Amberlyst-15-catalyzed synthesis of 2-substituted 1,3-benzazoles in water under ultrasound

Article abstract of DOI:10.1080/00397911.2013.769605

A clean and general method has been developed for the synthesis of benzothiazole, benzoxazole, and benzimidazoles using Amberlyst-15 as a recyclable catalyst under ultrasound irradiation in water. The methodology is operationally simple, free from the use of hazardous organic solvents, and does not require the use of inert or anhydrous atmosphere. A number of 1,3-benzazole derivatives were prepared in good to excellent yields by using this methodology.

Full text of DOI:10.1080/00397911.2013.769605

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Amberlyst-15 Catalyzed Synthesis of 2-Substituted 1,3-
Benzazoles in Water Under Ultrasound
a b
a
a
c
D. Rambabu , P. Radha Krishna Murthi , Balakrishna Dulla , M. V. Basaveswara Rao
&
b
Manojit Pal
a
Department of Chemistry , K L University , Guntur , Andhra Pradesh , India
b
Institute of Life Sciences, University of Hyderabad Campus , Hyderabad , Andhra Pradesh ,
India
c
Department of Chemistry , Krishna University , Machilipatnam , Andhra Pradesh , India
Accepted author version posted online: 18 Jun 2013.
To cite this article: Synthetic Communications (2013): Amberlyst-15 Catalyzed Synthesis of 2-Substituted 1,3-Benzazoles in
Water Under Ultrasound, Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, DOI: 10.1080/00397911.2013.769605
To link to this article: http://dx.doi.org/10.1080/00397911.2013.769605
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AMBERLYST-15 CATALYZED SYNTHESIS OF 2-SUBSTITUTED 1,3-
BENZAZOLES IN WATER UNDER ULTRASOUND
1
,2
1
1
3,
D. Rambabu , P. Radha Krishna Murthi , Balakrishna Dulla , M. V. Basaveswara Rao ,
Manojit Pal2,
1
2
Department of Chemistry, K L University, Guntur, Andhra Pradesh, India, Institute of
Life Sciences, University of Hyderabad Campus, Hyderabad, Andhra Pradesh, India,
3
Department of Chemistry, Krishna University, Machilipatnam, Andhra Pradesh, India
Address correspondence to Manojit Pal, Institute of Life Sciences, University of
Hyderabad Campus, Gachibowli, Hyderabad 500 046, Andhra Pradesh, India, Tel.:+91
4
0 6657 1500; fax: +91 40 6657 1581; E-mail: manojitpal@rediffmail.com
Abstract
A clean and general method has been developed for the synthesis of benzothiazole,
benzoxazole and benzimidazoles by using Amberlyst-15 as a recyclable catalyst under
ultrasound irradiation in water.The methodology is operationally simple, free from the
use of hazardous organic solvents and does not require the use of inert or anhydrous
atmosphere. A number of 1,3-benzazole derivatives were prepared in good to excellent
yields by using this methodology.
[
Supplementary materials are available for this article. Go to the publisher’s online
edition of Synthetic Communications® for the following free supplemental resource(s):
Full experimental and spectral details.]
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KEYWORDS: Benzothiazole, benzoxazole, benzimidazole, Amberlyst-15
INTRODUCTION
The development of inexpensive, efficient and eco-friendly synthetic methods that are
mild, faster, functional group tolerant and free from the use of complicated work-up,
isolation and purification procedures is an important goal in modern organic synthesis.
Accordingly, we have devoted our effort towards the development of anefficient method
for the construction of 1,3-benzazole ring under clean reaction conditions.
