Transition metal salts catalysis in the aerobic oxidation of organic compounds Thermochemical and kinetic aspects and new synthetic developments in the presence of N-hydroxy-derivative catalysts

Abstract

Abstract The catalytic system formed by Mn(II) and Co(II) or Cu(II) nitrates has shown to be particularly effective for the oxidation of ketones and aldehydes by molecular oxygen under mild conditions. The process has a general character, but it is particularly selective for the oxidation of alkyl–aryl ketones to aromatic carboxylic acids, alkyl-cyclopropyl ketones to cyclopropane carboxylic acids and cycloalkanones to dicarboxylic acids. Mn salt plays a key role in this catalysis, which catalyses the enolisation of the carbonyl compound and initiates a free-radical redox chain with oxygen by an electron-transfer process. Co and Cu salts alone are inert, but they exalt the catalytic activity of the Mn salt, being more effective in the decomposition of the hydroperoxides. The same metal salt complexes, associated with TEMPO revealed particularly effective for the aerobic oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones under mild conditions (with air or oxygen at room temperature and atmospheric pressure); the mechanism of the catalysis is discussed. Thermochemical and kinetic investigations by EPR spectroscopy, concerning N-hydroxy compounds, have allowed to evaluate the Bond Dissociation Enthalpies (BDE) of several OH bonds and the absolute rate constants for the formation of the phthalimide-N-oxyl (PINO) radical from N-hydroxyphthalimide (NHPI) and for the hydrogen abstraction from several CH bonds by the PINO radical. The thermochemical and kinetic results have allowed us to explain the opposite catalytic behaviour of the two nitroxyl radicals, TEMPO and PINO, the former being an inhibitor of free radical processes, the latter a promoter of free-radical chain, and to develop new selective processes concerning the aerobic oxidation of alcohols, amines, amides, silanes and the substitution of heteroaromatic bases.