Bismuth(III)-catalyzed rapid and highly efficient synthesis of 2-aryl-1-arylmethyl-1H-benzimidazoles in water

Article abstract of DOI:10.1139/V07-140

2-Aryl-1-arylmethyl-1H-benzimidazoles are prepared in high yields under extremely mild conditions via the condensation of aryl-1,2-diamines with aromatic aldehydes using 10 m.ol% of bismuth triflate in water. The use of water and readily available bismuth

Full text of DOI:10.1139/V07-140

1
24
Bismuth(III)-catalyzed rapid and highly efficient
synthesis of 2-aryl-1-arylmethyl-1H-
benzimidazoles in water
J.S. Yadav, B.V. Subba Reddy, K. Premalatha, and K. Shiva Shankar
Abstract: 2-Aryl-1-arylmethyl-1H-benzimidazoles are prepared in high yields under extremely mild conditions via the
condensation of aryl-1,2-diamines with aromatic aldehydes using 10 mol% of bismuth triflate in water. The use of wa-
ter and readily available bismuth triflate makes this process quite simple, more convenient, and environment-friendly.
Key words: aryl-1,2-diamines, aromatic aldehydes, bismuth catalysis, benzimidazoles.
Résumé : On a préparé des 2-aryl-1-arylméthyl-1H-benzimidazoles avec des rendements élevés, sous des conditions
extrêmement douces, en faisant appel à une condensation d’aryl-1,2-diamines avec des aldéhydes aromatiques en utili-
sant 10 moles pour cent de triflate de bismuth dans l’eau. L’utilisation de l’eau et la grande accessibilité du triflate de
bismuth font que ce procédé est simple, très commode et environnementalement acceptable.
Mots-clés: aryl-1,2-diamines, aldéhydes aromatiques, catalyse par le bismuth, benzimidazoles.
[
Traduit par la Rédaction] Yadav et al. 128
Introduction
unsatisfactory yields, harsh reaction conditions, expensive
reagents, tedious work-up procedures, co-occurrence of sev-
eral side reactions, and poor selectivity. Since benzimidazole
derivatives are useful and important in the field of drugs and
pharmaceuticals, the development of simple, convenient,
high yielding, and environmentally benign protocols is desir-
able. Therefore, the search continues for a better catalyst for
the synthesis of benzimidazoles, in terms of operational sim-
plicity, economic viability, and selectivity.
Benzimidazoles are important biologically active
heterocycles possessing selective neuropeptide YY1 receptor
antagonists (1) and as 5-lipoxygenase inhibitors for use as
novel antiallergic agents (2), 5-HT3 antagonists in isolated
guinea pig ileum (3), poly(ADP-ribose) polymerase (PARP)
inhibitors (4), and factor Xa (Fxa) inhibitors (5). In addition,
they exhibit significant activity against several viruses, such
as HIV, herpes (HSV-1), RNA influenza, and human cyto-
megalovirus (HCMV) (6). Benzimidazole derivatives have
found commercial applications in veterinarian medicine as
anti-helmintic agents and in diverse human therapeutic ar-
eas, such as treatment of ulcers and as anti-histamines (7).
Medicinal chemists would certainly classify benzimidazoles
as privileged sub-structures for drug design, as they display
affinity towards a variety of enzymes and protein receptors
Recently, great attention has been focused on the use of
water as green solvent for various organic transformations
(
12). In addition to its abundance and for economical and
safety reasons, water has naturally become a substitute and
an alternative environmentally benign solvent in organic
synthesis. This becomes further sophisticated if these reac-
tions can be performed using inexpensive and readily avail-
able reagents. Lanthanide triflates are unique Lewis acids
that are currently of great interest (13). The high catalytic
activity, low toxicity, moisture and air tolerance, and their
recyclability make lanthanide triflates attractive alternatives
to conventional Lewis acids (14). However, lanthanide
triflates are rather expensive, and their use in large-scale
synthesis is limited. Therefore, cheaper and more efficient
catalysts are desirable. In this direction, bismuth triflate has
evolved as remarkable Lewis acid catalyst for effecting vari-
ous organic transformations (15). Compared with lanthanide
triflates, bismuth triflate is cheap and is easy to prepare,
even on a multi-gram scale, from commercially available
bismuth oxide and triflic acid (16). The use of bismuth
triflate for the synthesis of benzimidazoles has recently been
reported under solvent-free conditions (17).
