Nano ZnO catalyzed one-pot synthesis of benzimidazoles from o-phenylenediamine with aldehydes

Article abstract of DOI:10.14233/ajchem.2015.19076

Nano-ZnO was found to be a highly efficient and reusable heterogeneous catalyst for the one-pot synthesis of substituted benzimidazoles from aromatic aldehydes with o-phenylenediamine in moderate to good yield. The spent catalyst can be easily recovered and reused for five cycles with consistent activity.

Full text of DOI:10.14233/ajchem.2015.19076

Asian Journal of Chemistry; Vol. 27, No. 11 (2015), 4045-4048
A
SIAN
J
OURNAL OF HEMISTRY
C
http://dx.doi.org/10.14233/ajchem.2015.19076
Nano ZnO Catalyzed One-Pot Synthesis of
Benzimidazoles from o-Phenylenediamine with Aldehydes
1
1
1
2,*
KUN QIAN , XIN NIAN , GUAN-MING ZHU , DONG MEI CUI1,* and CHEN ZHANG
1College of Pharmaceutical Science, Zhejiang University of Technology, Hangzhou 310014, P.R. China
2School of Pharmaceutical Sciences, Zhejiang University, Hangzhou 310058, P.R. China
*Corresponding authors: Tel/Fax: +86 571 88320320; E-mail: cuidongmei@zjut.edu.cn; chenzhang@zju.edu.cn
Received: 3 February 2015; Accepted: 13 March 2015; Published online: 16 July 2015;
AJC-17399
Nano-ZnO was found to be a highly efficient and reusable heterogeneous catalyst for the one-pot synthesis of substituted benzimidazoles
from aromatic aldehydes with o-phenylenediamine in moderate to good yield. The spent catalyst can be easily recovered and reused for
five cycles with consistent activity.
Keywords: Nano-ZnO, Benzimidazole, Heterogeneous catalyst, o-Phenylenediamine, Aldehyde.
BrukerAvance spectrometer in CDCl3 or DMSO-d6 with Me4Si
as an internal standard.
INTRODUCTION
Benzimidazole derivatives are important intermediates
in many organic reactions and also exhibit various biological
activities such as antiulcers, antihypertensives, antivirals, anti-
fungals, anticancers and antihistamines1-7. The most common
method reported in the literature for synthesis of these com-
pounds involves the condensation of o-phenylenediamines and
carboxylic acids or their derivatives, but several drawbacks
remain associated with such reactions, including the use of harsh
acids and under high temperature conditions8-10. Synthesis of
benzimidazoles have also been reported using other approaches,
including the condensation of o-phenylenediamines with
aldehydes under oxidative conditions, or using homogeneous
General procedure for the synthesis of benzimidazoles:
To a reactor containing 1,2-phenylenediamine (1 mmol),
benzaldehyde (2.2 mmol) and Cl2CHCHCl2 (2 mL) was added
nano ZnO (0.1 mmol). The mixture was then stirred at 80 °C
until the reaction was completed as judged by TLC. The
mixture was filtered and the recovered nano ZnO catalyst was
washed with water and methanol with no further purification
before reuse. The filtrate was extracted with ethyl acetate (3 ×
10 mL). The organic layer was washed with brine, dried over
Na2SO4 and concentrated in vacuo. The residue was purified
by flash chromatography to give the pure product (Scheme-I).
catalysts such as Lewis acids, or heterogeneous catalyst11-16
.
R1
R1
R1
NH2
NH2
N
N
N
R2
In recent years, there has been a significant progress on the
use and design of metal nanoparticles as catalysts due to their
high catalytic activity, efficient recyclability and facile separa-
tion of products. There are a few reports available in the
literature for the use of nano crystalline ZnO as a heterogeneous
catalyst for various organic reactions17-19. Herein, we report
an efficient synthesis of benzimidazole derivatives from o-
phenylenediamines with aldehydes using nano ZnO catalyst.
R2
R2CHO
+
+
N
H
R2
1
2
3
4
Scheme-I: Synthesis of benzimidazoles from o-phenylenediamine with
aryl aldehyde
1-Benzyl-2-phenyl-1H-benzo[d]imidazole (3a): White
1
solid, m.p. 128-130 °C (lit.20 129-131 °C), yield: 87 %; H
NMR (500 MHz, CDCl3) δ 7.91 (d, J = 7.9 Hz, 1H), 7.76-7.67
(m, 2H), 7.51-7.46 (m, 3H), 7.38-7.31 (m, 4H), 7.27-7.21 (m,
2H), 7.13 (d, J = 7.0 Hz, 2H), 5.48 (s, 2H).
