Sulfamic acid as a recyclable and green catalyst for rapid and highly efficient synthesis of 2-arylbenzothiazoles in water at room temperature

Article abstract of DOI:10.1007/s13738-011-0059-y

2-Arylbenzothiazoles have been synthesized in water in good to excellent yield from direct condensation of various aromatic aldehydes with 2-aminothiophenol promoted by catalytic amount of sulfamic acid at room temperature. This method provides a simple,

Full text of DOI:10.1007/s13738-011-0059-y

J IRAN CHEM SOC (2012) 9:489–493
DOI 10.1007/s13738-011-0059-y
ORIGINAL PAPER
Sulfamic acid as a recyclable and green catalyst for rapid
and highly efficient synthesis of 2-arylbenzothiazoles
in water at room temperature
•
A. Rostami A. Yari
Received: 8 July 2011 / Accepted: 2 December 2011 / Published online: 19 January 2012
Ó Iranian Chemical Society 2012
Abstract 2-Arylbenzothiazoles have been synthesized in
water in good to excellent yield from direct condensation
of various aromatic aldehydes with 2-aminothiophenol
promoted by catalytic amount of sulfamic acid at room
temperature. This method provides a simple, rapid and
efficient protocol in terms of mild reaction conditions,
clean reaction profiles, small quantity of catalyst and
simple work-up procedure.
(2) Jacobson’s cyclization of thiobenzanilides [24].
Although these methods produce good results in many
instances, there is still a demand to devolve new mild
method for synthesis of benzthiazoles in the presence of
cheap and bench top catalyst.
The use of water as a sole medium of organic reactions
would greatly contribute to the development of environ-
mentally friendly processes. In recent years much attention
has been focused on acid-catalyzed organic reactions in
water and several reactions of this type have been already
identified [25].
Keywords Condensation reaction ꢀ Sulfamic acid ꢀ
Catalyst ꢀ 2-Arylbenzothiazoles
Experimental
Introduction
General
The exploration of privileged structures in drug discovery
is rapidly emerging theme in medicinal chemistry. Ben-
zothiazole and its derivatives belong to such immensely
important class of heterocyclic systems, owing to their
potent antitumor activity [1] and other important pharma-
ceutical utilities [2]. Also, they have been widely used in
industry as antioxidants and vulcanization accelerators that
highlight their synthetic necessity [3]. Numerous methods
have been reported in the literature for the synthesis of
benzothiazoles. The most popular approaches generally
involve: (1) condensation of 2-aminothiophenols with
carboxylic acids and their derivatives [4–6] or condensa-
tion with aldehydes under oxidative conditions [7–23] and
Chemicals including starting materials, solvents and sul-
famic acid were obtained from Merck chemical company
and used without purification. Melting points were
obtained in open capillary tubes and also were measured on
the Electrothermal 9100 apparatus. FT-IR spectra were
recorded on a Perkin Elmer RXI spectrometer. NMR
spectra were recorded on a Bruker Avance DPX 250 MHz
instrument. Chemical shifts (d) are given in ppm and
coupling constants (J) in Hz. Deuterated solvents were
used as lock and reference. Thin layer chromatography
(TLC) was performed on SIL G/UV 254 plates.
General procedure for the synthesis of benzothiazoles
in water
A. Rostami (&) ꢀ A. Yari
A mixture of an aromatic aldehyde (1 mmol) and 2-ami-
nothiophenol (1 mmol) in water (2 mL) was stirred at
room temperature in the presence of a catalytic amount of
Department of Chemistry, Faculty of Science,
University of Kurdistan, 6617715143 Sanandaj, Iran
e-mail: a.rostami@uok.ac.ir
123

