Photoredox-Induced Radical 6-exo-trig Cyclizations onto the Indole Nucleus: Aromative versus Dearomative Pathways

The investigation of photoredox-induced intra- and intermolecular radical [4+2] annulations of indoles confronted us with a puzzling dichotomous behavior of structurally closely related intermediate 3-indolyl radicals, which either undergo exclusive oxidation to tricyclic tetrahydropyridoindoles or reduction to benzindolizidine products under identical reaction conditions. A combined experimental and computational study revealed that only very subtle structural changes in the substrate-reactant complexes of the key radical intermediates with amine radical cations steer the divergent product selectivities, instead of the usual reactivity parameters such as ionization potentials or partial charges.