An efficient synthesis of 2-arylbenzothiazoles using silica sulfuric acid, oxalic acid and aluminum chloride hydrate as heterogeneous and homogeneous catalyst systems

Article abstract of DOI:10.1135/cccc2010111

A convenient method for the synthesis of 2-arylbenzothiazoles from condensation of aromatic/heteroaromatic aldehydes with 2-aminothiophenol has been developed using inexpensive, green and reusable solid acids - silica sulfuric acid (SSA), oxalic acid or A

Full text of DOI:10.1135/cccc2010111

Synthesis of 2-Arylbenzothiazoles
27
AN EFFICIENT SYNTHESIS OF 2-ARYLBENZOTHIAZOLES USING
SILICA SULFURIC ACID, OXALIC ACID AND ALUMINUM CHLORIDE
HYDRATE AS HETEROGENEOUS AND HOMOGENEOUS CATALYST
SYSTEMS
Behrooz MALEKI
Department of Chemistry, Sabzevar Tarbiat Moallem University, 397 Sabzevar, Iran;
e-mail: maleki@sttu.ac.ir
Received August 27, 2010
Accepted November 2, 2010
Published online December 6, 2010
A convenient method for the synthesis of 2-arylbenzothiazoles from condensation of
aromatic/heteroaromatic aldehydes with 2-aminothiophenol has been developed using in-
expensive, green and reusable solid acids – silica sulfuric acid (SSA), oxalic acid or
AlCl3·6H2O as efficient catalysts. The reactions occurred under mild conditions to afford the
corresponding 2-arylbenzothiazoles in good to excellent yields.
Keywords: Acidity; Aldehydes; Antibiotics; Antibacterial activity; Antiaromaticity;
Antifungal activity; 2-Arylbenzothiazoles; Silica sulfuric acid; Oxalic acid; AlCl3·6H2O;
2-Aminothiophenol.
2-Arylbenzothiazoles are an important class of biologically active hetero-
cyclic compounds which are becoming increasingly attractive due to their
pharmacological and industrial importance1. Several functionalized 2-aryl-
benzothiazoles are widely used in medicinal chemistry and have been
found to exhibit a wide spectrum of biological effects including anti-
tumour, anticonvulsant, and antiviral activities2–5. In addition, several
industrial applications have been reported for these compounds as anti-
oxidants6–7, fluorescents8 and photochromic agents9.
Thus, these advantages described to benzothiazoles make them to con-
tinue attracting the attention of researchers for the development of effi-
cient approaches towards these compounds10–13. The methods previously
reported for the synthesis of 2-arylbenzothiazoles include: (i) the condensa-
tion of 2-aminothiophenol with aldehydes14–20, carboxylic acids21–25, acid
chlorides26–27 and esters28, and (ii) the cyclization of thiobenzanilides29–31
.
Other synthetic routes employ microwave-assisted reaction of 2-amino-
thiophenol with β-chlorocinnamaldehydes32, palladium-catalyzed direct
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 1, pp. 27–37
© 2011 Institute of Organic Chemistry and Biochemistry
doi:10.1135/cccc2010111