(
8). Consequently, a variety of methods have been developed
for the preparation of substituted benzimidazoles (9). Of
these, one of the most traditional methods involves the con-
densation of an o-phenylenediamine with carboxylic acid or
its derivatives (10). Subsequently, several improved proto-
cols have been developed for the synthesis of benzimi-
dazoles via the condensation of o-phenylenediamines with
aldehydes in the presence of acid catalysts under various re-
action conditions (11). However, many of these methods suf-
fer from certain drawbacks, including longer reaction times,
Received 23 September 2007. Accepted 20 November 2007.
Published on the NRC Research Press Web site at
canjchem.nrc.ca on 4 January 2008.
1
J.S. Yadav, B.V. Subba Reddy, K. Premalatha, and
K.S. Shankar. Division of Organic Chemistry, Indian
Institute of Chemical Technology, Hyderabad 500 007, India.
Results and discussion
Inspired by recent reports on the use of Bi(OTf) in aque-
ous conditions (18) and our interest on the catalytic use of
3
1Corresponding author (e-mail: yadav@iict.res.in).
Can. J. Chem. 86: 124–128 (2008)
doi:10.1139/V07-140
© 2008 NRC Canada

Yadav et al.
125
Scheme 1.
CHO
NH2
10 mol% Bi(OTf)3
N
N
+
H O; RT
2
NH2
1
2
3a
Bi(OTf) (19), we herein report a mild and rapid synthesis of
ous acids, environmentally unfavorable organic solvents,
and harsh reaction conditions, which makes it useful and en-
vironment-friendly for the synthesis of biologically relevant
benzimidazole scaffolds.
3
2
-aryl-1-arylmethyl-1H-benzimidazoles using a catalytic
amount of Bi(OTf) in water. Accordingly, treatment of o-
3
phenylenediamine (1) with benzaldehyde (2) in the presence
of 10 mol% of Bi(OTf) in water resulted in the formation
3
of benzimidazole (3a) in 97% yield (Scheme 1).
Experimental
This result provided the incentive for further study of re-
actions with various substituted aryl 1,2-diamines and aryl
aldehydes. Interestingly, several mono- and di-substituted
aryl 1,2-diamines reacted efficiently with aldehydes to give
the corresponding benzimidazoles (Table 1). The present
protocol is effective for aromatic aldehydes bearing either
electron-rich or electron-deficient substituents on the
aromatic ring (Table 1, entries b–h). Similarly, α-
naphthaldehyde (Table 1, entry j) and heteroaromatic alde-
hydes (Table 1, entries k–m) also reacted well with o-
Melting points were recorded on Buchi R-535 apparatus
and are uncorrected. IR spectra were recorded on a
PerkinElmer FTIR 240-c spectrophotometer using KBr op-
1
tics. H NMR spectra were recorded on Gemini 200 and
Varian Bruker 300 spectrometers in CDCl using TMS as in-
3
ternal standard. Mass spectra were recorded on a Finnigan
MAT 1020 mass spectrometer operating at 70 eV. TLC was
monitored on 0.25 mm pre-coated silica-gel plates (60F-
2
54).
phenylenediamine
to
furnish
the
corresponding
benzimidazoles in good yields. In all cases, the reactions
proceeded rapidly at room temperature with high efficiency
and were complete within a short time. The products were
General procedure
A mixture of the 1,2-diamine (1.0 mmol), aldehyde
(2 mmol), and Bi(OTf)3 (10 mol%) in water (2 mL) was
stirred at room temperature for the specified time (see Ta-
ble 1). After completion of the reaction as indicated by TLC,
the reaction mixture was extracted with ethyl acetate (2 ×
1
13
characterized by H, C NMR, IR, and mass spectroscopy
and also by comparison with authentic samples (11). Unlike
reported methods, the present protocol does not require
harsh or drastic conditions to produce benzimidazoles. The
effects of various metal triflates, such as Cu(OTf) , In(OTf) ,
1
0 mL). Evaporation of the solvent, followed by purification
on silica gel (Merck, 100–200 mesh, ethyl acetate/hexane
1:5)), afforded pure benzimidazole.
