An efficient and eco-friendly tungstate promoted zirconia (WO x /ZrO2) solid acid catalyst for the synthesis of 2-aryl benzimidazoles

Article abstract of DOI:10.1007/s10562-010-0340-1

An eco-friendly catalytic amount (0.3 g) of tungstate-promoted zirconia (WO _x /ZrO₂) solid acid catalyst has been explored in the synthesis of 2-aryl benzimidazoles using aryl aldehydes and o-phenylenediamine. This is one of the fundamental aspects in green chemistry for simple experimental and product isolation protocol combined with easy recovery and reusability of the catalyst. It is expected to contribute in the development of clean and environment friendly strategy for synthetic protocols. The incorporated promoter showed strong influence on the surface and bulk properties of zirconia. The catalytic activity results suggest that the methodology adopted offers significant improvements for the synthesis of benzimidazole derivatives with respect to yields, simplicity and green aspects by avoiding toxic conventional catalysts and solvents. Henceforth, a good substrate conversion and excellent product selectivity were obtained over tungstate promoted zirconia solid acid catalyst.

Full text of DOI:10.1007/s10562-010-0340-1

Catal Lett (2010) 137:63–68
DOI 10.1007/s10562-010-0340-1
An Efficient and Eco-Friendly Tungstate Promoted Zirconia
(WOx/ZrO2) Solid Acid Catalyst for the Synthesis of 2-Aryl
Benzimidazoles
•
Ramya V. Shingalapur Kallappa M. Hosamani
Received: 27 February 2010 / Accepted: 28 March 2010 / Published online: 13 April 2010
Ó Springer Science+Business Media, LLC 2010
Abstract An eco-friendly catalytic amount (0.3 g) of
tungstate-promoted zirconia (WOx/ZrO2) solid acid cata-
lyst has been explored in the synthesis of 2-aryl benzimi-
dazoles using aryl aldehydes and o-phenylenediamine. This
is one of the fundamental aspects in green chemistry for
simple experimental and product isolation protocol com-
bined with easy recovery and reusability of the catalyst. It
is expected to contribute in the development of clean and
environment friendly strategy for synthetic protocols. The
incorporated promoter showed strong influence on the
surface and bulk properties of zirconia. The catalytic
activity results suggest that the methodology adopted offers
significant improvements for the synthesis of benzimid-
azole derivatives with respect to yields, simplicity and
green aspects by avoiding toxic conventional catalysts
and solvents. Henceforth, a good substrate conversion and
excellent product selectivity were obtained over tungstate
promoted zirconia solid acid catalyst.
green chemistry. Many organic reactions such as alkyl-
ation, acylation, isomerization, nitration, esterification and
rearrangements like Pinacol, Meerwin, Beckmann, Ritter
reactions etc., are accomplished by the homogeneous acid
catalysts. All these acid-catalyzed reactions are almost
carried out by employing conventional mineral acids like
H2SO4, HNO3 and HF or Lewis acids such as AlCl3 and
BF3. Thus, these classical acid catalyzed organic reactions
have been exploited as one of the most important basic
reactions in organic chemistry for their use in polymer and
pharmaceutical syntheses. However, all these procedures
suffer from one or more of the following disadvantages
such as low yields, use of expensive, lachrymatory or
hygroscopic reagents, highly inflammable or potentially
explosive solvents, high temperatures, a special oxidation
process or long reaction times, tedious work-up procedures
and co-occurrence of several side reactions.
Henceforth, number of methods has been developed for
the synthesis of benzimidazole derivatives including the
condensation of carboxylic acids [1, 2], acid chlorides [3],
nitriles [4], orthoesters [5–7], amides [8], esters [9] or
aldehydes [10–13] with o-phenylenediamine (OPD). In
view of the environmental and economical reasons, there is
an ongoing effort to replace such conventional homoge-
neous catalysts with newer solid green acid catalysts. This is
mainly due to the distinct advantage of solid acid catalysts
such as non-toxicity, non-corrosiveness, ease of handling,
less expensive and easy to recover and reuse [14–21].
