Oxidation of Cyclohexane by High-Valent Iron Bispidine Complexes: Tetradentate versus Pentadentate Ligands

The iron-bispidine-catalyzed oxidation of cyclohexane with H2O2, where either a tetradentate or a pentadentate bispidine ligand is coordinated to the iron center, yields up to 35% cyclohexanol and cyclohexanone (alcohol/ketone ratio of up to 4). Product distribution (including O-18 labeling studies), kinetic isotope effects, and the ratio of tertiary/secondary alcohols with adamantane as a substrate (3 degrees/2 degrees) suggest that (i) H abstraction by a ferryl complex is the rate-determining step and that the emerging cyclohexyl radical is short-lived, (ii) there is a parallel reaction involving oxidation by OH radicals, and (iii) there are considerable differences in the reaction pathways between the tetradentate and pentadentate ligand catalyst. These interpretations are fully supported by a DFT-based computational analysis.