2
3
and Ce(OTf) were screened. Of these catalysts, bismuth(III)
3
(
triflate was found to be the most effective in terms of con-
version and reaction rates. Interestingly, 10 mol% of
1
-Benzyl-2- phenyl-1H-benzo[d]imidazole (3a)
Sc(OTf) was found to be an equally effective for this con-
3
Solid; mp 139–141 °C. IR (KBr) νmax: 3031, 2947, 1452,
version. In the absence of catalyst, the reaction did not pro-
ceed in water even after long reaction times (12–24 h).
–
1 1
1
5
7
8
348, 1162, 967, 697 cm . H NMR (200 MHz, CDCl ) δ:
3
.45 (s, 2H), 7.08–7.12 (m, 3H), 7.21–7.30 (m, 5H), 7.35–
Compared with conventional Lewis acids, Bi(OTf) is partic-
3
.44 (m, 3H), 7.62 (dd, J = 8.0 and 2.0 Hz, 2H), 7.84 (d, J =
.0 Hz, 1H). EI-MS m/z (%): 284 (M , 45), 156 (10), 142
ularly attractive because it can be used in water (18). Since a
+
water solution of Bi(OTf) is acidic (16), it may be possible
3
(
30), 92 (100).
that the true catalyst is TfOH produced from the hydrolysis
of Bi(OTf) . However, TfOH is not as effective as Bi(OTf)
3
3
to catalyze the condensation of o-phenylenediamine with
benzaldehyde (10% TfOH at 25 °C, 3 h, 65%), thus suggest-
ing that a Lewis acid is likely involved in activating the alde-
hyde. Furthermore, the recovered aqueous solution of
1-(4-Methylbenzyl)-2-(4-methylphenyl)-1H-
benzo[d]imidazole (3b)
Solid; mp 128–130 °C. IR (KBr) νmax: 3024, 2917, 2861,
–
1 1
1411, 1249 cm . H NMR (200 MHz, CDCl ) δ: 2.34 (s,
3
Bi(OTf) could be reused in further reactions with gradual
3H), 2.40 (s, 3H), 5.37 (s, 2H), 6.96 (d, J = 8.1 Hz, 2H),
7.11–7.26 (m, 6H), 7.55 (d, J = 8.3 Hz, 2H), 7.79 (d, J =
3
decrease in activity. For instance, treatment of o-
phenylenediamine with benzaldehyde over 10 min gave
+
7.9 Hz, 2H), EI-MS m/z (%): 312 (M , 92), 208 (26), 105
9
7%, 85%, and 71% yield, respectively, over three cycles.
(100), 77 (10).
This observation clearly shows the reusability of the cata-
lyst. The scope and generality of this process is illustrated
with respect to various aryl 1,2-diamines and aldehydes, and
the results are summarized in Table 1.
In conclusion, we have described a mild, convenient, and
efficient protocol for the synthesis of highly substituted
benzimidazoles using bismuth(III) triflate as the novel cata-
lyst. This method not only offers substantial improvements
in reaction rates and yields but also avoids the use of hazard-
1
-(4-Methoxybenzyl)-2-(4-methoxyphenyl)-1H-
benzo[d]imidazole (3c)
Solid; mp 121–123 °C. IR (KBr) ν : 3064, 3043, 2959,
max
–
1
1
2943, 1604, 1452, 1243, 750 cm . H NMR (200 MHz,
CDCl ) δ: 3.65 (s, 3H), 3.85 (s, 3H), 5.38 (s, 2H), 6.78–6.85
(m, 3H), 6.95–7.05 (m, 3H), 7.08–7.35 (m, 4H), 7.62 (d, J =
8.0 Hz, 2H). EI-MS m/z (%): 344 (M , 64), 329 (6), 299
(30), 223 (100), 91 (64).
3
+
©
2008 NRC Canada

1
26
Can. J. Chem. Vol. 86, 2008
Table 1. Bi(OTf) -catalyzed synthesis of N-substituted benzimidazoles in water.
3
©
2008 NRC Canada

Yadav et al.
127
1
-(4-Hydroxybenzyl)-2-(4-hydroxyphenyl)-1H-
(m, 4H), 7.46–7.53 (m, 2H), 7.82–7.85 (m, 1H). EI-MS m/z
(%): 296 (M , 78), 172 (6), 109 (2), 97 (100), 77 (6).
