New Palladium-catalysed Reduction of Acids to Aldehydes

A catalyst generated in situ from palladium acetate and tricyclohexyl phosphine catalyses the reduction of carboxylic acids to aldehydes. The reducing agent is sodium hypophosphite in combination with pivalic anhydride (Goossen L J et al, Chem Commun, 2002, 837). This follows on the work of Yamamoto’s group, who established that mixed anhydrides formed with pivalic anhydride can be reduced at high pressure (Nagayama K et al, Bull Chem Soc, Japan, 2001, 74, 1803). The reaction proceeds best in THF, but water must be present or the reaction is very slow. If too much water is added, the intermediate anhydrides are hydrolysed. Aliphatic, aromatic and β- unsaturated acids all work, though it has to be said that yields are only moderate. Perhaps further optimisation would improve these ketones, nitriles and carbon-carbon double bonds are not reduced.