Acidic ionic liquid [(CH2)4SO3HMIM] [HSO4]: A green media for the simple and straightforward synthesis of 2,4,5-trisubstituted imidazoles

Article abstract of DOI:10.1080/00397910903219377

2,4,5-Trisubstituted imidazoles have been synthesized in excellent yields (85-95%) in the presence of 5mol% of [(CH₂)4SO₃HMIM] [HSO₄] as a Brnsted acidic ionic liquid. Copyright

Full text of DOI:10.1080/00397910903219377

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Acidic Ionic Liquid [(CH2)4SO3HMIM]
[HSO4]: A Green Media for the Simple
and Straightforward Synthesis of 2,4,5-
Trisubstituted Imidazoles
Majid M. Heravi a , Masoumeh Zakeri a , Narges Karimi a , Mina
Saeedi a , Hossien A. Oskooie a & Niloofar Tavakoli-Hosieni b
a Department of Chemistry, School of Sciences, Alzahra University,
Vanak, Tehran, Iran
b Department of Chemistry, Faculty of Sciences, Islamic Azad
University, Mashhad Branch, Mashhad, Iran
Version of record first published: 08 Jun 2010.
To cite this article: Majid M. Heravi , Masoumeh Zakeri , Narges Karimi , Mina Saeedi , Hossien A.
Oskooie & Niloofar Tavakoli-Hosieni (2010): Acidic Ionic Liquid [(CH2)4SO3HMIM][HSO4]: A Green
Media for the Simple and Straightforward Synthesis of 2,4,5-Trisubstituted Imidazoles, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
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Synthetic Communications1, 40: 1998–2006, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903219377
ACIDIC IONIC LIQUID [(CH2)4SO3HMIM][HSO4]: A
GREEN MEDIA FOR THE SIMPLE AND
STRAIGHTFORWARD SYNTHESIS OF
2,4,5-TRISUBSTITUTED IMIDAZOLES
Majid M. Heravi,1 Masoumeh Zakeri,1 Narges Karimi,1
Mina Saeedi,1 Hossien A. Oskooie,1
and Niloofar Tavakoli-Hosieni2
1Department of Chemistry, School of Sciences, Alzahra University, Vanak,
Tehran, Iran
2Department of Chemistry, Faculty of Sciences, Islamic Azad University,
Mashhad Branch, Mashhad, Iran
2,4,5-Trisubstituted imidazoles have been synthesized in excellent yields (85–95%) in
the presence of 5 mol% of [(CH2)4SO3HMIM][HSO4] as a Brønsted acidic ionic liquid.
Keywords: Brønsted acidic ionic liquid; multicomponent reaction; one-pot synthesis; solvent-free
conditions; trisubstituted imidazole
INTRODUCTION
Solvent-free reactions and use of ionic liquids[1] in organic synthesis, parti-
cularly those based on imidazolium cations,[2] have gained attention in recent years.
Some of the unique physical and chemical properties of imidazolium cations, such as
thermal stability, negligible vapor pressure, recyclability, ability to dissolve a wide
range of organic and inorganic compounds, fast reaction rate, selectivity, and tend-
ency to immobilize starting materials and catalyst, make them an attractive substi-
tute to various volatile and hazardous organic solvents.[3] There are many reports
that acid=base-catalyzed reactions in organic solvents can be smoothly carried out
in functionalized ionic liquids containing acidic=basic counterions or cations in the
absence of any additional catalysts. Examples are sulfonic ionic liquids as dual
solvent catalysts in Fischer esterification, alcohol dehydrodimerization, pinacol rear-
rangements,[4] Friedel–Crafts alkylations[5] and nitration of aromatic compounds,[6]
carboxyl-functionalized ionic liquids for acetalization[7] and deoximation,[8]
guanidine-based ionic liquids as bases in Henry reactions,[9] amino-functionalized
ionic liquid for microwave-assisted synthesis of 4H-pyrans,[10] and Brønsted
Received April 11, 2009.
Address correspondence to Majid M. Heravi, Department of Chemistry, School of Sciences,
Alzahra University, Vanak, Tehran, Iran. E-mail: mmh1331@yahoo.com
1998

