Aerobic Alcohol Oxidation with a Cerium-Phenanthrolinedione Complex: A Mimic of Lanthanide-Based Methanol Dehydrogenase

The development of efficient catalytic systems that use an environmentally friendly oxidant, such as molecular oxygen, remains an ongoing challenge in the oxidative transformation of hydrocarbons. Herein, we report the synthesis and characterization of a bioinspired cerium(III) catalyst bearing a commercially available 1,10-phenanthroline-5,6-dione (phd) ligand, an inexpensive and efficient molecular model of the pyrroloquinoline quinone (PQQ) cofactor. This Ce-phd complex, in which the cerium(III) ion acts as a Lewis acid center and the phd ligand regulates the redox process, shows good reactivity in the catalytic oxidation of various alcohols using air as an oxidant and no need for any cocatalyst, base, desiccant, or solvent pretreatment. Secondary benzylic and aliphatic alcohols and aromatic primary alcohols were converted to the corresponding ketones and aldehydes with good yields, respectively. In addition, this Ce-phd complex exhibited good chemoselectivity for the oxidation of alcohols over other labile groups, the secondary alcohols over primary alcohols, and benzylic alcohols over aliphatic alcohols by intramolecular and intermolecular competitive reactions. The catalytic reaction mechanism is proposed as a possible hydride transfer process on the basis of the studies of probe substrates, the kinetic isotope effect (KIE), Hammett plot, UV-vis spectra analysis, and stoichiometric reactions. The present results provide not only a practical mimic of PQQ-dependent methanol dehydrogenases (MDH) with earth-abundant cerium and a simple ligand but also an efficient, selective, and sustainable approach to the catalytic aerobic oxidation of alcohols.