In this context, new avenues for various solid acid cat-
alytical systems have been introduced including hetero-
polyacids, ion exchange resins (Amberlyst and Nafion-H),
zeolites and clays. Amongst, zirconia supported oxoanions
have been investigated [22], and the solid superacid cata-
lyst, sulfated zirconia have also been gained much attention
due to its high activity to catalyze many reactions even at
Keywords Aryl aldehydes Á o-Phenylenediamine Á
2-Aryl benzimidazoles Á Tungstate-promoted zirconia Á
WOx/ZrO2
1 Introduction
Solid green acid catalysts play an important role in the
green synthesis and transformations in organic chemistry.
Thus, a straightforward, efficient and environmentally
acceptable synthetic protocol is an important aspect in
R. V. Shingalapur Á K. M. Hosamani (&)
P.G. Department of Studies in Chemistry, Karnatak University,
Pavate Nagar, Dharwad 580003, India
e-mail: dr_hosamani@yahoo.com
123

64
R. V. Shingalapur, K. M. Hosamani
low temperatures [23–25]. Thus, modified zirconia solid
acid catalyst has been used for Mannich reaction, for the
synthesis of a-amino nitriles, diphenylureas, coumarines,
1,5-benzodiazepines, and acylation of alcohols, phenols
and amines and protection of carbonyl compounds [26–28].
Interest in the benzimidazole chemistry, there has been a
exploration of biologically potent benzimidazole molecules
for their significant physiological and pharmacological
activities [29] and also encompasses a diverse range of
pharmacological activities including anti-microbial, anti-
inflammatory, analgesic, anti-histamine, anti-ulcerative,
anti-oxidant, anti-proliferative, anti-allergic, anti-tumour,
anti-kinase, epilepsy, diabetes, anti-fertility, anti-tubercu-
lar, anti-cancer, cytotoxicity and anti-HIV-1.
The IR spectra were recorded on a Nicolet Impact-410 FT-
IR Spectrophotometer using KBr pellets. 1H NMR and 13
C
NMR spectra were recorded on a Bruker AC-300F
300 MHz spectrometer in CDCl3/DMSO solvents using
TMS as an internal standard with 1H resonant frequency of
300 MHz and 13C resonant frequency of 75 MHz. D2O
exchange was applied to confirm the assignment of the
signals of NH protons. The mass spectra were recorded on
Shimadzu GCMS-QP2010S at 70 eV. The elemental
analysis was carried out by using Heraus CHN rapid ana-
lyzer. All the compounds gave C, H and N analysis within
0.4% of the theoretical values. The homogeneity of the
compounds was described by TLC on aluminum silica gel
60 F254 (Merck) detected by UV light (254 nm) and iodine
vapours.
As in recent times more emphasis has been given to
develop clean process, it is essential to find a suitable alter-
nate catalyst to meet the requirements. In continuation to our
quest in organic synthesis employing inexpensive catalysts
[2, 30] use of bifunctional solid acid catalysts seems to be the
right solution. To the best of our knowledge, there has been
no report for the utilization of solid acid catalyst such as
mixed metal oxides consisting of WOx/ZrO2 for the green
synthesis of benzimidazole derivatives. Hence, it is a right
time to explore such solid green acid catalysts in organic
reactions to synthesize clean and green compounds.
2.2 Catalyst Preparation
Zirconium hydroxide was prepared from zirconium oxy-
chloride by hydrolysis with dilute aqueous ammonia
solution. For this purpose, the requisite quantity of
ZrOCl2Á8H2O (Aldrich, GR grade) was dissolved in doubly
distilled water and to this clear solution, aqueous NH3 was
added drop-wise with vigorous stirring until the pH of the
solution reached 8. The obtained precipitate was washed
with hot distilled water several times until free from
chloride ions and dried at 393 K for 24 h. On the obtained
Zr(OH)4, a nominal 10 wt% WOx was deposited by
adopting a wet impregnation method. To achieve this, the
requisite quantity of ammonium metatungstate (Aldrich,
AR grade) was dissolved in doubly distilled water and to
this clear solution the desired quantity of oven dried
Zr(OH)4 was added and excess of water was evaporated on
a water-bath. The resulting cake was oven dried at 393 K
for 24 h and calcined at 923 K for 6 h in a flow of oxygen.