EXPERIMENTAL
2-Phenyl-1H-benzo[d]imidazole (4a):White solid, m.p.
Nano ZnO was purchased from Aladdin Chemistry Co.
Ltd. Under otherwise noted, other materials were obtained
from commercial suppliers and used without further
purification. 1H NMR (500 MHz) spectra were recorded on a
1
299-301 °C (lit.21 290-292 °C), yield: 10 %; H NMR (500
MHz, CDCl3) δ 8.13-8.01 (m, 2H), 7.67-7.65 (m, 2H), 7.51-
7.47 (m, 3H), 7.32-7.28 (m, 2H).

4046 Qian et al.
Asian J. Chem.
1-(2-Methyl-benzyl)-2-p-tolyl-1H-benzo[d]imidazole
7.50 (d, J = 7.4 Hz, 1H), 7.23-7.13 (m, 2H), 7.11 (d, J = 8.1
Hz, 1H), 6.13 (s, 2H).
(3b): Light yellow solid, m.p. 128-130 °C (lit.22 129-130 °C),
1
yield: 63 %; H NMR (500 MHz, CDCl3) δ 7.87 (d, J = 8.1
4-((2-(4-(Dimethylamino)phenyl)-1H-benzo[d]imidazol-
1-yl)methyl)-N,N-dimethylbenzenamine (3g): White solid,
Hz, 1H), 7.60 (d, J = 8.1 Hz, 2H), 7.33-7.29 (m, 1H), 7.27-
7.26 (m, 2H), 7.25-7.19 (m, 2H), 7.15 (d, J = 8.0 Hz, 2H),
7.02 (d, J = 8.0 Hz, 2H), 5.43 (s, 2H), 2.42 (s, 3H), 2.35 (s, 3H).
2-p-Tolyl-1H-benzo[d]imidazole (4b): Brown solid, m.p.
1
m.p. 182-185 °C (lit.28 182 °C), yield: 79 %; H NMR (500
MHz, CDCl3) δ 7.83 (d, J = 7.9 Hz, 1H), 7.64 (d, J = 8.9 Hz,
2H), 7.26-7.25 (m, 1H), 7.24-7.16 (m, 2H), 7.04 (d, J = 8.9
Hz, 2H), 6.75 (d, J = 8.9 Hz, 2H), 6.70 (d, J = 8.9 Hz, 2H),
5.39 (s, 2H), 3.03 (s, 6H), 2.95 (s, 6H).
1
275-276 °C (lit.21 262-264 °C), yield: 33 %; H NMR (500
MHz, CDCl3) δ 12.83 (s, 1H), 8.07 (d, J = 8.0 Hz, 2H), 7.62-
7.54 (m, 2H), 7.37 (d, J = 8.0 Hz, 2H), 7.20-7.18 (m, 2H),
2.39 (s, 3H)
1-(4-Methoxybenzyl)-2-(4-methoxyphenyl)-1H-
benzo[d]imidazole (3c): White solid, m.p. 129-130 °C (lit.23
129-131 °C), yield: 62 %; 1H NMR (500 MHz, CDCl3) δ 7.88
(d, J = 7.9 Hz, 1H), 7.67 (d, J = 8.6 Hz, 2H), 7.33-7.31 (m,
1H), 7.25-7.24 (m, 2H), 7.05 (d, J = 8.6 Hz, 2H), 6.99 (d, J =
8.6 Hz, 2H), 6.89-6.86 (m, 2H), 5.42 (s, 2H), 3.87 (s, 3H),
3.81 (s, 3H).
2-(4-Methoxyphenyl)-1H-benzo[d]imidazole (4c):
White solid, m.p. 236-238 °C (lit.21 222-224 °C), yield: 5 %;
1H NMR (500 MHz, CDCl3) δ 8.09 (d, J = 8.8 Hz, 2H), 7.69-
7.67 (m, 2H), 7.31-7.28 (m, 2H), 6.84 (d, J = 8.8 Hz, 2H),
3.77 (s, 3H).
