Organophotoredox-Catalyzed Intermolecular Oxa-[4+2] Cycloaddition Reactions

An intermolecular oxa-[4+2] cycloaddition
reaction promoted by
a thioxanthylium photoredox catalyst under irradiation with green
light has been developed. The reaction of ortho-quinone
methides with styrenes smoothly affords the desired cycloadducts.
Especially styrenes bearing electron-donating groups are efficiently
transformed in this reaction. This method represents a sustainable
way to carry out oxa-[4+2] cycloaddition reactions using only a catalytic
amount of a photocatalyst and visible light.