Photochemistry of Aromatic α,β-Epoxy Ketones. Substituent Effects on Oxirane Ring-Opening and Related Ylide Behavior

Article abstract of DOI:10.1021/jo00216a003

Upon 337.1-nm laser excitation, chalcone epoxides containing donor/acceptor substituents at para positions of phenyl and benzoyl groups undergo triplet-mediated ring opening to carbonyl ylides observable by broad absorption spectra (λ_max^Y=520-600 nm, ε_max^Y (13-27) x 10³ M^-1 cm^-1 in benzene) on a microsecond time scale (τ_Y=0.4-24 μs in benzene).The short-lived, carbonyl-type triplets (τ_T=0.8-100 ns) giving rise to ylides are monitored in some cases by direct transient absorption on a nanosecond time scale and, for all systems, are probed by quenching studies with 1-methylnaphthalene and 2,5-dimethyl-2,4-hexadiene.Substituent effects on ylide absorption maxima, ylide decay kinetics, reactivity toward dipolarophiles and methanol, and precursor triplet lifetimes are discussed in the light of charge delocalization in dipolar structures, variation in HOMO/LUMO energies, complexity of thermal processes contributing to ylide decay, and energy gap between an ylide triplet and its triplet carbonyl precursor (ring closed).