28
Maleki:
coupling of benzothiazoles with arylbromides33, palladium-catalyzed con-
densation of arylhalides with 2-aminothiophenol followed by dehydrative
cyclization34, reaction of copper(I) thiobenzoate with 2-iodoanilines35, and
the aquatic reaction of the corresponding thioamidinium salts and
2-aminothiophenol36. Recently, some simple one-step methods of synthe-
ses of 2-arylbenzothiazoles have been found to be effective using perchloric
acid-doped polyaniline37, ceric ammonium nitrate (CAN)38, tungstophos-
phoric acid impregnated zirconium phosphate39, zirconium(IV) oxide chlo-
ride and anhydrous copper(II) sulfate40, Dowex 50W 41, activated carbon
44
under oxygen as catalysts42, p-toluenesulfonic acid43, H2O2/Fe(NO3)3
,
solid heteropolyacid supported on silica gel (Cu3/2PMo12O40/SiO2)45, alum
(KAl(SO4)2·12H2O)46, Bakers’ yeast47, trichloroisocyanuric acid48 and silica
sulfuric acid under microwave irradiation49. However, many of these
methods have one or more disadvantages such as high reaction tempera-
ture, expensive metallic compounds, requirement for excess of reagenets,
harsh reaction conditions, air sensitive catalysts, multistep processes, pro-
longed reaction times, use of toxic and costly catalysts, requirement for
hazardous and carcinogenic organic solvents. To avoid these drawbacks, de-
velopment of simpler, safe, inexpensive, green and efficient protocols are
highly demanded. In our previous work, we reported for the first time a
simple, efficient and eco-friendly procedure for the synthesis of 2-aryl-
benzothiazoles utilizing glacial acetic acid as a non-toxic and inexpensive
catalyst under microwave irradiation and solvent-free conditions50.
RESULTS AND DISCUSSION
We describe a simple, efficient and practical method for a one-pot synthesis
of 2-arylbenzothiazoles using catalytic amount of silica sulfuric acid (SSA),
oxalic acid or AlCl3·6H2O as catalysts in acetonitrile, EtOH/H2O (1:1) and
MeOH/H2O (20:1), respectively (Scheme 1).
Oxalic acid is the most acidic among the simple dicarboxylic acids having
a first pKa value of 1.27. Application of oxalic acid as a catalyst was previ-
SCHEME 1
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 1, pp. 27–37

Synthesis of 2-Arylbenzothiazoles
29
ously reported for one-pot synthesis of 2-aryl-1-arylmethyl-1H-benzimid-
azoles and 2,4,5-triaryl-1H-imidazoles51, dealumination of zeolite52, and
synthesis of quinoxalines53. These special activities inherent to oxalic acid
prompted us to explore this catalyst for the synthesis of 2-arylbenzo-
thiazoles in a model reaction between 2-aminothiophenol (1 mmol) and
4-methoxybenzaldehyde (1 mmol) in different solvents and various
amounts (mole %) of oxalic acid. The product 2-(4-methoxyphenyl)benzo-
thiazole was isolated in 86% yield under the optimum reaction condition,
that is the use of EtOH/H2O (1:1) as solvent and 10 mole % of oxalic acid
catalyst at 80 °C. Similarly, several other aromatic aldehydes (1a–1p) re-
acted well under the present reaction conditions to afford the correspond-
ing 2-arylbenzothiazoles (2a–2p) in high yields (Table I).
TABLE I
Condensation of 2-aminothiophenol with aromatic/heteroaromatic aldehydes catalyzed by
oxalic acid (10 mole %) in EtOH/H2O (1:1)
M.p., °C
(Het)Ar
1a–1p
Producta
Time, min
Yield, %b
Found
Reported
Ph
2a
2b
2c
2d
2e
2f
30
40
25
20
40
50
25
30
30
25
30
10
30
30
20
20
80
74
86
78
74
68
86
84
83
80
84
90
86
80
72
78
111–112
103–105
120–121
127–128
82–84
112–11441
101–10341
120–12141
127–12817
8517
53–5417
13241
83–8441
160–16041
115–11741
101–10348
162–16420
181–18241
134–13517
137–13848
9917
2-MeOC6H4
4-MeOC6H4
2-HOC6H4
4-MeC6H4
2-MeC6H4
4-BrC6H4
52–54
2g
2h
2i
132–133
82–83
3-BrC6H4
4-Me2NC6H4
4-ClC6H4
4-FC6H4
157–159
116–118
100–102
161–162
179–180
132–133
136–137
100–101
2j
2k
2l
4- CNC6H4
3-NO2C6H4
Pyridyl-4-yl
Pyridyl-2-yl
Thienyl-2-yl
2m
2n
2o
2p
a
All the products were characterized from spectral (1H, 13C NMR, MS and IR) data which
b
were in accord with those of the literature cited. Isolated yields.
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 1, pp. 27–37