+
benzo[d]imidazole (3d)
Solid; mp 111–113 °C. IR (KBr) ν : 3420, 3089, 3045,
max
–
1
1
2
955, 1605, 1455, 1245, 750 cm . H NMR (200 MHz,
2-(Furan-2-yl)-1-((furan-2-yl)methyl)-1H-
benzo[d]imidazole (3l)
CDCl ) δ: 5.28 (s, 2H), 6.87–6.90 (m, 3H), 7.02–7.08 (m,
3
3
H), 7.18–7.25 (m, 4H), 7.78 (d, J = 8.0 Hz, 2H). ESI-MS
Solid; mp 88–89 °C. IR (KBr) νmax: 3378, 3150, 1605,
–
1 1
m/z (%): 317 (M + 1).
1332, 1007, 735 cm . H NMR (200 MHz, CDCl ) δ: 5.65
3
(
s, 2H), 6.18 (d, J = 7.7 Hz, 1H), 6.24–6.26 (m, 1H), 6.59–
1
-(4-Fluorobenzyl)-2-(4-fluorophenyl)-1H-
6.61 (m, 1H), 7.17–7.32 (m, 4H), 7.40–7.44 (m, 1H), 7.61–
7.62 (m, 1H), 7.71–7.74 (m, 1H). EI-MS m/z (%): 264 (M ,
80), 141 (20), 81 (100), 53 (30).
+
benzo[d]imidazole (3e)
Solid; mp 110–112 °C. IR (KBr) νmax: 3032, 2935, 2835,
–
1 1
1
5
7
465, 1423, 1078, 750 cm . H NMR (200 MHz, CDCl ) δ:
3
.45 (s, 2H), 6.92–7.18 (m, 4H), 7.21–7.32 (m, 5H), 7.62–
2-(Pyridine-2-yl)-1-(pyridine-2-yl)methyl)-1H-
benzo[d]imidazole (3m)
.78 (m, 3H). ESI-MS m/z (%): 321 (M + 1).
Solid; mp 128–123 °C. IR (KBr) νmax: 3055, 2993 1675,
–
1 1
1
-(4-Chlorobenzyl)-2-(4-chlorophenyl)-1H-
1442, 1350, 1256, 1135 cm . H NMR (200 MHz, CDCl3)
δ: 5.32 (s, 2H), 7.03–7.10 (m, 2H), 7.25–7.35 (m, 5H),
7.45–7.53 (m, 3H), 7.82–7.85 (m, 2H). ESI-MS m/z (%):
287 (M + 1).
benzo[d]imidazole (3f)
Solid; mp 137–139 °C. IR (KBr) νmax: 3038, 2923, 2853,
–
1 1
473, 1439, 1091 cm . H NMR (200 MHz, CDCl ) δ: 5.39
3
1-Benzyl-2-phenyl-1H-benzo[d]imidazole-5-
yl)(phenyl)methanone (3n)
+
7
.5 Hz, 2H). EI-MS m/z (%): 352 (M , 52), 227 (4), 192
(
30), 125 (100), 90 (13).
Solid; mp 195–197 °C. IR (KBr) νmax: 3053, 2995, 1673,
–
1 1
1
453, 1350, 1236, 1145 cm . H NMR (200 MHz, CDCl )
3
1
-(4-Bromobenzyl)-2-(4-bromophenyl)-1H-
δ: 5.50 (s, 2H), 7.05–7.12 (m, 2H), 7.21–7.39 (m, 5H),
benzo[d]imidazole (3g)
7.41–7.52 (m, 5H), 7.62–7.75 (m, 3H), 7.81–7.90 (m, 2H),
8
.25 (d, J = 7.8 Hz, 1H). ESI-MS m/z (%): 389 (M + 1).
Solid; mp 160–162 °C. IR (KBr) νmax: 3045, 2928, 1569,
–
1 1
1
2
468, 1091 cm . H NMR (200 MHz, CDCl ) δ: 5.42 (s,
3
H), 7.01 (d, J = 7.8 Hz, 2H), 7.15–7.35 (m, 3H), 7.42–7.59
1-Benzyl-5-methyl-2-phenyl-1H-benzo[d]imidazole (3o)
(
(
m, 6H), 7.78 (d, J = 7.8 Hz, 1H). ESI-MS m/z (%): 441
M + 1).
