A novel polymeric catalyst for the one-pot synthesis of 2,4,5-triaryl-1H-imidazoles

Article abstract of DOI:10.1007/s12039-012-0248-y

An efficient synthesis of 2,4,5-trisubstituted imidazoles is achieved by three component cyclocondensation of benzil or benzoin, aldehyde and ammonium acetate by using novel polymeric catalyst [poly(AMPS-co-AA)] under solvent-free conditions. The key advantages of this process are high yields, shorter reaction times, easy work-up, purification of products by non-chromatographic method and the reusability of the catalyst. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-012-0248-y

c
J. Chem. Sci. Vol. 124, No. 3, May 2012, pp. 717–722. ꢀ Indian Academy of Sciences.
A novel polymeric catalyst for the one-pot synthesis
of 2,4,5-triaryl-1H-imidazoles
ALI MOHAMMADIa,∗, HOSSEIN KESHVARIb, REZA SANDAROOSc, HAMED ROUHIa
and ZEINALABEDIN SEPEHRb
aDepartment of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, P.O. Box 91775,
Mashhad, Iran
bDepartment of Chemistry, Tarbiat Moallem University of Sabzevar, P.O. Box 9617976487, Sabzevar, Iran
cDepartment of Chemistry, Faculty of Science, University of Birjand, P.O. Box 97175/615, Birjand, Iran
e-mail: a.mohammady62@yahoo.com
MS received 8 August 2011; revised 29 November 2011; accepted 20 January 2012
Abstract. An efficient synthesis of 2,4,5-trisubstituted imidazoles is achieved by three component cyclo-
condensation of benzil or benzoin, aldehyde and ammonium acetate by using novel polymeric catalyst
[poly(AMPS-co-AA)] under solvent-free conditions. The key advantages of this process are high yields, shorter
reaction times, easy work-up, purification of products by non-chromatographic method and the reusability of
the catalyst.
Keywords. 2,4,5-Trisubstituted imidazoles; solvent-free synthesis; multicomponent reaction;
poly(AMPS-co-AA); heterogeneous catalysts.
1. Introduction
from 1,2-dicarbonyl compounds, aldehydes and ammo-
nia to obtain 2,4,5-triphenylimidazole.6,7 There are sev-
eral methods for the synthesis of 2,4,5-triarylimidazoles
using ZrCl4,8 zeolites HY/silica gel,9 NaHSO3,10 sul-
phanilic acid,11 iodine,12 ceric ammonium nitrate,13
oxalic acid,14 ionic liquids15 and also by microwave
irradiation using acetic acid.16 Each of the above meth-
ods for this reaction has its own merits, while some
of the methods are plagued by the limitations of poor
yield, longer reaction time, difficult work-up and efflu-
ent pollution.5 Therefore, the development of a new
mild method to overcome these disadvantages still
remains a challenge for organic chemists. One of the
aims we have in mind is to introduce a new catalyst
for synthesis of 2,4,5-trisubstituted imidazoles with cost
effectiveness and mild condition in high yields. In 1963,
Merrifield introduced a modified technique that over-
came the problems associated with classical multistep
synthesis. This technique has been used in the produc-
tion of large amount of products. However, recently,
the chemistry of functional polymers has received great
attention and became a practical method for the effi-
cient preparation of novel chemical libraries.17 Poly-
meric reagents have recently been developed for use in
simple processes such as epoxidation, oxidation, acyla-
tion, halogenation, and Wittig reactions. In all of these
applications, the advantages of the selectivity, insolu-
bility, and reusable capacity of the polymeric reagent
Multicomponent reactions (MCRs) have drawn great
interest in modern organic synthesis and medicinal
chemistry because they are one-pot processes bring-
ing together three or more components and show high
atom economy and high selectivity.1,2 MCRs have con-
tributed in convergent synthesis of complex and impor-
tant organic molecules from simple and readily avail-
able starting materials, and have emerged as power-
ful tools for drug discovery.3,4 The imidazole nucleus
is a fertile source of biologically important molecules.
Compounds containing imidazole moiety have many
pharmacological properties and play important roles
in biochemical processes. They are well-known as
inhibitors of P38MAP kinase, fungicides, herbicides,
antiinflammatory agents, antithrombotic agents, plant
growth regulators and therapeutic agents. In addition,
they are used in photography as photosensitive com-
pounds. Some substituted triarylimidazoles are selec-
tive antagonists of the glucagons receptor and inhibitors
of IL-1 biosynthesis.5 Radziszewski and Jaap proposed
the first synthesis of the imidazole core in 1882, starting
∗For correspondence
717

718
Ali Mohammadi et al.
