Bi Bi Fatemeh Mirjalili* and
Roya Soltani
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
First published on 14th June 2019
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 β-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.
Kaolin or hydrated aluminum silicate (Al2O3·2SiO2·2H2O)18 has been used in numerous fields such as in the medicine, paint, ceramic, rubber, paper, petroleum and glass industries.19 Moreover, 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 synthesis and characterization of nano-kaolin/Ti4+/Fe3O4 for the synthesis of 4H-pyrimido[2,1-b]benzothiazoles via the condensation reaction of ethyl acetoacetate, aromatic aldehydes, and 2-amino benzothiazole.
Fig. 1 FTIR spectra of (a) nano-kaolin, (b) Fe3O4, (c) nano-kaolin/Ti4+, and (d) nano-kaolin/Ti4+/Fe3O4. |
The particle size of nano-kaolin/Ti4+/Fe3O4 was studied using field 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 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.
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 confirms the catalytic superparamagnetic property (Fig. 5). The saturation magnetization (Ms) values of Fe3O4 and nano-kaolin/Ti4+/Fe3O4 were 50 and 23 emu g−1, respectively. 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.
The X-ray diffraction (XRD) pattern of nano-kaolin/Ti4+/Fe3O4 is shown in Fig. 6. According to the XRD pattern, the signals at 2θ = 13°, 20°, 25°, 41°, 46°, 61° and 68° indicate the presence of kaolin. The four signals at 2θ = 21°, 27°, 39° and 50° indicate the existence of SiO2. Moreover, the signals at 2θ = 31°, 36°, 44°, 58° and 63° are related to Fe3O4. Presumably, three other peaks at the 2θ value of 37°, 43° and 55° 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 β-keto ester, aromatic aldehydes 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 conditions for the synthesis of 4H-pyrimido[2,1-b]benzothiazole under solvent-free conditions correspond to the utilization of 0.03 g of catalyst at 100 °C.
Entry | Catalyst (g) | Solvent/condition | Time (h) | Yieldb (%) |
---|---|---|---|---|
a The amount ratios of 2-aminobenzothiazole (mmol), benzaldehyde (mmol) and ethyl acetoacetate (mmol) equals to 1:1:1.b Isolated yield.c Nano-kaolin/Ti4+/Fe3O4. | ||||
1 | Fe3O4 (0.03) | —/100 °C | 1.5 | 52 |
2 | Kaolin (0.03) | —/100 °C | 1.5 | 72 |
3 | Nano-kaolin/Ti4+ (0.03) | —/100 °C | 1.5 | 78 |
4 | Catalystc (0.03) | H2O/reflux | 3 | 50 |
5 | Catalystc (0.03) | C2H5OH/reflux | 4 | 30 |
6 | Catalystc (0.03) | —/80 °C | 2 | 82 |
7 | Catalystc (0.03) | —/90 °C | 2 | 75 |
8 | Catalystc (0.03) | —/110 °C | 1.5 | 95 |
9 | Catalystc (0.025) | —/100 °C | 1.5 | 65 |
10 | Catalystc (0.03) | —/100 °C | 1.5 | 95 |
11 | Catalystc (0.04) | —/100 °C | 1.5 | 78 |
According to the conditions optimized for the model reaction, 4H-pyrimido[2,1-b]benzothiazole derivatives were synthesized by the reaction of various aromatic aldehydes, 2-aminobenzothiazole and ethylacetoacetate in the presence of the nano-kaolin/Ti4+/Fe3O4 catalyst. Based on the results tabulated in Table 2, the effects of the electron and the nature of the substituent on aldehyde significantly affected the time and yield of the reaction. The presence of electron-withdrawing groups in aryl aldehyde rings increased the activity of the aldehyde group and their yields were compared with that of electron-donating groups; the structure of the product was identified using melting point, FTIR, and 1H-NMR spectral results.
