Using a domestic microwave oven for synthesis of benzo[d]thiazole derivatives

Abstract. Reaction of o-aminothiophenol and aldehydes, a novel method to make benzo[d]thiazole derivatives in one step, was accelerated with a domestic microwave oven and gave 22 benzo[d]thiazole derivatives in excellent yield (up to 98%), within short reaction time (3-4 min) along with other advantages like mild reaction conditions and safer environmental conditions: in air, short time, no solvent, no catalyst, ease of purification. Some benzo[d]thiazole derivatives’ structures were confirmed by NMR and MS analysis.

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127 HNUE JOURNAL OF SCIENCE DOI: 10.18173/2354-1059.2018-0037 Natural Sciences 2018, Volume 63, Issue 6, pp. 127-135 This paper is available online at USING A DOMESTIC MICROWAVE OVEN FOR SYNTHESIS OF BENZO[d]THIAZOLE DERIVATIVES Duong Quoc Hoan 1 , Nguyen My Linh 1 , Phan Thi Hoa 1 , Hoang Thi Nhu Quynh 1 and Vu Thi Anh Tuyet 2 1 Faculty of Chemistry, Hanoi National University of Education 2 Faculty of Science, Lang Son College of Education Abstract. Reaction of o-aminothiophenol and aldehydes, a novel method to make benzo[d]thiazole derivatives in one step, was accelerated with a domestic microwave oven and gave 22 benzo[d]thiazole derivatives in excellent yield (up to 98%), within short reaction time (3-4 min) along with other advantages like mild reaction conditions and safer environmental conditions: in air, short time, no solvent, no catalyst, ease of purification. Some benzo[d]thiazole derivatives’ structures were confirmed by NMR and MS analysis. Keywords: Domestic microwave oven, benzo[d]thiazole, solvent-free, green chemistry. 1. Introduction Microwave was the first designed during the Second World War II, but it was not until 1986 microwave ovens were used to accelerate organic reaction by Gedye, Majetich and their co-workers [1, 2]. Then, the a range of organic reactions could be accelerated under microwave conditions, the use of microwave dielectric heating in organic, inorganic and organometallic chemistry has expanded very rapidly and now there are more than 2000 papers describing the application of this technique for the synthesis of new compounds [3, 4]. Nowadays, microwave- accelerated organic synthesis is very popular such as microwave-accelerated metal catalysis [5]; heterocyclic chemistry [6]; microwave-assisted reductions [7]; speed and efficiency in the production of diverse structures: microwave-assisted multi-component reactions [8]. Benzo[d]thiazole derivatives show remarkable bioactivities such as: anti-cancer activities [9, 10], antimicrobials [11], anti-inflammatory [12]. Therefore, its synthesis has attracted chemists’ attention. There are many methods for synthesis of benzo[d]thiazole derivatives containing. For instance, Hu et al. reported that a straightforward synthesis of 2-arylbenzothiazoles from 2-aminothiophenol and aryl aldehydes in air/DMSO oxidant system is operationally simple, proceeds without catalysts, tolerates a wide range of functionalities, and provides desired products in good to excellent yields [13]; A simple, green, and efficient method enables the synthesis of benzoxazoles and benzothiazoles from o-amino(thio)phenols and aldehydes using samarium triflate as a reusable acid catalyst under mild reaction conditions in aqueous medium [14, 15]. Received June 15, 2018. Revised July 13, 2018. Accepted July 20, 2018. Contact Duong Quoc Hoan, e-mail address: hoandq@hnue.edu.vn Duong Quoc Hoan, Nguyen My Linh, Phan Thi Hoa, Hoang Thi Nhu Quynh and Vu Thi Anh Tuyet 128 Moreover, a copper-catalyzed condensation of 2-aminobenzenethiols with nitriles is able to enable an efficient and convenient synthesis of 2-substituted benzothiazoles. The developed method is