A small portion of the hydrous zirconia was calcined at
923 K for 6 h to make unpromoted ZrO2. Depending on
the calcination temperatures, WOx/ZrO2 catalysts were
designated as WZ-temp (temp means the calcination tem-
perature). WZ-amorphous catalyst represents the amor-
phous WOx/ZrO2 catalyst.
Therefore, the use of eco-friendly green solid acid cat-
alysts for the organic synthesis and transformation reac-
tions in the liquid phase under hazardous free solvents or
environmentally benign solvents has represented the ideal
green chemical technology protocols from both environ-
mental and economical point of view. Thus, combining the
potent pharmacological activities and green compounds
would re-inforce greening of such eco-friendly organic
reactions. To the best of our knowledge, this will be one of
the promising approaches for the green synthesis of benz-
imidazole derivatives.
With reference to the research work on green catalysts
[31, 32] and in order to explore the expertise in green
chemistry, we are currently exploiting tungstate promoted
zirconia solid catalyst for the green synthesis. In the
present research investigation, we are reporting such green
solid acid catalyzed synthetic protocol for the synthesis of
benzimidazole derivatives [33–35]. However, efforts are
still ongoing to develop clean and eco-friendly strategy for
the synthesis of novel organic compounds.
2.3 Materials and Methods
A mixture of o-phenylenediamine (1.0 mmol), different
aromatic aldehydes (1.0 mmol), and a pinch of WZ catalyst
(0.3 g) in 10 ml 1,4-dioxane were stirred for 10 min and
refluxed further at 100 °C for an appropriate time (Table 3)
which was monitored by TLC. After completion of the
reaction, catalyst was recovered by simple filtration and
reused. The products were recovered from the filtrate,
concentrated on a rotatory evaporator and chromato-
graphed on a silica gel column to offer pure products. All
2 Experimental
2.1 Instrumentation
The melting points of the products were determined by
open capillaries on a Buchi apparatus and are uncorrected.
123

2-Aryl Benzimidazoles
65
the compounds were characterized by comparing their
physical and spectral data with those of reported com-
300 MHz): d 12.30 (br, 1H, s, –NH), 7.21–8.05 (8H, m);
13C NMR (DMSO-d6, 75 MHz): d 141.8, 130.2, 138.0,
125.1, 120.5, 115.1; EIMS: m/z 210 (M ? H)?; Anal.
Calcd. for C13H10N2O (210): C, 74.27; H, 4.79; N, 13.33.
Found: C, 74.26; H, 4.78; N, 13.32.
1
pounds. The m.p., IR, H NMR, 13C NMR and EIMS data
of all synthesized compounds are given below-
2.4 Product Characterization Data
2.4.6 2-(4-Methylphenyl)-1H-benzimidazole 3f
2.4.1 2-Phenyl-1H-benzimidazole 3a
Light yellow solid: m.p. = 261–263 °C [34]; IR (KBr):
White solid: m.p. = 287–288 °C [33]; IR (KBr): 3334,
3340, 3016, 2854, 1625, 1439, 1288 cm-1 1H NMR
;
3198, 1625, 1510, 1287 cm-1 1H NMR (DMSO-d6,
;
(DMSO-d6, 300 MHz): d 2.24 (3H, s), 12.21 (br, 1H, s,
–NH), 7.12–7.90 (m, 8H); 13C NMR (DMSO-d6, 75 MHz):
d 21.4, 116.8, 122.5, 126.3, 138.0,140.8; EIMS: m/z 208
(M ? H)?; Anal. Calcd. for C14H12N2 (208): C, 80.74; H,
5.81; N, 13.45. Found: C, 80.82; H, 5.78; N, 13.41.
300 MHz): d 12.93 (1H, s, –NH), 7.92 (2H, d, J = 1.3 Hz),
7.47 (m, 1H), 7.22 (4H, m), 6.95 (2H, s); 13C NMR
(DMSO-d6, 75 MHz): d 142.8, 136.1, 135.2, 129.4, 120.2,
114.9; EIMS: m/z 194.0 (M ? H)?; Anal. Calcd. for
C13H10N2 (194): C, 80.39; H, 5.19; N, 14.42. Found: C,
80.35; H, 5.21; N, 14.39.
