ZnO-beta zeolite: As an effective and eco-friendly heterogeneous catalyst for the synthesis of benzothiazole derivatives

Article abstract of DOI:10.1016/j.cclet.2009.10.020

A cheap and recyclable ZnO-beta zeolite was used as catalyst for the synthesis of benzothiazole derivatives. This method provides several advantages such as environmental friendliness, short reaction times, high yields, simple work-up procedure and cataly

Full text of DOI:10.1016/j.cclet.2009.10.020

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Chinese Chemical Letters 21 (2010) 421–425
www.elsevier.com/locate/cclet
ZnO-beta zeolite: As an effective and eco-friendly heterogeneous
catalyst for the synthesis of benzothiazole derivatives
Santosh S. Katkar a, Pravinkumar H. Mohite b, Lakshman S. Gadekar a,
Kaluram N. Vidhate a, Machhindra K. Lande a,
*
a Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad 431004, MS, India
b Chemical Engineering and Process Development Division, National Chemical Laboratory, Pune 411008, MS, India
Received 19 August 2009
Abstract
A cheap and recyclable ZnO-beta zeolite was used as catalyst for the synthesis of benzothiazole derivatives. This method
provides several advantages such as environmental friendliness, short reaction times, high yields, simple work-up procedure and
catalyst was successfully reused for four cycles without significant loss of activity.
# 2009 Machhindra K. Lande. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: ZnO-beta zeolite; Benzothiazole; Cyclocondensation; Heterogeneous catalyst
Zeolites have been used as catalysts in the petroleum refining and chemical industries [1,2]. The catalytic activity of
H-beta zeolite can be adjusted by ion-exchange with metal ions and acid treatment [3,4]. Zinc loaded zeolites are used
for organic transformation, such as Heck reaction [5a], aromatization [5b], dehydrogenation of small paraffins [5c]
and aromatization of in situ generated ethylene [6].
2-Aryl substituted benzothiazoles are studied as antitumour [7a], antimicrobial [7b], antifungal [7c] agents as well
as imaging agents for b-amyloid [7d], inhibition of enzymes such as cyclooxygenase [8a], acetylcholine esterase [8b]
and plant growth regulation [8c].
In view of importance of benzothiazole and their derivatives, many methods have been reported for the synthesis of
benzothiazole, such as palladium-catalyzed Suzuki biaryl coupling of 2-bromobenzothiazole with arylboronicacids
[9a], coupling of benzothiazoles with aryl bromides [9b] and the reaction between thiophenols and aromatic nitriles
[9c]. More recently, the reaction also catalyzed by silica gel [10], iodine [11] and Backer’s yeast [12]. However, many
of these methods are associated with several disadvantages such as long reaction time, drastic reaction conditions, very
expensive reagents, low yields and tedious work-up procedures. The main disadvantage is that the catalyst is destroyed
during the work-up procedure and cannot be recovered and reused. Therefore, it is important to find a simple, cheap,
recoverable and reusable catalyst for the synthesis of benzothiazoles. To the best of our knowledge is concerned the
use of ZnO-beta zeolite as a catalyst in the synthesis of benzothiazoles has not been reported yet.
* Corresponding author.
E-mail address: mkl_chem@yahoo.com (M.K. Lande).
1001-8417/$ – see front matter # 2009 Machhindra K. Lande. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2009.10.020

