Reductions with Sulfur Compounds

Hydrogen sulfide is probably the oldest reducing agent applied in organic chemistry, having been used by Wohler in 1838. It is a very toxic gas with a foul odor (m.p. —82.9°, b.p. —60.1°, density 1.19) which is sparingly soluble in water (0.5% at 10°, 0.4% at 20°, 0.3% at 30°) and more soluble in anhydrous ethanol (1 g in 94.3 ml at 20°) and ether (1 g in 48.5 ml at 20°). It is usually applied in basic solutions in pyridine [236], in piperidine [237] and most frequently in aqueous ammonia [238, 239, 240] where it acts as ammonium sulfide or hydrosulfide Procedure 42, p. 216). 

Similar reducing effects are obtained from alkali sulfides, hydrosulfides and polysulfides [241]. A peculiar reaction believed to be due to sodium polysulfide formed in situ by refluxing sulfur in aqueous-ethanolic sodium hydroxide is a conversion of p-nitrotoluene to p-aminobenzaldehyde [242]. Oxidation of the methyl group by the polysulfide generates hydrogen sulfide which then reduces the nitro group to the amino group. 

The reducing properties of organic compounds of sulfur, such as methyl mercaptan, show up in partial reduction of trigeminal to geminal dihalides [243]. Dimethyl sulfide reduces hydroperoxides to alcohols and ozonides to aldehydes while being converted to dimethyl sulfoxide [244]. 

The main field of applications of hydrogen and alkali sulfides is reduction of nitrogen functions in nitro compounds [236, 240], nitroso compounds 

More abundant are reductions with sodium sulfite which is applied in aqueous solutions (solubility 24%). Its specialties are reduction of peroxides to alcohols [257], of sulfonyl chlorides to sulfinic acids [252], of aromatic diazonium compounds to hydrazines [253], and partial reduction of geminal polyhalides [254] Procedure 43, p. 216). 

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Palladium catalysts are more often modified for special selectivities than platinum catalysts. Palladium prepared by reduction of palladium chloride with sodium borohydride Procedure 4, p. 205) is suitable for the reduction of unsaturated aldehydes to saturated aldehydes [i7]. Palladimn on barium sulfate deactivated with sulfur compounds, most frequently the so-called quinoline-5 obtained by boiling quinoline with sulfur [34], is suitable for the Rosenmund reduction [i5] (p. 144). Palladium on calcium carbonate deactivated by lead acetate Lindlar s catalyst) is used for partial hydrogenation of acetylenes to cw-alkenes [36] (p. 44). 

The stoichiometry of lithium aluminum hydride reductions with other compounds such as nitriles, epoxides, sulfur- and nitrogen-containing com-... 

Organic compounds of sulfur and phosphorus are included in the section on reduction with sulfur and phosphorus compounds (p. 32, 35). 

Presumably a 2 -oxy-2-aminobiphenyl is intermediate in the conversion of 292 with ammonia at 135°C to 1,3,6,8-tetranitrocarbazole. The p-benzoquinones 293 (R = H, Me or OMe) gave 294 on reduction with hydrogen-palladium-acid and 295 on reduction with sulfur dioxide. Compound 294 on oxidation with iron(III) chloride and compound 295 on reduction with hydrogen-nickel produced the 3-hydroxycarbazoles 296. ... 

Reduction of sulfur compounds 4-38 Coupling of aromatic acyl halides, with decarbonylation... 

Evidence (132) in favor of partial structure CXXXIII for C-curarine comes from the ozonolysis of tetrahydronorcurarine, in which only the ethylidene double bonds have been reduced. The UV-spectrum of this tetrahydro product is as that of C-curarine itself, and thus the central part of the molecule (CXXXII or CXXXIII) has been unaffected in the conversion into the tetrahydro derivative. After treatment of the ozonolysis products with active zinc, acid hydrolysis, and finally reduction with sulfur dioxide, the reaction mixture yields strychanone (LXXIV), indicating the 16,17 and 16, 17 positions for the central double bonds of C-curarine as in CXXXIII. It is pointed out, however, that this evidence is not conclusive, and that the illustrated mechanism can be written which would account for the formation of 1 molecule of strychanone from a compound with partial structure CXXXII (132). 

Nickel is determined by the gravimetric dimethyl-glyoxime procedure after reduction of the compound with sulfur dioxide and hydrochloric acid. Iodine is determined as silver iodide after reduction with sulfur dioxide in sulfuric acid medium. Total active oxygen is calculated by measuring the quantity of iodine liberated from potassium iodide in acidic solution. Alkali metals are determined as sulfates in the filtrates from the nickel determinations. 

Reduction of Sulfur Compounds. Sulfur compounds react differently with LiAHTj depending on the mode of covalent attachment of the hetero atom and its oxidation state. Reductive... 

In 1888, Foerster (91), reproducing the same reaction with dianisyl-thiourea, demonstrated that the compound he obtained (59) could lose a sulfur atom by reduction with tin and hydrochloric acid to form a product analogous to N-phenylpiperidine (60). 