O
O
CHO
or
poly(AMPS-co-AA)
N
+
+ NH4OAc
Solvent-free
110oC
O
N
H
R
R
1a-l
2a-l
OH
Figure 1. Crosslinked poly (AMPS-co-AA) catalysed synthesis of 2,4,5-trisubstituted imidazole.
are taken.18 Polymers are a large family of compounds 3. General procedure
that provide a variety of compositions and properties
3.1 Preparation and reusability of the crosslinked
poly (AMPS-co-AA) catalyst
that can be useful in catalysis. Insoluble polymers offer
suitable substrates for the heterogenization of homoge-
neous catalysis, but even the attachment of catalysts to
soluble polymers have advantages, since being macro-
molecules, these polymers can easily be separated from
the reaction mixture by the addition of a co-solvent or
by ultrafiltration.19
With these assumptions, we report here the solvent-
free synthesis of 2,4,5-trisubstituted imidazoles using
poly(2-acrylamido-2-methyl propane sulphonic acid)
(AMPS) crosslinked with N,Nꢁ-methylene bisacry-
lamide (MBA) as a novel catalyst under classical heat-
ing (figure 1).
We examined a wide variety of aromatic aldehy-
des with various substituents to establish the cata-
lytic importance of this catalyst for this reaction.
A wide range of ortho-, meta- and para-substituted
aromatic aldehydes undergo this one-pot multicompo-
nent synthesis with benzil or benzoin and ammonium
acetate to afford 2,4,5-trisubstituted imidazoles in good
yields.
Crosslinked poly (AMPS-co-AA) was prepared accord-
ing to the literature,27 by polymerization of 2-
acrylamido-2-methyl-1-propane sulphonic acid (AMPS)
and acrylic acid (AA) as the monomers in water, in
the presence of a crosslinking agent N,Nꢁ-methylene
bisacrylamide (MBA) and potassium persulphate (KPS)
as a free radical initiator (figure 2). The structure of the
polymer was characterized by FTIR analysis. When the
crosslinked poly (AMPS-co-AA) was used in the reac-
tion and the reaction was completed, the polymeric cata-
lyst was separated by simple filtration by diluting with
hot ethanol and then oven dried in 110 ◦C for 24 h. The
recycled catalyst was employed consecutively for three
reactions and no significant loss in its efficiency was
observed.
3.2 General procedure for preparation of 2a–l
A mixture of aldehyde (1 mmol), benzil or benzoin
(1 mmol), ammonium acetate (5 mmol) and crosslinked
poly (AMPS-co-AA) (0.03 g), as a catalyst, in a 20 ml
glass tube was stirred at 110 ◦C for 25–35 min. After
completion of the reaction, appropriate amounts of hot
EtOH (96%) was added and the mixture stirred for
10 min. then, the catalyst separated by filtration. The fil-
trate was concentrated in vacuo to remove the ethanol.
The residue was washed with cold water and crystal-
lized from hot ethanol to afford the pure products.
2. Experimental
Melting points were measured with an Electrothermal
9100 apparatus. IR spectra were recorded with a Vari-
an 3100 FTIR spectrometer. CHN analyses were per-
formed on Exeter Analytical Inc. ‘Model CE-400 CHN
1
Analyzer’. H and 13C NMR spectra were recorded
with a BRUKER DRX-400 AVANCE spectrometer
at 298 ◦K and 75.47 MHz, respectively. NMR spectra
were obtained on solutions in DMSO-d6. All the prod-
3.3 Selected spectral data
ucts were known compounds,20–26 which were char-
1
acterized by IR and H NMR spectral data and their 3.3.a. 2,4,5-Triphenyl-1H-imidazole (2a): Yield: (92%,
melting point compared with literature reports.
272 mg); Mp 273–275◦C; FTIR (KBr, cm−1): 3451,

A novel polymeric catalyst for the one-pot synthesis
719
O
O
O
O
SO3H
+
+
N
H
N
N
OH
H
H
AMPS
AA
MBA
KPS
CONHC(CH3)2CH2SO3H
H
C
*
C
*
H2
CO2H
NHCOCH2CONHCH
CH2
crosslinked poly (AMPS-co-AA)
Figure 2. Preparation of crosslinked poly (AMPS-co-AA).
