Nanoporous aluminosilicate catalyst with 3D cage-type porous structure as an efficient catalyst for the synthesis of benzimidazole derivatives

Article abstract of DOI:10.1016/j.tetlet.2010.07.132

Herein we demonstrate for the first time, the synthesis of benzimidazoles through the coupling of aldehydes with o-phenylenediamine by using highly acidic nanoporous aluminosilicate with 3D structure and cage-type pores as the catalyst. The catalyst resulted in excellent yields in short reaction times presumably due to its high acidity, large pore diameter, high surface area, and cage-type 3D porous structure.

Full text of DOI:10.1016/j.tetlet.2010.07.132

Tetrahedron Letters 51 (2010) 5195–5199
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Nanoporous aluminosilicate catalyst with 3D cage-type porous structure
as an efficient catalyst for the synthesis of benzimidazole derivatives
Murugulla A. Chari , D. Shobha , El-Refaie Kenawy , Salem S. Al-Deyab , B. V. Subba Reddy a,c,
Ajayan Vinu
a
a
a
b
b
a,
*
International Center for Materials Nanoarchitectonics, WPI Research Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
Department of Chemistry, Petrochemicals Research Chair, Faculty of Science, King Saud University, PO Box 2455, Riyadh 11451, Saudi Arabia
NIMS-IICT Materials Research Center, Indian Institute of Chemical Technology, Hyderabad 500 007, India
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Herein we demonstrate for the first time, the synthesis of benzimidazoles through the coupling of alde-
hydes with o-phenylenediamine by using highly acidic nanoporous aluminosilicate with 3D structure
and cage-type pores as the catalyst. The catalyst resulted in excellent yields in short reaction times pre-
sumably due to its high acidity, large pore diameter, high surface area, and cage-type 3D porous
structure.
Received 28 May 2010
Revised 15 July 2010
Accepted 23 July 2010
Available online 30 July 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Catalysis
Heterocycles
Nanoporous
Heterogeneous
Benzimidazoles
1
. Introduction
The benzimidazole derivatives are potent biologically active
environmental friendly heterogeneous catalysts for the synthesis
of benzimidazoles. Heterogeneous catalysts such as polymer-sup-
4 2
ported hypervalent iodine reagents and KHSO /SiO have been
1
0
compounds and exhibit antiviral, antiulcer, antihypertension, and
anticancer properties.1 These compounds have been receiving a
lot of attention in the recent years owing to their excellent biolog-
ical activities. Generally, the synthesis of benzimidazoles involves
the treatment of 1,2-phenylenediamine either with carboxylic
acids under strongly acidic conditions or with aldehydes under
used for the synthesis of benzimidazoles. Unfortunately, these
catalysts possess poor textural characteristics such as a low surface
area and pore volume which limit the efficiency of the catalysts.
Zeolites and nanoporous materials have been widely used as heter-
ogeneous catalysts in several organic transformations owing to
their excellent textural characteristics such as high surface area,
2
,3
11,12
oxidative conditions
using various oxidants such as 2,3-di-
uniform pores, large pore volume, and high thermal stability.
chloro-5,6-dicyanobenzoquinone, molecular iodine, oxone, and
Unfortunately, organic reactions involving two or three bulky reac-
tants cannot be carried out using zeolite catalysts as the size of
their pore diameter is too small. On the other hand, the nanopor-
ous materials can offer large pore diameter and high surface area,
wherein the reactant molecules freely diffuse inside the channels
and access the active sites. In addition, enhanced reaction rates,
greater selectivity, easy handing of these materials, and simple
workup are the added advantages of these materials. Among
various heterogeneous solid acid porous catalysts, nanoporous cat-
alysts with three-dimensional (3D) structures are more advanta-
geous than the catalysts with one-dimensional (1D) nanoporous
structures for various organic transformations. The 3D structure
of the catalyst can provide more adsorption sites and allow easy
diffusion of the reactants and products than 1D structure which
supports the resistant to pore blocking and coke formation. Re-
cently, we have reported the synthesis of AlKIT-5, which is highly
4
,5
FeCl
fonic acid, borontrifluoride etherate, In(OTf)
Bi(OTf)3, and ionic liquids have been employed as the catalysts
3
Á6H
2
O. Subsequently, several reagents such as p-toluenesul-
3
, Yb(OTf) , Sc(OTf)
3
3
,
for the synthesis of benzimidazoles.6
–10
Although the reaction
was efficiently promoted by the above catalysts, they suffer from
one or more disadvantages such as high cost, long reaction times,
occurrence of several side reactions, drastic reaction conditions,
difficulty in separation of the products from the reaction mixture,
and strong oxidizing nature. In addition, these catalysts are highly
corrosive and cannot be regenerated which make them less envi-
ronmental friendly and not attractive for commercialization. Thus,
it is highly imperative to find out stable, cheap, recyclable, and
*
Corresponding author. Fax: +81 29 860 4706.
