Hydrolysis of carboxyesters promoted by vanadium(V) oxyanions

Hydrolysis of carboxylic esters p-nitrophenyl acetate (pNPA), p-nitrophenyl butyrate (pNPB) and p-nitrophenyl trimethyl acetate (pNPTA) was examined in oxovanadate solutions by means of H-1 and V-51 NMR spectroscopy. In the presence of a mixture of oxovanadates, the hydrolysis of carboxyester bonds in pNPA proceeds under physiological conditions (37 degrees C, pD = 7.4) with a rate constant of k(obs) = 3.0 10(-5) s(-1) representing an acceleration of at least one order of magnitude compared to the uncatalyzed cleavage. EPR and NMR spectra did not show evidence for the formation of paramagnetic species, excluding the possibility of V(+5) reduction to V(+ 4), and indicating that the cleavage of the carboxyester bond is purely hydrolytic. The pH dependence of k(obs) revealed that the hydrolysis is slow in acidic media but rapidly accelerates in basic solutions. Comparison of the rate profile with the concentration profile of polyoxovanadates shows a clear overlap of the k(obs) profile with the concentration of monovanadate (V-1). Kinetic experiments at 37 degrees C using a fixed amount of pNPA and increasing amounts of V-1 permitted the calculation of catalytic (k(c) = 1 x 10(-4) s-1) and formation constant for the pNPA-V-1 complex (K-f = 17.5 M-1). The V-51 NMR spectra of a reaction mixture revealed broadening and shifting of the V-51 NMR resonances of the V-1 and V-2 upon addition of increasing amount of pNPA, suggesting a dynamic exchange process between vanadates and pNPA, occurring via a rapid association-dissociation equilibrium. The origin of the hydrolytic activity of vanadate is most likely a combination of its nucleophilic nature and the chelating properties which can lead to the stabilization of the transition state.