Raney nickel reaction with sulfur compound

Thiols and thioethers,413 both alkyl and aryl, can be desulfurized by hydrogenolysis with Raney nickel.414 The hydrogen is usually not applied externally, since Raney nickel already contains enough hydrogen for the reaction. Other sulfur compounds can be similarly desulfurized, among them ... 

Only a limited number of examples are known of applications of thietanes in organic synthesis. Prominent among these examples would be electrophilic ring opening reactions leading to polyfunctional sulfur compounds (33)-(37), utilization of 3-thietanones (55) and metal complexes (87) derived therefrom as oxyallyl zwitterion equivalents in cycloaddition reactions, synthesis of dipeptide (63) with a /3-thiolactone, Raney nickel desulfurization of thietanes (e.g. 120 cf. Table 7) as a route to gem-dimethyl compounds, and desulfurization of thietanes (e.g. 17) in the synthesis of cyclopropanes (also see Table 7). 

Mozingo (20) has shown with a wide variety of compounds that Raney nickel catalyst in the presence of a solvent and with only its adsorbed hydrogen can cleave either reduced or oxidized sulfur from the remainder of the molecule at a moderate temperature. Two courses for the reaction can be postulated ... 

The desulfurization of organic sulfur compounds with Raney Ni using far greater amounts than a catalytic is not a catalytic hydrogenolysis in the strict sense but involves a stoichiometric chemical reaction, since the catalyst itself is converted into nickel sulfide. However, since it has found many useful applications with Raney Ni (a hydrogenation catalyst), this reaction has been treated in this section. 

Thiolate ions react with a-(alkylthio)carbonyl compounds to afford disulfides and the corresponding reduced ketone (equation 26). The reaction apparently involves direct nucleophilic attack by thiolate on the sulfur atom of the alkylthio group. Other soft bases, such as cyanide ion, thiourea and tertiary phosphines, also effect this conversion. Raney nickel of course readily desulfurizes a-alkylthiocarbonyl compounds. The reaction is quite selective for example, the ester, ketone and alkenic moieties of (43) are unaffected by the Raney nickel treatment (equation 27). Raney nickel reduction of (44) is reported to proceed with retention of configuration in ethanol and with inversion in acetone. Telluride salts also desulfurize a-alkylthio ketones. ... 

---