Copper-Catalyzed Imino C–N Bond Formation with Aryl Boronic Acids under Aerobic Conditions

Abstract

A copper-catalyzed, ligand- and base-free C–N cross-coupling reaction of iminochromenes with arylboronic acids in a nontoxic ethanol solvent at room temperature under air is developed. The product yields are found to be good to moderate. The conservation of the sensitive imine functional group implies the mildness of this protocol

Key words

iminochromenes - copper - arylboronic acid - cross-coupling - aerobic conditions

• References and Notes

• 1 Also called iminocoumarins.
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For books, see:
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• 16 General Procedure for the Copper-Catalyzed Coupling of Iminochromene with Arylboronic Acids To a round-bottomed flask equipped with a stir bar, the iminochromene (1 mmol), Cu(OAc)₂·H₂O (20 mol%), and arylboronic acid (2.3 mmol) were added. EtOH (5 mL) was added, and the reaction mixture was stirred for 24 h at r.t. After completion of the reaction as monitored by TLC, the organic phase was concentrated under reduced pressure, and the solid residue was fractioned in EtOAc (20 mL) and H₂O (10 mL) thrice. The organic phase was separated and dried on anhydrous Na₂SO₄. The solvent was removed in vacuum, and the crude residue was purified by column chromatography with EtOAc–PE to afford the respective N-arylated chromene. Physical and spectroscopic data given below are for compound 3a and may be taken as representative. For further details, see the Supporting Information. (Z)-N-3-Diphenyl-2H-chromen-2-imine (3a) Yellow solid (249 mg, 93% yield); mp 112–113 °C. IR (KBr): ν = 1737, 1643, 1587, 1485, 1365, 1230, 1116 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.79–7.72 (m, 2 H), 7.46–7.30 (m, 7 H), 7.29 (s, 1 H), 7.20–7.13 (m, 2 H), 7.13–7.05 (m, 1 H), 7.02 (d, J = 8.2 Hz, 1 H). ¹³C NMR (100 MHz, CDCl₃): δ = 152.9, 148.9, 146.6, 136.7, 133.6, 133.3, 132.1, 130.1, 129.1, 128.7, 128.4, 128.3, 128.0, 127.2, 123.9, 123.4, 122.8, 122.5, 120.4, 115.6, 115.3. ESI-HRMS: m/z calcd for C₂₁H₁₆NO [M + H]⁺ : 298.1226; found: 298.1277.
• 17 See the Supporting Information.