Synthesis of benzimidazoles/benzothiazoles by using recyclable, magnetically separable nano-Fe2O3 in aqueous medium

Article abstract of DOI:10.1007/s13738-017-1107-z

An efficient, simple, ecofriendly and cost-effective method has been developed for the synthesis of benzimidazole/benzothiazole derivatives by a two-component reaction, involving 1,2-diamino benzene/2-amino thiophenol and substituted aromatic aldehydes us

Full text of DOI:10.1007/s13738-017-1107-z

J IRAN CHEM SOC
DOI 10.1007/s13738-017-1107-z
ORIGINAL PAPER
Synthesis of benzimidazoles/benzothiazoles by using recyclable,
magnetically separable nano‑Fe O in aqueous medium
2
3
Dileep Kommula · Sri Rama Murty Madugula1
1
Received: 27 January 2017 / Accepted: 14 March 2017
©
Iranian Chemical Society 2017
Abstract An efficient, simple, ecofriendly and cost-effec-
tive method has been developed for the synthesis of ben-
zimidazole/benzothiazole derivatives by a two-component
reaction, involving 1,2-diamino benzene/2-amino thio-
phenol and substituted aromatic aldehydes using recycla-
ble nano‑Fe O catalyst (10 mol%) in water afforded with
excellent yields (75–85%). The most important feature of
this protocol is short reaction times, high yields, aqueous
reaction medium, efficient recycling and high stability of
the catalyst.
mixture with an external magnet because it is simpler and
more efficient than conventional separation i.e filtration or
centrifugation [2, 3]. Among them, iron oxide nanoparticles
(Fe O NPS) have gained attention toward various organic
2
3
transformations [4, 5].
Benzimidazole/benzothiazole moieties are important
scaffolds in pharmaceutical applications, and these moie-
ties appear in many drugs encompassing a broad range of
activities (Fig. 1). Benzimidazole/benzothiazole deriva-
tives were associated with a wide variety of medicinal, bio-
logical activities such as antifungal, antiviral, antibacterial,
anticancer, anti-inflammatory, antiulcer, antihypertensive,
antihistaminic, anticonvulsant, and antiparkinsonian activi-
ties [6–11]. They are also widely used in organic synthe-
sis as intermediates. In particular, benzimidazoles are use-
ful in controlling the diseases such as hypertension [12],
ischemia–reperfusion injury [13], as well as obesity [14].
Several methodologies have been reported for the syn-
thesis of benzimidazole/benzothiazoles. Mirkhani et al.
[15] have developed the synthesis of 2-imidazolines and
bis-imidazolines by using starting materials ethylenedi-
amine and nitriles in the presence of sulfur under ultrasonic
irradiation. The synthesis of benzimidazoles was reported
by Lin and co-workers [16] taking starting materials phe-
nylenediamines and aldehydes using molecular iodine.
Hornberger et al. [17] described one-pot synthesis of dis-
ubstituted benzimidazoles from 2-nitroanilines with pal-
ladium charcoal as a catalyst in the presence of trimethyl
orthoformate and catalytic pyridinium p-toluenesulfonate
2
3
Keywords Benzimidazoles/benzothiazoles · Nano-iron
oxide · Magnetic separation · Water
Introduction
Nanochemistry is an emerging research field in modern
science. The nanoscale catalysts can provide higher surface
areas and lower coordinating sites, which are responsible for
the higher catalytic activity [1]. In recent years, magnetic
nanoparticles (MNPs) have gained attention as a useful
group of heterogeneous catalysts and in view of their recov-
ery. Magnetic nanoparticles provide a convenient method
for the separation of magnetized species from the reaction
Electronic supplementary material The online version of this
article (doi:10.1007/s13738-017-1107-z) contains supplementary
material, which is available to authorized users.
(PPTS) at room temperature. Das et al. [12] reported the
*
Dileep Kommula
kommula.dilipreddy@gmail.com
synthesis of benzimidazoles from 1,2-phenylenediamine,
with aldehydes by using (bromodimethyl)sulfonium bro-
mide at room temperature. Recently Ramesh et al. [18]
reported the aqueous phase synthesis of benzimidazoles/
benzothiazoles using recyclable β-cyclodextrin.
