Synthesis of 1,2-disubstituted benzimidazoles, 2-substituted benzimidazoles and 2-substituted benzothiazoles in SDS micelles

Article abstract of DOI:10.1039/c000047g

A practical and convenient synthetic method has been developed for the facile synthesis of 1,2-disubstituted benzimidazoles, 2-substituted benzimidazoles and 2-substituted benzothiazoles. The method described has the benefits of operational simplicity, excellent yields, and high chemoselectivity. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c000047g

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www.rsc.org/greenchem | Green Chemistry
Synthesis of 1,2-disubstituted benzimidazoles, 2-substituted benzimidazoles
and 2-substituted benzothiazoles in SDS micelles†
Kiumars Bahrami,*a,b Mohammad M. Khodaei*a,b and Akbar Nejatia
Received 6th January 2010, Accepted 30th April 2010
First published as an Advance Article on the web 24th May 2010
DOI: 10.1039/c000047g
A practical and convenient synthetic method has been developed for the facile synthesis of
1,2-disubstituted benzimidazoles, 2-substituted benzimidazoles and 2-substituted benzothiazoles.
The method described has the benefits of operational simplicity, excellent yields, and high
chemoselectivity.
in addition, several require higher temperatures and costly
Introduction
reagents. Moreover, one of the major limitations of these
Benzimidazole derivatives have versatile pharmacological
methodologies is that they show poor selectivity in terms of N-1
properties1 based on their presence in both clinical medicines2
substitution, which results in the formation of two compounds
and compounds with broad ranges of biological functions.3
(i.e., the formation of 2-substituted benzimidazole along with
Also, compounds with benzothiazole skeletons are of
1,2-disubstituted benzimidazole as a mixture).
paramount interest because of their antitumor,4 anticancer,5
Developing environmentally benign and economical synthe-
and antibacterial6 activities. Thus synthesis of this heterocyclic
ses is an area of research that is being vigorously pursued, and
nucleus is therefore of continuing interest, but few methods are
avoiding the use of harmful organic solvents is a fundamental
available for the synthesis of 1,2-disubstituted benzimidazoles.
strategy to achieving this. One of the most attractive alternatives
to organic solvents is water, which has witnessed increasing
popularity due to being inexpensive, readily available and
environmentally benign. In addition, reactions in aqueous media
The more important ones include: N-alkylation of 2-substituted
benzimidazole in the presence of a strong base,7 N-alkylation
of o-nitroanilides followed by reductive cyclization,8 cyclocon-
densation of N-substituted o-aminoanilides,9 and condensation
illustrate unique reactivities and selectivities that are not usually
of N-substituted phenylenediamines with the sodium salt of
a-hydroxybenzylsulfonic acid.10 In addition, 1,2-disubstituted
benzimidazoles can also be accessed by direct one-step con-
densation of 1,2-phenylenediamines with aryl aldehydes under
the influence of a variety of acid catalysts.11 However, the last
protocol is currently most popular, probably because of the ease
of accessibility of substituted aryl aldehydes.
Two protocols are usually followed for the synthesis
of 2-substituted benzimidazoles. The first is the coupling
of 1,2-phenylenediamines with carboxylic acids or their
derivatives,12 and the second route involves condensation of
1,2-phenylenediamine and aldehydes followed by oxidative
cyclodehydrogenation.13
observed in organic media.17 However, organic reactions in water
are often limited in scope due to poor solubility of the organic
compounds. A possible new way to improve the solubility of
substrates is the use of surface-active compounds that can form
micelles.18
Under ambient conditions, surfactant molecules can aggre-
gate in an aqueous phase to form micelles with a hydrophobic
core and a hydrophilic corona. The use of micellar surfactants
as catalysts is widespread, and has been investigated in detail for
various reactions in aqueous solutions.19
As a part of our continued activities in this area,20 we report
for the first time a simple and efficient method for the synthesis
of 1,2-disubstituted benzimidazoles through the reactions of
1,2-phenylenediamine with aryl aldehydes in aqueous micellar
media, using sodium dodecylsulfate (SDS), which simultane-
ously functions as a catalyst to promote the reactions and
as a surfactant to assist in solubilizing the organic substrates
(Scheme 1). SDS was chosen since it forms micelles in water, can
solubilize organic compounds, and has been used successfully
in a number of organic reactions as a catalyst.21
Many methods are available for the preparation of
2-substituted benzothiazoles,14 but the most popular
approaches generally involve condensation–dehydration of
2-aminothiophenol with carboxylic acids,15 or condensation
with aldehydes under oxidative conditions.16
Although these methods are quite satisfactory, many of them
employ considerable amounts of hazardous organic solvents;
aDepartment of Chemistry, Razi University, Kermanshah, 67149, Iran.
