Z-Selective Addition of Diaryl Phosphine Oxides to Alkynes via Photoredox Catalysis

Radical addition to alkynes is known to predominantly yield thermodynamically more stable E-alkenes. Control of stereoselectivity in these reactions, and the isolation of the higher-energy Z-alkenes, remain an important challenge in chemical synthesis. Herein, direct synthesis of Z-alkenylphosphine oxides via visible-light-induced radical addition to alkynes in water is reported. This protocol was effective with various terminal and internal alkynes, affording products with high Z-stereoselectivity. Moreover, this transformation was demonstrated on gram scale. Mechanistic studies support the following conclusions: (1) the reaction proceeds via free-radical addition; (2) the choice of K2CO3 as aqueous base is crucial to the transformation; and (3) pi-pi stacking interaction greatly improves Z-selectivity.