Magnetically separable and recyclable g-C3N4 nanocomposite catalyzed one-pot synthesis of substituted imidazoles

Article abstract of DOI:10.1007/s13738-017-1284-9

A green, reliable synthetic method and highly efficient one-pot three-component synthesis of 2,4,5-trisubstituted imidazoles from 1,2-diketones, aldehydes and ammonium acetate in the presence of recyclable magnetic graphitic carbon nitride nanocomposite (Fe₃O₄@g-C₃N₄) has been developed. Various imidazole derivatives were obtained in moderate to excellent yields and high purity after recrystallization from ethanol. Interestingly, the low-cost Fe₃O₄@g-C₃N₄ a nontoxic and inexpensive catalyst showed excellent recyclability using an external magnet without loss of parent catalytic activity even after ten cycles and its provides new opportunities for the truly environmental friendliness methodology.

Full text of DOI:10.1007/s13738-017-1284-9

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-017-1284-9
ORIGINAL PAPER
Magnetically separable and recyclable g‑C3N4 nanocomposite
catalyzed one‑pot synthesis of substituted imidazoles
Tahereh Soleymani Ahooie1 · Najmedin Azizi2 · Issa Yavari1 · Mohammad Mahmoudi Hashemi1
Received: 20 July 2017 / Accepted: 23 December 2017
© Iranian Chemical Society 2018
Abstract
A green, reliable synthetic method and highly efcient one-pot three-component synthesis of 2,4,5-trisubstituted imidazoles
from 1,2-diketones, aldehydes and ammonium acetate in the presence of recyclable magnetic graphitic carbon nitride nano-
composite (Fe3O4@g-C3N4) has been developed. Various imidazole derivatives were obtained in moderate to excellent yields
and high purity after recrystallization from ethanol. Interestingly, the low-cost Fe3O4@g-C3N4 a nontoxic and inexpensive
catalyst showed excellent recyclability using an external magnet without loss of parent catalytic activity even after ten cycles
and its provides new opportunities for the truly environmental friendliness methodology.
Keywords Substituted imidazoles · g-C3N4 · Multicomponent reactions · Fe3O4@g-C3N4
Introduction
unique place in the feld of medicinal chemistry, and vari-
ous prescribed drugs such as cimetidine, etomidate and the
well-known antifungal were imidazole heterocycles (Fig. 1).
Trisubstituted imidazole derivatives are widely used as
organic functional materials, such as to resist composition on
textile, fuorescent whiteners on textile, electroluminescent
compounds [12]. In the green chemistry, imidazole deriva-
tives are used as green solvents in form of ionic liquids, and
as N-heterocyclic carbenes chemistry as powerful steering/
controlling elements in transition metal catalysis [13]. Over
the past few decades, various methods have been devel-
oped for the synthesis of imidazole derivatives in the litera-
ture [14]. One of the most important and practical method
involves the condensation of benzil or benzoin, benzalde-
hyde, and ammonium acetate [15–29].
One of the main challenges in heterocyclic chemistry is
the design and synthesis of diverse highly functionalized
complex molecules [1–3]. Therefore, devising reactions that
achieve multibond formation in one operation is becom-
ing one of the major challenges in green chemistry [4–6].
Multicomponent reactions (MCRs), in which three or more
reactants are combined in a single chemical step enjoy an
outstanding status, represent a fascinating tool to achieve
this goal [7–9].
Imidazole functional group and more specifically
2,4,5-triarylimidazoles play important roles in numerous
bioactive compounds [10]. Its derivatives are widely dis-
tributed throughout nature in a variety of natural products
such as histamine, histidine, biotin, alkaloids, peptides and
nucleic acids [11]. Imidazole derivatives have occupied a
Carbon-based nanomaterials have become a hotspot due
to the extraordinary and unique physical, chemical, and
mechanical properties and extensive research eforts are
being made to utilize these materials for a wide variety of
applications in catalysis, optoelectronic materials, electrodes
of fuel cells and biomedical felds [30–37]. Among the car-
bon materials, graphitic carbon nitrides (g-C3N4) exhibit
distinctly diferent properties, such as its excellent thermal
conductivity, high electrical mobility and great mechani-
cal strength [38–40]. Furthermore, g-C3N4 can be easily
synthesized via solid state synthesis without any organic
solvents from cheap materials such as melamine or urea
derivatives and insoluble in most solvents [41–43]. Due to
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s13738-017-1284-9) contains
supplementary material, which is available to authorized users.
