Excitation of acyclic trisubstituted alkenes via triplet energy transfer for diastereo- and enantioselective [2 + 2] photocycloaddition.

Photochemistry offers a powerful platform for enabling unique bond cleavage and formation. However, achieving precise stereochemical control while suppressing persistent racemic background reactions remains a fundamental challenge in this field. Herein, we report a highly enantio- and diastereoselective [2 + 2] photocycloaddition of 2-cyanoacrylamides-acyclic trisubstituted alkenes-using a dual catalytic system. Despite non-selective triplet energy transfer to both catalyst-bound and unbound substrates, high stereoselectivity is achieved through a dual Curtin-Hammett kinetic control strategy. This method features broad substrate scope, accommodating diverse arenes and alkenes, and enables the construction of α-chiral quaternary nitriles as well as contiguous quaternary stereocenters. Notably, stereoconvergent transformations are observed from mixtures of varied E/Z isomers and rotamers, and dearomative cycloadditions proceed efficiently across a range of aromatic systems. Mechanistic studies support a pathway involving fast ring-closure kinetics of matched diradical intermediates, with the cyano group playing a critical role.