Copper photosensitizers unlock endergonic cycloadditions of strained bicyclobutanes under visible light

Researchers report heteroleptic copper(I)–BINAP photocatalysts with extended triplet lifetimes that enable endergonic, triplet-energy–transfer cycloadditions between strained bicyclo[1.1.0]butanes and alkenes under visible light. This approach moderates radical generation, suppresses alkene polymerization, broadens substrate scope to electron-deficient alkenes, enynes, dienes, and even aliphatic alkenes, and enables synthesis of bicyclo[2.1.1]hexanes, highlighting copper-catalyzed energy-transfer photocatalysis as a versatile tool for visible-light driven transformations.