Photochemistry of bicyclo[2.2.2]Octene-2,3-dione. Intersystem crossing viaan excimer

Irradiations of the title compound (10) in the visible region of the spectr um resulted in competing allylic rearrangement to cyclobutanedione 11 and p hotobisdecarbonylation to cyclohexadiene (12). The product ratio depended o n experimental variables including solvent, concentration, presence of adde d sensitizer or quencher, and temperature. It is suggested that rearrangeme nt occurs from the singlet state and bisdecarbonylation via an excimer whic h undergoes intersystem crossing and decay to 12. Comparison with the photo chemistry of a variety of related alpha-diketones indicates that competitio n between the two processes is sensitive to a variety of structural factors . The cyclobutanedione 11 underwent bisdecarbonylation to 12 upon direct an d sensitized irradiations. (C) 1999 Elsevier Science S.A. All rights reserv ed.