ACIDIC IONIC LIQUID [(CH2)4SO3HMIM][HSO4]
1999
ionic liquid with hydrogen sulfate counterion as the catalyst in the synthesis of
coumarines.[11] The dual role of ionic liquids as solvents and promoters will dramati-
cally expand their potential use in scaleup and clean production of significant mole-
cules. However, it is more desirable to conduct those acid=base-catalyzed
transformations in neutral ionic liquids to avoid prolonged and complicated proce-
dures for preparation of functionalized ionic liquids.
Imidazole and its derivatives play important roles in the fields of biology and
pharmacology as well as chemistry.[12–15] Imidazole ring systems are frequently
found in numerous naturally occurring and synthetic molecules such as fungi-
cides,[12] herbicides,[13] and therapeutic agents.[14] Also, recent advances in green
chemistry and organometallic catalysis have extended the application of imidazoles
as ionic liquids.[15] In addition to these important applications, imidazole derivatives
are ideal scaffolds to make libraries of anti-inflammatory, anti-allergic, and anal-
gesic[16] drug-like compounds and to generate inhibitors of P38 MAP kinase.[17]
Because of their wide range of biological, industrial, and synthetic applications,
those have recently received a great deal of attention. The synthesis of 2,4,5-
trisubstituted imidazoles has been carried out by condensation of benzil, aldehyde,
and ammonium acetate in the presence of acetic acid[18] and some common Lewis
acid catalysts such as Yb(OTf)3,[19] NbCl3,[19] LaCl3,[19] FeCl3,[19] AlCl3,[19]
Al2O3,[20] NiCl2,[21] [HBim]BF4,[22] and molecular iodine.[23]
Many of the reported synthetic protocols for the synthesis of imidazoles suffer
from one or more disadvantages such as harsh reaction conditions, poor yields,
prolonged time period, and use of hazardous and often expensive acid catalysts.
Moreover, the synthesis of these heterocycles has usually been carried out in
polar solvents such as ethanol, methanol, acetic acid, dimethylformamide
(DMF), and dimethylsulfoxide (DMSO), leading to complex isolation and recovery
procedures.[22] Hence, the development of clean, safe, effective, economical,
high-yielding, and mild environmentally benign protocols is still desirable and is in
much demand.
During the course of our studies on the synthesis of substituted imidazole com-
pounds[24] and because of our interest in multicomponent reactions,[25] this report
deals with the simple, mild, and rapid synthesis of 2,4,5-trisubstituted imidazoles
in good yields using [(CH2)4SO3HMIM][HSO4] (Scheme 1) as an ionic liquid catalyst
under solvent-free conditions.
For our investigations, [(CH2)4SO3HMIM][HSO4] was prepared according to
the literature procedure.[26] The efficiency of the reaction is mainly affected by the
amount of [(CH2)4SO3HMIM][HSO4] and temperature. The best results have been
Scheme 1. Structure of Brønsted acidic ionic liquid.

2000
M. M. HERAVI ET AL.
Table 1. Synthesis of 2-(2-chloro-phenyl)-4,5-diphenyl-1H-imidazole in the
presence of [(CH2)4SO3HMIM][HSO4] in different solvents
Entry
Solvent
Temperature (ꢀC)
Time
Yield (%)
1
2
3
4
5
EtOH
CHCl3
H2O
78
60
4 h
4 h
70
65
100
100
120
15 h
15 h
15 min
Trace
55
95
H2O (2 mL)
—
obtained at 120 ꢀC with an amount of 5 mol% of [(CH2)4SO3HMIM][HSO4] in
solvent-free conditions. It is important to note that in the absence of [(CH2)4SO3HMIM]
[HSO4], the reaction yield was negligible even at 120 ꢀC after 4 h. As is evident from
Table 1, all reactions gave rise to excellent isolated yields in a relatively short reac-
tion time. In addition, these good results can also be attributed to the stronger
Brønsted acidity of counteranion HSOꢁ4 .
The effect of solvent was studied by carrying out the reactions in different sol-
vents. Among the tested solvents, such as ethanol, chloroform, and water, and a
solvent-free system, the latter gave the best results for the condensation of benzil,
benzaldehyde, and ammonium acetate (Table 1, Scheme 2).
To show the generality of this method, the optimized condition was used for
the synthesis of other imidazole derivatives (Table 2). The products are known
1
and were characterized by infrared (IR) and H NMR and through comparison of
their physical properties with those reported in the literature.
One of the advantages of this ionic liquid is its ability to be recycled from the
reaction medium. We were able to separate catalyst from the reaction medium easily
by extraction with water. The catalyst was recovered by evaporation of the water of
the aqueous layer at 50–60 ꢀC in a vacuum oven and reused for the same experiment.
The results of both experiments were almost identical.
In conclusion, we have developed a mild, convenient, and efficient protocol for
the synthesis of 2,4,5-triaryl imidazoles via the condensation of 1,2-diketones such as
benzoin with aromatic aldehydes and ammonium acetate using a Brønsted ionic
liquid as a recyclable medium as well as promoter. The process gives rise to excellent
isolated yields of 2,4,5-triaryl imidazoles in short reaction times (5–15 min).
Moreover, the mild reaction conditions, easy workup, clean reaction profiles, lower
catalyst loading, and cost efficiency render this approach an interesting alternative to
the existing methods.
Scheme 2.

