Al(HSO4)3/silica gel as a novel catalytic system for the ring opening of epoxides with thiocyanate anion under solvent-free conditions

Article abstract of DOI:10.1016/j.cclet.2009.10.030

For the first time, metal hydrogen sulfates and phosphates/silica gel have been studied as efficient and powerful solid acid catalysts in the ring opening of epoxides with thiocyanate anion. The most significant result was obtained by Al(HSO₄)₃/SiO₂ which afforded the corresponding β-hydroxy thiocyanates under mild reaction conditions and in very short reaction times. The cheapness, availability of the catalyst, ease of procedure and work-up make this method attractive for the organic synthesis.

Full text of DOI:10.1016/j.cclet.2009.10.030

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Chinese Chemical Letters 21 (2010) 146–150
www.elsevier.com/locate/cclet
Al(HSO4)3/silica gel as a novel catalytic system for the ring opening
of epoxides with thiocyanate anion under solvent-free conditions
*
Ali Reza Kiasat , Arash Mouradzadegun, Somayeh Elahi, Mehdi Fallah-Mehrjardi
Department of Chemistry, College of Science, Shahid Chamran University, Ahvaz, Iran
Received 15 June 2009
Abstract
For the first time, metal hydrogen sulfates and phosphates/silica gel have been studied as efficient and powerful solid acid
catalysts in the ring opening of epoxides with thiocyanate anion. The most significant result was obtained by Al(HSO4)3/SiO2 which
afforded the corresponding b-hydroxy thiocyanates under mild reaction conditions and in very short reaction times. The cheapness,
availability of the catalyst, ease of procedure and work-up make this method attractive for the organic synthesis.
# 2009 Ali Reza Kiasat. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Metal hydrogen sulfates; Solid acid catalysts; Ring opening of epoxides; b-Hydroxy thiocyanates
Acid catalysts are of fundamental importance because of their ubiquity [1]. In view of green chemistry, the
substitution of harmful liquid acids by reusable solid acids as catalysts in organic synthesis is the most promising
application [2]. Recently, solid acids as environmentally friendly and economically viable catalyst, are increasing
continuously owing to their ease of handling, low cost, high catalytic activity, process simplicity and comparably easy
work-up [3]. In addition, solid acids, in many cases, show better selectivity than mineral acids toward a specific isomer
in a mixture of reaction products [4]. Finally, supported solid acid provides more effective catalytic reactivity by their
much extended surface area [5].
The development of versatile and efficient methods for the synthesis of b-hydroxy thiocyanates continues to be an
active area of research, because they play an important role in the realm of organic chemistry, such as biological,
pharmaceutical and agricultural chemistry [6]. From a fundamental viewpoint, ring opening of epoxides by
thiocyanate anion is a well-established route to thiiranes, achieved commonly through the corresponding b-hydroxy
thiocyanates as intermediates [7,8]. It has been reported that a portion of hydroquinone [9] or DDQ [6] is required to
stabilize the produced b-hydroxy thiocyanate, and to inhibit conversion to the respective thiirane. Although a few
useful reagents reported for the ring opening of epoxide to the thiocyanohydrin [10–14], however, to the best of our
knowledge, there have been no previous studies on the use of metal hydrogen sulfates (M(HSO4)n) and metal
dihydrogen phosphates (M(H2PO4)n) as effective catalysts for regioselective ring opening of epoxides in the literature.
Looking for ‘‘cleaner’’ alternatives to classical syntheses [7,15] and in developing green chemistry [16–18] and
also in pursuing our new interest in developing inexpensive and non polluting reagents [18–21], we report herein
* Corresponding author.
E-mail address: akiasat@scu.ac.ir (A.R. Kiasat).
1001-8417/$ – see front matter # 2009 Ali Reza Kiasat. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2009.10.030

A.R. Kiasat et al. / Chinese Chemical Letters 21 (2010) 146–150
147
studies of M(HSO4)n/SiO2 and M(H2PO4)n/SiO2 as efficient catalysts for the ring opening of epoxides with
thiocyanate anion at room temperature under solvent-free conditions.
1. Experimental
1.1. Preparation of metal hydrogen sulfates and phosphates supported on silica gel
A 50 mL suction flask was equipped with a constant pressure dropping funnel. The gas outlet was connected to a
vacuum system through an adsorbing solution (water) and an alkali trap. Anhydrous metal chlorides (10 mmol) were
charged in the flask and appropriate amount of concentrated sulfuric acid or phosphoric acid (20 mmol for MgCl2 and
ZnCl2 and 30 mmol for AlCl3) was added dropwise over a period of 45 min at room temperature. HCl evolved
immediately. After completion of the addition, the mixture was shaken for 30 min, while the residual HCl was
eliminated by suction. Finally, the obtained pale-brown solid materials were ground with silica gel (weight ratio of
silica gel to obtained catalyst was 2:1) in an agate mortar to form an intimate mixture.
1.2. Typical procedure for the preparation of b-hydroxy thiocyanates catalyzed by Al(HSO4)3/SiO2 under solvent-
free conditions
A mortar was charged with epoxide (1 mmol), NH4SCN (1.2 mmol) and Al(HSO4)3/SiO2 (0.3 mmol base on
catalyst). The mixture was ground with a pestle until the TLC assay showed complete disappearance of the starting
material. On completion of the reaction, the reaction mixture was poured into ethyl acetate (10 mL) and filtered to
remove the solids. The solid residue was then washed with ethyl acetate (5 mL) and the filtrate washed with water
(20 mL). The organic layer was dried over anhydrous sodium sulfate and solvent evaporated under reduced pressure to
give the corresponding b-hydroxy thiocyanates in excellent isolated yields. For styrene oxide, further purification was
achieved by preparative TLC or by silica gel column chromatography.
2. Results and discussion
Metal hydrogen sulfates and phosphates were easily prepared from the corresponding anhydrous metal chlorides
and sulfuric or phosphoric acid, respectively [22,23]. The reactions are very clean and did not require a work-up
procedure because HCl evolves as a by-product from the reaction vessel immediately (Scheme 1).
Initially we attempt to investigate the possible catalytic properties of these solid acids (I–VI) with simpler systems
such as phenyl glycidyl ether (1 mmol) with NH4SCN (2 mmol). Under solvent-free conditions, comparative results
led us to introduce Al(HSO4)3/SiO2 as the most effective system (Table 1).
Encouraged by this observation, further experiments were designed to optimizing this conversion in the presence of
Al(HSO4)3/SiO2. To determine the relative molar ratio of reactants and catalyst we arranged a systematic experiment.
The best results were obtained when 0.3 mmol of catalyst and 1.2 mmol of reactant were used (Table 2).
The effect of the solvent on the performance of promoted catalyst was also investigated (Table 3). The results
clearly revealed that, although ethyl acetate and acetonitrile represented a good compromise in reflux conditions, yet
the best results were obtained under solvent-free conditions.
The generality of this process was explored with respect to various epoxides (Table 4). In all cases, very clean
reactions were observed and the structure of all the products was determined from spectral assignments (IR, 1H and
13C NMR) through comparisons with data from authentic samples [18]. As shown in Table 4, cyclohexene oxide as a
cycloalkyl epoxide reacted smoothly in a SN2 fashion with NH4SCN in the presence of catalytic amount of
Scheme 1.

