Nano-kaolin/Ti4+/Fe3O4: A magnetic reusable nano-catalyst for the synthesis of pyrimido[2,1-: B] benzothiazoles

Article abstract of DOI:10.1039/c9ra01767d

Herein, nano-kaolin/Ti⁴⁺/Fe₃O₄ as a new magnetic nano-catalyst was synthesized, and its structural properties were characterized using various techniques such as Fourier transform infrared (FTIR) spectroscopy, field emissi

Full text of DOI:10.1039/c9ra01767d

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Nano-kaolin/Ti4+/Fe3O4: a magnetic reusable
nano-catalyst for the synthesis of pyrimido[2,1-b]
benzothiazoles†
Cite this: RSC Adv., 2019, 9, 18720
*
Bi Bi Fatemeh Mirjalili
and Roya Soltani
Herein, nano-kaolin/Ti4+/Fe3O4 as a new magnetic nano-catalyst was synthesized, and its structural
properties were characterized using various techniques such as Fourier transform infrared (FTIR)
spectroscopy, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy
(TEM), X-ray diffraction (XRD), a vibrating sample magnetometer (VSM), thermogravimetric analysis (TGA)
and energy-dispersive X-ray spectroscopy (EDX). This catalyst was used for the synthesis of pyrimido[2,1-
b]benzothiazoles via the one-pot condensation of 2-aminobenzothiazole, an aldehyde and b-keto ester
under solvent-free conditions at 100 ꢀC. This simple protocol has many advantages such as easy
workup, high product yields, short reaction times and reusability of the catalyst.
Received 8th March 2019
Accepted 24th May 2019
DOI: 10.1039/c9ra01767d
rsc.li/rsc-advances
synthesis and characterization of nano-kaolin/Ti4+/Fe3O4 for the
synthesis of 4H-pyrimido[2,1-b]benzothiazoles via the condensa-
Introduction
tion reaction of ethyl acetoacetate, aromatic aldehydes, and 2-
amino benzothiazole.
Pyrimido[2,1-b]benzothiazoles are an important category of
fused heterocycles, with the benzothiazole ring having
diverse pharmaceutical and industrial properties.1 These
compounds have shown various biological activities such as
anti-bacterial,2 anti-tumor,3,4 anti-inammatory,5,6 anti-
viral,7 anticonvulsant,8 anti-cancer9 and antifungal activi-
ties.10 These compounds have been synthesized via the
condensation reaction of 2-amino benzothiazole, b-keto
ester and aldehydes. Previously, various catalysts, such as
thiamine hydrochloride (VB1),11 Fe3O4@nano-cellulose-
TiCl,12 nano-TiCl2/cellulose,13 nanocellulose/BF3/Fe3O4,14
nano-Fe3O4@SiO2–TiCl3,15 kaolin,16 and tetrabutylammo-
nium hydrogen sulfate (TBAHS),17 have been applied for the
preparation of these compounds. However, the drawback of
these protocols is the high cost of the catalyst.
Kaolin or hydrated aluminum silicate (Al2O3$2SiO2$2H2O)18 has
been used in numerous elds such as in the medicine, paint,
ceramic, rubber, paper, petroleum and glass industries.19 More-
over, one of the most valuable application of kaolin is as promoter
in chemical industry;20 in recent years, magnetic nanoparticles
(Fe3O4) have been used due to their advantages such as high
stability, low toxicity and easy separation from reaction media.21–23
On the other hand, single atoms,24–27 such as transition metal
atoms and their ions,28,29 have been widely studied and used for the
promotion of organic reactions. In this study, we report the
Results and discussion
Herein, nano-kaolin/Ti4+/Fe3O4 was prepared as a new
catalyst in two-steps. At rst, TiCl4 was added to the mixture
of nano-kaolin and CH2Cl2. The resulting white powder
(nano-kaolin/Ti4+) was added to the suspension of nano
Fe3O4 in CH2Cl2 under the ultrasonic condition for the
formation of nano-kaolin/Ti4+/Fe3O4 as a brown magnetic
powder (Scheme 1). The morphology and structure of
kaolin/Ti4+/Fe3O4 were studied by various techniques such
as FTIR, FE-SEM, TEM, XRD, VSM, TGA and EDX. The FTIR
spectra of kaolin, Fe3O4, nano-kaolin/Ti4+ and nano-kaolin/
Ti4+/Fe3O4 are shown in Fig. 1. The absorption bands at 3342
and 3620 cmꢁ1 correspond to the stretching vibrations of
the O–H bond. The band at 538 cmꢁ1 is attributed to Fe/O in
the Fe3O4 nanoparticles. The Al–O and Si–O stretching
vibration bands are visible at 420–550 and 1050–1100 cmꢁ1
respectively, in the FTIR spectrum of nano-kaolin/Ti4+
Fe3O4.
