Photoredox-catalyzed direct formylation of alkenes to access α,β-unsaturated aldehydes

α,β-Unsaturated aldehydes are ubiquitous and versatile synthetic building blocks in organic synthesis. Nevertheless, the direct construction of such skeletons via site-selective β-formylation of readily available alkenes still remains challenging and largely unexplored. Herein, we disclose a visible-light photoredox-catalyzed selective β-formylation strategy of various alkenes, employing bench-stable α-chloro-N-methoxyphthalimide as a practical formyl radical precursor. This mild photoredox protocol delivers diverse α,β-unsaturated aldehydes with broad substrate generality toward electron-rich/deficient, sterically hindered, cyclic and heterocyclic olefins, and is suitable for late-stage modification of complex molecules. Mechanistic studies confirm a radical-polar crossover catalytic cycle involving formyl radical addition and carbocation induced elimination.