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

A 2-alkenoylpyridine-bound N,N'-dioxide-Tb complex has been found to absorb visible light to reach the excited state, leading to the direct visible-light-excited catalytic enantioselective [2+2] cycloaddition of 2-alkenoylpyridines to various alkenes in the absence of an additional photosensitizer. Diverse enantioenriched cyclobutanes were successfully obtained (yields up to 70 %, >19:1 d.r., 92 % ee). The new chiral terbium(III) complex features a bathochromic shift, lower excitation energy, and facile intersystem crossing due to paramagnetic and heavy-atom effects, which enable the antenna 2-alkenoylpyridines to be excited. For comparison, a chiral N,N'-dioxide-Sc complex in combination with [Ru(bpy) ]Cl was efficient in the enantioselective photocycloaddition reactions of 2'-hydroxychalcones with alkenes, thereby revealing that both substrates and metal salts have significant effects on the reaction.