30
Maleki:
Silica sulfuric acid (SSA) is another eco-friendly, versatile solid acid which
has recently found much interest as a catalyst54–58 in various organic trans-
formations. These salient catalytic activities allocated to silica sulfuric acid
have encouraged us, in second part of the present work, to examine its cata-
lytic efficiency in reaction of 2-aminothiophenol with aldehydes for the
synthesis of 2-arylbenzothiazoles. Among various solvents like CH3CN,
MeOH, EtOH, EtOAc, CH2Cl2 and CCl4 used for the condensation of
2-aminothiophenole (1 mmol) with 4-methoxybenzaldehyde (1 mmol) as
model reaction, acetonitrile was the solvent of choice as best results in
terms of the yield was obtained using 0.05 g of SSA per mmol of the alde-
hyde when the reaction was run at 80 °C for 20 min (Table II).
Similarly, to establish the scope and usefulness of this methodology,
a number of reactions were conducted and the isolated products are sum-
marized in Table III. Comparison of the experimental results summarized
in Table I with those given in Table III indicates a general increase in reac-
tion rates and also, in some cases, slight improvement of the yields when
SSA is used as a catalyst.
Recently, Shingare and co-Workers reported one-step synthesis of 2-sub-
stituted benzothiazoles using silica sulfuric acid under microwave irradia-
tion (domestic MW oven). It has been shown that application of microwave
irradiation reduce the reaction time and increases the product yield49. The
use of domestic microwave ovens involves disadvantages such as: without
control of temperature, power and pressure inside the reaction vessel.
TABLE II
Optimization of the silica sulfuric acid-catalyzed condensation of 4-methoxybenzaldehyde (1c)
with 2-aminothiophenol
Solvent
Amount of catalyst, g
Time, min
Yield, %a
CH2Cl2
CCl4
0.05
0.05
0.05
0.05
0.05
0.05
0.03
0.07
50
50
20
20
20
20
30
35
40
30
62
54
60
86
80
84
EtOH
MeOH
EtOAc
MeCN
MeCN
MeCN
a
Isolated yields.
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 1, pp. 27–37

Synthesis of 2-Arylbenzothiazoles
31
Therefore, these studies carried out using domestic microwave ovens pres-
ent incomplete experimental information, and the reaction conditions
have poor reproducibility59.
Finally, in search for other more robust and efficient acidic catalysts for
the synthesis of 2-arylbenzothiazoles, we chose to explore the catalytic effi-
ciencies of several solid halides like AlCl3·6H2O, ZnCl2 and SbCl3. Among
these, AlCl3·6H2O appeared as the most effective in catalyzing the conden-
sation of benzaldehyde with 2-aminothiophenol as a model reaction. To
optimize the reaction conditions, different solvents such as MeOH, EtOH,
EtOAc, CH2Cl2, CCl4 and CH3CN were examined under various amounts of
TABLE III
Condensation of 2-aminothiophenol with aromatic/heteroaromatic aldehydes catalyzed by
silica sulfuric acid (0.05 g) in CH3CN
M.p., °C
(Het)Ar
1a–1p
Producta
Time, min
Yield, %b
Found
Reported
Ph
2a
2b
2c
2d
2e
2f
25
25
20
20
40
50
30
20
30
25
35
10
15
20
15
15
82
81
86
78
74
70
88
82
87
84
80
96
90
82
76
80
113–114
103–105
119–120
127–128
82–84
112–11441
101–10341
120–12141
127–12817
8517
53–5417
13241
83–8441
160–16141
115–11741
101–10348
162–16420
181–18241
134–13517
137–13848
9917
2-MeOC6H4
4-MeOC6H4
2-HOC6H4
4-MeC6H4
2-MeC6H4
4-BrC6H4
53–55
2g
2h
2i
132–133
82–83
3-BrC6H4
4-Me2NC6H4
4-ClC6H4
4-FC6H4
157–159
116–118
102–104
161–162
179–180
132–133
137–138
99–100
2j
2k
2l
4- CNC6H4
3-NO2C6H4
Pyridyl-4-yl
Pyridyl-2-yl
Thienyl-2-yl
2m
2n
2o
2p
a
All the products were characterized from spectral (1H, 13C NMR, MS and IR) data which
b
were in accord with those of the literature cited. Isolated yields.
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 1, pp. 27–37

