Heterocycl. Commun. 2020; 26: 1–5 Research Article Open Access Xiaofeng Yu*, Zhen Zhang, Renyuan Song, Liping Gou and Guangrong Wang Synthesis of 2-aryl-benzothiazoles via Ni-catalyzed coupling of benzothiazoles and aryl sulfamates https://doi.org/10.1515/hc-2020-0001 disadvantageous because 2-aminothiophenol is extre- Received October 28, 2019; accepted December 02, 2019. mely unstable in air and highly toxic. In addition, benzyl Abstract: 2-Aryl-benzothiazoles have been successfully alcohol, benzylamine and aromatic aldehydes are also synthesized via a simple coupling reaction between ben- very reactive materials. zothiazoles and aryl sulfamates using a nickel catalyst. Recent studies have shown that novel synthetic The nickel catalyst is inexpensive, reusable and commer- methods which involve the direct coupling of benzothia- cially available. In addition, the use of highly expensive zoles with arylating agents avoid the use of highly toxic palladium catalysts and unstable raw materials has been and reactive materials. But, there are some disadvantages avoided. 2-Aryl-benzothiazoles bearing various substitu- in these reports, such as using expensive palladium as a ents on the aryl groups were obtained in good yield. catalyst [28, 29], and the use of unstable aromatic iodides [29] or aromatic aldehydes [30] as raw materials. Keywords: 2-Aryl-benzothiazoles, Aryl sulfamates, Coup- In this work, inexpensive and commercially available ling reaction nickel is used as a catalyst. In addition, inexpensive and stable aryl sulfamates are used as electrophiles. Moreover, Introduction the nickel catalyst can be recycled three times. 2-Aryl-benzothiazoles are an important class of nitrogen- containing heterocyclic compounds, which are present in Results and discussion numerous natural products and synthetic compounds. Due to their biological and pharmacological properties, Initially, the reaction of benzothiophene and p-toluenesul- 2-aryl-benzothiazoles exhibit a wide range of biological fonate was used to evaluate the catalytic activity of various activities, including insecticidal [1], antifungal [2], nema- nickel compounds (0.2 equivalent) using 1,10-phen·H2O ticidal [3], weed killing [2, 3], and plant growth regulation (1,10-phenanthroline·H2O) as a co-catalyst. As listed activities in agriculture[3]. In medicine, these compounds in Table 1, the results showed that NiBr2(DME) (DME = exhibit anti-tumor [4-7] and anti-bacterial activity [8, 9], 1,2-dimethoxyethane) exhibited the best catalytic activity and are also used in the treatment of Alzheimer’s disease with 68% yield (Table 1, entry 4). When the co-catalyst [10, 11]. Therefore, these compounds have received sustai- was changed to an amino acid the reaction yielded little ned attention from researchers in organic synthesis. or no product. This means that NiBr2 with 1,10-phen is a The conventional method used to prepare 2-aryl-benzo- highly active catalyst. Moreover, it was easier to generate thiazoles involves reacting 2-aminothiophenol with benzyl NiBr2 with 1,10-phen ligands by replacing DME with 1,10- alcohol [12], benzylamine [13-16], benzonitrile [12, 17, 18] phen, rather than directly from 1,10-phen and NiBr2. The or aromatic aldehyde [19-27]. However, this approach is yield was increased to 92% by increasing the amount of NiBr2(DME) to 40 mol% (Table 1, entry 9). * Corresponding author: Xiaofeng Yu, School of Materials After completion of the reaction, the reaction mixture and Chemical Engineering, Bengbu University, Bengbu, Anhui was filtered, and the starting material and product were 233030, P. R. China, e-mail: [email protected] washed with diethyl ether. The catalyst residue was then Renyuan Song, Liping Gou and Guangrong Wang, School of charged with lithium t-butoxide, along with a small Materials and Chemical Engineering, Bengbu University, Bengbu, Anhui 233030, P. R. China amount of 1,10-phenanthroline and used in the reaction Zhen Zhang, College of Pharmacy and Biological Engineering, again Repeated use of the catalyst showed that it was Chengdu University, Sichuan 610106, P. R. China active for three cycles (table 1, entry 14-16). Open Access. © 2020 Yu et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution alone 4.0 License. 