Electrode-mediated Wacker oxidation of cyclic and internal olefins

Abstract

An improved method for the electrode-mediated oxidations of olefins by palladium(II) is described. Current efficiencies from 80% to 95% were obtained in oxidations of 1-decene, styrene, trans-2-octene, and cyclohexene in which perchloric acid was added to a chloride-free solution of a palladium(II) acetate catalyst. The palladium(0) was reoxidized to palladium(II) by reaction with catalytic amounts of benzoquinone, which was, in turn, regenerated by anodic oxidation. Addition of varying amounts of perchloric acid did not affect the current efficiency but accelerated the oxidation reaction, up to a concentration of approximately 0.15 M. The current efficiency remained high (>80%) over the course of the electrode-mediated oxidations of 1-decene, trans-2-octene, and cyclohexene. At the end of the reactions, when the substrate was depleted, a drastic decrease in the current was observed, indicating that the catalytic cycle leading to product was primarily responsible for the electrochemical reaction. It also was shown that the rates of the electrochemical reactions were generally slower than those of homogeneous reactions in which a stoichiometric amount of benzoquinone was used, indicating that the electrochemical regeneration of benzoquinone was mass transport limited at the highest concentrations of perchloric acid. This is in contrast to other reports in the literature that suggested that the homogeneous (non-electrochemical) reactions were actually slower. Reasons for the discrepancy between these results are discussed.