An efficient synthesis of benzoxazoles using silica-supported tin exchanged silicotungstic acid catalyst

Article abstract of DOI:10.1016/j.molcata.2010.06.009

An efficient method for the preparation of benzoxazoles by reacting aminophenols with orthoesters in the presence of silica-supported tin exchanged silicotungstic acid catalyst under mild conditions with high yield and selectivity is demonstrated. The catalysts were characterized by FT-IR and pyridine adsorbed FT-IR. The catalyst exhibited consistent activity upon recycling and is highly active compared to other catalysts. The role of Lewis acidity of the catalysts and plausible reaction mechanism is presented.

Full text of DOI:10.1016/j.molcata.2010.06.009

Journal of Molecular Catalysis A: Chemical 328 (2010) 119–123
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journal homepage: www.elsevier.com/locate/molcata
An efficient synthesis of benzoxazoles using silica-supported tin exchanged
silicotungstic acid catalyst
∗
A. Srivani, K.T. Venkateswar Rao, P.S. Sai Prasad, N. Lingaiah
Catalysis Laboratory, I&PC Division, Indian Institute of Chemical Technology, Hyderabad 500607, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient method for the preparation of benzoxazoles by reacting aminophenols with orthoesters in the
presence of silica-supported tin exchanged silicotungstic acid catalyst under mild conditions with high
yield and selectivity is demonstrated. The catalysts were characterized by FT-IR and pyridine adsorbed
FT-IR. The catalyst exhibited consistent activity upon recycling and is highly active compared to other
catalysts. The role of Lewis acidity of the catalysts and plausible reaction mechanism is presented.
Received 18 January 2010
Received in revised form 24 March 2010
Accepted 7 June 2010
Available online 15 June 2010
©
2010 Elsevier B.V. All rights reserved.
Keywords:
Aminophenol
Orthoesters
Benzoxazole
Tin
Silicotungstic acid
Silica
1
. Introduction
properties. These catalysts have the ability to accept and release
electrons and possess high proton mobility [12,13]. These are
mainly used as acid and oxidation catalysts.
Benzoxazole and substituted benzoxazoles are important com-
pounds due to their biological activity and diverse medicinal uses
such as gram-positive antibacterial agents, antibiotics, antipara-
sitic, anti-inflammatory, elastase inhibitors, anti-stress, ulcer and
anti-cancer agents [1–3]. Benzobisoxazoles are also used as build-
ing blocks for novel organic semiconductors because conjugated
small molecules and polymers are well suited for use in organic
semi conducting applications [4]. Common synthetic routes to ben-
zoxazole involves the coupling of aldehydes and carboxylic acids
or their derivatives with 2-aminophenol in the presence of strong
acids at high temperatures and high reaction time [5–10].
Unacceptability of the conventional liquid acid catalysts gives
a major impetus to search for cleaner process. Such cleaner tech-
nologies are possible by making use of environmentally friendly
catalysts like solid acids. In recent years, the use of heterogeneous
solid acids received more interest for use as catalysts in different
organic reactions [11]. Heterogeneous solid acid catalysts are more
preferable over conventional acids as these are easily recoverable
from reaction mixture by simple filtration and reused. Among dif-
ferent solid acid catalysts heteropolyacids (HPA) attained much
attention due to their unique properties such as well-defined struc-
ture, Brønsted acidity, and possibility to modify their acid–base
The major disadvantages of HPAs as catalysts lie in their low
thermal stability, less surface area (1–10 m /g) and solubility in
2
polar media. HPAs can be made eco-friendly insoluble acid catalysts
with high thermal stability and high surface area by exchanging its
proton with metal ion and also supporting them on suitable sup-
ports. The support provides an opportunity to spread HPAs over a
large surface area, which generally increases catalytic activity.
In the present communication, we report silica-supported tin
exchanged silicotungstic acid (SnSTA) as a solid acid catalyst for
the synthesis of benzoxazoles from reaction of 2-aminophenol with
orthoesters under mild reaction conditions. The role of metal salts
of heteropoly tungstate in generating Lewis acidity and plausible
reaction mechanism is elucidated.
2
. Experimental
2
.1. Preparation of catalysts
A series of tin exchanged silicotungstic acid supported on sil-
ica (SnSTA/SiO ) catalysts were prepared by wet impregnation
2
method. Required quantities of silicotungastic acid (STA) and
stannous chloride were dissolved in minimum amount of water.
Calculated amount of silica was added to the above solution and
aged for about 1 h. The excess water was evaporated on a water
bath. The amount of tin was taken to replace two protons of STA
∗ Corresponding author. Tel.: +91 40 27193163; fax: +91 40 27160921.
E-mail address: nakkalingaiah@iict.res.in (N. Lingaiah).
1
381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.06.009