2.4.7 2-(4-Methoxyphenyl)-1H-benzimidazole 3g
2.4.2 2-(2-Chlorophenyl)-1H-benzimidazoles 3b
Light yellow solid: m.p. = 225–227 °C [33]; IR (KBr):
White solid: m.p. = 231–233 °C [34]; IR (KBr): 3421,
3068, 2886, 1637, 1464, 1245 cm-1; 1H NMR (DMSO-d6,
300 MHz): d 12.21 (br, 1H, s, –NH), 7.25–7.95 (8H, m);
13C NMR (DMSO-d6, 75 MHz): d 145.0, 132.5, 138.2,
120.8, 119.5, 114.1; EIMS: m/z 219.0 (35Cl [M ? H]?);
Anal. Calcd. for C13H9N2Cl (229): C, 68.28; H, 3.97; N,
12.25. Found: C, 68.25; H, 3.99; N, 12.22.
3339, 3000, 2928, 1609, 1440, 1292 cm-1 1H NMR
;
(DMSO-d6, 300 MHz): d 12.70 (1H, s), 8.15 (2H, d,
J = 1.75 Hz), 7.66 (1H, s), 7.42 (1H, s), 7.18 (4H, m), 3.76
(3H, s); 13C NMR (DMSO-d6, 75 MHz): d 56.3, 115.6,
119.1, 121.9, 122.6, 126.7, 127.8, 129.8, 135.3, 139.9,
144.1, 151.7; EIMS: m/z 225 (M ? 1)?; Anal. Calcd. for
C14H12N2O (224): C, 74.98; H, 5.39; N, 12.49. Found: C,
75.00; H, 5.38; N, 12.45.
2.4.3 2-(4-Chlorophenyl)-1H-benzimidazoles 3c
Light yellow solid: m.p. = 291–293 °C [34]; IR (KBr):
3338, 2923, 1624, 1448, 1487 cm-1; 1H NMR (DMSO-d6,
300 MHz): d 12.55 (br, 1H, s, –NH), 8.14 (2H, d,
J = 1.52 Hz), 7.61 (1H, s), 7.23–7.28 (3H, s), 7.12–7.04
(2H, s); 13C NMR (DMSO-d6, 75 MHz): d 142.5, 137.1,
133.9, 129.8, 120.5, 114.1; EIMS: m/z 219.0 (35Cl
[M ? H]?); Anal. Calcd. for C13H9N2Cl (229): C, 68.28;
H, 3.97; N, 12.25. Found: C, 68.29; H, 3.94; N, 12.27.
2.4.8 2-(1H-Benzimdazole-2-yl)-6-methoxy-phenol 3h
(Unknown)
White solid: m.p. = 185–187 °C; IR (KBr): 3360, 2923,
2843, 1612, 1495, 1272 cm-1 1H NMR (DMSO-d6,
;
300 MHz):
d
12.70 (s, 1H), 7.12–7.98 (8H, d,
J = 1.54 Hz), 3.62 (s, 3H); 13C NMR (DMSO-d6,
75 MHz): d 55.2, 115.1, 120.6, 122.8, 125.4, 137.2, 141.2,
149.1; EIMS: m/z 235 (M ? 1)?; Anal. Calcd. for
C14H12N2O2 (240): C, 69.99; H, 5.03; N, 11.66. Found: C,
69.95; H, 5.05; N, 11.62.
2.4.4 2-(1H-Benzimidazole-2-yl)-phenol 3d
White solid: m.p. = 235–237 °C [33]; IR (KBr): 3378,
3459, 2859, 1614, 1495 cm-1 1H NMR (DMSO-d6,
;
2.4.9 4-(1H-Benzimidazole-2-yl)-phenylamine 3i
300 MHz): d 12.48 (br, 1H, s, –NH), 7.18–7.93 (8H, m);
13C NMR (DMSO-d6, 75 MHz): d 21.4, 140.8, 138.0,
126.3, 122.5, 116.8; EIMS: m/z 208 (M ? H)?; Anal.
