Highly selective transfer hydrogenation from formates to the C:C double bond of -keto-,-unsaturated carboxylicacids catalyzed by Pd/C

A Pd/C catalytic system is highly active and selective at 80-90°C in the hydrogen transfer reaction from sodium formate to the C:C double bond of PhCOCH:CHCO2H to yield the corresponding -keto acid PhCOCH2CH2CO2H. When the substrate is allowed to react with a deficit of formate salt and in the presence of a relatively large amt. of catalyst, the yield, with respect to the formate, is practically quant. Since it is known that Pd/C catalyzes the decompn. 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:CHCO2H is relatively fast and occurs in the first step of the catalytic cycle. Moreover, water, at low concn., has a beneficial effect on the reaction rate, which passes through a max. with changes in its concn. The reaction rate increases also with the increasing concn. of formate and of substrate. These results suggest that water competes with the other reacting mols. 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.