1
,3-Benzazoles e.g. benzothiazoles, benzoxazoles and benzimidazoleshave
shownexciting and diverse biological and pharmaceutical properties and are therefore
considered as privileged structures in the design and discovery of new drugs.[1–6] The
most commonly used synthetic method for accessing 1,3-benzazoles includes
condensation of 2-aminothiophenol, 2-aminophenol, or 2-phenylenediamine with
substituted aldehydes,[7–9] nitriles,[10] acylchlorides,[11] or carboxylic acids.[12–15] Recently,
a useful and one-potsynthesis of benzimidazoles and benzothiazoles has been
accomplished from alcohols [16] which however involvedthe use of an oxidant required
for the oxidation step.[17,18] In the view of the fact that many of these methods though
effective some of them however, suffer from drawbacks, such as low products yields,
longer reaction time, high temperature, cumbersome preparation of catalyst along with
the use of non-recyclable catalysts development of a facile, environment friendlyand
inexpensive method leading to 1,3-benzazoles is highly desirable. During the last three
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decades ultrasound mediated reactions have emerged as efficient and attractive
methodologies in organic synthesis.[19–21] These reactions are generally more convenient
and advantageous because of shorter reaction time, mild conditions and high yields of
products [22,23] compared to the traditional methods. The heterogeneous catalysis on the
other hand have played a central role in various organic transformations.[24–26] As an
inexpensive and commercially available heterogeneous catalyst Amberlyst-15 attracted
our attention [27,28] due to its non-hazardous nature and easy removal from the reaction
mixture e.g. via simple filtration. Due to our continuing interest in 1,3-benzazole
derivatives [29] we now report Amberlyst-15 mediated one pot and eco-friendly synthesis
of 2-(hetero)aryl-1,3-benzazoles (3-5) via the reaction of 2-amino(thio)phenol /1,2-
phenylenediamine (1a-c) with various carboxylic acids(2) under ultrasound in water
(
Scheme 1). We also report the synthesis of 2-alkyl/aryl-1,3-benzazoles (7-9) from N-
acyl derivatives of 2-amino(thio)phenol /1,2-phenylenediamine (6) under similar reaction
conditions (Scheme 2).
RESULTS AND DISCUSSION
Initially, the Amberlyst-15 mediated reaction of 4-chlorobenzoicacid (2a) with 2-
aminophenol(1b) under ultrasoundirradiation(SONOREX SUPER RK 510H model with
ultrasonic peak output: 640 W) was chosen as a model reaction and was performed under
various conditions (Table 1). The reaction afforded the expected product 4a in 85% yield
whencarried outin dichloromethane (DCM) for 120 min (Table 1, entry 1). The use of
other solvents such as methanol and toluene was examined but did not improve the
product yield (Table 1, entry 2 and 3). However, almost quantitative yield was achieved
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when the reaction was carried out in waterthat was completed within 60 min (Table 1,
entry 4). The catalyst recovered from the reaction mixture was also tested for its
recyclability at least for three times and the desired product was isolated in good yield in
each run (Table 2). To understand the role of catalyst and ultrasound we performed the
following experiments. Thereaction was carried out in the absence of Amberlyst-15 and
ultrasound when 4a was isolated only in 32% yield after 180 min (Table 1, entry 5).
When the catalyst was omitted but not ultrasound the product 4a was obtained in 71%
yield (Table 1, entry 6). All these observations indicated that the use of Amberlyst-15
under ultrasound was optimum for the synthesis of 4a in waterandAmberlyst-15 was
identified as an efficient and reusable catalyst for the present reaction. Notably, all these
reactions were carried out in open air and therefore were free from the use of
inert/anhydrous atmosphere thereby avoiding possible pressure development in a closed
reaction vessel especially in large scale synthesis.
To extend the scope and generality of this methodology a variety of aryl acids (2)
including the isonicotinic acid (2f) were reacted with 2-phenylenediamine (1a), 2-amino
phenol (1b) or 2-amino thiophenol (1c)in the presence of Amberlyst-15 under the
conditions of entry 4 of Table 1. The reaction proceeded well in all these cases to give the
corresponding benzimidazoles (3), benzoxazole (4) and benzothiazole (5)in excellent
yields (Table 3). Acids containing various substituents such as electron donating F, Cl,
Meand electron withdrawing NO were well tolerated and did not affect the product
2
yields (Table 3, entries 1-4, 7-10 and 13-15). While the methodology worked well with
(
hetero)aromatic acids the use of aliphatic acids were found to be less effective under the
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conditions employed. We then examined the intramolecular cyclization of a variety of N-
acyl derivatives of 2-amino(thio)phenol /1,2-phenylenediamine (6) under similar reaction
conditions (Scheme 2) and the results are summarized in Table 4. The key starting
materials (6) required were synthesized[30, 31] via the reaction of appropriate aromatic
amines with an acid anhydride or acid chloride. A variety of 2-alkyl/aryl-1,3-benzazoles
(
7-9) were prepared smoothly in good yield by using this methodology (Table 4).
