A green and efficient method for synthesis of benzimidazoles using nano-Fe3O4 in PEG-400/H2O aqueous system under ambient conditions at room temperature

Article abstract of DOI:10.1002/aoc.3145

In this paper, a green and facile protocol was described which was efficient for synthesis of benzimidazoles using nano-Fe₃O₄ catalyst with continuous bubbling of air as the oxidant in PEG-400/H ₂O aqueous system at room temperature. This protocol afforded the target products in good to excellent yields and the catalytic system could be recycled and reused without significant loss of catalytic activity. Copyright

Full text of DOI:10.1002/aoc.3145

Full Paper
Received: 11 December 2013
Revised: 5 February 2014
Accepted: 24 February 2014
Published online in Wiley Online Library: 20 April 2014
(wileyonlinelibrary.com) DOI 10.1002/aoc.3145
A green and efficient method for synthesis of
benzimidazoles using nano-Fe O in PEG-400/
3
4
H O aqueous system under ambient conditions
2
at room temperature
Zhao-gang Wang, Jie Zhu, Zhou-shuo Zhu, Jian Xu and Ming Lu*
In this paper, a green and facile protocol was described which was efficient for synthesis of benzimidazoles using nano-Fe
3 4
O
catalyst with continuous bubbling of air as the oxidant in PEG-400/H O aqueous system at room temperature. This protocol
2
afforded the target products in good to excellent yields and the catalytic system could be recycled and reused without
significant loss of catalytic activity. Copyright © 2014 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: benzimidazoles; nano-Fe O ; PEG-400/H O aqueous system; room temperature
3
4
2
conventional heterogeneous catalysts. Among various MNPs,
Fe MNPs have been used as a high-performance support
Introduction
3 4
O
Benzimidazoles are very useful compounds for development in
because they are inexpensive, environmentally benign, have
good chemical stability and are conveniently recovered after
the pharmaceutical industry, and exhibit significant biological
[
1]
activity against several viruses, such as human immunodefi-
reaction by simple magnetic separation using, for example,
[
1]
[2]
[16]
ciency virus (HIV), herpes simplex virus type 1 (HSV-1),
influenza
Fe O -supportted sulfamic acid,
Fe O -supported dual
3 4
3
4
[3]
[1]
[17]
and human cytomegalovirus.
Although many
acidic ionic liquid,
Fe O -supported palladium–bipyridine
3 4
[
18]
[19]
methods are reported for synthesis of benzimidazole, there are
two methods for the synthesis of benzimidazole compounds
generally. One is the condensation of carboxylic compounds with
o-phenylenediamines, which is usually performed under strongly
acidic conditions, high temperature or microwave irradiation.
The other is oxidative dehydrogenation of Schiff bases, often
generated from phenylenediamine and aldehydes. Various
oxidants such as 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
complex, Fe O -supported Pd(0) or Fe O -supported pybox
3 4 3 4
[
20]
copper(I) catalyst.
In addition, Fe O4 MNPs has also been
3
demonstrated as an efficient catalyst for many useful chemical
[
21]
transformations, including thiolysis of epoxides, dehydrogena-
[
22]
[23]
tion of ethylbenzene,
synthesis of diverse N-heterocycles,
[
24]
synthesis of α-aminonitriles,
synthesis of decahydroacridine
[
25]
[26]
derivatives,
synthesis of quinoxalines,
Sonogashira–
[
27]
[28]
Hagihara reactions,
Friedel–Crafts acylation reaction,
[
4]
[5]
[6]
[7]
[8]
[9]
[29]
(
(
DDQ), FeCl3, MnO , H O , Pb(OAc)4, Oxone, PhI
trimethylsilyl protection of hydroxyl groups, three-component
coupling of aldehyde, alkyne and amine and N-Boc protection
of amines.
Recently, many aqueous-phase studies have been reported.
