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

✍ Scribed by G. Cavinato; L. Toniolo

• Publisher: Elsevier Science
• Year: 1996
• Tongue: English
• Weight: 631 KB
• Volume: 105
• Category: Article
• ISSN: 1381-1169
• DOI: 10.1016/1381-1169(95)00145-x

✦ Synopsis

The catalytic system PdCI,(PPh,),-HCI is highly active and selective in the hydrogen transfer reaction from H,O-CO to PhCOCH,CHOHCOOH which yields the corresponding y-keto acid PhCOCH,CH,COOH, with concomitant evolution of C02. An increase of temperature, pressure of carbon monoxide and catalyst concentration have a beneficial effect on the reaction rate, which appears to be of the first order in the substrate and passes through a maximum when varying the concentration of HCI. It is proposed that one important function of HCI is to give rise to chloride PhCOCH,CHCICOOH 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 H,O and CO on the metal center. The yield passes through a maximum on increasing the concentration of H,O. This trend is attributed to the fact that. on one hand, H,O favors the formation of the Pd-COOH species, while, on the other hand, it may compete with other reacting molecules for coordination to the metal center. Moreover. H,O does not favor the formation of the chloride. When employed in relatively high concentration, the catalyst precursor has been recovered as a complex of palladium(O), Pd,(CO),(PPh,), or Pd(CO)(PPh,),, the latter in the presence of PPh,. The reduction to palladium(O) takes place only in the presence of H,O and is likely to occur via the intermediacy of a Pd-COOH species, which after CO, evolution gives the reduced complex probably via reductive elimination of HCI from the hydride intermediate truns-PdHCl(PPh,),.

Moreover, PhCOCH=CHCOOH in combination with HCI (equivalent to PhCOCH,CHClCOOH) reacts with Pd(COXPPh,), to give the hydrogenated product PhCOCH,CH2COOH

and PdCI,(PPh,),.

On the basis of these results, and knowing that HCl reacts with Pd(CO@Ph,), to give the hydride PdHCl(PPh,),, it is proposed that the catalytic cycle proceeds through the following steps: (i> H,O and CO interact with the metal center of the precursor yielding a Pd-COOH species, (ii) this gives off CO, with formation of a hydride. (iii) this interacts with chloride PhCOCHzCHCICOOH to yield the product PhCOCH,CH2COOH and the palladium(I1) precursor back to the catalytic cycle.