Catalytic Photoredox Carbobromination of Unactivated Alkenes with α-Bromocarbonyls via the Mechanistically Distinct Radical-Addition Radical-Pairing Pathway
Harshvardhan Singh, Raj Kumar Tak, Dhruba P. Poudel, Ramesh Giri
Abstract
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 Br 3 – ), rather than by the alkyl radical (R • ), and the product is generated by the pairing of Br • (or Br 2 •– ) and R •, instead of the combination of Br – and a carbocation (R + ).