1
Medicinal Chemistry and Pharmacology Division, CSIR–
Indian Institute of Chemical Technology, Hyderabad 500
0
07, India
1
3

J IRAN CHEM SOC
Fig. 1 Some of the important
benzimidazole/benzothiazole
containing drugs
However, the above-mentioned methods have been asso-
ciated with different draw backs such as the use of hazard-
ous organic solvents, strongly acidic conditions, expensive,
moisture-sensitive catalysts, or tedious workup condi-
tions as well as low yields. In continuation of our earlier
efforts of novel environmentally benign methodologies
using magnetically recyclable iron oxide nanoparticles [19,
visualized by UV light or I2 stain. Column chromatogra-
phy was carried out with Merck 60–120 sized mesh silica
gel using ethyl acetate and hexane as eluent. All products
1
were characterized by their NMR and MS spectra. The H
1
3
NMR (300 MHz) spectra and C NMR (75 MHz) spec-
tra were recorded on Bruker Avance 300 nuclear magnetic
resonance spectrometer taking the compounds in CDCl3
using TMS as an internal standard. The chemical shifts (δ)
were reported in parts per million (ppm) downfield from
TMS and coupling constants nJ values were expressed in
Hz. Melting point was measured on Electrothermal 9100
Melting point apparatus. Surface morphology was recorded
on Hitachi S-3000N scanning electron microscope. X-ray
diffraction (XRD) pattern was obtained on a Bruker D8
Advance X-ray powder diffractometer.
2
0]. Herein, we report an efficient one-pot protocol for the
synthesis of benzimidazole/benzothiazole derivatives by a
two-component reaction, involving 1,2-diamino benzene/2-
amino thiophenol and substituted aromatic aldehydes for
the first time promoted by recyclable iron oxide nanoparti-
cles in aqueous medium (Scheme 1).
Experimental section
General experimental procedure for the synthesis
of benzimidazoles/benzothiazoles using nano‑Fe O
Instruments and reagents
2
3
All chemicals, reagents, and the catalyst were obtained
from Aldrich (Sigma-Aldrich, Saint Louis, MO, USA) or
Spectrochem Pvt. Ltd. (Mumbai, India) and were used
without further purification. The reactions were monitored
by thin-layer chromatography (TLC) using Merck silica
gel glass plate (60 F254, 0.25 mm), and TLC plates were
The aldehyde (1.0 mmol) and 1,2-diamino benzene/2-amino
thiophenol (1.0 mmol) were added to a suspension of nano-
Fe O (10 mol%) in water 3 mL. The reaction mixture was
heated to reflux and monitored by TLC until total conversion
of the starting materials. After completion of the reaction, the
catalyst was separated with the aid of a magnet. The separated
2
3
Scheme 1 Synthesis of benzo-
thiazoles/benzimidazoles
CHO
NH2
X
N
R
nano-Fe O
2
3
°
H O, 80 C, 2-3 h
2
XH
R
X=NH, S
R= H, NO , Cl, CH , OCH , OC H
3 3
2 2 5
1
3

J IRAN CHEM SOC
catalyst was washed several times with methanol, dried under
vacuum. The aqueous layer was extracted with ethyl acetate
2‑(3,4,5‑Trimethoxyphenyl)benzo[d]thiazole (Table 2,
Entry 6)
(
3 × 10 mL).The combined organic layers were washed with
1
water, saturated brine solution, and dried over anhydrous
Na SO . The combined organic layers were evaporated under
H NMR (300 MHz, CDCl ) δ = 3.99 (s, 9H, -OCH ) 7.13
3
3
(s, 1H, H-Ar), 7.33 (s, 1H, H-Ar), 7.51–7.48 (m, 2H, H-Ar),
2
4
reduced pressure, and the resulting crude product was puri-
fied by column chromatography using ethyl acetate and hex-
ane (1:9) as an eluent. The identity and purity of the products
7.89 (d, 1H, J = 7.6 Hz, H-Ar), 8.06 (d, 1H, J = 8.2 Hz,
1
3
H-Ar); C NMR (75 MHz, CDCl ) δ = 56.2, 60.9, 104.6,
3
121.4, 122.9, 125.0, 126.3, 134.8, 153.4, 153.8; MS(ESI):
1
13
+
were confirmed by H, C NMR, and mass spectra.
m/z = 302 [M + H] .
