Facile conversion of epoxides to thiiranes with ammonium thiocyanate catalyzed with oxalic acid

Article abstract of DOI:10.1080/10426500307898

Epoxids are efficiently converted to the corresponding thiiranes by ammonium thiocyanate (NH₄SCN) in the presence of catalytic amounts of oxalic acid with excellent isolated yields under mild and nonaqueous reaction condition. This conversion performed under both conventional heating and microwave conditions. Distinct rate enhancement is observed under microwave irradiation.

Full text of DOI:10.1080/10426500307898

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Phosphorus, Sulfur, and Silicon and the Related
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Facile Conversion of Epoxides to Thiiranes with
Ammonium Thiocyanate Catalyzed with Oxalic Acid
Foad Kazemi a & Ali Reza Kiasat a
a Shahid Chamran University , Ahvaz, Iran
Published online: 27 Oct 2010.
To cite this article: Foad Kazemi & Ali Reza Kiasat (2003) Facile Conversion of Epoxides to Thiiranes with Ammonium
Thiocyanate Catalyzed with Oxalic Acid, Phosphorus, Sulfur, and Silicon and the Related Elements, 178:6, 1333-1337, DOI:
10.1080/10426500307898
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Phosphorus, Sulfur, and Silicon, 178:1333–1337, 2003
C
Copyright Taylor & Francis Inc.
ISSN: 1042-6507 print
DOI: 10.1080/10426500390200648
FACILE CONVERSION OF EPOXIDES TO
THIIRANES WITH AMMONIUM THIOCYANATE
CATALYZED WITH OXALIC ACID
Foad Kazemi and Ali Reza Kiasat
Shahid Chamran University, Ahvaz, Iran
(Received August 20, 2002; accepted October 24, 2002)
Epoxids are efficiently converted to the corresponding thiiranes by am-
monium thiocyanate (NH4SCN) in the presence of catalytic amounts
of oxalic acid with excellent isolated yields under mild and nonaque-
ous reaction conditions. This conversion performed under both conven-
tional heating and microwave conditions. Distinct rate enhancement is
observed under microwave irradiation.
Keywords: Ammonium thiocyanate; epoxide; microwave irradiation;
oxirane; thiirane
Thiiranes are probably the most interesting class of cyclic sulfides both
from a theoretical and synthetically point of view.1 They serve as use-
ful precursors for the synthesis of olefins2 and other functional group
moieties,3 and so their synthesis is of fundamental interest. Various
methods already have been proposed in the literature for the synthesis
of these heterocyclices.4–20 The most efficient route reported so far is
based on the conversion of oxiranes into the corresponding thiiranes
by an oxygen-sulfur exchange reaction. Sulfur introducing agents
such as inorganic thiocyanates,4,5 thiourea,5 phosphine sulfides,6 3-
methylbenzothiazol-2-thione,7 dimethylthioformamide,8 silica gel sup-
ported KSCN,9 and polymer supported thiocyanate10 have been used
for this purpose. Most of these reagents suffer from disadvantages such
as relatively long reaction times, low yields of the products, increase
in pH of the reaction media, requirement of aqueous reaction condi-
tions and drastic reaction conditions required to generate the thiolate
ion, so that accurate conditions control is necessary to avoid extensive
decomposition and/or polymerization of the formed thiirane.
Address correspondence to Foad Kazemi, Chemistry Department, College of Sciences,
Shahid Chamran University, Ahvaz, Iran.
1333

1334
F. Kazemi and A. R. Kiasat
Recently, interest in the development of effective catalysts for the
mild conversion of epoxide to thiirane has prompted reports of many
catalysts,11–19 which are effective in accomplishing this transformation.
In continuation of our studies for the conversion of oxiranes to
thiiranes,15–19 we now report conditions whereby various types of thi-
iranes can be conveniently synthesized in high isolated yields from the
corresponding oxiranes under mild nonaqueous reaction conditions by
ammonium thiocyanate in the presence of catalytic amounts of oxalic
acid.
RESULTS AND DISCUSSION
We examined the catalytic ability of oxalic acid for conversion of epox-
ides to thiiranes with ammonium thiocyanate in dry acetonitrile un-
der reflux conditions. This catalyst acted very efficiently and it was
observed that only 0.2 molar equivalent of the catalyst is required to
convert epoxides to their corresponding thiiranes (Eq. 1) in excellent
yields within 0.5–1.75 h.
The effects of other solvents such as CCl4, CH2Cl2, n-hexane, and
t-butanol also were studied, but in comparison with acetonitrile the re-
action times were longer and the yields were considerably lower. With
this approach, different types of epoxides are converted to the corre-
sponding thiiranes as exclusive and virtually pure products according
to TLC and 1HNMR. The obtained results are summarized in Table I.
The promoting effect of oxalic acid was definitely confirmed by reac-
tion of styrene oxide with ammonium thiocyanate under similar reac-
tion conditions, without adding oxalic acid. It is observed that in the ab-
sence of oxalic acid, ammonium thiocyanate was much less efficient and
GC and TLC analysis of the reaction mixture show only trace amount
of styrene sulfide as product.
Microwave heating and its application in organic chemistry for a va-
riety of reactions have been developed successfully, and in the past few
years there have been a tremendous amount of interest in this area. Re-
markable decrease in reaction times and, in some cases, clean reaction
and better yields have been reported with microwave irradiation.20–21
Therefore, it seemed interesting to complete this work by studying
the reactivity of this catalyst for the conversion of epoxides to their
corresponding thiiranes with ammonium thiocyanate in dry acetoni-
trile under microwave irradiation.