1
2856, 1636, 1490; H NMR (400 MHz, DMSO-d6): 3.3.d. 2-(4-Methoxyphenyl)-4,5-diphenyl-1H-imidazole
δ 12.69 (s, 1H), 8.09 (d, J = 7.7 Hz, 2H), 7.56–7.22 (2d): Yield: (85%, 277 mg); Mp 230–233◦C; FTIR
(m, 13H); 13C NMR (75 MHz, DMSO-d6): δ 145.6, (KBr, cm−1): 3425, 3029, 2956, 1610, 1495, 1249;
137.2, 135.2, 131.2, 130.4, 128.7, 128.5, 128.3, 128.2, 1HNMR (400 MHz, DMSO-d6): δ 12.50 (s, 1H), 8.03
127.8, 127.2, 126.6, 125.3; Anal. Calcd for C21H16N2: (d, J = 8.5 Hz, 2H), 7.50–7.05 (m, 12H), 3.82 (s, 3H);
C, 85.11; H, 5.44; N, 9.45. Found: C, 85.18; H, 5.49; 13C NMR (75 MHz, DMSO-d6): δ 159.5, 145.7, 128.4,
N, 9.33.
127.7, 126.8, 123.1, 114.1, 55.2; Anal. Calcd for
C22H18N2O: C, 80.96; H, 5.56; N, 8.58. Found: C,
80.90; H, 5.51; N, 8.63.
3.3.b. 2-(4-Chlorophenyl)-4,5-diphenyl-1H-imidazole
(2b): Yield: (95%, 313 mg); Mp 264–266◦C; FTIR
(KBr, cm−1): 3452, 3065, 1635, 1323; 1H NMR
(400 MHz, DMSO-d6): δ 12.78 (s, 1H), 8.11 (d, J =
8.4 Hz, 2H), 7.56–7.23 (m, 12H); 13C NMR (75 MHz,
DMSO-d6): δ 146.3, 130.3, 129.9, 129.2, 128.5, 127.4,
127.0, 126.4, 125.5, 125.2, 123.3, 116.3; Anal. Calcd
for C21H15N2Cl: C, 76.24; H, 4.57; N, 8.47. Found: C,
76.20; H, 4.61; N, 8.41.
3.3.e. 4-(4,5-Diphenyl-1H-imidazol-2-yl)-phenol (2e):
Yield: (90%, 280 mg); Mp 265–267◦C; FTIR (KBr,
1
cm−1): 3590, 3454, 3284, 3064, 1701, 1283; H NMR
(400 MHz, DMSO-d6): δ 12.40 (s, 1H), 9.75 (s, 1H),
7.90 (d, J = 8.3 Hz, 2H), 7.54–7.21 (m, 10H), 6.86 (d,
2H); 13C NMR (75 MHz, DMSO-d6): δ 157.7, 146.1,
136.6, 135.4, 131.3, 128.6, 128.3, 127.5, 127.3, 127.0,
126.8, 126.3, 121.6, 115.4; Anal. Calcd for C21H16N2O:
C, 80.75; H, 5.16; N, 8.97. Found: C, 80.79; H, 5.22;
N, 8.91.
3.3.c.
4,5-Diphenyl-2-p-tolyl-1H-imidazole (2c):
Yield: (85%, 263 mg); Mp 231–232◦C; FTIR (KBr,
cm−1): 3449, 3034, 1611, 1495, 1320; 1H NMR
(400 MHz, DMSO-d6): δ 12.59 (s, 1H), 7.98 (d, J =
8.2 Hz, 2H), 7.54–2.21 (m, 12H), 2.35 (s, 3H); 13C
NMR (75 MHz, DMSO-d6): δ 145.6, 137.6, 136.9,
135.2, 131.1, 129.2, 128.6, 128.3, 128.1, 127.9, 127.6,
127.0, 126.4, 125.1, 20.8; Anal. Calcd for C22H18N2: C,
85.13; H, 5.85; N, 9.03. Found: C, 85.23; H, 5.79; N,
8.97.
3.3.f. 2-(4-Fluorophenyl)-4,5-diphenyl-1H-imidazole
(2f): Yield: (86%, 270 mg); Mp 250–252◦C; FTIR
(KBr, cm−1): 3316, 2993, 2470, 1660, 1210, 1169, 874,
1
719, 639; H NMR (400 MHz, DMSO-d6): δ 12.82 (s,
1H), 8.28 (d, J = 7.1 Hz, 2H), 7.55–7.03 (m, 12H);
13C NMR (75 MHz, DMSO-d6): δ 165.4, 137.3, 131.1,
129.8, 128.9, 127.7, 127.2, 126.6, 125.9, 125.5, 124.1,

720
Ali Mohammadi et al.
