Photocatalytic [2 + 2] cycloadditions of enones with cleavable redox auxiliaries.

α,β-Unsaturated 2-imidazolyl ketones undergo [2 + 2] cycloaddition with a variety of Michael acceptors upon irradiation with visible light in the presence of Ru(bpy)(3)(2+). Cleavage of the imidazolyl auxiliary from the cycloadducts affords cyclobutane carboxamides, esters, thioesters, and acids that would not be accessible from direct cycloaddition of the corresponding unsaturated carbonyl compounds.