Based on the observations of Schemes 1 and 2 a probable mechanism of the present
Amberlyst-15 mediated reaction can be presented (Scheme 3). Amberlyst-15 being a
macro reticular polystyrene based ion exchange resin havinghighly acidic sulfonic group
serves as a source of strong acid. In the present case it allows protonation of the
carboxylic acid moiety of 1 thereby facilitating the formation of mixed anhydride type of
intermediate E-1 which is highly susceptible to a nucleophilic attack by the amine 2.
Thus the reaction seems to proceed via the formation of an amideE-2in situthe reaction
rate of which is greatly accelerated by the ultrasound irradiation used.[32] The
intermediacy of the amide E-1 was further supported by the results presented in Scheme
2
and Table 3. The amide thus generated undergoes intramolecular cyclization aided by
the Amberlyst-15 to give the desired product (3-5).Once again the reaction rate of the
cyclization step was accelerated by the ultrasound. To gain further evidence, the
Amberlyst-15 mediated cyclization of 6a was carried out at 90 ºC in the absence of
ultrasound when the desired product 7a was isolated in low yield after 3h. Overall, both
catalyst and ultrasound played key roles in the present reaction.
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CONCLUSIONS
In conclusion, anew andefficient method has been developed for the preparationof
benzothiazoles, benzoxazoles, and benzimidazoles via Amberlyst-15 mediated
reactions under ultrasound in water. To the best of our knowledge this is the first
example of synthesizing 1,3-benzazoles using Amberlyst-15 as a catalyst under
ultrasound. The merit and disadvantages of the methodology are discussed. The
methodology is simple, inexpensive and environment friendly and does not require
the use of inert or anhydrous atmosphere.Thecatalyst can be recovered and reused.
The operational simplicity and good yields of products are the other advantages of
this methodology. Overall, the present methodology allows a quick and safer access
to the library of novel small molecules based on diverse 1,3-benzazole framework
especially useful for medicinal chemistry / new drug discovery and therefore may find
wide usage.
EXPERIMENTAL
General Procedure For The Preparation Of Compound 3-5
To a solution of o-phenylenediamine 1a, o-amino phenol 1b or o-amino thiophenol
1
c(1.0 mmol) and aromatic acid (2, 1.2 mmol) in water (5 mL) was added Amberlyst-15
(
10%, w/w) and the mixture was irradiated with ultrasound (40 KHz) continuously at 90
ºC till the completion of the reaction (monitored by TLC) as indicated in Table 3. The
solid separated was filtered, washed with diethyl ether (2 x 5 mL), dried and treated with
EtOAc (15 mL). After stirring for 10 min the mixture was filtered to remove the insoluble
catalyst. The filtrate was collected and concentrated under vacuum. The solid obtained
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was purified by recrystallization (column chromatography in few cases) to afford the
desired products 3, 4 or 5.
General Procedure For The Preparation Of Compound 7-9
To a mixture of N-acyl or benzoyl derivative (6, 1 mmol) in water (5 mL) was added
amberlyst-15 (10%, w/w) and the mixture was irradiated with ultrasound (40 KHz)
continuously at 90 ºC till the completion of the reaction (monitored by TLC) as indicated
in Table 4. The solid separated was filtered, washed with diethyl ether (2 x 5 mL), dried
and treated with EtOAc (15 mL). After stirring for 10 min the mixture was filtered to
remove the insoluble catalyst. The filtrate was collected and concentrated under vacuum.
The solid obtained was purified by recrystallization (column chromatography in few
cases) to afford the desired products 7, 8 or 9.
Recycling Of The Catalyst
After completion of the reaction the solid was filtered, washed with diethyl ether, dried
and treated with EtOAc. After stirring for 10 min the mixture was filtered, the residue
was collected, washed with diethyl ether and dried under vacuum for 1 h. This recovered
catalyst was then reused.
SUPPORTING INFORMATION
Full experimental detail, spectral data. This material can be found via the
‘
‘Supplementary Content’’ section of this article’s Webpage.
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ACKNOWLEDGEMENTS
The authors thank the management of Institute of Life Sciences for continuous support
and encouragement.