E-mail: kbahrami2@hotmail.com, mmkhoda@razi.ac.ir; Fax: +98
(831)4274559; Tel: +98 (831)4274559
bNanoscience and Nanotechnology Research Center (NNRC), Razi
University, Kermanshah, 67149, Iran
† Electronic supplementary information (ESI) available: 1H NMR
spectra for selected compounds. See DOI: 10.1039/c000047g
Scheme 1 Selective synthesis of 1,2-disubstituted benzimidazoles.
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Table 1 Optimization of the reaction conditionsa
Table 2 Synthesis of 1,2-disubstituted benzimidazoles in SDS micellar
solutiona
Entry
SDS (mol%)
Yield (%)b
Entry R
Time/min Yield (%)b Mp/◦C (lit.)
Ref.
1
2
3
4
5
0
5
7
10
15
23c
40
70
98
98
1
2
3
4
5
6
7
8
C6H5
22
25
10
27
98
97
94
93
75
93
95
90
98
95
0
132 (132)
127 (129–130)
226 (222)
11d
11d
23
4-MeOC6H4
4-HOC6H4
4-MeC6H4
126–128 (128–130) 11i
4-(Me2N)C6H4 20
253 (255)
163 (158–159)
134 (136)
304 (306–308)
125 (130)
96 (94)
11i
24
11g
25
11g
11d
—
a Reaction conditions: The reactions were performed with 1,2-
phenylenediamine (1 mmol) and benzaldehyde (2 mmol) in the presence
of different amount of SDS as catalyst for 22 min, at 25 ◦C in H2O
(5 mL) as solvent. b Isolated yields. c Reaction not complete after 5 h.
2-ClC6H4
4-ClC6H4
4-NO2-C6H5
2-Pyridyl
2-Furyl
10
22
30
8
23
120
9
10
11
CH3(CH2)2
—
a The products were characterized by comparison of their spectroscopic
and physical data with authentic samples synthesized by reported
procedures. b Yields refer to pure isolated products.
Results and discussion
We started this synthesis by examining the reaction of 1,2-
phenylenediamine (1 mmol) with benzaldehyde (2 mmol) as
a model reaction. As shown in Table 1, the use of SDS
allowed the direct conversion of 1,2-phenylenediamine into the
corresponding 1,2-disubstituted benzimidazole in a yield of 98%
in water (5 mL) at 25 ◦C (Table 1, entry 4). The use of more
than 10 mol% of SDS did not enhance chemical yield (Table 1,
entry 5).
It is noteworthy that in a control experiment (Table 1,
entry 1), no significant promotion was observed under similar
reaction conditions in the absence of SDS, and only a low
yield was obtained after 5 h. SDS is an emulsifying agent,
which catalyzes this reaction and forms stable colloidal particles
in the presence of the substrates in water, and this colloid
formation plays an important role in acceleration of the
reactions.21e
Fig. 1 Proposed model for the synthesis of 1,2-disubstituted benzimi-
dazole in water in the presence of SDS.
To test the generality of this reaction, a series of aromatic
aldehydes was subjected to the optimal reaction conditions
(Table 2). By using water as solvent at 25 ◦C, 1,2-disubstituted
benzimidazoles with various functional groups were obtained
in excellent yields. Among the reactions of different aromatic
aldehydes, no significant distinction on the yields of target prod-
ucts was observed. Even the sensitive substrate furfuraldehyde
(Table 2, entry 10) produced the corresponding 1,2-disubstituted
benzimidazole without any difficulty. All substrates gave their
corresponding 1,2-disubstituted benzimidazoles exclusively as a
single product. However, butyraldehyde failed to react under the
present reaction conditions.
The scope of this system has been successfully extended to
the synthesis of 2-substituted benzimidazoles, which represents
the first synthesis of these compounds in water through such
transformations (Scheme 2). The optimum conditions were 1,2-
phenylenediamine (1 mmol) and aldehyde (1 mmol), in the
presence of (NH4)2S2O8 (1 mmol) and SDS (10 mol%) in water
at 25 ◦C (Fig. 2).
A possible mechanism for the reaction consists of a two-step
sequence involving the micelle-promoted formation of the N,N-
dibenzylidene-1,2-phenylenediamine derivative followed by ring
closure. Aromatization then takes place by a deprotonation–
reprotonation process.22
The catalytic effect of micellar sodium dodecyl sulfate in
this reaction can be explained as follows. In the micellar
solution, 1,2-phenylenediamine and aryl aldehyde, which are
both hydrophobic, are forced inside the hydrophobic core of the
micelles, thus allowing the reaction to take place more easily
(Fig. 1).
