N,N'-Dioxide-Metal Complexes Mediated Enantioselective [2+2] Photocycloadditions of Enones and Olefins with Visible Light.

2-Alkenoyl pyridine-bound N,N'-dioxide/TbIII complex was found to be able to absorb visible light to reach its excited state, leading to direct visible-light-excited catalytic enantioselective [2+2] cycloaddition of 2-alkenoyl pyridines with various alkenes in the absence of additional photosensitizer. Diverse enantioenriched cyclobutanes were obtained in good results (up to 70% yields, >19:1 d.r., 92% ee). The new chiral terbium(III) complex features bathochromic shift, lower excitation energy, and facile intersystem crossing due to the paramagnetic and heavy-atom effects, enabling the antenna 2-alkenoyl pyridines to be excited. Comparatively, chiral N,N'-dioxide/ScIII complex in incorporation with Ru(bpy)3Cl2 was efficient in enantioselective photocycloadditions of 2'-hydroxychalcones with alkenes, revealing that both substrates and metal salts displayed significant effects on the reaction.