Highly efficient and simple protocol for synthesis of 2,4,5-triarylimidazole derivatives from benzil using fluorinated graphene oxide as effective and reusable catalyst

Article abstract of DOI:10.1007/s11164-016-2847-6

Abstract: Efficient synthesis of 2,4,5-triarylimidazole derivatives with simple workup is described using benzil, different aldehydes, and ammonium acetate in presence of fluorinated graphene oxide (A-MFGO) as effective catalyst that can be easily recover

Full text of DOI:10.1007/s11164-016-2847-6

Res Chem Intermed
DOI 10.1007/s11164-016-2847-6
Highly efficient and simple protocol for synthesis
of 2,4,5-triarylimidazole derivatives from benzil using
fluorinated graphene oxide as effective and reusable
catalyst
H. D. Hanoon1 E. Kowsari1 M. Abdouss1
•
•
•
M. H. Ghasemi1 H. Zandi2
•
Received: 13 October 2016 / Accepted: 21 December 2016
Ó Springer Science+Business Media Dordrecht 2017
Abstract Efficient synthesis of 2,4,5-triarylimidazole derivatives with simple workup is
described using benzil, different aldehydes, and ammonium acetate in presence of flu-
orinated graphene oxide (A-MFGO) as effective catalyst that can be easily recovered and
reused for several times without significant loss in activity. The simple preparation of the
catalyst, short reaction time, mild conditions, high yield, and low cost make this pro-
cedure economically attractive and highly suitable for synthesis of 2,4,5-triarylimidazole
derivatives.
Graphical Abstract
Electronic supplementary material The online version of this article (doi:10.1007/s11164-016-2847-6)
contains supplementary material, which is available to authorized users.
&
E. Kowsari
kowsarie@aut.ac.ir
1
2
Department of Chemistry, Amirkabir University of Technology, Hafez Avenue, No. 424,
Tehran, Iran
Department of Chemistry, Yadegar-e-Imam Khomeini (RAH), Shahre-Rey Branch, Islamic
Azad University, Tehran, Iran
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H. D. Hanoon et al.
Keywords 2,4,5-Triarylimidazole Á Graphene oxide Á Benzil Á Catalysis Á Synthesis
Introduction
Graphene oxide (GO), which can be obtained from graphite by decoration with
oxygen functional groups, easily decomposes and may convert to smaller
components under heat treatment, a routine condition for fluorination of carbon.
We selected GO to make a composite in this work because of its high surface area,
nontoxicity, and ability to be functionalized with different chemical groups. The
most important feature of GO is that it can be properly prepared on large scale from
normal graphite at low cost. GO is similar to benzene and polycyclic aromatic
hydrocarbons in that it comprises sp2 and sp3 carbon atoms [1–3]. GO can be
densely decorated with hydroxyl, epoxy, carbonyl, and carboxyl functionalities on
the surface and edges of the sheet, providing a variety of opportunities for chemical
modification and transformation [4–6]. Recently, we envisaged that GO could be a
promising starting material for cost-effective, large-scale preparation of fluorinated
graphene (FG) if oxygen-containing functionalities could be successfully trans-
formed into C–F bonds.
Fluorination stands out among the most effective chemical techniques to change
the physiochemical properties of carbon materials, since further substitution can be
promptly accomplished [7, 8]. The C–F bond exhibits excellent oxidative and
thermal stability [9]. In addition, fluorine is the most electronegative element in the
Periodic Table, while C–F bonds possess high polarity and low surface free energy.
Because of the unique nature of covalent fluorine atoms, FG has different properties
compared with GO. FG with fluorine atoms covalently functionalized on GO
therefore represents an innovative two-dimensional carbon material [10]. Experi-
mental methods for synthesis of FG can be divided into two approaches: (1) direct
fluorination of graphene sheets using XeF2 and plasma (CF4 and SF6) as fluorination
agents [11, 12], and (2) exfoliation of graphite fluoride by mechanical cleavage or
ultrasonication in sulfolane, isopropanol, or ionic liquid [13, 14]. Recently,
fluorination of GO utilizing hydrogen fluoride, a corrosive reagent, was described
[15]. However, an effective, convenient, environmentally friendly, and solution-
based synthetic route to FG is required for mass production to study its properties
and practical applications. FG has attracted interest for technological applications,
for instance, for use in lubricants [16]. Therefore, many experimental and
theoretical studies [17, 18] describe its synthesis or application in organic synthesis
as a catalyst. FG has been utilized in biomedical applications. FG is in vogue once
more due to the recent scientific and technological interest in graphene [19].