ACIDIC IONIC LIQUID [(CH2)4SO3HMIM][HSO4]
2001
Table 2. Synthesis of 2,4,5-trisubstituted imidazoles
Mp (ꢀC)
(min) (%) Found Reported Ref.
Time Yield
Entry
Aldehyde
Products
1
10
90
95
210–212 212
22
22
2
3
4
5
6
7
8
5
226–227 222
227–228 227–228
259–262 261–262
270–272 272
243–245 248
190–192 188
231–233 233
7
92
88
95
85
95
95
28
28
27
22
22
22
5
15
15
15
10
(Continued )

2002
M. M. HERAVI ET AL.
Table 2. Continued
Mp (ꢀC)
Time Yield
(min) (%) Found
Entry
Aldehyde
Products
Reported Ref.
9
15
10
85
295–297 297
27
22
10
90
270–272 269
1
Note. All the compounds in Table 2 were characterized on the basis of H NMR and IR spectral data,
which were consistent with those of reported in literature.
EXPERIMENTAL
Melting points were measured with an Electrothermal engineering LTD 9200
apparatus and are uncorrected. IR spectra were recorded on a Perkin-Elmer RX1
Fourier transform (FT)–IR spectrometer. 1H NMR spectra were recorded on a Var-
ian 300-MHz spectrometer using tetramethylsilane (TMS) as an internal reference.
Products were identified by comparison of melting points and spectroscopic data
with those reported.
Preparation of [(CH2)4SO3HMIM][HSO4]
A mixture of 1-methylimidazole (0.2 mol) and 1,4-butane sultone (0.2 mol) was
charged into a 150-mL conical flask. The mixture was stirred at room temperature
for 4 days until it turned into a solid. The white solid zwitterion that formed
was washed repeatedly with ether, filtered to remove nonionic residues, and dried
in vacuum. Then, the white solid zwitterions were slowly added dropwise to a stoi-
chiometric amount of concentrated sulfuric acid (98%, 10.9 mL) in a 150-mL conical
flask at 0 ꢀC. The mixture was stirred at 80 ꢀC for 6 h. The product was washed with
diethyl ether and dried in vacuo at 50 ꢀC for 2 h to get the viscous clear
[(CH2)4SO3HMIM][HSO4].[26]
Synthesis of 2,4,5-Trisubstituted Imidazoles: General Procedure
A mixture of benzil (1 mmol), aromatic aldehyde (1 mmol), and ammonium
acetate (4 mmol) were ground in a mortar until a homogeneous powder was formed.
A 10-mL, round-bottom flask was charged with this homogeneous powder and
[(CH2)4SO3HMIM][HSO4] (0.05 mmol). The mixture was heated at 120 ꢀC with
stirring for 5–15 min. After completion of the reaction (monitored by thin-layer