* Najmedin Azizi
azizi@ccerci.ac.ir
1
Department of Chemistry, Science and Research Branch,
Islamic Azad University, Tehran, Iran
2
Chemistry and Chemical Engineering Research Center
of Iran, P.O. Box 14335-186, Tehran, Iran
Vol.:(0123456789)
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Journal of the Iranian Chemical Society
Fig. 1 Examples of approved
synthetic imidazole drugs
these characteristics, recently attracted great attention, as it
provides a green and potential route for clean energy produc-
tion and environmental remediation with wide application
[44–46]. However, g-C3N4 are relatively less explored as
catalysis in organic synthesis [47–50].
over 2 h at 90 °C, and cooled to room temperature, washed
with water and ethanol, separated magnetically, and dried
overnight at 60 °C under vacuum.
General procedure
A mixture of 1,2-diketone (0.5 mmol), aldehyde (0.5 mmol),
ammonium acetate (1.5 mmol) and Fe3O4@g-C3N4 (20 mg)
in ethanol (1 mL) was heated at 78 °C with stirring for
120–200 min. After the completion of the reaction, ethanol
was added and the Fe3O4@g-C3N4 was separated magneti-
cally using a bar magnet. The reaction mixture was allowed
to cool to room temperature to recrystallize from ethanol
to get the pure fnal product. All compounds were known
and characterized on the basis of their melting point and by
comparison with those reported in the literature.
Experimental
General
All starting materials and solvents such as aldehydes, dik-
etones, melamine FeCl2 and FeCl3 were commercially avail-
able. Melting points were determined on Buchi 535 melting
point apparatus. 1H and 13C NMR spectra were recorded on
500 MHz NMR spectrometer using DMSO-d6 as solvent.
Chemical shifts were expressed in (ppm) downfeld from
TMS. All the reactions were monitored by thin-layer chro-
matography (TLC) with UV light as detecting agents.
Results and discussion
Preparation of magnetic g‑C3N4 catalyst
Based on the aforementioned fndings, and in a continuation
of our interest in green chemistry-oriented organic synthesis
in novel solvent [40–43], herein, we report a simple and ef-
cient three-component condensation of reactions of benzoin,
benzaldehyde, and ammonium acetate to synthesize highly
functionalized benzopyran and pyran derivatives using mag-
netic nanoparticle–graphitic carbon nitride, Fe3O4@g-C3N4
under mild conditions. The use of Fe3O4@g-C3N4 not only
is an excellent nanocomposite for magnetic separation of
catalyst, but also provides the acidic environment required
to accomplish the reaction without the need for an external
acid.
The g-C3N4 nanosheet was prepared by directly calcinating
melamine in the air. In order to prepare that, an alumina
crucible with a loose cover with 2 g melamine was placed
in a mufe furnace. Then, the programed temperature was
set up to 550 °C for 3 h with a ramp rate of 5 °C min−1.
After cooling down to room temperature naturally, a light
yellow powder was obtained [33]. The g-C3N4 nanosheets
were prepared by exfoliation of as prepared bulk g-C3N4 in
acidic water. In the next step, Fe3O4/g-C3N4 was prepared by
reported methods [34]. The g-C3N4 (500 mg) was dispersed
in a 300 mL of ethanol/water (1:1) using sonication for 4 h at
room temperature. Then, FeCl3·6H2O (1.838 g, 0.0216 mol)
and FeCl2·4H2O (0.703 g, 0.0108 mol) were dissolved in
the solution using sonication and 10 mL of ammonia solu-
tion (NH4OH) was quickly injected into the mixed solution.
Under nitrogen atmosphere, the reaction mixture was stirred
Initially, in order to investigate the multicomponent syn-
thesis of imidazole using a new and greener nanocatalyst,
magnetically active g-C3N4 nanocomposite has been syn-
thesized without the generation of any hazardous product
in the simplest manner as reported in the literature [34] and
the formation of nanocomposite was confrmed by scanning
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Journal of the Iranian Chemical Society
electron microscopy (SEM) and Fourier transformed infra-
red (FT-IR).
formation of a heterostructure (Fig. 3b). Energy-dispersive
X-ray spectrometer (EDS) results are shown in Fig. 4 clearly
detected the presence of Fe, O, C, and N which indicated
that the Fe3O4 nanoparticle is deposited on the surface of
g-C3N4 sheets (Fig. 4).
