Tungstate sulfuric acid as an efficient catalyst for the synthesis of benzoxazoles and benzothiazoles under solvent-free conditions

Article abstract of DOI:10.1016/j.crci.2013.03.009

Tungstate sulfuric acid (TSA) has been found to be an efficient and reusable catalyst for the synthesis of benzoxazole and benzothiazole derivatives via reactions of orthoesters with o-aminophenols or o-aminothiophenols in high yields.

Full text of DOI:10.1016/j.crci.2013.03.009

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CRAS2C-3734; No. of Pages 6
C. R. Chimie xxx (2013) xxx–xxx
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Full paper/Memoire
Tungstate sulfuric acid as an efficient catalyst for the synthesis of
benzoxazoles and benzothiazoles under solvent-free conditions
*
Mahnaz Farahi, Bahador Karami , Masume Azari
Department of Chemistry, Yasouj University, Yasouj 75918–74831, Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
Tungstate sulfuric acid (TSA) has been found to be an efficient and reusable catalyst for the
synthesis of benzoxazole and benzothiazole derivatives via reactions of orthoesters with
o-aminophenols or o-aminothiophenols in high yields.
Received 18 January 2013
Accepted after revision 21 March 2013
Available online xxx
´
ß 2013 Published by Elsevier Masson SAS on behalf of Academie des sciences.
Keywords:
Heterogeneous catalysis
Tungstate sulfuric acid
Benzoxazoles
Benzothiazoles
Orthoesters
1. Introduction
these compounds have prompted wide studies for their
synthesis. The most frequently used synthetic method to
Heterogeneous solid catalysts have gained much
attention in recent years, as they possess a number of
advantages, such as cleaner reaction, easier work-up,
reduced reaction time, and eco-friendliness [1,2]. These
considerations are currently driving our effort to develop
heterogeneous organic transformations. Tungstate sulfuric
acid (TSA) is an alternative to sulfuric acid that was
synthesized by the reaction of anhydrous sodium tung-
state with chlorosulfonic acid [3]. This new inorganic solid
acid has been applied as an efficient catalyst to a variety of
organic transformations [4–7].
There has been considerable interest in benzoxazole
and benzothiazole derivatives, not least because of their
value for a variety of industrial [8,9], biological [10,11],
and medicinal chemistry uses [12,13]. Furthermore,
these compounds have shown different pharmacological
activities, such as anti-HIV [14,15], antitumor [16,17],
anticancer [18,19], antibacterial [20,21], and anti-inflam-
matory [22,23] properties. The extensive applications of
produce benzoxazole and benzothiazole derivatives
consists of the condensation of o-aminophenol or o-
aminobenzenethiol with substituted aldehydes, nitriles,
acyl chlorides, or carboxylic acids [13]. However, the
majority of these methods suffer from certain drawbacks,
for instance, the use of hazardous solvents and catalysts,
high cost, long reaction times, and drastic reaction
conditions.
In continuation of our interest in the use of hetero-
geneous solid catalysts [24–26], we wish to report herein a
very simple procedure for the synthesis of benzoxazole (4)
and benzothiazole (5) derivatives using TSA as a superior
solid acid catalyst (Scheme 1).
2. Results and discussion
In a first set of experiments, the solvent-free reaction of
o-aminophenol (1 mmol) and triethyl orthoacetate
(1.1 mmol) was performed at room temperature in the
absence of the catalyst as a model reaction. No benzox-
azole product was synthesized, even after 24 h. Therefore,
the model reaction was investigated in the presence of
different catalysts, under various conditions (Table 1).
* Corresponding author.
E-mail address: karami@mail.yu.ac.ir (B. Karami).
1631-0748/$ – see front matter ß 2013 Published by Elsevier Masson SAS on behalf of Acade´mie des sciences.
http://dx.doi.org/10.1016/j.crci.2013.03.009
Please cite this article in press as: Farahi M, et al. Tungstate sulfuric acid as an efficient catalyst for the synthesis of
benzoxazoles and benzothiazoles under solvent-free conditions. C. R. Chimie (2013), http://dx.doi.org/10.1016/
j.crci.2013.03.009

