Melamine sulfonic acid: A recoverable catalyst for the ecofriendly synthesis of thiocyanohydrins under solvent-free conditions

Article abstract of DOI:10.1080/00397910903100734

Melamine sulfonic acid as a novel and efficient solid acid catalyst has been synthesized and used in the ring opening of epoxides under solvent-free conditions. This catalyst afforded the corresponding thiocyanohydrins with good regioselectivity and very short reaction times. The catalyst can be easily separated and reused several times without appreciable loss of activity. The availability and recoverability of the catalyst with easy operation and workup make this method attractive for organic synthesis. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910903100734

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Melamine Sulfonic Acid: A Recoverable
Catalyst for the Ecofriendly Synthesis of
Thiocyanohydrins Under Solvent-Free
Conditions
a
a
Ali Reza Kiasat & Mehdi Fallah-Mehrjardi
a
Chemistry Department, Faculty of Sciences, Shahid Chamran
University, Ahvaz, Iran
Published online: 23 Apr 2010.
To cite this article: Ali Reza Kiasat & Mehdi Fallah-Mehrjardi (2010) Melamine Sulfonic Acid:
A Recoverable Catalyst for the Ecofriendly Synthesis of Thiocyanohydrins Under Solvent-Free
Conditions, Synthetic Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 40:10, 1551-1558, DOI: 10.1080/00397910903100734
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1
Synthetic Communications , 40: 1551–1558, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903100734
MELAMINE SULFONIC ACID: A RECOVERABLE
CATALYST FOR THE ECOFRIENDLY SYNTHESIS
OF THIOCYANOHYDRINS UNDER SOLVENT-FREE
CONDITIONS
Ali Reza Kiasat and Mehdi Fallah-Mehrjardi
Chemistry Department, Faculty of Sciences, Shahid Chamran University,
Ahvaz, Iran
Melamine sulfonic acid as a novel and efficient solid acid catalyst has been synthesized and
used in the ring opening of epoxides under solvent-free conditions. This catalyst afforded the
corresponding thiocyanohydrins with good regioselectivity and very short reaction times.
The catalyst can be easily separated and reused several times without appreciable loss of
activity. The availability and recoverability of the catalyst with easy operation and workup
make this method attractive for organic synthesis.
Keywords: Melamine sulfonic acid; ring opening of epoxide; solid acid catalyst; thiocyanohydrin
Industrial chemists are widely adopting the concept of ‘‘green chemistry’’ to meet the
key scientific challenges of protecting human health and the environment while sim-
[
1]
ultaneously achieving commercial profitability. To realize this goal, in recent years,
significant articles have appeared reporting efficient solvent-free reactions by grind-
[
2]
ing. This technique has many advantages such as reduced pollution, low cost,
process simplicity, and easy workup. In addition, because of environmental
acceptability, recently more attention has been paid to the application of inorganic
[3]
acidic salts in organic synthesis. Solid acids have many advantages such as
simplicity in handling, decreased reactor and plant corrosion problems, and more
environmentally safe disposal. Also, wastes and by-products can be minimized or
avoided by using solid acids in developing cleaner synthetic routes.
The development of versatile and efficient methods for the synthesis of b-
hydroxy thiocyanates has still been an active area of research, because b-hydroxy
thiocyanates are important intermediates in agricultural biological and pharmaceu-
[4]
tical chemistry. As a suitable route to thiiranes, the reaction of epoxides with thio-
cyanate anion has been widely studied. It has been proved that this reaction is
[
5]
achieved thorough the b-hydroxy thiocyanate as an intermediate. For this
synthesis, it has been reported that the presence of some hydroquinone or
[
6]
Received April 19, 2009.
Address correspondence to Ali Reza Kiasat, Chemistry Department, Faculty of Sciences, Shahid
Chamran University, Ahvaz 61357-4-3169, Iran. E-mail: akiasat@scu.ac.ir
1
551

