Theoretical Investigation of Knowles Hydroamination Based on Systematic Exploration of Oxidation/Reduction Pathways for Photoredox-Catalyzed Radical Process

Systematic exploration of reaction paths based on quantum chemical calculations revealed the entire mechanism of Knowles’s light-promoted catalytic intramolecular hydroamination via radical processes. Bond formation/cleavage competes with single electron transfer (SET) from the catalyst/substrate to substrate/catalyst. All these processes were theoretically described by reactions through transition states in the same electronic state and non-radiative transitions through the seam of crossings (SX) between different electronic states. This study determined the energetically favorable reaction path by combining the reaction path searches and the SX geometry searches, and then discusses the entire reaction mechanism. Such a calculation was achieved by establishing a novel computational approach that represents SET as a non-adiabatic transition between substrate's PESs for different charge states adjusted based on the catalyst's redox potential. Finally, we uncovered the whole picture of the reaction process, in which N atom of the substrate is oxidized by photoredox catalyst via SET, the resulting aminium radical is added to alkene, and the hydroamination product is produced after SET process accompanying protonolysis with MeOH. The present calculations showed that the reduction and proton transfer proceed concertedly. Also, in the reduction process, there are SET paths leading to both the product and the reactant, and the redox potentials of the catalyst change the contribution of the SET path leading to the product.