Mechanism of oxidations with H2O2 catalyzed by vanadate anion or oxovanadium(V) triethanolaminate (vanadatrane) in combination with pyrazine-2-carboxylic acid (PCA): Kinetic and DFT studies

The oxovanadium(V) triethanolaminate complex (vanadatrane, 2), in the presence of pyrazine-2-carboxylic acid (PCA) and H2O2, is shown to act as a catalyst for the efficient oxidation of isopropanol to acetone (without other solvents) and of cyclohexane (in MeCN) to cyclohexyl hydroperoxide at low temperature (40-50 degrees C). This catalytic system, that is similar to the previously discovered "vanadate ion (1)-PCA-H2O2" reagent, is investigated by detailed spectroscopic, kinetic, and DFT methods which allow us to establish the mechanism of the oxidations. The kinetic and spectroscopic studies reveal that both catalytic systems activate C-H bonds via a common reaction mechanism. The V-51 NMR data show that catalytic cycles involve the same intermediate species. Calculations by the DFT method allow us to explore intimate details of the mechanism of free radical generation and to introduce modifications to the mechanistic proposals published previously. Thus, a "water-assisted" mechanism of H-transfer steps is suggested and shown to be more effective than the "PCA-assisted" or "robot's arm" mechanism. The formation of both HOO center dot and HO center dot radicals is more favorable along the cycle based on complexes containing only one peroxo fragment V(OO) ("cycle I") than in the cycle based on diperoxo species containing V(OO)(2) fragments ("cycle III"). The generation of the reactive HO- radicals occurs via the addition of H2O2 to the V-IV complex [V(OO)(OH)(pca)] (pca = anionic basic form of PCA) to produce [V-IV(OO)(OH)(pca)(H2O2)], followed by water-assisted H-transfer to the HO-ligand (the rate-limiting stage of the overall process), isomerization, and O-OH bond cleavage. The pca ligand is found to play a key role as a stabilizer of the transition state for the H-transfer. (C) 2009 Elsevier Inc. All rights reserved.