Synthesis of 1,2-disubstituted benzimidazoles and 2-substituted benzothiazoles catalyzed by HCl-treated trans-3,5-dihydroperoxy-3,5-dimethyl-1, 2-dioxolane

Article abstract of DOI:10.3987/COM-10-12068

The catalytic effect of trans-3,5-dihydroperoxy-3,5-dimethyl-1,2-dioxolane in the presence of HCl has been explored in one-pot condensation reaction of o-phenylenediamines and o-aminobenzenethiol with a variety of aldehydes into their corresponding 2-aryl

Full text of DOI:10.3987/COM-10-12068

HETEROCYCLES, Vol. 81, No. 12, 2010
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HETEROCYCLES, Vol. 81, No. 12, 2010, pp. 2855 - 2863. © The Japan Institute of Heterocyclic Chemistry
Received, 18th September, 2010, Accepted, 1st November, 2010, Published online, 2nd November, 2010
DOI: 10.3987/COM-10-12068
SYNTHESIS OF 1,2-DISUBSTITUTED BENZIMIDAZOLES AND 2-
SUBSTITUTED BENZOTHIAZOLES CATALYZED BY HCl-TREATED
TRANS-3,5-DIHYDROPEROXY-3,5-DIMETHYL-1,2-DIOXOLANE
Davood Azarifar,* Kaveh Khosravi, Zohreh Najminejad, and Khadijeh
Soleimani
Department of Chemistry, Buali Sina University, Zip Code 65178, Hamedan, Iran.
E-mail: azarifar@basu.ac.ir
Abstract – The catalytic effect of trans-3,5-dihydroperoxy-3,5-dimethyl-1,2-
dioxolane in the presence of HCl has been explored in one-pot condensation
reaction of o-phenylenediamines and o-aminobenzenethiol with a variety of
aldehydes into their corresponding 2-aryl-1-arylmethylimidazoles and 2-
arylbenzothiazoles respectively. The reactions were conducted under mild
conditions in MeCN at room temperature to afford the products in excellent
yields.
Benzimidazole and benzothiazole nuclei are of significant importance to medicinal industry.1-4 Many
benzimidazole and benzothiazole derivatives exhibit vital biological activities as neuropeptide YY1
receptor antagonism,5 5-lipoxygenase inhibitor for use as antiallergic agents,6 factor Xa (FXa) inhibitors,7
and poly (ADP-ribose) polymerase (PARP) inhibitors.8 In addition, these classes of heterocyclic
systems have found commercial applications in several therapeutic area as antiulcer, antihypertensive,
antimicrobial,9 antituberculosis,10 antifungal,11 antitumor,12 and antihistamine agents,13 as well as agents
in treatment of interstitial cystitis.14 Also, these compounds have found industrial uses as antioxidants,15
vulcanization accelerators,16 fluorescents,17 photochromic agents,18 and are considered as very important
intermediates in organic reactions.19 Therefore, medicinal and industrial chemists consider these
benzimidazole- and benzothiazole-containing heterocycles to be promising compounds and pay
considerable attention towards the development of efficient approaches to these compounds.
Almost all the existing methods for the synthesis of these heterocycles including the condensation of
carboxylic acids,20 acid chlorides,21 nitriles,22 orthoesters,23 amids,24 esters,25 or aldehydes26 with
o-aminobenzenethiols and o-phenylenediamines (OPD) suffer from certain synthetic shortcomings such