1-(2-Methoxybenzyl)-2-(2-methoxyphenyl)-1H-
benzo[d]imidazole (3d): Light yellow solid, m.p. 157-158
°C (lit.22 150-152 °C), yield: 77 %; 1H NMR (500 MHz, CDCl3)
δ 7.86 (d, J = 8.0 Hz, 1H), 7.55 (dd, J = 7.5, 1.6 Hz, 1H), 7.50-
7.41 (m, 1H), 7.29-7.27 (m, 1H), 7.24-7.19 (m, 3H), 7.08-
7.04 (m, 1H), 6.97 (d, J = 8.4 Hz, 1H), 6.85 (d, J = 8.4 Hz,
1H), 6.79-6.76 (m, 1H), 6.70 (d, J = 6.9 Hz, 1H), 5.25 (s, 2H),
3.80 (s, 3H), 3.60 (s, 3H).
1-(4-Fluorobenzyl)-2-(4-fluorophenyl)-1H-benzo[d]-
imidazole (3h): Brown solid, m.p. 68-70 °C (lit.23 82-84 °C),
1
yield: 34 %; H NMR (500 MHz, CDCl3) δ 7.87 (d, J = 8.0
Hz, 1H), 7.87-7.64 (m, 2H), 7.36-7.32 (m, 1H), 7.29-7.26 (m,
1H), 7.23-7.15 (m, 3H), 7.09-7.01 (m, 4H), 5.41 (s, 2H).
2-(4-Fluorophenyl)-1H-benzo[d]imidazole (4h): Light
yellow solid, m.p. 266-268 °C (lit.21 248-250 °C), yield: 60 %;
1H NMR (500 MHz, CDCl3) δ 9.57 (br, 1H), 8.07-8.03 (m,
2H), 7.85-7.70 (m, 1H), 7.65-7.45 (m, 1H), 7.31-7.29 (m, 2H),
7.24-7.19 (m, 2H).
1-(4-Chlorobenzyl)-2-(4-chlorophenyl)-1H-benzo[d]-
imidazole (3i): Yellow solid, m.p. 139-140 °C (lit.22 136-137
°C), yield: 12 %; 1H NMR (500MHz, CDCl3) δ 7.88 (d, J =
8.0 Hz, 1H), 7.60 (d, J = 8.5 Hz, 2H), 7.46 (d, J = 8.5 Hz, 2H),
7.36-7.28 (m, 4H), 7.1 (d, J = 8.0 Hz, 1H), 7.04 (d, J = 8.5 Hz,
2H), 5.41 (s, 2H).
2-(4-Chlorophenyl)-1H-benzo[d]imidazole (4i): Brown
1
solid, m.p. 296-298 °C (lit.21 284-286 °C), yield: 68 %; H
NMR (500 MHz, CDCl3) δ 13.01 (s, 1H), 8.19 (d, J = 8.4 Hz,
2H), 7.67-7.53 (m, 4H), 7.31-7.15 (m, 2H).
1-(2-Chlorobenzyl)-2-(2-chlorophenyl)-1H-benzo[d]-
imidazole (3j): Light yellow solid, m.p. 160-163 °C (lit.29 158-
159 °C), yield: 57 %; 1H NMR (500MHz, CDCl3) δ 7.91 (d, J
= 8.0 Hz, 1H), 7.53 (d, J = 8.0 Hz, 1H), 7.50-7.41 (m, 2H),
7.38-7.31 (m, 3H), 7.28-7.26 (m, 1H), 7.25-7.15 (m, 2H), 7.07
(t, J = 7.5 Hz, 1H), 6.64 (d, J = 7.7 Hz, 1H), 5.38 (s, 2H).
2-(2-Chlorophenyl)-1H-benzo[d]imidazole (4j): Light
yellow solid, m.p. 216-218 °C (lit.25 234-235 °C), yield: 36 %;
1H NMR (500 MHz, CDCl3) δ 10.25 (s, 1H), 8.48 (dd, J = 7.7,
1.9 Hz, 1H), 7.90-7.84 (m, 1H), 7.56 (m, 1H), 7.53 (m, 1H),
7.49-7.41 (m, 2H), 7.33 (m, 2H).
