Research Journal of Chemical Sciences 
______
______________________________
______
____
 
ISSN 2231
-
606X
 
 
 
Vol. 
2
(
4
), 
1
8
-
23
, 
April 
(201
2
)
 
 
Res.J.Chem.Sci.
 
 
 
International Science Congress Association
 
 
 
 
    
18
 
Synthesis of
 
Substituted Imidazoles 
via
 
a Multi
-
Component Condensation 
Catalyzed by 
p
-
toluene Sulfonic Acid, PTSA
 
 
Kumar Vikrant
1
*, Mamgain Ritu
2
 
and Singh Neha
1
 
1
Department of Chemistry, 
Acharya Narendra Dev College (University of Delhi), New Delhi
-
110019, INDIA
 
2
Department of Chemistry, Graphic Era University, Dehradun, Uttarakhand, INDIA
 
 
Available online at: 
www.isca.in
 
(Received
 
25
th
 
January 
201
2
, 
revised
 
10
th
 
March 
201
2
, accepted
 
17
th
 
March 
201
2
)
 
 
Abstract
 
A robust and reliable one pot synthetic method has been developed for 2,4,5
-
trisubstituted and 1,2,4,5
-
tetra substituted 
imidazoles. The synthetic sequence, 
via a multi
-
component condensation 
catalyzed by 
p
-
toluenesulfonic acid (PTSA)
, provides 
good isolated yields under mild conditions. The structural features have been arrived at from their micro analytical, infra r
ed, 
mass and 
1
H NMR spectral data. Short synthesis, mild reaction conditions, 
inexpensive reagents and high yield illustrate the 
utility of this approach.
 
 
Keywords
:
 
Multicomponent reaction, one
-
pot synthesis, substituted imidazoles, 
p
-
toluenesulfonic acid,
 
(PTSA).
 
 
 
Introduction
 
Imidazole 
represents an
 
important class of compound being the 
main components of many naturally occurring products, as well 
as synthetic derivatives. Imidazole ring ha
s
 
been of great 
interest for organic chemist due to their useful biological and 
pharmacological aspects. They ac
t as inhibitors of p38 MAP 
kinase
1
,
 
B
-
Raf kinase
2
,
 
transforming growth factor b1 (TGF
-
b1) 
type 1 activin receptor
-
like kinase (ALK5)
3
,
 
cyclooxygenase
-
2 
(COX
-
2)
4
 
and biosynthesis of interleukin
-
1 (IL
-
1)
5
.
 
Appropriately substituted imidazoles are extensively
 
used as 
glucagon receptors
6
 
and CB1 cannabinoid receptor antagonists
7
,
 
modulators of P
-
glycoprotein (P
-
gp)
-
mediated multidrug 
resistance (MDR)
8
,
 
antitumor
9
, 
antibacterial
10
 
and anti
-
inflammatory agents. It can also be employed as 
fungicides
, 
herbicides
11
 
and plant growth regulators
12
. 
Imidazoles as ionic 
liquid and imidazole related N
-
 
hetrocyclic carbenes are well 
known green solvent. 
 
 
Due to pharmacological properties and 
industrial
 
application
s
 
the preparation of imidazole
 
has been attracted considera
ble 
attention in recent years.
 
Various synthetic protocols have been 
developed for the synthesis 
of Imidazole such as 
the hetero
-
Cope rearrangement
13
, four
-
component condensation of 
arylglyoxals, primary amines, carboxylic acids and isocyanides 
on Wang res
in
14
, reaction of N
-
(2
-
oxo)
-
amides with ammonium 
tri fluoroacetate, 1, 2
-
aminoalcohols in the presence of PCl
5
, 
diketones, aldehyde, amine and ammonium acetate in 
phosphoric acid and in acetic acid, organo catalyst in acetic acid 
as well as H
2
SO
4
 
and DMSO. Several microwave (MW) 
assisted syntheses of imidazoles from 1, 2
-
diketones and 
aldehydes in the presence of a variety of catalysts such as silica
-
gel, silica
-
gel/HY, Al
2
O
3
, DMF, acetic acid
15
, ZrCl
4
16
, 
NiCl
2
.6H
2
O
17
 
and ionic liquid
18
 
has been r
eported.
 
