Synthesis, characterization and antibacterial evaluation of some novel derivatives of 2-bromomethyl-benzimidazole

Article abstract of DOI:10.4314/bcse.v28i3.14

In the present study, synthesis, and biological evaluation of some novel derivatives of 2-bromomethyl-benzimidazole were investigated. The structures of the new synthesized compounds were characterized by IR, ¹H-NMR, ¹³C-NMR, mass spectrometry and elemental analysis and screened for antimicrobial activity against two Gram-positive strains (Staphylococcus aureus and Bacillus subtilis) and two Gram-negative strains (Escherichia coli and Pseudomonas aeruginosa). Results indicate that new compounds showed moderate to good activity when compared with standard antibiotic Ciprofloxacin and Streptomycin.

Full text of DOI:10.4314/bcse.v28i3.14

Bull. Chem. Soc. Ethiop. 2014, 28(3), 451-456.
Printed in Ethiopia
DOI: http://dx.doi.org/10.4314/bcse.v28i3.14
ISSN 1011-3924
 2014 Chemical Society of Ethiopia
SYNTHESIS, CHARACTERIZATION AND ANTIBACTERIAL EVALUATION OF
SOME NOVEL DERIVATIVES OF 2-BROMOMETHYL-BENZIMIDAZOLE
*
Abbas Ahmadi
Department of Medicinal Chemistry, Pharmaceutical Sciences Branch, Islamic Azad University,
Tehran, Iran
(
Received December 30, 2013; revised August 27, 2014)
ABSTRACT. In the present study, synthesis, and biological evaluation of some novel derivatives of 2-
bromomethyl-benzimidazole were investigated. The structures of the new synthesized compounds were
1
13
characterized by IR, H-NMR, C-NMR, mass spectrometry and elemental analysis and screened for
antimicrobial activity against two Gram-positive strains (Staphylococcus aureus and Bacillus subtilis) and two
Gram-negative strains (Escherichia coli and Pseudomonas aeruginosa). Results indicate that new compounds
showed moderate to good activity when compared with standard antibiotic Ciprofloxacin and Streptomycin.
KEY WORDS: Benzimidazole, Antibacterial activity, Bromo derivatives, Gram-positive strains, Gram-negative
strains
INTRODUCTION
Infectious microbial diseases remain pressing problems worldwide, because resistance to a
number of antimicrobial agents among variety of clinically significant species of
microorganisms has become an important global health problem. One way to battle with this
challenge is the conscious usage of the currently marketed antibiotics; the other is the
development of novel antimicrobial agents [1]. The benzimidazole core is classified by
medicinal chemists as one of the privileged sub-structures for drug design, in light of the affinity
they display towards a variety of enzymes and protein receptors [2]. Benzimidazoles are
heterocyclic compounds in which benzene is fused with five membered ring containing two
heteroatoms. Both heteroatoms are nitrogen (N), which are present at non-adjacent position [3].
The benzimidazole moiety is an important heterocyclic ring, a wide variety of benzimidazole
derivatives are known for their chemotherapeutic importance and antimicrobial activities [4-9],
antifungal activity [10-12], anti-inflammatory [13] and antioxidant [14-18] in this context. Due
to great potential of the moiety, synthesis of benzimidazole derivatives was carried out to
evaluate their antimicrobial. In generally benzimidazoles are readily formed by heating o-
phenylenediamines with carboxylic acids or aldehydes. For example, benzimidazole itself is
produced by heating o-phenylenedamine with 90% formic acid or formaldehyde [19].
In this research, some novel bromo-derivatives of 2-bromomethyl-benzimidazole (a-e in
Figure 1) were synthesized and screened for antibacterial activity against various strains of
Staphylococcus aureus, Bacillus subtilis, Escherichia coli and Pseudomonas aeruginosa.
EXPERIMENTAL
General. All chemicals and solvents were obtained from E-Merck and Sigma-Aldrich and used
without further purification. All melting points are uncorrected and taken with an electrothermal
melting point apparatus (Electrothermal Eng. Ltd, Essex, UK). IR spectra were determinate in
1
13
KBr on a Shimadzu Dr-8031 instrument. The H and C-NMR spectra of the synthesized
compounds were measured in DMSO-d or CDCl solution and TMS as the internal standard
using a Varian Mercury 400, 400MHz instrument. All Chemical shifts were reported as δ (ppm)
6
3
_
*
_________
Corresponding author. E-mail: a-ahmadi@kiau.ac.ir; ahmadikiau@yahoo.com

