PHOTOCHEMISTRY OF 1,5-HEXADIEN-3-ONES - WAVELENGTH-DEPENDENT SELECTIVITY IN INTRAMOLECULAR ENONE-OLEFIN PHOTOADDITIONS

The photochemistry of ten 1,5-hexadien-3-ones in methanol is studied over the wavelength range of 313-366 nm, by using monochromatic light. With the goal of understanding the unusual wavelength-dependent selectivity observed for 5-methyl-1,5-hexadien-3-one (1), quantum yield measurements, structure reactivity studies, triplet sensitization, and quenching experiments are performed. While six of the ten dienones studied show selectivities that are independent of irradiation wavelength from 313 to 366 nm, four of the dienones exhibit wavelength-dependent selectivities that are similar to that observed in 1, thus establishing that 1 is not unique, as previously believed. Triplet sensitization studies suggest that the wavelength dependence results, largely, from a single-state alpha-cleavage reaction that competes with triplet cycloaddition. A variety of triplet quenchers were ineffective at inhibiting these reactions. Some possible mechanisms are discussed.