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HETEROCYCLES, Vol. 81, No. 12, 2010
as rigid conditions, poor yields, co-occurrence of several side reactions, use of toxic and costly catalysts
and/or solvents.
Other synthetic routes employ microwave-assisted reaction of o-aminobenzenethiol with β-chloro
cinnamaldehydes,27 palladium-catalyzed direct coupling of benzothiazoles with arylhalides,28
palladium-catalyzed condensation of arylhalides with o-aminobenzenethiol followed by dehydrative
cyclization,16 and reaction of copper(I) thiobenzoate with 2-iodoanilines.15 In recent years, several simple
one-step procedures have been developed for the synthesis of benzothiazoles and benzimidazoles using
various catalytic systems.29 However, these methods also suffer from the above-mentioned drawbacks.
Indeed, the development of new and more benign synthetic methods is still in high demand. Bahrami et
al.,30 have utilized H2O2 in HCl in oxidative cyclocondensation of phenylenediamines with aldehydes to
synthesize benzimidazoles. Mamedov et al.,31 have reported a novel synthesis of benzimidazoles from
quinoxalinones and arylenediamines. Recently, use of silica gel-supported heteropoly acid,32 and
silica-supported ZnCl2,33 in the synthesis of both benzothiazoles and benzimidazoles have been reported.
Polymer-bound esters,34 and PEG-mediated systemes35 have received considerable interest in the
synthesis of benzothiazoles and benzimidazoles.
In continuation of our studies to explore new and cleaner approaches towards the heterocyclic
compounds,36,37 and also the synthesis and applications of gem-dihydroperoxides as versatile and high
potent oxygen transfer agents in various organic transformations,38-40 we report herein the novel use of
trans-3,5-dihydroperoxy-3,5-dimethyl-1,2-dioxolane in combination with HCl to effect the one-pot
condensation of o-phenylenediamines and o-aminobenzenethiol with aldehydes into corresponding 2-
aryl-1-arylmethylbenzimidazoles and 2-arylbenzothiazoles respectively. Trans-3,5-dihydroperoxy-3,5-
dimethyl-1,2-dioxolane has been prepared based on our previously reported method,51 as a white
crystalline solid in high yield (85%) from the SnCl2.2H2O-catalyzed reaction of acetylacetone with
aqueous (30%) hydrogen peroxide (Scheme 1).
The reactions proceeded almost rapidly under mild conditions at room temperature in acetonitrile to
convert the aldehydes 2 into 2-aryl-1-arylmethylbenzimidazoles 3 (Table 1) and 2-arylbenzothiazoles 4
(Table 2) respectively in high to excellent yields (Scheme 2).

HETEROCYCLES, Vol. 81, No. 12, 2010
2857
R
SH
NH2
R
N
N
N
S
NH2
NH2
Ar
Ar
ArCHO
O
O
O
O
Me
OOH
Me
HOO
OOH
Me
/ HCl
/ HCl
HOO
Me
2
Ar
4a-p
MeCN, rt
3a-q (R = H, Me)
MeCN, rt
Scheme 2
In order to establish the conditions for the titled reactions, we initially examined a model reaction
choosing o-phenylenediamine (1 mmol)/or o-aminobenzenethiol (1 mmol) and 4-chlorobenzaldehyde (2.2
mmol) (entry 2) as test compounds. We also examined different solvents such as EtOH, Et2O, CH2Cl2,
hexane, MeCN, 1,4-dioxane and DMF. It was found that, the best yields for the corresponding
benzimidazole (96%) and benzothiazole (94%) are obtained when the reactions were conducted under
stirring at room temperature in acetonitrile as the solvent of choice. Use of the aldehyde in slightly excess
amount (10%) was found useful to prevent the formation of 2-arylbenzimidazole as the probable side
product.39 Stirring for further prolonged times or the use of excess amount of the reagent 1 (1.2 mmol) in
the reaction resulted in comparable or in some cases reduced yields of the products.
To develop the scope of the reaction, various other aldehydes carrying different substituents were
subjected to condensation with o-phenylenediamine or o-aminobenzenethiol under the optimized reaction
conditions (stirring at room temperature in MeCN). The experimental results are summarized in Tables 1
and 2. Almost all the reactions proceeded smoothly with high selectivity in relatively short reaction times
to afford the corresponding 2-aryl-1-arylmethyl-1H-benzimidazoles 3 and 2-aryl-1H-benzothiazoles 4
respectively in 98-88% and 96-85% yields.
It is important to note that, trans-3,5-dihydroperoxy-3,5-dimethyl-1,2-dioxolane operates quite safely in
these reactions with no explosion as long as the reactions are conducted at room temperature. Also, this
compound tends to smoothly decompose on standing at normal temperatures. However, like many other
peroxidic compounds, trans-3,5-dihydroperoxy-3,5-dimethyl-1,2-dioxolane is expected to be potentially
explosive. Therefore, as mentioned in the experimental section, special precautions in handling of this
compound are required, particularly heating to higher temperatures should be avoided.
A possible mechanism to explain the formation of the 1,2-disubstituted benzimidazoles 3 from o-
phenylenediamine and aldehydes 2 under the promotion of the oxidant 1 in the presence of HCl is
depicted in Scheme 3. According to our previous report,37 and as suggested by Zelenin et al.,41 the
reaction probably occurs with the formation of diarylidene-o-phenylenediamine 5, followed successively
by cyclization and 1,3-hydride transfer. As shown in this Scheme, the first condensation step to yield the
diarylidene-o-phenylenediamine 5 intermediate is probably catalyzed by the stronger Cl+ ion which is