Imidazoles are a class of nitrogen-containing heterocyclic compounds that
include pharmaceutical agents with antibacterial [20], antitumor [21], and
antiinflammatory actions [22], herbicides [23], fungicides [24], as well as inhibitors
of mammalian 15-LOX [25] and B-Raf kinase [26]. Imidazole moieties form the
fundamental structural skeleton of numerous significant biological molecules such
as histamine and histidine [27, 28]. Besides, substituted imidazoles are used in
synthesis of several ionic liquids [29] applied in the new approach known as green
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Highly efficient and simple protocol for synthesis of…
chemistry. They are also used in photography as photosensitive compounds [30].
Due to their wide range of pharmacological, biological, and industrial activities,
synthesis of imidazoles has received extensive attention in organic synthesis
[31–36]. Over the years, numerous methods for synthesis of highly substituted
imidazoles have been published in literature. Generally, synthesis of 2,4,5-
trisubstituted imidazoles via three-component cyclocondensation of 1,2-diketone,
a-hydroxyketone or a-ketomonoxime with aldehyde and ammonium acetate can
be catalyzed by various catalysts, including ytterbium trifoliate [37], ytterbium
perfluorooctanesulfonate [38], InCl3Á3H2O [39], acetic acid [40], glyoxylic acid
[41], oxalic acid [42], ionic liquids [43], silica sulfuric acid [44], cellulose
sulfuric acid [45], ceric(IV) ammonium nitrate [46], sodium bisulfate [47],
anhydrous FePO4 [48], and nanocrystalline magnesium oxide [49]. Many of these
methods suffer from one or more disadvantages, such as longer reaction time,
poor yield, expensive reagents and catalyst, difficult workup, or excessive use of
reagents and catalyst, finally resulting in generation of large amounts of
dangerous waste. It is therefore important to develop more convenient procedures
for synthesis of these compounds. Hence, development of simple, efficient, high-
yielding, environmentally friendly approaches using new catalysts for synthesis
of imidazoles remains a challenge for organic chemists. Changing conventional,
toxic Brønsted and Lewis acid catalysts for environmentally benign, easily
recoverable solid catalysts is an active area of research. In recent years,
employment of solid acids as catalysts has received great attention in diverse
fields of organic synthesis [50, 51]. Solid acid catalysts offer various advantages,
such as catalyst reuse, environmental acceptability compared with liquid acid
catalysts, and easy separation of products. Exploration of the design and activity
of such novel specific catalysts could greatly improve the efficiency of a wide
range of organic syntheses.
One of our aims is to introduce a new safe, efficient, easy to handle, reusable
catalyst for synthesis of 2,4,5-triarylimidazole derivatives. In this study, a simple
and inexpensive method was used for synthesis of A-MFGO by employing graphene
oxide (Scheme 1). We also describe an excellent procedure for preparation of 2,4,5-
triarylimidazole derivatives using A-MFGO as a solid acid catalyst that is both
efficient and reusable. The method reported herein is not cumbersome, thus we
consider that the methodology is a good addition to the list of procedures available
for synthesis of 2,4,5-triarylimidazole derivatives (Scheme 2).
Experimental
Materials and methods
All chemicals used in this work were purchased from Merck or Fluka and used
without further purification. Melting points were recorded using an Electrother-
mal 9100 apparatus. Fourier-transform infrared (FTIR) spectroscopy (Bruker
ALPHA) was used to determine the quality and composition of the materials.
1
Reactions were monitored by thin-layer chromatography (TLC). H and 13C NMR
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H. D. Hanoon et al.
Scheme 1 Synthesis procedures for catalyst A-MFGO
spectra were measured on a Bruker (300 MHz) spectrometer using dimethyl
sulfoxide (DMSO)-d6 as solvent and tetramethylsilane (TMS) as internal standard.
Elemental (C, H, N) analyses were performed using a Heraeus CHN-O-Rapid
analyzer. Field-emission scanning electron microscopy (FESEM) images were
captured using a Hitachi S4160 cold field-emission instrument. X-ray diffraction
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Highly efficient and simple protocol for synthesis of…
Scheme 2 Synthesis of some useful 2,4,5-triarylimidazole derivatives using A-MFGO
(XRD) analysis was performed using an INEL Equinox 3000 diffractometer.
Energy-dispersive X-ray spectroscopy (EDS) measurements were performed using a
SAMx analyzer. X-ray photoelectron spectroscopy (XPS) was performed on a VG
Scientifics ESCALAB 250 spectrometer with an Al Ka irradiation source
(1486.7 eV) at 15 kV and 10 mA.
