Abstract
2-Aryl-benzothiazoles have been successfully synthesized via a simple coupling reaction between benzothiazoles and aryl sulfamates using a nickel catalyst. The nickel catalyst is inexpensive, reusable and commercially available. In addition, the use of highly expensive palladium catalysts and unstable raw materials has been avoided. 2-Aryl-benzothiazoles bearing various substituents on the aryl groups were obtained in good yield.
Introduction
2-Aryl-benzothiazoles are an important class of nitrogen-containing heterocyclic compounds, which are present in numerous natural products and synthetic compounds. Due to their biological and pharmacological properties, 2-aryl-benzothiazoles exhibit a wide range of biological activities, including insecticidal [1], antifungal [2], nematicidal [3], weed killing [2, 3], and plant growth regulation activities in agriculture [3]. In medicine, these compounds exhibit anti-tumor [4, 5, 6, 7] and anti-bacterial activity [8, 9], and are also used in the treatment of Alzheimer’s disease [10, 11]. Therefore, these compounds have received sustained attention from researchers in organic synthesis.
The conventional method used to prepare 2-aryl-benzothiazoles involves reacting 2-aminothiophenol with benzyl alcohol [12], benzylamine [13, 14, 15, 16], benzonitrile [12, 17, 18] or aromatic aldehyde [19, 20, 21, 22, 23, 24, 25, 26, 27]. However, this approach is disadvantageous because 2-aminothiophenol is extremely unstable in air and highly toxic. In addition, benzyl alcohol, benzylamine and aromatic aldehydes are also very reactive materials.
Recent studies have shown that novel synthetic methods which involve the direct coupling of benzothiazoles with arylating agents avoid the use of highly toxic and reactive materials. But, there are some disadvantages in these reports, such as using expensive palladium as a catalyst [28, 29], and the use of unstable aromatic iodides [29] or aromatic aldehydes [30] as raw materials.
In this work, inexpensive and commercially available nickel is used as a catalyst. In addition, inexpensive and stable aryl sulfamates are used as electrophiles. Moreover, the nickel catalyst can be recycled three times.
Results and discussion
Initially, the reaction of benzothiophene and p-toluenesulfonate was used to evaluate the catalytic activity of various nickel compounds (0.2 equivalent) using 1,10-phen·H2O (1,10-phenanthroline·H2O) as a co-catalyst. As listed in Table 1, the results showed that NiBr2(DME) (DME = 1,2-dimethoxyethane) exhibited the best catalytic activity with 68% yield (Table 1, entry 4). When the co-catalyst was changed to an amino acid the reaction yielded little or no product. This means that NiBr2 with 1,10-phen is a highly active catalyst. Moreover, it was easier to generate NiBr2 with 1,10-phen ligands by replacing DME with 1,10-phen, rather than directly from 1,10-phen and NiBr2. The yield was increased to 92% by increasing the amount of NiBr2(DME) to 40 mol% (Table 1, entry 9).
Synthesis of 2-(p-CH3Ph)benzothiazoles by benzothiazoles with p-CH3PhOSO2NMe2a,b
| Entry | Catalyst (mol%) | Ligand (mol%) | Yield/% |
|---|---|---|---|
| 1 | NiBr2⋅6H2O (20) | 1,10-phen⋅H2O(20) | 51 |
| 2 | NiBr2 (20) | 1,10-phen⋅H2O(20) | 50 |
| 3 | Ni(COD)2 (20)e | 1,10-phen⋅H2O(20) | NRc |
| 4 | NiBr2(DME) (20)d | 1,10-phen⋅H2O(20) | 68 |
| 5 | Ni(acac)2(20)f | 1,10-phen⋅H2O(20) | 10 |
| 6 | NiCl2⋅6H2O (20) | 1,10-phen⋅H2O(20) | 45 |
| 7 | NiBr2(DME) (20) | 1,10-phen(20) | 31 |
| 8 | NiBr2(DME) (30) | 1,10-phen⋅H2O(30) | 81 |
| 9 | NiBr2(DME) (40) | 1,10-phen⋅H2O(40) | 92 |
| 10 | NiBr2(DME) (20) | - | NRc |
| 11 | NiBr2(DME) (20) | L-proline | tracec |
| 12 | NiBr2(DME) (20) | L-lysine | NRc |
| 13 | NiBr2(DME) (20) | L-valine | tracec |
| 14 | NiBr2(DME) (40)f | 1,10-phen⋅H2O(40) | 90 |
| 15 | NiBr2(DME) (40)g | 1,10-phen⋅H2O(40) | 89 |
| 16 | NiBr2(DME) (40)h | 1,10-phen⋅H2O(40) | 85 |
a) Reaction conditions: benzothiazoles (0.1 mmol ), p-CH3PhO-SO2NMe2 (0.1 mmol ), tBuOLi(0.2 mmol ), 1,10-phen⋅H2O, NiBr2(DME), dioxane (3 ml), 120 oC, N2 atmosphere. b) Isolated yields. c) By 1H NMR. d) DME = 1,2-dimethoxyethane. e) COD = 1,5-Cyclooctadiene. f) acac = acetylacetone. f) First recovered catalyst. g) Second recovered catalyst. h) Third recovered catalyst.
