Acyclic acid anhydrides oxidatively add to Pd(styrene)(PMe3)2 with C-O bond cleavage to give corresponding acyl(carboxylato)palladium(II) complexes 3a-3e that are isolated and identified. The acetato ligand in 3a is exchanged with added acetic acid. Treatment of 3a with formic acid at room temperature yields acetaldehyde, acetic acid and CO2. Reaction of 3a with dihydrogen liberates acetaldehyde, acetic acid and ethanol. Catalytic hydrogenolysis of octanoic and benzoic anhydrides into aldehydes and carboxylic acids was found to be catalyzed by Pd(PPh3)4.
The aryl trifluoroacetates oxidatively added to a zerovalent palladium complex 2a with acyl-O bond cleavage under mild conditions to give the corresponding trans-(aryloxo)(trifluoroacetyl)palladium complexes 3a—3c. But 4-nitrophenyl trifluoroacetate reacted with 2a to yield cis-[Pd(OC6H4-4-NO2)2(PMe3)2] 3d, which was produced with C–O bond activation followed by disproportionation reaction. In contrast, benzyl trifluoroacetates reacted with the Pd(0) complex with benzyl–O bond fission to form benzyl(trifluoroacetato)palladium complexes 4a—4d. Complexes 4a—4d are in equilibrium in solutions between trans and cis isomers, with the proportion of the cis isomer increasing in polar solvents. Palladium-catalyzed carbonylation of benzyl trifluoroacetate has been achieved in the presence of benzyl alcohol and triethylamine to yield benzyl phenylacetate.
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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 converted 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.
Carboxylic acids are catalytically hydrogenated into corresponding aldehydes by palladium complexes in the presence of pivalic (trimethylacetic) anhydride under H2. Less hindered aliphatic and aromatic acids are reduced to aldehydes in high yields. The method is also applicable to hydrogenation of dibasic carboxylic acids and carboxylic acids containing heterocyclic groups.
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