Fourier transform infrared (FT-IR) spectroscopy reveals
(Fig. 2) the chemical structure of nanocomposite. The broad
peak in the region from 2800 to 3500 cm−1 is contributed by
the N–H stretching of amine groups and absorbed water. As
shown in Fig. 2, the characteristic bands at 1167, 1302, 1478
and 1660 cm−1 are attributed to either trigonal C–N(–C)–C
or bridging C–NH–C units. Besides, a broad peak from 650
to 760 cm−1 and 550 to 650 cm−1 corresponding to Fe–O
proves the existence of Fe3O4 in the composite.
To obtain the optimized reaction conditions, we ini-
tiated our research by examining the reaction of benzil
(0.5 mmol), benzaldehyde (0.5 mmol), and ammonium
acetate (1.5 mmol) at ambient temperature in ethanol as
green solvent in the presence of Fe3O4@g-C3N4 as safe
catalyst (Table 1). The use of 20 mg of Fe3O4@g-C3N4
in ethanol resulted a 38% yield of the desired product at
room temperature (Table 1). Further investigations revealed
that the reaction was also afected by the reaction tempera-
ture and solvent, and the optimal conditions are 20 mg of
Fe3O4@g-C3N4 at 80 °C in ethanol. Changing the solvent
from ethanol to other solvents, such as acetonitrile, water,
tetrahydrofuran, toluene and DMF was inefective for this
Figure 3 shows SEM images of pure g-C3N4 and
Fe3O4/g-C3N4 nanocomposite. Figure 3a presents represent-
ative SEM images of pure g-C3N4 nanosheets with diameters
of about 32 nm, that a crumpled and rippled structure, as
a result of deformation upon the exfoliation and restack-
ing process. After combining with Fe3O4, the nanoparticles
are sparsely distributed on the g-C3N4 and resulting in the
55
54
52
50
48
46
44
42
40
887.64cm-1
588.83cm-1
808.96cm-1
38
36
3156.94cm-1
34
32
30
28
26
1636.85cm-11410.68cm-1
1243.42cm-1
1571.47cm-11323.28cm-1
23
4000
3500
3000
2500
2000
1500
1000
500 400
cm-1
Fig. 2 FTIR spectra of nanocomposite
Fig. 3 SEM image of (g-C3N4) and nanocomposite
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Journal of the Iranian Chemical Society
substrates and the results are depicted in Table 1. As it is
shown in Table 1, all reactions were proceeded in good
short times (120–200 min) and in all cases desired products
were obtained in good-to-excellent yields (72–95%). A wide
range of aromatic aldehydes including electron-withdrawing
groups as well as electron-donating groups on the aromatic
ring underwent the reaction with benzil and ammonium ace-
tate to aford the desired products in good isolated yields.
Aromatic aldehydes with electron-donating groups such as
methyl and methoxy aforded good-to-excellent yields, while
aldehydes with electron-withdrawing substituents such as
nitro and halogen performed poorly and require long reac-
tion times. Notably, the reaction also accommodated ali-
phatic aldehydes and heterocyclic aldehydes, and afording
the desired imidazole product in a moderate yield (Table 1,
entry 16). The obtained results have revealed that the reac-
tion conditions are very general and clean and can easily
be varied for diferent aldehydes. Furthermore, the isolated
compounds were obtained in a simpler manner after crystal-
lization in pure form without further purifcation. The scope
of this novel eco-friendly system was then examined using
diferent 1,2-diketones. The reactions with 1,2-diketones
bearing electron-donating substituents proceeded efciently
to aford various 2,4,5 trisubstituted imidazoles in moderate
yields (Table 2, entries 12–14). When electron-withdrawing
1,2-diketones were employed, lower yields were obtained
(Table 2, entries 15, 16). Because of the mild conditions,
active functional groups including chloro, bromo, nitro, and
methoxy could tolerate.
Fig. 4 The EDX analysis of nanocomposite
Table 1 Solvent screening for the synthesis of imidazole
CHO
Ph
Ph
O
O
Fe3O4@g-C3N4 ( 20 mg)
Solvent, (1 mL), 2 h
Ph
Ph
N
+
Ph
N
H
NH4OAc
Entry
Solvent
Temp. (°C)
Yieldsa (%)
1
2
3
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Water
CH3CN
DMF
THF
rt
38
42
74
95
80
95
68
54
78
45
42
65
40
60
78
78
78
80
80
80
80
80
80
4
5b
6c
7
One of the main advantages of the Fe3O4@g-C3N4 as
catalyst is its rapid separation via external magnetic feld.
Thus, we turned our attention to the reusability of the
Fe3O4@g-C3N4 that is essential for industrial applications.
