510 Abbas et al.
Asian J. Chem.
on a MAT-112-S machine at 70 eV.All the reactions were moni-
tored by TLC using Pre-coated Kieselgel-60 HF254 TLC plates.
Synthesis of 2(3H)-benzimidazolone: 1,2 Phenylene
diamine (0.05 mol) and urea (0.015 mol) were placed in a 100
mL round bottom flask, dissolved in 25 % THF/water solution
and stir vigorously, then added 1-2 drops of acetic acid as a
catalyst. The reaction mixture was refluxed on oil bath at 100-
120 ºC for 4.5 h and then for 0.5 h at 135 ºC to remove the
remaining ammonia gas. Completion of the reaction was conti-
nuously checked by TLC. After completion of the reaction,
the mixture was stayed for a long time near 5 h at room tempe-
rature. The resulting precipitates were filtered and washed with
ethanol thrice and then recrystallized in methanol. The preci-
pitates were filtered, dried (anhydrous MgSO4) for overnight.
The product obtained was pale yellow in colour with good
yield and sharp m.p. Elemental analysis: C7H6N2O (134):
calcd. C 62.68, H 4.51, N 20.89; found C 62.70, H 4.53, N
20.91.
If comparison is made between the synthetic route avail-
able in literature and new proposed method, very encouraging
results are achieved and it is found to be a very facile method
for preparation of biological active bezimidazolone and its
derivative.
The methods found in literature to obtain benzimidazolone
by the reaction of phosgene with o-phenylenediamine has some
disadvantages due to formation of hydrochloric acid, which
requires special safety arrangements due to its toxic nature.
Similarly the method with urea in solid phase reaction has
also some disadvantages, like the use of high temperature,
continuous removal of ammonia gas and difficulty in stirring
the reaction mixture.
Discrepancies have been eliminated in our proposed new
and convenient method for the preparation of benzimidazolone
with better yield. The products obtained in result of this most
promising synthetic approach are in good yield (Table-1).
TABLE-1
PHYSICAL DATA OF 2(3H)-BENZIMIDAZOLONES
AND 5-NITRO-2(3H)-BENZIMIDAZOLONE
Synthesis of 5-nitro-2(3H)-benzimidazolone: 5-Nitro-
1,2-phenylene diamine (0.05 mol) and urea (0.015 mol) were
placed in a 100 mL round bottom flask, dissolved in 25 %
THF/water solution and stir vigorously, then added 1-2 drops
of acetic acid as a catalyst. The reaction mixture was refluxed
on oil bath at 100-120 ºC for 4.5 h and then for 0.5 h at 135 ºC
to remove the remaining ammonia gas. Completion of the
reaction was continuously checked by TLC. After completion
of the reaction, the mixture was stayed for a long time near
5 h at room temperature. The resulting precipitates were filtered
and washed with ethanol 3 times and then recrystallized in
methanol. The precipitates were filtered, dried (anhydrous
MgSO4) for overnight. The product was yellowish brown in
colour.Elemental analysis: C7H5N3O3 (179.13): calcd. C 46.92,
H 2.81, N 23.46 ; found C 46.88, H 2.86, N 23.54.
R1
H
N
R2
R3
Entry Compound name
O
N
H
R1
H
H
R2
H
R3
H
H
Yielda
(%)
m.p.
(ºC)
1
1
2(3H)-benzimid-
azolones
93.7
310
5-Nitro-2(3H)-
benzimidazolone
NO2
87
306
aGiven yield after column chromatography; Rf value = 42 (in pure
ethyl acetate)
The structures of all the synthesized compounds were
established by modern spectroscopic techniques and purity
ascertained by elemental analysis which supports the idea of
this work. The reaction Scheme-II elaborates the synthesis
method.
RESULTS AND DISCUSSION
2-(3H)-Benzimidazolone and its derivative 5-nitro-2(3H)-
benzimidazolone were synthesized in liquid phase of THF/
water by the reaction of o-phenylene diamine with urea in the
presence of acetic acid as a catalyst as shown in Scheme-II.
The contrasting results were obtained by this attempt as
compared to reported methods. A number of such compounds
with different substitution patterns have been synthesized9,10
to check their medicinal properties.
The FT-IR spectra showed a bond at 1740 cm-1 and 1110
cm-1 for -NCO stretching and C-H aromatic vibration as shown
in Table-2. On the other hand when H is replaced by NO2
group at positon No. 5 of benzene ring then it is observed an
Ar-NO2 asymmetric stretching at 1520 cm-1 which give the
clear indication of introduction of -NO2 group as shown in
Table-3.
1
H
In H-NMR spectra of 2(3H)-benzimidazolone, signals
for NH protons were observed in a downfield region in the
range of 10.59 ppm. The protons H 4, 5, 6, 7 are appeared at
H
NH2
NH2
O
N
THF/Water
O
Reflux under argon
NH2 (1-2 drop of acetic acid)
H2N
N
H
TABLE-2
FT-IR SPECTRAL DATA OF
2(3H)-BENZIMIDAZOLONES WHEN R=H
N
ν (cm-1)
1740
OH
Entry
Functional group behaviour
N
H
1
2
3
4
5
6
7
-NCO stretching
C-H aromatic vib
C-H vib deformation
C=C aromatic vib
=C-H aromatic stretching
-NH free group
1110
H
N
O2N
NH2
O2N
O
830-880
1385-1395
3182-3190
3250-3400
3350-3400
THF/Water
O
Reflux under argon
NH2
(1-2 drop of acetic acid)
H2N
N
H
NH2
Scheme-II: Synthesis of 2(3H)-benzimidazolone and 5-nitro-2(3H)-benzimi-
dazolone
C=O overtone