Sensitized Photooxygenation. 3. Mechanistic Studies on the Singlet Oxygenation of 5,6-Disubstituted 3,4-Dihydro-2H-pyrans

Article abstract of DOI:10.1021/jo00307a022

Studies on the photooxygenation of four 5,6-disubstituted 3,4-dihydro-2H-pyrans under singlet oxygen (¹Δ_g) conditions are presented.For the 6-methyl-substituted substrates (1a,c), two hydroperoxides (2,3) are formed as the primary products, the ratio of which is solvent dependent, with the polar solvent favoring the production of 2.Hock cleavage of 5-hydroxyperoxide 2a and 2c is observed to occur via the possible intermediacy of dioxetane 4a and 4c.For the 6-phenyl analogue (1b,d), dioxetanes are observed to form directly, which decompose to ring-cleavageproducts (5b,d) readily. 6-Hydroperoxide 3a and 3c isomerize to 4-hydroperoxide 6a and 6c thermally, which further transform to 4-alcohol 7a and epoxy alcohols 8a and 8c.The singlet oxygenation reaction shows very significant effects of substitution and solvent.Both 6-phenyl and 5-acetyl substituents and polar solvents favor the "dioxetane-mode" process.Kinetic studies by the competition method indicate that the overall reaction rate decreases with increasing temperature, giving rise to a very significant negative enthalpy of activation.The experimental data is interpreted in terms of a prior formation of a reversible exciplex, which collapses to perepoxides, and the possible involvement of a zwitterionic transition state or intermediate along the "dioxetane" path.