Photoinduced Cobalt Catalysis for Direct and Slow‐Release Homologation of Electron‐Deficient Alkenes Using Acetylene

Homologation is of high value in organic synthesis, yet the two‐carbon extension of electron‐deficient alkenes remains challenging. Here we report a photoinduced cobalt‐catalyzed homologation of electron‐withdrawing‐group (EWG)‐substituted alkenes using acetylene as a C2 building block and water as a hydrogen source. Complementing existing methods that are largely limited to non‐electron‐deficient alkenes, this protocol enables both the direct homologation of electron‐deficient alkenes and a slow‐release approach that employs readily accessible β ‐chloro compounds as alkene precursors. The method demonstrates broad functional group compatibility and enables efficient synthesis of valuable compounds, including bioactive pheromones and fragrance molecules. Mechanistic studies reveal that the catalytically active Co(III)─H species forms via oxidative addition of Co(I) to a proton source. This work provides a practical and complementary platform for alkene homologation.