Direct Conversion of Cyclohexane into Adipic Acid with Molecular Oxygen Catalyzed by N-Hydroxyphthalimide Combined with Mn(acac)2 and Co(OAc)2

Direct conversion of cyclohexane into adipic acid was achieved by the use of the radical catalyst, N-hydroxyphthalimide (NHPI), in the presence of a small amount of a transition metal. For instance, cyclohexane could be converted into adipic acid in 73% selectivity at 73% conversion under atmospheric oxygen (1 atm) in the presence of NHPI (10 mol %) and Mn(acac)2 (1 mol %) at 100 °C for 20 h. ESR measurements show that the formation of phthalimide N-oxyl generated from NHPI with O2 was assisted by Co(II) species. Thus, the oxidation of cyclohexane to adipic acid was found to be enhanced by the addition of a small amount of Co(OAc)2 to the NHPI/Mn(acac)2 system. The NHPI-catalyzed oxidation of cyclohexane provides an attractive direct method which has long been desired in the chemical industry for the manufacturing of adipic acid. The present oxidation seems to be the first practical environmentally friendly process, in which nitric acid is not used as the oxidant, for the production of adipic acid from cyclohexane.