SENSITIZED PHOTOOXYGENATION .3. MECHANISTIC STUDIES ON THE SINGLET OXYGENATION OF 5,6-DISUBSTITUTED 3,4-DIHYDRO-2H-PYRANS

Studies on the photooxygenation of four 5,6-disubstituted 3,4-dihydro-2H-pyrans under singlet oxygen (1Δg) conditions are presented. For the 6-methyl-substituted substrates (la,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-hydroperoxide 2a and 2c is observed to occur via the possible intermediacy of dioxetane 4a and 4c. For the 6-phenyl analogue (lb,d), dioxetanes are observed to form directly, which decompose to ring-cleavage products (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”. © 1990, American Chemical Society. All rights reserved.