Photoinduced Reaction of Triarylphosphines with Alkenes Forming Fused Tricyclic Phosphonium Salts

Herein reported is a photoinduced reaction of triarylphosphines with alkenes in the presence of carbonic acid. A drastic structural change occurs to produce fused tricyclic phosphonium salts having a phosphatricyclo­[4.3.0.02.9]­nonene core structure. The reaction was marked by dearomatizing transannulation of the phenyl ring, 1,2-migration of the phosphorus atom, and incorporation of an alkene. The highly endergonic process is promoted by cooperation of visible light with a photocatalyst under mild reaction conditions.