2
2 2
[
10]
[11]
[12]
[13]
[30]
OAc)2,
Na S O ,
benzofuroxan
and K [Fe(CN) ]
have
2
2
5
4
6
[
31]
been employed in the reaction. Although these methods are
effective, some suffer from one or more deficiencies such as the
use of strong proton acid, high temperature, long reaction time
and recovery of catalysts. Recently, air or oxygen has been used
[
32–37]
Water, as the safest reaction medium, is cheap and widely
distributed on the Earth, and possesses significant advantages
of mild reaction conditions, anti-vice, non-polluting to the
environment and effectively preventing loss of product, due to
its low solubility of organic products compared with organic
solvents. Furthermore, water could provide reactions with a
new reaction micro-environment, which is obviously different
from traditional solvents, and promote notably improved
selectivity of organic reaction. However, owing to its own unique
[
3]
as a green oxidant for the synthesis of benzimidazoles because
of its inexpensive, non-toxic, and inexhaustible benign nature.
[14]
Kawashita et al.
have reported that 2-phenylbenzimidazole
was prepared from o-phenylenediamine and benzaldehyde using
[
15]
2
O /activated carbon system in 64% yield at 120°C for 26 h. Lin
has also reported the condensation of benzaldehyde with
o-phenylenediamine in 85% yield using air as the oxidant in
1
,4-dioxane at 100°C for 16 h. These methods are effective and
practical; however, some still have one or more drawbacks such
as high temperature and low product yield. Therefore, much
effort is still needed to develop an eco-friendly method.
In the past decade, magnetic nanoparticles (MNPs) have
emerged as a new class of catalyst owing to ultrafine size and
high surface area, which exhibit higher catalytic activity than
* Correspondence to: Ming Lu, School of Chemical Engineering, Nanjing
University of Science and Technology, Nanjing 210094, China. E-mail:
luming302@126.com
School of Chemical Engineering, Nanjing University of Science and Technology,
Nanjing 210094, China
Appl. Organometal. Chem. 2014, 28, 436–440
Copyright © 2014 John Wiley & Sons, Ltd.

Green and efficient method for synthesis of benzimidazoles
physical properties, such as high molecule cohesive energy density
and dielectric constant, the aqueous phase has notable features of
a lipophobic effect during organic reactions, in which most organic
compounds are non-soluble, thereby severely limiting the applica-
tion of water in organic reactions. One of the most common
approaches is to add surfactants to the aqueous phase to achieve
the effect of increasing solubility and performing its hydrophobic
effect, which plays a role in the protection of some sensitive
reagents to water. However, this method still suffers from the
deficiency of cumbersome after-treatment, involving a demul-
sification operation of the reaction solution, in which the oil phase
is dispersed in aqueous solution to form an emulsion when
performed with addition of surfactants and vigorous stirring.
Polyethylene glycol (PEG), a commercially available, non-toxic and
recyclable polymer, has been widely used in organic transformation
(KBr). Transmission electron microscopy (TEM) was performed
using a JEM-2100 instrument. Melting points were determined
on a PerkinElmer differential scanning calorimeter. Elemental
analysis was performed using an Elementar Vario Micro
1
spectrometer. H NMR spectra were recorded on a Bruker Avance
III (500 MHz) spectrometer with TMS as an internal standard.
HPLC experiments were performed using a liquid chromatograph
(Shimazhu LC-20AT, Japan).
Experimental Procedure
Preparation of Fe
3 4
O MNPs
Fe MNPs were prepared by the co-precipitation method
3 4
O
according to a previously reported procedure. First, FeCl .6H
O (2.0 g) were dissolved in distilled water
50 ml) in a three-necked round-bottom flask (150 ml). The
3
2
O
[38–45]
as an environmentally benign solvent,
which is has potential as
(
(
2 2
5.4 g) and FeCl .4H
a substitute for volatile organic solvents, e.g. in N-arylation of
amines, cross-coupling reaction, asymmetric dihydroxylation, aldol
reaction, Baylis–Hillman reaction and multi-component condensa-
tion. On the other hand, PEG is an amphiphatic molecule, which
presents some interesting characteristics of a surfactant, owing to
its special structural properties, with high solubility in water and
most common organic compounds. Also, the recovery of products
is more easily achieved than using surfactants under mild reaction
resulting solution was stirred with mechanical stirring for 5 h. A
solution of aqueous ammonia (10 ml, 25 wt%) was then added
to the solution in a drop-wise manner over a 30 min period.
The reaction mixture was then stirred under room temperature
for 3 h. The resulting Fe
and washed thoroughly with distilled water and ethanol three
times, then dried under vacuum for 12 h to give Fe MNPs.
