A functional phenoxazinone synthase model based on dioximatomanganese(II): Kinetics and mechanism of the catalytic oxidation of 2-aminophenols by dioxygen

The dimeric dioximatomanganese(II) complex [Mn2(HL)2](BPh4)2 (1) (where H2L is [HONC(CH3)C(CH3)NCH2CH2]2NH) dissociates to the monomer [Mn(HL)]+ (2) upon dissolution in MeOH. Complex 2 is a functional phenoxazinone synthase model catalyzing the base-assisted oxidative dimerization of 2-aminophenol (H3ap) to 2-amino-3H-phenoxazine-3-one (apx) by dioxygen under ambient conditions via an o-benzoquinone monoimine (bqmi) intermediate, which, however, cannot be directly detected due to its high reactivity. If H3ap is replaced by 2-anilino-4,6-di-tert-butylphenol (3), a structural analog where the required positions are blocked, oxidation occurs without dimerization, affording the corresponding N-substituted o-quinone monoimine. Free radical intermediates were detected by ESR spectroscopy in both reactions. The kinetics of the catalytic oxidation of H3ap was studied spectrophotometrically and the corresponding rate equation established. It is consistent with a mechanism involving the formation of a catalyst-H2apO2 ternary complex, which decomposes in the rate-determining step, generating a semiquinone monoimine undergoing noncatalytic oxidative dimerization to apx.

The dioximatomanganese(II) complex [Mn(HL)]+ (2) is a functional phenoxazinone synthase model, catalyzing the oxidative dimerization of 2-aminophenol to 2-amino-3H-phenoxazine-3-one (apx) via an o-benzoquinone monoimine (bqmi) intermediate. The kinetic behavior is consistent with the rate-determining decomposition of a ternary catalyst-substrate-dioxygen intermediate to bqmi followed by non-catalytic transformation to apx. Figure optionsDownload as PowerPoint slide