Homogeneous catalysis of oxovanadium(IV) in the oxidation of substituted 4-oxo acids by bromate in acid medium: A mechanistic study

Oxovanadium(IV)-catalyzed oxidation of substituted 4-oxo-4-phenyl-butanoic acids (4-oxo acids) by bromate in acid medium leads to the formation of malonic acid and the corresponding benzoic acid. The reaction exhibits first order each in [bromate] and [acid], less than unity order each in [4-oxo acid] and [oxovanadium(IV)], and also shows solvent isotope effect (k(D2O)/k(H2O) similar to 1.8). The reaction is failed to induce the polymerization of acrylonitrile. The decrease in the rate of reaction with increase in dielectric constant of the medium is observed with all the studied 4-oxo acids. Electron-releasing substituents in the phenyl ring accelerate the rate of oxidation to a large extent, the rate retardation by electron-withdrawing substituents, though perceptible, is not very much. The linear free-energy relationship is characterized by a smooth curve, concaves downward, in the Hammett's plot of log k versus sigma, however, Hammett' plots are linear with excellent correlation coefficient, at four temperatures, when Brown and Okamoto's sigma(+) values are used. The rho(+) values are negative and decrease with increase in temperature. From the intersection of the lines in the Hammett and Arrhenius plots, the isokinetic relationship is discussed. The mechanistic pathway involving the cyclic oxidant-substrate-catalyst ternary complex is proposed, wherein the oxidation state of V(IV) is unaltered. The reaction is an example of the neighboring group participation and intramolecular catalysis. (C) 2010 Elsevier B.V. All rights reserved.