1
20
A. Srivani et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 119–123
(
H SiW12O40). The content of tin exchanged STA was varied from 5
4
to 25 wt% on silica. The dried catalysts were kept for further drying
◦
◦
in a hot air oven at 120 C for 12 h and finally calcined at 300 C for
2
h.
Scheme 1. Synthesis of benzoxazole from aminophenol and orthoesters.
Table 1
Effect of SnSTA content on silica for the synthesis of benzoxazole.
2.2. Characterization of catalysts
The FT-IR spectra were recorded on a Bio-Rad Excalibur series
spectrometer using the KBr disc method.
Catalyst
Conversion (%)
Selectivity (%)
Benzoxazole
Ether
The nature of the acid sites (Brønsted and Lewis) of the catalysts
samples were determined by in situ Fourier Transform Infrared
5
1
1
SnSTA/SiO2
0SnSTA/SiO2
5SnSTA/SiO2
58
100
100
100
100
95.4
100
100
–
–
–
–
4.6
–
–
70.6
76.8
85.4
87.2
50.6
58
(
FT-IR) spectroscopy with chemisorbed pyridine. The Brønsted and
Lewis acidities were estimated by identifying peak areas corre-
sponding to the acid sites.
20SnSTA/SiO2
2
2
5SnSTA/SiO2
0STA/SiO2
SnCl2
2.3. General reaction procedure
Reaction conditions: 2-aminophenol (1 mmol), triethylorthoformate (3 mmol),
CH3CN (solvent): 4 ml, catalyst weight: 50 mg, reaction temperature 60 C.
◦
A mixture of aminophenol (1 mmol), triethylorthoformate
(
2
3 mmol) and catalyst (50 mg) in acetonitrile (4 ml) were taken in
3
.2. Synthesis of benzoxazole
5 ml two-necked round bottom flask. The reaction mixture was
◦
allowed to stir at 60 C on oil bath. The progress of the reaction was
monitored by using TLC. After ensuring the completion of reac-
tion the catalyst was separated by filtration. The products were
identified by GC–MS (SHIMADZU-2010) analysis by separating the
products on a DB-5 column.
Silica-supported tin exchanged STA catalysts were evaluated for
the synthesis of benzoxazole. The reaction of 2-aminophenol with
triethylorthoformate over these catalysts was carried (Scheme 1)
and the results are shown in Table 1. The STA supported on silica
catalyst (without tin) was also prepared and studied for compari-
son. The conversion of 2-aminophenol increased with increase in
the content of SnSTA on silica and attained maximum at a loading of
3
3
3
. Results and discussion
2
0 wt%. The 20 wt% SnSTA/SiO catalyst showed about 85% conver-
2
sion of 2-aminophenol with 100% selectivity towards benzoxazole.
The parent catalyst 20 wt% STA/SiO2 showed only 50.6% conver-
sion. The salts of tin also tested for this reaction and SnCl2 showed
about conversion of 58%. The presence of both tin and heteropoly
anion leads to enhanced activity. The exchange of Sn resulted in
substantial increase in the conversion of 2-aminophenol. The reac-
tion profiles of these catalysts suggest that these are highly selective
in getting the desired product with maximum conversion.
The variation in the activity with change in STA content is related
to the high dispersion of active component on silica. There is not
much variation in conversion beyond 20-wt% of SnSTA on silica.
This suggests that bulk nature of STA was attained at high loadings.
As the 20-wt% SnSTA supported on silica showed better activity
for this reaction, the reaction profile was studied over this catalyst
and the results are shown in Fig. 2. The conversion of aminophenol
is increased with increase in reaction time and attained complete
.1. Characterization
.1.1. Fourier transform-infrared spectroscopy (FT-IR)
FT-IR spectra of tin exchanged silicotungstic acid supported on
silica are shown in Fig. 1. The catalyst showed four characteristic
bands in the region 1100–800 cm
−1
.
IR bands were observed at 977, 921, 876, and 786 cm−1, which
are assigned to the stretching vibrations of W O, Si–O, W–Oc–W,
W–Oe–W, respectively of Keggin ion [14]. The FT-IR patterns sug-
gest the existence intact tin exchanged STA Keggin structure on
silica. The results support that it is possible to prepare supported
metal exchanged heteropoly tungstate in single step.
Fig. 1. FT-IR Spectra of silica-supported tin exchanged silicotungstic acid catalysts.
Fig. 2. Reaction profile for the synthesis of benzoxazole over SnSTA/SiO2.