1552
A. R. KIASAT AND M. FALLAH-MEHRJARDI
Scheme 1. Preparation of MSA.
[
,3-dichloro-5,6-dicyanobenzoquinone (DDQ) is required to stabilize the produced
4]
2
b-hydroxy thiocyanate and to inhibit its conversion to thiirane. Although a few use-
ful reagents have been reported for the ring opening of epoxides to the thiocyanohy-
[
7–11]
drins,
some of these methods are not always fully satisfactory and suffer from
disadvantages such as relative poor yields, long reaction times, use of toxic solvents
or catalysts, high cost of reagents, poor regioselectivity, difficulty in workup, or
harsh reaction conditions. Therefore, it seems that there is still a need for the devel-
opment of novel methods that proceed under mild and economically appropriate
conditions.
[
12–14]
In continuation of our research on using water as a reaction medium
our interest in developing a true catalyst using inexpensive and nonpolluting
reagents,
and
[
14–17]
here we report the regioselective ring opening of epoxides with thio-
cyanate anion in the presence of melamine sulfonic acid (MSA) as catalyst at room
temperature under solvent-free conditions.
As described in Scheme 1, MSA, a novel and efficient solid acid, was easily pre-
pared by addition of chlorosulfonic acid to melamine under an N atmosphere at
2
room temperature. The reaction is very easy and clean, because the evolved HCl
gas can be removed from the reaction vessel immediately. This solid acid is not
soluble in water or common polar or nonpolar organic solvents.
To ascertain the possible catalytic property of this solid acid, the reaction of
, 3-epoxypropyl phenyl ether (1 mmol) with NH SCN (1.5 mmol) in the presence
4
2
of a catalytic amount of melamine sulfonic acid was studied under solvent-free con-
ditions. This reaction was completed after 3 min and the corresponding thiocyanohy-
drin was obtained, whereas in the absence of catalyst, this reaction was completed
after 10 min and corresponding thiirane was obtained as the only product (Scheme 2).
The results of the ring opening of 2,3-epoxypropyl phenyl ether in the presence
of MSA using various solvents are summarized in Table 1. The results clearly show
that although this reaction proceeded in refluxing acetonitrile, the best result was
Scheme 2. Ring opening of 2,3-epoxypropyl phenyl ether with and without catalyst.

SOLVENT-FREE SYNTHESIS OF THIOCYANOHYDRINS
1553
Table 1. Solvent optimization in the ring opening of 2,3-epoxypropyl phenyl ether (1 mmol)
4
by NH SCN (1.5 mmol) in the presence of MSA (0.3 mmol)
No.
Solvent=condition
Time (h)
Result
No reaction
1
2
3
4
5
6
7
8
CH
CH
2
Cl
Cl
2
2
=rt
=reflux
6
2
6
Thiocyanohydrin (60%)
Thiocyanohydrin (40%)
Thiocyanohydrin þ thiirane
Thiocyanohydrin (45%)
Thiocyanohydrin (85%)
Thiocyanohydrin þ thiirane
Thiocyanohydrin (94%)
H
H
2
O=rt
O=reflux
6
2
1
CH
CH
3
CN=rt
6
3
3
CN=reflux
PEG=rt
—=silica gel (0.5 g)
6
3 min
obtained under solvent-free conditions. Although the MSA can be stabilized the
produced b-hydroxy thiocyanate under solvent-free conditions and without using silica
gel, the presence of silica gel (0.5 g) for facile grinding of reaction media is necessary.
To show the general applicability of this method, we extended it to a variety of
epoxides (Scheme 3, Table 2), and in all cases very clean reactions were observed.
The structure of all the products were settled from their analytical and spectral
1
13
(IR, H NMR, and C NMR) data and by direct comparison with authentic
samples.
[
14]
The regioselectivity of the ring opening of epoxides is thoroughly dependent on
the mechanism of the reaction and particularly on steric and electronic factors.
1
13
According to H and C NMR spectra, unlike those obtained from epoxides carry-
ing electron-withdrawing groups [which gave the regioisomer (I) based on steric
effect], styrene oxide and 1,2-butene oxide gave regioisomer (II) according to more
important electronic effect. Furthermore, cycloalkyl epoxides were reacted smoothly
in an S 2 fashion with NH SCN in the presence of a catalytic amount of MSA to
N
4
afford the corresponding b-hydroxy thiocyanates in excellent isolated yields. The
stereochemistries of the ring-opening products were found to be trans from the coup-
1
ling constants of the ring protons in H NMR spectrums (Scheme 4).
By comparison, the obtained results for the conversion of 2,3-epoxypropyl
phenyl ether to the corresponding thiocyanohydrin in the presence of acidic catalysts
are given in Table 3. As shown in Table 3, MSA has greater efficiency and shorter
reaction time than other catalysts.
In conclusion, we have found that MSA can be easily prepared from inexpen-
sive and nontoxic materials and can serve as an excellent catalyst in the conversion of
a variety of epoxides carrying activated and deactivated groups into their corre-
sponding b-hydroxy thiocyanates under solvent-free conditions. The advantages of
the present protocol, such as environmental friendliness, short reaction times, sim-
plicity in operation, the low cost of catalyst, high regioselectivity and stereoselectivity,
Scheme 3. Ring opening of epoxides to the thiocyanohydrins.