applicable to a wide range of nitriles containing different functional groups furnishing excellent yields of the corresponding products [16]. Decarboxylative redox cyclization strategy enables the synthesis of 2-substituted benzothiazoles from o-chloronitroarenes and arylacetic acids in the presence of elemental sulfur/N-methylmorpholine under metal- and solvent-free conditions [17] and so on [18-20]. However, these methods take some disadvantages such as long time, using solvents for reactions causing high expense and environmental problems. Our previous paper reported that a domestic microwave oven could deal these issues for organic synthesis [21]. Taking advantages of microwave in organic synthesis, this paper shows the results of using a domestic microwave oven in synthesis of benzo[d]thiazole derivatives as a green chemistry method in organic synthesis. 2. Content 2.1. Experiments and synthetic procedure 2.1.1. Experiments Solvents and other chemicals were purchased from Sigma-Aldrich, Merck Corp, Aladdin, Vietnam or other China’s companies were used as received, unless indicated. The 1H NMR spectra were recorded on the Bruker Avance 500 NMR spectrometer in DMSO-d6 in The Vietnam Academy of Science and Technology. Chemical-shift data for each signal was reported in ppm units. Domestic Sanyo microwave oven, Sanyo EM - S1065, 800W Microwave Power, made in Thailand 2005, was used to carry out the reactions. 2.1.2. Synthetic procedure General procedure: To a mixture of o-aminothiophenol (0.34 mL, 2.1 mmol, 152 g/mol) and an aldehyde (2.0 mmol) in a 250 mL beaker was irradiated 3-4 min at 400W power level. The progress of reaction was monitored with TLC in every 30 seconds. The mixture was then dissolved in ethyl acetate and n-hexane and stood at room temperature to form solid. 2-(4,5-dimethoxy-2-nitrophenyl)benzo[d]thiazole (5) 1 H NMR (500 MHz, DMSOd6)  7.69 (s, 1H), 7.33 (s, 1H), 6.98 (dd, J = 8.0, 1.0 Hz, 1H), 6.92 (td, J = 8.0, 1.0 Hz, 1H), 6.79 (d, J = 7.0 Hz, 1H), 6.64 (td, J = 7.5, 1.0 Hz, 1H), 3.86 (s, 3H), 3.76 (s, 3H). 2-(3,4-dimethoxyphenyl)benzo[d]thiazole (6) 1 H NMR (500 MHz, DMSOd6)  8.11 (s, 1H), 8.02 (d, J = 8.0 Hz, 1H), 7.65 (d, J = 2.0 Hz, 1H), 7.63 (dd, J = 8.5, 2.5 Hz, 1H), 7.52 (td, J = 8.0, 1.0 Hz, 1H), 7.43 (td, J = 8.0, 1.0 Hz, 1H), 7.13 (d, J = 8.5 Hz, 1H), 3.89 (s, 3H), 3.86 (s, 3H). 4-(benzo[d]thiazol-2-yl)-2-bromo-6-methoxyphenol (7) IR v (cm -1 ) 3320(br), 3270, 3138, 3000, 2856, 1577, 1510; 1 H NMR (500 MHz, DMSOd6)  10.29 (s, 1H, OH), 8.10 (d, J = 8.0, 1H, H2), 8.02 (d, J = 8.0 Hz, 1H, H5) 7.75 (d, J = 1.0 Hz, 1H, H13), 7.52 (t, J = 7.5 Hz, 1H, H4), 7.43 (t, J = 7.5 Hz, 1H, H3), 7.06 (d, J = 1.0 Hz, H9), 3.96(s, 3H, H14); 13 C NMR (125 MHz, DMSOd6)  165.9 (C7), 153.4 (C6), 148.6 (C10), 146.8 (C11), 134.3 (C1), 126.5 (C4), 125.2(C3), 125.0 (C8), 123.5 (C13), 122.5 (C5), 122.1(C2), 109.6(C12), 109.2 (C9), 56.3 (C14); EI-MS m/z: [M+H] + Calcd for C14H11 79 BrNO2S 336.0, found 335.8; [M+H] + Calcd for C14H11 81 BrNO2S 338.0, found 337.8; [M-H] + Calcd for C14H9 79 BrNO2S 334.0, found 333.8; [M-H] + Calcd for C14H9 81 BrNO2S 336.0, found 335.8. Using a domestic microwave oven for synthesis of benzo[d]thiazole derivatives 129 2-(4-nitrophenyl)benzo[d]thiazole (9) 1 H NMR (500 MHz, DMSOd6)  8.38 (d, J = 9.0 Hz, 2H), 8.33 (d, J = 8.5 Hz, 2H), 8.21 (d, J = 8.0 Hz, 1H), 8.13 (d, J = 8.5 Hz, 1H), 7.60 (t, J = 8.0 Hz, 1H), 7.53 (t, J = 8.0 Hz, 1H). 4-(benzo[d]thiazol-2-yl)-N,N-dimethylaniline (10) 1 H NMR (500 MHz, DMSOd6)  8.03 (dd, J = 8.0, 1.0 Hz, 1H), 7.93 (d, J = 8.0 Hz, 1H), 7.89 (d, J = 9.0 Hz, 2H), 7.46 (td, J = 8.0, 1.0 Hz, 1H), 7.35 (td, J = 8.0, 1.0 Hz, 1H), 6.82 (d, J = 9.0Hz, 2H), 3.02 (s, 6H). 