Highly selective transfer hydrogenation from formates to the C=C double bond of gamma-keto-alpha,beta-unsaturated carboxylic acids catalyzed by Pd/C

A Pd/C catalytic system is highly active and selective at 80-90 degrees C in the hydrogen transfer reaction from sodium formate to the C=C double bond of PhCOCH=CHCOOH to yield the corresponding gamma-keto acid PhCOCH(2)CH(2)COOH. When the substrate is allowed to react with a defect of formate salt and in the presence of a relatively large amount of catalyst, the yield, with respect to the formate, is practically quantitative. Since it is known that Pd/C catalyzes the decomposition of alkali formates, this result suggests that the hydrogen transfer to the substrate is much faster than the hydrogen evolution or that there are no catalytic sites able to absorb only the formate anions and water, and that the chemisorption of the substrate PhCOCH=CHCOOH is relatively fast and occurs in the first step of the catalytic cycle. Moreover, water, at low concentration, has a beneficial effect on the reaction rate, which passes through a maximum with changes in its concentration. The reaction rate increases also with the increasing concentration of formate and of substrate. These results suggest that water competes with the other reacting molecules for activation on only one type of active centers. It is also proposed that the catalytic cycle then proceeds through the adsorption of the hydrogen donors, followed by a hydride abstraction from absorbed formate, Hydrogen transfer from this hydride and water to the substrate yields the products and regenerates the catalyst, after the desorption steps.