Graphene oxide-chitosan bionanocomposite: A highly efficient nanocatalyst for the one-pot three-component synthesis of trisubstituted imidazoles under solvent-free conditions

Article abstract of DOI:10.1039/c5ra03355a

In the present work, a green, rapid, convenient and eco-friendly method for the synthesis of 2,4,5-trisubstituted imidazoles is described. In this method, we used 1,2-diketone or α-hydroxy ketone, an aldehyde and ammonium acetate in the presence of graphene oxide-chitosan bionanocomposite, which acted as an efficient nanocatalyst. This protocol has many advantages such as short reaction time, high yield, easy separation of the catalyst and solvent-free condition. We used differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR) and atomic force microscopy (AFM) to confirm that the nanocomposite was formed, and FT-IR, ¹H and ¹³C NMR analyses were performed for the confirmation of the synthesized products.

Full text of DOI:10.1039/c5ra03355a

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Graphene oxide–chitosan bionanocomposite: a
highly efficient nanocatalyst for the one-pot three-
component synthesis of trisubstituted imidazoles
under solvent-free conditions
Cite this: RSC Adv., 2015, 5, 33177
*
Ali Maleki and Reza Paydar
In the present work,
a green, rapid, convenient and eco-friendly method for the synthesis of
2,4,5-trisubstituted imidazoles is described. In this method, we used 1,2-diketone or a-hydroxy ketone,
an aldehyde and ammonium acetate in the presence of graphene oxide–chitosan bionanocomposite,
which acted as an efficient nanocatalyst. This protocol has many advantages such as short reaction time,
high yield, easy separation of the catalyst and solvent-free condition. We used differential scanning
calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR) and atomic force microscopy (AFM) to
confirm that the nanocomposite was formed, and FT-IR, 1H and 13C NMR analyses were performed for
the confirmation of the synthesized products.
Received 23rd February 2015
Accepted 24th March 2015
DOI: 10.1039/c5ra03355a
www.rsc.org/advances
several methods for the construction of imidazoles have been
developed.
Introduction
Among these methods, the one-pot reaction of diketones is
extensively used for the synthesis of 2,4,5-trisubstituted imid-
azoles.2,3 Various catalysts can be used in this reaction such as
heteropolyacids,4 uranyl nitrate hexahydrate supported on
acidic alumina,5 cellulose sulfuric acid,6 Y(TFA),7 Mo(IV),8
Eu(OTf)3,9 SbCl3/SiO2,10 ZrCl4,11 scolecite,12 NiCl2$6H2O,13
H14[NaP5W30O110],14 iodine,15 MoO3/SiO2,16 silica sulfuric
acid,17,18 L-proline,19 PEG-400,20 Cu(TFA)2,21 p-TSA/TBAI,22
lipase,23 NBS,24 Zr(acac)4,25 and different uoroboric acid
derived catalysts.2 Nevertheless, many of these approaches
suffer from a long reaction time, low yield, acidic media, diffi-
cult work-up, expensive catalysts and excessive use of reagents
and catalysts. Therefore, introducing a new green catalyst
having good recyclability is of prime importance.
Recently, nanoscience has been one of the hot topics among
chemists, physicists, biologists and other scientists because of
its novel and unique applications in various elds. We are living
in an era in which everything is becoming smaller and smaller in
size with improvements in properties. Use of this science in the
catalysis of various reactions, especially in organic reactions,
appears to be essential and important. For comprehending the
importance of this science in the catalytic eld, we have intro-
duced a green nanocomposite that has precious properties such
as thermal stability, mechanical strength and a wide surface.
