Supported bis(maltolato)oxovanadium complexes as catalysts for cyclopentane and cyclooctane oxidations with dioxygen

Carbamated silica gel supported bis(maltolato)oxovanadium(IV or V) complexes, [VO(ma)2] (1), [VO(py)(ma)2] (2) (py = pyridine), cis-[VO(OCH3)(ma)2] (3) and cis-[VO(OC2H5)(ma)2] (4), were used as catalysts for the oxidation of cyclopentane and cyclooctane by molecular oxygen in a batch reactor, under moderate reaction conditions (typically p(O2) ca. 10–14 atm, at 150–160 °C). The corresponding ketones, i.e. cyclopentanone and cyclooctanone are the main products (up to 10 and 25% yields, respectively) of the cycloalkane oxidations, but cyclopentanol and cyclooctanol are also formed in smaller amounts. The supported complex 1 shows the best activity and selectivity. The reaction is further promoted by the use of some heteroaromatic acids as co-catalysts, namely 2-pyrazine-carboxylic acid. Experiments with radical traps suggest the involvement of a free-radical mechanism.

Graphical abstractBis(maltolato)oxovanadium(IV or V) complexes anchored on carbamate modified silica catalyse, under mild or moderate conditions, the direct and solvent free oxidation (with O2) of cycloalkanes (cyclopentane and cyclooctane) to the corresponding cycloketones as the main products, while the cycloalcohols are obtained in lower amounts.Figure optionsDownload full-size imageDownload as PowerPoint slide