Kinetics and mechanism of the cobaloxime(II)-catalysed oxidation of 2-aminophenol by dioxygen. A phenoxazinone synthase model involving free-radical intermediates

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Abstract
Cobaloxime(II) derivatives catalysed the oxidative dehydrogenation of 2-aminophenol (ap) to 2-amino-3H-phenoxazin-3-one by dioxygen under ambient conditions. The 2-aminophenoxyl radical (ap˙) and a cobalt-bound dimeric radical have been detected as intermediates. The kinetics was followed by monitoring dioxygen uptake. The cobaloxime(II) concentration is very low during the reaction as evidenced by ESR spectroscopy. According to the proposed mechanism, in the rate-determining step superoxocobaloxime abstracts an H atom from ap via a hydrogen-bonded intermediate, affording the ap˙ radical. In the steady state cobaloxime(III) predominates and the active catalyst is generated in low concentration via its reduction by the ap˙ radical. The system studied serves as a model of phenoxazinone synthase and calls attention to the possible involvement of radical intermediates in the enzymatic reaction.