Synthesis of catalyst
Synthesis of graphene oxide (GO)
GO was synthesized from graphite powder and purified according to literature
procedure [52]. Graphite (10 g) and sodium nitrate (7.5 g) were mixed together,
followed by addition of 98% (621 g) sulfuric acid under constant stirring; the
mixture was stirred while being cooled in an ice-water bath. KMnO4 (45 g) was
added gradually to the above solution for about 1 h, while keeping the temperature
lower than 10 °C for 2 h to prevent overheating and explosion. The mixture was
allowed to stand for 5 days at room temperature with gentle stirring to obtain highly
viscous liquid, which was added to H2SO4 aqueous solution (1000 mL) with stirring
in an ice bath; the resultant mixture was further stirred for 2 h. After removing the
ice bath, H2O2 (10%, 30 g), slowly and with caution, was added to the above liquid,
and the mixture was stirred for 2 h. Then, the resultant mixture was allowed to stand
for 2 days in order to precipitate. The resultant mixture was purified by filtration and
washed with 3 wt% H2SO4 and 0.5 wt% H2O2 aqueous solution. The mixture was
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H. D. Hanoon et al.
filtered for further purification. The filter cake was washed twice with distilled
water, then the resulting GO was dried and stored for subsequent use.
Synthesis of fluorinated graphene oxide (FGO)
FGO was synthesized by a simple reaction between GO dispersion and HF. GO
powder (1 g) and N-methyl-2-pyrrolidinone (NMP) (100 mL) was sonicated at
room temperature for 15 min to obtain completely exfoliated, monolayer GO
dispersion. After sonication, GO suspension was transferred to an autoclave.
Afterwards, HF (20 mL) was added and maintained at 180 °C for 30 h. The
autoclave was naturally cooled to room temperature. The final product was obtained
by successively washing with excess tetrahydrofuran (THF) and deionized water to
remove impurities and dried in an oven at 60 °C for 3 h.
Modification of FGO with diamine (MFGO)
Butylethylimidazolium bromide (4 g) ionic liquid (IL) was dissolved in NMP
(20 mL), then triphenyl phosphate (TPP) (4 mL) was added to a 50-mL, round-
bottomed flask [53]. FGO (1 g) and 1,6-diaminohexane (2 g) were sequentially added
to the above mixture. The whole solution was heated at 100 °C for 48 h. The resulting
product was isolated by filtration of the mixture through sintered glass and washed
with methanol (3 9 35 mL), followed by drying at 60 °C for 24 h to obtain MFGO.
Functionalized MFGO with 1,3-propane sultone (A-MFGO)
A-MFGO was obtained by reaction of MFGO (0.5 g) with 1,3-propane sultone (1 g)
in DMSO (20 mL). The mixture was refluxed at 160 °C for 4 days. After reaction
completion, the reaction mixture was cooled to room temperature. The resulting
product was isolated by filtration of the mixture over sintered glass and washed with
methanol (3 9 35 mL). Then, the solid product was added to HCl aqueous solution
(40 mL, 1:3), and the mixture was stirred at ambient temperature for 20 min and left
for 2 days. Afterwards, the separated solid was filtered using sintered glass,
followed by washing three times with water to remove residual acid. Finally, the
catalyst was removed by filtration and drying at 60 °C for 24 h.
Synthesis of 2,4,5-triarylimidazole derivatives
A mixture of benzil (1.05 g, 5.0 mmol), 3-nitrobenzaldehyde (0.755 g, 5.0 mmol),
and ammonium acetate (1.54 g, 20.0 mmol) in ethanol (15 mL) was taken, and
A-MFGO as catalyst (0.15 g) was added at room temperature. The resulting mixture
was stirred for 2 h at reflux temperature. Upon reaction completion (monitored by
TLC), dichloromethane (10 mL) was added with stirring for 15 min at ambient
temperature. The catalyst was recovered by filtration and washed with dichlor-
omethane for reuse in the next cycle (up to five runs) (Fig. 4). Then, the filtrate was
poured into cold water (20 mL) and stirred at room temperature for 10 min. The
crude product was obtained after addition of water to the organic layer. Two phases
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Highly efficient and simple protocol for synthesis of…
of solution were extracted, then the solvent was evaporated. The solid product was
washed with water, dried, and recrystallized from ethanol to obtain 2-(3-
nitrophenyl)-4,5-diphenylimidazole (4c) (0.998 g, yield 95%). The same procedure
was used for all other compounds. All 2,4,5-triarylimidazole derivatives were fully
characterized by usual spectroscopic techniques.