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HETEROCYCLES, Vol. 81, No. 12, 2010
likely generated in situ by the effect of HCl on trans-3,5-dihydroperoxy-3,5-dimethyl-1,2-dioxolane 1.
This is confirmed by conducting the reaction with benzaldehyde at room temperature in the absence of
the oxidant 1 which resulted in very low yield (24%) of the expected product 3a after 6 h stirring at room
temperature. However, a separately prepared diarylidene-o-phenylenediamine from the reaction of o-
phenylenediamine with two equimolar amount of benzaldehyde in AcOH at 100 ºC, was rapidly cyclized
to 3a (94%) under a catalytic amount of HCl. This can be indicative that the Cl+ ion is most likely
involved as a much stronger Lewis acid in the first condensation step to provide the intermediate 5.
Cl-
HO
O
O
H3O+
O
O
O
Cl- + H3O+
- ClOH
O
Me
O OH
Me
H2O2
+
Me
- ClOH
HOO
HCl
- H2O
Me
Me
Cl+OH-
Me
1
HOO
Cl
H+
N
H
O+ OH-
Ar
R
R
R
N
N
N
R
1,3-H shift
NH2
NH2
Ar
Ar
Ar
Ar
H
H
N
N
3
Ar
5
Ar
Scheme 3
Similarly, the same in situ generation of Cl+ ion is probably responsible for the activation of the
aldehydes in the first condensation step with o-aminobenzenethiol to yield an intermediate arylidene-o-
mercaptophenylamine 6. Successive cyclization of the intermediate 6 in the presence of HCl followed by
oxidation affords the expected products 3 (Scheme 4).45,46
Cl-
HO
O
O
H3O+
O
O
O
Cl - + H3O+
- ClOH
O
Me
O OH
Me
H2O2
+
Me
- ClOH
HOO
Me
Me
S
HCl
Cl+OH-
Me
1
HOO
SH
- H2O
Cl
O+ OH-
S
SH
NH2
Ar
[O]
Ar
H
Ar
H
N
N
H
Ar
N
4
H+
6
Scheme 4

