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

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 + ).

α-Bromoacetate as a Mild and Safe Brominating Agent in the Light-Driven Vicinal Dibromination of Unactivated Alkenes and Alkynes

“…During our studies on photoredox-catalyzed alkene carbobromination of 4-phenylbut-1-ene ( 1 ) with α-bromocarbonyls via a radical addition–radical pairing (RARP) mechanism, we observed that exposure of the reaction to air completely shunned carbobromination and induced selective 1,2-dibromination (Table , entry 1). A control experiment in the absence of the photocatalyst (4-CzIPN) at 440 nm showed that the dibromination still proceeded in good yield (entry 2).…”