A Rhodium Catalyst Superior to Iridium Congeners for Enantioselective Radical Amination Activated by Visible Light

A bis-cyclometalated rhodium(III) complex catalyzes a visible-light-activated enantioselective alpha-amination of 2-acyl imidazoles with up to 99% yield and 98% ee. The rhodium catalyst is ascribed a dual function as a chiral Lewis acid and, simultaneously, as a light-activated smart initiator of a radical-chain process through intermediate aminyl radicals. Notably, related iridium-based photoredox catalysts reported before were unsuccessful in this enantioselective radical C-N bond formation. The surprising preference for rhodium over iridium is attributed to much faster ligand-exchange kinetics of the rhodium complexes involved in the catalytic cycle, which is crucial to keep pace with the highly reactive and thus short-lived nitrogen-centered radical intermediate.