HETEROCYCLES, Vol. 81, No. 12, 2010
2859
Table 1. Trans-3,5-dihydroperoxy-3,5-dimethyl-1,2-dioxolane-promoted synthesis of 2-aryl-1-
arylmethyl-1H-benzimidazoles 3a-r in HCla
Entry
Productb
Ar
R
Time
(min)
Yieldc (%)
Mp (°C)
Found
Reported
1
2
3a
3b
C6H5
H
20
30
96
94
130-132
135-137
133-13542
13744
15544
125-12742
187-18843
129-13142
4-ClC6H4
H
H
3
4
3c
2-ClC6H4
4-MeC6H4
28
25
90
95
154-156
126-128
3d
H
H
H
5
6
3e
3f
4-NCC6H4
24
21
90
93
185-187
132-134
4-MeOC6H4
7
3g
3h
3i
2-MeOC6H4
4-HOC6H4
2-HOC6H4
4-Me2NC6H4
4-O2NC6H4
2-O2NC6H4
4-NCC6H4
2-Furyl
H
24
25
27
32
32
30
25
42
28
30
27
94
90
88
87
91
90
88
85
93
90
90
152-154
256-258
210-212
154-156
190-192
116-118
188-190
96-98
15144
8
H
254-25643
207-20843
152-15542
19244
9
H
10
11
12
13
14
15
16
17
3j
H
3k
3l
H
H
11844
3m
3n
3o
3p
3q
H
187-18843
9644
H
C6H5
Me
Me
Me
188-190
160-162
182-184
New
4-MeC6H4
4-MeOC6H4
New
New
a Conditions: aldehyde 2 (2.2 mmol), o-phenylenediamine (1 mmol), reagent 1 (1 mmol), MeCN (8 mL),
HCl (3 mmol), room temperature.
b All the products are known.42-44
c Isolated yield.
Table 2. Trans-3,5-dihydroperoxy-3,5-dimethyl-1,2-dioxolane-promoted synthesis of 2-aryl-1H-
benzothiazoles 4a-p in HCla
Entry
Productb
Ar
Time
(min)
Yieldc (%)
Mp (°C)
Reported36
Found
1
2
4a
4b
C6H5
15
94
90
111-113
110-112
116-117
4-ClC6H4
17
22
114-116
3
4
4c
2-ClC6H4
4-MeC6H4
88
92
84-86
87-89
81-83
85-87
4d
16
5
6
4e
4f
2- MeC6H4
17
18
90
92
50-52
52-54
4-MeOC6H4
120-122
119-121
7
4g
4h
4i
2-MeOC6H4
4-HOC6H4
2-HOC6H4
2-O2NC6H4
3-O2NC6H4
4-NCC6H4
4-BrC6H4
20
22
21
14
16
13
13
17
91
89
88
89
91
88
92
90
100-102
228-230
126-128
131-133
180-182
163-165
128-130
82-84
99-102
8
227-228
122-124
133-135
182-184
165-166
129-131
84-86
9
10
11
12
13
14
4j
4k
4l
4m
4n
3-BrC6H4
15
16
4o
4p
4-FC6H4
2-Furyl
13
21
90
87
100-102
101-103
98-100
100-103
a Conditions: aldehyde 2 (1 mmol), o-aminobenzenethiol (1 mmol), reagent 1 (1 mmol), MeCN (8
mL), HCl (3 mmol), room temperature.
bAll the products are known.36
c Isolated yield.

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HETEROCYCLES, Vol. 81, No. 12, 2010
In summary, trans-3,5-dihydroperoxy-3,5-dimethyl-1,2-dioxolane in combination with HCl has been
explored as a highly selective and efficient oxygen releasing reagent to catalyze the condensation
reactions of o-phenylenediamines and o-aminobenzenethiol with aromatic aldehydes. The reactions
proceed under mild conditions at room temperature in acetonitrile to afford the corresponding 2-aryl-1-
arylmethyl-1H-benzimidazoles and 2-aryl-1H-benzothiazoles respectively in high to excellent yields.
EXPERIMENTAL
Chemicals used in this work were purchased from Fluka and Merk chemical companies and used without
1
purification. IR spectra were recorded on a Perkin Elmer GX FT IR spectrometer from KBr pellets. H
13
and C NMR spectra were measured for samples in CDCl3 with a JEOL FX 90Q instrument at 90 and
22.5 MHz respectively, using Me4Si as internal standard. Melting points were measured on a SMPI
apparatus.
Caution: Although we did not encounter any problem with trans-3,5-dihydroperoxy-3,5-dimethyl-1,2-
dioxolane 1, it is potentially explosive and should be handled with precautions; all reactions should be
carried out behind a safety shield inside a fume hood and transition metal salts or heating should be
avoided.
General procedure for the synthesis of 2-aryl-1-arylmethyl-1H-benzimidazoles 3a-q:
To a mixture of o-phenylenediamine (1 mmol) and aldehyde 2 (2.2 mmol) in MeCN (8 mL), was added
trans-3,5-dihydroperoxy-3,5-dimethyl-1,2-dioxolane 1 (1 mmol). Then 37% aqueous HCl (3 mmol) was
added and the resulting mixture was stirred at room temperature for an appropriate time (Table 1). After
completion of the reaction as monitored by TLC (n-hexane/EtOAc; 2:1), 3M Na2SO3 (1 mL) was added
to quench the reaction and stirring was let to continue for a further 15 min. Then, the reaction mixture was
diluted with distilled water (15 mL) and filtered to leave a solid which was crystallized from EtOH (96%)
to obtain pure product 3 (Table 1). All the products were characterized by their physical and spectral (IR,
1H NMR and 13C NMR) analysis and compared with the reported data.42-44
1-Benzyl-5-methyl-2-phenyl-1H-1,3-benzimidazole (3o)
º
Mp 188-190 C; IR (KBr) (νmax/cm-1): 3051, 2926, 1594, 1549, 1502, 1448, 13.71, 1060, 711, 694. MS,
m/z (%): 298 (M+). 1H NMR (90 MHz, CDCl3): δH 2.44 (3H, s, CH3), 5.50 (2H, s, CH2), 7.00-7.68 (13H,
m, Ar-H). 13C NMR (22.5 MHz, DMSO-d6): δC 20.2, 47.7, 110.0, 119.4, 122.6, 123.3, 125.0, 128.0,
129.8, 132.8, 134.5, 135.3, 142.5, 145.3, 149.5, 151.6, 154.3. Anal. Calcd for C21H18N2: C, 84.56; H,
6.04; N, 9.39%. Found: C, 84.52; H, 5.92; N, 9.34%.
5-Methyl-1-(4-methylbenzyl)-2-(4-methylphenyl)-1H-1,3-benzimidazole (3p)

