F. Rajabi et al.
often generated in situ from the condensation of
phenylenediamines with aldehydes [11]. The first method
often requires strong acidic conditions, sometimes combined
with very high temperatures. The second method offers a
broad applicability because of the availability of a large
variety of aldehydes and oxidative reagents (i.e. nitroben-
zene, 1,4-benzoquinone, air, heteropoly acids, MnO2,
Pb(OAc)4, H2O2/HCl etc.). An alternative protocol for the
synthesis of benzimidazoles is the one-pot tandem synthesis
from alcoholsin the presence of bifunctional catalysts, where
alcohols are in situ oxidized to aldehydes followed by cyclo-
condensation with amine compounds [12–14]. However,
multistep processes, drastic reaction conditions, tedious
work-up procedures and low yields are among major draw-
backs of these methods. The additional serious drawbacks
relate to the use of homogeneous catalysts that are somewhat
modified in the work-up procedure and cannot be recovered.
In the light of these premises, the design of heterogeneous
catalysts for a direct, simple and highly selective
benzimidazole synthesis will be highly beneficial.
In continuation of our work on the development of low
loaded, environmentally sound, affordable, stable and se-
lective heterogeneous catalysts for greener organic reac-
tions, herein we report the use of a previously reported
reusable effective catalytic system based on cobalt (II) on a
mesoporous silica support (Co/SBA-15) for the synthesis
of benzimidazole compounds under mild reaction condi-
tions. The proposed catalytic system possesses inherent
advantages for benzimidazole synthesis including high
activity and stability, low metal loading in the catalyst as
well as the use of a relatively inexpensive transition metal
(Co) system and an unprecedented low catalyst loading in
the reaction (0.004 mmol Co, 0.014 mg catalyst).
2 Experimental
2.1 Preparation of SBA-15 Supported Cobalt
Nanocatalyst (Co/SBA-15)
In recent years, the use of heterogeneous catalysts has
received considerable interest in the synthesis of various
benzimidazole derivatives. Different oxide based supported
or unsupported catalysts including aluminosilicates [15],
iron oxide [16], cobalt oxide [17], ZnO [18], CuO [19] and
MoO3 [20] have already been explored. Supported
heteropolyacid catalysts have also shown very good effi-
ciency in such processes [21–23]. Recently, an efficient
room temperature synthesis of benzimidazole has been
described using zeolite catalysts under shorter reaction
time [24]. Excellent yields of 2-aryl-1-arylmethyl-1H-
benzimidazoles were also obtained using highly reusable
Amberlite IR-120 in aqueous media [25]. Expensive gold
catalysts on different support (e.g. TiO2, Al2O3, ZnO,
polyurea and hydrotalcite) have been recently utilised for
the synthesis of benzimidazoles from 2-nitroanilines under
mild reaction conditions [26]. In the course of reaction,
2-nitroanilines was hydrogenated to o-phenylenediamine
which subsequently underwent cyclization in presence of
CO2 and H2. An interesting organic–inorganic hybrid
porous iron–phosphonate material containing both mi-
cropores and mesopores and a high Fe loading (26.7 wt%)
has also been explored by Dutta et al., which showed ex-
cellent catalytic activity for the synthesis of benzimidazole
derivatives under mild conditions [27].
Salicylaldehyde (2 mmol, 0.244 g) was added to an excess
of absolute MeOH, to which 3-aminopropyl (trimethoxy)
silane (2 mmol, 0.352 g) was subsequently added. The
solution instantly became yellow due to imine formation.
After 3 h, cobalt (II) acetate, Co(OAc)2Á2H2O (1 mmol,
0.248 g) was added to the solution and the mixture further
stirred for 3 h to allow the new ligands to complex the
cobalt. A colour change from pink to olive green is ob-
˚
served. Mesoporous silica (average pore diameter 60 A,
3 g) was activated by refluxing in concentrated hy-
drochloric acid (6 M) and then washed thoroughly with the
deionized water and dried before undergoing chemical
surface modification. This activation treatment readily
hydrolyses the siloxane Si–O–Si bonds to Si–OH species
which will be key to anchor the cobalt complex. The
complex and the activated silica were then mixed and the
mixture stirred overnight. The solvent is removed using a
rotary evaporator, and the resulting olive green solid dried
at 80 °C overnight. The final product was washed with
MeOH and water (to remove all physisorbed metal species)
until the washings were colourless. Further drying of the
solid product was carried out in an oven at 80 °C for 8 h.
2.2 General Procedure for the Synthesis
of Benzimidazoles
Most reported heterogeneously catalysed protocols
nevertheless require high catalyst loadings, expensive and/
or sophisticated catalysts to be developed, low catalyst
stability or high loadings of metals [15–27]. Consequently,
the design of low metal loaded catalysts featuring a good
stability, activity and environmentally friendly nature can
provide additional advantages to existing methodologies in
the synthesis of benzimidazole derivatives.
In a typical run, a 50 mL round bottom flask with a mag-
netic stir bar was charged with aldehyde (1 mmol), o-
phenylenediamine (1 mmol), Co/SBA-15 nanocatalyst
(0.004 mmol, 0. 014 g) and ethanol (2 mL) under oxygen
atmosphere at 60 °C for 4 h. The progress of reaction was
monitored by TLC (EtOAc: Hexane = 1:5). After reaction
completion, the solvent was removed under vacuum and
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