2
‑Phenyl‑1H‑benzo[d]imidazole (Table 2, Entry 7)
2
‑Phenylbenzo[d]thiazole (Table 2, Entry 1)
1
1
H NMR (300 MHz, CDCl ) δ = 5.47 (s, 1H, –NH), 7.10
H NMR (300 MHz, CDCl ) δ = 7.42–7.36 (m, 2H, H-Ar)
3
3
(
7
d, 1H, J = 6.8 Hz, H-Ar), 7.29–7.27 (m, 2H, H-Ar), 7.49–
7
8
.52–7.47 (m, 3H, H-Ar), 7.90 (d, 1H, J = 7.9 Hz, H-Ar),
13
.44 (m, 2H, H-Ar), 7.68–7.64 (m, 3H, H-Ar), 8.07 (d, 1H,
.11–8.07 (m, 3H, H-Ar),; C NMR (75 MHz, CDCl )
3
1
3
J = 6.2 Hz, H-Ar); C NMR (75 MHz, CDCl ) δ = 121.7,
δ = 121.5, 123.1, 125.1, 126.2, 127.5, 128.9, 130.9;
3
+
1
26.3, 128.2, 129.2, 129.9, 151.5; MS(ESI): m/z = 195
MS(ESI): m/z = 212 [M + H] .
+
[M + H] .
2
‑(4‑Fluorophenyl)benzo[d]thiazole (Table 2, Entry 2)
2
‑(4‑Chlorophenyl)‑1H‑benzo[d]imidazole (Table 2, Entry
1
8)
H NMR (300 MHz, CDCl ) δ = 7.18 (t, 2H, J = 7.0 Hz,
3
H-Ar), 7.49 (t, 1H, J = 7.1 Hz, H-Ar), 7.89 (d, 1H, J = 7.6 Hz,
1
13
H NMR (300 MHz, CDCl ) δ = 7.57 (d, 1H, J = 7.6 Hz,
H-Ar), 8.09–8.05 (m, 4H, H-Ar); C NMR (75 MHz, CDCl )
3
3
H-Ar), 7.41 (d, 2H, J = 7.6 Hz, H-Ar), 7.32–7.20 (m,
δ = 115.9, 116.1, 121.5, 123.0, 125.1, 126.3, 129.3, 129.4,
+
3H, H-Ar), 7.12 (d, 1H, J = 7.6 Hz, H-Ar), 7.01 (d, 1H,
1
34.9, 153.9, 166.6; MS(ESI): m/z = 230 [M + H] .
1
3
J = 7.6 Hz, H-Ar); C NMR (75 MHz, CDCl ) δ = 121.5,
3
1
23.1, 125.1, 126.2, 127.5, 128.9, 130.9, 154.4; MS(ESI):
2
‑(4‑Bromophenyl)benzo[d]thiazole (Table 2, Entry 3)
+
m/z = 229 [M + H] .
1
H NMR (300 MHz, CDCl ) δ = 7.43–7.38 (m, 1H, H-Ar),
3
2
‑(4‑Nitrophenyl)‑1H‑benzo[d]imidazole (Table 2, Entry 9)
7
.53–7.48 (m, 1H, H-Ar), 7.65–7.61 (m, 2H, H-Ar), 7.91
(
(
d, 1H, J = 7.9 Hz, H-Ar), 7.99–7.94 (m, 2H, H-Ar), 8.08
1
H NMR (300 MHz, CDCl ) δ = 7.32–7.25 (m, 3H, H-Ar),
1
3
3
d, 1H, J = 8.1 Hz, H-Ar),; C NMR (75 MHz, CDCl )
3
13
7
.44–7.40 (m, 3H, H-Ar), 7.66–7.64 (m, 2H, H-Ar);
C
δ = 121.6, 123.2, 125.4, 126.4, 128.8, 132.1, 135.0, 154.0,
NMR (75 MHz, CDCl ) δ = 110.3, 114.6, 119.7, 122.6,
+
3
1
66.6; MS(ESI): m/z = 290 [M + 2] .