After completion of the reaction, the reaction mixture was filtered, and the starting material and product were washed with diethyl ether. The catalyst residue was then charged with lithium t-butoxide, along with a small amount of 1,10-phenanthroline and used in the reaction again Repeated use of the catalyst showed that it was active for three cycles (table 1, entry 14-16).
A wide variety of aryl sulfamates and benzothiazoles were investigated with regard to the effect on the yield of their corresponding 2-aryl-benzothiazole compounds. Table 2 shows that the reaction of various aryl sulfamates gave the desired products in high yield. The product yields obtained using aryl sulfamates bearing electron-donating groups were higher than those containing electron-withdrawing groups [p-N(CH3)2PhOSO2NMe2 (95%), p-CF3PhOSO2NMe2 (85%)]. Sterically hindered aryl sulfamates gave higher product yields; the yield obtained using o-CH3OPhOSO2NMe2 is higher than its corresponding m- and p-derivatives. The product yield obtained using aryl sulfamates containing a naphthalene ring was the highest. This was attributed to the positively charged intermediate being stabilized by the electron-donating groups and large conjugated ring.
Synthesis of 2-arylbenzothiazoles from benzothiazoles with ArOSO2NMe2a,b
|
a) Reaction conditions: benzothiazoles (0.1 mmol ), ArOSO2NMe2 (0.1 mmol ), tBuOLi(0.2 mmol ),1,10-phen⋅H2O(0.04 mmol ), NiBr2(DME)(0.04 mmol ), dioxane (3 ml), 120 oC, N2 atmosphere. b) Isolated yields.
Conclusions
An inexpensive, reusable and commercially available nickel catalyst can be used to efficiently catalyze the coupling reaction between benzothiazoles and aryl sulfamates to prepare a variety of 2-aryl-benzothiazole compounds. The aryl group in the aryl sulfamate starting material can contain electron-withdrawing groups (such as -C6H4CF3, -C6H4OCH3, and -C6H4Cl), electron-donating groups (such 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 palladium catalysts, and unstable aromatic iodine or aromatic aldehydes as starting materials. In future work, substituted benzothiazole compounds will be studied as raw materials.
Experimental
General procedure for the synthesis of 2-aryl-benzothiazole
A Schlenk tube was charged with the benzothiazole (0.10 mmol) and aryl sulfamate (0.10 mmol) [31], followed by lithium t-butoxide (0.20 mmol), nickel catalyst (0.4 equivalent), and co-catalyst (0.4 equivalent). The 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 mixture was stirred at 120°C for 12 h. The reaction mixture was extracted with Et2O. The combined organic phases were dried with anhydrous Na2SO4, filtered, concentrated under vacuum and the resulting residue purified by column chromatography on silica gel.
Catalyst recovery experiment
After completion of the catalytic reaction, the reaction mixture was filtered and the filter cake was washed three times with diethyl ether. The powder solid was charged with two equivalents of lithium t-butoxide and 0.4 equivalents of 1,10-phenanthroline and was used in the reaction again (table 1, entry 14-16).