Graphene is a single-layer of sp2-hybridized carbon atoms
placed in a two-dimensional lattice, which has attracted
tremendous attention in recent years for possessing exceptional
thermal, mechanical, and electrical properties. Graphene oxide
(GO) is generally produced by the treatment of graphite using
Multicomponent reactions (MCRs), as an important organic
synthesis approach, are one-pot processes, which involve reac-
tion of three or more accessible components to form a single
product that incorporates essentially most or all the atoms of
the starting materials. MCRs strategies grant remarkable
advantages over conventional bimolecular reactions owing to
their convergence, atom-economy, operational simplicity,
structural diversity and short synthetic pathway. MCRs have
recently gained a new dimension in the eld of designing
methods to produce elaborate libraries of biologically active
compounds and new molecular frameworks for potential drugs
with diverse pharmacological activities.1
In recent years, the importance of imidazoles in biological
systems has attracted considerable interest because of their
chemical and biochemical properties, and compounds with an
imidazole ring system also have many pharmacological prop-
erties and can play an important role in biochemical processes,
and accordingly they have gained considerable interest among
chemists. The heterocyclic compounds including 2,4,5-trisub-
stituted imidazoles possess versatile pharmaceutical actions
such as being anti-bacterial, anti-tumor agents, anti-
inammatory, p38 MAP kinase inhibitors, glucagon receptor
antagonists and inhibitors of mammalian 15-LOX.2,3 These
propellants have engendered considerable interest among
synthetic organic and medicinal chemists in recent years. Thus,
Catalysts and Organic Synthesis Research Laboratory, Department of Chemistry, Iran
University of Science and Technology, Tehran 16846-13114, Iran. E-mail: maleki@
iust.ac.ir; Fax: +98-21-73021584; Tel: +98-21-77240540-50
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ultrasonically mixed for 1 h, N,N0-dicyclohexylcarbodiimide
(DCC) and 4-dimethylaminopyridine (DMAP) were then added
into the abovementioned suspension and the resulting solution
was allowed to stand at room temperature for 48 h. The
resulting solid was isolated by centrifugation and then washed
with ortho-dichlorobenzene (ODCB) to remove unreacted chi-
tosan. The mixture was subsequently washed completely with
Fig. 1 The structures of chitosan (I) and GO–chitosan (II).
ꢀ
water, methanol and acetone. Finally, it was dried at 60 C for
24 h under vacuum.
FT-IR spectroscopy was used for the conrmation of the
formation of amide bond between GO and chitosan. The IR
spectra of GO, chitosan and GO–chitosan are compared in
Fig. 2. In the IR spectrum of GO, dominant peaks appear at 1035
cmꢁ1, 1225 cmꢁ1, 1623 cmꢁ1 and 1739 cmꢁ1. The peak placed at
1035 cmꢁ1 is assigned to the stretching vibration of the C–O–C
bonds of epoxy or alkoxy functionalities. The peak placed at
1225 cmꢁ1 is attributed to C–OH bonds. The peak placed at
1623 cmꢁ1 is attributed to C]C bonds. The peak located at
1739 cmꢁ1 is attributed to C]O bonds. In the IR spectrum of
chitosan, dominant peaks appear at 1593 and 1029 cmꢁ1, these
peaks are related to the C]O bond of NHCO and bending
vibration of NH2, respectively. In the spectrum of GO–chitosan,
the peak present at 1594 cmꢁ1 is attributed to the stretching
vibration of the C]O bond of NHCO, and the peak located at
1038 cmꢁ1 is attributed to the N–H bending of NH2. A peak at
1739 cmꢁ1 is absent in the IR spectrum of GO–chitosan, which
Scheme
imidazoles.
1 GO–chitosan-catalyzed synthesis of substituted
strong mineral acids and oxidizing agents, which nally inter-
rupt the delocalized electronic structure of graphite and induce
a variety of oxygen-based chemical functionalities on the
surface of graphite. While the precise structure of GO is
uncertain, it is thought that hydroxyl and epoxy groups are
present in the maximum value on the basal plane with
carboxylic acid groups mainly distributed at the edge of the
sheets.26
Chitosan, as a partially deacetylated derivative of chitin, is a
linear polysaccharide consisting of b-(1,4)-linked D-glucosamine
residues with variable randomly located N-acetyl-glucosamine
groups on it (Fig. 1, I). As
a natural polymer, poly-
aminosaccharide chitosan has attracted considerable attention
due to its unique properties such as biological renewability,
biodegradability, non-antigenicity and biocompatibility. It also
has several chemical, medicinal and industrial applications.27
In this study, we have graed chitosan to GO (Fig. 1, II). GO–
chitosan is more thermally stable than chitosan because of the
presence of amide bonds.
In continuation of our previous work on the preparation and
application of nanocatalysts in MCRs,28 herein we have prepared
and characterized GO–chitosan as a green nanocatalyst and used
it for a convenient and effective synthesis of 2,4,5-trisubstituted
imidazoles 4a–o through a one-pot three-component reaction of
benzoin 1 or benzil 2, benzaldehyde 3 and NH4OAc under
solvent-free conditions at 120 ꢀC (Scheme 1).
To the best of our knowledge, this is the rst report on the
application of GO–chitosan nanocomposite as a catalyst in
organic reactions. Thermal stability was one of the most
important factors for selecting this catalyst for this reaction.
Moreover, GO–chitosan nanocomposite has good biocompati-
bility and biodegradability.29
Results and discussion
The functionalization of GO with chitosan was rst performed
by procedures described in the literature.29 For functionaliza-
tion, GO and chitosan were added into a round-bottom ask,
which was previously charged with DMF. The mixture was Fig. 2 FT-IR spectra of chitosan, GO and GO–chitosan.
