Enantioselective Friedel-Crafts alkylations catalysed by well-defined iridium and rhodium half-sandwich complexes

Abstract

Aqua-complexes (SM,RC)-[CpM{(R)-prophos}(H 2O)][SbF6]2 (M = Rh 1, Ir 2) catalysed the alkylation of α,β-unsaturated aldehydes with aromatics and heteroaromatics but in some cases, mixtures of products were obtained. Complexes 1 and 2 were also used to activate nitroalkenes for the Friedel-Crafts alkylation of a variety of aromatics and heteroaromatics, in particular, 1,3,5-trimethoxybenzene. For this substrate, the monoalkylated adduct was obtained in quantitative yield with enantioselectivities of up to 73% ee being achieved. The intermediate catalyst/nitroalkene was isolated and characterized and the complex catalyst/adduct was detected spectroscopically. From these data a plausible catalytic cycle is proposed. © 2011 Elsevier Ltd. All rights reserved.