A green chemical synthesis of 2-alkylbenzimidazoles from 1,2-phenylenediamine and propylene glycol, or alcohols mediated by Ag-TiO 2/clay composite photocatalyst

Article abstract of DOI:10.1246/cl.2007.1060

Benzimidazole derivatives were synthesized in good yield from 1,2-phenylenediamine (PD) and various alcohols by photocatalytic cyclization with TiO₂ /acidic clay composite catalyst using UV-A and solar light. Doping silver on TiO₂ enhances product yield and selectivity of the photocatalytic synthesis of benzimidazoles. The higher efficiency of nano-Ag-doped TiO₂ in solar light makes this synthesis a green chemical process. Copyright

Full text of DOI:10.1246/cl.2007.1060

1
060
Chemistry Letters Vol.36, No.8 (2007)
A Green Chemical Synthesis of 2-Alkylbenzimidazoles from 1,2-Phenylenediamine
and Propylene Glycol, or Alcohols Mediated by Ag–TiO /Clay Composite Photocatalyst
2
Ã
K. Selvam and M. Swaminathan
Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India
(Received May 31, 2007; CL-070588; E-mail: chemsam@yahoo.com)
Benzimidazole derivatives were synthesized in good yield
Table 1. Photocatalytic cyclization of 1,2-phenylenediamine
with propylene glycol under different conditions
from 1,2-phenylenediamine (PD) and various alcohols by photo-
catalytic cyclization with TiO2/acidic clay composite catalyst
using UV–A and solar light. Doping silver on TiO2 enhances
product yield and selectivity of the photocatalytic synthesis of
benzimidazoles. The higher efficiency of nano-Ag-doped TiO2
in solar light makes this synthesis a green chemical process.
Irr. time 2MBZ BZ Conversion
Conditionsa
/
h
/% /%
/%
Clay/TiO2/dark/air
TiO2/air/light
Clay/air/light
8
0
14
0
0
3
0
17
0
8
8
0
TiO2/clay (2%)/air/light
TiO2/clay (5%)/air/light
TiO2/clay (5%)/air/solar light
a
8
48
60
58
18
30
30
66
90
88
8
10
Benzimidazole and its derivatives are important class of bi-
1
oactive molecules in the field of drugs. The classical synthesis
of benzimidazole and its derivatives involves the facile conden-
All reactions were performed with a mole ratio of (1:2) in acetoni-
À1
trile (20 mL) solvent. TiO2 = 200 mg, airflow = 8.1 mL s
.
sation of aromatic o-diamine and carbonyl compounds under
2
strong acidic, or harsh dehydrating conditions. A number of
photocatalytic chemical transformations using semiconductor
found to be more efficient photocatalyst for the production of
MBZ and BZ. This photocatalytic reaction using TiO2/clay
5%) composite catalyst was also carried out with PD and pri-
2
(
3
,4
particles (CdS and TiO2) have been reported. Combining the
photocatalytic activity of TiO2 with utility of zeolites for photo-
mary alcohols such as ethanol, propanol and butanol. The results
are summarized in Table 2. It was observed that the reaction pro-
ceeded efficiently with anhydrous primary alcohols giving re-
spective 2-alkylbenzimidazoles in good yield. Benzimidazole
was not formed with primary alcohols. When air was replaced
by nitrogen the reaction did not yield any product of our interest.
Based on the results we propose the following mechanism
5
induced cyclisation is advantageous because this integrated
photocatalyst absorbent matrix utilizes absorption with ad-
vanced oxidation by photocatalyst. Performance of these photo-
catalysts has been improved to extend their light absorption and
6
conversion capacity to the visible region of the solar spectrum.
In our laboratory, we had investigated the use of these modified
photocatalysts in wastewater detoxification.
7
(
Scheme 1). 1,2-Phenylenediamine being a basic reactant is ad-
In the present work, we report a simple ecofriendly intermo-
lecular cyclization of 1,2-phenylenediamine (PD) with propyl-
ene glycol (PG) or primary alcohols in the presence of TiO2,
sorbed completely on the strong acidic clay and then propylene
glycol undergoes oxidation to keto aldehyde at the oxidizing site
of TiO2. It is already established that alcohols can be easily oxi-
dized on semiconductor surfaces to corresponding aldehyde or
8
,13
Ag–TiO2/acidic clay (K10 montmorillonite), and air.