1-(3-Chlorobenzyl)-2-(3-chlorophenyl)-1H-benzo[d]-
imidazole (3k)30: Colourless oil, yield: 16 %; 1H NMR (500
MHz, CDCl3) δ 7.90 (d, J = 7.9 Hz, 1H), 7.73 (s, 1H), 7.50 (t,
J = 8.9 Hz, 2H), 7.44-7.34 (m, 2H), 7.31 (t, J = 9.3 Hz, 3H),
7.24 (d, J = 7.9 Hz, 1H), 7.14 (s, 1H), 6.96 (d, J = 6.8 Hz, 1H),
5.43 (s, 2H).
2-(3-Chlorophenyl)-1H-benzo[d]imidazole (4k): Light
yellow solid, m.p. 242-244 °C (lit.25 236-238 °C), yield: 79 %;
1H NMR (500 MHz, DMSO-d6) δ 13.05 (s, 1H), 8.23 (t, J =
1.6 Hz, 1H), 8.19-8.10 (m, 1H), 7.69 (d, J = 7.8 Hz, 1H),
7.62-7.55 (m, 3H), 7.35-7.10 (m, 2H).
1-(4-Bromobenzyl)-2-(4-bromophenyl)-1H-benzo[d]-
imidazole (3l): Brown solid, m.p.159-160 °C (lit.22 139-
141 °C), yield: 19 %; 1H NMR (500 MHz, CDCl3) δ 7.88 (d,
J = 8.0 Hz, 1H), 7.60 (d, J = 8.3 Hz, 2H), 7.52 (d, J = 8.3
Hz, 2H), 7.48-7.46 (m, 2H), 7.36-7.32 (m, 1H), 7.29-7.25
(m, 1H), 7.19 (d, J = 8.0 Hz, 1H), 6.96 (d, J = 8.2 Hz, 2H),
5.38 (s, 2H).
2-(2-Methoxyphenyl)-1H-benzo[d]imidazole (4d):
White solid, m.p. 164-166 °C (lit.21 156-158 °C), yield: 16 %;
1H NMR (500 MHz, DMSO-d6) δ 12.11 (s, 1H), 8.33 (dd, J =
7.7, 1.7 Hz, 1H), 7.69-7.56 (m, 2H), 7.52-7.46 (m, 1H), 7.25
(d, J = 8.3 Hz, 1H), 7.23-7.15 (m, 2H), 7.14-7.10 (m, 1H),
4.04 (s, 3H).
1-(3-Methoxybenzyl)-2-(3-methoxyphenyl)-1H-
1
benzo[d]imidazole (3e)24: Colourless oil, yield: 63 %; H
NMR (500 MHz, CDCl3) δ 7.92 (d, J = 7.9 Hz, 1H), 7.38-7.31
(m, 2H), 7.28-7.23 (m, 5H), 7.05-7.02 (m, 1H), 6.85-6.82 (m,
1H), 6.71-6.66 (m, 2H), 5.43 (s, 2H), 3.74 (s, 3H), 3.73 (s, 3H).
2-(3-Methoxyphenyl)-1H-benzo[d]imidazole (4e):
White solid, m.p. 218-220 °C (lit.25 205-206 °C), yield: 36 %;
1H NMR (500 MHz, DMSO-d6) δ 12.89 (s, 1H), 7.78-7.75
(m, 2H), 7.67 (d, J = 7.7 Hz, 1H), 7.54 (d, J = 7.4 Hz, 1H),
7.48-7.45 (m, 1H), 7.23-7.119 (m, 2H), 7.08-7.05 (m, 1H),
3.87 (s, 3H).
2-(Benzo[d][1,3]dioxol-5-yl)-1-(benzo[d][1,3]dioxol-5-
ylmethyl)-1H-benzo[d]imidazole (3f):White solid, m.p. 176-
179 °C (lit.26 170-171 °C), yield: 48 %; 1H NMR (500 MHz,
CDCl3) δ 7.84 (d, J = 8.0 Hz, 1H), 7.32-7.17 (m, 5H), 6.90 (d,
J = 8.0 Hz, 1H), 6.76 (d, J = 8.0 Hz, 1H), 6.59-6.57 (m, 2H),
6.05 (s, 2H), 5.97 (s, 2H), 5.37 (s, 2H).
