On the mechanism of the hydrogen transfer from H2O-CO to -keto--hydroxy carboxylic acids to yield -keto acidscatalyzed by a PdCl2(PPh3)2 precursor in combination with hydrochloric acid

The catalytic system PdCl2(PPh3)2-HCl is highly active and selective in the hydrogen transfer reaction from H2O-CO to PhCOCH2CH(OH)COOH, which yields the corresponding -keto acid PhCOCH2CH2COOH, with concomitant evolution of CO2. An increase of temp., pressure of carbon monoxide and catalyst concn. have a beneficial effect on the reaction rate, which appears to be of the first order in the substrate and passes through a max. when varying the concn. of HCl. It is proposed that one important function of HCl is to give rise to chloride PhCOCH2CHClCOOH which interacts with a palladium hydride that takes origin from the decarboxylation of a species having a Pd-COOH moiety, which in turn results from the interaction of H2O and CO on the metal center. The yield passes through a max. on increasing the concn. of H2O. This trend is attributed to the fact that, on the one hand, H2O favors the formation of the Pd-COOH species, while, on the other hand, it may compete with other reacting mols. for coordination to the metal center. Moreover, H2O does not favor the formation of the chloride. When employed in relatively high concn., the catalyst precursor has been recovered as a complex of palladium(0), Pd3(CO)3(PPh3)3 or Pd(CO)(PPh3)3, the latter in the presence of PPh3. The redn. to palladium(0) takes place only in the presence of H2O and is likely to occur via the intermediacy of a Pd-COOH species, which after CO2 evolution gives the reduced complex probably via reductive elimination of HCl from the hydride intermediate trans-PdHCl(PPh3)2. Moreover, PhCOCH:CHCOOH in combination with HCl (equiv. to PhCOCH2CHClCOOH) reacts with Pd(CO)(PPh3)3 to give the hydrogenated product PhCOCH2CH2COOH and PdCl2(PPh3)2. On the basis of these results, and knowing that HCl reacts with Pd(CO)(PPh3)3 to give the hydride PdHCl(PPh3)2, it is proposed that the catalytic cycle proceeds through the following steps: (i) H2O and CO interact with the metal center of the precursor yielding a Pd-COOH species, (ii) this gives off CO2 with formation of a hydride, (iii) this interacts with chloride PhCOCH2CHClCOOH to yield the product PhCOCH2CH2COOH and the palladium(II) precursor back to the catalytic cycle