Many of the 
synthetic protocols for the synthesis of imidazole reported so far 
suffer from one or more disadvantages such as harsh reaction 
conditions, poor yields, prolonged time period, use of hazardous 
and often expensive acid catalysts. 
So, de
velopment of an 
improved synthetic protocol for the generation of
 
substituted 
imidazole to lead optimization is of considerable interest.
 
 
As a part of our ongoing effort
s
 
towards the development of 
new procedure for the highly substituted heterocycles thr
ough 
multi
-
component reaction
19
, we have discovere
d an efficient and 
environment
 
friendly procedure for the synthesis of substituted 
imidazole. 
Here we describe the synthesis of highly substituted 
imidazoles by one
-
pot condensation of benzil with a substituted 
benzaldehyde, ammonium acetate and aniline in the presence of 
p
-
toluenesulphonic acid (PTSA) a non
-
toxic and inexpensive 
catalyst.
 
 
Materia
l and Method
s
 
Chemicals used in the experiment were of analytical grade.  
Analytical TLC’s were performed on pre
-
coated Merck silica 
gel 60 F254 plates; the spots were detected either under UV 
light or by placing in iodine chamber. Melting points were 
dete
rmined using a Thomas Hoover melting point apparatus and 
are uncorrected. IR spectra were obtained on Perkin
-
Elmer 
FTIR
-
1710 spectrophotometer using Nujol film. 
1
H NMR and 
13
C NMR spectra were recorded on a Bruker Advance 
Spectrospin at 300 and at 75 MHz, 
respectively, using TMS as 
internal standard. 
 
 
General procedure for the synthesis of 2,4,5
-
triarylimidazole 
(4a
-
4d)
: 
A mixture of benzil 1
 
(10 mmol), ammonium acetate 2
 
(20 mmol), aromatic aldehyde 
3a
-
3d 
(20 mmol) and PTSA (5 
mol
 
%) stirred at 80 �C in ethanol (5 ml) for the appropriate 
time as mentioned in 
t
able 1
.
 
The completion of reaction was 
monitored by TLC.
 
After completion of reaction, the reaction 
Research Journal of Chemical Sciences 
______
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_
____
 
ISSN 2231
-
606X
 
 
Vol. 
2
(
4
), 
1
8
-
23
, 
April 
(201
2
)
 
  
Res.J.Chem.Sci
 
International Science Congress Association
 
 
19
 
mixture was cooled to room temperature and diluted with excess 
of cold water
. The solid imidazole products that separated out, 
were filtered, washed with excess of water and was further 
recrystallized with 9:1 acetone
-
water to result a pure compound 
of 2,4,5
-
triarylimidazole (4a
-
4d)
,
 
s
cheme 1.
 
 
General procedure for the synthesis 
of 1,2,4,5
-
tetraarylimidazole (
6
a
–
6
d)
: 
A mixture of benzil 
5
 
(10 mmol), 
ammonium acetate 2 (10 mmol), aniline 
5
 
(10 mmol), aromatic 
aldehyde 
3
a
-
3
d 
(10 mmol), and PTSA (5 mol %) stirred at 80 
�C in ethanol (5 ml) for the appropriate time as mentioned in 
t
ab
le 2.
 
The completion of reaction was monitored by TLC. To 
obtain pure compounds of 1,2,4,5
-
tetraarylimidazole (
6
a
-
6
d) 
after completion of reaction, work
-
up procedure followed was 
similar to the synthesis of 2,4,5
-
triarylimidazole. The structures 
of all the products were unambiguously established on the basis 
of their spectral analysis (IR, 
1
H, 
13
C NMR and CHN data)
, 
sc
heme 2
. 
 