,
/
The particle size of nano-kaolin/Ti4+/Fe3O4 was studied
using eld emission scanning electron microscopy (FE-
SEM) and transmission electron microscopy (TEM) and
found to be less than 100 nm (Fig. 2).
Energy-dispersive X-ray spectroscopy (EDS) was used to
Department of Chemistry, College of Science, Yazd University, Yazd, P.O.Box
89195-741, Iran. E-mail: fmirjalili@yazd.ac.ir; Fax: +98 3538210644; Tel: +98
3531232672
determine the percentage of elements in nano-kaolin/Ti4+
/
Fe3O4 (Fig. 3). The percentage of Fe, Si, Al, Ti, Cl, O and K in
nano-kaolin/Ti4+/Fe3O4 was 37.4, 25.5, 12.6, 11.0, 8.2, 4.2
and 1.2, respectively.
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra01767d
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Fig. 1 FTIR spectra of (a) nano-kaolin, (b) Fe3O4, (c) nano-kaolin/Ti4+, and (d) nano-kaolin/Ti4+/Fe3O4.
The thermal stability (TG-DTA) of nano-kaolin/Ti4+/Fe3O4
was studied by thermogravimetric analysis (TGA) in the
temperature range of 50–800 ꢀC (Fig. 4). According to Fig. 4,
ꢀ
at 100–250 C, the catalyst weight was reduced by 4%; this
could be related to the removal of moisture and bonded
water. Moreover, the catalyst lost 80% of its weight in the
temperature range of 250–600 ꢀC probably due to the
collapse of the kaolin network. According to the TGA curve,
this catalyst is stable up to 230 ꢀC and suitable for reactions
ꢀ
that are carried out at temperatures below 230 C.
The vibrating sample magnetometer (VSM) pattern of the
catalyst at room temperature shows that the coercivity value
is zero, and there is no hysteresis loop and remanence; this
conrms the catalytic superparamagnetic property (Fig. 5).
The saturation magnetization (Ms) values of Fe3O4 and
nano-kaolin/Ti4+/Fe3O4 were 50 and 23 emu gꢁ1, respec-
tively. Although the magnetization of the catalyst is lower
than that of Fe3O4, the proposed catalyst can be easily
separated from the solution using an external magnet.
Scheme 1 Preparation of nano-kaolin/Ti4+/Fe3O4.
The X-ray diffraction (XRD) pattern of nano-kaolin/Ti4+
/
Fe3O4 is shown in Fig. 6. According to the XRD pattern, the
signals at 2q ¼ 13ꢀ, 20ꢀ, 25ꢀ, 41ꢀ, 46ꢀ, 61ꢀ and 68ꢀ indicate the
Fig. 2 (a) FE-SEM and (b) TEM images of nano-kaolin/Ti4+/Fe3O4.
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Fig. 3 The EDX spectra of nano-kaolin/Ti4+/Fe3O4.
Fig. 4 Thermal gravimetric analysis of nano-kaolin/Ti4+/Fe3O4.
presence of kaolin. The four signals at 2q ¼ 21ꢀ, 27ꢀ, 39ꢀ and
50ꢀ indicate the existence of SiO2. Moreover, the signals at
2q ¼ 31ꢀ, 36ꢀ, 44ꢀ, 58ꢀ and 63ꢀ are related to Fe3O4.
Presumably, three other peaks at the 2q value of 37ꢀ, 43ꢀ and
55ꢀ revealed that Ti was bonded to kaolin and Fe3O4.
The catalytic activity of the catalyst was investigated for
the synthesis of 4H-pyrimido[2,1-b]benzothiazole using the
three-component reaction of b-keto ester, aromatic alde-
hydes and 2-aminobenzothiazole. To select optimum
conditions, the reaction of ethyl acetoacetate, benzaldehyde
and 2-amino benzothiazole was studied as a model reaction
under different conditions. According to Table 1, the best
Fig. 5 Magnetization loops of (a) Fe3O4 and (b) nano-kaolin/Ti4+
Fe3O4.
/
Fig. 6 XRD pattern of nano-kaolin/Ti4+/Fe3O4.