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J IRAN CHEM SOC (2012) 9:489–493
sulfamic acid (0.004 g, 0.04 mmol) for appropriate time
(Table 2). The reaction was monitored by TLC (eluent:
n-hexane/EtOAC: 7/3). After the reaction was completed,
solid product was isolated by filtration and washed with
water. The crud product was recrystallized from aqueous
ethanol to give pure product. All of the products are known
obtained with CH3OH, EtOH, CH3CN, CH2Cl2, Et2O and
n-hexane as solvents (Table 1, entries 6–9).
With the optimal conditions, the generality and the
applicability of this method was further examined for the
synthesis of 2-arylbenzothiazoles from condensation of
2-aminothiophenol with structurally diverse aromatic
aldehydes including different type of substituted benzal-
dehydes, 1-naphthaldehyde, 2-naphthaldehde, 9-anthra-
ldehyde and terephthaldehyde (Table 2).
1
and were characterized by IR, H NMR, and by melting
point comparisons with those of authentic samples.
The reaction proceeds smoothly to afford the desired
products with good to excellent yields within short times.
The substitution groups on the aromatic ring have no
obvious effect on the yields and reaction time under the
aforementioned optimal conditions. The condensation
reactions of 2-aminophenol or katekol with aromatic
aldehydes were not completed in the presence of catalytic
amount of sulfamic acid in water under reflux conditions
even after prolonged reaction times.
Results and discussion
In continuation of our study on the development of green
synthetic methodologies [26, 27], herein we report a sim-
ple, efficient, environmentally benign and practical method
for the synthesis of 2-arylbenzothiazoles from the con-
densation of 2-aminothiophenol with aromatic aldehydes in
the presence of catalytic amount of sulfamic acid in water
at room temperature (Scheme 1).
It should be noted that no added stoichiometric or
excessive oxidant other than air was provided.
The synthesis of 2-arylbenzothiazole with this method is
dependent on solvent and the amount of sulfamic acid
(Table 1). The investigation of the reaction conditions for
the synthesis of 2-(4-methoxy) phenyl benzothiazole from
condensation of 2-aminothiophenol and 4-methoxybenzal-
dehyde demonstrated that 0.04 mmol of the catalyst in
water at room temperature was optimal for obtaining the
desired reaction (Table 1, entry 3). Interestingly, it was also
observed that higher amount of catalyst did not improve the
results (Table 1, entry 4–5); also, inferior results were
Interestingly, the catalyst is recycled at least three times
without considerable loss in its activity; the turnover
number (TON) and turnover frequency (TOF) for the sul-
famic acid catalyst in the synthesis of 2-phenylbenzo-
thiazole reached 92.5 and 57.8 h-1, respectively (Table 2,
entry 1).
To investigate the feasibility of applying this method on
a preparative scale, we performed the synthesis of 2-phen-
ylbenzothiazole on a 50-mmol scale of starting material. As
expected, the reaction proceeded, similar to the case in a
smaller scale (Scheme 2).
SH
+ ArCHO
S
N
NH2SO3H (Cat.)
H2O, rt
The proposed mechanism for the formation of
2-substituted benzothiazoles is shown in Scheme 3. It is
proposed that sulfamic acid is a source for the formation of
H?, which makes the carbonyl and imine groups suscep-
tible to nucleophilic attack. In the second step the actual
oxidant is the oxygen of air, whereas in the atmosphere of
nitrogen the reaction was not completed even after pro-
longed reaction times.
Ar
NH2
Scheme 1 Sulfamic acid catalyzed synthesis of 2-arylbenzothiazols
Table 1 Effect of solvent and catalyst evaluation in synthesis of
2-(4-methoxy) phenylbenzothiazole at room temperature
In order to learn the efficiency and greenness of this
method, we compared our obtained results for the syn-
thesis of 2-phenyl benzothiazole with the best of the
well-known data from the literature (Table 3). As shown
in the Table 3, many of the previously reported meth-
odologies suffer from one or more disadvantages such as
(1) requirement of excess oxidizing agents (entries 1–3,
8), (2) using high temperature or microwave-assisted
baking (entries 1–6), (3) using transitional metal and
corrosive catalyst (entries 1–6), (4) special efforts for the
preparation of catalyst (entry 6), (5) prolonged reaction
time (entries 1,7), (6) high load of catalyst (entries 4, 7),
(7) using voltaic and toxic organic solvents (entries 1, 4,
Entry
Solvent
Catalyst
(mol%)
Time
(min)
Yield
(%)a
1
2
3
4
5
6
7
8
9
a
H2O
None
120
60
Trace
100
100
100
100
100
100
100
100
H2O
2
4
H2O
25
H2O
20
4
70
CH3OH
EtOH
CH3CN
n-Hexane
CH2Cl2
22
4
100
50
4
4
4 h
5 h
4
Conversion
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J IRAN CHEM SOC (2012) 9:489–493
491
Table 2 Sulfamic acid (0.04 mmol) catalyzed synthesis of 2-arylbenzthiazoles in water (2 mL) at room temperature from direct condensation of
aromatic aldehydes (1 mmol) with 2-aminothiophenol (1 mmol)
Entry
1
Aldehyde
Product
Time (min)
Yield (%)a
Mp (°C) [Ref]
96, 92, 93, 89b
218–219 [15]
CHO
20, 23, 26, 26
S
N
2
3
4
25
25
25
78
94
96
79–81 [16]
121–122 [14]
135 [21]
CHO
S
N
Me
Me
CHO
CHO
S
OMe
N
MeO
MeO
S
OMe
Me
N
MeO
OMe
5
40
93
173–174 [10]
CHO
S
N
N
Me
F
Me2N
6
7
37
45
89
81
100–101 [10]
80–82 [16]
CHO
S
N
F
CHO
S
N
Cl
Cl
8
40
40
25
89
92
95
115–117 [10]
133 [11]
CHO
CHO
S
N
Cl
Cl
9
S
N
Br
Br
Br
10
164 [28]
Br
CHO
OH
S
N
HO
HO
11
20
96
130–131 [14]
CHO
OH
S
N
12
13
25
25
94
90
227–229 [11]
121–122 [16]
CHO
S
N
OH
HO
S
N
CHO
NO2
O2N
14
15
S
N
30
30
97
229–230 [10]
128–130 [11]
CHO
NO2
O2N
93c
S
N
CHO
16
28
94
129–130 [13]
S
N
CHO
123

