SYNTHESIS OF GAMMA-KETOCARBOXYLIC ACIDS VIA REDUCTION OF GAMMA-KETO-ALPHA-HYDROXYCARBOXYLIC ACIDS WITH CARBON-MONOXIDE CATALYZED BY A PD-HCL SYSTEM

A Pd-HCl catalytic system is highly active in the synthesis of gamma-k etoacids of type ArCOCH2CH2COOH via reduction with CO of the ketohydro xy acids ArCOCH2CHOHCOOH. Typical reaction conditions are: P(CO): 20-3 0 atm; Pd/substrate/H2O/HCl = 1/400-1000/800-3000/100-1000 (mol); temp erature: 100-110-degrees-C; [Pd]: 10(-3) to 10(-2) M; solvent: dioxane ; reaction time: 1-2 h. The reaction occurs in high yield only when th e palladium precursor is used in combination with HCl and in the prese nce of H2O. Under the reaction conditions employed, the palladium(II) complex used as catalyst precursor decomposes to palladium metal. Pd/C is also highly active. It is proposed that the catalytic cycle procee ds through the following steps: (i) The chloride ArCOCH2CHClCOOH, whic h forms in situ from the starting substrate and HCl, undergoes oxidati ve addition to reduced palladium with formation of a catalytic interme diate having a Pd-[CH(COOH)CH2COPh] moiety. (ii) Interaction of H2O an d CO on the metal yields an intermediate having also a carbohydroxy li gand, (HOOC)-Pd-[CH(COOH)CH2COPh]. (iii) This intermediate, after beta -hydride abstraction from the carbohydroxy ligand, gives off CO2 and r eductive elimination gives product PhCOCH2CH2COOH. Alternatively, HCl may react with the intermediate proposed in step (i), yielding directl y the product and a Pd(II) species, which is reduced by CO to a Pd(0) species, which starts another catalytic cycle.