Water mediated chemoselective synthesis of 1,2-disubstituted benzimidazoles using o-phenylenediamine and the extended synthesis of quinoxalines

Article abstract of DOI:10.1039/b914286j

By applying water as the reaction medium, the one-pot synthesis of 1,2-disubstituted benzimidazoles has been achieved in excellent efficiency and selectivity at room temperature via trimethylsilyl chloride promoted reaction of o-phenylenediamine with aldehyde. This green catalyst system has also been successfully extended to the synthesis of quinoxalines via the reaction of o-phenylenediamine with α-bromoketone. Water displayed a specific functionality in mediating the selectivity, and remarkable advantages over organic solvents in terms of yields as well as in the work up procedure of the reactions. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b914286j

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www.rsc.org/greenchem | Green Chemistry
Water mediated chemoselective synthesis of 1,2-disubstituted
benzimidazoles using o-phenylenediamine and the extended
synthesis of quinoxalines†
Jie-Ping Wan, Shi-Feng Gan, Jian-Mei Wu and Yuanjiang Pan*
Received 29th May 2009, Accepted 14th July 2009
First published as an Advance Article on the web 29th July 2009
DOI: 10.1039/b914286j
By applying water as the reaction medium, the one-pot synthesis of 1,2-disubstituted
benzimidazoles has been achieved in excellent efficiency and selectivity at room temperature via
trimethylsilyl chloride promoted reaction of o-phenylenediamine with aldehyde. This green
catalyst system has also been successfully extended to the synthesis of quinoxalines via the
reaction of o-phenylenediamine with a-bromoketone. Water displayed a specific functionality in
mediating the selectivity, and remarkable advantages over organic solvents in terms of yields as
well as in the work up procedure of the reactions.
condition turned out to be the most facile and effective
Introduction
method to synthesize 2-substituted benzimidazole 414 and
1,2-disubstituted benzimidazole 3.15 On the other hand, the
synthesis of quinoxalines was usually achieved by the reac-
tions of o-phenylenediamine with dicarbonyl compounds16,
the oxidative cascade reactions of o-phenylenediamine with
a-hydroxyketones17, epoxides18 and diols19 in the presence of
either noble metal or additional oxidants/microwave assistance.
The synthesis using multicomponent reactions has also been
recently reported.20 Interestingly, as an equivalent chemical pre-
cursor of a-hydroxyketones, a-bromoketones has been claimed
in much less cases as reaction partners of o-phenylenediamine
to prepare quinoxalines.21
Benzimidazole and quinoxaline are ubiquitous hetero-
cyclic units in pharmaceuticals and bioactive natural prod-
ucts. Benzimidazole derivatives bear versatile pharmacological
properties1 based on their presence in both clinical medicines2
and compounds of broad biological functions.3 In addition,
the treatment potency of benzimidazoles in diseases such as
ischemia-reperfusion injury4a, hypertension,4b obesity 4c etc. have
been recently reported. Quinoxaline is also a heteroaromatic unit
of extensive interests owing to its occurrence in many biocides5a,
pharmaceuticals5b and various biofunctional molecules.5c More-
over, quinoxalines displayed practical utilities in the fields of
organic semiconductor materials6 and organic synthons.7
Among the reactions of o-phenylenediamine with aldehyde,
the selectivity in forming 1,2-disubstituted benzimidazole 3 and
2-substituted benzimidazole 4 is an issue of high interest. As a
large body of protocols have been established for synthesizing
benzimidazoles of type 414, relatively rare methods have been
reported to directly prepare 1,2-disubstituted benzimidazole 3
with ideal selectivity.15 Recently, several elegant catalyst systems
have been developed to prepare 3 in excellent chemoselectivity by
employing water as the medium at the presence of organometal-
lic catalysts.15d–f Given the desire for developing more economi-
cal and facile methods of less environmental impact in organic
synthesis, we wish to report herein the trimethylsilyl chloride
(TMSCl)22 promoted benzimidazole synthesis in water. One
highly interesting aspect of this protocol is that water as the
medium displayed specific inducing effect for the selective forma-
tion of 1,2-disubstituted benzimidazole 3 since previous report
on the TMSCl promoted, organic solvent (DMF) mediated
condensation of o-phenylenediamine with aldehyde furnishes
product 4 as the major product (Scheme 1).23 In addition,
the application scope of this highly green catalyst system
has been successfully extended to the effective synthesis of
quinoxaline by using o-phenylenediamine and a-bromoketones,
which represents the first synthesis of quinoxalines in water
through such transformations.