A. Srivani et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 119–123
121
Fig. 4. Influence of reaction temperature for the preparation of benzoxazole over
SnSTA/SiO2 catalyst.
Fig. 3. Pyridine adsorption FT-IR Spectra of SnSTA/SiO2 catalyst.
The scope of the reaction for the present catalyst was explored
and the results are presented in Table 2. The catalyst is highly active
for the synthesis of substituted benzoxazole within 15 min of reac-
tion time with near 100% selectivity towards the desired products.
The chloro or nitro group containing aminophenols were reacted
with ethyl or methyl orthoesters with quantitative yields in short
time. It is noteworthy to mention that the catalyst gives a selec-
tive product when different substrates are used for the synthesis
of substituted benzoxazole. The metal exchanged heteropoly acids
are working as metal salts of triflic acids.
conversion with in 30 min. The catalyst showed high conversion
even within 5 min of reaction time. The reported acid catalysts such
as rare earth metal triflates, mineral acids took 1–5 h for reason-
able conversion for similar reactions [4,10]. The high activity of the
present catalyst is due to the presence of both Lewis and Brønsted
acid sites as noticed from pyridine adsorbed FT-IR spectra shown in
Fig. 3. The IR bands related to both Lewis and Brønsted acid peaks
are clearly seen in the inset of Fig. 3. The present protocol requires
strong acidity and the catalyst possess strong acidity resulting due
to the presence of Sn. The presence of Sn results in generation of
Lewis acidity [15]. The plausible reaction mechanism for the syn-
thesis of benzoxazole from aminophenol with triethylorthoformate
is shown in Scheme 2. The Lewis acidic sites of the Sn exchanged
STA catalyst might be participating in the reaction by accepting the
lone pair of electron from amine group of aminophenol. The high
activity of the tin exchanged STA compared to the STA (Table 1),
suggests the participation of Lewis acidic sites also for this reac-
tion. The conversion obtained with tin salts further supports the
role of tin in enhancing the overall activity of tin exchanged STA
catalysts.
3.3. Recyclability of the catalysts
After completion of the reaction, the catalyst was separated by
simple filtration and washed 2–3 times with acetonitrile. Later the
◦
catalyst was dried in air oven at 120 C for 1 h and reused. The recov-
ery of the catalyst is quantitative and is varied with in 95–90%. The
results obtained by recycling of the catalyst are shown in Table 3.
The conversion of the aminophenol after four cycles was almost
constant and no loss of catalytic activity upon reuse. The recycling
result suggests heterogeneity of the catalyst.
The influence of reaction temperature on the synthesis of ben-
zoxazole over 20 wt% SnSTA/SiO2 catalysts was evaluated and the
results are shown in Fig. 4. The conversion of 2-aminophenol
increased with increase in reaction temperature with near 100%
selectivity to benzoxazole. The catalysts showed reasonable con-
version of about 35% conversion even at 30 C with in 15 min. These
results suggest that the catalyst activity is comparable with that of
homogeneous acid catalysts, which are generally more active.
3.4. Comparison of different catalysts with SnSTA/SiO2
The present catalyst SnSTA/SiO2 was compared with the
reported catalysts for the synthesis of benzoxazole. The com-
parative results are shown in Table 4. When compared to other
catalysts, present catalyst showed high conversion with 100% selec-
◦
◦
tivity to benzoxazole at 60 C temperature within 15 min. Where
as the other reported acid catalysts took about 15–240 min at high
Scheme 2. Plausible reaction mechanism for the synthesis of benzoxazole over tin exchange silicotungstic acid.