Calcd. for C13H10N2O (210): C, 74.27; H, 4.79; N, 13.33
Found: C, 74.30; H, 4.75; N, 13.35.
Yellowish white crystals: m.p. = 246–248 °C, lit
m.p. = 248 °C [35]; IR (KBr): 3351, 3062, 2923, 1610,
1491, 1281 cm-1 1H NMR (DMSO-d6, 300 MHz): d
;
12.42 (s, 1H), 6.92–7.75 (d, 8H, J = 1.50 Hz), 4.55 (s,
2H); 13C NMR (DMSO-d6, 75 MHz): d 115.7, 122.1,
126.0, 138.3, 140.9; EIMS: m/z 209 (M ? 1)?; Anal.
Calcd. for C13H11N3 (209): C, 74.62; H, 5.30; N, 20.08.
Found: C, 74.60; H, 5.29; N, 20.10.
2.4.5 4-(1H-Benzimidazole-2-yl)-phenol 3e
Yellow solid: m.p. = 218–220 °C [33]; IR (KBr): 3357,
1609, 1510, 1444, 1247 cm-1 1H NMR (DMSO-d6,
;
123

66
R. V. Shingalapur, K. M. Hosamani
O
2.4.10 2-(3-Nitrophenyl)-1H-benzimidazole 3j
NH2
NH2
N
WOx -ZrO2
(0.3 g)
1,4-dioxane, 100oC
H
+
R
R
Pale yellow solid: m.p. = 210–212 °C [33]; IR (KBr):
3350, 2924, 1609, 1491, 1281 cm-1; 1H NMR (DMSO-d6,
N
H
R = a) -H, b) 2-Cl, c) 4-Cl, d) 2-OH, e) 4-OH, f) 4-CH3, g) 4-OCH3,
h) 2-OH,3-OCH3 , i) 4-NH2, j) 3-NO2, k) 4-NO2, l) Furfural
300 MHz):
d
12.51 (s, 1H), 6.98–7.80 (d, 8H,
J = 1.47 Hz); 13C NMR (DMSO-d6, 75 MHz): d 115.7,
122.1, 126.0, 137.9, 141.9, 142.9, 146.9; EIMS: m/z 235
(M ? 1)?; Anal. Calcd. for C13H9N3O2 (239): C, 65.27; H,
3.79; N, 17.56. Found: C, 65.29; H, 3.80; N, 17.54.
Scheme 1 Synthesis of 2-aryl substituted benzimidazoles using
tungstate-promoted zirconia (WOx/ZrO2) catalyst
2.4.11 2-(4-Nitrophenyl)-1H-benzimidazole 3k
Table 1 Search for optimal solvent for the synthesis of 2-aryl-1H-
benzimidazoles 3(a–l)
Pale red solid: m.p. = 308–310 °C [33]; IR (KBr): 3353,
3050, 2924, 1609, 1491, 1280 cm-1; 1H NMR (DMSO-d6,
300 MHz): d 12.38 (br, 1H, s, –NH), 7.27–7.85 (m, 8H);
13C NMR (DMSO-d6, 75 MHz): d 149.2, 141.9, 140.8,
136.3, 124.5, 115.9; EIMS: m/z 238 (M ? 1)?; Anal.
Calcd. for C13H9N3O2 (239): C, 65.27; H, 3.79; N, 17.56.
Found: C, 65.29; H, 3.75; N, 17.52.
Entry Solvent
Time
(h)a
Yield
(%)b
Temp. in °C
1
2
1,4-Dioxane
4.5
10
95
69
100
100
N,N-Dimethyl
formamide
3
4
5
Toluene
8
6
8
65
70
73
100
80
Acetonitrile
Tetrahydrofuran
65
2.4.12 2-Fural-1H-benzimidazole 3l
The mixture of o-phenylenediamine (0.01 mol) and aromatic alde-
hydes (0.01 mol) with WZ catalyst (0.3 g)
White solid: m.p. = 284–286 °C [33]; IR (KBr): 3352,
3017, 2923, 1625, 1441, 1288 cm-1; 1H NMR (DMSO-d6,
300 MHz): d 12.47 (br, 1H, s, –NH), 6.54 (s, 2H, –CH), 7.2
(s, 1H, –CH), 7.5–7.82 (m, 4H); 13C-NMR (DMSO-d6,
75 MHz): d 152.0, 142.7, 105, 110.4, 136.2, 120.3; EIMS:
m/z 182 (M ? 1)?; Anal. Calcd. for C11H8N2O (184): C,
71.73; H, 4.38; N, 15.21. Found: C, 71.70; H, 4.35; N,
15.19.
