Catalytic Photoredox Carbobromination of Unactivated Alkenes with α-Bromocarbonyls via the Mechanistically Distinct Radical-Addition Radical-Pairing Pathway.

We disclose a catalytic photoredox carbobromination of unactivated alkenes with α-bromocarbonyl compounds under a blue LED light. The reaction proceeds with α-bromoesters, α-bromonitriles and α-bromo-γ-lactones along with terminal and 1,2-disubstituted internal alkenes. Reactions with indenes and 1,1-disubstituted alkenes generate alkylated alkenes. Mechanistic studies by product selectivity and three-way competitive crossover experiments suggest that the reaction operates by a radical-addition radical-pairing (RARP) mechanism. The catalytic turnover is achieved by a single electron reduction of PC by Br (or Br ), rather than by alkyl radical (R•), and the product is generated by the pairing of Br• (or Br•) and R•, instead of the combination of Br and a carbocation (R).