32
Maleki:
the catalyst AlCl3·6H2O. The best results in terms of the yields were ob-
tained when 5 mole % of AlCl3·6H2O was used in MeOH/H2O (20:1) as the
solvent of choice and the reaction was run at room temperature for 30 min.
These optimized reaction conditions were similarly extended to a series of
other aldehydes and the corresponding products were isolated as given in
Table IV. It is necessary to mention that, although the catalytic efficiency
of AlCl3·6H2O as a nontoxic and inexpensive catalyst has already been ex-
plored in a variety of organic transformations, the use of AlCl3·6H2O in the
present report to catalyze the synthesis of 2-arylbenzothiazoles is hitherto
unreported.
TABLE IV
Condensation of 2-aminothiophenol with aromatic/heteroaromatic aldehydes catalyzed by
AlCl3·6H2O (5 mole %) in MeOH/H2O (20:1) at room temperature
M.p., °C
(Het)Ar
1a–1p
Producta
Time, min
Yield, %b
Found
Reported
Ph
2a
2b
2c
2d
2e
2f
30
30
20
40
20
40
30
25
20
30
40
15
20
50
60
90
76
92
80
84
78
92
88
92
86
83
94
89
74
68
111–112
103–105
119–120
127–128
84–85
112–11441
101–10341
120–12141
127–12817
8517
53–5417
13241
83–8441
160–16141
115–11741
101–10348
162–16420
181–18241
134–13517
137–13848
9917
2-MeOC6H4
4-MeOC6H4
2-HOC6H4
4-MeC6H4
2-MeC6H4
4-BrC6H4
52–54
2g
2h
2i
132–133
82–83
3-BrC6H4
4-Me2NC6H4
4-ClC6H4
4-FC6H4
160–162
116–118
102–104
161–162
179–180
134–136
136–138
100–102
2j
2k
2l
4- CNC6H4
3-NO2C6H4
Pyridyl-4-yl
Pyridyl-2-yl
Thienyl-2-yl
2m
2n
2o
2p
60
70
a
All the products were characterized from spectral (1H, 13C NMR, MS and IR) data which
b
were in accord with those of the literature cited. Isolated yield.
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Synthesis of 2-Arylbenzothiazoles
33
A comparison of the efficiency of this method with selected previous
methods is collected in Table V. The results show that this method is su-
perior to some previously reported methods in terms of yields and reaction
times.
The presence of either silica sulfuric acid, oxalic acid or AlCl3·6H2O in the
reaction as the catalyst can possibly bring about the generation of protons
required to promote the reactions51–58. As previously suggested37–50, the re-
action may proceed through the formation of imine I from aldehydes and
2-aminothiophenol. Subsequent cyclization to 2,3-dihydrobenzothiazole II
followed by oxidation in air would afford 2-arylbenzothiazoles (Scheme 2).
It is important to mention that, when the same reaction with silica sulfuric
acid, oxalic acid or AlCl3·6H2O was carried out under nitrogen atmosphere
(in absence of oxygen), the reactions stopped at the benzothiazoline II
TABLE V
Comparison of methods for the synthesis of 2-arylbenzothiazoles 2
Aldehyde
Conditions
Time
Yield, %
1c
20 min
24 h
24 h
6 h
30 min
10 h
92
80
78
90
71
89
91
91
AlCl3·6H2O, MeOH/H2O (20:1), r.t.
Bakers’ yeast, CH2Cl2, r.t.47
CAN, MeOH, r.t.38
[PmIm]Br, solvent-free, 80 °C 15
I2, DMF, 100 °C 18
Dowex 50W, H2O, 80 °C 41
H2O2, Fe(NO3)3·9H2O, solvent-free, 50 °C 44
MnO2, SiO2, solvent-free, MW oven, 140 °C 11
22 min
4 min
1a
SSA, CH3CN, 80 °C
25 min
24 h
3 h
24 h
12 h
6 h
82
75
79
75
85
80
CAN, MeOH, r.t.38
Activared carbon, O2, Xylene, 50 °C 42
Bakers’ yeast, CH2Cl2, r.t.47
Dowex 50W, H2O, 80 °C 41
[PmIm]Br, solvent-free, 80 °C 15
1m
Oxalic acid, EtOH/H2O (1:1), 80 °C
P-TSA, H2O, 80 °C 43
30 min
300 min
35 min
24 h
24 h
8 h
86
84
78
71
88
88
I2, DMF, 100 °C 18
Bakers’ yeast, CH2Cl2, r.t.47
CAN, MeOH, r.t.38
Dowex 50W, H2O, 80 °C 41
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 1, pp. 27–37