E-mail address: vinu.ajayan@nims.go.jp (A. Vinu).
0
040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.07.132

5
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M. A. Chari et al. / Tetrahedron Letters 51 (2010) 5195–5199
acidic and possesses 3D mesostructure with Fm3m symmetry, high
surface area, large pore volume, cage type pores, and high acidity.
The catalytic activity of the AlKIT-5 catalysts has been studied for
various organic transformations including the synthesis of triazolo
H
N
NH2
O
H
AlKIT-5
CH CN, Reflux
1
3
+
R
R
3
N
NH2
1
2
3
indazolones,
a-aminophosphonates, and dihydropyrimidinones. It
has been found that the catalyst is extremely stable and showed
Scheme 1. Synthesis of benzimidazoles using AlKIT-5 as catalyst.
superior activity in most of the organic transformations.1
4–20
How-
ever, to the best of our knowledge, there has been no report avail-
able on the synthesis of benzimidazoles using AlKIT-5 catalysts in
the open literature so far.
(-)
AlKIT-5
O
H
AlKIT-5
N. . H
H
N
NH2
2
(
+)O
Herein we report for the first time a simple, convenient, and
efficient method for the synthesis of benzimidazole and its deriva-
tives by condensation of 1,2-phenylenediamine with aldehydes
under acetonitrile reflux conditions using AlKIT-5 as the catalyst.
Initially, we have attempted the condensation of 1,2-phenylenedi-
amine (1.2 mmol) with benzaldehyde (1.0 mmol) using AlKIT-5
catalyst with different nSi/nAl ratios (Table 1 and Scheme 1). It
was found that the amount of Al in the catalyst plays a significant
role in controlling the activity of the catalyst. Among the catalysts
studied, AlKIT-5(10) was found to be the best, giving a high yield.
This was mainly due to the fact that AlKIT-5(10) has the highest Al
content and the presence of each aluminum atom causes the
appearance of an extra negative charge in the aluminosilicate
framework of AlKIT-5 which must be compensated by proton in
the form of bridged hydroxyls, resulting in Brönsted acid sites. This
makes the AlKIT-5(10) more acidic than the other AlKIT-5 catalysts
and is responsible for its high activity. It should also be mentioned
that the specific surface area and specific pore volume of the AlKIT-
Ar
Ar
+
C
Ar
N
H
Ar
-H O
2
NH
H
N
H
.
.
2
Air
N
N
H
Scheme 2. A plausible mechanism for the synthesis of benzimidazoles.
matic, heteroaromatic, and aliphatic underwent smooth conver-
sion to afford a wide range of benzimidazoles (Table 2). Aromatic
aldehydes containing both electron-donating and electron-with-
drawing groups worked well in this reaction. The catalyst was also
found to be very active for the preparation of benzimidazoles from
an acid-sensitive aldehyde such as furfural (3l) and a sterically hin-
dered aldehyde, 2-naphthaldehyde (3m). Substituted aldehydes
were used with similar success to provide the corresponding benz-
imidazoles in high yields. The products were formed in high yields
(76–95%). The reaction was also carried out with substituted o-
phenylenediamines using AlKIT-5(10) under reflux condition. The
catalyst showed excellent activity and afforded the corresponding
derivatives of benzimidazole in good yields. The structures of the
products were determined from their spectral data (NMR, IR, and
MS) and also by comparison with authentic samples.5 Several
examples illustrating this novel and general method for the syn-
thesis of benzimidazoles are also summarized in Table 2.
