Alkenylation and Allylation Reactions of Alkyl Halides Using Photo Catalyst

Abstract

Radical addition/beta-fragmentation sequence has long been utilized for alkenylation and allylation of alkyl halides, in which key alkyl radicals are generated from alkyl halides by S(H)2 reaction typically using tributyltin radical. In this article, we describe modernized alkenylation and allylation of alkyl halides, in which alkyl radical generation is carried out by electron transfer under photoirradiation with the use of transition metal catalyst, which enables us to carry out the radical alkenylation and allylation without the use of radical initiators nor chain transfer reagents. Indeed, radical alkenylation of alkyl iodides using alkenyl bromides proceeded well by the use of a Pd/light combined system involving Hantzsch ester. Alkenyl and allyl sulfones worked better, since the system can obviate the use of Hantzsch ester. Mechanistically spontaneous reductive elimination of PhSO2I is suggested to recover the Pd catalyst. We also discuss allylation of gem-difluoromethylene-containing alkyl halides using allyl sulfones, which is best performed by the use of a visible light photoredox system based on Ru-catalyst.