Characterisation of Reactive Intermediates Generated During Photolysis of 4-Acetoxy-4-aryl2,5-cyclohexadienones: Oxenium lons and Aryloxy Radicals

Abstract

Aryloxenium ions 1 are reactive intermediates that are isoelectronic with the better known arylcarbenium and arylnitrenium ions. They are proposed to be involved in synthetically and industrially useful oxidation reactions of phenols. However, mechanistic studies of these intermediates are limited. Until recently, the lifetimes of these intermediates in solution and their reactivity patterns were unknown. Previously, the quinol esters 2 have been used to generate 1, which were indirectly detected by azide ion trapping to generate azide adducts 4 at the expense of quinols 3, during hydrolysis reactions in the dark. Laser flash photolysis (LFP) of 2b in the presence of O₂ in aqueous solution leads to two reactive intermediates with λ[max] 360 and 460 nm, respectively, while in pure CH₃CN only one species with λ[max] 350 nm is produced. The intermediate with λ[max] 460 nm was previously identified as 1b based on direct observation of its decomposition kinetics in the presence of N₃⁻, comparison to azide ion trapping results from the hydrolysis reactions, and photolysis reaction products (3b). The agreement between the calculated (B3LYP/6-31G(d)) and observed time-resolved resonance Raman (TR⁳) spectra of 1b further confirms its identity. The second intermediate with λ[max] 360 nm (350 nm in CH₃CN) has been characterised as the radical 5b, based on its photolytic generation in the less polar CH₃CN and on isolated photolysis reaction products (6b and 7b). Only the radical intermediate 5b is generated by photolysis in CH₃CN, so its UV-vis spectrum, reaction products, and decay kinetics can be investigated in this solvent without interference from 1b. In addition, the radical 5a was generated by LFP of 2a and was identified by comparison to a published UV-vis spectrum of authentic 5a obtained under similar conditions. The similarity of the UV-vis spectra of 5a and 5b, their reaction products, and the kinetics of their decay confirm the assigned structures. The lifetime of 1b in aqueous solution at room rempurature is 170 ns. This intermediate decays with first-order kinetics. The radical intermediate 5b decomposes in a biphasic manner, with lifetimes of 12 and 75 μs. The decay processes of 5a and 5b were successfully modeled with a kinetic scheme that included reversible formation of a dimer. The scheme is similar to the kinetic models applied to describe the decay of other aryloxy radicals.