[2 + 2] Photocycloadditions of Conformationally Constrained Styrenes Enabled by Triplet Lifetime Extension

We report an intermolecular, cross-selective [2 + 2] cycloaddition between cyclic styrenes and olefin acceptors proceeding via visible-light irradiation, enabled by the extended triplet lifetime of cyclic styrenes. A broad range of olefins, including electron-deficient and simple alkenes, (hetero)styrenes, arylimines, vinyl boronate esters, among others, underwent efficient cycloaddition with cyclic styrenes constrained by four-, five-, and rigidified six-membered rings, providing rapid access to sp³-rich fused cyclobutane scaffolds. Notably, the [2 + 2] cycloaddition between two electronically similar styrenes, which typically gives statistical mixtures of homo- and heterodimers, proceeded with excellent chemoselectivity and yield between cyclic and acyclic styrenes without requiring a large excess of one reactant, facilitated by the significant difference in triplet lifetime between the reacting partners. Mechanistic studies and DFT calculations support a Dexter energy transfer mechanism of substrate activation and underscore the critical role of triplet lifetime extension in enabling the observed reactivity. The utility of this method is demonstrated in the total synthesis of lindleyanin, requiring only two steps.