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

Investigations into the selectivity of intermolecular alkyl radical additions to C-O- 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 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, an electron transfer, after reversible radical addition, which eventually propagates the radical chain.