General and Efficient Intermolecular [2+2] Photodimerization of Chalcones and Cinnamic Acid Derivatives in Solution through Visible‐Light Catalysis

[2+2] Photocycloaddition, for example, the dimerization of chalcones and cinnamic acid derivatives, is a unique strategy to construct cyclobutanes, which are building blocks for a variety of biologically active molecules and natural products. However, most attempts at the above [2+2] addition have focused on solid‐state, molten‐state, or host–guest systems under ultraviolet‐light irradiation in order to overcome the competition of facile geometric isomerization of nonrigid olefins. We report a general and simple method to realize the intermolecular [2+2] dimerization reaction of these acyclic olefins to construct cyclobutanes in a highly regio‐ and diastereoselective manner in solution under visible light, which provides an efficient solution to a long‐standing problem. Circle of light: Visible‐light‐induced energy transfer can be used to construct cyclobutanes from nonrigid olefins in a highly regio‐ and diastereoselective manner through [2+2] photocycloaddition. Chalcones and cinnamic acid derivatives were reacted to give cyclobutanes with high yields at room temperature.