Oxidations by the reagent "O-2-H2O2-vanadium derivative-pyrazine-2-carboxylic acid'. Part 12. Main features, kinetics and mechanism of alkane hydroperoxidation

Various combinations of vanadium derivatives (n-Bu4NVO3 is the best catalys t) with pyrazine-2-carboxylic acid (PCA) catalyse the oxidation of saturate d hydrocarbons, RH, with hydrogen peroxide and air in acetonitrile solution to produce, at temperatures < 40 degreesC, alkyl hydroperoxides, ROOH, as the main primary products. These compounds are easily reduced with tripheny lphosphine to the corresponding alcohols, which can then be quantitatively determined by GLC. Certain aminoacids similar to PCA can play the role of c o-catalyst; however the oxidation rates and final product yields are lower for picolinic and imidazole-4,5-dicarboxylic acids, while imidazole-4-carbo xylic and pyrazole-3,5-dicarboxylic acids are almost inactive. The oxidatio n is induced by the attack of a hydroxyl radical on the alkane, RH, to prod uce alkyl radicals, R-.. The latter further react rapidly with molecular at mospheric oxygen. The peroxyl radicals, ROO., thus formed can be converted to alkyl hydroperoxides. We conclude on the basis of our kinetic investigat ion of the oxidation of cyclohexane that the rate-limiting step of the reac tion is the monomolecular decomposition of the complex containing one coord inated PCA molecule: V-V(PCA)(H2O2)-->V-IV(PCA)+HOO.+H+. The V-IV species t hus formed reacts further with a second H2O2 molecule to generate the hydro xyl radical according to the equation V-IV(PCA)+H2O2-->V-V(PCA)+HO.+HO-. Th e concentration of the active species in the course of the catalytic proces s has been estimated to be as low as [V(PCA)H2O2] approximate to 3.3 x 10(- 6) mol dm(-3). The effective rate constant for the cyclohexane oxidation (d [ROOH]/dt = k(eff)[H2O2](0)[V](0)) is k(eff)=0.44 dm(3) mol(-1) s(-1) at 40 degreesC, the effective activation energy is 17 +/-2 kcal mol(-1). It is a ssumed that the accelerating role of PCA is due to its facilitating the pro ton transfer between the oxo and hydroxy ligands of the vanadium complex on the one hand and molecules of hydrogen peroxide and water on the other han d. For example: (pca)(O=)V . . .H2O2--> (pca)(HO-)V-OOH. Such a "robot's ar m mechanism" has analogies in enzyme catalysis.