Fig. 2 Photographs of the reaction of 1,2-phenylenediamine with
benzaldehyde in the presence of (NH4)2S2O8 and SDS H2O at 25 ◦C:
(a) at the start of the reaction; (b) in the middle of the reaction; and (c)
at the end of the reaction.
Aromatic aldehydes with electron-donating and electron-
withdrawing groups both participated in this reaction equally
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Table 3 Synthesis of 2-substituted benzimidazoles and 2-substituted benzothiazoles in SDS micellar solutiona
Entry
R
Y
X
Time/min
Yield (%)b
Mp/◦C (lit.)
Ref.
1
2
3
4
5
6
7
8
C6H5
H
H
H
H
H
H
H
H
H
H
H
Me
Me
Me
Me
NO2
NO2
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
S
22
25
27
20
10
28
22
20
40
23
10
18
24
20
25
32
35
120
120
120
20
20
24
20
22
24
24
15
16
28
10
20
97
97
93
95
94
96
95
94
90
95
99
98
96
94
97
94
95
15
12
Trace
95
95
96
97
96
94
95
98
95
96
99
95
293 (295)
223–226 (226)
271 (270)
232 (228–229)
275 (279)
248 (247–248)
300 (301)
203 (207–208)
200 (201–203)
286 (288)
222–224 (218)
236 (240–241)
195 (190–191)
171 (169)
180 (180–183)
203 (207–208)
302 (305–308)
173–175 (177–178)
147–148 (148–149)
—
114 (113–114)
121 (119–120)
85–86 (85)
28a
13a
13a
28b
28c
28d
28e
28f
13d
13a
13a
28g
28h
28i
4-MeOC6H4
4-MeC6H4
4-(Me2N)C6H4
4-HOC6H4
4-FC6H4
4-ClC6H4
3-NO2-C6H4
=
9
C6H5CH CH
2-Furyl
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
2-Pyridyl
C6H5
4-MeC6H4
4-MeOC6H4
4-FC6H4
C6H5
4-ClC6H4
CH3
CH3(CH2)2
CH3(CH2)4
C6H5
4-MeOC6H4
4-MeC6H4
4-(Me2N)C6H4
4-HOC6H4
4-FC6H4
4-ClC6H4
20b
28h
28h
12b
28l
—
28d
28d
28j
S
S
S
S
S
S
S
S
173 (170–171)
231 (231–232)
98–100 (98–99)
114 (119–120)
113 (110–111)
228 (225–226)
132 (135–136)
134 (129–130)
106 (103–104.5)
28b
28k
28m
28m
15c
28d
20b
28b
15c
=
C6H5CH CH
4-NO2-C6H4
CHO-C6H4
2-Pyridyl
S
S
S
2-Furyl
a The products were characterized by comparison of their spectroscopic and physical data with authentic samples synthesized by reported procedures.
b Yields refer to pure isolated products.
Scheme 2 Selective synthesis of 2-substituted benzimidazole and 2-
substituted benzothiazoles.
well – apparently, the nature and position of substitution on
the aryl ring does not make much difference in reactivity.
Similarly, heterocyclic aldehydes and those containing an a,b-
Scheme 3 A possible pathway for synthesis of 2-aryl benzimidazoles.
unsaturated group afforded excellent yields of the products,
without formation of any side-products. This condensation–
oxidation procedure is fairly general, and several functionali-
ties, including hydroxyl and conjugated carbon–carbon double
bonds, are tolerated (Table 3).
Nevertheless, this protocol has its limitations. Aliphatic alde-
hydes such as acetaldehyde, butylaldehyde and hexylaldehyde
show extremely poor yields of the desired products (Table 3,
entries 18–20).
For the oxidation of cyclic intermediates, we selected ammo-
nium persulfate, (NH4)2S2O8, because it is a very strong oxidant,
very soluble in water, nonhazardous and relatively cheap.26 In
this case, acts both as a peroxide booster (providing additional
nascent oxygen to the aqueous solution) and as a mild oxidizing
agent27 (Scheme 3).
Employing the same procedure given above but using
2-aminothiophenol instead of 1,2-phenylenediamine, the cor-
responding 2-aryl benzothiazole derivatives were obtained in
excellent yields with various substituents (Scheme 2; Table 3,
entries 21–32). The limitations on the use of the aldehydes were
similar to those for 1,2-phenylenediamines.
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Conclusion
Acknowledgements
In summary, a practical and convenient synthetic method in
aqueous media using SDS as the surfactant catalyst (10 mol%)
has been developed for the facile synthesis of 1,2-disubstituted
benzimidazoles, 2-substituted benzimidazoles and 2-substituted
benzothiazoles. The operational simplicity, excellent yields of the
products, and high chemoselectivity are the main advantages of
this method, and furthermore, this procedure is cheap, safe and
environmentally benign.
We are thankful to the Razi University Research Council for
partial support of this work.
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