a
All the reactions monitored by TLC
b
Isolated yields
Table 2 Effect of WZ catalyst concentration for the synthesis of
2-aryl-1H-benzimidazoles 3(a–l)
Entry
Catalyst concentration (g)
Time (h)a
Yield (%)b
1
2
3
4
5
0.1
0.2
0.3
0.4
0.5
4.0
3.5
3.5
4.0
5.0
70
80
95
79
80
3 Results and Discussion
In continuation of our interest in the application of heter-
ogeneous acids on the development of useful synthetic
methodologies [2, 30] for biologically important hetero-
cycles, we have observed that benzimidazoles can be
synthesized efficiently by the treatment of o-phenylenedi-
amine with aromatic aldehydes using tungstate-promoted
zirconia (WOx/ZrO2) catalyst.
The mixture of the o-phenylenediamines (0.01 mol) and aromatic
aldehydes (0.01 mol) with different weights of WZ catalyst in 1,4-
dioxane at 100 °C
a
All the reactions were monitored by TLC
b
Isolated yields
The reaction proceeded as per Scheme 1. Initially, to
optimize the reaction conditions, the effect of solvent and
catalyst was carried out with benzaldehyde, o-phenylene-
diamine as a model. It was observed that this reaction goes
well in 1,4-dioxane among the commonly used organic
solvents such as N,N-dimethyl formamide, acetonitrile,
toluene, tetrahydrofuran (Table 1, entries 1–5). From these
results we concluded that 1,4-dioxane takes less time and
the comparative yield was high. To evaluate the quantity of
WZ catalyst, 0.1–0.5 g of WZ was used and the products
were obtained in 70–95% yields. To verify the efficiency of
catalyst loading we have investigated the amount of WZ on
the reaction. At mild temperature no product was formed in
the absence of WZ. Therefore, decreasing the quantity of
catalyst from 0.2 to 0.1 g drastically reduced the yield from
95 to 70%. However, increase in the quantity of catalyst
from 0.4 to 0.5 g reduced the yield from 95 to 79%
(Table 2, entries 1–5). From this, we concluded that 0.3 g
of catalyst as optimum amount of the catalyst for this
reaction. Lower catalytic activities were observed when
typical organic solvents such as N,N-DMF, Toluene,
CH3CN and THF were employed which is probably
interfering with the active sites of the catalyst.
123

2-Aryl Benzimidazoles
67
Under identical conditions, the unpromoted ZrO2 cata-
lyst exhibited insignificant product yields. Using optimized
reaction parameters, a number of 2-substituted aryl benz-
imidazoles (3a–l) were synthesized. The time required and
the yields of the compounds are given in Table 3. Aromatic
aldehydes possessing both electron-donating and electron-
withdrawing groups were employed for benzimidazole
formation and in all the cases, the yields were excellent
(Table 3). The method is suitable for the preparation of
benzimidazoles from an acid sensitive aldehyde such as
furfuraldehyde (Table 3, entry 3l).