3-hydroxy(benzo[d]thiazol-2-yl)phenol (14) 1 H NMR (500 MHz, DMSOd6)  9.91 (s, 1H), 8.13 (dd, J = 8.0, 0.5 Hz, 1H), 8.05 (d, J = 8.0 Hz, 1H), 7.55 (dd, J = 7.5, 1.5 Hz, 1H), 7.52 (d, J = 2.0 Hz, 1H), 7.50 (dd, J = 7.5, 1.5 Hz, 1H), 7.46 (td, J = 7.5, 1.5 Hz, 1H), 7.37 (t, J = 8.0 Hz, 1H), 6.98 (m, 1H). 2-(2-nitrophenyl)benzo[d]thiazole (21) 1 H NMR (500 MHz, DMSOd6)  8.08 (dd, J = 8.0, 1.0 Hz, 1H), 7.81 (dd, J = 8.0, 1.5 Hz, 1H), 7.77 (td, J = 8.0, 1.0 Hz, 1H), 7.56 (td, J = 8.5, 2.0 Hz, 1H), 6.93 (td, J = 8.0, 1.5 Hz, 1H), 6.77 (dd, J = 8.0, 1.0 Hz, 1H), 6.73 (d, J = 8.0 Hz, 1H), 6.63 (td, J = 8.5, 1.0 Hz, 1H). 2.2. Results and discussion 2.2.1. Synthesis In order to optimize the condition for benzo[d]thiazole cyclization under irradiation with a domestic microwave oven, it was first screened under a conventional method based on the reposted result by Hu et al. They reported that dimethyl sulfoxide (DMSO), 60 C and 6 h is the best condition for this reaction [13]. It was found that the cyclization of benzaldehyde and o- aminothiophenol gave the same reported by Hu et al. in 92 % yield after 3 entries for 6 h. In addition, conventional method was carried out without solvent at different temperature values: at 60 C [13], 100, 150 and 240 C in which 60 C was the temperature of reaction in DMSO, meanwhile 240 C is the boiling point of o-aminothiolphenol. It was found that at 60 C of conventional method, it gave 25 % yield for up to 6; yield of 70 % was got at 150 C. The highest yield was at 200 C in 82 %. Unfortunately, at 240 C (close to o-aminothiolphenol’s boiling point: 234 C) gave lower yield because of burning, Table 1. Table 1. Optimization of benzo[d]thiazole cyclization reaction between benzaldehyde and o-aminothiophenol Conventional method Microwave method [13] In this work In this work Solvent DMSO No solvent No solvent Time 6 h 6 h 6 h 6 h 6 h 10s 20s 25s 30s 35s 40s 3min Tem. (C) 60 60 150 200 240 80 120 150 240 280 burned - Yield (%) * 92 25 70 82 60 - - - - - - 95 * After purification by recrystallization in mixture of n-hexane and ethyl acetate Duong Quoc Hoan, Nguyen My Linh, Phan Thi Hoa, Hoang Thi Nhu Quynh and Vu Thi Anh Tuyet 130 In contrast, the microwave method was carried out without solvents as shown in the conventional method in this work, too by using the Sanyo domestic microwave. In our previous work [21], we reported that the medium power level was the best choice for condensation reaction; therefore, in this work, the medium power level was selected. First of all, benzaldehyde and o- aminithiophenol was mixed well with a glass rod. Then the mixture was irradiated for 10s, 20s, 25s, 30s, 35s and 40s to record temperature at each stage of time. It showed that after 10s, its temperature reached 80 C, and the temperature increased very fast at 240 C after 30s of irradiation. After period of 40 seconds, the mixture turned black because of over heat. Amazingly, after 3 minutes, the reaction was completed that was confirmed with TLC in 95% yield after purification. Detailed protocol was shown in the experimental part, Table 1. Table 2. Microwave assisted benzo[d]thiazole synthesis Compounds R1 R2 R3 R4 Time (min.) %Yield Melting point (ºC) Observed References 1 H H H H 4 95 115-116 [22],[23] [24] [25] [26] 2 H OCH3 OH H 4 97 162-164 [27] 3 H OCH3 OH NO2 4 98 177-178 [28] 4 H OCH3 NO2 H 3 89 163-165 40 5 NO2 H OCH3 OCH3 5 91 135-136 - 6 H OCH3 OCH3 H 4 93 130-131 [29] 7 H OCH3 OH Br 4 95 186-187 - 8 H H OH H 6 90 225-226 [30] 9 H H NO2 H 3 92 228-230 [25] [30] 10 H H N(CH3)2 H 5 95 160-162 [32] 11 H H OCH3 H 4 85 119-120 [25] [26] 12 H NO2 H H 3 90 181-182 [32] 13 H H Cl H 4 90 115-117 [33], [34] [25] 14 H OH H H 4 90 161-163 [27] 15 H H CH3 H 6 85 86-87 [35] Using a domestic microwave oven for synthesis of benzo[d]thiazole derivatives 131 When the optimization was in hand, 21 benzo[d]thiazole derivatives were synthesized successfully, Scheme 