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Table 1 The reaction of 2-aminobenzothiazole, benzaldehyde and ethyl acetoacetate in the presence of nano-kaolin/Ti4+/Fe3O4 under various
conditionsa
Entry
Catalyst (g)
Solvent/condition
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
Fe3O4 (0.03)
Kaolin (0.03)
—/100 ꢀC
—/100 ꢀC
—/100 ꢀC
H2O/reux
C2H5OH/reux
—/80 ꢀC
1.5
1.5
1.5
3
4
2
52
72
78
50
30
82
75
95
65
95
78
Nano-kaolin/Ti4+ (0.03)
Catalystc (0.03)
Catalystc (0.03)
Catalystc (0.03)
Catalystc (0.03)
Catalystc (0.03)
Catalystc (0.025)
Catalystc (0.03)
Catalystc (0.04)
—/90 ꢀC
2
—/110 ꢀC
—/100 ꢀC
—/100 ꢀC
—/100 ꢀC
1.5
1.5
1.5
1.5
9
10
11
a
b
The amount ratios of 2-aminobenzothiazole (mmol), benzaldehyde (mmol) and ethyl acetoacetate (mmol) equals to 1 : 1 : 1. Isolated yield.
Nano-kaolin/Ti4+/Fe3O4.
c
conditions for the synthesis of 4H-pyrimido[2,1-b]benzo-
According to the conditions optimized for the model
thiazole under solvent-free conditions correspond to the reaction, 4H-pyrimido[2,1-b]benzothiazole derivatives were
ꢀ
utilization of 0.03 g of catalyst at 100 C.
synthesized by the reaction of various aromatic aldehydes,
Table 2 The synthesis of 4H-pyrimido[2,1-b]benzothiazole derivativesa
Melting point
Entry
R
Product
Time (h)
Yieldb (%)
Observed
Reported
Ref.
1
2
3
4
5
6
7
8
H
4-NO2
4-Cl
IVa
IVb
IVc
IVd
IVe
IVf
IVg
IVh
IVi
IVj
1.5
0.5
2.2
1.6
3
95
98
95
92
90
50
78
80
94
70
85
60
178–180
172–173
141–143
111–113
210–214
120–124
131–133
222–224
259–261
133–135
170–172
227–229
178–180
170–172
140–142
110–114
110–112
123–125
130–132
222–224
260–263
133–135
171–173
225–227
23
30
15
31
30
14
17
23
12
13
12
15
4-Br
4-OH
2-NO2
2-Cl
3-NO2
3-OH
2,4-(Cl)2
2-OEt
3,4-(OH)2
1
2.2
0.5
1.25
1.4
1.1
0.9
9
10
11
12
IVk
IVl
a
One mmol of 2-aminobenzothiazole, aldehyde and ethyl acetoacetate was used. b Isolated yield.
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Fig. 7 The reusability experiment data of the catalyst.
Fig. 8 The FTIR spectrum of the reused catalyst.
2-aminobenzothiazole and ethylacetoacetate in the pres-
The proposed mechanism for the synthesis of 4H-pyr-
ence of the nano-kaolin/Ti4+/Fe3O4 catalyst. Based on the imido[2,1-b]benzothiazoles is shown in Scheme 2. In this
results tabulated in Table 2, the effects of the electron and reaction, the Ti4+ cation in the catalyst acts as a Lewis acid
the nature of the substituent on aldehyde signicantly and activates the carbonyl groups in the substrates. At rst,
affected the time and yield of the reaction. The presence of the aldehyde (I) as an electrophile and b-keto esters (II) as
electron-withdrawing groups in aryl aldehyde rings active methylene compounds produce the alkene (IV) via the
increased the activity of the aldehyde group and their yields Knoevenagel reaction. Then, the 2-aminobenzothiazole (III)
were compared with that of electron-donating groups; the reacts with the alkene (IV) via a Michael addition reaction,
structure of the product was identied using melting point, and an iminium ion (V) is formed. Subsequently, using
1
FTIR, and H-NMR spectral results.
proton transfer and intramolecular cyclization, the 4H-pyr-
To investigate the recovery and reuse function of nano- imido[2,1-b]benzothiazole derivative(VII) is formed.