Table 4 Comparison of catalytic activity of WZ (WOx/ZrO0) cata-
lyst with several other catalysts for the synthesis of 2-aryl-1H-benz-
imidazoles 3aa
Entry Catalysts
Quantity of catalyst Yield (%)
1
2
3
4
6 N HCl
10 ml
70
75
88
80
Phosphotungstic acid
p-Toluenesulfonic acid
0.01 mol
0.01 mol
0.01 mol
Wells-Dawson
heteropolyacid
5
6
a
Glyoxalic acid
WZ
0.01 mol
0.3 g
72
95
Reactions were carried out under same reaction conditions
for a comparative evaluation of the efficacy of WZ with those
of other catalysts reported recently for the synthesis of
Reaction condition: The mixture of the o-phenylenediamine
(0.01 mol) and aryl aldehydes (0.01 mol) with WZ catalyst (0.3 g) in
1,4-dioxane at 100 °C for about 5 h
biologically active potent 2-substituted benzimidazoles via
this route (Table 4). The table shows that the yield of the
desired product in the presence of WZ was comparably higher
than other catalysts used. Additionally, the present method is
indeed more expeditious than the rest of the methods. Con-
sequently, our method can certainly be considered as a con-
venient alternative to the other eco-friendly catalysts used for
one-pot synthesis of substituted benzimidazoles via this route.
The biological activities of synthesized substituted benz-
imidazole derivatives are under progress.
Table 3 Synthesis of 2-substituted benzimidazoles 3(a–l) using a
WZ (WOx/ZrO2) catalyst
Time
(hr)b
5.0
Yield
(%)c
92
Entry
Aryl aldehydes
Product
O
H
N
3a
N
H
Cl
N
O
Cl
3b
3c
5.0
4.5
92
95
H
N
H
O
N
Cl
H
N
H
In summary, WOx/ZrO2 was found to function as mild
and effective catalyst for the synthesis of 2-arylbenzimi-
dazoles under mild conditions by the rapid condensation of
various aryl aldehydes with o-phenylenediamine. In this,
the solvents were selected based on the solubility of reac-
tants and dispersion of catalyst. It is observed that the best
results are obtained in the least polar solvent (1,4-dioxane).
Hence, we have attributed lower yields and higher reaction
time due to the inference with active sites of catalyst which
is more likely with more polar solvents. To verify the role
of air as an oxidant in this reaction, the experiment was
also carried out under nitrogen atmosphere. The yields and
reaction time are almost same and identical. Hence, the
role of catalyst is very important in its optimum condition
with 1,4-dioxane. Thus, this method has advantages over
other reported methods in terms of product with better
yields, selectivity, operational simplicity (easy work up of
reactions), and eco-friendly (non-corrosive reagent).
Cl
HO
N
O
OH
3d
3e
3f
5.0
4.5
5.5
90
96
92
H
N
H
O
N
OH
H
N
H
OH
O
O
N
CH3
H
N
H
CH3
N
3g
3h
3i
5.0
5.5
5.0
95
90
92
OMe
H
N
H
OMe
OMe
HO
N
O
O
OH
OMe
H
H
N
H
N
NH2
N
H
NH2
NO2
O
O
NO2
3j
5.5
90
N
H
N
H
N
3k
3l
5.0
5.5
93
90
NO2
H
N
H
NO2
4 Conclusions
O
N
O
N
H
H
O
The incorporation of tungsten into zirconia is favourable in
the formation of the tetragonal phase, which inhibits the
sintering of the support and enhances the acid strength of
these solids by slight reduction of the WOx species. The
WOx/ZrO2 catalyst having the tetragonal structure of zir-
conia is one of the best catalysts for the synthesis of
The mixture of the o-phenylenediamines (0.01 mol) and aromatic
aldehydes (0.01 mol) with 0.3 g of WZ catalyst in 1,4-dioxane at
100 °C
a
All the reaction monitored by TLC
b
Isolated yields
123

68
R. V. Shingalapur, K. M. Hosamani
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benzimidazole derivatives in medicinal chemistry as well
as in green chemistry. More than the acidic properties,
the mesoporous structure of tungstated zirconia favours the
dispersion of the tungsten species, which improves the
transport of reactants and products in the solid and avoids
the secondary reactions promoted by the diffusion con-
straints that slow down the removal of initial isomerization
of the products from the catalyst. Henceforth, this meth-
odology works well and is environmentally benign eco-
friendly procedure, which would prove beneficial to both
academia and industry for the socio-economic change.
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Acknowledgement This research work is financially supported by
the University Grants Commission (UGC), New Delhi 110 002. (Ref.
No F.33-296/2007 (SR) dated 28-02-2008.
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