Visible-Light Driven [2+2] Photocycloadditions between Benzophenone and C=C Bonds in Unsaturated Lipids

The [2 + 2] photocycloaddition of alkenes and carbonyls is of fundamental interest and practical importance, as this process is extensively involved in oxetane-ring constructions. While individual carbonyl group or alkene moiety has been utilized as photoactive species for oxetane formations upon ultraviolet photoexcitation, direct excitation of the entire non-covalent complex involving alkene and carbonyl substrates to achieve [2 + 2] pho-tocycloadditions is rarely addressed. Herein, complexes with non-covalent interactions between benzophenone and C=C bonds in unsaturated lipids have been successfully characterized and for the first time a [2 + 2] cy-cloaddition leading to the formation of oxetanes has been identified under visible-light irradiation. The mech-anism of this reaction is distinctly different from the well-studied Paterno−Buchi reaction. The entire com-plexes characterized as dimeric proton-bonded alkene and carbonyl substrates can be excited under visible light, leading to electron transfe...