Catalytic oxidations mediated by metal ions and nitroxyl radicals

Abstract

The use of nitroxyl radicals, alone or in combination with transition metals, as catalysts in oxidation processes is reviewed, from both a synthetic and a mechanistic viewpoint. Two extremes of reactivity can be distinguished: stable (persistent) dialkylnitroxyls, such as TEMPO, and reactive diacylnitroxyls, derived from N-hydroxy imides, such as N-hydroxyphthalimide (NHPI). NHPI catalyzes a wide variety of free radical autoxidations, improving both activities and selectivities by increasing the rate of chain propagation and/or decreasing the rate of chain termination. In the absence of metal cocatalysts improved conversions and selectivities are obtained in the autoxidation of hydrocarbons to the corresponding alkyl hydroperoxides. In combination with transition metal cocatalysts, notably cobalt, NHPI and related compounds, such as N-hydroxysaccharin (NHS), afford effective catalytic systems for the autoxidation of hydrocarbons, e.g. toluenes to carboxylic acids and cycloalkanes to the corresponding ketones. Stable dialkylnitroxyl radicals, exemplified by TEMPO, catalyze oxidations of, e.g. alcohols, with single oxygen donors such as hypochlorite via the intermediate formation of the corresponding oxoammonium cation. Alternatively, in conjunction with transition metals, notably ruthenium and copper, they catalyze aerobic oxidations of alcohols via metal-centred dehydrogenation. The role of the TEMPO is to facilitate regeneration of the catalyst (Ru and Cu). In contrast, oxoammonium cations are involved in the aerobic oxidation of alcohols catalyzed by the copper-dependent oxidase, laccase, in combination with TEMPO. This different mechanistic pathway is attributed to the much higher redox potential of the copper(II) in the enzyme.