34
Maleki:
stage, which never proceeded to benzothiazoles 2a–2p. This surely proves
that aerial oxygen is not essential for benzothiazoline formation, though it
is absolutely essential for the oxidation step leading to the formation of
benzothiazoles.
SCHEME 2
CONCLUSION
In conclusion, an efficient one-pot procedure has been developed for the
synthesis of 2-arylbenzothiazoles via condensation of 2-aminothiophenol
with aldehydes catalyzed by either silica sulfuric acid (SSA), oxalic acid or
AlCl3·6H2O. Mild and simple experimental/product isolation procedure,
green aspects avoiding hazardous solvents, use of eco-friendly and less toxic
reagents, shorter reaction times and high yields of the products are the ad-
vantages of the present procedure.
EXPERIMENTAL
Solvents, reagents, and chemical materials were obtained from Aldrich (USA), Merck
(Germany) and Fluka (Switzerland) chemical companies and purified prior to use. Melting
points were determined in open capillary tubes in a BI Branstead Electrothermal Cat No.
IA9200 apparatus and are uncorrected. Nuclear magnetic resonance spectra were recorded on
JEOL FX 90Q using tetramethylsilane (TMS) as an internal standard. Infrared spectra were re-
corded on a PerkinElmer GX FT IR spectrometer (KBr pellets).
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 1, pp. 27–37

Synthesis of 2-Arylbenzothiazoles
35
Synthesis of 2-Arylbenzothiazoles from Condensation of 2-Aminothiophenol with
Aldehydes. Typical procedures
Synthesis with oxalic acid: 2-Aminothiophenol (1 mmol) and aromatic/heteroaromatic alde-
hydes 1a–1p (1 mmol) were dissolved in EtOH/H2O (1:1 v/v). Oxalic acid (10 mole %) was
added to the above solution and the mixture was stirred at 80 °C until completion (TLC,
ethyl acetate–hexane 2:8). After the complete conversion of the substrate, water (20 ml) was
added and the mixture was allowed to stand at room temperature for 3–4 h. The formed
crystals of the pure product were collected by filtration and dried. After isolation of the
product, the filtrate was extracted with CHCl3 (2 × 30 ml). The aqueous layer (including
oxalic acid) was separated and the solvent was evaporated to obtain pure oxalic acid. The re-
cycled catalyst was used for the next run under identical reaction conditions.
Synthesis with silica sulfuric acid: 2-Aminothiophenol (1 mmol) and aromatic/hetero-
aromatic aldehydes 1a–1p (1 mmol) were dissolved in 10 ml of acetonitrile. Silica sulfuric
acid (0.05 g) was added to the above solution at room tempetrature and the stirred mixture
was refluxed. The progress of the reaction was monitored by TLC (hexane–ethyl acetate 8:2).
After the complete conversion of the substrate as indicated by TLC analysis, the insoluble
silica sulfuric acid was removed by filtration and washed with CH3CN (2 × 5 ml). The recy-
cled catalyst was used for the next run. The solvent was evaporated under reduced pressure
to give the products 2a–2p, which were recrystalized from EtOH (96%).
Synthesis with AlCl3·6H2O: 2-Aminothiophenol (1 mmol) and aromatic/heteroaromatic al-
dehydes 1a–1p (1 mmol) were dissolved in MeOH/H2O (20:1 v/v). AlCl3·6H2O (5 mole %) was
added to the above solution and the mixture was stirred at room temperature until comple-
tion (TLC, ethyl acetate–hexane 2:8). After completion, the solvent was evaporated and
washed with water to give the crude products 2a–2p. Then, the residue was recrystalized
from (EtOH, 2 × 5 ml) to afford the pure product.
The author wish to thank the University of Sabzevar Tarbiat Moallem in Sabzevar for financial
support to carry out this research and he also thanks Ms N. Rahiminezhad for her assistance.
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