Desulfurization, catalytic Raney nickel

Aliphatic and aromatic sulfides undergo desulfurization with Raney nickel [673], with nickel boride [673], with lithium aluminum hydride in the presence of cupric chloride [675], with titanium dichloride [676], and with triethyl phosphite [677]. In saccharides benzylthioethers were not desulfurized but reduced to toluene and mercaptodeoxysugars using sodium in liquid ammonia [678]. This reduction has general application and replaces catalytic hydrogenolysis, which cannot be used [637]. 

Base-hydrolysis of the. lO -acetoxy-isotwistane 246 (3.2.3.) yielded the corresponding alcohol 137, which was also characterized as its tosylate 267 (3.2.3.). The azide 287 (3.2.3.) was catalytically reduced (H2/raney-nickel) to the 10° -amino-iso-twistane 330. Oxidation of the isotwistanol 137 with Jones-reagent gave the ketone 331, which was transformed via desulfuration with raney-nickel of the corresponding thioketal 332 to unsubstituted 2,7-dioxa-isotwistane 128, see also 2.2.2.1.). 

The most catalytically active metals are Ni, Pd, Pt, and Rh. Nickel is used extensively in hydrogenation. It is frequently used in skeletal form as Raney nickel (Ra-Ni or RNi). The hydrogenation of almost all hydrogenatable functional groups can be accomplished over some form of Ra-Ni. Ra-Ni is also useful for desulfurization of organic compounds, but this is a stoichiometric reaction, not a catalytic reaction. 

Catalytic hydrogenation of thiophene poses a problem since noble metal catalysts are poisoned, and Raney nickel causes desulfurization. Best catalysts proved to be cobalt polysulfide [425], dicobalt octacarbonyl [426], rhenium heptasulfide [5i] and rhenium heptaselenide [54]. The last two require high temperatures (230-260°, 250°) and high pressures (140, 322 atm) and give 70% and 100% of tetrahydrothiophene (thiophane, thiolene), respectively. 

The sulfonyl group in sulfones resists catalytic hydrogenation. Double bonds in a, -unsaturated sulfones are reduced by hydrogenation over palladium on charcoal (yield 94%) [686, 687] or over Raney nickel (yield 62%) without the sulfonyl group being affected [686]. In p-thiopyrone-1,1-dioxide both double bonds were reduced with zinc in acetic acid but the keto group and the sulfonyl group survived [655]. Raney nickel may desulfurize sulfones to hydrocarbons [673]. 

Ring closure to an episulfide is a feasible reaction for thiocarbonyl ylides. In most cases, the sulfur is further extruded under the reaction conditions to afford an olefin as the final product. This cascade transformation has been utilized by Danishefsky and co-workers in their total synthesis of ( )-indolizomycin (Scheme 16)." In the Danishefsky s approach, diazo ketone 137 is treated with a catalytic amount of Rh2(OAc)4 to generate thiocarbonyl ylide 138, which cyclizes to give episulfide 139. This episulfide isomerizes to mercaptan 140, which is then desulfurized by partially deactivated W-2 Raney nickel. 

The compound in which the 3-keto group is reduced to a hydrocarbon interestingly still acts as an orally active progestin. The preparation of this compound starts with the hydrolysis of dihydrobenzene (13-2) to afford 19-nortestosterone (15-1). Reaction with ethane-1,2-thiol in the presence of catalytic acid leads to the cyclic thioacetal (15-2). Treatment of this intermediate with Raney nickel in the presence of alcohol leads to the reduced desulfurized derivative (15-3). The alcohol at 17 is then oxidized by any of several methods, such as chromic acid in acetone (Jones reagent), and the resulting ketone (15-4) treated with hthium acetylide. There is thus obtained the progestin lynestrol (15-5) [18]. 

Veralinine, a minor alkaloid from Veratrum album subsp. lobelianum, also has the rearranged 22,26-epiminocholestane skeleton (74). From chemical and spectroscopic evidence this Veratrum base is regarded as (22S,25S)-22,26-epimino-17/3-methyl-18-Jior-cholesta-5,12-dien-3 iS-ol (118). This structure was confirmed by correlation with veralkamine. The ketone 115 prepared from veralkamine was treated with ethanedi-thiol. Desulfurization of the resultant thioketal 119 with Raney nickel yielded the C-16 deoxo compound 120, which is identical with (22S,25S)-22,26-acetyl-epimino-17 3-methyl-18-7ior-5a,13a-cholestan-3j8-ol, also prepared from veralinine (118) via catalytic hydrogenation... 

Catalytic hydrogenation of l-chloropyrrolo[l,2-a]quinoxalines results in removal of the halogen atom. Apparently the dechlorinated products are themselves reducible, and care must be taken in dechlorinations to allow the absorption of only one mole of hydrogen. A recent paper describes reduction of the 4,5 bond in pyrrolo[l,2-a]quinoxalines. 1,5-Dihydro-1-0X0 compounds are resistant to hydrogenation at atmospheric pressure. Reduction of the 1-oxo compound 117 over palladium on charcoal at 2000 psi gave an unidentified tetrahydro derivative. " The dioxo compounds 118 have been reduced with lithium aluminum hydride to give the hexahydro products 119. Heating 4,5-dihydro-4-thioxo derivatives with Raney nickel results in formation of the aromatic, desulfurized compound. Stannous chloride has been used to reduce... 

The desulfurization studies were performed in a batch reactor over a Raney nickel catalyst, and ethanol was used as the solvent. To avoid hydrogenation, the catalytic studies were performed at low temperature with continuous stirring. The dependence of the rate of desulfurization on the... 

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. 

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