The effect of the solvents affecting the catalytic activity of the
AlKIT-5(10) was also investigated under the optimized reaction
conditions and the results are given in Table 3. Among various sol-
vents like THF, acetonitrile, ethanol, and methanol used for this
transformation, acetonitrile and methanol were found to be good
solvents. Finally, recycling experiments were conducted to find
out the stability of the catalyst after the reaction. The catalyst
was separated by centrifuge and reused after activation at 500 °C
for 3–4 h. The efficiency of the recovered catalyst was verified with
entry 1. Using the fresh catalyst, the yield of product 3a was 95%,
while the recovered catalyst in the three subsequent runs gave
the yield of 94%, 92%, and 90%, respectively.
5
(10) are higher than those of AlKIT-5 materials with less Al con-
tent. These factors are highly critical for enhancing the adsorption
and the transport of the reactant molecules on the surface and
thereby increasing the catalytic activity. As the AlKIT-5(10) cata-
lyst was found to be the most active, this catalyst was used for fur-
ther studies.
It is noteworthy that no product was observed at room temper-
ature or under reflux conditions in the absence of a catalyst. How-
ever, when the reaction was carried out using AlKIT-5(10) catalyst
under reflux conditions, the product 3a with a yield of 95% was
achieved within 4 h. The effect of the catalyst weight on the activ-
ity of the AlKIT-5(10) was also investigated (Table 1). It was ob-
served that the yield of the final product increases with
increasing the amount of the catalyst in the reaction mixture.
The yield of the final product increases from 34% to 95% by increas-
ing the catalyst weight from 50 to 150 mg.
Mechanistically, the reaction proceeds via the activation of
aldehyde by AlKIT-5 followed by imine formation. The resulting
imine further reacts with another amine group of 1,2-phenylenedi-
amine resulting in the formation of dihydroimidazole which subse-
quently undergoes aromatization under the reaction conditions to
give the benzimidazole as shown in Scheme 2.
Encouraged by these results, we examined several aromatic
aldehydes under the optimized reaction conditions. This condensa-
tion proceeded smoothly in acetonitrile under reflux conditions
and was completed within 5 h. Several aldehydes including aro-
In conclusion, benzimidazole and its derivatives were synthe-
sized by the coupling of aldehydes with o-phenylenediamine using
AlKIT-5 catalyst with different Al contents. Benzimidazoles could
successfully be synthesized using the AlKIT-5 catalyst with acid
Table 1
Effect of the textural parameters and the weight of the AlKIT-5 catalysts in the synthesis of benzimidazole 3a
2
3
Catalyst
Weight (mg)
A
BET (m /g)
V
p
(cm /g)
D
p
BJH (nm)
Acidity (mmol/g)
Yield (%)
AlKIT-5(10)
AlKIT-5(10)
AlKIT-5(10)
AlKIT-5(28)
AlKIT-5(44)
50
100
150
150
150
989
989
989
815
713
0.68
0.68
0.68
0.56
0.45
6.0
6.0
6.0
5.6
5.2
0.50
0.50
0.50
0.32
0.14
34
72
95
89
78
Reaction conditions: substrates: 1,2-phenylenediamine, benzaldehyde, reaction time: 4 h, reaction temperature: reflux, solvent: ace-
tonitrile, D : pore diameter, V : pore volume, ABET: specific surface area.
p
p

M. A. Chari et al. / Tetrahedron Letters 51 (2010) 5195–5199
5197
Table 2
Synthesis of benzimidazole derivatives using AlKIT-5 catalyst
Entry
Aldehyde
CHO
Producta (3)
Time (h)
4.0
Yieldb (%)
H
N
a
3a
3b
3c
3d
3e
95
95
92
84
92
N
HO2N
N
NO2
CHO
b
c
4.0
4.0
4.0
4.0
N
H
N
O N
CHO
NO2
CH3
2
N
H
N
O N
2
d
e
CHO
N
H
N
H C
CHO
3
N
Cl
Cl
H
N
f
3f
3g
3h
3i
4.5
4.5
4.5
4.5
4.5
84
90
90
88
88
Cl
CHO
Cl
Cl
N
H
N
g
h
i
Cl
CHO
N
H
N
HO
OH
CHO
N
H
N
H CO
CHO
OCH3
OCH3
OCH3
3
N
H CO
3
H
N
j
3j
H CO
CHO
3
N
H
N
CHO
k
3k
4.5
88
N
H
N
O
O
l
CHO
3l
3m
3n
4.5
4.5
5.0
90
82
78
N
H
N
CHO
m
n
N
H
N
H C
3
CHO
N
CH3
H
N
o
CHO
3o
5.0
76
H3C
N
CH3
Me
Cl
N
p
q
r
3p
3q
3r
5.0
6.0
7.0
85
80
70
CHO
N
H
N
CHO
CHO
N
H
O N
2
N
N
H
a
All products were characterized by NMR, IR, and mass spectroscopy.
