Highly selective palladium catalyzed hydrogen transfer from H2OCO to the CC double bond of β-benzoylacrylic acid

A PDHCl catalytic system is highly active and selective in the hydrogen transfer from H₂OCO to the olefinic double bond of the unsaturated γ-ketoacid PhCOCHCHCOOH to PhCOCH₂CH₂COOH. Typical reaction conditions are: P_CO: 20–30 atm; Pd/substrate/H₂O/HCl = 1/400–1000/800–3000/100–1000 (mol); temperature: 100–110°C; [Pd]: 10⁻³ to 10⁻² M; solvent: dioxane; reaction time: 1–2 h. High yields are obtained only when the palladium catalyst is used in combination with HCl. When a palladium(II) catalyst precursor is employed, extensive decomposition to palladium metal occurs. Pd/C shows also high activity. The proposed catalytic cycle proceeds through the following steps. (i) Addition of HCl to the olefinic double bond of the starting substrate gives the chloride PhCOCH₂CHClCOOH, which oxidatively adds to “reduced palladium”, with formation of a catalytic intermediate having a Pd[CH(COOH)CH₂COPh] moiety. “Reduced palladium” is the metal coordinated by other atoms of palladium, and/or by carbon monoxide. (ii) H₂O and CO react on the metal center of this species giving an intermediate having also a carbohydroxy ligand, (HOOC)Pd[CH(COOH)CH₂COPh]. (iii) β-hydride abstraction from the carbohydroxy ligand gives a hydride HPd[CH(COOH)CH₂COPh], with evolution of CO₂. (iv) Finally, reductive elimination of the product PhCOCH₂CH₂COOH returns the catalyst to the catalytic cycle. Alternatively, protonolysis of the intermediate 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 PhCOCHCHCOOH is allowed to react with a stoichiometric amount of Pd/C, in the presence of HCl and of CO and in the absence of H₂O, PhCOCH₂CH₂COOH is formed in a significant amount.