Highly selective palladium catalyzed hydrogen transfer from H 2OCO to the CC double bond of β-benzoylacrylic acid

Abstract

A PDHCl catalytic system is highly active and selective in the hydrogen transfer from H 2OCO to the olefinic double bond of the unsaturated γ-ketoacid PhCOCHCHCOOH to PhCOCH 2CH 2COOH. Typical reaction conditions are: P CO: 20–30 atm; Pd/substrate/H 2O/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 PhCOCH 2CHClCOOH, which oxidatively adds to “reduced palladium”, with formation of a catalytic intermediate having a Pd[CH(COOH)CH 2COPh] moiety. “Reduced palladium” is the metal coordinated by other atoms of palladium, and/or by carbon monoxide. (ii) H 2O and CO react on the metal center of this species giving an intermediate having also a carbohydroxy ligand, (HOOC)Pd[CH(COOH)CH 2COPh]. (iii) β-hydride abstraction from the carbohydroxy ligand gives a hydride HPd[CH(COOH)CH 2COPh], with evolution of CO 2. (iv) Finally, reductive elimination of the product PhCOCH 2CH 2COOH 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 H 2O 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 H 2O, PhCOCH 2CH 2COOH is formed in a significant amount.