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Table 1
Comparison of catalytic ability of catalysts (I–VI)
No.
Catalyst
Time (min)
Product (yield, %)
1
2
3
4
5
6
Al(HSO4)3/SiO2
Al(H2PO4)3/SiO2
Mg(HSO4)2/SiO2
Mg(H2PO4)2/SiO2
Zn(HSO4)2/SiO2
Zn(H2PO4)2/SiO2
5
10
10
10
10
10
Thiocyanohydrin (91)
Thiocyanohydrin (90)
Thiocyanohydrin (88)
Thiocyanohydrin (60) + thiirane (21)
Thiocyanohydrin (90)
Thiocyanohydrin (92)
Table 2
Optimization of reaction conditions for ring opening of 1 mmol of phenyl glycidyl ether under solvent-free conditions (RT).
No.
Al(HSO4)3/SiO2 (mmol)
NH4SCN (mmol)
Time (min)
Result (yield, %)
1
2
3
4
5
6
7
0.5
0.5
0.5
0.4
0.3
0.2
0.1
1.5
1.2
1
3
3
Thiocyanohydrin (90)
Thiocyanohydrin (92)
10
4
Thiocyanohydrin (86)
1.2
1.2
1.2
1.2
Thiocyanohydrin (89)
5
Thiocyanohydrin (91)
8
Thiocyanohydrin (55) + thiirane (20)
Thiocyanohydrin (46) + thiirane (40)
11
Table 3
Solvent optimization in the ring opening of phenyl glycidyl ether (1 mmol) by NH4SCN (1.2 mmol) in the presence of Al(HSO4)3/SiO2 (0.3 mmol).
No.
Solvent/condition
Time (h)
Result (yield, %)
1
2
3
4
5
6
Ethyl acetate/r.t.
Ethyl acetate/ref.
Acetonitrile/r.t.
Acetonitrile/ref.
n-Hexane/r.t.
2
1
2
1
1
1
Thiocyanohydrin (40)
Thiocyanohydrin (73)
Thiocyanohydrin (37)
Thiocyanohydrin (88)
Thiocyanohydrin (51) + polymer
Thiocyanohydrin (22) + polymer
Dichloromethane/r.t.
Table 4
Reaction of various epoxides with NH4SCN in the presence of Al(HSO4)3/SiO2
No.
1
Substrate
Product(s)a
Time (min)
Yields (%)b
96
5
5
6
2
3
91
96

A.R. Kiasat et al. / Chinese Chemical Letters 21 (2010) 146–150
149
Table 4 (Continued )
No.
4
Substrate
Product(s)a
Time (min)
5
Yields (%)b
96
5
6
6
5
96
96
7
5
95
8
9
6
97
94
10
10
5
95
a
Products were identified by comparing their physical and spectral data with those of authentic samples.
Isolated yield.
b
Al(HSO4)3/SiO2 to afford the corresponding b-hydroxy thiocyanate in excellent yield. The stereochemistry of the ring
opening product was found to be in the trans-configuration as determined from the coupling constants associated with
the 1H NMR spectral resonances of the ring protons in. It is also noteworthy to mention that various functionalities can
change the regioselectivity of the ring opening of epoxides, dependent on the mechanism of the reaction and
particularly on steric and electronic factors. According to 1H and 13C NMR spectra, in our experimental conditions,
unlike those obtained from aliphatic epoxides which gave regioisomer I due to steric forces, styrene oxide gave
regioisomer II in agreement with more important electronic effect.
In summary, we have described an efficient, simple and convenient protocol for the ring opening of certain epoxides
using Al(HSO4)3/SiO2 as a catalytic system. In addition to its efficiency, simplicity and mild reaction conditions, this
procedure provides high yields of b-hydroxy thiocyanates in short reaction times with high selectivity. The use of
inexpensive Al(HSO4)3/SiO2 catalytic system makes this process quite simple, more convenient, cost-effective and
environmentally friendly.
Acknowledgment
The authors are thankful to the Shahid Chamran University Research Council for partial financial support of this
work.
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