Regioselective hydroxylation of aromatic acids with hydrogen peroxide proceeds readily in the presence of a family of aminopyridine iron(II) complexes. Hydroxylation of aromatic hydrocarbons is enhanced by carboxylic acid additives. Incorporating electron-donating substituents in the ligands increases the redox reactivity of iron(II) complexes, and enables spectroscopic observation of a new intermediate. Kinetic studies of the reactions between iron(II) complexes and H2O2 identified an (L)FeIII-OOH species which does not directly attack the aromatic ring, but serves a precursor to an active oxidant. Independently generated (L)FeIV=O does not hydroxylate aromatic substrates. Heterolytic pathway of productive O-O bond cleavage in ferric peroxo intermediates was observed when a mechanistic probe MPPH was used in place of H2O2. In the presence of H2O2 and carboxylic acids, a new low-spin Fe(III) intermediate was observed in the systems containing electron-rich aminopyridine complexes; available data suggest Fe(III)-acylperoxo formulation for this species. The new FeIIIO3CR intermediate appears to undergo heterolytic O-O bond cleavage to afford an active oxidant, which has yet to be directly observed. Diiron(III) oxo-carboxylato-bridged complexes are inactive in aromatic hydroxylations with H2O2.

Hydrogen peroxide activation at non-heme iron:  Mechanistic insights into aromatic hydroxylation

248th National Meeting (2014)

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