A Synthesis of Benzothiazoles and Indoles by Direct C(sp²)–I ­Activation Catalyzed by Copper(II) on Silica-Coated Magnetite Nanoparticles

Abstract

N-Substituted 2-amino-1,3-benzothiazoles and 2,3-disubstituted indoles were prepared by C–S and C–C cross-coupling reactions of 2-iodoanilines with isothiocyanates or 1,3-dicarbonyl compounds, respectively, in the presence of magnetic nanoparticles functionalized with a copper(II)–Schiff base complex as an efficient and reusable catalyst.

Key words

cross-coupling - benzothiazoles - indoles - copper - catalysis - Schiff bases

• References

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• 3 N-Substituted 1,3-Benzothiazol-2-amines 2a–f; General Procedure A mixture of the appropriate isothiocyanate 1 (1.1 equiv), 2-iodoaniline (1 equiv), CTAB (10 mol%), DABCO (2 equiv), and magnetic catalyst (15 mg) in H₂O (4 mL) was stirred at 50 °C for 2 h then cooled to r.t. The catalyst was removed by using an external magnet, and the solution was extracted with EtOAc (3 × 10 mL). The extracts were dried (MgSO₄), filtered, and concentrated under vacuum to give a crude product that was purified by column chromatography. N-Phenyl-1,3-benzothiazol-2-amine (2a) White solid; yield: 210 mg (93%); mp 157–160 °C. IR (KBr): 3459, 1635 cm⁻¹. ¹H NMR (400 MHz, CDCl₃): δ = 7.02 (t, ³ J _HH = 7.2 Hz, 1 H), 7.61 (t, ³ J _HH = 7.2 Hz, 1 H), 7.30–7.41 (m, 3 H), 7.15 (d, ³ J _HH = 7.7 Hz, 1 H), 7.76–7.84 (m, 3 H), 10.48 (s, 1 H). ¹³C NMR (100 MHz, CDCl₃): δ = 118.2, 119.7, 121.5, 122.5, 122.8, 126.4, 129.5, 130.5, 141.2, 152.6, 162.1.
• 4 2,3-Disubstituted Indoles 4a–f; General Procedure A mixture of the appropriate 1,3-dicarbonyl compound 3 (1.1 equiv), 2-iodoaniline (1 equiv), CTAB (10 mol%), K₂CO₃ (1 equiv), and magnetic catalyst (20 mg) in H₂O (4 mL) was stirred at 50 °C for 3 h then cooled to r.t. The catalyst was removed by using an external magnet, and the solution was extracted with EtOAc (3 × 10 mL). The extracts were dried (MgSO₄), filtered, and concentrated under vacuum to give a crude product that was purified by column chromatography. Methyl 2-Methyl-1H-indole-3-carboxylate (4a) Beige solid; yield: 160 mg (85%); mp 164–166 °C. IR (KBr): 3016, 1690, 1449, 1209, 10905 cm⁻¹. ¹H NMR (400 MHz, CDCl₃): δ = 2.72 (s, 3 H), 3.93 (s, 3 H), 7.16−7.30 (m, 3 H), 8.08 (d, J= 7.7 Hz, 1 H), 8.67 (br s, 1 H). ¹³C NMR (100 MHz, CDCl₃): δ = 14.2, 50.8, 104.3, 110.4, 121.2, 121.7, 122.3, 127.2, 134.5, 144.2, 166.7.