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CRAS2C-3734; No. of Pages 6
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M. Farahi et al. / C. R. Chimie xxx (2013) xxx–xxx
R1
R1
N
X
NH2
R2C(OEt)3
TSA (1mol%)
R2
solvent-free
80-90 oC
XH
4 X=O
5 X=S
1 X = O
2 X = S
3
Scheme 1. Tungstate sulfuric acid-catalyzed synthesis of benzoxazoles and benzothiazoles.
Table 1
Optimization of the reaction.
NH2
N
Conditions
Catalyst
MeC(OEt)3
Me
OH
O
Entry
Solvent
Catalyst (mol%)
Temperature (8C)
Time (min)
Yield (%)a
1
None
None
None
None
None
None
None
None
None
EtOH
CH3CN
H2O
None
90–100
60–70
60–70
60–70
60–70
60–70
60–70
80–90
100
360
240
180
120
25
50
60
70
50
87
85
80
96
95
65
60
45
2
ZnCl2 (5)
MgBr2 (5)
H2SO4 (5)
TSA (1)
TSA (5)
TSA (10)
TSA (1)
TSA (1)
TSA (1)
TSA (1)
TSA (1)
3
4
5
6
30
7
30
8
8
9
10
10
11
12
70
120
120
180
70–80
100
a
Isolated yields.
As it can be seen, the best results were obtained by
performing the model reaction in the presence of 1 mol% of
TSA at 80–90 8C under solvent-free conditions.
orthoester is activated by TSA to give 6 and then 7. Next, o-
aminophenol or o-aminothiophenol attacks 7, affording 8,
which is then activated by the catalyst to give 9.
Cyclization of 10 gives 11, which in turn is activated by
the catalyst to afford the final product.
The reusability of the catalyst is important for the large-
scale operation and from an industrial point of view.
Therefore, the recovery and reusability of TSA was
examined. Hence, TSA was regenerated from the
model reaction by washing with chloroform and drying
at 120 8C.
The major advantages of the presented protocol over
the existing methods can be seen by comparing our results
with those of some recently reported procedures, as shown
in Table 3. For example use of o-benzenedisulfonimide
required long reaction times. And use of Silica sulfuric acid
as catalyst requires high temperature.
Then, the generality of the procedure was evaluated by
the reaction of various orthoesters (3) with o-aminophe-
nols (1) and o-aminothiophenols (2) under optimized
reaction conditions. It was found that both electron-rich
and electron-poor o-aminophenols (Me, Cl and NO2 as
donating and withdrawing groups) reacted well in this
process to afford the corresponding products in good to
excellent yields. The obtained results are summarized in
Table 2.
The products were isolated and characterized by
physical and spectroscopic techniques and they were
compared with authentic samples [28–32].
The probable mechanism for the formation of benzox-
azoles and benzothiazoles is outlined in Scheme 2. Initially,
R2
EtO
OEt
OEt
R2C(OEt)3
-EtOH
WHSO6
WHSO6
OSO3H
OSO3
H
6
H2N
R1
R1
H
N
Et
O
H
N
R1
R2
EtO
OEt
R1
WHSO6
OSO3H
H
R2
HX
WHSO6
OEt
R1
OEt
OEt
OSO3
XH
XH
7
X= O, S
9
8
H
N
H
N
R1
R1
OEt
N
X
WHSO6
R2
OSO3
R2
R2
OEt
X
XH
11
10
Scheme 2. The proposed mechanism for tungstate sulfuric acid-catalyzed formation of benzoxazoles and benzothiazoles.
Please cite this article in press as: Farahi M, et al. Tungstate sulfuric acid as an efficient catalyst for the synthesis of
benzoxazoles and benzothiazoles under solvent-free conditions. C. R. Chimie (2013), http://dx.doi.org/10.1016/
j.crci.2013.03.009

