Catalytic autoxidation of benzoquinone dioximes with nitrogen oxides: steric effects on the preparation of monomeric dinitrosobenzenes

Abstract

A convenient catalytic method for the autoxidation of quinone dioximes to dinitrosobenzenes with dioxygen is based on the presence of small amounts of nitrogen oxides. The catalytic cycle is deduced from the facile chemical oxidation of quinone dioxime to dinitrosobenzene with stoichiometric amounts of the 1-electron oxidant, nitrosonium—either as the NO⁺BF₄^– salt or the disproportionated ion pair NO⁺NO₃^– derived from nitrogen dioxide. The regeneration of NO⁺ occurs by the subsequent oxidation of nitric oxide (NO) with dioxygen to nitrogen dioxide followed by the disproportionation to nitrosonium nitrate in the presence of electron-rich donors. Indeed, dioximes of various p-benzoquinones are shown to be strong reducing agents by transient electrochemistry. Electrochemical oxidation also leads to dinitrosobenzenes in good yields at anodic potentials of ca. 1.3 V. The substitution of p-dinitrosobenzene with bulky alkyl groups stabilizes the monomeric form, which is otherwise extensively associated.