Hydrogenation of Oxygen- and Sulfur-containing Aromatic Ring Systems

6 Hydrogenation of Oxygen- and Sulfur-containing Aromatic Ring Systems 

Benzofuran is completely hydrogenated by use of rhodium catalysts to give mainly the cis stereoisomer [42]. If Pd/C in acetic acid is used, partial hydrogenation of the oxygen-containing ring can be accomplished. 

Because of the strong coordination of sulfur to metal surfaces, sulfur-containing molecules are very effective catalyst poisons. Nevertheless, a few examples of the hydrogenation of such molecules have been reported. Thiophene can be hydrogenated to tetrahydrothiophene by use of rhenium heptasulfide [44] under harsh conditions (250 °C and 300 atm hydrogen) or with a large excess of palladium in methanolic sulfuric acid [45]. In the synthesis of biotin, stereoselective civ-hydrogenation of a tri-substituted thiophene was achieved with Pd/C in acetic acid [46]. 

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