It
was reported earlier that composite catalyst TiO2-acidic zeolite
induced a photocatalytic cyclization of PD and propylene glycol
resulting in the formation of 2-methylquinoxaline and quinoxa-
1
0
ketone. The keto aldehyde undergoes further oxidation form-
ing acetic acid and formic acid. The acetic acid and formic acid
generated in situ reacts with PD to give 2MBZ and BZ. Thermal
reactions for the formation of 2MBZ and BZ from PD and acetic
9
line. To the best our knowledge, this paper describes for the
first time the synthesis of 2-alkylbenzimidazoles under mild
conditions using solar light.
1
1
acid and formic acid have been reported. When the photocata-
lytic reaction between PD and PG was carried out in presence of
TiO2/zeolite the products formed were 2-methylquinoxaline and
quinoxaline. As per the mechanism proposed, first 2-methylqui-
noxaline was formed by the cyclization of keto aldehyde with
PD then the 2-methylquinoxaline undergoes oxidative decarbox-
We first investigated the photocatalytic cyclization reaction
of PD with PG using UV–A or solar light under different condi-
tions. The results are presented in Table 1. It was observed that
photocatalytic formation of 2-methylbenzimidazole (2MBZ)
and benzimidazole (BZ) from PG with PD required light, TiO2,
acidic clay, and air for higher yield. When carried out using
conventional liquid acid (HCl) or clay without TiO2 the reaction
did not yield any product. However, PD and PG when irradiated
with TiO2 under similar conditions, a small amount of 2MBZ
9
Table 2. Yield obtained after 15 h irradiation of UV–A light/
a
solar light on photocyclization of PD and primary alcohols with
TiO2/clay (5%) catalyst
(
14%) was formed.
With TiO2/clay (2%) composite catalyst 48% of 2MBZ and
Products/%
Reactants
UV
Solar
18% BZ were formed on irradiation by UV–A light. When the
percentage of clay was increased to 5%, 60% of 2MBZ and
3
2
change in the percentage of clay or TiO2 in the TiO2/clay (5%)
did not give better yield. Hence TiO2/acidic clay (5%) was
PD + PG
2MBZ–62, BZ–30
2MBZ–35
2EtBZ–75
2MBZ–60, BZ–30
2MBZ–16
2EtBZ–60
PD + EtOH
PD + PrOH
PD + BuOH
0% of BZ were formed. Whereas in solar light 58% of
MBZ and 30% of BZ were formed on 10 h irradiation. Further
2PrBZ–63
2PrBZ–48
aCondition same as in Table 1.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.8 (2007)
1061
Step 1
Table 3. Comparison of photocatalytic activities of TiO2 and
Ag–TiO catalysts in UV and solar light
OH
OH
O
2
Ag−TiO
2
/clay (5%)/h
ν
Air
Irr. time
2MBZ/%
BZ/%
O
Conditionsa
/
h
UV Solar UV Solar
H3C
H
O
OH
C
H3C
O
C
Ag−TiO /h
ν
2
OH
C
CH3
C
O + H
C
O
+
TiO2/clay (5%)/air
Ag–TiO /clay (5%)/air
10
8
60
59
62
46
67
56
30
33
30
34
30
30
C
Air
O
OH
H
O
2
Step 2
TiO2-P25/clay (5%)/air
8
NH2
aCondition same as in Table 1.
CH3
C
Ag−TiO
2
/clay (5%)/h
ν
N
+
+
Air
O
NH2
NH2
HO
N
CH3
is found to be efficient than TiO2-P25 and TiO2 (anatase) for
the synthesis of 2-alkylbenzimidazole under solar radiation
and hence the use of Ag–TiO2 makes this synthesis as green
technological process. The photocyclization efficiency of
TiO2/acidic clay is much more higher than the TiO2/zeolite.
H
H
C
N
NH2
O
OH
N
H
Scheme 1. Proposed reaction mechanism for the formation of
MBZ and BZ.