1
22
A. Srivani et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 119–123
Table 2
Synthesis of substituted benzoxazole over SnSTA/SiO2 catalyst.
Aminophenol
Orthoester
Product
Time (min)
15
Conversion (%)
85.4
Selectivity (%)
100
HC-(OEt)3
HC-(OMe)3
HC-(OEt)3
HC-(OMe)3
HC-(OEt)3
HC-(OMe)3
15
15
15
15
15
83.2
86.5
86.1
83.6
100
100
100
100
100
83.4
◦
Reaction conditions: aminophenol (1 mmol), orthoester (3 mmol), catalyst weight: 50 mg, CH3CN (solvent): 4 ml, reaction temperature 60 C.
Table 3
Recycling results of SnSTA/SiO2 catalyst.
S. no.
No. of recycles
Conversion (%)
Selectivity (%)
Catalyst recovery (%)
1
2
3
4
0
1
2
3
85.4
83.6
81.7
79.4
100
100
100
100
–
95
93
90
Table 4
Comparison of different catalyst with SnSTA/SiO2.
◦
S. no.
Substrate 1
Substrate 2
Catalyst
Reaction time
Temperature ( C)
Yield (%)
References
◦
1
2
3
4
5
6
o-Hydroxyamino pyridine
Aminophenol
Aminophenol
Aminophenol
Diamino benzenediol
Aminophenol
Orthoester
Carboxylic acid
Aldehyde
Triethylorthoester
Orthoester
Triethylorthoester
p-TsOH
–
15 min
4 h
10 min
1 h
100
C
60
97
78
89
64
85
[10]
[9]
[8]
[5]
◦
PS-PPh3/CCl3CN
Activated carbon
ZrOCl2.8H2O
H2SO4
Microwave, 150
C
◦
120
C
◦
85
C
◦
130
C
[4]
◦
SnSTA/SiO2
15 min
60
C
Present study
temperature to get reasonable conversion. Moreover, most of the
reported catalysts are homogenous in nature and used in stoichio-
metric quantities.
Acknowledgement
A. Srivani thanks Council of Scientific and Industrial Research
CSIR), India for financial support in the form of Junior Research
(
Fellowship.
4
. Conclusions
Silica-supported tin exchanged STA catalysts were prepared
References
with intact Keggin structure. SnSTA/SiO2 catalyst is demonstrated
as an efficient solid acid catalyst for the synthesis of benoxazoles
under mild reaction conditions with in short reaction time. The
catalyst is active even at ambient temperatures with high activity
and selectivity. The exchange of tin with the protons of STA results
in generating the Lewis acidity and there by results in substantial
increase in catalytic activity for the synthesis of benzoxazole. The
involvement of Lewis acid sites for this reaction was explained by
plausible reaction mechanism. The catalyst is reusable with consis-
tent activity
[
[
1] M. Radi, S. Saletti, M. Botta, Tetrahedron Lett. 49 (2008) 4464–4466.
2] T. Ertan, I. Yildiz, B. l Tekiner-Gulbas, K. Bolelli, O. Temiz-Arpaci, S. Ozkan, F.
Kaynak, I. Yalcin, E. Aki, Eur. J. Med. Chem. 44 (2009) 501–510.
[
[
3] S. Alper-Hayta, M. Arisoy, O. Temiz-Arpaci, I.E. Yildiz, A. Semiha O¨zkan, F.
Kaynak, Eur. J. Med. Chem. 43 (2008) 2568–2578.
4] J.F. Mike, A.J. Makowski, M. Jeffries-EL, Org. Lett. 10 (2008) 4915–4918.
[5] I. Mohammadpoor-Baltork, A.R. Khosropour, S.F. Hojati, Catal. Commun. 8
(2007) 1865–1870.
[
6] M.M. Heravi, S. Sadjadi, H.A. Oskooie, R.H. Shoar, F.F. Bamoharram, J. Chin.
Chem. Soc. 55 (2008) 890–895.
[
7] J. Bonnamour, C. Bolm, Org. Lett. 10 (2008) 2665–2667.

A. Srivani et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 119–123
123
[
8] Y. Kawashita, N. Nakamichi, H. Kawabata, M. Hayashi, Org. Lett. 5 (2003)
713–3715.
9] Y. Wang, K. Sarris, D.R. Sauer, S.W. Djuric, Tetrahedron Lett. 47 (2006)
823–4826.
10] M. Doise, F. Dennin, D. Blondeau, H. Sliwa, Tetrahedron Lett. 31 (1990)
155–1156.
11] I.V. Kozhevnikov, Chem. Rev. 98 (1998) 171–198.
[12] T. Okuhara, N. Mizuno, M. Misono, Appl. Catal. A: Gen. 222 (2001) 63–77.
[13] M. Misono, I. Ono, G. Koyano, A. Aoshima, Pure Appl. Chem. 72 (2000)
1305–1311.
[14] K.M. Parida, S. Mallick, J. Mol. Catal. A: Chem. 275 (2007) 77–83.
[15] K.-I. Shimizu, H. Furukawa, N. Kobayashi, Y. Itaya, A. Satsuma, Green Chem. 11
(2009) 1627–1632.
3
[
4
[
[
1