Yield refers to pure products after purification. Reaction conditions: reaction temperature: reflux, solvent: acetonitrile, catalyst: AlKIT-5(10), catalyst weight: 150 mg.
b
sensitive, sterically hindered, and substituted aromatic and ali-
phatic aldehydes. The catalyst is highly active, stable, and could
be reused several times without much loss of its activity. The het-
erogeneous nature, high surface area, large pore volume, and high
acidity of the catalyst make the preparation of benzimidazoles sim-
ple, convenient, and practical. As this catalyst is highly active, this

5
198
M. A. Chari et al. / Tetrahedron Letters 51 (2010) 5195–5199
Table 3
135.10, 133.29, 129.91, 128.20, 127.78, 122.32, 121.62, 118.52,
Effect of the various solvents on the activity of the AlKIT-5 catalyst in the synthesis of
benzimidazoles
1
8 2 2
11.40. HRMS (EI) m/z calcd for C13H N Cl 262.0065, found
2
62.0068.
Name of the catalyst
Name of the solvent
Reaction time (h)
Yield (%)
Compound 3g: 2-(4-chlorophenyl)-1H-benzimidazole (Table
2):5 solid, mp 287–289 °C, IR (KBr):
m 3189, 2980, 1625 cm ; H
À1 1
AlKIT-5(10)
AlKIT-5(10)
AlKIT-5(10)
AlKIT-5(10)
Acetonitrile
Methanol
Ethanol
THF
4
4
4
4
95
94
89
80
NMR (DMSO-d
6
, 300 MHz): d 12.74 (br s, 1H), 8.17 (d, J = 9.0 Hz,
13
2
H), 7.62 (m, 1H), 7.60–7.57 (m, 3H), 7.22–7.19 (m, 2H);
C
NMR (DMSO-d
6
, 125 MHz): d 151.80, 143.75, 134.50, 130.12,
Reaction conditions: substrate = 1,2-phenylenediamine (1.2 mmol), benzaldehyde
1.0 mmol), amount of the AlKIT-5 catalyst = 150 mg, reaction temperature = reflux
temperature.
129.10, 128.20, 126.20, 122.32, 121.62, 118.52, 111.10. HRMS (EI)
m/z calcd for C13 Cl, 228.0454, found 228.0453.
Compound 3h: 2-(3-hydroxy phenyl)-1H-benzimidazole (Table
(
9 2
H N
À1
1
2): solid, mp 280–282 °C, IR (KBr):
m
3185, 2989, 1620 cm
; H
could also be used for several acid-catalyzed organic transforma-
tions and could replace the existing homogenous catalysts which
are hazardous and currently being used in the industry.
NMR (300 MHz, DMSO-d
6
): d 12.77 (br s, 1H), 9.69 (br s, OH),
7
.59–7.49 (m, 4H), 7.325 (s,1H), 7.17–7.20 (d, 2H), 6.88 (s, 1H);
1
3
C NMR (DMSO-d
31.38, 129.91, 122.37, 121.53, 118.76, 117.16, 116.89, 113.30,
O, 210.0793, found
6
, 125 MHz): d 157.71, 151.31, 143.71, 134.91,
1
2
. Experimental section
111.21. HRMS (EI) m/z calcd for C13
10.0799.