452
Abbas Ahmadi
values. The mass spectra were recorded on a LCQ ion trap mass spectrometer (Thermo Fisher.
San Jose.CA, USA), equipped with an EI source. Elemental analyses were carried out using a
Perkin-Elmer, CHN elemental analyzer model 2400 and within ± 0.4% of the theoretical values.
Br
N
OH
N
NH
NH
(
I)
(II)
Br
N
N
Br
N
N
N
N
Br
O
O
Br
(a)
(b)
(c)
Br
Br
N
Br
Br
N
N
N
O
O
S
O
O
Br
(
d)
(e)
Br
Figure 1. Structural formula for the intermediates (I and II) and final (a-e) synthesized
compounds.
Synthesis of compounds I and II
(1H-Benzimidazol-2-yl)-methanol (I). This compound was prepared based on a known method
with some modification [19, 20]. Mixture of o-phenylenediamine (27 g, 0.25 mol) in 500 mL 4
M HCl and glycolic acid (34.2 g, 0.45 mol) in 250 mL 4 M HCl stirred and heated under reflux
for 2 h. Then slowly cooled and resultant mixture was basified with aqueous sodium hydroxide
solution. The solid obtained, filtered, dried and re-crystallized with hot water (Scheme 1). (I:
o
white to light brown powder, m.p. 171-174 C, 89.1% yield).
2
-Bromomethyl-1H-benzimidazole (II). Thionyl bromide is used for some brominations of
certain α,β-unsaturated carbonyls, and it also converts alcohols to alkyl bromides [21]. To a
solution of I (5.92 g, 0.04 mol) in 75 mL cyclohexane as a nonpolar solvent and 2 mL N,N-
dimethylformamide as a catalyst, was drop wise added thionyl bromide (5 mL).The addition of
N,N-dimethylformamide as a catalyst was found to accelerate the reaction significantly through
2
a Vilsmeier–Haack type SOBr –DMF complex [22]. The mixture was refluxed for 10 h. The
excess of thionyl bromide was evaporated under vacuum and residue dissolved in cyclohexane
and washed with water (3×50 mL). The organic layer was dried over sodium sulfate and
o
evaporated to obtain the product (Scheme 1). Beige to dark yellow powder, m.p. 158-162 C,
Bull. Chem. Soc. Ethiop. 2014, 28(3)