2
References and Notes
ylation in presence of TiO2/zeolite to give quinoxaline. But
in presence of TiO2/clay (5%) composite catalyst we found
the simultaneous formation of 2MBZ and BZ. This is possible
only by the cyclization of PD with acetic acid or formic acid.
This reveals that in presence of TiO2/clay, the keto aldehyde
formed, undergoes further oxidation by TiO2 to give acetic acid
and formic acid.
The formation of acid intermediates with this catalyst
was also confirmed by products obtained from PD and primary
alcohols (Table 2). Ethanol, propanol, and butanol formed only
1
2
3
4
A. F. Pozharskii, A. D. Garnovskii, A. M. Simonov, Russ.
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7180.
2
-methyl-, 2-ethyl-, and 2-propylbenzimidazoles by cyclization
5
6
V. Ramamurthy, D. R. Corbin, N. J. Turro, Z. Zhang, M. A.
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of PD with the corresponding acids. Benzimidazole was not
formed in these reactions because there was no chance of getting
formic acid as in propylene glycol. Among the primary alcohols,
the efficiency of cyclization is high with propanol, and low with
ethanol. This is because of the presence of electron donor methyl
group in propanol. The less efficiency of butanol when compared
to propanol is due to the steric effect of bulky propyl group. The
yields of 2-methylquinoxaline and quinoxaline in presence of
TiO2/zeolite were only 22.5 and 12.6% respectively. But in
presence of TiO2/clay the photocyclization was highly efficient,
giving 60% 2MBZ and 30% BZ.
The photocatalytic cyclization reaction of PD with PG
was carried out with the composite clay catalysts prepared
using TiO2 (anatase), TiO2-P25, and Ag–TiO2 and the results
are shown in Table 3. With UV light Ag/TiO2 and TiO2-P25
are equally efficient giving 92% yield in 8 h. But TiO2 (anatase)
gives only 90% yield at a longer irradiation time of 10 h.
Under solar irradiation Ag/TiO2 is more efficient showing
7
8
9
All reactions were performed with mole ratio 1:2 of 1,2-
phenylenediamine and propylene glycol in 25 mL of acetoni-
trile using 200 mg of Ag–TiO2/acidic clay (5%) catalyst in
a cylindrical round-bottomed quartz photoreactor of 50 mL
À1
capacity under aerated conditions (8.1 mL s ). The loss of
solvent due to evaporation was compensated periodically.
The reaction mixture was irradiated with 365 nm light (8
lamps of 8 W each) for 8 h. After irradiation, the samples
were subjected to GC analysis for the determination of the
yield. The same experimental setup was used for solar light.
The separation of products was performed by column chro-
matography on a silica-gel column by eluting with a co-sol-
vent of dichloromethane and methanol (4:1 v/v). The struc-
97% conversion in 8 h. The order of activity of these catalysts
in solar light is Ag/TiO2 (97%) > TiO2-P25 (86%) > TiO2
(
anatase) (80%, 10 h).
The higher activity of Ag–TiO2 catalyst in solar light has
1
tures of the products were confirmed with H NMR,
13
C NMR, and GC-Mass spectra (ref. 13).
6
already been proved in the degradation of azo dyes. The Ag
particles act as electron traps, enhancing the electron–hole
separation and subsequent transfer of the trapped electron to
the adsorbed O2.12
The formation of 2-alkylbenzimidazoles during the photo-
cyclization of PD and PG/primary alcohols at ambient temper-
ature with UV/solar light is reported over TiO2/acidic clay
9
K. V. Subba Rao, M. Subramanyam, Chem. Lett. 2002, 234.
10 E. Borgarello, E. Pollizetti, Chem. Ind. 1983, 65, 474.
11 W. O. Pool, H. J. Harwood, A. W. Ralston, J. Am. Chem.
Soc. 1937, 59, 178.
12 A. Henglein, J. Phys. Chem. 1979, 83, 2209.
13 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
(5%) photocatalyst for the first time. A mechanism involving
the cyclization of PD with the acid is proposed. Nano Ag–TiO2
Published on the web (Advance View) July 21, 2007; doi:10.1246/cl.2007.1060