KINETICS AND MECHANISM OF THE COBALOXIME(II)-CATALYZED OXIDATION OF 2-AMINOPHENOL BY DIOXYGEN - A PHENOXAZINONE SYNTHASE MODEL INVOLVING FREE-RADICAL INTERMEDIATES

Cobaloxime(II) derivatives catalysed the oxidative dehydrogenation of 2-aminophenol(ap) to 2-amino-3H-phenoxazin-3-one by dioxygen under amb ient conditions. The 2-aminophenoxyl radical (ap(.)) and a cobalt-boun d dimeric radical have been detected as intermediates. The kinetics wa s followed by monitoring dioxygen uptake. The cobalbxime(II) concentra tion is very low during the reaction as evidenced by ESR spectroscopy. According to the proposed mechanism, in the rate-determining step sup eroxocobaloxime abstracts an H atom from ap via a hydrogen-bonded inte rmediate, affording the ap(.) radical. In the steady state cobaloxime( III) predominates and the active Catalyst is generated in low concentr ation via its reduction by the ap(.) radical. The system studied serve s as a model of phenoxazinone synthase and calls attention to the poss ible involvement of radical intermediates in the enzymatic reaction.