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3
Table 2
Synthesis of benzoxazole and benzothiazole derivatives in the presence of tungstate sulfuric acid (1 mol%).
Entry
2-Aminophenol or 2-aminothiophenol
Orthoester
HC(OEt)3
Product
Time (min)
7
Yield (%)a
92
M.P. (8C) [Reference]
4a
Oil [27]
NH2
N
OH
O
N
4b
4c
4d
MeC(OEt)3
EtC(OEt)3
HC(OEt)3
9
4
7
98
98
90
Oil [27]
NH2
CH3
OH
O
N
Oil [27]
NH2
Et
N
OH
O
43–45 [27]
H3C
H3C
H3C
O2N
O2N
O2N
Cl
H3C
H3C
H3C
O2N
O2N
O2N
Cl
NH2
OH
O
N
4e
4f
MeC(OEt)3
EtC(OEt)3
HC(OEt)3
6
12
10
90
88
84
Oil [27]
NH2
CH3
Et
OH
O
N
29–30 [27]
146–148
NH2
OH
O
N
4g
NH2
O
N
OH
4h
4i
MeC(OEt)3
EtC(OEt)3
9
80
93
128–129
150–152
NH2
CH3
Et
O
N
OH
10
NH2
O
OH
4j
HC(OEt)3
MeC(OEt)3
EtC(OEt)3
4
7
96
97
90
36–38 [27]
53–55 [27]
58–59 [27]
NH2
N
OH
O
N
4k
4l
Cl
Cl
NH2
CH3
OH
O
N
10
Cl
Cl
NH2
Et
OH
O
5a
5b
5c
a
HC(OEt)3
MeC(OEt)3
EtC(OEt)3
7
4
6
89
91
94
Oil [27]
Oil [27]
Oil [27]
NH2
N
SH
S
NH2
N
CH3
SH
S
NH2
N
Et
SH
S
Isolated yields.
Table 3
Comparison of the results for the preparation of 2-methyl benzoxazole with various used catalysts.
Catalyst
Mol%
Time (min)
Temperature
Yield (%) [Reference]
o-Benzenedisulfonimide
Ga(OTf)
5
120–240
5–90
R.T.
85–93 [28]
71–98 [29]
91–97 [30]
80–93 [31]
85–97 [32]
10
R.T.
HBF4–SiO2
2
45–60
1–10
R.T.
Bi salts
0.5–4
50 mg
85 8C
85 8C
Silica sulfuric acid
2–10
R.T.: room temperature.
Please cite this article in press as: Farahi M, et al. Tungstate sulfuric acid as an efficient catalyst for the synthesis of
benzoxazoles and benzothiazoles under solvent-free conditions. C. R. Chimie (2013), http://dx.doi.org/10.1016/
j.crci.2013.03.009

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Fig. 1. (a) Powder X-ray diffraction pattern of the tungstate sulfuric acid (TSA); (b) FT-IR spectra of H3OSO(WO2)OSO3H (TSA).
3. Characterization of the catalyst
1702, 1620, 1290, 1060, 1005 and 860 cmÀ1 in the
catalyst’s spectra reveal bonds in both anhydrous sodium
tungstate and the –OSO3H group. Hence, the titration of
the catalyst with NaOH (0.1 N) was done. Firstly, 1 mmol of
catalyst was dissolved in 100 mL of water. Therefore, the
titration with NaOH (0.1 N) in the presence of phe-
nolphthalein was carried out. At the endpoint of the
titration, 4 mmol of the titrant was consumed. Also, based
on potentiometric data, the pKa1 value for TSA is 3.17 and
pKa2 is 8.78. The result of the catalyst’s titration showed
four acidic valences because of hydrolysis in aqueous
solvent. The X-ray fluorescence (XRF) data of TSA indicates
the presence of WO4 and SO3 in this catalyst (Table 4).
Fig. 1a shows the X-ray diffraction analysis (XRD)
patterns of TSA. It was reported that high-degree mixing of
W–S in chlorosulfonic acid often led to the absence of the
XRD pattern for anhydrous sodium tungstate. The broad
peak around 25.78 (2u) (u is the Bragg angle) from the
smaller inset could be attributed to the linking of WO3 into
the chlorosulfonic acid.
The FT-IR spectra of anhydrous sodium tungstate and
TSA are shown in Fig. 2b. The spectrum of TSA shows the
characteristic bonds of anhydrous sodium tungstate and
chlorosulfonic acid. The wavenumbers 3406, 1820, 1725,
Table 4
XRF data of HO3SO(WO2)OSO3H (TSA).
Compound
Concentration (%W/W)
WO4
SO3
19.49
0.317
0.190
0.056
0.023
0.015
0.014
79.82
99.93
Na2O
Cl
CuO
Fe2O3
CaO
LOIa
Total
a
Fig. 2. Reusability study of the catalyst in the model reaction.
Loss on ignition.
Please cite this article in press as: Farahi M, et al. Tungstate sulfuric acid as an efficient catalyst for the synthesis of
benzoxazoles and benzothiazoles under solvent-free conditions. C. R. Chimie (2013), http://dx.doi.org/10.1016/
j.crci.2013.03.009

