Novel Catalysis of Hydroquinone Autoxidation with Nitrogen Oxides

An efficient catalytic method is described for the preparative conversion of hydroquinones to quinones with dioxygen under mild conditions. The use of the gaseous nitrogen oxide (NOx) catalyst allows a simple workup procedure for the isolation of quinones in essentially quantitative yields by merely removing the low-boiling solvent dichloromethane in vacuo. The mechanism of the catalytic autoxidation of hydroquinones is ascribed to the critical role of nitrosonium (NO+) in the oneelectron oxidation of hydroquinone, followed by the reoxidation of the reduced nitric oxide (NO) with dioxygen. An extensive series of complex interchanges among various NOx, species in nitrogen-(V), -(IV), -(III), and -(II) oxidation states, coupled with stepwise oxidation of hydroquinone via a successive series of one-electron/proton transfers, form the critical components of the catalytic cycle.