Synthesis of some novel derivatives of 2-bromomethyl-benzimidazole
453
-
1
8
(
9.1% yield. IR (KBr, cm ): 3345 (N-H str), 3045 (Ar-C-H str), 2952 (aliphatic C-H str) , 1682
1
C=N str), 1448 (C=C str), 1292 (C-N str), 667 (C-Br); H-NMR (CDCl , δ/ppm): 4.64 (s, 2H,
3
13
CH ),4.89 (s, 1H, NH), 7.20-7.25 (m, 2H, Ar-H), 7.54-7.57 (m, 2H, Ar-H); C-NMR (CDCl ,
2 3
δ/ppm): 45.1 (CH -Br), 114.8 (benzimidazole, CH=), 121.6 (benzimidazole, CH=), 135.2
2
(
benzimidazole, C), 140.1 (N-C=N). Anal. calcd. for C+
8 7 2
H N Br: C, 45.50; H, 3.31; N, 13.27%.
Found: C, 45.31; H, 3.28; N, 13.20%. MS, m/z: 211[M] .
NH
NH
2
2
O
OH
N
OH
HCl (aq)
+
C
OH
NH
(
I)
N
OH
Br
N
SOBr2 / Cyclohexane
Reflux
NH
NH
(
I)
(
II)
Scheme 1. Preparation for synthesis of intermediates (I and II).
General procedure for the preparation of the compounds (a-e). To a solution of II (10.5 g, 0.05
mol), anhydrous potassium carbonate (5.52 g, 0.04 mol) in acetone (25 mL), appropriate Br-R-
Cl (0.06 mol) was added slowly (Scheme 2). The mixture was refluxed for 12 h and hot mixture
was poured into cold water (300 mL), extracted with ethyl acetate, dried over anhydrous sodium
sulphate and concentrated under vacuum to give the desired product.
2
1
1
-Bromo-1-(2-bromomethyl-benzimidazole-1-yl)-ethanone (a). Reddish yellow powder, m.p.
o
-
1
43-145 C, 90.3% yield. IR (KBr, cm ): 3078 (Ar-C-H str), 2921 (aliphatic-C-H str),
1
648(C=O), 1590 (C=N str), 1452 (C=C str), 1302 (C-N str), 675 (C-Br); H-NMR (CDCl
, Br, C=O), 4.75 (s, 2H, CH , Br), 7.21-7.28 (m, 2H, Ar-H), 7.60-7.70
m, 2H, Ar-H); C-NMR (CDCl , δ/ppm): 29.4, 37.4 (CH -Br), 119.8 (benzimidazole, CH=),
24.4 (benzimidazole, CH=), 140.2 (benzimidazole, C), 145.8 (N-C=N), 193.8 (C=O). Anal.
OBr : C, 36.15; H, 2.40; N, 8.43%. Found: C, 36.05; H, 2.28; N, 8.35%. MS,
3
,
δ/ppm): 4.61 (s, 2H, CH
(
1
calcd. for C10
m/z: 333[M+H] .
2
2
13
3
2
H
8
N
2
2
+
1
-(3-Bromobenzyl)-2-bromomethyl-1H-benzimidazole (b). Pale orange powder, m.p. 153-155
-1
o
C, 88.7% yield. IR (KBr, cm ): 2966 (Ar-C-H str), 2921 (aliphatic-C-H str), 1661(C=O), 1560
1
(
C=N str), 1458 (C=C str), 1302 (C-N str), 675 (C-Br); H-NMR (CDCl , δ/ppm): 4.56 (s, 2H,
3
13
2 2
CH -Br), 4.89 (s, 2H, CH -N), 7.11-7.33 (m, 4H, Ar-H), 7.40-7.75 (m, 4H, benzimidazole); C-
NMR (CDCl , δ/ppm): 27.4 (CH -N), 52.8 (CH -Br), 119.8-133.1 (phenyl, C), 123.8-138.3
3
2
2
(
benzimidazole, C), 145.8 (N-C=N). Anal. calcd. for C15 +
12 2 2
H N Br : C, 47.37; H, 3.16; N, 7.37%.
Found: C, 47.30; H, 3.02; N, 7.29%. MS, m/z: 381[M+H] .
(
2-Bromomethyl-benzimidazole-1-yl)-(3-bromophenyl)-methanone (c). Yellowish orange
o
-
1
powder, m.p. 160-162 C, 87.3% yield. IR (KBr, cm ): 3077 (Ar-C-H str), 2965 (aliphatic-C-H
str), 1720(C=O), 1580 (C=N str), 1488 (C=C str), 1278 (C-N str), 690 (C-Br); H-NMR (CDCl ,
1
3
1
3
δ/ppm): 4.61 (s, 2H, CH
NMR (CDCl , δ/ppm): 27.4 (CH
43.1 (N-C=N), 189.1 (C=O). Anal. calcd. for C H N OBr : C, 45.68; H, 2.53; N, 7.10%.
2
-Br), 7.40-7.75 (m, 4H, benzimidazole), 7.30-7.95 (m, 4H, Ar-H); C-
3
2
-Br), 114.8-136.1 (benzimidazole, C),120.2-138.5 (phenyl, C),
1
1
5
10
2
2
+
Found: C, 45.60; H, 2.48; N, 7.05%. MS, m/z: 395[M+H] .
Bull. Chem. Soc. Ethiop. 2014, 28(3)