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J IRAN CHEM SOC (2012) 9:489–493
Table 2 continued
Entry
17
Aldehyde
Product
Time (min)
70
Yield (%)a
86c
Mp (°C) [Ref]
218–221 [22]
CHO
S
N
18
100
83c,d
259–261 [23]
N
S
S
N
OHC
CHO
Alltheproductsareknownandwere characterizedbyIR, 1H NMRandbymeltingpointcomparisons withthose ofauthenticsamples[7, 10, 11, 14, 21]
a
Isolated yields
b
Catalyst was reused for four times
c
The reactions were performed in methanol
d
Reaction conditions: in order to preparation of bis-benzthiazoles: 2-aminothiophenol (2 mmol), terephthaldehyde (1 mmol), sulfamic acid
(8 mg, 0.08 mmol), MeOH (4 mL)
7). We believe that the present method helps avoid
the disadvantages within the previously reported
methodologies.
CHO
SH
S
N
NH2SO3H (2 mmol)
+
H2O(100 ml), rt, 20 min
NH2
In summary, the significant advantages of this proce-
dure are (a) the use of the eco-friendly, cost-effective,
commercially available, reusable and chemically stable
catalyst, (b) the procedure does not involve the use of
any additional oxidants, (c) short reaction time, mild
reaction conditions and very easy work-up, (d) avoiding
hazardous and toxic solvent, (e) good to high yields of
products, (f) practical large-scale synthesis of benzo-
thiazols, (g) environmentally benign a chemical proce-
dure both for academia and industry, (h) the procedure
conforms to several of the guiding principles of green
chemistry. All of these ensure an efficient route and
provide an attractive and variable alternative to the pre-
viously reported procedures for the synthesis of 2-aryl-
benzthiazoles from condensation of aromatic aldehydes
with 2-aminothiophenol.
50 mmol
50 mmol
Yield=95 %
Scheme 2 Sulfamic acid catalyzed synthesis of 2-phenylbenzthiaz-
ole in large scale
HS
HS
H3O
O
OH
N
H2N
H
-H2O
-H
S
N
S
air oxidation
-H2
N
H
Scheme 3 A plausible mechanism for synthesis of 2-arylbenzothiaz-
ols in the presence of catalytic amount of sulfamic acid
Table 3 Comparison of the
Entry Catalyst
Condition
Time Yield Reference
(min) (%)
activity of various catalysts for
the synthesis of
2-phenylbenzothiazole from the
condensation of benzaldehyde
and 2-aminothiophenol
1
2
3
4
5
6
7
8
9
4-Methoxy-tempo (5 mol%), O2 (1 atm) Xylene, 100 °C
9 h
13
80
97
97
88
83
95
75
95
96
[6]
H2O2, CAN (10 mol%)
H2O2, Fe(NO3)3 (10 mol%)
I2 (50 mol%)
SF, 50 °C
SF, 50 °C
DMF, 100 °C
[bmim]PF6, 80 °C
SF, M.W 1,350 W
CH2Cl2, rt
[7]
4
[8]
25
[14]
RuCl3 (5 mol%), air
30
[17]
Cu3/2PMo12O40 (9 mol%), SiO2
Bakers’ Yeast (2 g)
15
[12]
24 h
20
[18]
(NH4)2S2O8 (10 mol%), SDS
Sulfamic acid (4 mol%)
water, rt
[19]
water, rt
20
Present work
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J IRAN CHEM SOC (2012) 9:489–493
493
Acknowledgments We are grateful to the University of Kurdistan
Research Councils for partial support of this work.
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