Efficient [2+ 2] heterodimerizations of dissimilar acyclic enones can be accomplished upon visible light irradiation in the presence of a ruthenium (II) photocatalyst. Similar cycloadditions under standard UV photolysis conditions are inefficient and unselective. Nevertheless, a diverse range of unsymmetrical tri-and tetrasubstituted cyclobutane structures can be produced in good yields and excellent diastereoselectivities using this ...