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S.S. Katkar et al. / Chinese Chemical Letters 21 (2010) 421–425
1. Experimental
All the chemicals were purchased from Merck, S. D. Fine India and used without further purification. The X-ray
powder diffraction patterns of catalyst were recorded by using Bruker 8D X-ray diffractometer using CuKa radiation
of wavelength = 1.54056 A8. FT-IR spectra were recorded on JASCO FT-IR-4100, Japan. Scanning electron
microscope (SEM) image with energy dispersive X-ray spectroscopy (EDS) was obtained on JEOL, JSM-6330 LA
operated at 20.0 kVand 1.0000 nA. Brunnauer–Emmett–Teller (BET) surface area has been measured by means of N2
adsorption at 77 K preformed on a Quantachrome CHEMBET 3000 instrument. Temperature programmed desorption
(TPD) measurements were carried out on a Quantachrome CHEMBET 3000 TPR/TPD instrument.
Tetraethyl orthosilicate (TEOS), was added to a mixture of tetraethyl ammonium hydroxide (TEAOH), sodium
hydroxide (NaOH) and an aqueous solution of aluminium sulphate (Al2(SO4)3) and stirred at room temperature for
24 h. Then this mixture was hydrothermally treated at 120 8C for 96 h in autoclavable bottle, and then was cooled at
room temperature, solid material was filtered and washed with deionised water, dried at 80 8C for 6 h and calcined at
550 8C for 12 h. H-form of beta zeolite was prepared by ion-exchange with 1 M ammonium acetate solution at 80 8C
for 10 h and the resulting product was calcined at 550 8C for 8 h. H-beta zeolite was modified by impregnation method
by mixing H-beta zeolite with aqueous solution of zinc acetate. The resultant mixture was heated at 80 8C for 8 h, dried
and calcined at 550 8C for 8 h.
A mixture of aldehyde (2 mmol), 2-aminothiophenol (2 mmol) and ZnO-beta zeolite catalyst (0.1 gm) was refluxed
in ethanol (10 mL) for the time mentioned in Table 2. The reaction was monitored by TLC. After completion of
reaction, the reaction mixture filter in hot condition to separate the catalyst, poured into crushed ice and the solid
product was filtered and recrystallized.
Fig. 1 shows the FT-IR spectrum of H-beta zeolite and ZnO-beta zeolite. In the present work IR bands was found at
567 cmÀ1 and 517 cmÀ1 corresponds to H-beta zeolite [13]. In case of zinc modified beta zeolite disappearance of
peak at 619 cmÀ1 was observed. EDS analysis of ZnO-beta zeolite gives 2.75 mass% zinc and 2.57 mass% aluminium
(Fig. 2). Total acidity and specific surface area of ZnO-beta zeolites were calculated from TPD and BET
measurements; it was found that the total acidity 0.703 mmol/g and specific surface area 137 m2/g.
Fig. 1. FT-IR spectra of H-beta zeolite (a), ZnO-beta zeolite (b).
Fig. 2. EDS spectrum of ZnO-beta zeolite.

S.S. Katkar et al. / Chinese Chemical Letters 21 (2010) 421–425
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2. Result and discussion
Fig. 3 shows the XRD pattern of H-beta and ZnO-beta zeolite. The powder XRD pattern is the typical type for H-
beta zeolite by comparing it with the reference H-beta zeolite [14]. It was observed that, there is no visible difference
between H-beta zeolite and zinc modified beta zeolite, only the intensity of the peaks decreases than the parent zeolite.
It is clear that beta zeolite contain typical diffraction peaks (2u = 7.88 and 22.38).
SEM images of H-beta zeolite and ZnO-beta zeolite are presented in Fig. 4. As can be seen from Fig. 4 (A and B)
particles are well agglomerated, a slight change has been observed in ZnO-beta zeolite may be due to the ZnO particles
(Fig. 4B).
In view of our continued interest in the development of novel synthetic methodologies for the synthesis of
heterocyclic compounds [15,16], herein, we would like to report a simple, efficient and rapid method for the synthesis
of benzothiazoles.
Initially, the reaction of benzaldehyde (2 mmol) and 2-aminothiophenol (2 mmol) was chosen as a model reaction
for the optimization of solvent effect and catalyst amount (Table 1, entries 1–8). Reaction in 1,4-dioxane,
dimethylformamide (DMF), dichloromethane (DCM) solvent gave low product yields even after 120 min, 160 min,
and 135 min (Table 1, entries 1, 2, 3). Although the yields were moderate in case of acetonitrile and methanol (Table 1,
entries 4 and 5). We found ethanol and 0.1 g of ZnO-beta zeolite catalyst an efficient reaction medium in terms of
reaction time as well as yield (Table 1, entry 6). It is noteworthy to mention that in the absence of catalyst, no product
was found even after 12 h. These results indicate that the catalyst exhibits a high catalytic activity in this
transformation.
Encouraged by these results, a variety of aldehyde with electron donating and withdrawing substituents was used
for the highly selective synthesis of benzothiazoles in the presence of ZnO-beta zeolite under similar experimental
conditions. However, the variations in the yields were very little and both substituted aromatic aldehydes such as 4-
nitro and 2-nitro as well as heterocyclic aldehyde furfuraldehyde gave the desired products in excellent yields
Fig. 3. XRD pattern of H-beta zeolite and ZnO-beta zeolite.
Fig. 4. SEM images of H-beta zeolite (A), ZnO-beta zeolite (B).