1554
A. R. KIASAT AND M. FALLAH-MEHRJARDI
Table 2. MSA-catalyzed regioselective thiocyanation of epoxides
Time
(min)
Yields
b
(%)
a
No.
1
Substrate
Product(s)
c
2
3
88 (89:11)
c
2
3
85 (94:6)
3
94
4
5
6
3
3
3
85
90
88
7
2
95
8
9
3
2
85
90
1
0
1
10
86
84
1
4
a
Products were identified by comparison of their physical and spectral data with those of authentic
samples.
b
Isolated yields.
According to gas chromatographic (GC) analysis.
c

SOLVENT-FREE SYNTHESIS OF THIOCYANOHYDRINS
1555
Scheme 4. Stereoselectivity in ring opening of cycloalkyl epoxides.
Table 3. Comparison of catalytic ability of acidic catalysts in the thiocyanation of
2,3-epoxypropyl phenyl ether under solvent-free condition
No.
Catalyst
Time (min)
Yield (%)
94
1
2
3
4
5
6
7
8
MSA
SSA
3
10
8
[15]
88
85
[16]
Dowex
PEG-SO
3
H
5
—
[17]
92
B(HSO
Mg(HSO
Zn(HSO
Al(HSO
4
)
3
4
4
)
2
10
10
5
85
89
91
4 2
)
4 3
)
and excellent yields of products, make this new catalytic process as an attractive
alternative to current methodologies.
We recently reported the synthesis of thiocyanohydrins in the presence of
[
17]
poly(ethylene glycol) (PEG)-SO H as an acidic phase-transfer catalyst. Unfortu-
3
nately, the use of recycled PEG-SO H resulted in a substantial loss of its activity
3
as the yield of products dropped to a low value. The most ideal synthetic method-
ology could be defined as a system wherein 100% atom economy is preserved, the
catalyst is recycled, and excess of reagent remains throughout in the medium and
does not lose activity for several runs. In this procedure, ring opening of epoxides
with NH SCN in the presence of catalyst furnished the corresponding thiocyanohy-
4
drins in excellent yields. The catalyst, the additional ammonium thiocyanate
(0.5 mmol), and silica gel were conveniently separated from the reaction mixture
by simple filtration. The mixture was reused three consecutive times with only a
slight variation in the yields of the corresponding products. Table 4 demonstrates
the usability of the catalyst after three runs in thiocyanation of 2,3-epoxypropyl
phenyl ether. The facile nucleophilic ring opening of epoxides with ammonium
thiocyanate in the atom economical method is described in Scheme 5.
a
Table 4. Reusability of the catalyst
Experiment
Cycle
Time (min)
Yield (%)
1
2
3
1
2
3
3
3
3
94
93
90
a
All reactions carried out under similar conditions.

