Hydroquinone Oxidation by Redox‐active Guanidines and Thioguanidines: Faster Conversion to High‐Potential Than to Low‐Potential Quinones

Quinones are important oxidants in biology and synthetic chemistry. Herein, we study the kinetics of the oxidation of hydroquinones to quinones with redox-active guanidines and thioguanidines. In the first section, we report the synthesis and characterization of the first redox-active aromatic compounds with two, three, and four thioguanidino groups and compare their redox properties with the corresponding oligoguanidines. The stable salts obtained upon chemical two-electron oxidation of the tetra-thioguanidine were then applied in dehydrogenative P-P coupling reactions at room temperature, demonstrating their superior proton-coupled electron-transfer reactivity compared with previously used oligoguanidines. Oxidation of hydroquinones and halogenated derivatives to the 1,4-benzoquinones is also much faster with the new tetrathioguanidine than with oligoguanidines. Seemingly paradoxically, for tetraguanidines and tetrathioguanidines, oxidations to high-potential halogenated quinones are faster than those to low-potential quinones (due to proton-coupled electron transfer (PCET)), motivating the use of redox-active guanidines as redox mediators.