2-(Benzo[d][1,3]dioxol-5-yl)-1H-benzo[d]imidazole
(4f): Light yellow solid, m.p. 266-267 °C (lit.27 245-247 °C),
yield: 51 %; 1H NMR (500 MHz, DMSO-d6) δ 12.74 (s, 1H),
7.73 (d, J = 8.1 Hz, 1H), 7.69 (s, 1H), 7.62 (d, J = 7.4 Hz, 1H),

Vol. 27, No. 11 (2015)
Nano ZnO Catalyzed One-Pot Synthesis of Benzimidazoles 4047
TABLE-1
2-(4-Bromophenyl)-1H-benzo[d]imidazole (4l): Brown
1
solid, m.p. 301-303 °C (lit.21 293-295 °C), yield: 65 %; H
OPTIMIZATION OF NANO ZnO CATALYZED SYNTHESIS
OF BENZIMIDAZOLES FROM o-PHENYLENEDIAMINE
AND BENZALDEHYDEa
NMR (500 MHz, CDCl3) δ 13.01 (s, 1H), 8.12 (d, J = 8.5 Hz,
2H), 7.78 (d, J = 8.5 Hz, 2H), 7.68 (d, J = 7.8 Hz, 1H), 7.54
(d, J = 7.8 Hz, 1H), 7.35-7.09 (m, 2H).
ZnO
(mol %)
Temp.
(°C)
Yield of Yield of
Entry
Solvent
3a (%)b
87
4a (%)b
10
2-(Pyridin-3-yl)-1H-benzo[d]imidazole (4m): White
1
2
3
4
5
6
7
8
9
20
10
10
10
20
20
20
20
20
20
20
CHCl2CHCl2
CHCl2CHCl2
CHCl2CHCl2
CHCl2CHCl2
CHCl3
80
80
60
120
60
80
80
80
80
80
80
1
solid, m.p. 264-266 °C (lit.31 246-248 °C), yield: 56 %; H
74
25
51
45
NMR (500MHz, DMSO-d6) δ 13.12 (s, 1H), 9.35 (d, J = 1.9
Hz, 1H), 8.70-8.68 (m, 1H), 8.52-8.49 (m, 1H), 7.70 -7.59
(m, 3H), 7.28-7.15 (m, 2H).
53
29
74
23
H2O
67
29
1-Benzyl-5-nitro-2-phenyl-1H-benzo[d]imidazole (3n):
C2H5OH
1,4-dioxane
CH3CN
63
35
1
Red solid, m.p.148-149 °C (lit.32 148 °C), yield: 12 %; H
30
69
NMR (500 MHz, CDCl3) δ 8.27 (dd, J = 8.8, 2.1 Hz, 1H),
8.19 (m, 1H), 7.92 (d, J = 8.8 Hz, 1H), 7.76-7.72 (m, 2H),
7.57-7.50 (m, 2H), 7.40-7.35 (m, 4H), 7.11-7.08 (m, 2H), 5.57
(s, 2H).
5-Nitro-2-phenyl-1H-benzo[d]imidazole (4n): Brown
solid, m.p. 124-127 °C (lit.33 147.1-149.0 °C), yield: 61 %; 1 H
NMR (500 MHz, DMSO-d6) δ 13.63 (s, 1H), 8.65-39 (m, 1H),
8.24-8.22 (m, 2H), 8.16-8.14 (m, 1H), 7.90-7.65 (m, 1H), 7.64-
7.56 (m, 3H).
44
55
10
11
THF
DMSO
42
51
38
54
aThe reactions were performed with o-phenyldiamine (1.0 mmol),
benzaldehyde (2.2 mmol) and nano ZnO in solvent (1 mL) for 1 h.
bIsolated yields.
TABLE-2
NANO ZnO CATALYZED SYNTHESIS OF BENZIMIDAZOLESa
Time
(h)
Yield of
Yield of
Entry
R1
R2
3 (%)
4 (%)
RESULTS AND DISCUSSION
1
2
3
4
5
6
7
8
9
10
11
12
13
14
H
H
H
H
H
H
H
H
H
H
H
H
H
C6H5
1
5
5
2
5
3
4
5
3
5
5
5
5
3a: 87
3b: 63
3c: 63
3d: 77
3e: 63
3f: 48
3g: 79
3h: 34
3i: 12
3j: 57
3k: 16
3l: 19
–
4a: 10
4b: 33
4c: 5
4d: 16
4e: 36
4f: 51
–
4h: 60
4i: 68
4j: 36
4k: 79
4l: 65
4m: 56
4n: 61
4-CH3C6H5
4-CH3OC6H5
2-CH3OC6H5
3-CH3OC6H5
3,4-CH2O2C6H3
4-(CH3)2NC6H5
4-FC6H5
Initially, the reaction between o-phenylenediamine (1
mmol) and benzaldehyde (2.2 mmol) was selected for the
synthesis of benzimidazole as a model reaction for optimizing
the reaction conditions. The results are summarized in Table-1.
