Organosulfur compounds

ORGANOSULFUR COMPOUNDS 2.1. C-chiral hydroxy sulfides and derivatives 

Searching for a method of synthesis of enantiopure lamivudine 1, the compound having a monothioacetal stereogenic centre, Rayner et al. investigated a lipase-catalysed hydrolysis of various racemic a-acetoxysulfides 2. They found out that the reaction was both chemoselective (only the acetate group was hydrolysed with no detectable hydrolysis of the other ester moieties) and stereoselective. As a result of the kinetic resolution, enantiomerically enriched unreacted starting compounds were obtained. However, the hydrolysis products 3 were lost due to decomposition. In this way, the product yields could not exceed 50% (Equation 1). The product 2 (R = CH2CH(OEt)2) was finally transformed into lamivudine 1 and its 4-epimer.  

Pseudomonas fluomsoens lipase pH 7, phosphate buffer, %uOMe 

When a reverse procedure was applied, i.e. enzymatic acetylation of racemic 3, formed in situ from the appropriate aldehydes and thiols, the reaction proceeded under the conditions of dynamic kinetic resolution and gave enantiomerically enriched acetates 2 with 65-90% yields and with ees up to 95% (Equation 2). It must be mentioned that the addition of silica proved crucial, as in its absence no racemization of the initially formed substrates 3 occurred and the reaction stopped at the 50% conversion. 

When a, (3-diacetoxysulfides 2, = ACOCH2, were subjected to enzyme- 

Although thiols may be prepared by the nucleophilic displacement of an alkyl halide, the resultant thiol may react further to give a thioether. Consequently, methods have been introduced which reduce the nucle-ophilicity of the sulfur and thus the tendency for this second step to 

Thiols are more acidic than the corresponding alcohols and are converted into their salts using alkali metal hydroxides or alkoxides. The thiolate salts are powerful nucleophiles, a property which has been used in the cleavage of methyl esters and methyl ethers. The combination of a hard acid and a soft base such as lithium propanethiolate (Li SPr) may favour attack on the methyl group of an ester and lead to alkyl-oxygen fission of the ester, rather than the more common addition of a nucleophile to the carbonyl group and consequently acyl-oxygen fission. This is particularly useful in the hydrolysis of hindered esters. 

The ease of fission of the S H bond under homolytic conditions means that thiols are often used as hydrogen donors in free radical reactions. 

Hydrogenolysis of the C-S bond can be achieved both by dissolving metal systems (sodium in liquid ammonia) or by catalytic methods, particularly with a finely divided reactive form of nickel known as Raney nickel. When the latter is combined with dithioacetal formation, using either ethanedithiol or propane-1,3-dithiol, the result is a mild method for reducing a carbonyl group to a methylene group. 

The formation of salts from sulfides is a common process. Thus reaction of dimethyl sulfide with methyl iodide gives trimethylsulfonium iodide (MejS T). 

Carbonyl difluoride was fotmd to cleave C=S linkages between 50-85 C, over 1 h, to replace thiono sulfur by fluorine (632). Thus, Al,Af-dimethyl thiocarbamoyl fluoride reacts with COF3 according to the following stoicheiometry  

This reaction contrasts markedly to the difflcult conversion of the -C(0)F group to -CF3 using COFj the reaction of benzoyl fluoride or benzoic acid to give 1,1,1-trifluorotoluene (Section 13.14.6.3.3), for example, requiring particularly forcing conditions. 

In a similar fashion, tetramethylthiuram disulfide gave both the W,M-dimethylcarbamoyl fluoride and 1,1,1-trifluorodimethylamine but in view of Equation (13.32), this last reaction should be capable of forming two molar equivalents of the trifluorodimethylamine under suitable conditions [632]. 

Carbonyl difluoride reacts with two molar equivalents of dimethylsulfoxide, in a sealed tube at room temperature, according to the following reaction [1204b]  

The sulfide is formed with a yield of 70%, the reaction mechanism is presumably analogous to that described for the reaction of phosgene with dmso (Section 10.6.5.1). When an excess of COFj is used, CHjFOC(0)F is formed in a yield of 38% [1204b]. 

Methanethiol and other lighter alkyl thiols ate fairly common air pollutants that have ultragarlic odors both 1- and 2-butanethiol ate associated wiA skunk odor. Gaseous methanethiol is used as an odorant leak-detecting additive for natural gas, propane, and butane it is also employed as an intermediate in pesticide synthesis. A toxic, irritating volatile liquid with a strong garlic odor, 2-propene-l-thiol (allyl mercaptan) is a typical alkenyl mercaptan. Benzenethiol (phenyl mercaptan) is the simplest of the aromatic thiols. It is a toxic liquid with a severely repulsive odor. 

Alkyl sulfides or thioethers contain the C—S—C functional group. The lightest of these compounds is dimethyl sulfide, a volatile liquid (bp 38°C) that is mod tely toxic by ingestion. It is now known to be a major source of gaseous sulfur entering 

Organic derivatives of thiourea (R represents hydrocarbon substituents) 

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Acetic anhydride is a useful solvent in certain nitrations, acetylation of amines and organosulfur compounds for mbber processing, and in pesticides. Though acetic acid is unexceptional as a fungicide, small percentages of anhydride in acetic acid, or in cold water solutions are powerful fungicides and bactericides. There are no reports of this appHcation in commerce. It is possible that anhydride may replace formaldehyde for certain mycocidal apphcations. 

