Oxidation of phenols by molecular oxygen catalysed by transition metal complexes. Comparison between the activity of various cobalt and manganese complexes and the role of peroxy intermediates

Abstract

The oxidation reactions of hindered phenols by molecular oxygen catalysed by monomeric and polymeric cobalt–Schiff base complexes, cobalt and manganese porphyrins, and (pyridine)cobaloxime are described; the rate and selectivity of these reactions are very dependent on the catalyst and on the solvent.

A new product has been isolated, 1,3,5-tri-t-butyl-4-oxocyclohexa-2,5-dienylperoxy(pyridine)co-baloxime, and was fully characterised by its elemental analysis and spectroscopic methods. The reactivity of peroxy compounds derived from 2,4,6-tri-t-butylphenol which are postulated as intermediates in the oxidation of this phenol has been studied. Thermal decomposition of 1,3,5-tri-t-butyl-4-oxocyclohexa-2,5-dienyl(pyridine)cobaloxime indicates that the formation of this complex from the phenol, O₂, and (pyridine)cobaloxime(II) is reversible and that it is converted into 2,6-di-t-butyl-1,4-benzoquinone only in the presence of a proton source. The corresponding hydroperoxide is probably an intermediate in this transformation as its decomposition in the presence of the cobalt(II) or manganese(III) complexes yields the same final products as the overall oxidations.