Synthesis of 2-substituted benzimidazoles and benzothiazoles using Ag2CO3/Celite as an efficient solid catalyst

Article abstract of DOI:10.1007/s13738-015-0592-1

An efficient and simple approach for the synthesis of 2-substituted benzimidazoles and benzothiazoles through a coupling of 1,2-phenylenediamines and 2-aminothiophenols with variety of aryl aldehydes in ethanol at 70 °C using Ag₂CO₃/Celite as solid catalyst is described. The procedure features short reaction time, excellent yields and simple workup.

Full text of DOI:10.1007/s13738-015-0592-1

J IRAN CHEM SOC
DOI 10.1007/s13738-015-0592-1
ORIGINAL PAPER
Synthesis of 2‑substituted benzimidazoles and benzothiazoles
using Ag CO /Celite as an efficient solid catalyst
2
3
Ebrahim Soleimani · Mohammad Mehdi Khodaei ·
Hossein Yazdani · Parisa Saei · Javad Zavar Reza
Received: 9 October 2014 / Accepted: 2 January 2015
©
Iranian Chemical Society 2015
Abstract An efficient and simple approach for the syn-
thesis of 2-substituted benzimidazoles and benzothiazoles
through a coupling of 1,2-phenylenediamines and 2-ami-
nothiophenols with variety of aryl aldehydes in ethanol at
the use of microwave irradiation [22–24]. Another approach
for the synthesis of these compounds is including conden-
sation of 1,2-phenylenediamines and 2-aminothiophenol
with aldehydes [25–33] under oxidative conditions using
various oxidative and catalytic reagents [25–54] such as such
as nitrobenzene [25], 1,4-benzoquinone [26], PhI(OAc)2
[27], Zn-proline [28], (DDQ) [29], MnO [30], Pb(OAc)
7
0 °C using Ag CO /Celite as solid catalyst is described.
2 3
The procedure features short reaction time, excellent yields
and simple workup.
2
4
[
31], oxone [32], NaHSO [33], H O /HCl [34], CAN [35],
3 2 2
Keywords Benzimidazole · Benzothiazole ·
iodine [36], FeCl ·6H O [37], In(OTf) [38], Yb(OTf) [39],
3
2
3
3
1
,2-Phenylenediamine · 2-Aminothiophenol ·
Sc(OTf) [40], Cu(OTf) [41], KHSO [42], ZrOCl ·8H O
3
2
4
2
2
Ag CO /Celite
[43], ZrCl [44], HfCl [44], boron trifluoride etherate [45],
2
3
4
4
cobalt complexes [46], cerium complexes [35].
Although the reactions were efficiently promoted by the
above-mentioned conditions, they often employed homo-
geneous catalysts and also some of those methods suffer
from one or more disadvantages, such as usage of stoichio-
metric reagent, prolonged reaction times and difficulty in
separation of the products and recovery of the catalyst from
the reaction mixture. Therefore, the discovery of mild and
practicable, stable, cheap, recyclable, and ecofriendly het-
erogeneous catalysts for the synthesis of 2-substituted ben-
zimidazoles and benzothiazoles are still in serious demand
that can attract the attention of researchers.
The benzimidazoles and benzothiazoles are important class
of heterocyclic compounds that exhibit a wide range of
biological properties such as antiviral, antiulcer, antihyper-
tension, antifungal, antihistaminic, and anticancer [1–11].
These compounds are also good versatile intermediate for
synthesis of many important organic compounds [12, 13].
The most common approach for the synthesis of 2-substi-
tuted benzimidazoles and benzothiazoles involves the treat-
ment of 1,2-phenylenediamines and 2-aminothiophenol with
carboxylic acids [14–18] or their derivatives [19–21], under
strongly acidic conditions with very high temperatures or
Ag CO /Celite (Fetizon’s reagent) [55], a very mild oxi-
2
3
dant and easily available oxidizing agent, has been widely
used in organic reactions [56–63]. Moreover, this reagent
was extensively used for oxidation of alcohols. But the
catalytic application of this reagent has not been care-
fully studied in the synthesis of 2-substituted benzimida-
zoles and benzothiazoles until now. During our research in
organic synthesis, we have found that Ag CO /Celite could
effectively catalyze the synthesis of 2-substituted benzimi-
dazoles and benzothiazoles.
Due to the aforementioned reasons, and as a part of our
ongoing research to develop new synthetic methodologies
E. Soleimani · M. M. Khodaei · H. Yazdani · P. Saei
Department of Chemistry, Razi University,
Kermanshah 67149-67346, Iran
E. Soleimani (*)
Department of Chemistry, College of Sciences, Kermanshah
Branch, Islamic Azad University, Kermanshah, Iran
e-mail: e_soleimanirazi@yahoo.com; e-soleimani@razi.ac.ir
2
3
J. Zavar Reza
Department of Biochemistry, Faculty of Medicine, Shahid
Sadoughi University of Medical Sciences-Yazd, Yazd, Iran
1
3

