Replacement of sulfur by hydrogen

In practice the sulfo group is replaced by hydrogen on hydrolysis by water or dilute sulfuric acid or hydrochloric acid under pressure at temperatures up to 200°. 

Raney nickel has been widely used for reductive desulfurization.558 The course of the reduction depends greatly on the hydrogen content of the metal a deactivated catalyst gives considerable yields of dimerization products, whereas highly active Raney nickel favors hydrogenation. Since no solvent effect has been observed in this reductive desulfurization, the choice of solvent is unrestricted inter alia, water, a lower alcohol, or dioxan is usually employed. 

Aliphatic and aromatic thiols are reduced in good yield by boiling them with Raney nickel in alcohol containing sodium hydroxide or ammonia. According to Robins and Hitchings,559 6-methyl-7-phenylpyrido[2,3- /]pyrimidin-4-ol is prepared as follows  

2-Mercapto-6-methyl-7-phenylpyrido[2,3- /]pyrimidin-4-ol (6 g m.p. 240-242°) is dissolved in 95% ethanol (1.81) and concentrated aqueous ammonia (150 ml). Raney nickel (18-20 g) is added and the reaction mixture is boiled for 6 h under reflux on the water-bath. Then the catalyst is filtered off and washed with boiling 95 % ethanol (300 ml). The united filtrates are concentrated in a vacuum to 100 ml and the pH is adjusted to 5 by dilute acetic acid. On cooling, white needles separate, having m.p. 245-248° which is raised to 248-250° by re-crystallization from aqueous ethanol. 

Thioesters can be similarly reduced to aldehydes 560 further, the sulfur in heterocyclic rings, e.g., in thiazole,561 can be eliminated with ring fission and examples are also known of reduction of sulfoxides562 and sulfones.663 Al- 

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Replacement of sulfur by hydrogen Thiophene ring opening... 

Thus, the reaction of 1,4-dioxane with sulfur tetrafluoride and hydrogen fluoride in the presence of small amounts of sulfur chlorides or chlorine (0.3 0.5 %) at 185-220°C results in the replacement of 3-5 hydrogen atoms by fluorine to give mixtures of a polyfluoro-1,4-dioxane 1 or 3 and 2-fluoroethyl-l,2,2,2-tetrafluoroethyl ether (2) in a ratio dependent on the reaction temperature.237 Using technical grade sulfur tetrafluoride without addition of a sulfur chloride or chlorine, this reaction is not reproducible, but when carefully purified sulfur tetrafluoride is used the reaction does not give fluorinated products, instead a tar is formed.237... 

Asinger s studies demonstrated that product formation is sensitive to the ratio of sulfur to ketone (1), the structure of the ketone, the replacement of ammonia by amines, the temperature and the medium. Room temperature (20-25 °C) reactions in which the ratio of sulfur to ketones is 0.5 favors the formation of 3-thiazoline, 2, as shown in Figure 1. The formation of 5-alkylidene-3-thiazolines, 3, sometimes competes with the formation of 3-thiazolines such is the case when aryl ketones such as l-phenylpropan-2-one and l-phenylbutan-2-one are employed (4). Also the additional presence of hydrogen sulfide promotes the generation of 1,2,4-trithiolanes and 1,2,4,5-tetrathiolanes from ketones ana aldehydes at the expense of 3-thiazoline formation (11-12). Increasing the S/ketone ratio to 8 favors the formation of the 3-imidazoline-5-thione (5), a product which has a greater tendency to result from aryl methyl ketones (3). 

Takken (2) identified thiazoles and 3-thiazolines from the reaction of 2,3-butanedione and 2,3-pentanedione with ammonia, acetaldehyde and hydrogen sulfide at 20 °C. Study of tetramethylpyrazine (5) also showed that it can be readily formed in 3-hydroxy-2-butanone and ammonia model reaction at 22 C. Recent study of the model reaction of 3-hydroxy-2-butanone and ammonium acetate at low temperature revealed an interesting intermediate compound, 2-(l-hydroxyethyl)-2,3,4-trimethyl-3-oxazoline, along with 2,4,5-trimethyloxazole, 2,4,5-trimethyl-3-oxazoline, and tetramethylpyrazine were isolated and identified 4,5). We hypothesized that with the introducing of H2S, replacement of oxygen by sulfur could happen and sulfur-containing heterocyclic compoimds such as thiazoles and thiazolines could be formed along with oxazoles, oxazolines and pyrazines. 

A method that is important preparatively and is based on replacement of oxygen by sulfur consists of treating oxiranes with sodium hydrogen sulfite 389 it leads to sodium / -hydroxy sulfonates 390... 

Sulfur or the thiol group may be substituted for functional groups as well as for hydrogen in the presence of nickel sulfide catalysts. Simultaneous replacement of oxygen by sulfur in carbonyl compounds and hydrogenation to thiols may be carried put over nickel sulfide catalysts. 

The replacement of oxygen by sulfur results in a red shift of the first absorption band of around 0.21 eV. However, the substitution of the terminal hydrogen for the methyl group does not modify the band position, fir the four derivatives there is a flux of charge from the phenolic part toward the rest of the molecule. This flux is larger in pCA and pCMe, 0.22 e, than in pCTA and pCTMe , 0.13 e. Consequently, the delocalization of the charge is larger in the excited state of the 0X0 derivatives that becomes more stable than the thio derivatives. 

Catalytic hydrogenation is mostly used to convert C—C triple bonds into C C double bonds and alkenes into alkanes or to replace allylic or benzylic hetero atoms by hydrogen (H. Kropf, 1980). Simple theory postulates cis- or syn-addition of hydrogen to the C—C triple or double bond with heterogeneous (R. L. Augustine, 1965, 1968, 1976 P. N. Rylander, 1979) and homogeneous (A. J. Birch, 1976) catalysts. Sulfur functions can be removed with reducing metals, e. g. with Raney nickel (G. R. Pettit, 1962 A). Heteroaromatic systems may be reduced with the aid of ruthenium on carbon. 

Butyrolactone and hydrogen sulfide heated over an alumina catalyst result in replacement of ring oxygen by sulfur (151). 

Sulfur tetrafluoride [7783-60-0] SF, replaces halogen in haloalkanes, haloalkenes, and aryl chlorides, but is only effective (even at elevated temperatures) in the presence of a Lewis acid catalyst. The reagent is most often used in the replacement of carbonyl oxygen with fluorine (15,16). Aldehydes and ketones react readily, particularly if no alpha-hydrogen atoms are present (eg, benzal fluoride [455-31-2] from benzaldehyde), but acids, esters, acid chlorides, and anhydrides are very sluggish. However, these reactions can be catalyzed by Lewis acids (HP, BF, etc). 

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

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