3 4
O MNPs were collected with a magnet
3 4
O
[
46]
conditions. In 2012, Zolfigol
synthesis of benzimidazoles in excellent yields (85–97%) using a
nano-Fe /O system. In continuation of Zolfigol’s investigations,
here we report a simple, mild and efficient method for the synthesis
of benzimidazoles using Fe nanoparticles with continuous
bubbling of air as the oxidant in PEG-400/H O aqueous solution at
and co-workers reported the
3 4
O
2
Typical Procedure for Synthesis of Benzimidazole
Compounds
3 4
O
o-Phenylenediamine 1.08 g (10 mmol) and benzaldehyde 1.06 g
(10 mmol) were thoroughly mixed in PEG400/H O aqueous solution
(V(PEG400/H O)= 1:2, 15 ml) in a 25 ml three-necked flask equipped
with a water-cooled condenser and a gas inlet with vigorous
stirring at room temperature. Then, Fe nanoparticles 0.46 g
2
room temperature (Scheme 1). This protocol afforded high
catalytic activity and mild conditions due to the surfactant-like
property of the PEG molecule, which increased the solubility of
organic compounds and effectively transferred the two phases as
one component. Also, the reaction products were easily recovered
in the organic phase by adding ether to the reaction system, due
to the presence of water which could reduce the viscosity of PEG.
Moreover, the nano-catalyst could be conveniently recovered and
reused without significant agglomeration and sintering, and the
2
2
3 4
O
(2 mmol) were added, and air was continuously bubbled by a small
pump at an average speed of 60 bubbles per minute. When the
reaction was complete, as monitored by TLC analysis, Fe O
3 4
nanoparticles were collected by magnetic force, subsequently
washed with ethanol and dried under vacuum to give recycled
2
PEG-400/H O aqueous system could also be reused for further
Fe O nanoparticles. The reaction mixture was extracted with ether
3 4
cycles by adding new substrates.
three times, and the organic layer was dried using anhydrous
magnesium sulfate and rotary evaporation under reduced
pressure. The residue was purified by column chromatograph on
a silica gel using n-hexane–ethyl acetate (10:1) as eluent to afford
the pure product in 92% yield; m.p. 292–294°C. The next run was
performed under identical reaction conditions.
Experimental
General Information
All starting materials were purchased from commercial sources
and used without further treatment. X-ray diffraction (XRD) was Results and Discussion
performed using a Bruker Avance D8 XRD instrument. IR spectral
analysis was performed using a Nicolet IS-10 FT-IR spectroscope
XRD, FT-IR adsorption and TEM analysis of Fe O MNPs are shown
in Fig. 1. The position and relative intensities of all peaks
3
4
(2θ = 30.1°, 35.4°, 43.1°, 53.4°, 57°, 62.6°), which are assigned to
(
2 2 0, 3 1 1, 4 0 0, 4 2 2, 5 1 1, 4 4 0) were consistent with the
[
47]
JCPDS file (JCPDS19-0629), indicating the crystalline cubic spinel
of Fe
À1
O
3 4
MNPs. The strong FT-IR adsorption at 578 cm was the
Fe―O stretching vibration and the broad band at around 3500–
À1
3
000 cm was attributed to adsorbed solvents. Also, the control
experiment of FT-IR adsorption showed no obvious differences
in characteristic adsorption peaks between fresh Fe MNPs
and reused Fe MNPs. These results revealed that the catalyst
was stable and could endure the reaction conditions. TEM im-
3 4
O
3 4
O
2
Scheme 1. Diagram of the catalytic process in the PEG-400/H O
aqueous system.
ages of nano-Fe
3
O
4
show that the synthesized nanoparticles
Appl. Organometal. Chem. 2014, 28, 436–440
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

Z.-g. Wang et al.
different parts of the molecule for the two phases, making their
surfaces turn to face the inside of the PEG molecule. This could
increase the oil–water contact area and effectively reduce
surface tension.