Spectral data for products
Spectroscopic data
2,4,5-Triphenylimidazole (4a) M.p.: 271–273 °C; IR (KBr) t (cm-1): 1601
1
(C=N), 3038 (Ar–H), 3446 (NH) cm-1; H NMR (400 MHz, DMSO-d6) d (ppm):
7.24–7.58 (m, 13H, Ar–H), 8.09–8.11 (m, 2H, Ar–H), 12.71 (s, NH); 13C NMR
(100 MHz, DMSO-d6) d (ppm): 125.7, 127.0, 127.6, 128.3, 128.6, 128.7, 128.9,
129.1, 129.2, 130.7, 130.8, 131.5, 131.6, 135.7, 137.6, 145.9, 146.0; Anal. calcd. for
C21H16N2 (296.37): C, 85.11; H, 5.44; N, 9.45. Found: C, 84.91; H, 5.42; N, 9.41.
2-(2-Nitrophenyl)-4,5-diphenylimidazole (4b) M.p.: 234–237 °C; IR (KBr) t
1
(cm-1): 1352 and 1526 (NO2), 1626 (C=N), 3069 (Ar–H), 3427 (NH) cm-1; H
NMR (400 MHz, DMSO-d6) d (ppm): 7.25–7.50 (m, 10H, Ar–H), 7.66 (td,
J1 = 8.0, J2 = 1.2 Hz, 1H, Ar–H), 7.81 (td, J1 = 7.6, J2 = 1.2 Hz, 1H, Ar–H),
7.95 (dd, J1 = 8.0, J2 = 0.8 Hz, 1H, Ar–H), 8.01 (dd, J1 = 8.0, J2 = 1.2 Hz, 1H,
Ar–H), 12.99 (s, NH); 13C NMR (100 MHz, DMSO-d6) d (ppm): 123.9, 124.5,
127.2, 127.5, 128.5, 128.7, 129.2, 129.7, 130.0, 130.3, 131.1, 132.6, 135.2, 138.0,
141.5, 148.8; Anal. calcd. for C21H15N3O2 (341.36): C, 73.89; H, 4.43; N, 12.31.
Found: C, 73.85; H, 4.41; N, 12.21.
2-(3-Nitrophenyl)-4,5-diphenylimidazole (4c) M.p.: 316–319 °C; IR (KBr) t
1
(cm-1): 1348 and 1522 (NO2), 1629 (C=N), 3064 (Ar–H), 3424 (NH) cm-1; H
NMR (400 MHz, DMSO-d6) d (ppm): 7.27–7.60 (m, 10H, Ar–H), 7.81 (t,
J = 8.0 Hz, 1H, Ar–H), 8.23–8.25 (m, 1H, Ar–H), 8.53–8.55 (m, 1H, Ar–H), 8.98
(t, J = 2.0 Hz, 1H, Ar–H), 13.14 (s, NH); 13C NMR (100 MHz, DMSO-d6) d
(ppm): 119.9, 123.1, 127.3, 127.6, 128.6, 128.8, 128.9, 129.2, 129.7, 130.9, 131.0,
131.1, 131.6, 132.2, 132.3, 135.2, 138.1, 143.7, 143.9, 148.8; Anal. calcd. for
C21H15N3O2 (341.36): C, 73.89; H, 4.43; N, 12.31. Found: 73.85; H, 4.41; N, 12.21.
2-(4-Nitrophenyl)-4,5-diphenylimidazole (4d) M.p.: 237–240 °C; IR (KBr) t
1
(cm-1): 1343 and 1518 (NO2), 1639 (C=N), 3050 (Ar–H), 3423 (NH) cm-1; H
NMR (400 MHz, DMSO-d6) d (ppm): 7.32 (d, J = 8.8 Hz, 2H, Ar–H), 7.61 (d,
J = 8.4 Hz, 1H, Ar–H), 7.85 (dd, J1 = 7.2, J2 = 2.0 Hz, 2H, Ar–H), 7.97 (d,
J = 8.8 Hz, 2H, Ar–H), 8.17–8.44 (m, 7H, Ar–H), 13.75 (s, NH); 13C NMR
(100 MHz, DMSO-d6) d (ppm): 123.3, 124.2, 124.4, 124.5, 124.8, 126.8, 128.4,
129.2, 129.4, 131.1, 135.9, 146.0, 146.6, 147.9, 149.4, 151.1, 163.3; Anal. calcd. for
C21H15N3O2 (341.36): C, 73.89; H, 4.43; N, 12.31. Found: C, 73.86; H, 4.41; N,
12.25.
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H. D. Hanoon et al.