There are currently a number of synthetic methodologies
available for both the synthesis of benzimidazoles and quinox-
alines. Generally, the condensation of o-phenylenediamines
and carboxylic acids (or their derivatives such as nitriles,
imidates, orthoesters) had been widely used for the benzimi◦da-
zole synthesis, but harsh dehydrating conditions (170–180 C)
are usually required.8 Alternative approaches such as palla-
dium catalyzed tandem carbonylation–cyclization reaction of
o-phenylenediamine9, palladium catalyzed tandem dehydration-
coupling reaction of 2-bromoaniline10, rhodium catalyzed
hydroformylation reaction of N-alkenyl phenylenediamines11,
reductive cyclization reaction of o-nitroaniline with aldehydes12,
solid-phase supported synthesis13 etc. have been also de-
veloped to prepare functionalized benzimidazoles. However,
directly employing the condensation–aromatisation reac-
tion of o-phenylenediamines and aldehydes under oxidative
Department of Chemistry, Zhejiang University, Hangzhou, 310027,
People’s Republic of China. E-mail: panyuanjiang@zju.edu.cn; Fax: +86
57187951629; Tel: +86 571 87951264.
† Electronic supplementary information (ESI) available: General exper-
imental procedure, analytical data as well as copies of 1H and 13C NMR
spectra of all products. Copies of 1H NMR and ESI-MS+ spectra of
3a-d2. See DOI: 10.1039/b914286j
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Scheme 1 Selective synthesis of 2- and 1,2-substituted benzimidazole as well as quinoxaline.
together with TMSCl, which further demonstrated that acid
is not the sole promoting factor of the protocol reaction. The
reaction conditions were finally optimized as follows: 1 equiv
mole TMSCl in 2 mL water at room temperature (entry 13).
Following the determination of the optimal reaction condi-
tions, the application scope of this reaction has been examined
by subjecting different diamines and aldehydes. By using water as
solvent at room temperature, 1,2-disubstituted benzimidazoles
with various functional groups were obtained in excellent
selectivity and yields (Table 2). Among the reactions of different
aromatic aldehydes, no significant distinction on the yield
Results and discussion
We initially employed 1:1 mol o-phenylenediamine and ben-
zaldehyde for reaction in water in the presence of TMSCl. To
our surprise, the reaction did not proceed to give expected
2-substituted benzimidazole as expected, instead, the 1,2-
disubstituted benzimidazole 3 was obtained as a single product
after simple filtration. The excellent chemoselectivity mediated
by water inspired us to further investigate this transformation.
Various conditions have then been designed to determine the
efficiency of this model reaction using o-phenylenediamine and
2 equiv mol of aldehydes (Table 1). Different acids, solvents as
well as reaction temperature have been screened and TMSCl
turned out to be the proper promoter to give ideal result. It is
interesting that TMSCl in this reaction serves more than the
acid source in water according to the remarkably better results
in both the product yield and selectivity from these entries than
those using acids (entries 2 and 3). In addition, the target product
was isolated in 35% yield when 1 equiv mol of K2CO3 was added
Table 2 Water mediated synthesis of 1,2-disubstituted benzimidazolesa
Entry
R¢
R
Product
Yield (%)b
Table 1 Synthesis of 1-benzyl-2-phenyl-1H-benzo[d]imidazole under
different conditionsa
1
2
3
4
5
Ph
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
3a
3b
3c
3d
3e
3f
3g
3h
3i
87
85
78
72
75
80
85
91
88
83
85
79
92
75
75
65
51
80
43
78
70
66
trace
4-CH3C6H4
4-CH3OC6H4
4-OHC6H4
4-FC6H4
4-ClC6H4
4-BrC6H4
4-NO2C6H4
3-NO2C6H4
3-OHC6H4
2-OHC6H4
2-CH3OC6H4
2-BrC6H4
2-ClC6H4
furan-2-yl