Organosulfur Compounds. These compounds, Hsted in Table 8, are used in a variety of appHcations, including cooling water, paint, and metalworking. Methylenebisthiocyanate hydroly2es rapidly at a pH above 8 to cyanate ion which complexes with ferric iron to poison the cytochrome systems (36). 

Cross-country gas pipelines generally must odorize the normally odorless, colorless, and tasteless gas ia urban and suburban areas, as is required of gas distribution companies. Organosulfur compounds, such as mercaptans, are usually used for this purpose, and code requires that the odor must be strong enough for someone with a normal sense of smell to detect a gas leak iato air at one-fifth the lower explosive limit of gas—air mixtures. The latter is about 5%, so the odorant concentration should be about 1%, but most companies odorize more heavily than this as a safety precaution. 

E. Block, Reactions of Organosulfur Compounds, Academic Press, Inc., New York, 1978, p. 278. 

Organosulfur Adsorbates on Metal and Semiconductor Surfaces. Sulfur compounds (qv) and selenium compounds (qv) have a strong affinity for transition metal surfaces (206—211). The number of reported surface-active organosulfur compounds that form monolayers on gold includes di- -alkyl sulfide (212,213), di- -alkyl disulfides (108), thiophenols (214,215), mercaptopyridines (216), mercaptoanilines (217), thiophenes (217), cysteines (218,219), xanthates (220), thiocarbaminates (220), thiocarbamates (221), thioureas (222), mercaptoimidazoles (223—225), and alkaneselenoles (226) (Fig. 11). However, the most studied, and probably most understood, SAM is that of alkanethiolates on Au(lll) surfaces. 

Fig. 11. Surface-active organosulfur compounds that form monolayers on gold (a) alkanethiol (b) dialkyl disulfide (c) dialkyl sulfide (d) alkyl xanthate ...

The chemistry of organic sulfur compounds is very rich and organosulfur compounds are incorporated into many molecules. Thiols, or mercaptans as they were originally called, are essential as feedstocks in the manufacture of many types of mbber (qv) and plastics (qv). They are utilized as intermediates in agricultural chemicals, pharmaceuticals (qv), ia flavors and fragrances, and as animal feed supplements. Many reviews have been undertaken on the chemistry of the thiols, regarding both their preparation and their reactions (1 7). 

The heat capacities and entropies of organic compounds, including many thiols, have been compiled (19,20). The thermochemistry of thiols and other organosulfur compounds has been extensively reviewed (21). 

J. W. Eitzgerald, "Naturally Occurring Organosulfur Compounds in Soil," inJ. O. Ntiagu, ed.. Sulfur in the Environment, Part II Ecological Impacts, Wiley-lnterscience, New York, 1978, pp. 391—443. 

Multiple reactors achieve 95-96% conversion and recovery, and stringent air pollution legislation has now pushed this to 99%. A similar sequence of reactions is used for sulfur production from crude oil except that the organosulfur compounds must first be removed from the refinery feed and converted to H2S by a hydrogenation process before the sulfur can be recovered. 

Thiophene, benzo[d]thiophene and dibenzo[d,thiophene ring opening 97SL643. 

Y. Pico, A. J. H. Eouter, J. J. Vreuls and U. A. Th Brinkman, On-line tr-ace-level enrichment gas cliromatography of triazine herbicides, organophosphoms pesticides and organosulfur compounds from drinking and surface waters . Analyst 119 2025-2031 (1994). 

Although desulfurization is a process, which has been in use in the oil industry for many years, renewed research has recently been started, aimed at improving the efficiency of the process. Envii onmental pressure and legislation to further reduce Sulfur levels in the various fuels has forced process development to place an increased emphasis on hydrodesulfurization (HDS). For a clear comprehension of the process kinetics involved in HDS, a detailed analyses of all the organosulfur compounds clarifying the desulfurization chemistry is a prerequisite. The reactivities of the Sulfur-containing structures present in middle distillates decrease sharply in the sequence thiols sulfides thiophenes benzothiophenes dibenzothio-phenes (32). However, in addition, within the various families the reactivities of the Substituted species are different. 

Sulfur in crude oils is mainly present in the form of organosulfur compounds. Hydrogen sulfide is the only important inorganic sulfur compound found in crude oil. Its presence, however, is harmful because of its corrosive nature. Organosulfur compounds may generally be classified as acidic and non-acidic. Acidic sulfur compounds are the thiols (mercap-tans). Thiophene, sulfides, and disulfides are examples of non-acidic sulfur compounds found in crude fractions. Extensive research has been carried out to identify some sulfur compounds in a narrow light petroleum fraction.Examples of some sulfur compounds from the two types are ... 

TABLE 10. NMR chemical shifts of methyl protons of various organosulfur compounds (after References 43 and 44)... 

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