Visible-light photocatalytic E to Z isomerization of activated olefins and its application for the syntheses of coumarins

Photocatalytic isomerization of thermodynamically stable E-alkene to less stable Z-alkene has been the subject of numerous studies, being successfully achieved mainly under UV irradiation. Recent development of visible light photoredox catalysis has witnessed it emerging as a powerful tool for the access of new structural complexity and many challenging targets. Herein, we report a visible light-promoted E to Z isomerization of cinnamates. When E-isomer of cinnamates was irradiated with blue light in the presence of an organo-photocatalyst, fac-Ir(ppy)₃, Z-isomer was exclusively obtained in high yields and with good selectivity. The mild, convenient reaction condition has made this protocol an effective synthetic methodology, which was subsequently implemented in an efficient synthesis of coumarins.