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 blue-light-emitting diode (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 Br₃^-), rather than by the alkyl radical (R^•), and the product is generated by the pairing of Br^• (or Br₂^•-) and R^•, instead of the combination of Br^- and a carbocation (R⁺).