Asymmetric Friedel–Crafts addition of indoles to N-sulfonyl aldimines catalyzed by Cu(ii) chiral amino alcohol based Schiff base complexes

Abstract

Recyclable copper(II) chiral amino alcohol based Schiff base complexes smoothly catalysed the Friedel–Crafts alkylation of indole with aryl aldimine in good yields (98%) and with enantioselectivities up to 97%. The effects of ligand structure, solvent, metal source and temperature on the reaction were also studied. The catalytic system worked very well several times retaining its performance. To understand the mechanism of the catalytic Friedel–Crafts addition reaction, a kinetic investigation was carried out with different concentrations of the catalyst Cu(II)–L₂, indole and N-(3-nitrobenzylidene)-4-methylbenzene sulfonamide as a model substrate. The Friedel–Crafts alkylation reaction of N-(3-nitrobenzylidene)-4-methylbenzene sulfonamide was first order with respect to the concentration of the catalyst and the nucleophile but did not depend on the initial concentration of the substrate (aryl aldimine). An appropriate mechanism of the Friedel–Crafts alkylation reaction is proposed.