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–TbIII 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–ScIII complex in combination with [Ru(bpy)3]Cl2 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. Exciting terbium complex: The terbium complex of a novel four‐carbon‐linked L4‐RaPr2 ligand efficiently promotes the visible‐light‐excited [2+2] cycloaddition reactions of 2‐alkenoylpyridines with diverse alkenes to produce diverse enantioenriched cyclobutanes (see scheme). A plausible mechanism for the photocycloaddition reaction is discussed. [ABSTRACT FROM AUTHOR]