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

Aqua-complexes (SM,RC)-[Cp∗M{(R)-prophos}(H2O)][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.

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