2-(3-Methoxyphenyl)-4,5-diphenylimidazole (4e) M.p.: 225–227 °C; IR (KBr) t
(cm-1): 1240 (C–O), 1592 (C=N), 2964 (-CH), 3059 (Ar–H), 3416 (NH) cm-1; 1H
NMR (400 MHz, DMSO-d6) d (ppm): 3.85 (s, 3H, CH3), 6.95–6.98 (m, 1H, Ar–H),
7.23–7.57 (m, 11H, Ar–H), 7.67–7.71 (m, 2H, Ar–H), 12.71 (s, NH); 13C NMR
(100 MHz, DMSO-d6) d (ppm): 55.7, 110.6, 114.7, 118.1, 127.0, 127.5, 128.3,
128.7, 129.0, 129.2, 130.3, 131.5, 132.1, 135.6, 137.5, 145.8, 160.0; Anal. calcd. for
C22H18N2O (326.39): C, 80.96; H, 5.56; N, 8.58. Found: C, 80.94; H, 5.53; N, 8.54.
2-(4-Methoxyphenyl)-4,5-diphenylimidazole (4f) M.p.: 235–238 °C; IR (KBr) t
(cm-1): 1249 (C–O), 1659 (C=N), 2960 (-CH), 3059 (Ar–H), 3422 (NH) cm-1; 1H
NMR (400 MHz, DMSO-d6) d (ppm): 3.83 (s, 3H, CH3), 7.06 (dd, J1 = 6.8,
J2 = 2.0 Hz, 2H, Ar–H), 7.23–7.67 (m, 7H, Ar–H), 7.80–8.05 (m, 5H, Ar–H), 12.54
(s, NH); 13C NMR (100 MHz, DMSO-d6) d (ppm): 55.7, 114.6, 123.6, 126.9, 127.2,
127.6, 128.1, 128.6, 128.8, 129.1, 129.5, 129.6, 129.7, 130.0, 130.1, 131.7, 132.7,
135.8, 136.0, 137.3, 146.1, 159.9; Anal. calcd. for C22H18N2O (326.39): C, 80.96;
H, 5.56; N, 8.58. Found: C, 80.92; H, 5.53; N, 8.54.
2-(3-Chlorophenyl)-4,5-diphenylimidazole (4g) M.p.: 211–213 °C; IR (KBr) t
(cm-1): 778 (C–Cl), 1603 (C=N), 3060 (Ar–H), 3423 (NH) cm-1 1H NMR
;
(400 MHz, DMSO-d6) d (ppm): 7.26–7.58 (m, 12H, Ar–H), 8.07 (dt, J1 = 8.0,
J2 = 1.2 Hz, 1H, Ar–H), 8.17 (t, J = 2.0 Hz, 1H, Ar–H), 12.87 (s, NH); 13C NMR
(100 MHz, DMSO-d6) d (ppm): 124.2, 125.1, 127.2, 127.6, 128.4, 128.8, 129.1,
131.2, 132.7, 134.1, 135.4, 137.9, 144.4; Anal. calcd. for C21H15ClN2 (330.81): C,
76.24; H, 4.57; N, 8.47. Found: C, 76.22; H, 4.54; N, 8.42.
2-(4-Bromophenyl)-4,5-diphenylimidazole (4h) M.p.: 209–212 °C; IR (KBr) t
(cm-1): 604 (C–Br), 1635 (C=N), 3064 (Ar–H), 3421 (NH) cm-1 1H NMR
;
(400 MHz, DMSO-d6) d (ppm): 7.25–7.72 (m, 10H, Ar–H), 7.82–8.06 (m, 4H, Ar–
H), 12.82 (s, NH); 13C NMR (100 MHz, DMSO-d6) d (ppm): 121.9, 127.1, 127.5,
127.6, 128.4, 128.7, 128.9, 129.1, 129.2, 129.9, 130.0, 130.1, 131.3, 131.4, 132.2,
132.7, 135.4, 136.0, 137.8, 144.8, 144.9; Anal. calcd. for C21H15BrN2 (375.26): C,
67.21; H, 4.03; N,7.47. Found: C, 67.20; H, 4.00; N, 7.45.
4,5-Diphenyl-2-(thiophen-2-yl)imidazole (4i) M.p.: 256–259 °C; IR (KBr) t
(cm-1): 849 (C–S), 1658 (C=N), 3057 (Ar–H), 3424 (NH) cm-1 1H NMR
;
(400 MHz, DMSO-d6) d (ppm): 7.16–7.34 (m, 3H, Thiophene H), 7.39–7.96 (m,
10H, Ar–H), 12.81 (s, NH); 13C NMR (100 MHz, DMSO-d6) d (ppm): 127.1, 127.6,
128.0, 128.2, 128.3, 128.4, 128.7, 128.8, 129.2, 130.0, 130.1, 131.2, 131.3, 132.7,
134.3, 134.4, 135.3, 136.0, 137.3; Anal. calcd. for C19H14N2S (302.39): C, 75.47; H,
4.67; N, 9.26. Found: C, 75.44; H, 4.64; N, 9.22.