Characterization Data of the Products
2-p-Tolyl-benzothiazole (a) [17, 21, 32]
Yellow solid; 92% yield; mp 87–89°C. IR (KBr): 3081, 2957, 1449, 1431, 965, 840,758, 450 cm–1. 1H NMR (400 MHz, CDCl3): δ = 8.06 (d, J = 8.0 Hz, 1 H), 7.98 (d, J = 7.9 Hz, 2 H), 7.88 (d, J = 8.0 Hz, 1 H), 7.47 (t, J = 7.6 Hz, 1 H), 7.36 (t, J = 8.2 Hz, 1 H), 7.28 (t, J = 8.5 Hz, 2 H), 2.40 (s, 3 H). 13C NMR (100 MHz, CDCl3): δ = 168.1, 154.1, 141.2, 135.0, 131.1, 130.0, 127.4, 126.3, 125.3, 123.2, 121.4, 21.6. HRMS (EI): m/z [M]+ calcd for C14H11NS: 225.06; found: 225.09.
2-[4-(Trifluoromethyl)phenyl]-benzothiazole (b) [17, 33]
White solid; 85% yield; mp 160–163°C. IR (KBr): 3044, 1630, 1454, 1425, 1073, 976, 840, 761, 624, 450 cm–1. 1H NMR (400 MHz, CDCl3): δ = 8.20 (d, J = 7.9 Hz, 2 H), 8.10 (d, J = 7.9 Hz, 1 H), 7.91 (d, J = 7.9 Hz, 1 H), 7.74 (d, J = 7.9 Hz, 2 H), 7.50 (t, J = 7.9 Hz, 1 H), 7.43 (t, J = 7.9 Hz, 1 H). 13C NMR (100 MHz, CDCl3) δ 165.8, 154.3, 135.9, 134.1, 130.3 (q, J = 31.3 Hz), 125.5, 126.5, 125.4 (q, J = 3.5 Hz), 122.9 (q, J = 270.9 Hz), 122.1, 120.0. HRMS (EI): m/z [M]+ calcd for C14H8F3NS: 279.03; found: 279.08.
2-[4-(N, N-dimethyl)phenyl]-benzothiazole (c) [17, 34]
White solid; 95% yield; mp 167–169°C. IR (KBr): 3088, 2880, 1450, 1309, 1233, 1050, 965, 749, 715, 415. 1H NMR (400 MHz, CDCl3): δ = 7.96 (t, J = 7.5 Hz, 3H), 7.89 (d, J = 7.8 Hz, 1H), 7.50 (t, J = 7.5 Hz, 1H), 7.38 (t, J = 7.4 Hz, 1H), 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, 120.1, 118.2, 110.3, 39.5. HRMS (EI): m/z [M]+ calcd for C15H11N2S: 254.09; found: 254.11.
2-(3-Methoxyphenyl) benzothiazole (d) [21, 35]
White solid; 89% yield; mp 88–90°C. IR (KBr): 3061, 2855, 1630, 1510, 1340, 887, 750, 711, 681 cm–1. 1H NMR (400 MHz, 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, 1 H), 7.15-7.11 (m, 2 H), 6.96 (d, J = 7.4 Hz, 1 H), 3.55 (s, 3 H). 13C NMR (100 MHz, CDCl3): δ = 160.1, 156.1, 152.9, 125.2, 120.5, 116.1, 123.9, 122.5, 119.6, 117.4, 109.3, 106.1, 101.2, 51.1. HRMS (EI): m/z [M]+ calcd for C14H11NOS: 241.06; found: 241.09.
2-(2-Methoxyphenyl) benzothiazole (e) [21]
White solid; 93% yield; mp 110–112°C. IR (KBr): 3055, 2850, 1628, 1520, 1361, 755, 723 cm–1. 1H NMR (400 MHz, CDCl3): δ = 8.45 (d, J = 7.9, 1 H), 7.91 (d, J = 8.0 Hz, 1 H), 7.54 (t, J = 7.9 Hz, 1 H), 7.49 (d, J = 7.9 Hz, 1 H), 7.17 (t, J = 7.5, Hz, 1 H), 7.14-7.11 (m, 2 H), 7.06 (d, J = 7.4 Hz, 1 H), 6.96 (d, J = 7.9 Hz, 1 H), 3.45 (s, 3 H). 13C NMR (100 MHz, CDCl3): δ = 161.1, 155.1, 153.9, 135.1, 131.1, 125.2, 124.9, 122.5, 120.5, 118.3, 108.8, 105.2, 103.1, 52.8. HRMS (EI): m/z [M]+ calcd for C14H11NOS: 241.06; found: 241.11.