kaolin/Ti4+/Fe3O4, the catalyst was used for seven times in To investigate the performance of the nano-kaolin/Ti4+
/
the model reaction under identical conditions. Aer using Fe3O4 catalyst in synthesis of 4H-pyrimido[2,1-b]benzo-
the catalyst in the model reaction, it was isolated by an thiazole derivatives, condensation of benzaldehydes, 2-
external magnet, washed with ethanol and then dried at aminobenzothiazole and ethyl acetoacetate was employed
room temperature. The results indicate that the catalyst as the model reaction, and the results were compared with
nano-kaolin/Ti4+/Fe3O4 can be reused without any signi- those obtained using other reported catalysts. The results of
cant loss of its catalytic activity (Fig. 7). The FTIR spectrum this study are shown in Table 3. According to the obtained
of the reused catalyst shows that there is no change in the results, the nano-kaolin/Ti4+/Fe3O4 is one of the best cata-
catalyst structure during the recovery process (Fig. 8).
lyst for this purpose.
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Scheme 2 A proposed mechanism for the preparation of 4H-pyrimido[2,1-b]benzothiazole derivatives.
Table 3 Comparison of the nano-kaolin/Ti4+/Fe3O4 catalyst with other reported catalysts for the synthesis of ethyl-2-methyl-4-(phenyl)-4H-
pyrimido[2,1-b][1,3]benzothiazole-3-carboxylatea
Entry
Catalyst
Solvent/condition
Time (h)
Yieldb (%) (ref.)
1
2
3
4
5
6
7
TMGTc (0.08 g)
—/100 ꢀ
C
5
2
12
1.2
2
66 (31)
83 (17)
65 (32)
79 (30)
85 (33)
80 (14)
95 (this work)
TBAHSd (10 mol%)
Ethylene glycol/120 ꢀ
C
Acetic acid (10 mol%)
AlCl3 (10 mol%)
FeF3 (10 mol%)
Nano-cellulose/BF3/Fe3O4 (0.06 g)
Nano-kaolin/TiCl4/Fe3O4 (0.03 g)
Methanol/reux
—/60 ꢀ
—/80 ꢀ
C
C
—/100 ꢀ
—/100 ꢀ
C
C
1
1.5
a
b
c
One mmol of any substrate (2-aminobenzothiazole, benzaldehyde and ethyl acetoacetate) was used. Isolated yield. 1,1,3,3-N,N,N0,N0-
d
Tetramethylguanidinium triuoroacetate. Tetrabutylammonium hydrogen sulfate.
Conclusion
Experimental
General remarks
Due to the importance of green chemistry, we have intro-
duced the preparation and characterization of nano-kaolin/ A Bruker (DRX-400 Avance) NMR spectrometer was used to
Ti4+/Fe3O4 as a novel, eco-friendly and magnetically recy- obtain the NMR spectra. FTIR spectra were obtained using
clable heterogeneous catalyst. This catalyst was used to the Bruker, Equinox 55 spectrometer. Melting points were
synthesize 4H-pyrimido[2,1-b]benzothiazole derivatives via determined by the Buchi melting point B-540 B.V.CHI appa-
the one-pot three-component reaction of 2-amino- ratus. Field emission scanning electron microscopy (FE-SEM)
benzothiazole, aldehydes and ethyl acetoacetate under was carried out via Mira 3-XMU, and transmission electron
solvent-free conditions at 100 ꢀC. Some of the important microscopy (TEM) was conducted using Philips CM120 with
advantages of this protocol include short reaction times, the LaB6 cathode and the accelerating voltage of 120 kV. The
high yields, easy work-up procedure and easy separation of X-ray diffraction (XRD) pattern was obtained by the Philips
the catalyst with reusability.
X'pert MPD diffractometer equipped with a Cu Ka anode (k ¼
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ꢀ
1.54 A) in the 2q range from 10 to 80ꢀ. The VSM measure-
ments were performed using a vibrating sample magnetom-
eter (Meghnatis Daghigh Kavir Co. Kashan, Iran). The
thermogravimetric analysis (TGA) was conducted by
NETZSCH TG 209 F1 Iris. Energy-dispersive X-ray spectros-
copy (EDS) of the catalyst was conducted by the EDS instru-
ment Phenom pro X.
˚
Acknowledgements
The Research Council of Yazd University is gratefully acknowl-
edged for the nancial support for this work.
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General procedure for the synthesis of 4H-pyrimido[2,1-b]
benzothiazole derivatives
In a sand bath, a mixture of aldehyde (1 mmol), ethyl ace-
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nano-kaolin/Ti4+/Fe3O4 (0.03 g) was heated to 100 ꢀC. The
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