Amination of Benzoxazoles by Visible-Light Photoredox Catalysis

Abstract

An effective visible-light-promoted ‘radical-type’ coupling of benzoxazole with secondary amines has been developed. The broad substrate scope and mild reaction conditions make this procedure a practical and environmentally friendly method for the synthesis of 2-aminobenzoxazoles.

Key words

radical reaction - photochemistry - amination - heterocycles - iridium

• References and Notes

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• 19 One-Pot, Two-Step Amination of Benzoxazole with Various Amines An amine (1 mmol, 2 equiv) was added to a solution of N-chlorosuccinimide (149 mg, 1.1 mmol, 2.2 equiv) in CH₂Cl₂ (5 mL) at r.t. in the dark. After 3 h, Ir(dtbpy)(ppy)₂PF₆ (4 mg, 5 μmol, 0.01 equiv), Ph₃N (245 mg, 1 mmol, 2 equiv), and benzoxazole (60 mg, 0.5 mmol, 1 equiv) were added. The reaction rube was sealed and placed at a distance of 5 cm from 3 W blue LED and stirred for 60 h. After the reaction was complete, the solvent was evaporated under vacuo. The crude mixture was purified by flash column chromatography eluting with a mixture of PE–EtOAc. 2-(4-Morpholinyl)benzoxazole (3a) Known compound; yellow solid; mp 85–87 °C. ¹H NMR (400 MHz, CDCl₃): δ = 7.37 (d, J = 7.6 Hz, 1 H), 7.27 (d, J = 6.4 Hz, 1 H), 7.18 (t, J = 7.6 Hz, 1 H), 7.04 (t, J = 7.6 Hz, 1 H), 3.82 (t, J = 4.4 Hz, 4 H), 3.69 (t, J = 5.2 Hz, 4 H). ¹³C NMR (100 MHz, CDCl₃): δ = 162.0, 148.7, 142.8, 124.0, 120.9, 116.4, 108.8, 66.1, 45.7. 5-Methyl-2-(4-morpholinyl)benzoxazole (3b) Known compound; yellow solid; mp 117–118 °C. ¹H NMR (400 MHz, CDCl₃): δ = 7.16 (d, J = 0.8 Hz, 1 H), 7.12 (d, J = 8.4 Hz, 1 H), 6.83 (dd, J = 8.0, 0.8 Hz, 1 H), 3.80 (t, J = 4.4 Hz, 4 H), 3.67 (t, J = 5.2 Hz, 4 H), 2.39 (s, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 162.3, 147.0, 143.0, 133.8, 121.6, 116.9, 108.2, 66.2, 45.8, 21.5.