Chain propagation determines the chemo- And regioselectivity of alkyl radical additions to C-O: Vs. C-C double bonds

Abstract

Investigations into the selectivity of intermolecular alkyl radical additions to C-O- vs. C-C-double bonds in α,β-unsaturated carbonyl compounds are described. Therefore, a photoredox-initiated radical chain reaction is explored, where the activation of the carbonyl-group through an in situ generated Lewis acid-originating from the substrate-enables the formation of either C-O or the C-C-addition products. α,β-Unsaturated aldehydes form selectively 1,2-, while esters and ketones form the corresponding 1,4-addition products exclusively. Computational studies lead to reason that this chemo- and regioselectivity is determined by the consecutive step, i.e. an electron transfer, after reversible radical addition, which eventually propagates the radical chain.