(KBr, cm−1): 3448, 3068, 1526, 1350; 1H NMR
(400 MHz, DMSO-d6): δ 13.12 (s, 1H), 8.90 (s, 1H),
8.56 (d, J = 7.4 Hz, 1H), 8.21 (d, J = 7.6 Hz,
1H), 7.81 (d, J = 7.6 Hz, 1H), 7.56–7.30 (m, 10H);
13C NMR (75 MHz, DMSO-d6): δ 148.4, 143.4, 131.8,
131.2, 130.4, 128.7, 128.4, 127.1, 122.6, 119.4; Anal.
Calcd for C21H15N3O2: C, 73.89; H, 4.43; N, 12.31.
Found: C, 73.84; H, 4.47; N, 12.39.
Table 1. Optimization one-pot synthesis of trisubstituted
imidazoles under classical heating conditions.a
Entry poly(AMPS-co-AA) (g) T(◦C) Time(min) Yield(%)
1
2
3
4
5
6
7
8
0.01
0.01
0.01
0.01
0.02
0.02
0.02
0.03
0.03
0.03
0.04
100
110
120
130
110
120
130
110
120
130
110
60
55
45
40
50
30
35
25
25
30
30
50
56
70
75
70
78
80
95
95
85
90
4. Results and discussion
9
10
11
Several methods are used in the synthesis of these
trisubstituted imidazoles and their derivatives. In addi-
tion, the synthesis of these heterocycles has been usu-
ally carried out in polar organic solvents such as
ethanol, methanol, acetic acid, DMF and DMSO lead-
ing to complex isolation and recovery procedures.
These processes also generate waste containing cata-
lyst and solvent, which have to be recovered, treated
and disposed of. The toxicity and volatile nature of
many organic solvents, particularly chlorinated hydro-
carbons that are widely used in huge amounts for
organic reactions have posed a serious threat to the envi-
ronment.28 Thus, design of solvent-free catalytic reac-
tion has received more attention in recent times in the
area of green synthesis.29
aBenzil (1 mmol), 4-chloro benzaldehyde (1 mmol) and
ammonium acetate (5 mmol)
117.4; Anal. Calcd for C21H15N2F: C, 76.27; H, 4.54;
N, 8.41. Found: C, 76.22; H, 4.60; N, 8.43.
3.3.g. 2-(2-Methoxyphenyl)-4,5-diphenyl-1H-imidazole
(2g): Yield: (85%, 277 mg); Mp 212–213◦C; FTIR
1
(KBr, cm1): 3430, 3038, 2950, 1615, 1495; H NMR
(400 MHz, DMSO-d6): δ 11.82, (s, 1H), 8.02 (d, J =
7.4 Hz, 1H), 7.53–7.07 (m, 13H), 3.92 (s, 3H); 13C
NMR (75 MHz, DMSO-d6): δ 158.2, 146.2, 128.3,
127.5, 125.4, 123.8, 115.4, 55.3; Anal. Calcd for
C22H18N2O: C, 80.96; H, 5.56; N, 8.58. Found: C,
80.90; H, 5.63; N, 8.51.
Efficiency of this reaction is mainly affected by the
amount of catalyst, temperature and reaction time. For
getting the best conditions, initially we started the con-
densation of benzil (1 mmol), 4-chloro benzaldehyde
(1 mmol) and ammonium acetate (5 mmol) in the pres-
3.3.h.
2-(3-Nitrophenyl)-4,5-diphenyl-1H-imidazole ence of crosslinked poly (AMPS-co-AA) (0.01 g) as a
(2i): Yield: (85%, 289 mg); Mp 301–302◦C; FTIR catalyst at 100◦C for 55 min, which led to low yield
Table 2. Synthesis of 2,4,5-triaryl-1H-imidazoles (2a–l) using crosslinked poly(AMPS-co-AA) (0.03 g) under solvent-free
conditions.
