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

The
direct introduction of sp3 carbon fragments at the
β position of α,β-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 β-substituted aldehydes bearing
a variety of alkyl fragments.