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Table 1. The reaction of 1b and 2a in the presence of Amberlyst-15 under various
COOH
NH2
N
Cl
O
OH
Amberlyst
Solvent
Cl
conditions.a
1b
2a
4a
b
Entry
Solvent
DCM
Time (min)
Temp (ºC)
35-40
60-65
70-75
90
Yield (%)
1
2
3
4
5
6
120
120
120
50
85
MeOH
Toluene
Water
Water
Water
80
60
90
180
180
90
32c
71d
90
aReaction conditions: all the reactions were carried out using 2-aminophenol 1b (1.0
mmol), 4-chloro benzoic acid 2a (1.2 mmol) and Amberlyst-15 (10%, w/w) under
ultrasoundirradiation in open air.
bIsolated yield.
cThe reaction was carried out in the absence of Amberlyst-15 and ultrasound.
dThe reaction was carried out in the absence of Amberlyst-15.
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Table 2. Recyclability of the catalyst Amberlyst-15.a
Runs
1st
2nd
3rd
86
4th
82
Yield (%)
90
87
aReaction conditions: 2-aminophenol 1b (1.0 mmol), 4-chloro benzoic acid 2a (1.2
mmol) and Amberlyst-15 (10%, w/w) or recovered catalyst under ultrasoundirradiation in
open air.
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Table 3. Amberlyst-15 mediated synthesis of benzimidazoles 3, benzoxazoles 4, and
benzothiazoles 5 under ultrasound in water (Scheme 1).a
Entry
1
Amine (1)
Acid (2)
Products (3-5)
Time Yieldb
(
min)
(%)
6
0
90
1
1
a
a
2
2
a
3a
3b
3c
2
3
4
80
60
90
92
85
82
b
1
1
a
a
2
c
2
d
3
d
5
6
1
1
a
a
120
160
90
85
2
e
3
e
2
f
3
f
7
8
50
90
90
90
2
a
4
a
1
b
b
1
2b
4b
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9
1b
2c
60
85
4
c
1
0
1b
2d
9
0
80
90
4d
1
1
1
2
1b
120
160
2
e
4
e
1b
82
92
90
88
82
85
82
2
f
4
f
1
1
1
1
1
1
3
4
5
6
7
8
6
0
2
a
1
1
c
c
5a
90
80
2b
5b
1
1
1
c
c
c
2c
2d
2e
5
c
90
5d
120
180
5
e
1c
2
f
5
f
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a
Reaction conditions: all the reactions were carried out using 1 (1.0 mmol), an
appropriate acid 2 (1.0 mmol) and Amberlyst-15 (10%, w/w) in water at 90ºC under
ultrasound irradiation in open air.
bIsolated yield.
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Table 4. Amberlyst-15 mediated synthesis of benzoxazoles 7, benzothiazoles 8, and
benzimidazoles 9 under ultrasound in water.a
b
Entry
1
N-acyl derivative (6)
Product (7-9)
Time (min)
Yield (%)
3
6
0
0
92
7
a
6
a
NO
2
3
2
OH
NH
82
85
H C
O
3
7b
6b
O N
OH
NH
2
60
7
c
H C
O
3
6
c
OH
4
5
6
6
0
0
82
90
O N
NH
2
H C
O
3
7
d
6d
7
e
6
e
6
6
0
85
7
f
6
f
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6

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7
60
82
7
g
6
g
8
9
9
0
86
90
7h
6h
60
8
8
a
6
6
g
1
0
3
0
92
90
b
h
1
1
1
2
30
9
9
a
6
i
3
0
92
b
6
j
a
Reaction conditions: all the reactions were carried out using N-acyl derivative 6 (1.0
mmol), and Amberlyst-15 (10%, w/w) in water under ultrasound at 90 ºC in open air.
bIsolated yield.
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Scheme 1. Amberlyst-15 mediated synthesis of 2-(hetero)aryl-1,3-benzazoles (3-5) under
ultrasound.
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8

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Scheme 2. Amberlyst-15 mediated synthesis of 2-alkyl/aryl-1,3-benzazoles (7-9) under
ultrasound.
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Scheme 3. Proposed mechanism for the Amberlyst-15 mediated synthesis of 1,3-
benzazoles under ultrasound.
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0