1,2-Dioxetanes Derived from 4,5-Dimethyl-2,3-dihydrofuran and 4,5-Dimethyl-2,3-dihydrothiophene: Synthesis via Photooxygenation, Activation Parameters, and Excitation Properties

Article abstract of DOI:10.1021/jo00242a028

Photooxygenation of 4,5-dimethyl-2,3-dihydrofuran (1a) and 4,5-dimethyl-2,3-dihydrothiophene (1b) in a variety of solvents gave the respective 1,2-dioxetanes 2a,b (<2 + 2> cycloaddition) as major products (82-90percent) and the allylic hydroperoxides 3a,b and 4a,b (ene reaction) as minor products (10-18percent).The 2,3-dihydrofuran-derived dioxetane 2a shows higher thermal stability <ΔG(excit.) = 28.1 +/- 1.0 kcal/mol at 343 K> compared to other known alkoxy-substituted dioxetanes; but more remarkable is the 2,3-dihydrothiophene derivative 2b <ΔG(excit.) = 22.7 +/- 1.2 kcal/mol at 343 K>, the most stable sulfur-substituted dioxetane to date, isolable by molecular distillation.Concerning their excitation properties, both dioxetanes afford preferentially triplet excited state products during thermal decomposition, e.g. Φ^T = 3.2 +/- 0.5percent for 2a and Φ^T ca. 0.002percent for 2b, the latter being the first triplet exitation yield (Φ^T) for a sulfur-substituted dioxetane.Mechanistic rationales for the 1000-fold lower efficiency of generating excited states for the 2,3 -dihydrothiophene dioxetane 2b are presented.