2-(5-Methylfuran-2-yl)-4,5-diphenylimidazole (4j) M.p.: 260–263 °C; IR (KBr) t
(cm-1): 1022 (C–O), 1629 (C=N), 2918 (-CH), 3056 (Ar–H), 3445 (NH) cm-1; 1H
NMR (400 MHz, DMSO-d6) d (ppm): 2.39 (s, 3H, CH3), 6.27 (dd, J1 = 3.2,
J2 = 1.2 Hz, 1H, Furan H), 6.87 (d, J = 3.2 Hz, 1H, Furan H), 7.23–7.54 (m, 10H,
Ar–H), 12.71 (s, NH); 13C NMR (100 MHz, DMSO-d6) d (ppm): 13.9, 127.0, 127.6,
127.7, 128.2, 128.6, 128.7, 129.1, 130.0, 130.1, 131.2, 131.3, 132.7, 135.5, 136.1,
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Highly efficient and simple protocol for synthesis of…
137.3, 139.1, 139.3, 144.6; Anal. calcd. for C20H16N2O (300.35): C, 79.98; H, 5.37;
N, 9.33. Found: C, 79.94; H, 5.36; N, 9.32.
Results and discussion
Synthesis of catalyst
The catalyst A-MFGO was synthesized by a simple and inexpensive method as
shown in Scheme 1. A-MFGO was evaluated using various methods as follows:
XRD analysis
The XRD patterns of GO, FGO, MFGO, and A-MFGO samples are shown in Fig. 1.
˚
GO shows a peak at 11.3° corresponding to d-spacing of 7.77 A due to
incorporation of oxygen-containing functional groups on graphite sheets (Fig. 1a).
The XRD pattern of FGO (Fig. 1b) exhibits a new peak at 24.5°, corresponding to d-
˚
spacing of 3.61 A, while the GO peak at 11.3° disappeared, indicating successful
fluorination of GO. The decrease in the d-spacing between the layers in FGO is
attributed to removal of some oxygen functionalities from GO after fluorination.
Figure 1c, d presents the XRD patterns of MFGO and A-MFGO. Both modified
samples (MFGO and A-MFGO) showed peaks at 24.5° and 24.3°, with d-spacing of
˚
3.62 and 3.66 A, respectively. These d-spacing values are lower than the value of
˚
7.77 A for GO due to the modification, proving successful modification of GO by
introduction of 1,6-diaminohexane and 1,3-propane sultone groups. Furthermore,
this indicates that the reaction occurred on the edges of GO [54, 55].
FTIR spectral analysis
The FTIR spectrum was measured to distinguish between the functional groups on
GO and the three products (Fig. 2). The FTIR spectrum of GO (Fig. 2a) exhibited a
Fig. 1 XRD patterns of a GO, b FGO, c MFGO, and d catalyst
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H. D. Hanoon et al.
Fig. 2 FTIR spectra of a GO, b FGO, c MFGO, and d catalyst
broad band at 3419 cm-1 due to O–H bond stretching. The peak at 1721 cm-1 is
assigned to C=O bond stretching at the edges of GO. Other oxygen-containing
groups were also observed, for example, C–O bond in carboxylic acid at
1383 cm-1, C–O bond in epoxy group at 1222 cm-1, and C–O bond in alcohol
group at 1050 cm-1. These oxygen functional groups indicate that oxidation of
graphite was successful. The two peaks at 1222 and 1050 cm-1, corresponding to
C–O in epoxy and alcohol group, respectively, disappeared in the FTIR spectrum of
FGO (Fig. 2b), while a new peak corresponding to C–F bond was observed at
1167 cm-1, indicating successful fluorination of GO. The FTIR spectrum of MFGO
(Fig. 2c) exhibited slightly different absorption behavior. The peak at 1636 cm-1 is
attributed to C=O bond in amide group. Besides, the peak of –CH2– group is
observed at 2920 cm-1. These peaks confirm the reaction between –COOH groups
on GO sheet and 1,6-diaminohexane. Finally, the FTIR data of A-MFGO (Fig. 2e)
are clearly similar to those for MFGO, with the addition of a peak at 1035 cm-1
,
assigned to S=O bond in –SO3H group, and another peak at 741 cm-1, assigned to
C–S bond, indicating successful attachment of –SO3H group onto GO sheet
[56–58].