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S.S. Katkar et al. / Chinese Chemical Letters 21 (2010) 421–425
Table 1
Optimization of reaction conditions for the synthesis of 2-phenyl-1H benzothiazole (Table 2, compound 3a)a.
Entry
Solvent
Catalyst (g)
Time (min)
Yield (%)b
1
2
3
4
5
6
7
8
1,4-Dioxane
DMF
0.1
0.1
0.1
0.1
0.1
0.1
0.05
0.2
120
160
135
90
65
75
79
85
81
93
89
93
DCM
Acetonitrile
Methanol
Ethanol
Ethanol
Ethanol
100
60
70
60
a
All reactions were carried out at reflux condition.
Yield refers to isolated product.
b
Table 2
Synthesis of benzothiazole derivatives catalyzed by ZnO-beta zeolitea.
Entry
R
Time (min)
Yield (%)b
mp (8C)
Found
Reported
3a
3b
3c
3d
3e
3f
C6H5
60
65
90
75
85
60
55
53
72
70
85
93 (93, 93, 92, 91)c
114–115
84–85
113–114 [10]
85 [10]
4-CH3C6H4
4-NO2C6H4
4-OHC6H4
2-NO2C6H4
2-OHC6H4
2-ClC6H4
4-ClC6H4
4-OMeC6H4
2-OMeC6H4
Furyl
91
87
91
86
89
93
92
93
92
89
229–230
227–228
128–129
130–131
82–83
230–231 [10]
227 [10]
127–128 [10]
131–132 [10]
81–82 [10]
3g
3h
3i
117–118
122–123
102–103
102–104
117–118 [10]
121–122 [10]
101–102 [10]
102–104 [11]
3j
3k
a
All reactions were carried out at reflux condition.
Isolated yields.
b
c
Catalyst was reused for four times.
Scheme 1. Synthesis of benzothiazoles using 2-aminothiophenol (1) aldehydes and (2) ZnO-beta zeolite as a catalyst under reflux condition.
(Table 2, compound 3c, 3e and 3k). The products (3a–k) were confirmed by comparisons with authentic samples, IR,
1H NMR, mass spectra and melting points [17] (Scheme 1).
Next we investigate the possibility of recycling the catalyst for the same model reaction and the results of recycling
experiments are shown in Table 2. These results clearly indicate that the recovered ZnO-beta zeolite can be recycled
successfully without significant loss of activity. The catalyst having recycling performance is also an attractive
property for the environmental protection and economic reasons.
3. Conclusion
In conclusion, we have found a novel procedure for the preparation of benzothiazole derivatives. The main
advantages of this procedure include high yields, simple experimental procedure and reusability of the catalyst.
Therefore, the present method is expected to serve as an alternative process for the preparation of benzothiazole
derivatives.

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425
Acknowledgment
We are grateful to the Head, Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University,
Aurangabad 431004 (MS), India for providing the laboratory facility.
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