Intramolecular Crossed [2+2] Photocycloaddition through Visible Light-Induced Energy Transfer

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 benzobicyclo[3.1.1]­heptanones, 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.