 
 
Table
-
1
 
Analytical and physical data of the tri
-
substituted imidazole compounds
 
Entry
 
Product
 
R
 
Time (h)
 
Yield (%)
 
MP (obs)�C
 
M.P (rep)�C
 
3a
 
4a
 
H
 
1
 
90
 
275
 
273
 
3b
 
4b
 
CH
3
 
1.5
 
82
 
240
 
235
 
3c
 
4c
 
OCH
3
 
1.5
 
87
 
225
 
222
-
224
 
3d
 
4d
 
Cl
 
2
 
80
 
265
 
261
-
262
 
 
Table
-
2
 
Analytical and physical data of the tetra
-
substituted imidazole compounds
 
Entry
 
Product
 
R
 
Time (h)
 
Yield (%)
 
MP  (obs) �C
 
M.P (rep) �C
 
3
a
 
6
a
 
H
 
1
 
84
 
117
 
121
 
3
b
 
6
b
 
CH
3
 
1.5
 
81
 
188
 
185
 
3
c
 
6
c
 
OCH
3
 
1.5
 
91
 
186
 
184
 
3
d
 
6
d
 
Cl
 
2
 
85
 
154
 
151
 
 
 
 
 
Scheme
-
1
 
Synthesis of tri
-
substituted imidazoles
 
 
 
 
Scheme
-
2
 
Synthesis of tetra
-
substituted imidazoles
 
Research Journal of Chemical Sciences 
______
_
_
_______________________________
______________
_
____
 
ISSN 2231
-
606X
 
 
Vol. 
2
(
4
), 
1
8
-
23
, 
April 
(201
2
)
 
  
Res.J.Chem.Sci
 
International Science Congress Association
 
 
20
 
Results and Discussion
 
Careful literature analyses revealed
 
that a variety of
 
acidic 
catalyst have been used for this multicomponent reaction. It has 
been reported that 
p
-
toluenesulphonic acid (PTSA) acts as a 
mild, useful, non
-
toxic and inexpensive Lewis acid catalyst 
which makes the process convenient, more econ
omic and 
environmentally benign. The mild reaction conditions, 
operational simplicity and the excellent yields make the catalyst 
more versatile. The reaction is rapid, facile, and efficient and is 
devoid of unnecessary derivatization and generation of 
haza
rdous substance. Knowing the importance of PTSA we 
used it as a catalyst for these multicomponent reactions. We 
were please to know that high yield product was obtained on 
using only 5 mol% of PTSA in ethanol. A wide range of 
substituted aromatic aldehydes
 
underwent multicomponent 
condensation with benzil and ammonium acetate to give high 
yield tri
-
substituted imidazole. In addition to above reactants if 
aniline was added it resulted into tetra
-
substituted imidazole. 
We also tried aqueous ethanol and PEG as
 
solvent but the 
results were not satisfactory. 
All the utilized functionalities were 
found to be compatible under the reaction conditions
 
(t
able 1 
and 
t
able 2
).
 
 
Spectroscopic Data of the Synthesized Compounds: 
Compound 4
a
 
2, 4, 5
-
triphenyl 1H
-
 
imidazole
:
 
M.
P
. 27
5
0
C.
 
IR
 
(cm
-
1
,
 
Nujol): 338
5, 3116, 1638, 1522, 1420. 
1
H NMR
 
(DMSO
-
d
6,
): 
δ
 
= 
12.32 (s, 1H), 7.21
-
8.10 (m, 15H) ppm
. 
13
C NMR
 
(DMSO
-
d
6
): 
δ
 
123.2, 126.7, 128.2, 129.0,135.6 ppm; 
Anal. 
Calcd. (Found) for C
21
H
16
N
2
: C, 85.10 (85.01); H, 5.44 (5.51); 
N, 
9.44 (9.39).
 
 
Compound 
4b
 
2
-
(4
-
Methyl
-
phenyl)
-
4,5
-
dipheny
-
1H
-
imidazole:
 
M
.
P
. 2
4
0
0
C
. 
IR
 
(cm
-
1
, KBr): 2920, 1602, 1493, 1486, 1453 
1218
.
 
1
H NMR
 
(DMSO
-
d
6
): 
δ
 
= 
2.32(s, 3H), 7.39
-
8.49 (m, 
14H), 12.87 
ppm
; 
13
C NMR
 
(DMSO
-
d
6
): 
δ
 
= 48.8
, 126.1, 127.4, 
128.3, 128.9, 129.5, 130.6, 134.4, 138.2, 147.3 ppm; 
Anal. 
Calcd. (Found) for C
22
H
18
N
2
: C, 85.13 (84.90); H, 5.85 (5.66); 
N, 9.02 (9.31).
 
