Scope and reaction mechanism of CeO₂-catalyzed one-pot imine synthesis from alcohols and amines

The scope and reaction mechanism of CeO₂ catalysis in the imine formation from alcohols and amines were investigated. CeO₂ catalyst was applicable to the synthesis of imines from various alcohols and amines, and also to the synthesis of imines by homo-coupling of aromatic methylamines, although aliphatic alcohols and sterically hindered alcohols such as secondary alcohols were not suitable substrates for the CeO₂ catalyst system. Based on the kinetic and spectroscopic studies, the reaction mechanism of the imine formation from benzyl alcohol and aniline over CeO₂ was proposed. The reaction proceeds by the five steps: (i) dissociative adsorption of alcohol on CeO₂ to form the alkoxide adspecies and proton, (ii) oxidative dehydrogenation of the alkoxide adspecies to aldehyde adspecies by the lattice oxygen on CeO₂ along with reduction of Ce⁴⁺ to Ce³⁺, (iii) nucleophilic addition of amine to the aldehyde adspeices to produce carbinolamine adspecies, and (iv) dehydration of carbinolamine adspecies to the corresponding imine and H₂O. (v) Finally, oxidation of Ce³⁺ to Ce⁴⁺ by O₂ from air to regenerate CeO₂ catalyst. The rate-determining step is the step (ii), particularly the dissociation of benzylic C-H bond at the α-carbon of OH group in benzyl alcohol by the lattice oxygen of CeO₂, and the oxidation follows Mars-van Krevelen mechanism. The reactive lattice oxygen species are mobile ones at the redox sites of CeO₂, and the high mobility of the oxygen species is responsible for the high activity in the reaction. Based on the reaction mechanism, the low reactivity of the aliphatic alcohols and secondary alcohols can be explained by the steric hindrance, stability of the carbonyl adspecies and/or C-H bond dissociation energy.