Photoaddition of Biacetyl and Alkenes. Reaction Stereochemistry, Multiplicity, and Photokinetics

Article abstract of DOI:10.1021/ja00539a019

Quantum yields are reported for the photoaddition of biacetyl with the alkenes indene, 2,3-dimethyl-2-butene, furan, and 1,2-dimethoxyethene in benzene solution.The dependence of quantum efficiencies on alkene concentration is consistent with a photoaddition mechanism involving biacetyl triplets.The quenching of fluid solution biacetyl phosphorescence has been observed, and quenching constants correlate with the electron-donor ability (ionization potentials) of the alkenes.Photoaddition of biacetyl and 1,2-dimethoxyethene is nonstereospecific, and oxetane formation is accompanied by the isomerization of the starting alkene.Stereochemical results are used to estimate relative rates of cleavage, closure, and stereorandomizing bond rotation in biradicals, proposed intermediates in photoaddition.Exciplexes of triplet biacetyl and alkenes are proposed as primary photochemical intermediates (precursors to biradicals) on the basis of stereochemical, emission quenching, and other data.