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Highly efficient and simple protocol for synthesis of…
SEM and EDX analysis
Figure 3 shows SEM images of GO, FGO, MFGO, and A-MFGO. SEM is a useful
technique to study the surface morphology of samples. In the SEM image of GO in
Fig. 3a, a model folded morphology can be observed. After fluorination, less
accumulation of FGO sheets is observed in Fig. 3b. To check the composition of the
FGO, EDX analysis was also carried out (Fig. 4). The EDX spectrum revealed
presence of C, O, and F elements in FGO, confirming presence of fluorine atoms on
the surface of the FGO sheets and thus successful fluorination of GO. As shown in
Fig. 3c, MFGO showed less accumulation, clearly revealing the efficiency of the
modification method for prevention of restacking of GO sheets. Figure 3d shows a
SEM image of A-MFGO, which reveals large particles with low rank, indicating
presence of bulk carbon. This also suggests that –NH2 groups on MFGO sheet and
1,3-propane sultone reacted [59–61].
XPS analysis
The XPS results for the catalyst A-MFGO are shown in Fig. 5. The XPS survey
spectrum of A-MFGO reveals the presence of elements including sulfur, carbon,
Fig. 3 SEM images of a GO, b FGO, c MFGO, and d catalyst
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H. D. Hanoon et al.
Fig. 4 EDX spectrum of FGO
oxygen, nitrogen, and fluorine (Fig. 5a). The observation of the fluorine peak
demonstrates that fluorine atoms were introduced into the graphene oxide. The total
survey of the A-MFGO also displays presence of nitrogen and sulfur elements. The
C 1s region of the XPS spectrum of A-MFGO could be divided into eight
components at 284.6, 285.4, 285.8, 286.8, 287.1, 287.8, 288.2, and 288.8 eV
(Fig. 5b), associated with C–C, C–N, C–CF, C–S, C=O, O=C–N, C–F, and O=C–O
groups, respectively. Two fluorine groups are observed at 285.8 eV (C–CF) and
288.2 eV (C–F), while peaks of C–OH and C–O–C group did not appear. All these
observations support successful fluorination of GO. The peaks at 286.8 eV (C–S),
285.4 eV (C–N), and 287.8 eV (O=C–N) from the surface of the GO sheets indicate
successful functionalization of GO. Moreover, the XPS spectrum (Fig. 5c) showed a
peak at 168.2 eV corresponding to –SO3H group, indicating incorporation of
sulfonic acid group [62–65].
Optimization of reaction conditions
Firstly, the reaction between benzil (5 mmol), 3-nitrobenzaldehyde (5 mmol), and
ammonium acetate (20 mmol) was selected as model reaction (4c) to identify the
optimum reaction conditions (Scheme 2). Various solvents and catalysts were
utilized, and the results are presented in Tables 1 and 2. Ethanol with catalytic amount
of sulfuric acid was clearly the best choice for this reaction (Table 1, entry 3). The
efficiency of the various catalysts was also investigated. The best yield was obtained
with A-MFGO (Table 2, entry 6). In absence of catalyst, product formation was not
observed (Table 2, entry 1). The amount of A-MFGO was also evaluated using the
model reaction. The maximum yield was obtained with 0.15 g A-MFGO (Table 3,
entry 4). Further increase to 0.2 g did not change the yield (Table 3, entry 5).
After successful optimization of the reaction conditions, the series of 2,4,5-
triarylimidazole derivatives 4a–j were synthesized (Table 4). Under these
123

Highly efficient and simple protocol for synthesis of…
Fig. 5 a XPS survey spectrum of catalyst, b XPS carbon region spectrum for catalyst, and c sulfur region
of catalyst with curve fitting results
conditions, the reaction between a variety of aromatic aldehydes and benzil
(Scheme 2) was carried out in presence of A-MFGO as catalyst to synthesize 2,4,5-
triarylimidazole derivatives. Excellent product yield was observed, and reaction
workup was simple. The results obtained are summarized in Table 4. The structures
1
of all products obtained were characterized well by spectral analyses (FTIR, H
NMR and 13C NMR). Moreover, the melting points were recorded and compared
with those reported in literature, as given in Table 4 [66–72]. The FTIR spectra of
compounds 4a–j demonstrated peaks at 1592–1659 and 3416–3446 cm-1 for (C=N)
and (N–H) groups, respectively. The 1H NMR spectra exhibited bands at
12.54–13.75 ppm for N–H proton of imidazole moiety.
Finally, a mechanism for the synthesis of the 2,4,5-triarylimidazole derivatives
using the catalyst is proposed in Scheme 3. The initial step involves H-bond
123

H. D. Hanoon et al.
Table 1 Effect of solvent on synthesis of 2,4,5-triarylimidazole derivatives
CHO
H
N
O
Solvent
NH4OAc
+
+
N
R
O
R
B
A
C
Entry
Solvent
Mole ratio (A:B:C)
Time (min)
Yield (%)a
1
2
3
4
5
6
7
8
Acetonitrile
1:1:4
1:1:4
1:1:4
1:1:4
1:1:4
1:1.5:4
1:1:4
1:1:4
120
120
120
120
120
120
60
53
62
86
70
51
86
48
30b
Tetrahydrofuran
Ethanol
Dimethylformamide
Dichloromethane
Ethanol
Ethanol
Ethanol
60
Reaction conditions: catalyst (H2SO4, three drops), reflux
a
Isolated yield
b
Reaction performed at room temperature
Table 2 Synthesis of 2,4,5-triarylimidazole derivatives catalyzed by various catalysts
Entry
Catalyst
Yield (%)a
1
2
3
4
5
6
–
N.R.
