Metal catalysis in oxidation by peroxides. Part 3. Significance of ligand exchange and acid–base equilibria of vanadium(V) catalyst in oxidation by t-butyl hydroperoxide in alcohols

Abstract

Kinetic and spectroscopic evidence suggests that, in alcoholic solvents (ROH) and in the presence of t-butyl hydroperoxide, bis(acetylacetonato)oxovanadium(IV)[VO(acac)2] is rapidly converted to vanadate esters VO(OR)3. These vanadium(V) triesters undergo acid–base equilibria leading to anionic or cationic species depending on the acidity of the medium. Kinetic studies on the vanadium(V) catalysed oxidation of di-n-butyl sulphide by t-butyl hydroperoxide in methanol, ethanol, and propan-2-ol, respectively, at 25° indicate that the most active catalyst is the neutral species VO(OR)3. The significant depression of oxidation rates observed when vanadium anion species are present, even at low concentration, suggests that vanadium(V) species might be involved in aggregation phenomena which become more significant as the acidity decreases. Thus, the seemingly large differences observed in the rates of catalysed oxidation in the three alcohols mentioned stem from changes in the position of acid–base equilibria involving the catalyst species in the three solvents. Indeed, the dependence of oxidation rates upon the medium acidity shows that rather small differences in rates are found in the range where the neutral catalyst ester VO(OR)3 is the dominant species.