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

The oxidation of N-benzylidene-2-hydroxyaniline and its related derivatives by O-2 catalysed by [Co2L(mu-OH)] {H3L = 2,6-bis[(2-hydroxyphenyl)iminomet hyl]-4-methylphenol} in DMF at 363 K was investigated. The oxidation produc ts were the corresponding 2-substituted benzoxazoles. However, a side react ion leading to 2-aminophenoxazine-3-one via hydrolysis of the starting comp ound with water produced in the oxidation, followed by the oxidation of ami nophenol, was established. If a dehydrating agent such as anhydrous Na2SO4 or 4 Angstrom molecular sieves was present in the reaction mixture the isol ated 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 tha t without suggests that it was a non-radical process. The initial oxidation rates depended linearly on the concentration of catalyst. The rates also d epended linearly on the concentration of the organic substrate N-benzyliden e-2-hydroxyaniline and O-2 pressure when these variables were small ([subst rate] < 0.2 M, O-2 pressure < 70 kPa). At high concentration of substrate a nd O-2 pressure the rate showed saturation behavior. These kinetic data cou ld be satisfactorily accounted for by a mechanism with initial co-ordinatio n of substrate to [Co2L(mu-OH)], followed by the co-ordination of O-2 to th e catalyst, then the rate limiting step of oxidation of substrate. The form ation of the adduct between N-benzylidene-2-hydroxyaniline and catalyst at 298 K was characterized by UV-Vis spectroscopy. The variation of the initia l rates among the organic substrates could be explained by stereoelectronic effects. Moreover, the influence of acetic acid which slowed the initial r ate 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.