2 X. Yu et al.: Synthesis of 2-aryl-benzothiazoles via Ni-catalyzed coupling of benzothiazoles and aryl sulfamates Table 1. Synthesis of 2-(p-CH3Ph)benzothiazoles by benzothiazoles Table 2. Synthesis of 2-arylbenzothiazoles from benzothiazoles with a,b a,b with p-CH3PhOSO2NMe2 ArOSO2NMe2 S [Ni],Ligand S S S p-CH3PhOSO2NMe2 t NiBr2(DME) BuOLi, dioxane ArOSO2NMe2 Ar N N . N 1,10-phen H2O N Entry Catalyst (mol%) Ligand (mol%) Yield/% S S S CF3 N N N N 1 NiBr2⋅6H2O (20) 1,10-phen⋅H2O(20) 51 b 85% 95% 2 NiBr2 (20) 1,10-phen⋅H2O(20) 50 a 92% c 3 Ni(COD) (20)e 1,10-phen⋅H O(20) NRc O O S 2 2 S S 4 NiBr (DME) (20)d 1,10-phen⋅H O(20) 68 O 2 2 N f N N 5 Ni(acac)2(20) 1,10-phen⋅H2O(20) 10 6 NiCl2⋅6H2O (20) 1,10-phen⋅H2O(20) 45 d 89% e 93% f 91% 7 NiBr2(DME) (20) 1,10-phen(20) 31 S S 8 NiBr2(DME) (30) 1,10-phen⋅H2O(30) 81 Cl S N N 9 NiBr2(DME) (40) 1,10-phen⋅H2O(40) 92 N 10 NiBr (DME) (20) - NRc 2 g 90% h 96% i 94% c 11 NiBr2(DME) (20) L-proline trace c 12 NiBr2(DME) (20) L-lysine NR c a) Reaction conditions: benzothiazoles (0.1 mmol ), ArOSO2NMe2 13 NiBr2(DME) (20) L-valine trace t f (0.1 mmol ), BuOLi(0.2 mmol ),1,10-phen⋅H2O(0.04 mmol ), 14 NiBr2(DME) (40) 1,10-phen⋅H2O(40) 90 o g NiBr2(DME)(0.04 mmol ), dioxane (3 ml), 120 C, N2 atmosphere. 15 NiBr2(DME) (40) 1,10-phen⋅H2O(40) 89 h b) Isolated yields. 16 NiBr2(DME) (40) 1,10-phen⋅H2O(40) 85 a) Reaction conditions: benzothiazoles (0.1 mmol ), p-CH3PhO- t to prepare a variety of 2-aryl-benzothiazole compounds. SO2NMe2 (0.1 mmol ), BuOLi(0.2 mmol ), 1,10-phen⋅H2O, NiBr2(DME), o The aryl group in the aryl sulfamate starting material can dioxane (3 ml), 120 C, N2 atmosphere. b) Isolated yields. c) By 1 H NMR. d) DME = 1,2-dimethoxyethane. e) COD = 1,5-Cyclooctadiene. contain electron-withdrawing groups (such as -C6H4CF3, f) acac = acetylacetone. f) First recovered catalyst. g) Second -C6H4OCH3, and -C6H4Cl), electron-donating groups (such recovered catalyst. h) Third recovered catalyst. as -C6H4CH3, and -C6H4N(CH3)2), or large conjugated rings (such as -C10H7). This method avoids the use of unstable and highly toxic 2-aminothiophenol, highly expensive A wide variety of aryl sulfamates and benzothiazoles palladium catalysts, and unstable aromatic iodine or aro- were investigated with regard to the effect on the yield matic aldehydes as starting materials. In future work, sub- of their corresponding 2-aryl-benzothiazole compounds. stituted benzothiazole compounds will be studied as raw Table 2 shows that the reaction of various aryl sulfamates materials. gave the desired products in high yield. The product yields obtained using aryl sulfamates bearing electron-dona- ting groups were higher than those containing electron- Experimental withdrawing groups [p-N(CH3)2PhOSO2NMe2 (95%), p-CF3PhOSO2NMe2 (85%)]. Sterically hindered aryl sul- General procedure for the synthesis of famates gave higher product yields; the yield obtained 2-aryl-benzothiazole using o-CH3OPhOSO2NMe2 is higher than its correspon- ding m- and p-derivatives. The product yield obtained A Schlenk tube was charged with the benzothiazole using aryl sulfamates containing a naphthalene ring was (0.10 mmol) and aryl sulfamate (0.10 mmol) [31], follo- the highest. This was attributed to the positively charged wed by lithium t-butoxide (0.20 mmol), nickel catalyst intermediate being stabilized by the electron-donating (0.4 equivalent), and co-catalyst (0.4 equivalent). The groups and large conjugated ring. nitrogen atmosphere was replaced three times and 3 mL of dioxane was introduced to the reaction mixture. The Schlenk tube was placed into an oil bath and the reaction Conclusions mixture was stirred at 120°C for 12 h. The reaction mixture was extracted with Et2O. The combined organic phases An inexpensive, reusable and commercially available were dried with anhydrous Na2SO4, filtered, concentra- nickel catalyst can be used to efficiently catalyze the coup- ted under vacuum and the resulting residue purified by ling reaction between benzothiazoles and aryl sulfamates column chromatography on silica gel. X. Yu et al.: Synthesis of 2-aryl-benzothiazoles via Ni-catalyzed coupling of benzothiazoles and aryl sulfamates 3 Catalyst recovery experiment 6.75 (d, J = 8.7 Hz, 2H), 3.00 (s, 6H). 13C NMR (100 MHz, CDCl3) δ 168.0, 158.7, 155.3, 136.5, 130.1, 127.4, 125.4, 123.5, After completion of the catalytic reaction, the reaction 120.1, 118.2, 110.3, 39.5. HRMS (EI): m/z [M]+ calcd for mixture was filtered and the filter cake was washed three C15H11N2S: 254.09; found: 254.11. times with diethyl ether. The powder solid was charged with two equivalents of lithium t-butoxide and 0.4 equiva- 2-(3-Methoxyphenyl) benzothiazole (d) [21, 35] lents of 1,10-phenanthroline and was used in the reaction O again (table 1, entry 14-16). S N Characterization Data of the Products: White solid; 89% yield; mp 88–90°C. IR (KBr): 3061, 2855, 2-p-Tolyl-benzothiazole (a) [17, 21, 32] 1630, 1510, 1340, 887, 750, 711, 681 cm–1. 1H NMR (400 MHz, S CDCl3): δ = 8.23 (d, J = 8.0, 1 H), 7.90 (d, J = 7.8 Hz, 1 H), 7.54 (t, J = 7.4, Hz, 1 H), 7.49 (d, J = 7.5, Hz, 1 H), 7.17 (t, J = 7.5, Hz, N 1 H), 7.15-7.11 (m, 2 H), 6.96 (d, J = 7.4 Hz, 1 H), 3.55 (s, 3 H).