J IRAN CHEM SOC
Scheme 1 Synthesis of 2-sub-
stituted benzimidazoles and
benzothiazoles
O
NH2
N
Ag CO /Celite
EtOH, 70 C
R1
+
2
3
R1
R2
R2
o
H
X
XH
1
2
3
Table 1 Optimization of the reaction
NH2
O
N
Ag CO /Celite
2
3
+
Cl
Cl
Solvent
N
H
NH2
H
Entry
Solvent
EtOH
Time (h)
Catalyst (mol %)
Ag CO /Celite (25 mol %)
Temperature (°C)
Yield (%)
1
2
3
4
5
6
7
8
9
3
3
70
70
70
70
70
25
70
70
70
25
50
100
70
70
70
70
70
70
96
65
0
2
3
H O
Ag CO /Celite (25 mol %)
2 3
2
CH CN
3
Ag CO /Celite (25 mol %)
2 3
3
EtOAc
Toluene
CH Cl
3
Ag CO /Celite (25 mol %)
0
2
3
3
Ag CO /Celite (25 mol %)
0
2
3
3
Ag CO /Celite (25 mol %)
0
2
2
2
3
EtOH/H O (2:1)
3
Ag CO /Celite (25 mol %)
81
76
71
0
2
2
3
EtOH/H O (1:1)
3
Ag CO /Celite (25 mol %)
2 3
2
EtOH/H O (1:2)
3
Ag CO /Celite (25 mol %)
2 3
2
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
12
3
Ag CO /Celite (25 mol %)
2 3
Ag CO /Celite (25 mol %)
60
96
35
50
65
86
96
0
2
3
3
Ag CO /Celite (25 mol %)
2 3
3
Ag CO /Celite (5 mol %)
2 3
3
Ag CO /Celite (10 mol %)
2 3
3
Ag CO /Celite (15 mol %)
2 3
3
Ag CO /Celite (20 mol %)
2 3
3
Ag CO /Celite (30 mol %)
2 3
12
Without catalyst
[
64–67], we report herein simple and efficient in situ tan-
dem cyclocondensation–oxidation sequence reaction of
,2-phenylenediamine and 2-aminothiophenol with vari-
entries 1–2, 7–9), the results indicated that although water
and water/ethanol mixture were found to be effective for
this reaction system, the best result was obtained when etha-
nol was utilized as solvent. This effect can be explained by
a simple acid-catalysis mechanism facilitated by the strong
hydrogen bond interaction at the organic–water or ethanol
interface which stabilizes the reaction intermediate. Next,
we studied the model reaction in ethanol at different tem-
peratures (Table 1, entry 1 and entries 10–12). The reaction
rate increased as the temperature was raised. At 70 °C, the
maximum yield (96 %) was obtained in a reaction time of
3 h (Table 1, entry 1). Also, the model reaction was stud-
ied in ethanol at 70 °C using different amounts of Ag CO /
1
ous alkyl and aryl aldehydes for the preparation of 2-sub-
stituted benzimidazoles and benzothiazoles in ethanol at
7
0 °C using Ag CO /Celite as solid catalyst in excellent
2 3
yield (Scheme 1).
We chose the reaction of 1,2-phenylenediamine with
-chlorobenzaldehyde as a model system for the optimiza-
4
tion study (Table 1). First, we compared the reaction rate
in different solvents by measuring the isolated yield using
identical amounts of reactants and catalyst (25 mol % of
Ag CO /Celite) for a fixed reaction time of 3 h at 70 °C
2
3
2
3
(
Table 1, entries 1–6). The desired products were scarcely
Celite (Table 1, entry 1 and entries 13–17). The best results
were obtained with 25 mol % of Ag CO /Celite (Table 1,
obtained in aprotic solvents such as ethyl acetate, toluene,
acetonitrile, and dichloromethane (Table 1, entries 3–6). But
when the reaction was carried out using the protic solvents
such as ethanol, water and water/ethanol mixture (Table 1,
2
3
entry 1). Further work indicated that the best results are
obtained when the reaction is performed at 70 °C for 3 h
using Ag CO /Celite (25 % mol) in ethanol.
2
3
1
3