Entry | R | Product | Time (h) | Yieldb (%) | Melting point | Ref. | |
---|---|---|---|---|---|---|---|
Observed | Reported | ||||||
a One mmol of 2-aminobenzothiazole, aldehyde and ethyl acetoacetate was used.b Isolated yield. | |||||||
1 | H | IVa | 1.5 | 95 | 178–180 | 178–180 | 23 |
2 | 4-NO2 | IVb | 0.5 | 98 | 172–173 | 170–172 | 30 |
3 | 4-Cl | IVc | 2.2 | 95 | 141–143 | 140–142 | 15 |
4 | 4-Br | IVd | 1.6 | 92 | 111–113 | 110–114 | 31 |
5 | 4-OH | IVe | 3 | 90 | 210–214 | 110–112 | 30 |
6 | 2-NO2 | IVf | 1 | 50 | 120–124 | 123–125 | 14 |
7 | 2-Cl | IVg | 2.2 | 78 | 131–133 | 130–132 | 17 |
8 | 3-NO2 | IVh | 0.5 | 80 | 222–224 | 222–224 | 23 |
9 | 3-OH | IVi | 1.25 | 94 | 259–261 | 260–263 | 12 |
10 | 2,4-(Cl)2 | IVj | 1.4 | 70 | 133–135 | 133–135 | 13 |
11 | 2-OEt | IVk | 1.1 | 85 | 170–172 | 171–173 | 12 |
12 | 3,4-(OH)2 | IVl | 0.9 | 60 | 227–229 | 225–227 | 15 |
To investigate the recovery and reuse function of nano-kaolin/Ti4+/Fe3O4, the catalyst was used for seven times in the model reaction under identical conditions. After using the catalyst in the model reaction, it was isolated by an external magnet, washed with ethanol and then dried at room temperature. The results indicate that the catalyst nano-kaolin/Ti4+/Fe3O4 can be reused without any significant loss of its catalytic activity (Fig. 7). The FTIR spectrum of the reused catalyst shows that there is no change in the catalyst structure during the recovery process (Fig. 8).
The proposed mechanism for the synthesis of 4H-pyrimido[2,1-b]benzothiazoles is shown in Scheme 2. In this reaction, the Ti4+ cation in the catalyst acts as a Lewis acid and activates the carbonyl groups in the substrates. At first, the aldehyde (I) as an electrophile and β-keto esters (II) as active methylene compounds produce the alkene (IV) via the Knoevenagel reaction. Then, the 2-aminobenzothiazole (III) reacts with the alkene (IV) via a Michael addition reaction, and an iminium ion (V) is formed. Subsequently, using proton transfer and intramolecular cyclization, the 4H-pyrimido[2,1-b]benzothiazole derivative(VII) is formed.
To investigate the performance of the nano-kaolin/Ti4+/Fe3O4 catalyst in synthesis of 4H-pyrimido[2,1-b]benzothiazole derivatives, condensation of benzaldehydes, 2-aminobenzothiazole and ethyl acetoacetate was employed as the model reaction, and the results were compared with those obtained using other reported catalysts. The results of this study are shown in Table 3. According to the obtained results, the nano-kaolin/Ti4+/Fe3O4 is one of the best catalyst for this purpose.
Entry | Catalyst | Solvent/condition | Time (h) | Yieldb (%) (ref.) |
---|---|---|---|---|
a One mmol of any substrate (2-aminobenzothiazole, benzaldehyde and ethyl acetoacetate) was used.b Isolated yield.c 1,1,3,3-N,N,N′,N′-Tetramethylguanidinium trifluoroacetate.d Tetrabutylammonium hydrogen sulfate. | ||||
1 | TMGTc (0.08 g) | —/100 °C | 5 | 66 (31) |
2 | TBAHSd (10 mol%) | Ethylene glycol/120 °C | 2 | 83 (17) |
3 | Acetic acid (10 mol%) | Methanol/reflux | 12 | 65 (32) |
4 | AlCl3 (10 mol%) | —/60 °C | 1.2 | 79 (30) |
5 | FeF3 (10 mol%) | —/80 °C | 2 | 85 (33) |
6 | Nano-cellulose/BF3/Fe3O4 (0.06 g) | —/100 °C | 1 | 80 (14) |
7 | Nano-kaolin/TiCl4/Fe3O4 (0.03 g) | —/100 °C | 1.5 | 95 (this work) |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra01767d |
This journal is © The Royal Society of Chemistry 2019 |