10,11 The spectral data of some
new thiiranes are given below:
2-((4-Iodophenoxy)methyl)thiirane (3f)
Oil, 1H NMR (CDCl3, 400 MHz): δ = 7.20–6.79 (m, 4H, ArH), 4.10–3.90 (m, 2H,
OCH2), 3.26–3.22 (m, 1H, CH), 2.55–2.26 (m, 2H, CH2S). 13C NMR (CDCl3, 100 MHz):

SILICA CHLORIDE–CATALYZED SYNTHESIS OF THIIRANES
2163
δ = 155.4, 130.1, 124.9, 117.6, 74.8, 32.2, 22.8. Anal. calcd. for C9H9IOS: C 37.00, H
3.11, S 10.98; found C 37.18, H 3.02, S 11.09.
2-(4-Fluorophenyl)-3-phenylthiirane (3k)
Oil, 1H NMR (CDCl3, 400 MHz): δ = 7.70 (d, J = 8.4 Hz, 2H, ArH), 7.58 (d, J =
8.4 Hz, 2H, ArH), 7.26–7.18 (m, 5H, ArH), 2.71 (d, J = 7.2 Hz, 1H, CH), 2.42 (d, J =
7.2 Hz, 1H, CH). 13C NMR (CDCl3, 100 MHz): δ = 159.6, 138.2, 135.2, 130.8, 130.7,
128.5, 128.3, 125.9, 115.6, 115.5, 45.6. Anal. calcd. for C14H11FS: C 73.01, H 4.81, S
13.92; found C 73.14, H 4.96, S 13.76.
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