Thus, the recyclability of magnetic catalyst was investigated
in the model reaction (Fig. 5). Upon the completion of the
reaction, ethanol was added and the magnetic g-C3N4 was
readily recovered from the reaction mixture by magnetic
decantation. The recovered catalyst was washed with eth-
anol or ethyl acetate, and reused under the same reaction
conditions. As shown in Fig. 5, the catalyst Fe3O4@g-C3N4
could be recycled up to 10 times and no signifcant decrease
in the product yield was observed. The recovered catalyst
after six runs had no obvious change in structure as well as
morphology, referring to the FT-IR spectra and SEM image
in comparison with the fresh one.
8
9
10
11
12
Toluene
No solvent
aIsolated yields, b10 mg of catalyst, c50 mg of catalyst
transformation. Attempts to carry out the transformation in
the absence of a solvent resulted in 65% yield of the desired
product. Furthermore, in the lower amount of nanocatalyst
(5, 10 and 15 mg), longer reaction time was needed to com-
plete the reaction and higher amount of catalyst give nearby
the same results.
A plausible mechanism for these conversions is shown
in Fig. 6. Initially, nanocatalyst can activate the carbonyl
groups of aldehyde and benzil and coupling of the activated
aldehyde and benzil in with the in situ generated ammo-
nia from ammonium acetate occur. This intermediate A in
the presence of nanocatalyst condenses with benzil to form
intermediate B which in turn rearranges the trisubstituted
imidazoles by a [1, 5]-H shift.
With a suitable reaction system in hand, to defne the
scope of the environmentally friendly condensation reac-
tion, particularly in regard to library construction, this
methodology was evaluated by using a variety of aromatic,
heterocyclic and aliphatic aldehydes and 1,2-diketones. At
the beginning of the search for the scope of the aldehyde
substrate, ammonium acetate and benzil were used as model
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Journal of the Iranian Chemical Society
Table 2 Fe3O4@g-C3N4
R
R
R
R
O
O
O
catalyzed one-pot synthesis of
2,4,5-trisubstituted imidazoles
NH4OAc
N
R'
+
R'
H
N
H
Fe3O4@g-C3N4,
78 oC, 120-200 min.
Entry Products
1
Yieldsa (%) m.p. (°C) (rep.) m.p. (founded) Refs.
X = H
95
272
271–273
[15]
N
X
N
H
2
3
4
5
6
X = Cl
X = Br
X = Me
X = OMe 78
X = Cl
88
86
92
260–261
260–262
236–238
228–230
192–194
261–263
265–266
230–231
228
[15]
[15]
[15]
[15]
[18]
74
195–197
X
N
N
H
7
8
X = Me
X = NO2 78
77
204–207
311–313
204–206
295–296
[15]
[15]
N
N
H
X
9
10
X = Cl
80
74
295–297
247–251
294–296
260–262
[15]
[22]
N
S
N
H
11
72
78
161
160–170
202–204
[22]
[51]
N
N
H
12
X = H
201–203
MeO
N
X
N
H
MeO
13
14
15
X = Br
X = OMe 72
X = H
74
160–161
184–186
290–291
158–160
182–184
292–294
[52]
[51]
[53]
71
Cl
N
X
N
H
Cl
16
X = Me
70
263–264
260–261
[53]
aIsolated yields
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Journal of the Iranian Chemical Society
100
90
80
70
60
50
40
30
20
10
0
Fig. 5 Recyclability study of
Fe3O4@g-C3N4 in the model
reaction
1
2
3
4
5
6
Run
Fig. 6 The proposed mechanism
Finally, comparison of the magnetic carbon nitride cata-
lyzed synthesis of imidazole with a range of other reported
methodologies demonstrated the high yields, short reaction
times, low loading of nanocatalyst and eco-friendly nature
of the protocol (Table 3).
Conclusion
In summary, we have devised a general, practical and an
environmentally benign protocol for the synthesis of diverse
imidazole derivatives using Fe3O4@g-C3N4 nanocomposite
as catalyst via one-pot three-component condensation reac-
tion of an aldehyde, 1,2-diketones and ammonium acetate
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Journal of the Iranian Chemical Society
Table 3 Comparison between
the catalytic activity of the
nanocomposite with the
reported catalysts
Entry
Catalyst
Yield (%)
Refs.
1
2
3
4
5
6
7
8
9
NFS–PMA
PMA
Zr(acac)4
Dendrimer-PWA
SA-MNPs
Zeolite
Chitosan
Fe3O4@chitosan
Fe3O4@g-C3N4
94
72
94
94
98
80
80
95
95
[26]
[26]
[54]
[54]
[54]
[54]
[50]
[50]
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