HIGHLY SELECTIVE PALLADIUM-CATALYZED HYDROGEN-TRANSFER FROM H2O-CO TOTHE C=C DOUBLE-BOND OF BETA-BENZOYLACRYLIC ACID

A Pd-HCl catalytic system is highly active and selective in the hydrog en transfer from H2O-CO to the olefinic double bond of the unsaturated gamma-ketoacid PhCOCH=CHCOOH to PhCOCH(2)CH(2)COOH. Typical reaction conditions are: P-CO: 20-30 atm; Pd/substrate/H2O/HCl = 1/400-1000/800 -3000/100-1000 (mol); temperature: 100-110 degrees C; [Pd]: 10(-3) to 10(-2) M; solvent: dioxane; reaction time: 1-2 h. High yields are obta ined only when the palladium catalyst is used in combination with HCl. When a palladium(II) catalyst precursor is employed, extensive decomp osition to palladium metal occurs. Pd/C shows also high activity. The proposed catalytic cycle proceeds through the following steps. (i) Add ition of HCl to the olefinic double bond of the starting substrate giv es the chloride PhCOCH(2)CHClCOOH, which oxidatively adds to ''reduced palladium'', with formation of a catalytic intermediate having a Pd-[ CH(COOH)CH(2)COPh] moiety. ''Reduced palladium'' is the metal coordina ted by other atoms of palladium, and/or by carbon monoxide. (ii) H2O a nd CO react on the metal center of this species giving an intermediate having also a carbohydroxy ligand, (HOOC)-Pd-[CH(COOH)CH(2)COPh]. (ii i) beta-hydride abstraction from the carbohydroxy ligand gives a hydri de H-Pd-[CH(COOH)CH(2)COPh], with evolution of CO2. (iv) Finally, redu ctive elimination of the product PhCOCH(2)CH(2)COOH returns the cataly st to the catalytic cycle. Alternatively, protonolysis of the intermed iate formed in the first step yields directly the final product and a Pd(II) species, which is reduced by CO and H2O to palladium metal back into the catalytic cycle. This is supported by the fact that when PhC OCH=CHCOOH is allowed to react with a stoichiometric amount of Pd/C, i n the presence of HCl and of CO and in the absence of H2O, PhCOCH(2)CH (2)COOH is formed in a significant amount.