Metal–Nitroalkene and aci-Nitro Intermediates in Catalytic Enantioselective Friedel–Crafts Reactions of Indoles with trans-β-Nitrostyrenes

The half-sandwich aqua complex (SRh,RC)-[(η5-C5Me5)­Rh­{(R)-Prophos}­(H2O)]­[SbF6]2 (Prophos = propane-1,2-diylbis­(diphenylphosphane)) efficiently catalyzes the asymmetric reaction between N-methyl-2-methylindole and trans-β-nitrostyrenes (up to 94% ee). The metal–nitroalkene complex involved has been characterized by X-ray crystallography, and the aci-nitro intermediate complex has been spectroscopically detected. A plausible catalytic cycle is proposed.