Electrode-mediated Wacker oxidation of cyclic and internal olefins

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.On d\ue9crit une m\ue9thode am\ue9lior\ue9e d'oxydation des ol\ue9fines par le palladium(II), assist\ue9e par une \ue9lectrode. On a obtenu des efficacit\ue9s de courant allant de 80 \ue0 95% lors d'oxydations du d\ue9c-1-\ue9ne, du styr\ue9ne, du trans-oct-2-\ue9ne. et du cyclohex\ue9ne au cours desquelles on avait ajout\ue9 de l'acide perchlorique \ue0 une solution sans chlorure d'un catalyseur d'ac\ue9tate de palladium(II). Le palladium(0) est r\ue9-oxyd\ue9 en palladium(II) par une r\ue9action impliquant des quantit\ue9s catalytiques de benzoquinone qui est, \ue0 son tour, r\ue9g\ue9n\ue9r\ue9 par oxydation anodique. L'addition de quantit\ue9s variables d'acide perchlorique n'affecte pas l'efficacit\ue9 de courant; toutefois, la r\ue9action d'oxydation est acc\ue9l\ue9r\ue9e jusqu'\ue0 une concentration d'environ 0,15 M. Avec le d\ue9c-1-\ue9ne, le trans-oct-2-ene et le cyclohex\ue9ne, l'efficacit\ue9 du courant demeure \ue9lev\ue9e (>80%) pour l'ensemble des oxydations assist\ue9es par une \ue9lectrode. \ue0 la fin de r\ue9actions, lorsqu'il ne reste plus de substrat, on observe une dramatique diminution du courant qui indique que le cycle catalytique conduisant au produit est le principal responsable de la r\ue9action \ue9lectrochimique. On a montr\ue9 que la vitesse des r\ue9actions \ue9lectrochimiques est g\ue9n\ue9ralement plus lente que les r\ue9actions homog\ue9nes dans lesquelles on utilise une quantit\ue9 stoechiom\ue9trique de benzoquinone; ceci indique que, \ue0 des concentrations \ue9lev\ue9es d'acide perchlorique, la r\ue9g\ue9n\ue9ration \ue9lectrochimique de la benzoquinone est limit\ue9e par un transport de masse. Cette situation est en opposition avec d'autres rapports parus dans la litt\ue9rature qui sugg\ue9raient que les r\ue9actions homog\ue9nes (non-\ue9lectrochimiques) sont de fait plus lentes. On discute des raisons pouvant expliquer ces diff\ue9rences.Peer reviewed: NoNRC publication: Ye