Time-Resolved Study of the Solvent and Temperature Dependence of Singlet Oxygen (¹Δ_g) Reactivity toward Enol Ethers: Reactivity Parameters Typical of Rapid Reversible Exciplex Formation

Pulsed nitrogen laser excitation has been used to examine and compare the reactivities of four enol ethers toward singlet oxygen (¹Δ_g) in five solvents. The experimental values of ΔH^⧧ (0 ± 1.0 kcal mol⁻¹), ΔS^⧧ (−23 to −34 eu), and the overall reaction rate constants (10⁴–10⁸ L mol⁻¹ s⁻¹) are characteristic of excited-state processes involving rapid reversible exciplex formation. Solvent effects are small but in some cases significant. It is concluded that such effects are the result of solvation entropy requirements at the transition state leading from exciplex to product, particularly in the case of dioxetane formation. The possibility is raised that the reversible exciplex mechanism operates generally for O₂ (¹Δ_g) reactions.