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

We disclose a catalytic photoredox carbobromination of unactivated alkenes with -bromocarbonyl compounds under a 390 nm LED light. The reaction proceeds with -bromoesters, -bromonitriles and -bromo--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, radical probes and three-way competitive crossover experiments suggest that the reaction operates by a unique radical addition-radical pairing (RARP) mechanism. The catalytic turnover is achieved by a single electron reduction of PC•+ by Br‒, rather than by alkyl radical (R•), and the product is generated by the pairing of Br• and R•, instead of the combination of Br‒ and a carbocation (R+).