Oxidation of N-benzylidene-2-hydroxyaniline by dioxygen catalysed by a dicobalt complex†

Abstract

The oxidation of N-benzylidene-2-hydroxyaniline and its related derivatives by O₂ catalysed by [Co₂L(µ-OH)] {H₃L = 2,6-bis[(2-hydroxyphenyl)iminomethyl]-4-methylphenol} in DMF at 363 K was investigated. The oxidation products were the corresponding 2-substituted benzoxazoles. However, a side reaction leading to 2-aminophenoxazine-3-one via hydrolysis of the starting compound with water produced in the oxidation, followed by the oxidation of aminophenol, was established. If a dehydrating agent such as anhydrous Na₂SO₄ or 4 Å molecular sieves was present in the reaction mixture the isolated product yields were above 87%. The fact that the reaction rate of the oxidation process in the presence of a radical scavenger was similar to that without suggests that it was a non-radical process. The initial oxidation rates depended linearly on the concentration of catalyst. The rates also depended linearly on the concentration of the organic substrate N-benzylidene-2-hydroxyaniline and O₂ pressure when these variables were small ([substrate] < 0.2 M, O₂ pressure < 70 kPa). At high concentration of substrate and O₂ pressure the rate showed saturation behavior. These kinetic data could be satisfactorily accounted for by a mechanism with initial co-ordination of substrate to [Co₂L(µ-OH)], followed by the co-ordination of O₂ to the catalyst, then the rate limiting step of oxidation of substrate. The formation of the adduct between N-benzylidene-2-hydroxyaniline and catalyst at 298 K was characterized by UV-Vis spectroscopy. The variation of the initial rates among the organic substrates could be explained by stereoelectronic effects. Moreover, the influence of acetic acid which slowed the initial rate of oxidation and weak base (2,6-di-tert-butylpyridine) which had little effect on the rate could also be satisfactorily accounted for based on the acid–base properties of the proposed reaction intermediates.