454
Abbas Ahmadi
(
5-Bromo-2-methoxy-phenyl)-(2-bromomethyl-benzimidazole-1-yl)-methanone (d). Pale yellow
o
-
1
powder, m.p. 155-160 C, 85.9% yield. IR (KBr, cm ): 3067 (Ar-C-H str), 2945 (aliphatic-C-H
str), 1730(C=O), 1562 (C=N str), 1490 (C=C str), 1201 (C-N str), 1118 (C-O str), 715 (C-Br);
1
H-NMR (CDCl , δ/ppm): 3.69 (s, 3H, O-CH ), 4.70 (s, 2H, CH -Br), 6.85-7.90 (m, 3H, Ar-H),
3
3
2
1
3
7
.25-7.80 (m, 4H, benzimidazole); C-NMR (CDCl
3
2
, δ/ppm): 26.9 (CH -Br), 56.5 (C-O),
1
16.1-138.2 (benzimidazole, C), 113.9-139.4 (phenyl, C), 143.1 (N-C=N), 164.6 (Ph-O-), 192.2
Br : C, 45.28; H, 2.83; N, 6.60%. Found: C, 45.19; H, 2.79;
(
C=O). Anal. calcd. for C16
H
12
N
2
O
2
2
+
N, 6.54%. MS, m/z: 425[M+H] .
1
-(3-Bromo-benzenesulfonyl)-2-bromomethyl-1H-benzenimidazole (e). Cream white powder,
o -1
m.p. 169-171 C, 88.9% yield. IR (KBr, cm ): 3319 (S=O str), 3075 (Ar-C-H str), 2933
1
(
(
aliphatic-C-H str), 1566 (C=N str), 1495 (C=C str), 1276 (C-N str), 690 (C-Br); H-NMR
CDCl , δ/ppm): 4.67 (s, 2H, CH -Br), 7.21-7.80 (m, 4H, benzimidazole), 7.5-8.25 (m, 3H,
, δ/ppm): 25.9 (CH -Br), 122.9-142.4 (Phenyl, C),117.3-137.8
benzimidazole, C), 141.8 (N-C=N). Anal. calcd. for C H N O SBr : C, 39.07; H, 2.32; N,
3
2
13
Ar-H); C-NMR (CDCl
(
3
2
1
4
10
2
2
2
+
6
.51%. Found: C, 38.95; H, 2.28; N, 6.48%. MS, m/z: 431[M+H] .
N
Br
Br
N
a
Acetone/ K CO
bromoacetyl
2
3
+
Br-R-Cl
b
c
m-bromobenzyl
m-bromobezoyl
Reflux
NH
R =
N
d
5
-bromo-2-methoxybenzoyl
(
a-e)
(
II)
R
e
m-bromobenzenesulfonyl
Br-R-Cl for (a-e) :
Cl
Cl
O
Cl
O
Cl
O
S
O
O
O
C
Br
Cl
Br
Br
Br
Br
a
b
c
d
e
Scheme 2. Preparation rout for synthesis of new compounds (a-e).
Antimicrobial activity. The antibacterial activities of newly synthesized compounds were tested
by paper disc diffusion method [23, 24]. Using nutrient agar medium against following
microorganism: Staphylococcus aureus, Bacillus subtilis, (Gram positive) and Escherichia coli,
Pseudomonas aeruginosa (Gram negative).
Experimental procedure. In the paper disc-diffusion method, paper disc impregnated with
compounds dissolved in DMSO at conc. 50 and 100 µg/mL were used. The microorganism
culture was spread over nutrient agar media in Petri dishes, and then the disc impregnated with
the solution was placed on the surface of the media inoculated with the bacterial strain. The
o
plates were incubated at 35 C for 24 h. After incubation, the zones of inhibition around the disc
were observed. The zones of inhibition indicate that the compounds inhibit growth of
microorganism. Each testing is done in triplicate. Streptomycin and Ciprofloxacin at conc. 50
and 100 µg/mL were used as standard drugs for antibacterial activity. Results were interpreted
in terms of diameter (mm) of zone of inhibition.
Bull. Chem. Soc. Ethiop. 2014, 28(3)