1
27.1, 128.1, 130.6, 136.0, 123.0, 153.9,158.5, 160.2;
+
MS(ESI): m/z = 240 [M + H] .
2
‑(2‑Bromophenyl)benzo[d]thiazole (Table 2, Entry 4)
2
‑(p‑Tolyl)‑1H‑benzo[d]imidazole (Table 2, Entry 10)
1
H NMR (300 MHz,CDCl ) δ = 7.56–7.30 (m, 4H, H-Ar),
3
7
8
.73 (d, 1H, J = 7.9 Hz, H-Ar), 8.01–7.94 (m, 2H, H-Ar),
1
H NMR (300 MHz, CDCl ) δ = 2.44 (s, 3H, –CH ),
1
3
3
3
.15 (d, 1H, J = 7.9 Hz, H-Ar); C NMR (75 MHz,
3
.13 (s, 1H, –NH), 7.32 (d, 4H, J = 8.1 Hz, H-Ar), 8.12
CDCl ) δ = 121.3, 123.4, 125.4, 126.2, 127.4, 131.1,
13
3
(
d, 4H, J = 8.3 Hz, H-Ar); C NMR (75 MHz, CDCl )
+
3
1
32.0, 133.9, 152.5, 165.6; MS(ESI): m/z = 290 [M + 2] .
δ = 21.0, 110.4, 120.5, 123.2, 125.7, 128.8, 129.4,
1
29.7, 132.7, 137.7, 140.3, 161.4; MS(ESI): m/z = 209
+
2
‑(2‑Methoxyphenyl)benzo[d]thiazole (Table 2, Entry 5)
[M + H] .
1
H NMR (300 MHz, CDCl ) δ = 4.06 (s, 3H, -OCH ), 7.38–
3
3
2‑(4‑Methoxyphenyl)‑1H‑benzo[d]imidazole (Table 2,
Entry 11)
7
.35 (m, 2H, H-Ar), 7.49–7.44 (m, 2H, H-Ar), 7.50 (d, 1H,
J = 1.2 Hz, H-Ar), 8.09 (d, 1H, J = 8.2 Hz, H-Ar), 8.52 (d,
13
1
H, J = 1.6 Hz, H-Ar), 8.53 (d, 1H, J = 1.8 Hz); C NMR
1
H NMR (300 MHz, CDCl ) δ = 3.73 (s, 3H, -OCH ),
3
3
(
1
75 MHz, CDCl ) δ = 111.5, 121.1, 122.7, 124.5, 125.8,
3
6.79 (d, 1H, J = 8.6 Hz, H-Ar), 6.98–6.93 (m, 3H, H-Ar),
.65–7.46 (m, 3H, H-Ar), 8.11 (d, 1H, J = 8.6 Hz, H-Ar);
+
29.4, 131.7, 152.0, 157.1; MS(ESI): m/z = 242 [M + H] .
7
1
3

J IRAN CHEM SOC
1
3
C NMR (75 MHz, CDCl ) δ = 58.4,115.1, 119.0, 123.4,
123.3, 124.1, 124.8, 126.0, 126.4, 133.7, 153.6, 163.2; MS
(ESI): m/z = 251 [M + H] .
3
+
+
1
32.9, 154.8, 164.1, 166.6; MS(ESI): m/z = 225 [M + H] .
