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) π–π
stacking interaction greatly improves Z-selectivity.