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Herein, we present the intramolecular 2+2 cycloadditions of dienones promoted through sensitization, using a polypyridyl iridium(III) catalyst, to form bridged cyclobutanes. In contrast to previous examples of straight 2+2 cycloadditions, these efficient crossed additions were achieved under irradiation with visible light. The reactions delivered desired bridged benzobicycloheptanone products with excellent regioselectivity in high yields (up to 96%). This process is superior to previous syntheses of benzobicyclo3.1.1heptanones, which are readily converted to B-norbenzomorphan analogues of biological significance. Electrochemical, computational, and spectroscopic studies substantiated the mechanism of triplet energy transfer and explained the unusual regiocontrol.
Zhao et al. (2017) studied this question.