ACIDIC IONIC LIQUID [(CH2)4SO3HMIM][HSO4]
2003
chromatography, TLC), the mixture was cooled to room temperature and mixed
thoroughly with 3 ꢂ 10 ml of acetone. The mixture was filtered, and the solvent
was removed under reduced pressure to afford the crude product, which was purified
by recrystallization from acetone–water (15:1 V=V).
Data
2-(4,5-Diphenyl-1H-imidazol-2-yl)-phenol (4a). Mp: 210–212 ꢀC; IR (KBr):
1
v
max=cmꢁ1 1262, 1501, 1583, 1601, 3058, 3204, 3409; H NMR (CDCl3=DMSO-d6,
300 MHz) d 6.90–7.02 (d, J ¼ 7.5 Hz, 2H), 7.26–7.64 (m, 10H), 7.95–7.98 (d,
J ¼ 8.06 Hz, 2H), 9.08 (s, 1H), 12.74 (brs, 1H).
2-(4-Methoxy-phenyl)-4,5-diphenyl-1H-imidazole (4b). Mp: 226–227 ꢀC;
IR (KBr): vmax=cmꢁ1 1493, 1506, 1601, 1633, 3026, 3420; 1H NMR (CDCl3=
DMSO-d6, 300 MHz) d 3.09 (s, 3H), 7.03–7.05 (d, J ¼ 8.8 Hz, 2H), 7.29–7.61 (m,
10H), 8.06–8.09 (d, J ¼ 8.8 Hz, 2H), 12.52 (brs, 1H).
2-(4-Methyl-phenyl)-4,5-diphenyl-1H-imidazole (4c). Mp: 227–228 ꢀC; IR
(KBr): vmax=cmꢁ1 1250, 1528, 1613, 2958, 3407; 1H NMR (CDCl3=DMSO-d6,
300 MHz) d 3.09 (s, 3H, CH3), 7.03–7.83 (m, 12H), 8.02–8.052 (d, J ¼ 8.3 Hz, 2H),
12.60 (brs, NH).
2-(4-Chloro-phenyl)-4,5-diphenyl-1H-imidazole (4d). Mp: 259–262 ꢀC; IR
(KBr): vmax=cmꢁ1 1433, 1485, 1603, 3028, 3063, 3417; 1H NMR (CDCl3=DMSO-d6,
300 MHz) d 7.29–7.62 (m, 12H), 8.13–8.16 (d, J ¼ 8.3 Hz, 2H), 11.78 (brs, NH).
2-(2,4-Dihydroxyl-phenyl)-4,5-diphenyl-1H-imidazole (4e). Mp: 270–
1
272 ꢀC; IR (KBr): vmax=cmꢁ1 1195, 1568, 1604, 2960, 3011, 3124, 3384; H NMR
(CDCl3=DMSO-d6, 300 MHz) d 7.60–8.00 (m, 13H), 9.39 (s, 1H), 9.66 (s, 1H),
12.90 (brs, NH).
2-(4-Bromo-phenyl)-4,5-diphenyl-1H-imidazole (4f). Mp: 243–245 ꢀC; IR
(KBr): vmax=cmꢁ1 1261, 1485, 1583, 1645, 3029, 3417; 1H NMR (CDCl3=DMSO-d6,
300 MHz) d 7.21–7.54 (m, 10H), 7.66–7.68 (d, J ¼ 8 Hz, 2H), 8.80–8.03 (d, J ¼ 7.8 Hz,
2H), 12.77 (brs, 1H).
2-(2-Chloro-phenyl)-4,5-diphenyl-1H-imidazole (4g). Mp: 190–192 ꢀC; IR
1
(KBr): vmax=cmꢁ1 1201, 1446, 1479, 1602, 2956, 3063, 3434; H NMR (CDCl3=
DMSO-d6, 300 MHz) d 7. 18–7.50 (m, 13H), 8.50–8.52 (d, J ¼ 7.2 Hz, 1H), 11.31
(brs, 1H).
4-(4,5-Diphenyl-1H-imidazol-2-yl)-phenol (4h). Mp: 231–233 ꢀC; IR (KBr):
v
max=cmꢁ1 1215, 1638, 2998, 3022, 3409, 3596; 1H NMR (CDCl3=DMSO-d6,
300 MHz) d 6.93–6.97 (d, J ¼ 8 Hz, 2H), 7.52–7.87 (m, 10H), 7.88–7.92 (d, J ¼ 8.5 Hz,
Hz, 2H), 9.56 (s, 1H), 12.58 (brs, 1H).
2-(1-Naphthyl)-4,5-diphenylimidazole (4i). Mp: 295–297 ꢀC; IR (KBr):
v
max=cmꢁ1, 1433, 1506, 1549, 2969, 3033, 3422; 1H NMR (CDCl3=DMSO-d6,
300 MHz) d 7.18–7.63 (m, 13H), 7.77–7.98 (m, 3H), 9.00–9.07 (d, J ¼ 8.11 Hz, 1H),
12.57 (s, 1H).

2004
M. M. HERAVI ET AL.
2,4,5-Triphenyl-1H-imidazole (4j). Mp: 270–272 ꢀC; IR (KBr): vmax=cmꢁ1
1
1485, 1638, 2993, 3063, 3434; H NMR (CDCl3=DMSO-d6, 300 MHz) d 7.22–7.56
(m, 13H), 8.07–8.09 (d, J ¼ 7.8 Hz, 2H) 12.69 (brs, 1H).
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