Toxic or malodorous pollutants can be removed from industrial gas streams by reaction with hydrogen peroxide (174,175). Many Hquid-phase methods have been patented for the removal of NO gases (138,142,174,176—178), sulfur dioxide, reduced sulfur compounds, amines (154,171,172), and phenols (169). Other effluent treatments include the reduction of biological oxygen demand (BOD) and COD, color, odor (142,179,180), and chlorine concentration. 

Impurities can be removed by formation of a gaseous compound, as in the fire-refining of copper (qv). Sulfur is removed from the molten metal by oxidation with air and evolution of sulfur dioxide. Oxygen is then removed by reduction with C, CO, in the form of natural gas, reformed... 

Electrolytic reductions generally caimot compete economically with chemical reductions of nitro compounds to amines, but they have been appHed in some specific reactions, such as the preparation of aminophenols (qv) from aromatic nitro compounds. For example, in the presence of sulfuric acid, cathodic reduction of aromatic nitro compounds with a free para-position leads to -aminophenol [123-30-8] hy rearrangement of the intermediate N-phenyl-hydroxylamine [100-65-2] (61). 

The compound can be prepared from 2,4,6-trinitrophenol (picric acid [88-89-1]) by reduction with sodium hydrosulfide (163), with ammonia —hydrogen sulfide followed by acetic acid neutralization of the ammonium salt (164), with ethanolic hydrazine and copper (165), or electrolyticaHy with vanadium sulfate in alcoholic sulfuric acid (159). Heating 4,6-dinitro-2-benzamidophenol in concentrated HQ. at 140°C also yields picramic acid (166). 

A process development known as NOXSO (DuPont) (165,166) uses sodium to purify power plant combustion flue gas for removal of nitrogen oxide, NO, and sulfur, SO compounds. This technology reHes on sodium metal generated in situ via thermal reduction of sodium compound-coated media contained within a flue-gas purification device, and subsequent flue-gas component reactions with sodium. The process also includes downstream separation and regeneration of spent media for recoating and circulation back to the gas purification device. A full-scale commercial demonstration project was under constmction in 1995. 

Sulfonic acids are prone to reduction with iodine [7553-56-2] in the presence of triphenylphosphine [603-35-0] to produce the corresponding iodides. This type of reduction is also facile with alkyl sulfonates (16). Aromatic sulfonic acids may also be reduced electrochemicaHy to give the parent arene. However, sulfonic acids, when reduced with iodine and phosphoms [7723-14-0] produce thiols (qv). Amination of sulfonates has also been reported, in which the carbon—sulfur bond is cleaved (17). Ortho-Hthiation of sulfonic acid lithium salts has proven to be a useful technique for organic syntheses, but has Httie commercial importance. Optically active sulfonates have been used in asymmetric syntheses to selectively O-alkylate alcohols and phenols, typically on a laboratory scale. Aromatic sulfonates are cleaved, ie, desulfonated, by uv radiation to give the parent aromatic compound and a coupling product of the aromatic compound, as shown, where Ar represents an aryl group (18). 

A flow diagram for the system is shown in Figure 5. Feed gas is dried, and ammonia and sulfur compounds are removed to prevent the irreversible buildup of insoluble salts in the system. Water and soHds formed by trace ammonia and sulfur compounds are removed in the solvent maintenance section (96). The pretreated carbon monoxide feed gas enters the absorber where it is selectively absorbed by a countercurrent flow of solvent to form a carbon monoxide complex with the active copper salt. The carbon monoxide-rich solution flows from the bottom of the absorber to a flash vessel where physically absorbed gas species such as hydrogen, nitrogen, and methane are removed. The solution is then sent to the stripper where the carbon monoxide is released from the complex by heating and pressure reduction to about 0.15 MPa (1.5 atm). The solvent is stripped of residual carbon monoxide, heat-exchanged with the stripper feed, and pumped to the top of the absorber to complete the cycle. 

The catalyst commonly used in this method is 5 wt % palladium supported on barium sulfate inhibited with quinoline—sulfur, thiourea, or thiophene to prevent reduction of the product aldehyde. A procedure is found in the Hterature (57). Suitable solvents are toluene, benzene, and xylene used under reflux conditions. Interestingly, it is now thought that Rosenmund s method (59) originally was successful because of the presence of sulfur compounds in the xylene used, since the need for an inhibitor to reduce catalyst activity was not described until three years later (60). 

Among the measures which have successfully prevented metal dusting are the use of additives (steam, and compounds of S, As, Sb, and P) in the feed, reduction of pressure, reduction of temperature, and material change. The most common additives are sulfur compounds and steam. Susceptibility can be reduced by using a material in which the total percent of Cr plus two times the percent of Si is in excess of 22 percent. In some environments, a. small amount of a sulfur compound will stop the dusting. When sulfur compounds cannot be tolerated in the process stream, a combination of steam and an alloy with a Cr equivalent of over 22 percent may be most desirable. 

These reactions differ from those of sulfur tetrafluoride with carbonyl compounds in that a formal oxidation-reduction of the sulfur atoms m the thiocarbonyl compound and sulfur tetrafluoride molecule occurs, resulting in the formation of free sulfur and the complete utilization of the fluorine atoms in sulfur tetrafluoride. 

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