Study of the mechanism of redox transformations of sterically hindered N-aryl-o-iminoquinones

The mechanisms of redox transformations of sterically hindered [(1)IBQ]-[(3)IBQ], 9,10-iminophenanthraquinone [(4)IFQ], and o-aminophenol [(5)AP] have been studied by cyclic voltammetry. It has been shown that the reduction process in tetrahydrofuran consists of two consecutive reversible steps leading to the formation of a radical anion and a dianion. In the case of acetonitrile, only the first redox process is detected, which involves the steps of protonation and electron addition resulting in the formation of a monoanion. Quantum-chemical calculations indicate a lower degree of participation of the six-membered nonaromatic carbon ring in the spin density delocalization in the o-iminosemiquinonate radical anions as compared to o-benzosemiquinonate radical anions. The oxidation of iminoquinones results in the formation of an unstable radical cation, which undergoes further chemical transformations. An increase in the acidity of a medium leads to the formation of protonated forms with the reduction potential being shifted anodically. The introduction of screening groups makes it possible to suppress side reactions of hydrolysis and cyclization of oxidized o-aminophenol [(5)AP] forms. Therefore, the major product of complete electrolysis is o-iminobenzoquinone [(2)IBQ].