Catalytic transfer hydrogenation of benzylic and styryl compounds with palladium/carbon and 2-propanol. Selective removal of O-benzyl groups from carbohydrate derivatives

Abstract

2-Propanol is shown to be an effective hydrogen donor, in the presence of palladium/carbon, for catalytic transfer hydrogenation reactions of benzylic and styryl compounds, e.g., for the quantitative conversion of cinnamyl alcohol into 3-phenylpropanol. In the reduction of derivatives of cinnamic acid, decarboxylation also occurs. During the latter reaction, the phenyl group appears to function as an anchoring group for the palladium atom, because derivatives of phenylpropanoic acid are decarboxylated more readily than are those of phenylacetic acid. In hydrogenolysis reactions, aryl alcohols undergo disporportionation whereas benzyl ethers, represented by O-benzyl and related derivatives of carbohydrates, are smoothly cleaved. Although 2-propanol is less reactive as a hydrogen donor than formic acid, it exhibits greater selectivity, as illustrated by relative rates of hydrogenolysis of O-benzyl and O-benzylidene substituents. The use of alumina-supported palladium is more effective than palladium/carbon for promoting stepwise removal of O-benzyl substituents, as shown by the preparation of methyl 2-O-benzyl- and -3-O-benzyl-α-D-glucopyranoside from the 2,3-di-O-benzyl derivative. A possible relationship between the orientation of O-benzyl groups and the chemical shift differences between the diastereotopic benzylic protons is described.