Time (min)
Yields (%)b
Mp/ ◦C
Reported
Productsa
R
Benzil
Benzoin
Benzil
Benzoin
Found
2a
2b
2c
2d
2e
2f
2g
2h
2i
4-H
4-Cl
4-CH3
4-OMe
4-OH
4-F
2-OMe
3-Br
25
25
25
30
35
35
30
30
35
30
25
30
25
25
30
35
35
35
30
30
35
35
25
30
92
95
85
85
90
86
85
90
85
90
92
85
90
90
80
80
85
80
85
85
80
88
85
80
273–275
264–266
231–232
230–233
265–267
250–252
212–213
232–233
301–302
201–204
264–266
217–219
272–27420
262–26420
230–23221
228–23120
268–27022
250–25123
210–21122
231–23324
>30024
3-NO2
2-CH3
4-Br
2j
2k
2l
205–20725
263–26521
216–21826
3,4-diOMe
1
aAll the isolated products were characterized on the basis of their physical properties and IR, H- and 13C-NMR spectral
analysis and by direct comparison with authentic materials; bIsolated yields

A novel polymeric catalyst for the one-pot synthesis
721
(50%) of 2,4,5-trisubstituted imidazole. To enhance the ortho-, meta- and para-substitutions under solvent-free
yield of the desired product the temperature of the reaction classical heating conditions to establish the catalytic
was increased to 130◦C. With increasing the tempera- importance of crosslinked poly(AMPS-co-AA) for this
ture, the reaction time decreased and the productivity of reaction.
the reaction increased but was not very high. Hence, it
Generally, the synthetic procedure involves stirring
was thought worthwhile to carry out the reaction in the the mixture of aldehyde (1 mmol), benzil (1 mmol),
presence of higher amount of the catalyst. As indicated ammonium acetate (5 mmol) and crosslinked poly
in table 1, Maximum yield was obtained (95%) when (AMPS-co-AA) (0.03 g) for 25–35 min at 110◦C. The
the reaction was loaded with 0.03 g of the catalyst at corresponding results are given in table 2. We found that
110◦C. A further increasing of catalyst loading did not the reaction proceeded very efficiently either electron-
affect the yield (entry 11, table 1).
releasing or electron-withdrawing substituents on aryl
After optimizing the conditions, we applied this cat- ring of aldehyde.
alyst for synthesis of trisubstituted imidazoles by using
Also, due to direct use of benzoin rather than benzil
different aromatic aldehydes with a wide range of in the synthesis of imidazoles a significant improvement
H+
H
H
O
O
H
NH2
poly(AMPS-co-AA)
NH4OAc
NH3
[
]
HO
Ph
Ph
Ph
NH3 + HOAc
poly(AMPS-co-AA)
H
NH2
+H
H2N
H
NH2
NH3
O
-H2O
Ph
H
Ph
Ph
O
O
Ph
Ph
O
Ph
H
OH
H
+H
+H
H
H
H
O
O
O
N
N
N
Ph
Ph
Ph
Ph
Ph
Ph
H2N
O
+H
+H
H
H
-2H2O
-2H2O
Ph
Ph
N
N
H
H
air oxidation
H
Ph
Ph
N
N
H
Ph
Ph
H [1,5]
Ph
N
Ph
N
Ph
H
Figure 3. Probable mechanism for the formation of triarylimi-
dazoles using benzil or benzoin, ammonium acetate, aromatic
aldehydes and poly (AMPS-co-AA) as catalyst.

722
Ali Mohammadi et al.
in the synthesis toward the greener chemistry is rep-
resented. We have repeated the reaction with benzoin
instead of benzil and the desired product has been effi-
ciently produced. As indicated in table 2, when we used
benzoin instead of benzyl, the reaction time increased
and also the yield of the reaction decreased partially.
Possible mechanism for the crosslinked poly
(AMPS-co-AA) catalysed synthesis of trisubstituted
imidazoles has been given in figure 3.30 In summary,
this paper describes a convenient and efficient process
for the solvent-free synthesis of trisubstituted imida-
zoles through the three-components coupling of ben-
zil or benzoin, aldehydes and ammonium acetate using
crosslinked poly(AMPS-co-AA) as a catalyst. Reaction
profile is very clean and no by-products are formed.
All the synthesized imidazoles have been character-
ized on the basis of elemental and spectral studies. We
believe that this procedure is convenient, economic, and
a user-friendly process for the synthesis of trisubstituted
imidazoles of biological and medicinal importance.
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Lande M K 2009 Cent. Eur. J. Chem. 7 550
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5. Conclusion
We have been able to introduce an efficient and environ-
mentally friendly approach for the synthesis of biolog-
ically active trisubstituted imidazoles via condensation
of benzil or benzoin with various aromatic aldehydes
and ammonium acetate using crosslinked poly (AMPS-
co-AA) as a catalyst. High yields, easy work-up, purifi-
cation of compounds by non-chromatographic method
(crystallization only) and the reusability of the catalyst
are the key advantages of this method.
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