1. Results are shown in Table 2. It shows that no significance of difference in yield when the aldehydes have either withdrawing electron groups or donating electron groups in. This protocol also worked well in case of furfural. Scheme 1. Synthesis of benzo[d]thiazole derivatives 2.2.2. Structural determination Almost all benzo[d]thiazole derivatives were known; therefore, their melting points were checked carefully, except some compounds were investigated further with 1 H NMR spectrum such as compounds 5, 6, 7, 9, 10, 14 and 21, their 1 H NMR spectral analysis was addressed in the experimental section. These observed melting points were matched quite well in comparison with references, Table 2. In addition, compound 7 has not been reported so its structure was confirmed by IR, 1 H NMR, 13 C NMR, HSQC, HMBC and MS spectral methods, Figure 1. To assign all proton and carbon atoms of compound 7, it needed to get started at a known signal on HMBC spectrum, that was H14 at  3.96 ppm- a singlet which only had a correlation peaks with C10 at  148.6 ppm. Luckily, C10 had a weak correlation peak with H9 at  7.60 ppm (s, 1H) since it was close to the C10. Consequently, the known H9 was a key signal that was allowed us to confirm C11 ( 146.8 ppm), C13 ( 123.5 ppm) and C7 ( 165.9 ppm). H9 also helped to assigned C8 ( 125.0 ppm) with a small correlation peak. Therefore, C12 was at  109.6 ppm and H13 was at  7.75 ppm. The other part of compound 7 was more complicated because two quaternary carbons C1 and C6 that had positions quite the same each other. Fortunately, C6 linked to sp 2 nitrogen atom, on the other hand C1 boned with sp 3 sulfur atom. It referred that C1 must be in the stronger field. So, signal of carbon NMR at  134.3 ppm belonged to C1 and  153.4 ppm signal should be for C6. It’s worth to know that H2 and H5 are doublet peaks 16 OH H H H 6 95 125-126 [36] 17 Cl H H H 5 90 84-85 [30] 18 OH H OH H 7 75 197-199 [40] 19 H H F H 4 85 101-102 [38] 20 H NO2 OH H 3 95 135-137 [39] 21 NO2 H H H 3 92 135-136 [30] 22 4 85 97-99 [32] Duong Quoc Hoan, Nguyen My Linh, Phan Thi Hoa, Hoang Thi Nhu Quynh and Vu Thi Anh Tuyet 132 meanwhile H4 and H3 are triplet peaks. The signal at  8.10 ppm (d, J = 8.0, 1H) was for H2 since it had a correlation peak with the known C6. This result identified the C4 that was at  126.5 ppm. Similarly, the signal at  8.02 ppm (d, J = 8.0 Hz, 1H) was for H5 based on the known C1 at  134.3 ppm. The rest of assignment was detailed in the experimental section, Figure 1 (a). Compound 7 was also checked with EI-MS method. The results were matched with isotopic effects and nitrogen rule, Figure 1 (b, c). Other 1 H NMR spectra of compounds 5, 7, 9, 10, 14 and 21 had good match with their structures expectedly. Figure 1. (a) A part of HMBC and (b, c) MS spectra of compound 7 3. Conclusion Using domestic microwave oven accelerated the benzo[d]thiazole cyclization. The reaction condition was in air, without solvent, short time. It was completed with popular and cheap equipment in high yield after easy purification. Yields of reactions were up to 98%. Acknowledgements: This research is supported by the Hanoi National University of Education under the project code SPHN17-13. Using a domestic microwave oven for synthesis of benzo[d]thiazole derivatives 133 REFERENCES [1] Gedye, R., Smith, F., Westaway, K., Ali, H., Balderisa, L., Laberge, L. and Roasell, J., 1986. The use of microwave ovens for rapid organic syntheses, Tetrahedron Lett., 27, 279. [2] Giguere, R.J., Bray, T.L., Duncan, S.N. and Majetich, G., 1986. Application of commercial microwave ovens to organic syntheses. Tetrahedron Lett., 28, 4945-4948. [3] Macias, A., Alonso E., Pozo, C. D., Venturini, A., Gonzalez, J., 2004. 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