Hydrogenolysis of Carbon-Sulfur Bonds

Among the electron-rich alkenes, vinylsulfides are especially amenable to cation-radical reduction an important feature is the absence of hydrogenolysis of carbon-sulfur bonds. The reduction of [(phenylthio)methylene]cyclohexane is efficient (88%), and the retention of the phenylthio group clearly contrasts with catalytic hydrogenation (Mirafzal et al. 1993). This provides versatile functionality for further synthetic operations. 

Rhenium Sulfides and Selenides.208 209 These catalysts are characterized by their outstanding resistance to poisoning and minimal tendency to cause the hydrogenolysis of carbon-sulfur bonds than the base metal sulfides. Rhenium heptasulfide is easily prepared from boiling 6M hydrochloric acid solutions of perrhenate with hydrogen sulfide. It has been noted that occasional exposure of the dried, powdered catalyst to the atmosphere is not deleterious. 

Hydrogenolysis of carbon-sulfur bonds is a widely utilized reaction for removing sulfur from sulfur-containing organic compounds, and is known as desulfurization or hydrodesulfurization. Bougault et al. used Raney Ni for the first time for the desulfurization of aliphatic thioalcohols and disulfides in neutral and alkaline solution.126 Since then the reaction has been widely applied, for example, for organic syntheses, purification of solvents and substrates, structural studies, and determination of sulfur contents.127... 

The hydrogenolysis of carbon-sulfur bonds has found extensive use in both structure determination and synthesis. 52.158 a sulfur compound is desulfurized by refluxing it with a large excess of Raney nickel in a solvent, usually absolute... 

The hydrogenolysis of the carbon-sulfur bond by Raney nickel con-... 

It is of interest to note the ease with which a carbon-sulfur bond in an aromatic sulfide is cleaved. Simple refluxing of the compound in an ethanol suspension of Raney nickel is sufficient to effect hydrogenolysis,... 

Thiophene was also hydrogenated to tetrahydrothiophene over rhenium heptasulfide (Re2S7) at 230-260°C246 or rhenium heptaselenide (Re2Se7) at 250°C and at pressures greater than 30 MPa H2 (eq. 12.129),247 without accompanying hydrogenolysis of the carbon-sulfur bond that occurred extensively over molybdenum sulfide.246... 

Hydrogenolysis is analogous to hydrolysis and ammonolysis, which involve the cleavage of a bond induced by the action of water and ammonia, respectively. Chemical bonds that are broken by hydrogenolysis reactions include carbon—carbon, carbon—oxygen, carbon—sulfur, and carbon—nitrogen bonds. An example of hydrogenolysis is the hydrodealkylation of toluene to form benzene and methane ... 

Since sulfur is the most effective of all catalyst poisons, the hydrogenation of sulfur containing heterocycles is not easily accomplished unless there are no unshared electron pairs on the sulfur atom or the catalyst used is not affected by the poison. The hydrogenation of the cyelie sulfone, 58, takes palace over an excess of palladium in acetic acid at room temperature and atmospheric pressure (Eqn. 17.57). Thiophene, itself, can be hydrogenated to tetrahydrothiophene over rhenium heptasulfide at 250°C and 300 atmospheres of hydrogen or over a large excess of palladium in methanolic sulfuric acid at room temperature and 3-4 atmospheres. No hydrogenolysis of the carbon-sulfiir bond was observed in these reactions. 

Na or Li and ammonia, excellent yields. This is a very good method to remove a benzyl group from an amide and will usually work when hydrogenolysis does not. A dissolving metal reduction can be effected without cleavage of a sulfur-carbon bond. Note also the unusual selectivity in the cleavage illustrated below. This was attributed to steric compression. Primary benzyl amides are not cleaved under these conditions. ... 

Hydrogenation of diarylamines yields dialicyclic amines, but hydrogenolysis of the carbon-nitrogen bond also occurs. Dehydrogenation of DPA yields catbazole and alkyl derivatives of DPA give acridines. Dehydrogenation with sulfur gives phenothiazines (12). 

Only hydrogenolysis at carbon-carbon bonds is included under this heading. The hydrogenolysis of coal, coal tar, and heavy oils to light hydrocarbons has been given the most attention. These materials contain sufficient sulfur compounds to exclude the practical use of metaUic ca lysts. At the temperatures and pressures prevailing in these hydro-genolyses, 300-700° with pressures from 200 to 800 atmospheres, the stable nickel sulfide would be NijSj. 

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