Mechanism of palladium-carbon bond Oxidation: Dramatic solvent effect

Pentafluoroiodosylbenzene (C6F5IO) selectively oxidizes Pd-C bonds of a ser ies of cyclopalladated 2-(alkylthio)azobenzene complexes. The kinetics of o xygen atom insertion into the Pd-C bond of one representative compound has been studied in detail to understand the mechanism of this reaction. At 20 degrees C Pd-C bond oxidation takes place smoothly in acetonitrile at a rat e of 0.08 M-1 s(-1), whereas this reaction does not proceed in solvents suc h as dichloromethane and chloroform. The Delta H double dagger and Delta S double dagger values for this reaction are 55.5 +/- 3.5 kJ/mol and -75.7 +/ - 11.5 eu, respectively. Among other oxidants, hydroperoxy radical (for exa mple, t-BuOO.) is found to be extremely efficient, whereas the highly elect rophilic oxoiron(IV) porphyrin cation radical (oxene) is incapable of oxidi zing the Pd-C bond. Oxene, however, selectively oxidizes the thioether func tionality. These observations suggest that nucleophilic attack of the oxida nt molecule on palladium(II) could be the most crucial step prior to Pd-C b ond oxidation. A large negative value of Delta S double dagger supports an associative mechanism, and a smooth reaction in polar solvent supports a po lar intermediate structure. Hydroperoxides, in the presence of a catalytic amount of iron(III) porphyrin chloride, selectively oxidizes the Pd-C bond. This observation, coupled with the fact that oxene oxidizes the thioether functionality, indicates that oxene may not be the major reactive intermedi ate in hydroperoxide oxidations. On the basis of these experimental results , we have attempted to draw a plausible mechanism of Pd-C bond oxidation.