Hydrogenation of Sulfur Heterocycles

In fluid-solution systems, the plain hydrogenation of thiophenes to thioether products has been found to be catalyzed by various transition metal complexes surprisingly, none of these catalysts contains either molybdenum or tungsten, which are essential components of heterogeneous HDS catalysts [1 b]. Unpromoted M0S2, on the other hand, is quite active for the HDS of thioethers with no need of assistance by a late transition metal promoter [lb]. 

As a general trend, the homogeneous hydrogenation of thiophenes to thioethers is catalyzed by complexes that are not sterically demanding with relatively electrophilic metal centers (e. g., d metal ions such as Rh , Ir , Ru , or Os ). This is because moderate electron density and low steric hindrance at the metal favor the rf--C,C coordination mode of the thiophene (precursor to hydride migration) over rj -S binding (precursor to C-S bond cleavage) [7-10]. 

Valuable information on the mechanism of the regioselective BT hydrogenation by soluble metal complexes has been obtained by substituting deuterium for hydrogen gas in the reduction reactions catalyzed by tbe precursors [Rb(MeCN)3(Cp )](BF4)2 [7 a] and [Ru(MeCN)3(TRIPHOS)]BPh4 [7 a, 9 a]. 

Although still confined to laboratory scale, aqueous-biphase catalysis (cf. Section 3.1.1.1) and related variations such as supported liquid-phase catalysis (cf. Section 3.1.1.3.5) are emerging as viable techniques for the deep HDS of refined fuels [15]. 

In general, Ru-based catalysts are more efficient than Rh-based catalysts for the selective hydrogenation of BT to DHBT in water/hydrocarbon mixtures [19]. Rhodium forms much better catalysts for the hydrogenolysis of thiophenes to thiols (vide infra). 

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