10 2
H N
2
All chemicals and solvents were obtained from Aldrich and used
Compound 3j: 2-(3,4-dimethoxy phenyl)-1H-benzimidazole
À1
without further purification. Column chromatographic separations
(Table 2): solid, mp 180–182 °C, IR (KBr):
m
3192, 2985, 1629 cm
;
1
1
were carried out on silica gel 100–200 mesh size. The H NMR
H NMR (DMSO-d6, 300 MHz): d 12.79 (br s, 1H), 7.68 (d, J =
spectra of samples were recorded on a JEOL 300-MHz NMR spec-
8.0 Hz, 1H), 7.28–7.21 (m, 2H), 7.1–7.0 (d, 1H), 6.83–6.73
13
3
trometer using TMS as an internal standard in CDCl . Mass spectra
were recorded on a MALDI-MS.
(m, 2H), 6.45–6.42 (d, J = 9.0 Hz, 1H), 3.81 (s, 3H); C NMR
(DMSO-d , 125 MHz): d 153.17, 150.06, 148.65, 135.97, 129.35,
122.31, 121.62, 121.58, 118.9, 111.97, 110.27, 55.8. HRMS (EI)
m/z calcd for C15 254.1055, found 254.1061.
Compound 3k: 2-benzyl-1H-benzimidazole (Table 2):
6
The AlKIT-5 materials with different nSi/nAl ratios were synthe-
sized using polymeric Pluronic F127 as a template, and tetraethyl
orthosilicate (TEOS) and aluminum isopropoxide as the sources
of silicon and aluminum, respectively. In a typical synthesis, plu-
ronic F127 (5 g) was dissolved in concd HCl (3 g, 35 wt %) and dis-
tilled water (240 g). To this mixture, TEOS (24 g) and the required
amount of the aluminum source were added, and the resulting
mixture was stirred for 24 h at 45 °C. Subsequently, the reaction
mixture was heated for 24 h at 100 °C under static condition for
hydrothermal treatment. After hydrothermal treatment, the final
solid product was filtered off and then dried at 100 °C without
washing. The white colored product was calcined at 540 °C for
14 2 2
H N O
21b
solid,
H NMR
, 300 MHz): d 12.22 (br s, 1H), 7.52 (d, J = 9.0 Hz, 1H),
7.37 (d, 1H), 7.31 (m, 4H), 7.22 (m, 1H), 7.10 (d, 2H), 4.14 (s,
–
1
1
mp 184–186 °C; IR (KBr): 3047, 2966, 1625 cm
(DMSO-d
;
6
1
3
6
2H); C NMR (DMSO-d , 125 MHz): d 153.44, 149.36, 143.33,
137.60, 128.69, 128.41, 126.44, 121.55, 120.87, 118.19, 110.85.
Compound 3m: 2-(2-naphthyl)-1H-benzimidazole (Table 2): so-
À1
1
lid, mp 212–215 °C, IR (KBr):
m 3155, 2925, 2853, 1624 cm ; H
1
NMR (DMSO-d
13.02 (br s, 1H), 8.72 (br s, 1H), 8.32 (dd, J = 8.0, 2.0 Hz, 1H),
8.08–7.96 (m, 3H), 7.68–7.57 (m, 4H), 7.24–7.21 (m, 2H); 13
NMR (DMSO-d , 125 MHz): d 151.17, 133.38, 132.75, 128.45,
28.35, 127.71, 127.55, 127.01, 126.82, 125.73, 123.87.
Compound 3p: 5-methyl-2-phenyl-benzimidazole (Table 2):2
6 6
, 300 MHz): H NMR (DMSO-d , 300 MHz): d
1
0 h. The samples are denoted as AlKIT-5(x) where x denotes the
C
nSi/nAl ratio in the final product.