2-(4-Methoxyphenyl)benzothiazole (f) [17, 35]
White solid; 91% yield; mp 123–125°C. IR (KBr): 3045, 2848, 1558, 1432, 1356, 1311, 1110, 748, 725. 1H NMR (400 MHz, CDCl3) δ = 8.14-8.11 (m, 3H), 7.65 (d, J = 7.9 Hz, 1H), 7.50 (t, J = 7.3 Hz, 1H), 7.38 (t, J = 7.5 Hz, 1H), 7.09 (d, J = 8.6 Hz, 2H), 3.45 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 165.9, 159.7, 151.2, 130.5, 128.9, 124.3, 121.5, 120.4, 119.6, 117.8, 113.2, 53.4. HRMS (EI): m/z [M]+ calcd for C14H11NOS: 241.06; found: 241.09.
2-(4-Chlorophenyl)benzothiazole (g) [17, 34]
White solid; 90% yield; mp 116–118°C. IR (KBr): 3048, 2915, 1573, 1438, 1316, 1120, 759, 743. 1H NMR (400 MHz, CDCl3) δ = 8.19 (d, J = 8.1 Hz, 1H), 8.11 (d, J = 8.4 Hz, 2H), 7.85 (d, J = 8.0 Hz, 1H), 7.50 (d, J = 8.1 Hz, 1H), 7.44 (d, J = 8.4 Hz, 2H), 7.38 (d, J = 8.0 Hz, 1H). 13C NMR (100 MHz, CDCl3) δ 164.5, 155.0, 139.1, 134.2, 131.1, 128.9, 127.5, 126.1, 125.3, 120.3, 120.6. HRMS (EI): m/z [M]+ calcd for C13H8ClNS: 245.01; found: 245.05.
2-(Naphthalen-2-yl)benzothiazole (h) [21]
White solid; 96% yield; mp 128–130°C. IR (KBr): 3056, 2860, 1530, 1443, 1358, 1200, 1101, 1005, 958, 850, 751, 731 cm–1.1 H NMR (400 MHz, CDCl3): δ = 8.94 (d, J = 7.8 Hz, 1 H), 8.19 (d, J = 7.8 Hz, 1 H), 7.96 (t, J = 7.8 Hz, 1 H), 7.87 (d, J = 7.8 Hz, 2 H), 7.63 (t, J = 7.8 Hz, 1 H), 7.60-7.58 (m, 3 H), 7.41 (t, J = 7.8 Hz, 1 H). 13C NMR (100 MHz, CDCl3): δ = 165.2, 153.1, 133.1, 132.5,131.4, 130.2, 128.9, 127.5, 126.9, 126.5, 126.0, 125.7, 124.5, 124.0, 122.9, 122.1, 120.7. HRMS (EI): m/z [M]+ calcd for C17H11NS: 261.06; found: 261.06.
2-(Naphthalen-3-yl)benzothiazole (i) [17, 34]
White solid; 94% yield; mp 126–128 °C. IR (KBr): 3055, 2890, 1502, 1435, 1360, 1205, 998, 899, 758, 723. 1H NMR (400 MHz, CDCl3) δ 8.56 (s, 1H), 8.25 (d, J = 8.5 Hz, 1H), 8.10 (d, J = 8.0 Hz, 1H), 7.96-7.91 (m, 4H), 7.54-7.51 (m, 3H), 7.26 (t, J = 7.5 Hz, 1H). 13C NMR (100 MHz, CDCl3) δ 167.1, 155.1, 136.2, 135.2, 131.0, 129.8, 128.0, 127.6, 126.6, 125.8, 123.1, 122.7, 122.0, 121.3, 120.5, 119.9, 119.0. HRMS (EI): m/z [M]+ calcd for C17H11NS: 261.06; found: 261.08.
Acknowledgements
Financial support was received from the Natural Science Foundation of the Higher Education Institutions of Anhui Province (KJ2019A0856, KJ2017A573), Bengbu University funded project of scientific research startup for high-level talents (BBXY2018KYQD20) and the Natural Science Foundation of Bengbu University (2017GJPY06). This research was supported by the Anhui Provincial Engineering Laboratory of Silicon-based Materials, and the Separation and Purification Technology Research Center of Bengbu University.
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