Modulating endergonic triplet photosensitization for cycloadditions between bicyclo[1.1.0]butanes and alkenes.

While energy transfer photocatalysis has recently been used in the synthesis of multisubstituted polycyclic alkanes as benzene bioisosteres, the use of highly reactive noble-metal and organo-photocatalysts often leads to rapid generation of diradical intermediates, triggering polymerization side reactions and limiting functional-group tolerance. Here we show that heteroleptic copper(I)-BINAP complexes containing bidentate (pyrazolyl)pyridine ligands act as effective triplet photosensitizers for cycloadditions between bicyclo[1.1.0]butanes and alkenes. These copper catalysts, with their extended excited-state lifetimes, enable endergonic photosensitization for substrate activation, thereby moderating radical generation, suppressing alkene polymerization and broadening the substrate scope to include electron-deficient alkenes, enynes, dienes and even aliphatic alkenes. Bicyclo[2.1.1]hexanes with an aryl substituent at either the 3- or 2-position can be synthesized by activating either styrenes or bicyclo[1.1.0]butanes. These findings suggest that copper photocatalysis holds promise for enhancing the synthetic versatility of visible-light-driven energy transfer processes.