Autoxidation of ketones and esters in basic solution

Abstract

The autoxidation of ketones and esters in aprotic solvents containing strong bases such as alkoxides was investigated extensively. Attention was paid to the autoxidation of other weakly acidic substrates such as nitriles, Schiff bases, and phenylhydrazones. The primary products are the α-hydroperoxides, which can be isolated in high yields when the oxidation is carried out at low temperatures, thus preventing their decomposition.

The first step of the reaction is ionization of the substrate to yield a resonance-stabilized anion, which subsequently reacts with oxygen.

A kinetic study showed that in some cases the oxidation is of the first order in anion and in oxygen, whereas in other cases (e.g., aliphatic esters) the ionization is the rate-determining step. Both oxidation and ionization show low activation energies (<10 kcal mol^–1) in aprotic systems. Arguments are advanced for a non-radical mechanism involving interaction of anion and O₂ to yield the hydroperoxide anion in one step. By considering the energies of the different steps involved it is shown that the autoxidation is favoured by the substrate being a weak acid as well as having a low C–H bond strength.