2
1
‑(2‑Chlorophenyl)‑1H‑benzo[d]imidazole (Table 2, Entry
2)
Results and discussion
Chemistry
1
H NMR (300 MHz, CDCl ) δ = 4.92 (s, 1H, NH), 6.70
3
(
d, 1H, J = 8.3 Hz, H-Ar), 7.25 (s, 1H, H-Ar), 7.47 (s, 2H,
H-Ar), 7.93 (d, 3H, J = 5.8 Hz, H-Ar), 8.62 (s, 1H, H-Ar);
A preliminary survey of reaction conditions was conducted
with 2-amino thiophenol and benzaldehyde as a model
reaction (Table 2, entry 1). In the first case, the impact of
catalyst was tested (Table 1), and in the absence of cata-
lyst, no formation of product was obtained after stirring for
24 h (entry 1). The effect of solvents was also studied, and
it was observed that the reaction was effective in polar sol-
vents, such as H O, CH OH, CH CN, DMF, and DMSO,
13
C NMR (75 MHz, CDCl ) δ = 121.5, 123.1, 125.1, 126.2,
3
+
127.5, 128.9, 130.9, 158.2; MS(ESI): m/z = 229 [M + H] .
2
1
‑(4‑Ethoxyphenyl)‑1H‑benzo[d]imidazole (Table 2, Entry
3)
1
H NMR (300 MHz, DMSO-d ) δ = 7.59–7.55 (m, 2H,
6
2
3
3
H-Ar), 7.25–7.14 (m, 3H, H-Ar), 6.77 (d, 1H, J = 8.3 Hz,
whereas trace amount of product was observed in toluene
and THF (Table 1, entries 2–8). As indicated in Table 1,
the reaction could be progressed very efficiently in H O
(entry 2). This solvent was chosen as a green solvent for
the synthesis of benzimidazole/benzothiazole.
H-Ar), 6.92–6.88 (m, 2H, H-Ar), 4.08–3.94 (q, 2H, –CH –),
2
13
1
.43 (t, 3H, J = 7.5 Hz, –CH ); C NMR (75 MHz, DMSO-
3
2
d6): δ = 63.3, 110.3, 114.6, 119.7, 122.6, 127.1, 128.1, 130.6,
1
36.0, 143.0, 153.9, 258.5, 160.2; MS (ESI): m/z = 239
+
[M + H] .
To further improve the yield, the same reaction was car-
ried out in the presence of 5, 10, and 20 mol% of nano-
Fe O (Table 1, entries 2, 9, 10). From the obtained results
2
‑(1H‑Indol‑3‑yl)benzo[d]thiazole (Table 2, Entry 14)
2
3
shown in Table 1, it was clear that the yields depend on the
amount of catalyst and the optimum amount of catalyst was
10 mol% (entry 2). The reaction (10 mol% nano-Fe O ) in
water stirring at room temperature yileded only trace amount
of product even after prolonged reaction time (Table 1, entry
11). Thus, the optimized reaction conditions turned out to be
1
H NMR (300 MHz, CDCl ) δ = 7.48–7.27 (m, 5H, H-Ar),
3
7
.87 (d, 1H, J = 7.9 Hz, H-Ar), 7.93 (d, 1H, J = 2.8 Hz,
2
3
H-Ar), 8.02 (d, 1H, J = 8.1 Hz, H-Ar), 8.44 (d, 1H,
1
3
J = 7.1 Hz, H-Ar), 8.94 (s, 1H. –NH); C NMR (75 MHz,
CDCl ) δ = 111.7. 112.2, 120.9, 121.2, 121.7, 121.9,
3
Table 1 Optimization of the
reaction conditions
a
Entry Nanocatalyst Catalyst loading (mol%) Solvent
Time (h) Temperature (°C) Yield (%)
1
2
3
4
5
6
7
8
9
1
1
No catalyst
Fe O
–
H O
24
2
80
80
65
82
80
100
110
60
80
80
rt
0
2
10
10
10
10
10
10
10
5
H O
85
2
3
3
3
3
3
3
3
3
3
3
2
Fe O
CH OH
2
78
2
3
Fe O
CH CN
2
46
2
3
Fe O
DMF
2
39
2
Fe O
DMSO
2
35
2
Fe O
C H CH 24
Trace
Trace
76
2
6
5
3
Fe O
THF
24
2
2
Fe O
H O
2
2
0
1
Fe O
20
10
H O
2
86
2
2
Fe O
H O
24
Trace
2
2
Reaction conditions: aldehyde (1.0 mmol), 2-amino thiophenol/1,2-diamino benzene (1.0 mmol), nano-
Fe2O (10 mol%) in water (3 mL) 80 °C
3
a
Isolated yield
1
3

J IRAN CHEM SOC
Table 2 Synthesis of benzimidazoles/benzothiazoles
Reaction conditions: aldehyde (1.0 mmol), 2-amino thiophenol/1,2-diamino benzene (1.0 mmol), nano-Fe2O (10 mol%) in water (3 mL) 80 °C
3
a
Isolated yield
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J IRAN CHEM SOC
Recyclability of nano‑Fe O catalyst
2
3
Important features of the nano‑Fe O catalysts are reus-
2
3
ability, recyclability, and selectivity, which make them
