Hydrogenolysis disulfides

The disulfide intermediate was also hydrogenolyzed very efficiently at about 2-3 times the rate of the thiocyanate in selected solvents such as tetrahydrofuran at a lower temperature and pressure, as illustrated in Table 15.2. Of course, in the disulfide hydrogenolysis, no hydrogen cyanide was available as a coproduct to poison the palladium catalyst. 

Diaryl disulfides and diselenides add to alkynes to afford the (Z)-l, 2-bis(ar-ylthio)alkenes 193 and (Z)-l,2-bis(arylseleno)alkenes 194. Under CO pressure, carbonylative addition takes place to give thio esters and the selenoketones 195[I07], The selenoketones are converted into the /J-seleno-a, 3-unsaturated aldehydes 196 by Pd-catalyzed hydrogenolysis with HSnBu3[108,109],... 

Table 15.2. Comparison of Rates of Disulfide and Thiocyanate Hydrogenolysis.

The initial step of the adsorption of thiols on a Mo(100) surface is the formation of adsorbed thiolate groups. Phenyl thiolate is formed upon adsorption of thiophenol at 120 K on a clean Mo(110) surface.381 The thiolate intermediate subsequently undergoes competing C-S bond hydrogenolysis to form benzene, or C-S and C-H bond scission to form surface benzyne. The adsorption of thiophenol was also studied on a sulfur-covered Mo surface382 Phenyl disulfide is formed via S-H bond scission and S-S bond formation. The S-S linkage is oriented perpendicular and the phenyl ring parallel to the surface. 

In the base-assisted reactions, the hydrogenolysis products are recovered as sodium or potassium thiolates which can either be converted to thiols by acidification with protic acids, or be oxidized to disulfides by exposure to air.184,195 In turn, all of the Rh catalyst of the aqueous biphasic reactions remains in the polar phase for use in a further catalytic run after the thiolate product is extracted as thiol. 

The 3-methyl- and 3-phenyl-l,2,3-oxadiazolinium salts 96 and 97 are capable of oxidizing thiols to disulfides <1995MI817>. New dihydro-1,2,3-benzoxadiazoles, prepared by the reaction of 1,2-benzoquinones with diethyl azodicarboxylate (DEAD) or diisopropyl azodicarboxylate (DIAD) in the presence of triphenylphosphine (Section 5.03.9.4), have been shown to undergo catalytic hydrogenolysis to give phenols (Equation 12) <20050L5139>. 

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 sulfones and sulfonic acids over MoS3 requires higher temperatures (300-375°C) than for thiols, sulfides, or disulfides, except with dibenzyl sulfone, where hydrogenolysis to give toluene proceeded at 200-250°C. / -Tolu-... 

In crude oil sulfur is present as mercaptans (R-SH), sulfides (R-S-R ), disulfides (R-S-S-R ) and aromatic compounds, thiophenes and condensed derivatives. The aromatics are much more difficult to desulfurize and require more drastic conditions. The HDS process includes a complex succession of hydrogenolysis and hydrogenation steps, represented for dibenzothiophene in Figure 3. 

Hydrogenolysis of compounds with sulfur atoms attached to aromatic rings such as benzenethiols, and aryl sulfides, disulfides, sulfoxides and sulfones takes place on refluxing with Raney nickel or nickel boride. Sulfur combines with nickel, and hydrogen replaces the sulfur-containing group. 

The 4-hydroxyl of 8,9-0-isopropylidene derivative 4b has been protected as its 4-t-butyl-dimethyl-silyl ether 4e. Then, the 7-hydroxyl has been converted to its xantate ester 4f, by deprotonation with butyl litium, treatment with carbon disulfide and alkylation with methyl iodide. Deoxygenatiobn of the 7-position has been accomplished by heating with tributyl-tin hydride in xylenes. Then, the cleavage of acetonide and of silyl ether, by heating in 80% acetic acid, followed by hydrogenolysis to remove the benzyl ester, afford 7-deoxy-Neu%Ac-ctMe Im. 

General problems, as compared to the formation of 0-glycosides, are the incompatibility between catalytic hydrogenolysis and sulfur functions, which complicates the use of benzyl ethers as protecting groups, although Birch reduction might be an alternative, and the easy formation of disulfides from thiols, irrespective of if they are used as donors or acceptors. 

Molybdenum disulfide catalysts, promoted by cobalt or nickel, are used to remove organosulfur compoimds from crude petroleum by hydrogenolysis— hydrodesulfurisation or HDS [124,125,126], These compounds are imdesirable because they poison motor vehicle autoexhaust catalysts and bum to sulfur dioxide, an environmental pollutant. The most difficult compounds to desulfurise are sulfur heterocyclics thiophene, benzothiophene, dibenzothiophene and their methyl substituted derivatives. A typical reaction is the removal of sulfiir from thiophene ... 

The interaction of ammonia with carbon disulfide produces ammonium dithiocarbamate in solution, which reacts with 2-halo-ketones to produce thiazole-2-thiones similarly, methyl dithiocarbamate serves as a component for the construction of 2-methylthiothiazole, reducable to thiazole itself by hydrogenolysis, thus providing a good route to the unsubstituted heterocycle. °... 

Sulfur-containing pyrimidines are frequent intermediates in substitution reactions. Several methods exist for exchange of the thiol group with hydrogen, either by hydrogenolysis, especially by the use of Raney Nickel, or by oxidative desulfurization which involves a sulfinic acid intermediate. Oxidation may also lead to a sulfonic acid. The 2- and 4-/6-thiones are considerably more stable to air oxidation than 5-thiol derivatives. Disulfides are conveniently formed by oxidation with bromine. Oxidation with chlorine at low temperature can be used to prepare sulfonyl chlorides. These reactions have been summarized [Pg.187]

Although nearly aU unsaturated groups can be reduced with Raney nickel, this catalyst is most frequently used for the hydrogenolysis of sulfur compounds such as disulfides, thioacetals and thioamides. When freshly prepared, it contains 20-100 mL absorbed hydrogen per gram of nickel. The bound hydrogen can be partially replaced with tritium by... 

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