Electrochemical transformations and antiradical activity of asymmetrical RS-substituted pyrocatechols

Redox transformations of sulfides 1–8 combining a fragment of sterically hindered pyrocatechol with alkyl, cycloalkyl, and aromatic substituents were studied. The first step of electrooxidation of thioethers affords o-benzoquinones. The introduction of the redox-active thioether group extends the range of redox properties of pyrocatechols. In the second step, the thioether fragment is involved in the quasi-reversible anodic process, and the number of electrons participating in the electrode reaction depends on the structure of the hydrocarbon group bonded to the sulfur atom. The reactivity of compounds 1–8 toward O₂^•– was evaluated on the basis of the electrochemical data. Cyclopentyl, phenyl, or benzyl substituents in the thioether group exert a greater effect on the antiradical activity than the alkyl moieties. The formation of an o-semiquinolate radical anion in the reaction of pyrocatechol thioethers with KO₂ was detected by the ESR method. It was shown using the reaction with the stable 2,2-diphenyl-1-picrylhydrazyl radical as an example that RS-functionalized pyrocatechols show a higher antiradical activity compared to 3,5-di-tert-butylpyrocatechol.