 
Compound
 
4c
 
2
–
(4
–
Methoxy
–
phenyl)
–
4,5 diphenyl
–
1H
-
imidazole
: 
M.
P
: 225�C; IR (cm
−1
, Nujol): 3433, 1619, 1527; 
1
H 
NMR (CDCl
3
/DMSO
-
d
6
): 
δ
 
= 
 
3.72 (s, 3H), 6.92
–
6.96 (d, 2H), 
7.18
–
7.31 (m, 10H), 7.82
–
7.85 (d, 2H), 12.48 (br, s, NH) ppm; 
13
C NMR (CDCl
3
/DMSO
-
d
6
): 
δ
 
= 56.1, 114.3, 123.1, 126.3, 
126.6, 128.0, 128.3, 134.2, 146.0, 158.9 ppm; 
Anal. Calcd. 
(Found) for C
22
H
18
N
2
: C, 80.96 (81.01); H, 5.55 (5.49); N, 8.56 
(8.39).
 
 
Compound 
4d
 
2
-
(4
-
chlorophenyl)
-
4,5
-
diphenyl 1H imidazole
:
 
M
. p. 265�C. IR
 
(cm
-
1
, KBr):  3447, 1620, 1519
.
 
1
H NMR
 
(
CDCl
3
/DMSO
-
d
6
): 
δ
 
=
 
12.71 (br, s, 1H), 7.76
-
7.89(d, 2H),
 
7.44 
-
 
7.51  (d, 2H), 7.10
-
7.41
 
(m, 10H) ppm
. 
13
C NMR
 
(
CDCl
3
/DMSO
-
d
6
): 
δ
 
= 124.9, 126.3, 126.8, 128.1, 129.2, 
129.9, 132.3, 143.9 ppm; 
Anal. Calcd. (Found) for C
21
H
16
N
2
: C, 
76.25
 
(76.14); H, 
4.53
 
(4.55); N, 
8.47
 
(8.37).
 
 
Compound 
6
a
 
1,2,4,5
-
Tetraphenylimidazole
: M.
P
: 
117
�C; IR 
(cm
−1
, Nujol): 3008, 1621, 1521, 1421; 
1
H NMR 
(CDCl
3
/DMSO
-
d
6
) 
δ
 
= 7.10
–
7.91 (m, 20H) ppm; 
13
C NMR 
(CDCl
3
/DMSO
-
d
6
) 
δ
 
= 123.2, 124.5, 125.1, 126.0, 127.4, 128.7, 
128.8, 129.2, 129.5, 129.9, 136.9 ppm; Anal. 
Calcd. (Found) for 
C
27
H
20
N
2 
: C, 87.07 (87.10); H, 5.41 (5.39); N, 7.52 (7.45).
 
 
Compound 
6
b
 
2
-
(4
-
Methylphenyl)
-
1,4,5
-
triphenylimidazole: 
M.
P
: 188
�C; IR (cm
−1
, Nujol): 
1610, 1582; 
1
H NMR 
(CDCl
3
/DMSO
-
d
6
) 
δ
 
= 2.09 (s, 3H), 7.58 
-
 
6.80 (m, 19H); 
13
C 
NMR 
(CDCl
3
/DMSO
-
d
6
) 
δ
 
= 47.20, 120.06, 122.43, 126.76, 
127.06, 129.06, 132.80, 138.09, 141.87, 145.76 ppm;
 
Anal. 
Calcd. (Found) for C
28
H
22
N
2
: C, 87.01 (87.05); H, 5.74 (5.81); 
N, 7.23 (7.29).
 
 
Compound 
6
c
 
2
-
(4
-
Methoxy
-
phynyl)
-
1,4,5
-
triphenyl
-
1H
-
imidazole
:
 
M.
P
: 18
6
�C; IR (cm
−1
, Nujol): 1624, 1585; 
1
H NMR 
(CDCl
3
/DMSO
-
d
6
) 
δ
 
= 3.69 (s, 3H), 6.83
–
6.86 (d, 2H), 7.11
–
7.32 (m, 15H), 7.50
–
7.52 (d, 2H) ppm; 
13
C NMR 
(CDCl
3
/DMSO 
d
6
) 
δ
 
= 56.2, 113.8,  125.6, 126.3, 127.1, 128.0, 
128.3, 128.5,128.8, 129.4, 130.8, 
131.5, 131.9, 135.5, 136.9, 
138.4, 147.1, 159.5 ppm; Anal. Calcd. (Found) for C
28
H
22
N
2
O: 
C, 83.56 (83.58); H, 5.51 (5.45); N, 6.96 (6.90). 
 
