Direct Stereoselective Installation of Alkyl Fragments at the β-Carbon of Enals via Excited Iminium Ion Catalysis

The direct introduction of sp(3) carbon fragments at the beta position of a,beta-unsaturated aldehydes is greatly complicated by competing 1,2-addition manifolds. Previous catalytic enantioselective conjugate addition methods, based on the use of organometallic reagents or ground-state iminium ion activation, could not provide general and efficient solutions. We report herein that, by turning them into strong oxidants, visible light excitation of chiral iminium ions triggers a stereocontrolled radical pathway that exclusively affords highly enantioenriched beta-substituted aldehydes bearing a variety of alkyl fragments.