J IRAN CHEM SOC
Table 2 Synthesis of benzimidazole derivatives by using 1,2-phenylenediamines and aromatic aldehydes
NH2
O
H
N
Ag CO /Celite
EtOH, 70 C
2
3
+
R
R
o
N
H
NH2
1
2
3
Entry
R
X
Product
Yield (%)
Mp (°C)
Found
Reported
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
Ph
H
3a
3b
3c
3d
3e
3f
95
91
96
94
92
96
97
97
95
95
90
99
95
88
91
94
91
292
290–291 [68]
270 [69]
4-CH C H
H
269
3
6
4
4-FC H
H
246–247
279
247–248 [70]
279 [71]
6
4
4-OHC H
H
6
4
4-N(CH ) C H
4
H
229–230
300–302
>300
228–229 [72]
301 [73]
3
2
6
4-ClC H
H
6
4
4-O NC H
4
H
3g
3h
3i
324–326 [78]
295–296 [79]
>300
2
6
4-BrC H
H
293–295
>300
6
4
4-HCOC H
H
6
4
0
1
2
3
4
5
6
7
3-O NC H
4
H
3j
205–206
225–226
220–223
241–242
286–288
200–202
238–240
189–191
207–208 [75]
[227–228] [7–11]
218 [69]
2
6
2-CH OC H
H
3k
3l
3
6
4
2-Pyridyl
H
2-OHC H
H
3m
3n
3o
3p
3q
[242] [14–18]
288 [69]
6
4
2-Furyl
H
C H -CH=CH
H
201–203 [74]
240–241 [76]
190–191 [77]
6
5
Ph
CH3
CH3
4-CH C H
4
3
6
Table 3 Synthesis of benzothiazole derivatives by using 1,2-benzo-
thiazoles and aromatic aldehydes
To illustrate the need for Ag CO /Celite, the feasibility
2 3
of model reaction is checked in the absence of Ag CO /
2
3
NH2
Celite. No product was obtained at 70 °C even after 12 h.
Therefore, Ag CO /Celite is obviously an important com-
ponent of the reaction.
O
H
N
S
Ag CO /Celite
EtOH, 70 C
2
3
+
R
R
2
3
o
SH
1
2
4
With the optimized condition established above, our
next attempt was to extend the capability of this process to
two 1,2-phenylenediamines such as 1,2-phenylenediamine
and 4-methyl-1,2-phenylenediamine and various types of
aryl aldehydes. The results are summarized in Table 2. In
all cases, excellent yields were obtained, even in the pres-
ence of electron-donating or electron-withdrawing substitu-
ents on the aldehydes. This result confirms the reliability of
the present synthetic methodologies. Notably, this protocol
has also its limitations. Aliphatic aldehydes only provided
very low yields of the desired products or did not offer any
desired product.
To expand the synthetic scope of this protocol, we car-
ried out the reaction using 2-aminothiphenol instead of
o-phenylenediamine. The reaction worked well using vari-
ous substituted aldehydes furnishing the corresponding
benzimidazoles in excellent yields (Table 3).
Entry R
Product Yield Mp (°C)
(%)
Found
Reported
1
2
3
4
5
6
7
8
9
1
1
1
1
1
Ph
4-CH C H
4
4a
4b
93
90
93
94
95
96
97
95
96
90
94
98
95
93
114
113–114 [22]
85 [80]
86
3
6
4-N(CH ) C H 4c
170–172
231–232
100
170–171 [71]
231–232 [81]
98–99 [82]
119–120 [82]
225–226 [80]
170 [85]
3
2
6
4
4-OHC H4
4d
4e
4f
6
4-FC H4
6
4-ClC H4
117–119
226–227
170
6
4-O NC H
4
4g
4h
4i
2
6
4-NCC H4
6
4-CH COC H
179
178–180 [83]
103–104 [84]
[122] [86]
3
6
4
0
1
2
3
4
2-Furyl
4j
105
2-OHC H4
4k
4l
121
6
2-Pyridyl
135
129–130 [71]
2-OH-4-BrC H 3m
169–169.5 [170] [19–21]
109–111 110–111 [84]
6
4
As per a plausible mechanism, the reaction proceeds
via the activation of aldehyde by Ag CO /Celite followed
C H -CH=CH 3n
6
5
2
3
1
3

J IRAN CHEM SOC
NH2
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2 3
In conclusion, we have introduced the utilization of
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1
2
3
and efficient synthesis of 2-substituted benzimidazoles
and benzothiazoles through the condensation of 1,2-phe-
nylenediamines and 2-aminothiophenols with various aryl
aldehydes. This method offers some advantages in terms of
simplicity of performance, short reaction times, excellent
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Experimental
General procedure for the synthesis of 2-substituted
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2
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the products were filtered and washed with water. All of the
products are known compounds and characterized easily by
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Acknowledgments We gratefully acknowledge financial support
from the Iran National Science Foundation (INSF) and Research
Council of Razi University.
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