Mechanistic Investigation and Optimization of Photoredox Anti-Markovnikov Hydroamination

The reaction mechanism and the origin of the selectivity for the photocatalytic intermolecular anti-Markovnikov hydroamination of unactivated alkenes with primary amines to furnish secondary amines have been revealed by time-resolved laser kinetics measurements of the key reaction intermediates. We show that back-electron transfer (BET) between the photogenerated aminium radical cation (ARC) and reduced photocatalyst complex (Ir(II)) is nearly absent due to rapid deprotonation of the ARC on the sub-100 ns time scale. The selectivity for primary amine alkylation is derived from the faster addition of the primary ARCs (as compared to secondary ARCs) to alkenes. The turnover of the photocatalyst occurs via the reaction between Ir(II) and a thiyl radical; the in situ formation of an off-cycle disulfide from thiyl radicals suppresses this turnover, diminishing the efficiency of the reaction. With these detailed mechanistic insights, the turnover of the photocatalyst has been optimized, resulting in a >10-fold improvement in the quantum yield. These improvements enabled the development of a scalable flow protocol, demonstrating a potential strategy for practical applications with improved energy efficiency and cost-effectiveness.

Automated summary

The selectivity and reaction mechanism for the photocatalytic anti-Markovnikov hydroamination of unactivated alkenes with primary amines to produce secondary amines has been revealed through time-resolved laser kinetics measurements of key reaction intermediates. The absence of back-electron transfer between the photogenerated aminium radical cation and reduced photocatalyst complex leads to the selectivity for primary amine alkylation. The turnover of the photocatalyst occurs through a reaction between Ir(II) and a thiyl radical, with the in situ formation of an off-cycle disulfide suppressing this turnover and reducing reaction efficiency.