The catalytic system was investigated for the synthesis of
2-phenylbenzimidazole via a model reaction of o-phenylenediamine
with benzaldehyde. First, the reaction was performed in the
absence of air under an N atmosphere, when extremely little
2
product was observed by HPLC analysis. The result suggests that
oxygen played an oxidant role in this reaction. When the reac-
tion was run under a static atmosphere of air, the product yield
was good after a long reaction time of 12 h. Following on from
the above results, the reaction was subsequently run using
continuous bubbling of air, with the best performance in terms
of reaction time and product yield. This indicates that continu-
ous bubbling of air is more efficient than reaction just under
air atmosphere, probably as a result of the increase of gas–liquid
contact area and mass transfer efficiency. However, when the
reaction was run under continuous bubbling of oxygen, the
product yield was lower than using continuous bubbling of air,
in spite of lower reaction time, which can probably be attributed
to excessive oxidation of substrates in the pure oxygen atmo-
sphere (Fig. 2).
The reaction was then carried out in the presence of various
metal oxides as catalyst under the same conditions (Table 1,
entries 1–3 and 6). We were delighted to find that the reaction
with nano-Fe
high efficiency here could be attributed to the large surface area
and ultrafine size offered by the Fe nanoparticles, which
3 4
O completed in 5 h to give a yield of 92%. The
3 4
O
dispersed well during the reaction process to form a pseudo-
homogeneous system and alleviate the disadvantage of the
corresponding heterogeneous catalysts. The influence of catalyst
amount was investigated in the range of 0–40 mol% with a
maximum activity at 20 mol% (Table 1, entries 4–8). As control
experiments, we tested the effects of solvents by using
commercial solvents under the same reaction conditions (Table 1,
entries 6 and 9–14). The use of CH
2
2 3 2 5
Cl and CH COOC H afforded
low yields of product, whereas CH
3
CN and DMF gave moderate
yields. When the experiment was run in ethanol and methanol,
the results were good and similar. As we expected, the best
result was obtained with the PEG/H O system, which might be
2
attributed to the high binding capacity of the PEG/H O system
2
with oxygen derived from hydrogen bonding between molecules
and excellent oxygen solubility of the PEG polymer. Moreover,
Figure 1. XRD, FT-IR adsorption and TEM analysis: (a) fresh; (b) after
using five times.
were of uniform size (average size ~25 nm); the aggregation
phenomenon was not clearly presented after the catalyst
had been used five times. Notably, PEG polymer acts not only
as a reaction solvent but also as a green stabilizer, limiting
particle growth and effectively preventing their agglomeration
[
48,49]
and sintering.
As we know, PEGs are amphiphatic molecules as a result of
their special long chain structure consisting of ether bonds.
The oxygen atom of ether bonds could combine with a water
molecule to afford oxonium compounds, and the ―CH units
of PEG increase the solubility of organic compounds, which is
derived from the hydrophobic gathering effect of the hydrocar-
bon chain. The mechanism for transferring the two phases as
one component involves the PEG molecules being arranged
between the two phases through the separate affinity of
Figure 2. Effect of oxidant with different oxygen sources.
wileyonlinelibrary.com/journal/aoc
Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2014, 28, 436–440

Green and efficient method for synthesis of benzimidazoles
Table 1. Influence of reaction conditions on the synthesis of 2-phenyl
a
Table 2. Synthesis of benzimidazoles from aldehydes and diamines
a,b
benzimidazoles
b
Entry
X
R
1
R
2
Time (h) Yields (%)
c
Run
Catalyst
mol%)
Solvent
Temperature Yield
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
H
H
H
H
N
N
H
Ph
5
6
92
86
81
91
95
93
96
94
84
85
80
74
(
(°C)
(%)
H
6 4
4-OHC H
1
2
3
4
5
6
7
8
9
Al
2
O
3
(20)
(20)
PEG/H
PEG/H
PEG/H
PEG/H
PEG/H
PEG/H
PEG/H
PEG/H
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
r.t.
80
87
75
58
82
92
91
92
88
85
45
50
70
68
86
64
51
40
H
4-Me
2
NC
6
H
4
10
MnO
2
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
40
H
2-ClC
4-ClC
6
6
H
H
4
4.5
CuO (20)
H
4
4
4
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
(0)
H
4-BrC
6 4
H
(10)
(20)
(30)
(40)
(20)
(20)
(20)
(20)
(20)
(20)
(20)
(20)
(20)
(20)
H
2, 4-ClC
6
H
3
3
H
3-NO
Ph
2
C
6
H
4
5
4-NO
H
2
10
12
14
15
c
10
11
12
H
Ethanol
H
H
H
10
11
12
13
14
15
16
17
18
Methanol
5,6-Diamno
Dichloromethane
Ethyl acetate
Acetonitrile
DMF
a
Reaction conditions: diamines (10 mmol) and aldehyde (10 mmol),
nano-Fe , 0.46 g (2 mmol), PEG/H O (15 ml). The reaction was
monitored by TLC analysis.