40
CH3CO2H
H2SO4
86
[IL](bmim)SO3H
H3[P(W3O10)4]
A-MFGO
70
61
95
Reaction conditions: benzil:aldehyde:ammonium acetate (1:1:4 mol ratio), EtOH (15 mL), t = 120 min
a
Isolated yield
formation between catalyst and aldehyde, which activates the carbonyl group of the
aldehyde to give hydroxylamine intermediate [I]. Dehydration of hydroxylamine
leads to formation of imine intermediate [II]. Nucleophilic attack of another NH3 on
imine forms diamine intermediate [III]. This stage is followed by nucleophilic
attack of diamine intermediate [III] on activated carbonyl group of benzil to give
cycle intermediate [IV], which later undergoes release of a water molecule [V] and
proton transfer to produce 2,4,5-triarylimidazole derivatives [VI].
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Highly efficient and simple protocol for synthesis of…
Table 3 Effect of catalyst amount on synthesis of 2,4,5-triarylimidazole derivatives
Entry
Catalyst (g)
Yield (%)a
1
2
3
4
5
0.025
0.05
0.1
61
78
90
95
95
0.15
0.2
Reaction conditions: benzil:aldehyde:ammonium acetate (1:1:4 mol ratio), EtOH (15 mL), t = 120 min
a
Isolated yield
Table 4 Synthesis of 2,4,5-triarylimidazole derivatives using catalysts in EtOH
CHO
H
O
N
Catalyst
EtOH, reflux
NH4OAc
+
+
N
R
O
R
Entry
Aldehyde
Yield (%)a
A-MFGO
M.p. (°C)
References
H2SO4
1
2
3
4
5
6
7
8
9
10
Benzaldehyde
4a
4b
4c
4d
4e
4f
86
94
95
93
91
84
95
92
81
90
73
82
86
80
78
70
83
74
69
75
271–273
231–234
316–319
237–240
221–224
235–238
210–212
206–209
256–259
260–263
[66]
[67]
[68]
[66]
[69]
[70]
[71]
[71]
[72]
–
2-Nitrobenzaldehyde
3-Nitrobenzaldehyde
4-Nitrobenzaldehyde
3-Methoxybenzaldehyde
4-Methoxybenzaldehyde
3-Chlorobenzaldehyde
4-Bromobenzaldehyde
4g
4h
4i
Thiophene-2-carbaldehyde
5-Methylfurfural
4j
Reaction conditions: benzil:aldehyde:ammonium acetate (1:1:4 mol ratio), EtOH (15 mL), t = 120 min,
and catalysts (three drops of H2SO4 and 0.15 g A-MFGO)
a
Isolated yield
Reusability of catalyst
The reusability of the catalyst A-MFGO was also investigated using the model
reaction. After completion of the reaction, the catalyst was filtered and washed with
dichloromethane. Recovered catalyst could be dried and further reused in another
reaction. The recovered catalyst could be reused five times without addition of fresh
123

H. D. Hanoon et al.
Scheme 3 Proposed mechanism for synthesis of 2,4,5-triarylimidazole derivatives
catalyst (Fig. 6) with the same purity as the first run, indicating that A-MFGO
shows high catalytic activity.
Conclusions
A highly efficient approach with mild reaction conditions and operationally simple
protocol for synthesis of 2,4,5-triarylimidazole derivatives via condensation of
benzil, various aromatic aldehydes, and ammonium acetate as ammonia source in
ethanol is presented, using A-MFGO as inexpensive, recyclable, and effective
123

Highly efficient and simple protocol for synthesis of…
Fig. 6 Reusability of catalyst in reaction of benzil with 3-nitrobenzaldehyde
catalyst. The catalyst could be reused several times without activity loss. The easy
workup, high yield, simple operation, short reaction time, and safer reaction
conditions of this method using a reusable catalyst with simple preparation are the
highlights of this method. This research also provides interesting ideas regarding
fluorination and chemical modification of graphene oxide.
Acknowledgements The authors wish to gratefully thank the Research Affairs Division at Amirkabir
University of Technology (AUT), Tehran, for financial support.
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