Direct hydrogenation of carboxylic acids to corresponding aldehydes catalyzed by palladium complexes

A variety of carboxylic acids can be directly hydrogenated into the corresponding aldehydes in high yields by using homogeneous catalysts such as [Pd(PPh₃)₄] or combination of Pd(OAc)₂ with tertiary phosphines in the presence of an excess amount of 2,2-dimethylpropionic anhydride (pivalic anhydride). As a typical example, octanoic acid can be convened into octanal in 99% yield in 3 h at 80 °C under 3.0 MPa of H₂ in acetone in the presence of pivalic anhydride and a catalyst system composed of Pd(OAc)₂ + 5P(p-tol)₃. Such hydrogenation is widely applicable to various aliphatic, aromatic, and heterocyclic carboxylic acids as well as to di- and tribasic carboxylic acids. The process allows the presence of other functional groups such as ketonic carbonyl, nitrile, and ester groups and even internal C=C bonds. Heterogeneous palladium catalyst such as palladium on carbon also showed some catalytic activities.