セリウム錯体の一電子酸化還元特性を利用する酸素雰囲気下での有機化合物の触媒的な酸化反応

抄録

Lanthanides contain 15 elements from lanthanum to lutetium in a 4f block, and their distinctive feature is their large ionic radii, enabling them to have high coordination numbers in contrast to transition metals. Among these lanthanide complexes, cerium stands out due to its unique redox chemistry by having two stable oxidation states, cerium(III) and cerium(IV). Although cerium(IV) complexes are well-known as potent stoichiometric oxidants in organic synthesis, they exhibit catalytic performances upon introducing suitable supporting ligands in the cerium coordination sphere as well as utilizing their photo-responsive characters. Our exploration of cerium catalysis is summarized in this review, with emphasis on two characteristic features of cerium complexes, activation of dioxygen by cerium(III) to form peroxocerium(IV) species and photo-reduction of cerium(IV) carboxylates by cleaving the cerium(IV) -ligand covalent bonds to generate organic radicals. Characterization of cerium(III) and cerium(IV) complexes with elucidating the transitions of cerium species during the redox processes is included to propose the comprehensive reaction mechanisms. In the initial part of this review, cerium-catalyzed alcohol oxidation by two catalytic systems, i.e. 2,2,6,6-tetramethylpiperidine-N-oxyl(TEMPO)-assisted and -free conditions is described with clarifying the pivotal roles of TEMPO and the supporting phenoxide ligands during the catalytic cycle. In the later part, utilization of the photo-responsivity of cerium(IV) carboxylate clusters is summarized, and their UV-Vis measurements, DFT studies, and photo-catalytic transformation of carboxylic acids in the presence of cerium(IV) complexes are disclosed.