6
1
2a
2.1. General reaction procedure
solid, mp 243–245 °C. IR (KBr):
m
3262, 1745, 1586, 1445, 1261,
–
1
1
To a mixture of an aldehyde (1.0 mmol) and 1,2-phenylenedi-
1115, 969, 746 cm
;
6
H NMR (DMSO-d , 300 MHz): d 12.72 (br
amine (1.2 mmol) in acetonitrile (5 mL) under open air atmo-
sphere, AlKIT-5 (150 mg) was added. The resulting mixture was
stirred at reflux temperature for appropriate time (Table 2). After
completion of the reaction, as monitored by TLC, the reaction mix-
ture was diluted with ethyl acetate (20 mL) and the catalyst was
separated by filtration. The organic layer was concentrated under
reduced pressure and the crude product was purified by silica gel
column chromatography using ethyl acetate–n-hexane (1:9) as
s, 1H), 8.13 (d, J = 6.9 Hz, 2H), 7.45–7.55 (m, 5H), 7.00 (d,
J = 7.0 Hz, 1H), 2.41 (s, 3H); C NMR (DMSO-d , 125 MHz): d
6
1
3
151.3, 130.3 (3C), 129.7, 128.8, 128.9 (2C), 126.35 (2C), 126.25
(3C), 21.8.
Compound 3q: 5-chloro-2-phenyl-benzimidazole (Table 2):22b
solid, mp 212–214 °C. IR (KBr): 3580, 2918, 1580, 1430, 1270,
–
1
1
1108, 805, 693 cm
6
; H NMR (DMSO-d , 300 MHz): d 13.1 (br s,
1H), 8.15 (d, J = 8.0 Hz, 2H), 7.68–7.58 (m, 1H), 7.57–7.47 (m,
1
3
eluent to afford the pure benzimidazole. The spectral data are in
4H), 7.23–7.22 (dd, J = 8.0, 1.6 Hz, 1H);
6
C NMR (DMSO-d ,
full agreement with the data reported in the literature.5
tral data for the selected products:
,21,22
Spec-
125 MHz): d 153.0 (4C), 130.25, 129.6, 128.9 (2C), 126.5 (2C),
122.25 (3C). HRMS (EI) m/z calcd for C13H N Cl 228.0454, found
9 2
228.0458.
22a
Compound 3a: 2-phenyl-1H-benzimidazole (Table 2):
3a: 2-
21a
phenyl-1H-benzimidazole (Table 2):
047, 2966, 1462, 1411, 1276, 970, 744, 703 cm–1
300 MHz, DMSO-d ): d 12.89 (br s, 1H), 8.15–8.18 (m, 2H), 7.54
d, 1H) 7.50–7.54 (m, 4H), 7.19–7.20 (d, 2H); C NMR (DMSO-d
Solid, mp 293 °C, IR (KBr):
Compound 3r: 5-nitro-2-phenyl-benzimidazole (Table 2): solid,
mp 208–210 °C, IR (KBr): 3423, 3081, 1642, 1563, 1539, 1328,
, 300 MHz): d 9.50 (br s, 1H, NH), 8.46 (d,
J = 1.8 Hz, 1H), 8.20–8.18 (m, 2H), 8.12 (dd, J = 7.8, 1.8 Hz, 1H),
1
3
;
H NMR
m
1
(
(
6
758; H NMR (DMSO-d
6
13
6
,
1
3
1
1
1
25 MHz): d 151.8, 143.75, 134.96, 130.12, 129.7, 128.88, 126.39,
7.75 (d, J = 7.8 Hz, 1H), 7.59–7.57 (m, 3H, ArH); C NMR (DMSO-
6
d , 125 MHz): d 155.8 (3C), 142.7, 131.0, 129.1, 129.0 (2C), 127.0
22.47, 121.62, 118.82, 111.26. HRMS (EI) m/z calcd for C13
H
10
N
2
94.0000, found 194.0004.
(2C), 118.0 (3C).
Compound 3f: 2-(2,4-dichlorophenyl)-1H-benzimidazole (Table
À1
2
): solid, mp À218 to 220 °C, IR (KBr):
m
3192, 2984, 1622 cm
;
Acknowledgments
1
6
H NMR (DMSO-d , 300 MHz): d 12.41 (br s, 1H), 7.92 (d,
J = 9.0 Hz, 2H), 7.82 (m, 1H), 7.64–7.58 (m, 3H), 7.25–7.22 (m,
This work was financially supported by the Ministry of Educa-
tion, Culture, Sports, Science and Technology (MEXT) under the
1
3
2
6
H); C NMR (DMSO-d , 125 MHz): d 151.88, 148.80, 143.75,

M. A. Chari et al. / Tetrahedron Letters 51 (2010) 5195–5199
5199
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