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Table
1
Optimization of conditions in the reaction of benzil 2,
benzaldehyde 3 and NH4OAc under solvent-free conditions at 120 ꢀC
Entry
Catalyst
Loading (mg)
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
GO–chitosan
GO–chitosan
GO–chitosan
GO–chitosan
GO–chitosan
GO–chitosan
GO–chitosan
GO
—
1
5
120
120
60
30
20
10
10
10
12
Nil
Trace
50
90
92
95
95
75
70
8
10
12
15
15
15
Fig. 3 DSC curves for determining the Tg of chitosan and GO–
chitosan.
Chitosan
indicates the presence of the C]O bonds of carboxylic acids
and other carbonyl groups, because the mass ratio of GO to
chitosan is 1 : 9.29
the as-obtained structure of GO–chitosan. As indicated in the
scale bar of the AFM image, the average size distribution of GO–
chitosan composite was measured to be about 50 nm.
The thermal properties of GO–chitosan were investigated by
differential scanning calorimetry (DSC) (Fig. 3). An obvious
improvement in the thermal stability of GO–chitosan via glass
transition temperatures (Tg) can be observed in the DSC
diagram. The Tg of pristine chitosan occurs at 118–119 ꢀC, while
for GO–chitosan, Tg is 162 ꢀC. The Tg of polymers is affected by
the mobility of the polymer chains. It can be concluded from the
increase in Tg and the thermal stability of the composite in
comparison to pristine chitosan that the dispersion of GO in a
chitosan matrix hinders the mobility of chitosan chains
via electrostatic attractive forces and hydrogen bonding.
Furthermore, chemical bonds between chitosan chains and GO
sheets in GO–chitosan composite lms prevent the conforma-
tion of polymeric molecules and cause a high Tg for GO–chito-
san.29 Because imidazole is synthesised at 120 ꢀC, we need a
catalyst with high thermal stability, and obviously the high Tg of
GO–chitosan in comparison to pristine chitosan would be
useful nanocomposite catalyst in reactions carried out in high
temperatures.
To investigate the catalytic ability of GO–chitosan, a pilot
experiment was carried out using the reaction of benzil 2,
benzaldehyde 3 and ammonium acetate to obtain 2,4,5-tri-
phenyl-1H-imidazole 4a. Aer several tests, we realized that the
optimized conditions for the reaction was using 0.012 g of
GO–chitosan at 120 ꢀC under solvent-free conditions in a
reasonable and short reaction time (10 minutes) to produce a
high yield (95%) of 4a (Table 1, entry 6). It is noteworthy that the
desired product has not been obtained with such efficiency to
date under similar reaction conditions, neither in the absence
of any catalyst nor using other catalysts.
We have also investigated the individual catalytic potential
of GO and chitosan in the pilot experiment (Table 1, entries
8 and 9). Individually, GO catalyses the reaction in the same
time but the yield of the reaction is low (Table 1, entry 8). This
can be attributed to the carboxylic acid groups of GO, which in
the presence of amine created by the decomposition of
ammonium acetate cause an undesirable amidation reaction
that decreases the reaction efficiency. In addition, when the GO
is separated from the reaction mixture, it sticks to the lter
The nanostructured surface of the catalyst was conrmed by
atomic force microscopy (AFM). Fig. 4 shows the AFM image of
Fig. 4 AFM images of GO–chitosan nanocomposite.
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Table 2 Synthesis of 2,4,5-trisubstituted-1H-imidazoles 4a–o using GO–chitosan nanocatalyst under solvent-free conditions
Time (min)
Benzil
Yield (%)
Benzil
Entry
Product
Benzoin
Benzoin
Mp (ꢀC)
1
10
20
20
25
20
20
95
92
94
95
90
94
270–272
2
3
4
5
30
25
35
30
90
92
91
95
190–192
228–230
231–233
269–271
6
7
8
10
18
25
40
93
92
96
89
91
93
264–266
290–292
200–203
8
35
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Table 2 (Contd.)
Time (min)
Benzil
Yield (%)
Benzil
Entry
Product
Benzoin
30
Benzoin
92
Mp (ꢀC)
9
25
20
91
88
255–257
10
30
84
168–170
11
12
20
23
25
30
87
82
85
82
227–229
203–205
13
14
20
10
25
20
86
93
80
91
196–198
234–236
15
20
30
85
83
194–196
paper and as a result, the catalyst recyclability decreases. thermally stable than chitosan. The reaction is carried out at
Furthermore, the reaction in the presence of chitosan required 120 ꢀC under solvent-free conditions. Therefore, thermal
a longer time and a lower yield was obtained (Table 1, entry 9). stability is one of the most important factors in selecting a
Moreover, the colour of chitosan, in most cases, is similar to the suitable catalyst for this reaction.