green catalysts, especially considering from an environ-
mental point of view. In this regard, the life time as well
as reusability of the catalyst is verified. The recyclabil-
ity of nano-Fe O was studied using 2-amino thiophenol
2
3
and benzaldehyde as a model reaction. After completion
of reaction, the reaction mixture was cooled to room
temperature and the catalyst was recovered magnetically
and washed several times with methanol, dried at 60 °C
for 30 min. The recovered catalyst was reused directly
in the next cycle with fresh reactants, under the same
conditions, and it was reused for four times without
any change in activity (Fig. 2). The powder XRD pat-
terns of nano-Fe O catalyst before and after the reaction
Fig. 2 Recyclability of nano-Fe O
2
3
2
3
using 10 mol% nano-Fe O in water at 80 °C temperature
revealed no substantial variation was observed in the
powder XRD patterns of fresh and used catalyst, which
was revealing the retention of the original structure of
nano-Fe O in the used catalyst (Fig. 3). Further, The
2
3
for 2 h. With the optimized conditions defined, the scope of
the iron oxide catalyzed synthesis of benzimidazole/benzo-
thiazole derivatives was further expanded with a variety of
2
3
1
,2-diamino benzene/2-amino thiophenol and substituted
SEM image of the catalyst taken after the fourth cycle
of the reaction did not show any significant change in
the morphology and the size of the catalyst, which indi-
cates the retention of the catalytic activity after recycling
(Fig. 4).
aromatic aldehydes. The results are summarized in Table 2.
The aldehydes bearing either electron-withdrawing or elec-
tron-donating groups reacted satisfactorily to furnish the cor-
responding 2-substituted benzimidazole/benzothiazole.
Fig. 3 X-ray diffraction patterns of a native nano-Fe O , b nano-Fe O after fourth cycle
2
3
2
3
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J IRAN CHEM SOC
Fig. 4 SEM images of a nano-Fe O before use, b same nano-Fe O after fourth cycle
2
3
2
3
4
.
.
F. Shi, M.K. Tse, M.M. Pohl, A. Brückner, S. Zhang, M. Beller,
Angew. Chem. Int. Ed. 46, 8866 (2007)
N. Anand, K.H.P. Reddy, T. Satyanarayana, K.S.R. Rao, D.R.
Conclusion
5
In summary, we have developed a simple and efficient
protocol for the synthesis of benzimidazole/benzothia-
zole derivatives by a two-component reaction, involving
Burri, Catal. Sci. Technol. 2, 570 (2012)
6. V. Zaharia, A. Ignat, N. Palibroda, B. Ngameni, V. Kuete, C.N.
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1
,2-diamino benzene/2-amino thiophenol and substituted
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aromatic aldehydes catalyzed by recyclable nano-Fe O3
2
in water without using any toxic solvent or co-catalyst.
This simple and novel methodology will be useful to green
chemistry with some advantage that the reaction excludes
nontoxic, moisture-sensitive or hazardous catalysts and ele-
vated reaction temperatures, longer reaction times, and the
catalyst nano-Fe O is economically viable, readily avail-
able, easily handling. Moreover, this nanocatalyst can be
recycled and reused for four times without loss of catalytic
activity.
8. A. Kamal, M.N.A. Khan, K.S. Reddy, K. Rohini, Bioorg. Med.
Chem. 15, 1004 (2007)
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Acknowledgements The author DK is thankful to CSIR New Delhi,
India, for the award of research fellowships. The authors thank CSIR
New Delhi for financial support as a part of XII five year plan pro-
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