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5
4. Conclusion
appropriate time according to Table 4. After the comple-
tion of the reaction (as indicated by TLC), the mixture was
diluted with chloroform (10 mL) and the catalyst was
separated by filtration. Further purification was achieved
by column chromatography.
In conclusion, TSA was found to be a mild and efficient
catalyst for the formation of benzoxazoles and benzothia-
zoles. The use of this reusable catalyst under solvent-free
conditions has made this protocol practical, environmen-
tally friendly and economically attractive. The simple
work-up procedure, the mild reaction conditions, the short
reaction times, the high yields of products and the non-
toxicity of the catalyst are other advantages of the present
method.
5.5. Selected spectral data
5.5.1. 2-Methylbenzoxazole (4b)
Colorless oil, FT-IR:
n
max (KBr) = 3050, 2900, 1617, 1575,
= 2.61 (s,
1450, 1160 cmÀ1 1H NMR (400 MHz, CDCl3):
,
d
3H, CH3), 7.27 (m, 2H, CH arom), 7.44 (m, 1H, CH arom),
7.63 (m, 1H, CH arom) ppm, 13C NMR (100 MHz, CDCl3):
5. Experimental
d
= 14.2, 110.7, 119.5, 124.0, 124.4, 141.5, 150.8,
5.1. General
163.5 ppm.
XRD patterns were obtained with a Philips X Pert Pro X
5.5.2. 2,5-Dimethylbenzoxazole (4e)
diffractometer operated with a Ni-filtered Cu Ka radiation
Colorless oil, FT-IR:
n
max (KBr) = 3010, 2950, 1575, 1450,
= 2.39 (s, 3H,
1260, 610 cmÀ1, 1H NMR (400 MHz, CDCl3):
d
source. XRF spectroscopy spectra were recorded with the
X-ray fluorescence analyzer S4 Pioneer, Bruker, Germany.
The chemicals were purchased from Aldrich, Fluka and
Merck chemical companies and freshly used without
purification. The products were isolated and identified by
their spectral data. IR spectra were recorded on a FT-IR
JASCO-680 using KBr disks. The 1H NMR and 13C NMR
spectra were recorded with a Bruker 400 Ultrashield1
(400 MHz), with CDCl3 as the solvent.
CH3), 2.55 (s, 3H, CH3), 7.02 (d, J = 8.4 Hz, 1H, CH arom),
7.27 (d, J = 8.1 Hz, 1H, CH arom), 7.39 (s, 1H, CH arom) ppm,
13C NMR (100 MHz, CDCl3):
d = 14.1, 22.5, 109.6, 119.5,
125.3, 133.4, 141.6, 149.1, 163.4 ppm.
5.5.3. 5-Nitrobenzoxazole (4g)
Yellow solid, mp 146–148 8C, FT-IR: nmax (KBr) = 3055,
1620, 1560, 1462, 1169 cmÀ1 1H NMR (400 MHz, CDCl3):
,
d
= 7.27 (m, 1H, CH arom), 8.08 (m, 1H, CH arom), 8.18 (m,
1H, CH arom), 8.53 (s, 1H) ppm, 13C NMR (100 MHz,
CDCl3): = 111.42, 117.19, 121.75, 140.49, 145.47, 153.44,
5.2. Preparation of the catalyst
d
TSA was prepared via the previously reported proce-
dure [3]. First, anhydrous sodium tungstate (2 mmol,
5.876 g) was added to dry n-hexane (25 mL) in a 100-mL
round bottom flask, equipped with an ice bath and
overhead stirrer. Chlorosulfonic acid (4 mmol, 0.266 mL)
was then added dropwise to the flask during 30 min and
stirred for 1.5 h. Afterwards, the reaction mixture was
gradually poured into 25 mL of chilled distilled water
under agitation. The TSA was separated as a yellowish solid