HETEROCYCLES, Vol. 81, No. 12, 2010
2861
º
Mp 160-162 C; IR (KBr) (νmax/cm-1): 3026, 2905, 1597, 1573, 1497, 1396, 1332, 1134, 1048, 963, 823,
1
745, 690. MS, m/z (%): 326 (M+). H NMR (90 MHz, CDCl3): δH 2.24 (3H, s, CH3), 2.45 (3H, s, CH3),
13
2.52 (3H, s, CH3), 5.50 (2H, s, CH2), 7.10-7.88 (11H, m, Ar-H). C NMR (22.5 MHz, DMSO-d6): δC
20.4, 21.2, 21.4, 48.3, 110.3, 119.5, 122.8, 123.2, 125.6, 126.8, 128.6, 129.2, 129.7, 133.5, 135.8, 137.1,
139.4, 143.6, 154.7. Anal. Calcd for C23H22N2: C, 84.66; H, 6.74; N, 8.58%. Found: C, 84.63; H, 6.68; N,
8.52%.
1-(4-Methoxybenzyl)-2-(4-methoxyphenyl)-5-Methyl-1H-1,3-benzimidazole (3p)
Mp 182-184 ºC; IR (KBr) (νmax/cm-1): 3083, 3056, 3025, 2924, 1597, 1497, 1353, 1293, 1117, 1093, 973,
830, 691. MS, m/z (%): 358 (M+). 1H NMR (90 MHz, CDCl3): δH 2.50 (3H, s, CH3), 3.88 (3H, s, OCH3),
13
3.92 (3H, s, OCH3), 5.66 (2H, s, CH2), 7.00-7.98 (11H, m, Ar-H). C NMR (22.5 MHz, DMSO-d6): δC
20.8, 47.0, 55.6, 55.9, 110.1, 113.8, 119.1, 122.2, 123.1, 125.6, 128.0, 130.8, 131.9, 132.9, 135.6, 136.8,
144.4, 146.7, 152.2, 160.6. Anal. Calcd for C23H22N2O2: C, 77.09; H, 6.14; N, 7.82%. Found: C, 77.04; H,
6.10; N, 7.78%.
General procedure for the synthesis of 2-aryl-1H-benzothiazoles 4a-p:
To a solution of o-aminobenzenethiol (1 mmol) in MeCN (8 mL) was added the aldehyde 2 (1 mmol).
Trans-3,5-dihydroperoxy-3,5-dimethyl-1,2-dioxolane 1 (1 mmol) was then added to the mixture followed
by the addition of 37% aqueous HCl (3 mmol) and the resulting mixture was stirred for an appropriate
time (Table 2). After the completion of the reaction as monitored by TLC (n-hexane/EtOAc; 2:1), the
reaction was quenched with 3M aqueous solution of Na2SO3 (1 mL). Then, the reaction mixture was
diluted with water (15 mL) and filtered under reduced pressure to leave a solid product 4 which was
purified by crystallization from 96% EtOH (Table 2). All the products were characterized by their
physical and spectral (IR, 1H NMR and 13C NMR) analysis and compared with the reported data.36
ACKNOWLEDGEMENTS
We wish to thank the research council of Buali Sina University, Hamedan, Iran, for financial support to
carry out this research.
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