Selective Oxidation of Alkylbenzenes

Benzoic and hydroxybiphenyl carboxylic acids are widely used in the synthesis of drugs, biologically active compounds, and heat-resistant polymers. The known methods for preparation of aromatic hydroxy carboxylic acids include many stages, require not easily accessible starting compounds, and provide poor summary yields of the target products. 

A method was proposed for the preparation of p-hydroxybenzoic acid by oxidation of p-cresol with atmospheric oxygen in an acetic acid-acetic anhydride mixture under catalysis of cobalt acetate, manganese(II) acetate, and sodium bromide (Litvintsev et al. 1994). This procedure ensures 60% yield of p-acetoxybenzoic acid and 100% conversion of the initial p-cresol. 

It is known that the oxidation of alkyl-substituted aromatic hydrocarbons in acetic acid on metal bromide catalysis follows the one-electron transfer mechanism (Sheldon and Kochi 1981). The rate-determining stage is the one-electron transfer from the substrate to the metal ion in the highest oxidation state (Digurov et al. 1986). As a result, an unstable cation-radical is formed that 

The liquid-phase oxidation of p-methylacetophenone is important from practical and methodological points of view and deserves a concise consideration. The reaction is performed in acetic acid with the cobalt acetate catalyst. As shown by Obukhova et al. (2002), the catalyst detaches an electron from the substrate. The latter forms the cation-radical, which can dissociate in the following two ways  

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