Synthesis of some novel derivatives of 2-bromomethyl-benzimidazole
455
RESULTS AND DISCUSSIONS
Chemistry. In continuation of our interest to explore of new antimicrobial drugs, the efficient
syntheses of biologically active benzimidazole derivatives were applied in this study. Herein,
some novel bromo-derivatives of 2-bromomethyl-benzimidazole (a-e) in good yields were
prepared from 2-bromomethyl-1H-benzimidazole (II), appropriate bromo-hydrocarbon chloride
and anhydrous potassium carbonate in acetone (Figure 2). Therefore, initially, (1H-
benzimidazol-2-yl)-methanol (I) was synthesized by coupling of o-phenylenediamine with
organic acid based on a known method with some modification for increasing yield. TLC was
used for monitoring the progress of the reaction; the structures of products were assessed by
1
13
interpretation of obtained spectra. The spectroscopic data of IR, H-NMR, C-NMR, mass
spectra and elemental analysis were in agreement with the structure of the synthesized
compounds.
Antimicrobial activities. For evaluation of the biological activities of the compounds, (a-e)
were screened for in vitro antimicrobial activities against variety of bacteria, two Gram- positive
strains (Staphylococcus aureus and Bacillus subtilis) and two Gram-negative strains
(
Escherichia coli and Pseudomonas aeruginosa). The preliminary screening results for the
compounds (a-e) established that the newly synthesized compounds have not shown
antibacterial activity against E. coli, but the compounds (a), (b) and (e) showed the best
activities against B. Subtilis, and compounds (c) and (d) were slightly active against B. Subtilis.
Compound (c) showed the best activity against S. Aureus, compounds (d) was slightly active
against S. Aureus (Table 1 and 2).
Table 1. Zone of inhibition (mm±SD) of test sample and standard drug (Streptomycin and Ciprofloxacin)
against B. subtilis and E. coli.
Test samples
Diameter of zone of inhibition in mm [mean±SD (n = 3)]
B. subtilis E. coli
0 µg/mL
5
100 µg/mL
10.281±0.42
11.80±1.30
13.95±1.20
14.96±1.45
12.84±1.30
12.88±0.32
15.10±0.68
19.24±0.35
18.57±0.52
50 µg/mL
4.42±0.20
5.15±0.50
6.10±0.64
4.40±1.25
6.89±1.11
6.88±0.78
3.76±0.38
11.36±0.44
10.80±0.54
100 µg/mL
8.60±0.42
8.22±0.60
9.25±0.45
6.86±0.36
11.10±0.46
11.66±0.35
8.20±0.45
16.45±0.28
I
6.52±0.42
6.90±1.30
9.70±0.75
9.75±0.44
8.78±0.71
8.32±0.36
9.85±0.66
14.36±0.36
13.55±0.44
II
a
b
c
d
e
Ciprofloxacin
Streptomycin
15.85±0.35
Table 2. Zone of inhibition (mm±SD) of test sample and standard drug (Streptomycin and Ciprofloxacin)
against S. Aureus and P. aeureginosa.
Test samples
Diameter of zone of inhibition in mm [mean±SD (n = 3)]
S. Aureus P. aeureginosa
0 µg/mL 50 µg/mL
5
100 µg/mL
11.26±0.23
14.70±1.00
13.14±0.33
11.85±1.20
14.45±0.35
12.76±0.25
10.24±0.76
17.45±0.22
17.62±0.28
100 µg/mL
7.53±0.54
12.92±0.78
14.80±1.00
12.78±1.25
14.95±1.15
12.95±0.29
7.86±0.34
19.12±0.32
19.78±0.44
I
6.54±0.58
7.70±0.42
6.80±0.55
5.34±0.78
8.62±0.44
7.85±0.80
5.22±1.20
11.95±0.22
12.20±0.44
4.75±0.63
8.00±0.36
8.85±0.85
6.80±0.62
8.78±0.71
6.24±0.26
4.00±0.30
14.00±0.28
14.50±0.36
II
a
b
c
d
e
Ciprofloxacin
Streptomycin
Bull. Chem. Soc. Ethiop. 2014, 28(3)

456
Abbas Ahmadi
CONCLUSIONS
A series of some novel bromo-derivatives of 2-bromomethyl-benzimidazole were synthesized
and evaluated for their potential antimicrobial activities. Based on results, it can be concluded
that all the synthesized compounds showed good to moderate antimicrobial activities. The
results indicated that new antimicrobial compounds could be prepared by changing of different
substrates on various benzimidazole derivatives. These new compounds could be evaluated for
further pharmacological activities in other studies.
ACKNOWLEDGEMENT
The author gratefully acknowledges beneficial support of Pharmaceutical Sciences Branch,
Islamic Azad University for this research project.
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