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We report that Ru(bipy)3Cl2 can serve as a visible light photocatalyst for 2+2 enone cycloadditions. A variety of aryl enones participate readily in the reaction, and the diastereoselectivity in the formation of the cyclobutane products is excellent. We propose a mechanism in which a photogenerated Ru(bipy)3+ complex promotes one-electron reduction of the enone substrate, which undergoes subsequent radical anion cycloaddition. The efficiency of this process is extremely high, which allows rapid, high-yielding 2+2 cyclizations to be conducted using incident sunlight as the only source of irradiation.
Ischay et al. (2008) studied this question.