The reactions using 20 mol % nano ZnO with a size of about
30 nm proceeded efficiently to form the 2-phenyl benzimidazole
(3a) in 87 % isolated yield and a minor amount of 1-benzyl-2-
phenyl-1H-benzimidazole (4a) (10 %) at 80 °C in Cl2CHCHCl2
(Table-1, entry 1). When the loading catalyst was reduced to
10 mol %, the yield of the product 4a was increased signifi-
cantly up to 25 % (Table-1, entry 2). For comparison, it was
also observed that reaction temperatures higher or lower than
80 °C resulted in decreased yields of 3a (Table-1, entries 3 and
4).A number of common solvents were examined, Cl2CHCHCl2
appeared to be the best one for the synthesis of 1,2-disubstituted
benzimidazole 3a and DMSO resulted in the formation of 1-
substituted benzimidazole 4a as a main product (Table-1,
entries 1, 5-11).
4-ClC6H5
2-ClC6H5
3-ClC6H5
4-BrC6H5
3-Pyridinyl
NO2 C6H5
14
3n: 12
aThe reactions were performed with 1 (1.0 mmol), 2 (2.2 mmol) and
nano ZnO (20 mol %) in Cl2CHCHCl2 (1 mL) at 80 °C.
bIsolated yields.
benzene ring was tolerated for the reaction, obtaining the
corresponding benzimidazoles 3n and 4n in 12 % and 61 %
yields, respectively (Table-2, entries 4-5).
With the optimal reaction conditions in hand, the scope of
nano ZnO catalyzed synthesis of benzimidazoles was explored.
The results are summarized in Table-2. It was observed that
the reaction was found to be deeply impacted by the electronic
effects of the substituents on the aryl aldehyde and o-phenylene-
diamine. The aromatic aldehyde with an electron-donating
group such as Me or MeO groups on the benzene ring all gave
a mixture with the 1,2-disubstituted benzimidazoles as the
major products and small amounts of the 1-substituted benzimi-
dazoles in good yield. In sharp contrast, with an electron-
withdrawing substituent at the para- or meta-positions on the
aromatic ring, the formation of 2-aryl benzimidazoles 4 predo-
minated over 1-arylmethyl-2-aryl benzimidazoles 3, except
for the case of a ortho-Cl substituent at aryl aldehyde. Further-
more, 3-pyridinylaldehyde reacted with o-phenylenediamine
to afford only product 4m with 56 % yield (Table-2, entry 13).
In addition, o-phenyldiamine containing p-NO2 group on the
The reusability of nano ZnO catalyst was studied for the
reaction of o-phenylenediaminewith 4-dimethylaminobenzal-
dehyde in Cl2CHCHCl2. The results are summarized in Table-3.
The catalyst nano ZnO was recovered by a simple filtration
and reused with consistent activity even after the fifth cycle
(Table-3).
Conclusion
In conclusion, a very efficient procedure for the one-pot
synthesis of substituted benzimidazoles from aromatic aldehydes
and o-phenyldiamine has been developed using easily
recyclable heterogeneous nano ZnO catalyst. A wide range of
aryl aldehyde can be tolerated, giving the corresponding substi-
tuted benzimidazoles in good yields. The ratio of substituted
2-aryl benzimidazoles and substituted 1-arylmethyl-2-aryl
benzimidazoles is impacted by the electronic effects of the
substituents on aryl aldehydes.

4048 Qian et al.
Asian J. Chem.
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TABLE-3
REUSABILITY OF NANO ZnO FOR THE
REACTION o-PHENYLENEDIAMINEWITH
4-DIMETHYLAMINOBENZALDEHYDEa
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1
1
1
1
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74
58
58
60
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