1556
A. R. KIASAT AND M. FALLAH-MEHRJARDI
Scheme 5. Facile nucleophilic ring opening of epoxides in the atom economical method.
EXPERIMENTAL
Preparation of MSA
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. Melamine (1.26 g, 10mmol) was charged in the flask, and
chlorosulfonic acid (3.5 g, ca. 2 mL, 30 mmol) in CH Cl (10 mL) was added dropwise
2
2
over a period of 30min at room temperature. HCl gas was evolved immediately. After
completion of the addition, the mixture was shaken for 2 h, while the residual HCl was
eliminated by suction. Then the mixture was washed with excess CH Cl to remove the
2
2
unreacted chlorosulfonic acid. Finally, a white solid powder was obtained (3.3 g, 90%).
The IR spectrum of MSA showed the broad OꢀH absorption in the region
ꢀ
1
ꢀ1
between 2600 and 3600cm , strong S=O absorptions between 1150 and 1200cm ,
ꢀ1
ꢀ1
strong SꢀO absorption at 650 cm , and medium C=N absorption at 1620 cm . This
ꢁ
solid acid leads to a decrease in the pH from 7 to 2 and decomposed at 350 C.
Typical Procedure for the Preparation of b-Hydroxy Thiocyanates in
the Presence of MSA Under Solvent-Free Conditions
A mortar was charged with epoxide (1 mmol), NH SCN (1.5 mmol), MSA
4
(0.3 mmol), and silica gel (0.5 g). The mixture was ground with a pestle for the time

SOLVENT-FREE SYNTHESIS OF THIOCYANOHYDRINS
1557
specified in Table 2. On completion of the reaction, the reaction mixture was poured
into ether (10 mL) and filtered to remove the solids. The solid residue was washed
with ether (2 ꢂ 5 mL). The organic layer was dried over anhydrous Na SO , and
2
4
solvent evaporated under reduced pressure to give the corresponding b-hydroxy
thiocyanates in excellent isolated yields. For styrene oxide and 1,2-butene oxide,
further purification was achieved by preparative thin-layer chromatography (TLC)
or by silica-gel column chromatography.
Representative Examples of Spectra Data of Thiocyanohydrin
ꢀ
1 1
3
-Phenoxy-2-hydroxypropyl thiocyanate (3). IR (neat): ¼ 2156 cm ; H
NMR (CDCl ): d ¼ 3.13–3.31 (2H, d), 3.78 (1H, s), 4.01–4.05 (2H, d), 4.29 (1H,
3
1
3
m), 6.95 (2H, m), 7.02 (1H, m), 7.22–7.34 (2H, m); C NMR (CDCl ): d ¼ 37.39,
3
6
8.06, 69.51, 112.99, 114.64, 121.32, 129.88, 158.49.
ꢀ
1 1
3
-Allyloxy-2-hydroxypropyl thiocyanate (6). IR (neat): ¼ 2155 cm ; H
NMR (CDCl ): d ¼ 3.04–3.24 (3H, m), 3.53 (2H, d), 4.01–4.08 (3H, m), 5.19–5.29
3
1
3
(
1
2H, m), 5.87 (1H, m); C NMR (CDCl ): d ¼ 37.35, 69.17, 71.09, 71.59, 113.07,
3
17.46, 133.66.
ꢀ
1
2
-Hydroxy-3-thiocyanatopropyl methacrylate (7). IR (neat): ¼ 2157 cm
;
1
H NMR (CDCl ): d ¼ 1.85 (3H, m), 3.01–3.18 (2H, d), 3.40 (1H, m), 4.12 (1H,
3
1
3
s), 4.13–4.17 (2H, m), 5.56 (1H, m), 6.07 (1H, m); C NMR (CDCl ): d ¼ 18.40,
3
3
7.34, 66.14, 68.07, 112.80, 126.56, 135.23, 167.06.
ꢀ
1
1
; H
3
-Chloro-2-hydroxypropyl thiocyanate (8). IR (neat): ¼ 2157 cm
NMR (CDCl ): d ¼ 1.27 (2H, d), 3.10–3.28 (2H, m), 3.64–3.84 (1H, m), 4.20
3
1
3
(
1H, s); C NMR (CDCl ): d ¼ 37.31, 47.11, 69.88, 112.01.
3
-Hydroxycyclohexyl thiocyanate (9). IR (neat): ¼ 2151 cmꢀ ; H NMR
1 1
2
(
3
CDCl ): d ¼ 1.21–1.28 (4H, m), 1.69 (2H, m), 1.98 (2H, m), 3.13–3.16 (2H, m),
3
1
3
.34 (2H, m); C NMR (CDCl ): d ¼ 23.14, 25.20, 30.04, 31.42, 51.51, 79.12, 110.60.
3
ACKNOWLEDGMENT
We gratefully acknowledge the support of this work by the Chamran
University Research Council.
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