thiophene-2-yl
PhCH CH2
4-CH3C6H4
H
6
7
Entry
Catalyst (eq)
Solvent
T (◦C)
Yield (%)b
8d
9
1
2
3
no
H2O
H2O
H2O
H2O
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
70
50
rt
rt
25 (42)c
49 (18)
30
83
51
10
11c
12
13
14
15
16
17d
18
19
20
21
22
23
3j
3k
3l
PTSA (0.6)
HCl (0.6)
4
5
6
7
8
9
10
11
12
13
14d
TMSCl (0.6)
TMSCl (0.3)
TMSCl (0.6)
TMSCl (0.6)
TMSCl (0.6)
TMSCl (0.6)
TMSCl (0.6)
TMSCl (0.6)
TMSCl (0.6)
TMSCl (1.0)
TMSCl (1.0)
3m
3n
3o
3p
3q
3r
3s
3t
H2O
Hexane
Cyclohexane
Toluene
DMF
EtOH
H2O
H2O
H2O
H2O
43
41 (26)
25 (12)
40
46
70
62
87
35 (39)
=
H
3-CH3O
3-benzoyl
3-benzoyl
3-benzoyl
3-benzoyl
H
4-CH3OC6H4
4-ClC6H4
furan-2-yl
C2H5CHO
3u
3v
3w
a Unless specified, all reactions were carried out at 0.5 mmol diamine,
1.0 mmol aldehyde and open atmosphere. b All the yields in this section
were obtained from the silica gel chromatography isolated products for
accurate evaluation on the effect of reaction conditions. c The data in the
parentheses refer to the yield of 4a. d 1 equiv mol of K2CO3 was added.
a Unless specified, the reaction conditions are: 0.5 mmol diamine 1 and
1.0 mmol aldehyde 2 in 2 mL water stirred at rt for 5 h in the presence
of 0.5 mmol TMSCl. b Yield of purified product by simple filtration or
EtOH recrystallization. c The reaction mixture was heated at 85 ◦C for
8 h. d The product was purified by silical gel chromatography.
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of target products has been observed regardless of the high
diversity of the functional groups. An exception is the reaction
of salicylaldehyde (entry 11), which led to the formation of
the Schiff base product 5 (Scheme 2)24 under the standard
condition, instead, heating the reaction at 85 ◦C provided the
corresponding benzimidazole 3k. This unexpected phenomenon
was probably due to the existence of intramolecular hydrogen
bonds which prevented 5 from further cyclization to give 3k,
while heating the reaction could effectively break the energy
barrier of these hydrogen bonds to ensure the further process of
the reaction.
experiment in our study provided the first direct evidence to
support this mechanism.
Enlightened by the successful reaction of o-phenylene-
diamines with aldehydes in this system, we envisioned that func-
tional ketones might also incorporate into o-phenylenediamine
to give functional heterocyclic compounds. Despite many re-
ports demonstrated that a-hydroxyketones are the excellent
reaction partners to incorporate o-phenylenediamines to give
quinoxalines,17 we wish to construct the same scaffold using
a-bromoketones due to the easier access of this kind of
reactants.21 However, the expected transformation was not
detected when we employed the aforementioned room tempera-
ture catalysis condition for the reaction of o-phenylenediamine
with benzoyl bromomethane, instead, heating the reaction at
70 ◦C furnished anticipated quinoxaline 7a in good yield. This
modified method has been subsequently applied to the reaction
of o-phenylenediamine and a-bromoketones of various substi-
tutions (Table 3). According to the obtained data, this water-
mediated catalyst system bears general tolerance to functional
groups in the substrates, the 2-substituted quinoxalines were
isolated generally in satisfactory yields, which suggested that the
functional groups in the arene unit of a-bromoketones have no
evident influence on the reaction.
Scheme 2 Unexpected formation of Schiff base from salicylaldehyde.
Following the reactions of aromatic aldehydes, similar reac-
tions using aliphatic aldehydes have been investigated. As shown
in Table 2 (entries 17 and 23), cinnamaldehyde furnished corre-
sponding product 3q in modest yield whereas propanaldehyde
gave only trace amounts of target product.
