Metal-free and iron(II)-assisted oxidation of cyclohexane to adipic acid with ozone: a theoretical mechanistic study

Abstract Adipic acid has a tremendous applied importance. Its industrial synthesis requires harsh conditions and application/emission of hazardous HNO3 and N2O. Here, a mechanism of ecologically benign neat cyclohexane (CyH) oxidation with ozone into adipic acid under room temperature and radiation-free conditions, both metal-free and catalyzed by the iron(II) complex [FeCl2{κ3-HC(pz)3}] (pz = pyrazol-1-yl), was investigated by theoretical methods. Relatively high activation barriers of the rate limiting H-abstractions by O3 without a catalyst explain the experimentally observed low CyH conversion and selectivity toward adipic acid due to efficient accumulation of cyclohexanone. The active catalytic form [FeIV(=O)Cl2{κ3-HC(pz)3}] improves both CyH conversion and selectivity because it is a much more efficient H-abstractor compared to O3 and provides facile decomposition of CyH and cyclohexanone, opens up a new route to the formation of ortho-cyclohexanediol and then adipic acid, and prevents the accumulation of adipic anhydride – one of the stable intermediates under metal-free conditions.