Facile Conversion of Epoxides to Thiiranes
1335
TABLE I Conversion of Epoxides to Thiiranesa with NH4SCN Catalyzed
with Oxalic Acid (0.2 eq) in Dry Acetonitrile
Thermal
Microwave
Entry
1
Substrate
Product
Time (min) Yield%b Time (min) Yield%b
80
95
3
93
2
3
105
60
93
87
8
5
92
94
4
5
45
30
89
90
3
4
90
89
aProducts were identified by comparison of their physical and spectral data with
those of authentic samples.
bIsolated yield.
As shown in Table I, we have found a distinct rate enhancement
under microwave conditions where reactions were completed in just
3–8 min as compared to a few hours (0.5–1.75 h) under conventional
heating. The experimental procedure is very simple and involves stir-
ring the solution of oxirane (1 mmol) in acetonitrile (5 ml) with am-
monium thiocyanate (2 mmol) and oxalic acid (0.2 mmol) under reflux
condition for a few hours or irradiating in a microwave oven for a few
minutes.
The advantage of using oxalic acid as catalyst for the synthesis of thi-
iranes using ammonium thiocyanate is shown by comparing our results
with those previously reported in the literature (Table II). Contrary to
the previous reports, our procedure, especially under microwave condi-
tions, provides good yields of thiiranes in a comparatively short reaction
time.
CONCLUSION
We believe that the present protocols offer mild, simple, and efficient
methods for the preparation of thiiranes from oxiranes. In addition,
availability of the reagent and catalyst, high yields and short reaction
times make this method a useful addition to the present methodologies.

1336
F. Kazemi and A. R. Kiasat
TABLE II Comparison of Some Results Obtained from the
Reaction of Epoxides with SCN in the Presence of Oxalic Acid with
Those Reported from Other Methods
Styrene Oxide
Phenyl Glycidyl Ether
Time/Cond./Yield
Cyclohexene Oxide
Time/Cond./Yield
Time/Cond./Yield
Method
(min)
(%)
(min)
(%)
(min)
(%)
I
3/MW/95
8/MW/92
4/MW/89
II
80/4/95
180/4/72
—
270/4/93
45/r.t/95
60/r.t/98
105/4/93
—
30/4/90
2400/r.t/73
1200/ /65
840/4/82
30/r.t/92
45/4/93
III
IV
V
VI
VII
—
900/4/85
20/r.t/95
90/4/95
I = The present method (microwave heating).
II = The present method (Conventional heating).
III = SCN anion has been used without a catalyst.4
IV = Silica gel has been used as catalyst.9
V = Polymer supported thiocyanate has been used.10
VI = CAN has been used as catalyst.15
VII = RuCl3 has been used as catalyst.6
EXPERIMENTAL
General
IR spectra were recorded on a Shimadzo 450 spectrophotometer,
1HNMR spectra in CDCl3 on a Bruker Avance DPX instrument
(250 MHz). Products were characterized by comparison of their physi-
cal data, IR, and 1H NMR spectra with known samples. All yields refer
to isolated products. The purity determination of the products and re-
action monitoring were accomplished by TLC on silica gel polygram
SILG/UV 254 plates. Microwave reactions were carried out in Butane
made domestic microwave oven operating at 200 watt.
General Procedure for the Conversion of Oxiranes to
Thiiranes Under Conventional Heating
In a round-bottom flask (25 ml) equipped with a condenser and a mag-
netic stirrer, a solution of epoxide (1 mmol) in dry CH3CN (5 ml) and
ammonium thiocyanate (0.152 g, 2 mmol) was prepared. Then, oxalic
acid (0.018 g, 0.2 mmol) was added to the solution and the reaction mix-
ture was stirred magnetically under reflux condition for the appropriate
time according to Table I. The progress of the reaction was monitored
by TLC. To the reaction mixture, 10 ml of water was added and the

Facile Conversion of Epoxides to Thiiranes
1337
mixture was extracted with CHCl3 (2 10 ml). Evaporation of the sol-
vent followed by chromatography on a short column of silica gel (elut:
CCl4) gave the pure products in 87–95% isolated yields.
General Procedure for the Conversion of Oxiranes to
Thiiranes Under Microwave Irradiation
Ammonium thiocyanate (0.152 g, 2 mmol), oxalic acid (0.018 g,
0.2 mmol), and 5 ml of dry CH3CN were placed in a Pyrex round-
bottomed flask (25 ml) equipped with a reflux condenser (reflux con-
denser was outside the microwave oven). Epoxide (1 mmol) was added
to the resulting mixture which was then irradiated in a commercial
microwave oven (200 W) for 3–8 min. The progress of the reaction was
monitored by TLC. To the reaction mixture, 10 ml of water was added
and the mixture was extracted with CHCl3 (2 10 ml). Evaporation of
the solvent followed by chromatography on a short column of silica-gel
(elut: CCl4) gave the pure products in 89–94% isolated yields.
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