Surface hydroxyl on TiO2 promoted formation of α-C radical and oxaziridine intermediates in aerobic oxidation of amine into oxime

Constructing N−O bond in amines via aerobic oxidation is of high interest for producing cyclohexanone oxime; however, the mechanism remains disputed. Concerning TiO₂/NHPI co-catalyzed aerobic oxidation of cyclohexylamine, here a convincing mechanism involving α-C radical and oxaziridine as key intermediates is demonstrated. All the evidences from electron paramagnetic resonance, mass spectrometry, and product analysis with methylated substrates lead to the same story about the mechanism, where both radical translocation of cyclohexylamino radical and epoxidation of 1-hydroperoxycyclohexylamine are mediated by double proton transfer and are the key to convert amine into oxime. Near IR analysis indicate that surface hydroxyl groups on TiO₂ facilitate the double proton transfer process. Here the unveiled role of surface hydroxyl on TiO₂ highlights its potential for broader applications in amine activation. Additionally, the well-established radical mechanism provides valuable insight into the construction of N−O bonds in amines.