Investigation of 3d-transition metal acetates in the oxidation of substituted dioxolene and phenols

• MnII, FeII, CoII, NiII, CuII acetates probed for dioxolene and aminophenol oxidation.
• Mn acetate most active in oxidation of dioxolene and formation of 2-aminophenoxazinone from 2-aminophenol.
• Mechanistic studies show that the substrate might also act as ligand to form the active catalytic species.
• The reactive oxygen species involved in the oxidation is mostly peroxide and hydroxyl radical

Enzymatic reactions have inspired many chemists to design small molecule mimics that would perform the function of the enzymes in aqueous and or non-aqueous medium. Catechol oxidase (CO) and phenoxazinone synthase (PHS) are two multi-copper enzymes in nature, which has led to model complexes of Mn, Fe, Co, Ni, Cu. Based on our earlier work in this area we have probed the commercially available metal acetates of the above metals to establish a trend in reactivity for catalytic conversions similar to those of the two enzymes. The results show that Mn is the best 3d transition metal for similar catalysis. MnII acetate was found to convert 3,5-di-tert-butylcatechol (DTBC) to 3,5-di-tert-butylquinone (DTBQ) with a kcat of 1.3(1) × 103 h−1and for o-aminophenol (OAP) to 2-aminophenoxazinone (APX) conversion the kcat = 111(2) h−1, demonstrating efficient CO and PHS like activity. Kinetic studies show that DTBC oxidation follows a first order kinetics with respect to the substrate for each of those metal(II) acetates with activity order of Mn >> Co > Cu > Fe ≥ Ni. Through mechanistic investigation we found that the reactive oxygen species detected during the oxidation of DTBC is mostly hydroxyl radical for Mn, Fe and Co whereas Cu and Ni generate H2O2.

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