[2+2] Cycloaddition of 1,3-Dienes by Visible Light Photocatalysis

[2+2] Photocycloadditions of 1,3‐dienes represent a powerful yet synthetically underutilized class of reactions. We report that visible light absorbing transition metal complexes enable the [2+2] cycloaddition of a diverse range of 1,3‐dienes. The ability to use long‐wavelength visible light is attractive because these reaction conditions tolerate the presence of sensitive functional groups that might be readily decomposed by the high‐energy UVC radiation required for direct photoexcitation of 1,3‐dienes. The resulting vinylcyclobutane products are poised for a variety of further diversification reactions, and this method is consequently expected to be powerfully enabling in the synthesis of complex organic targets. Square dance: A convenient and highly functional‐group‐tolerant [2+2] cycloaddition of 1,3‐dienes that operates under visible light irradiation is reported. The vinylcyclobutane products serve as versatile synthetic intermediates in a range of further transformations as illustrated by a concise synthesis of the marine natural product (±)‐epiraikovenal. bpy=bipyridyl, ppy=phenylpyridyl.