Reactions of Transition-Metal-Olefin Complexes. III. Kinetics of the Oxidation of Substituted Styrenes Catalyzed by Palladium Salts in an Aqueous Tetrahydrofuran Solution

The kinetics of the oxidation of ring-substituted styrenes catalyzed by palladium(II) salts in a homogeneous aqueous tetrahydrofuran solution containing p-benzoquinone have been investigated at 30°C under a nitrogen atmosphere. The rate equation was found to be identical with the one previously found for styrene, i.e., −d[olefin]⁄dt=(kI+kII[Pd2+][olefin], where kI is the rate constant for ketone-formation and kII is the rate constant for aldehyde-formation. It was found that both the proton and the chloride ion affect the rate inhibition at high concentrations and the rate acceleration at low concentrations. The product ratio of the ketone to the aldehyde decreased monotonously with the increase in proton concentration and with the increase in chloride ion concentration. An increase in the ionic strength from 0.052 to 0.787 increased the rate constants, but the ratio of the products scarcely changed at all. The substituent effects for the sites of nucleophilic attack were specific; i.e., the electron-releasing substituents facilitated the formation of substituted acetophenone and diminished the formation of substituted phenylacetaldehyde. The reverse trend was found for the electron-withdrawing substituents. From these results, there is proposed a mechanism in which two active species, [ArCH=CH2Pd(H2O)-(OH)Cl] and [ArCH=CH2Pd(H2O)(OH)Cl2]−, are involved and in which the rate-determining step is the decomposition of the σ-complexes with the hydride shift to the carbonyl products.