product and this engenders problems during the separating of
Subsequently, to investigate the efficiency and applicability
the catalyst and makes the product indistinguishable from the of this catalyst in the three-component synthesis of
catalyst. If we are unable to recognize the product sediment, we 2,4,5-trisubstituted-1H-imidazoles 4a–o, the reaction was
cannot choose a suitable solvent for the dilution of mixture for extended to other substituted benzaldehydes 3 at 120 ꢀC under
the separation of catalyst. Furthermore, GO–chitosan is more solvent-free conditions (Table 2). From Table 2, it is obvious
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Scheme 2 Proposed mechanism for the synthesis of substituted imidazoles 4a–o.
that when benzoin 1 is used instead of benzil 2, the reaction recovered and reused several times in the subsequent runs
time is longer. Moreover, it is obvious that the reaction time using the same recovered catalyst without a considerable loss of
with 3-nitrobenzaldehyde is the lowest; this can be attributed to catalytic activity (Fig. 5).
the electron affinity of the nitro group that is placed at the meta
position of benzaldehyde 3.
Conclusions
A proposed mechanism for the synthesis of 2,4,5-trisubsti-
tuted-1H-imidazoles 4a–o is shown in Scheme 2. According to
In summary, we have introduced a GO–chitosan nanocomposite
the mechanism, ammonium acetate decomposes into ammonia
as a biocompatible and biodegradable heterogeneous nano-
and acetic acid, and this released ammonia is a nitrogen source.
catalyst that is applicable in the synthesis of organic
It can be proposed that the hydrogen atoms existing in
compounds. Efficient syntheses of 2,4,5-trisubstituted-1H-
ammonia and the hydroxyl groups of GO–chitosan are respon-
imidazoles were carried out starting from simple and readily
sible for the activation of carbonyl groups, and thus increase the
available precursors including benzoin or benzil, benzaldehyde
rate of the imine production through hydrogen bonding for the
and NH4OAc in the presence of a catalytic amount of GO–chi-
nucleophilic attack of amines (Scheme 2).30
tosan. A large number of unique properties of this catalyst were
Another important aspect of this active, non-toxic, and eco-
observed including a short reaction time, easy workup proce-
friendly heterogeneous nanocatalyst is its high degree of recy-
dure, reusability of the catalyst, high atom economy, excellent
clability. It has been shown that the GO–chitosan could be
yield and environmentally benign reaction condition.
Experimental section
General
All the solvents, chemicals and reagents were purchased from
Merck, Fluka and Aldrich. Melting points were measured on an
Electrothermal 9100 apparatus and are uncorrected. IR spectra
were recorded on a Shimadzu IR-470 spectrometer by the
method of KBr pellet. 1H and 13C NMR spectra were recorded on
a Bruker DRX-500 Avance spectrometer at 500 and 125 MHz,
respectivelꢁy.1DSC was measured by a TA-1A at the heating rate of
ꢀ
20 C min under N2 atmosphere and at a ow rate of 80 mL
minꢁ1. The tip radii were obtained from direct AFM measure-
ments of the tip apex region in tapping mode. The shell surface
was mounted on the AFM stage and the Triboscope recording
unit with transducers and leveling device was placed on the top
of a NanoScope III E 164 | 164 mm2 XY piezo scan base.
Fig. 5 Reusability of GO–chitosan for the synthesis of 2,4,5-triphenyl-
1H-imidazole 4a.
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All the products were known compounds and were identied
by comparison of their spectroscopic and analytical data with
authentic samples.
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Acknowledgements
The authors gratefully acknowledge the nancial support from
the Iran National Science Foundation (INSF) and partial
support from the Research Council of the Iran University of
Science and Technology.
Synthesis of GO
GO was prepared according to Hummers method.31 40 mL
H3PO4 (85.0%) and 360 mL of concentrated H2SO4 were added
into a three neck-ask, which was charged with 2 g of graphite
powder with continuous stirring. 12 g KMnO4 was gently added
into the reaction pot, which was cooled using an ice bath. The
cooling bath was removed and the reaction mixture was allowed
to stand at 45 ꢀC for 48 h to assure that the oxidation of graphite
had been completed. The reaction mixture was quenched by 2 L
of ice containing 20 mL of H2O2. The color of the mixture
changed from dark brown to bright yellow, which indicated the
formation of GO. The solid was collected via centrifugal sepa-
ration and it was washed three times with 10% aqueous solu-
tion of HCl followed by distilled water until a pH of 5–6 was
obtained. Ultimately, the resulting GO was washed with acetone
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Preparation of GO–chitosan nanocomposite
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