3
O
4
2
b
Products were purified by column chromatography and
PEG/H
PEG/H
PEG/H
PEG/H
2
2
2
2
O
O
O
O
1
confirmed by HPLC,
H NMR, elemental analysis and
60
melting point.
80
c
3
7% formaldehyde solution was used.
Reflux
a
Reaction conditions: o-phenylenediamines 1.08 g (10 mmol) and
benzaldehyde 1.06 g (10 mmol), nano-Fe 0.46 g (2mmol),
PEG/H O (15 ml). The reaction was stirred under ambient
3 4
O
formaldehyde (Table 2, entries 1 and 10), probably due to
the conjugated effect of the phenyl group of benzaldehyde,
which promoted an increase of the positive electrical property
of the carbon atom of aldehyde and facilitated nucleophilic
attack of the amine group. On the other hand, a diamine-
bearing electron-deficient group on the aromatic rings or
containing a hetero atom proceeded more inefficiently than
o-phenylenediamine in terms of longer reaction time and
lower yield (Table 2, entries 1, 9, 11 and 12).
2
conditions for 5 h.
b
c
Continuous bubbling of air as oxidant.
Yield of isolated product.
2
the PEG/H O system is thought to offer a practical advantage
over methanol or ethanol in regard to considerations of toxicity
and volatility. In addition, the impact of reaction temperature is
shown in Table 1, and it appears that room temperature gave
the highest product yield. A high reaction temperature (>30°C)
would cause a gradual decrease in conservation due to
escape of oxygen from the reaction system (Table 1, entries 6
and 15–18).
The reuse of catalyt is highly preferable for a green process.
Thus the recyclability of Fe O MNPs was investigated by
3 4
using o-phenylenediamine and benzaldehyde as a model
reaction. The catalyst was easily separated by using an
external magnet after the reaction and reused in a subse-
quent reaction. Nearly quantitative Fe O MNP catalyst could
3
4
Encouraged by the previous results, we continued to study the
scope of reaction, which could be extended to a series of
structurally variable arylamines and aromatic aldehydes. In most
cases, o-phenylenediamine reacted smoothly with various
benzaldehydes to give the corresponding products with good
yields. It appeared that benzaldehyde bearing electron-deficient
groups on the aromatic rings gave a higher yield of products
than those bearing electron-rich groups (Table 2, entries 2–8).
The reason might be that the electron-withdrawing groups
enhanced the electrophilicity of the carbon atom of the aldehyde
group to increase the reaction rate. In addition, the position of
the substitute group relative to the aldehyde group on the
aromatic rings also influenced the reaction rate and yield. The
sterically hindered o-chlorobenzaldehyde reacted with o-
phenylenediamine to give the target product in lower yield than
p-chlorobenzaldehyde (Table 2, entries 4 and 5). Moreover,
aromatic aldehydes like benzaldehydes reacted faster with o-
phenylenediamine than aliphatic aldehydes such as
be recovered from each run. In a test of five cycles, the
catalyst could be reused without significant loss of catalytic
activity (Fig. 3).
The mechanism for synthesis of benzimidazoles from
aromatic diamines and aldehydes is shown in Scheme 2. The
first step was dehydration to form a schiff base with
o-phenylendiamine. During the next steps, ring closure led to
a five-membered ring, and dehydrogenation with air oxidation
[50,51]
to form benzimidazole.
Conclusions
We have developed a green and efficient method for the
3 4
synthesis of benzimidazole compounds using Fe O MNPs with
continuous bubbling of air as the oxidant in PEG-400/H
2
O
aqueous solution at room temperature. Owing to the high
efficiency of nanomaterials and the surfactant-like property of the
Appl. Organometal. Chem. 2014, 28, 436–440
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

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[
[
[
[
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Appl. Organometal. Chem. 2014, 28, 436–440