Reduction to sulfinic acids

Reduction To sulfinic acid, ArS02H (Zn, HC1), and thiophenols, ArSH To RCHO... 

Reduction of perfluoroalkanesulfonyl halides leads to sulfinic acids, which are unstable in aqueous acid and give perfluoroalkanoic acids [75] (equation 16) A variety of reducing agents can be used... 

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). 

Chlorides of sulfonic acids can be reduced either partially to sulfinic acids, or completely to thiols. Both reductions are accomplished in high yields with lithium aluminum hydride. An inverse addition technique at a temperature of —20° is used for the preparation of sulfinic acids, while the preparation of thiols is carried out at the boiling point of ether [69S]. 

Thiols can be prepared by the reduction of sulfonyl halides652 with LiAIH4. Usually, the reaction is carried out on aromatic sulfonyl chlorides. Zinc and acetic acid, and HI. also give the reduction. Sulfonic acids have been reduced to thiols with a mixture of triphenyl-phosphine and either Ii or a diaryl disulfide.633 Disulfides RSSR can also be produced.634 For the reduction of sulfonyl chlorides to sulfinic acids, see 0-118. 

General directions for reduction of sulfonyl chlorides to sulfinic acids 543 A mixture of the sulfonyl chloride (20 g), sodium sulfite nonahydrate (50 g), and crushed ice (100 g) is shaken until the chloride has dissolved (about 3 h) the mixture is tested for alkalinity from time to time and dilute sodium hydroxide solution is added as necessary to prevent evolution of sulfur dioxide, whilst cooling by ice is maintained. Finally the mixture is filtered and the sulfinic acid is precipitated from the filtrate by gradual addition of concentrated hydrochloric acid. Yields average 80-90 %. 

General directions for reduction of sulfinic acids to thiols 461 The crude sulfinic acid is dissolved in water, four to five parts of zinc dust are added, and steam is passed through the mixture while 30 % sulfuric acid (somewhat more than equivalent to the zinc) is added gradually. The thiol distilling in the steam is taken up in ether, dried over sodium sulfate, recovered, and fractionated. 

Thiophene-2-sulfonic acid is a strong acid, similar to benzenesulfonic acid. It forms a sulfonyl chloride with phosphoryl chloride which on reduction with zinc yields thiophene-2-sulfinic acid. 

Sulfinic acids can be prepared by reduction of sulfonyl chlorides. Though mostly done on aromatic sulfonyl chlorides, the reaction has also been applied to alkyl compounds. Besides zinc, sodium sulfite, hydrazine, sodium sulfide, and other reducing agents have been used. For reduction of sulfonyl chlorides to thiols, see 19-57. 

The most commonly employed routes for the preparation of the / -sulfatoethylsulfone group, which is the essential structural feature of vinylsulfone reactive dyes, are illustrated in Scheme 8.5. One method of synthesis involves, initially, the reduction of an aromatic sulfonyl chloride, for example with sodium sulfite, to the corresponding sulfinic acid. Subsequent condensation with either 2-chloroethanol or ethylene oxide gives the / -hydroxyethylsulfone, which is converted into its sulfate ester by treatment with concentrated sulfuric acid at 20 30 °C. An alternative route involves treatment of an aromatic thiol with 2-chloroethanol or ethylene oxide to give the /Miydroxyethylsulfonyl compound which may then be converted by oxidation into the /Miydroxyethylsulfone. 

However, Phinney (8) has suggested on the basis of experiments with mutants of Neurospora that the biosynthesis of cysteine involves the coupling of sulfate with an organic compound (presumably cysteine sulfinic acid) followed by reduction to sulfide. He noted also that sulfate may be reduced stepwise to sulfide and may then enter the protein as such. But he observed this process proceeds less readily than when sulfate combines first with protein. 

More reliable reagents for the preparation of sulfinic acids are zinc [694, 695], sodium sulfide [249] and sodium sulfite [2S2. These reagents not only stop the reduction at the stage of the sulfinic acids (in the form of their salts) but do not reduce other functions present in the molecules. In the reduction of anthraquinone-l,5-disulfonyl chloride with sodium sulfide below 40° anthraquinone-l,5-disulfinic acid was obtained in 83.5% yield [249], and p-cyanobenzenesulfonyl chloride was reduced to p-cyanobenzenesulfinic acid in 87.4% yield [252]. 

Types of compounds are arranged according to the following system hydrocarbons and basic heterocycles hydroxy compounds and their ethers mercapto compounds, sulfides, disulfides, sulfoxides and sulfones, sulfenic, sulfinic and sulfonic acids and their derivatives amines, hydroxylamines, hydrazines, hydrazo and azo compounds carbonyl compounds and their functional derivatives carboxylic acids and their functional derivatives and organometallics. In each chapter, halogen, nitroso, nitro, diazo and azido compounds follow the parent compounds as their substitution derivatives. More detail is indicated in the table of contents. In polyfunctional derivatives reduction of a particular function is mentioned in the place of the highest functionality. Reduction of acrylic acid, for example, is described in the chapter on acids rather than functionalized ethylene, and reduction of ethyl acetoacetate is discussed in the chapter on esters rather than in the chapter on ketones. 

The sulfonic acids of these metallocenes can be converted to sulfonyl chlorides, sulfonamides, etc., by appropriate reagents. Reduction of ferrocene-sulfonyl chloride by lithium aluminum hydride produces the rapidly oxidized ferrocenethiol in quantitative yield (43). Both the sulfonic acid of cyclopentadienylmanganese tricarbonyl and the corresponding sulfinic acid (obtained by sodium sulfite reduction of the sulfonic acid) have been converted to sulfones (10). 

Early reports of an Movl-Cys complex have been shown to be erroneous because redox occurs to give the Mov complex [Mo2 03 (CysO)4]4 and cystine further electrochemical reduction to Mom is also possible. Oxidation of Cys coordinated to Co111 can give a sulfinic acid or a sulfenamide, according to the reagents and conditions used. 

Reduction of sulfonic acids. P4S,o in sulfolane reduces sulfonic acids (and sulfinic acids) directly to polysulfides in good yield. The products are reduced to thiols by LiAlH4 or NaBH4 (equation 1). 

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