Visible-Light-Induced Dehydrohalogenative Coupling for Intramolecular α-Alkenylation: A Way to Build Seven- and Eight-Membered Rings.

A visible-light-induced intramolecular α-alkenylation has been developed via metal-free dehydrohalogenative C(sp2)-C(sp2) coupling reaction to afford seven- and eight-membered rings. Extensive mechanistic studies prove that this reaction proceeds through a [2 + 2]-photocycloaddition, elimination, and retro-[2 + 2]-photocycloaddition process, with cyclobutane and cyclobutene being involved as key intermediates. This transformation is broadly applicable and highly stereoselective, yielding exclusively cyclic (1Z,3Z)-1,3-diene via photochemically allowed disrotation. This protocol excavates new applications of [2 + 2]-photocycloadditions, which may find their way in future olefin-olefin coupling reactions and medium-sized ring synthesis.