Facile Peroxidation of Cyclohexane Catalysed by In Situ Generated Triazole-Functionalised Schiff Base Copper Complexes

A set of facile room temperature catalytic systems for the oxidation of cyclohexane C–H bonds was developed from in situ generated triazole-functionalised Schiff base copper complexes. The combination of a new triazolium-functionalised Schiff base, [(E)-3-methyl-1-propyl-4-(2-(((2-(pyridin-2-yl)ethyl)imino)methyl)phenyl)-1H-1,2,3-triazol-3-ium hexafluorophosphate(V), 2] with a range of bench-top Cu(I) and Cu(II) salts (Cu2O, CuO, Cu(CH3CN)4PF6, CuSO4·5H2O, Cu2(OAc)4·2H2O, CuCl2, Cu(NO3)2·3H2O) as catalysts were screened under varying reaction conditions for the peroxidation of cyclohexane using hydrogen peroxide as a green source of oxygen. High conversions to oxidised products were obtained with up to 80% in 6 h for the 2/CuSO4·5H2O system at 1 mol% catalyst concentration under optimised reaction conditions. All the copper salts yielded the ketone–alcohol (K–A) oil containing varying ratios of cyclohexanol and cyclohexanone. The results also showed that at room temperature, the various in situ generated copper catalysts exclusively yielded only the K–A oil. Furthermore, by changing the reaction temperature to reflux in acetonitrile and depending on the starting substrate (cyclohexane, cyclohexanol or cyclohexanone), 23–100% of adipic acid was also obtained. The kinetics study for the peroxidation reaction reveals activation energy of 12.29 ± 2 kJ/mol following a copper initiated radical mechanism.