Solid; mp 170–172 °C. IR (KBr) ν : 3062, 2954, 1670,
max
–
1
1
1441, 1355, 1265, 1150, 750 cm . H NMR (200 MHz,
CDCl ) δ: 3.0 (s, 3H), 5.45 (s, 2H), 7.05–7.12 (m, 3H),
3
7
.28–7.33 (m, 4H), 7.43–7.51 (m, 4H), 7.59–7.61 (m, 2H).
1
-(4-Nitrobenzyl)-2-(4-nitrophenyl)-1H-
ESI-MS m/z (%): 299 (M + 1).
benzo[d]imidazole (3h)
Solid; mp 175–177 °C. IR (KBr) νmax: 3043, 2978, 2240,
–
1 1
1
(
(
470, 1352, 1188 cm . H NMR (200 MHz, CDCl ) δ: 5.45
1-Benzyl-5,6-dimethyl-2-phenyl-1H-
benzo[d]imidazole (3p)
3
s, 2H), 7.26–7.32 (m, 4H), 7.70–7.74 (m, 4H), 8.07–8.25
m, 4H). ESI-MS m/z (%): 375 (M + 1).
Solid; mp 187–189 °C. IR (KBr) νmax: 3422, 2919, 1448,
–
1
1
1
6
1
2
314, 697 cm . H NMR (200 MHz, CDCl ) δ: 2.35 (s,
3
H), 5.41 (s, 2H), 7.28–7.49 (m, 10H), 7.65 (d, J = 7.7 Hz,
H), 8.12 (d, J = 7.7 Hz, 1H). EI-MS m/z (%): 312 (M , 90),
22 (15), 141 (30), 119 (35), 91 (100).
2
-Cyclohexyl-1-(cyclohexyl methyl)-1H-
benzo[d]imidazole (3i)
+
Solid; mp 82–84 °C. IR (KBr) νmax: 3743, 3042, 2926,
–
1 1
1
0
2
581, 1454, 1344, 742 cm . H NMR (200 MHz, CDCl ) δ:
3
.995–2.0 (m, 22H), 3.88 (d, J = 7.8 Hz, 2H), 7.05–7.20 (m,
H), 7.68–7.75 (m, 2H). EI-MS m/z (%): 296 (M , 15), 241
4-(2-(4-(Dimethylamino)phenyl)-1H-benzo[d]imidazole-
1-yl)methyl-N,N-dimethylbenzamine (3q)
+
(
10), 213 (20), 141 (40), 83 (15), 41 (100).
Solid; mp165–167 °C. IR (KBr) νmax: 3062, 2954, 1670,
–
1
1
1
441, 1355, 1265, 1150, 750 cm . H NMR (200 MHz,
CDCl ) δ: 2.93 (s, 6H), 3.00 (s, 6H), 5.37 (s, 2H), 6.66–6.75
2
-(Napthalene-3yl)-1-(napthlen-3-yl)methyl)-1H-
3
(
m, 4H), 7.14–7.29 (m, 3H), 7.63–7.65 (m, 4H), 7.78 (d, J =
.9 Hz, 1H). ESI-MS m/z (%): 371 (M + 1).
benzo[d]imidazole(3j)
7
Solid; mp 122–124 °C. IR (KBr) νmax: 3075, 2955, 1443,
–
1 1
1
5
7
348, 1242, 1178, 968 cm . H NMR (200 MHz, CDCl ):
3
.65 (s, 2H), 7.21–7.35 (m, 4H), 7.42–7.52 (m, 5H), 7.69–
.75 (m, 2H), 7.81–7.98 (m, 6H), 8.18–8.22 (m, 1H). ESI-
Acknowledgements
MS m/z (%): 385 (M + 1).
KPL and KSS thank CSIR, New Delhi for the fellowship
awards.
2
-(Thiophen-2yl)-1-(thiophen-2yl)methyl)-1H-
benzo[d]imidazole (3k)
References
Solid; mp 150–152 °C. IR (KBr) νmax: 3095, 3072, 2929,
–
1 1
1
449, 1421, 1219 cm . H NMR (200 MHz, CDCl ): 5.71
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©
2008 NRC Canada

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2008 NRC Canada