 
Compound
 
6
d
 
2
-
(4
-
Chloro
-
phenyl)
-
1,4,5
-
triphenyl
-
1H
-
imidazole
: 
M.
P
: 154
�C, IR (cm
−1
, Nujol): 1620, 1584; 
1
H NMR 
(CDCl
3
/DMSO
-
d
6
) 
δ
 
= 7.21
–
7.55 (m, 15H), 7.64
–
7.67 (d, 2H), 
7.90
–
7.94 (d, 2H) ppm; 
13
C NMR (CDCl
3
/ DMSO
-
d
6
) 
δ
 
= 
123.6, 124.4, 126.1, 127.5, 128.0, 129.1, 130.5, 130.7, 132.5, 
134.1, 138.9, 144.1, 144.7 ppm; Anal. 
Calcd. (Found) For  
C
27
H
19
N
2
Cl: C, 79.70 (79
.78); H, 4.71 (4.69); N, 6.88 (6.81). 
 
 
Conclusion
 
I
midazoles enjoy
 
an outstanding status due to their biological 
importance. 
PTSA, a non toxic and inexpensive catalyst is 
optimized for the synthesis of tri
-
 
and tetra
-
 
substituted 
imidazoles. Existing 
synthetic approaches are currently 
somewhat limited by issues of poor yields, harsh reaction 
conditions, expensive catalysts etc. hence; the present one
-
pot 
synthetic method provides an alternate
 
methodo
logy to obtain 
excellent yield 
of product
,
 
under refl
ux condition with 5 mol % 
of PTSA.
 
 
 
 
Acknowledgements
 
Vikrant Kumar thanks the Principal, A. N. D. College (DU) for 
providing laboratory facilities. V. K. also thanks “Department of 
Biotechnology (DBT)” for support of this research (Grant No. 
BT/PR13120/GBD/27/188/2009).
 
 
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Research Journal of Chemical Sciences 
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_
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_
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ISSN 2231
-
606X
 
 
Vol. 
2
(
4
), 
1
8
-
23
, 
April 
(201
2
)
 
  
Res.J.Chem.Sci
 
International Science Congress Association
 
 
21
 
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and Tetrasubstituted 
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17.
 
Heravi M.M., Bakhtiari K., Oskooie H.A. and Taheri S., 
Synthesis of 2,4,5
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triaryl
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2. 
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279 
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Siddiqui S.A., Narkhede U.C., Palimkar S.S., Daniel T., 
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Room temperature ionic 
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-
triaryl imidazoles from aryl aldehydes and 1,2
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diketones or 
α
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hydroxyketone, 
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61
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3539 
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19.
 
Mamgain R., Singh R., Rawat D.S., DBU
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catalyzed three
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ynthesis of highly functionalized 
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46,
 
69 
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Research Journal of Chemical Sciences 
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_
_
_______________________________
______________
_
____
 
ISSN 2231
-
606X
 
 
Vol. 
2
(
4
), 
1
8
-
23
, 
April 
(201
2
)
 
  
Res.J.Chem.Sci
 
International Science Congress Association
 
 
22
 
 
 
Fig
ure
-
1a
 
NMR spectra of 
2, 4, 5
-
triphenyl 
1H
-
 
imidazole
 
 
 
 
Fig
ure
-
1b
 
NMR spectra of 
1,2,4,5
-
tetraphenylimidazole
 
Research Journal of Chemical Sciences 
______
_
_
_______________________________
______________
_
____
 
ISSN 2231
-
606X
 
 
Vol. 
2
(
4
), 
1
8
-
23
, 
April 
(201
2
)
 
  
Res.J.Chem.Sci
 
International Science Congress Association
 
 
23
 
 
Fig
ure
 
-
 
2a
 
IR spectra of 
2, 4, 5
-
triphenyl 
1H
-
 
imidazole
 
 
 
Fig
ure
-
2b
 
IR spectra of 
1,2,4,5
-
tetraphenylimidazole
 
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