The source of oxygen in cerium-catalyzed α-hydroxylation of β-dicarbonyl compounds

An investigation into the mechanism of the cerium-catalyzed α-hydroxylation of β-dicarbonyl compounds is presented. By 18O-labeling studies it was proved that dioxygen is the direct source of the HO-groups in the product. It was excluded that the HO-groups originate from nucleophilic attack of water to an electrophilic radical intermediate. Based on these results and on the stoichiometry of the reaction, a new hypothesis for the mechanism is proposed.