ON THE MECHANISM OF THE HYDROGEN-TRANSFER FROM H2O-CO TO GAMMA-KETO-ALPHA-HYDROXY CARBOXYLIC-ACIDS TO YIELD GAMMA-KETO ACIDS CATALYZED BY APDCL2(PPH(3))(2) PRECURSOR IN COMBINATION WITH HYDROCHLORIC-ACID

The catalytic system PdCl2(PPh(3))(2)-HCl is highly active and selecti ve in the hydrogen transfer reaction from H2O-CO to PhCOCH(2)CHOHCOOH which yields the corresponding gamma-keto acid PhCOCH(2)CH(2)COOH, wit h concomitant evolution of CO2. An increase of temperature, pressure o f carbon monoxide and catalyst concentration have a beneficial effect on the reaction rate, which appears to be of the first order in the su bstrate and passes through a maximum when varying the concentration of HCl. It is proposed that one important function of HCl is to give ris e to chloride PhCOCH(2)CHClCOOH which interacts with a palladium hydri de that takes origin from the decarboxylation of a species having a Pd -COOH moiety, which in rum results from the interaction of H2O and CO on the metal center. The yield passes through a maximum on increasing the concentration of H2O, This trend is attributed to the fact that, o n one hand, H2O favors the formation of the Pd-COOH species, while, on the other hand, it may compete with other reacting molecules for coor dination to the metal center. Moreover, H2O does not favor the formati on of the chloride. When employed in relatively high concentration, th e catalyst precursor has been recovered as a complex of palladium(0), Pd-3(CO)(3)(PPh(3))(3) or Pd(CO)(PPh(3))(3), the latter in the presenc e of PPh(3). The reduction to palladium(0) takes place only in the pre sence 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-P dHCl(PPh(3))(2). Moreover, PhCOCH=CHCOOH in combination with HCl (equi valent to PhCOCH(2)CHClCOOH) reacts with Pd(CO)(PPh(3))(3), to give th e hydrogenated product PhCOCH(2)CH(2)COOH and PdCl2(PPh(3))(2). On the basis of these results, and knowing that HCl reacts with Pd(CO)(PPh(3 ))(3) to give the hydride PdHCl(PPh(3))(2), it is proposed that the ca talytic cycle proceeds through the following steps: (i) H2O and CO int eract with the metal center of the precursor yielding a Pd-COOH specie s, (ii) this gives off CO2 with formation of a hydride, (iii) this int eracts with chloride PhCOCH(2)CHClCOOH to yield the product PhCOCH(2)C H(2)COOH and the palladium(II) precursor back to the catalytic cycle.