Conclusions
In summary, a practical and green synthetic method has been
developed for the facile synthesis of both 1,2-disubstituted
benzimidazoles and quinoxalines. In the presence of TMSCl,
a broad range of functional heterocyclic compounds have been
easily synthesized in water without using any additional surfac-
tant or oxidant. In addition, the isotope labeling experiment
has been firstly employed in the reaction of benzaldehyde
with o-phenylenediamine, which demonstrated the 1,3-hydride
migration process during the formation of 1,2-disubstituted
benzimidazoles.
In order to probe the possible mechanism of the reaction
forming 3, we designed the isotope labeling reaction using
benzaldehyde-d1 to incorporate with o-phenylenediamine. Un-
der the standard catalyst condition used in above experiment,
the product was isolated as full deuterium labeled product in the
form of 3a-d2 (Scheme 3 and see ESI† for the characterization).
Combining the formation of Shiff base product 5 observed in
previous experiments, we could therefore deduce the general
homogeneous reaction mechanism as outlined in Scheme 3. The
aldehyde firstly reacted with diamine to form Shiff base of type
5 as the intermediate, in the presence of electrophilic catalyst,
the intramolecular 1,3-hydride migration was induced to directly
give the 1,2-disubstituted benzimidazole. It is noteworthy that
although this kind of mechanism has been previously proposed
in literature15a,d , the results obtained from the isotope labeling
Experimental
Typical procedure for the synthesis of 1,2-disubstituted
benzimidazoles in water
1.0 mmol aldehyde 2 and 2 mL water were located in a round
bottom flask, and 0.5 mmol diamine was then added. Finally,
0.5 mmol TMSCl was injected to the mixture. The reaction
was stirred at room temperature for 5 h to form homogeneous
suspension. The suspension was then filtered and the residue
was washed with 10 mL water to give analytically pure product.
When necessary, the crude product was recrystallized with
ethanol.
1-(2-chlorobenzyl)-2-(2-chlorophenyl)-1H-benzo[d]imidazo le
◦
(3n). Pale yellow crystal; mp: 158–159 C;15a 1H NMR
(DMSO-d6, 500 MHz) d 7.76 (d, 1 H, J = 8.5 Hz), 7.62 (d,
1 H, J = 7.5 Hz), 7.55 (t, 1 H, J = 8.0 Hz),7.50 (d, 2 H, J =
8.0 Hz), 7.42 (t, 1 H, J = 7.5 Hz), 7.38 (d, 1 H, J = 8.0 Hz),
7.29 (t, 2 H, J = 4.5 Hz), 7.25 (d, 1 H, J = 7.5 Hz), 7.16 (t, 1 H,
J = 7.5 Hz), 6.65 (d, 1 H, J = 8.0 Hz), 5.43 (s, 2 H); 13C NMR
(DMSO-d6, 125 MHz) d 151.93, 143.67, 136.09, 134.58, 134.30,
133.36, 133.02, 132.77, 130.85, 130.63, 130.58, 130.54, 129.37,
Scheme
disubstituted benzimidazoles.
3
Plausible mechanism of the reaction furnishing 1,2-
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Table 3 Water mediated one-pot synthesis of quinoxalinesa
J = 8.0 Hz), 8.13 (t, 2 H, J = 8.5 Hz), 7.89–7.84 (m, 2 H),
7.61–7.57 (m, 3H); 13C NMR (DMSO-d6, 125 MHz) d 152.17,
144.96, 142.60, 142.28, 137.23, 131.83, 131.63, 131.11, 130.41,
130.32, 130.05, 128.66; ESI-MS [M+H+]: m/z = 207.
Acknowledgements
Entry Substrate 6
Product
Yield (%)b
62
The authors thank the NSFC of China (20775069), NSFC
of Zhejiang province (Z206510) and NCET-06–0520 of the
National Ministry of Education of China for financial support
on this work. We also thank professor Henry Rudler (Universite´
Pierre et Marie Curie) for his valuable discussion on this work.
1
7a
2
3
7b
57
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128.59, 128.50, 124.24, 123.44, 120.78, 112.14, 46.25; ESI-MS
[M+H+]: m/z = 353.
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