by filtration, washed with distilled water five times till the
filtrate showed a negative test for the chloride ion, and
dried at 120 8C for 5 h. The yield of the obtained yellowish
catalyst proved to be 98% after it was decomposed at
285 8C.
155.23 ppm.
5.5.4. 2-Methyl-5-nitrobenzoxazole (4h)
White solid, mp 128–129 8C, FT-IR: nmax (KBr) = 3100,
3000, 1610, 1572, 1515, 1445, 1340, 1160 cmÀ1 1H NMR
,
(400 MHz, CDCl3):
1H, CH arom), 8.09 (dd, JHH = 8.8 Hz, JHH = 2 Hz, 1H, CH
arom), 8.36 (d, J = 2 Hz, 1H, CH arom) ppm, 13C NMR
(100 MHz, CDCl3):
145.1, 154.5, 167.1 ppm.
d = 2.52 (s, 3H, CH3), 7.39 (d, J = 9.2 Hz,
3
4
d = 14.9, 110.4, 115.8, 120.7, 141.9,
5.5.5. 5-Chlorobenzoxazole (4j)
White
(KBr) = 3100,1600, 1576, 1445, 1160 cmÀ1
(400 MHz, CDCl3):
solid,
mp
36–38 8C,
FT-IR:
,
nmax
1H NMR
d
= 7.18 (d, J = 8.4 Hz, 1H, CH arom),
5.3. Reusability of the catalyst
7.32 (d, J = 8.8 Hz, 1H, CH arom), 7.59 (s, 1H, CH arom), 7.92
(s, 1H) ppm, 13C NMR (100 MHz, CDCl3):
126.2, 130.2, 141.4, 148.6, 153.7 ppm.
d = 111.7, 120.6,
At the end of the reaction of o-aminophenol (1 mmol),
triethyl orthoacetate (1.1 mmol) and TSA (1 mol%), the
catalyst was filtered, washed with chloroform, and dried at
120 8C for 1 h. Using the recycled catalyst for five
consecutive times in the model reaction afforded the
product with a gradual decrease in the reaction yield. The
results of these observations are shown in Fig. 2.
5.5.6. 5-Chloro-2-ethylbenzoxazole (4l)
White solid, mp 59–61 8C, FT-IR: nmax (KBr) = 3050,
2800, 1600, 1560, 1440, 1160 cmÀ1 1H NMR (400 MHz,
CDCl3): = 1.26 (t, J = 7.6 Hz, 3H, CH3), 2.76 (q, J = 7.6 Hz,
,
d
2H, CH2), 7.07 (m, 1H, CH arom), 7.2 (d, J = 8.4 Hz, 1H, CH
arom), 7.45 (d, J = 1.2 Hz, 1H, CH arom) ppm, 13C NMR
5.4. General procedure for the synthesis of benzoxazole and
benzothiazole derivatives
(100 MHz, CDCl3):
142.5, 149.4 ppm.
d = 10.7, 22.1, 110.9, 124.6, 124.9, 129.4,
To a mixture of orthoester (1.1 mmol) and o-amino-
phenol or o-aminothiophenol (1 mmol) was added TSA (1
mol%). The mixture was stirred at 80–90 8C for the
5.5.7. 2-Methylbenzothiazole (5b)
Pale yellow oil, FT-IR: nmax (KBr) = 3050, 2880,1617,
1575, 1450, 1160 cmÀ1, 1H NMR (400 MHz, CDCl3):
d = 2.84
Please cite this article in press as: Farahi M, et al. Tungstate sulfuric acid as an efficient catalyst for the synthesis of
benzoxazoles and benzothiazoles under solvent-free conditions. C. R. Chimie (2013), http://dx.doi.org/10.1016/
j.crci.2013.03.009

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d = 20.1, 121.3, 122.6,124.3, 126.1, 135.6, 153.3,
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Acknowledgement
We acknowledge the research council of Yasouj
University.
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Please cite this article in press as: Farahi M, et al. Tungstate sulfuric acid as an efficient catalyst for the synthesis of
benzoxazoles and benzothiazoles under solvent-free conditions. C. R. Chimie (2013), http://dx.doi.org/10.1016/
j.crci.2013.03.009