Unsymmetrical sulfoxides with sterically hindered catechol fragment: synthesis, structure, electrochemical...

New unsymmetrical sulfoxides containing a redox-active sterically hindered catechol fragment and a nonpolar hydrocarbon substituent at the sulfoxide group were synthesized via the oxidation of the corresponding catechol thioethers with hydrogen peroxide. The yield of the reaction products ranges from 42 to 89%. The crystal structures of catechol sulfoxides with isopropyl, cyclopentyl, adamantyl, benzyl and 1-naphthyl moieties were established by single-crystal X-ray analysis. The possibility of forming intra- and intermolecular hydrogen bonds has been shown for these compounds. The electrochemical behavior of sulfoxides was studied in comparison with that of the parent catechol thioethers. The redox transition corresponding to catechol fragment oxidation is shifted to the anodic region relative to the initial thioethers, which is attributed to the more electron-withdrawing nature of the S=O group. The second redox stage characterizes the transformation of the sulfoxide fragment and is observed in most cases at 1.82-1.91 V. The radical-scavenging activity and antioxidant properties of the unsymmetrical sulfoxides and their precursor thioethers were evaluated using the reaction with the 2,2'-diphenyl-1-picrylhydrazyl radical (DPPH) and the 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) radical cation (ABTS^+•). In both assays, the lowest IC₅₀ values among the studied catechol sulfoxides were found for compounds bearing isopropyl and tert-butyl substituents on the sulfoxide group.