Triplet Photochemistry of Vinyl Cyclopropenes: Mechanistic and Exploratory Organic Photochemistry

Our research on the triplet photochemistry of vinylcyclopropenes has dealt with a diverse series of systems, providing a series of experimental examples and mechanstic studies. It perhaps is not surprising that the reaction mechanisms have been controversial. The present study is theoretical and provides evidence for control by a critical T-1 diradical intermediate which has a high spin-orbit coupling with S-0 ground-state along the mechanistic pathway. The evidence now for the one pathway derives from independent generation and behavior of this diradical, prediction of the reaction regioselectivity in nine diverse examples, the S-0 and T-1 hypersurfaces in the reaction, arid an S-0-T-1 degeneracy with SOC for the critical diradical. This triplet diradical when independently generated gives the same regioselectivity observed in examples starting with the vinylcyclopropene triplet itself. The overall reaction provides a useful synthesis of cyclopentadienes.