Reduction carbon-sulfur bonds

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). 

The reductive capability of CgK has been a subject of interest (LA). Uses for CgK include the reductive cleavage of carbon-sulfur bonds (S5), the reductive alkylation of nitriles and esters (S6), and the reductive alkylation of aldehydes and ketones (S7). The activity of CgK has... 

Biodesulfurization (BDS) is the excision (liberation or removal) of sulfur from organosul-fur compounds, including sulfur-bearing heterocycles, as a result of the selective cleavage of carbon-sulfur bonds in those compounds by the action of a biocatalyst. Biocatalysts capable of selective sulfur removal, without significant conversion of other components in the fuel are desirable. BDS can either be an oxidative or a reductive process, resulting in conversion of sulfur to sulfate in an oxidative process and conversion to hydrogen sulfide in a reductive process. However, the reductive processes have been rare and mostly remained elusive to development due to lack of reproducibility of the results. Moderate reaction conditions are employed, in both processes, such as ambient temperature (about 30°C) and pressure. 

In the approach followed in this invention [29], a biocatalytic agent converts the sulfur heterocycles into different molecules that do not exhibit the hydrophobic interactions. This is achieved by selectively cleaving carbon-sulfur bonds. The selectivity of the biocatalytic agent employed is limited to the carbon-sulfur bonds and no attack to the carbon-carbon skeleton was reported. Thus, it is expected that the proposed biocatalytic reduction of viscosity would not diminish the fuel value of the treated petroleum liquids. The biocatalyst employed consisted of the strain ATCC No. 53968 (see Section 20 and references therein), in an aqueous culture conventionally prepared by fermentation under aerobic conditions. The fermenting bioreactor is fed with a suitable nutrient medium, which comprises a conventional carbon source (dextrose and glycerol are recommended carbon sources. To confer maximal biocatalytic activity for the desired cleavage of organic C—S bonds, the bacteria was kept in a state of sulfur deprivation. 

Addition to thioamides.s Alkyl- or aryllithiums add to the carbon-sulfur bond of aromatic thioamides to give adducts that are hydrolyzed to unsymmetrical ketones. Reduction of the adducts with LiAlHj before hydrolysis provides a-alkylated amines. 

The development of sulfone linkers, the exploration of sulfone based chemical transformations and cleavage strategies are an important objective in soHd-phase organic synthesis. This kind of Hnker (Tab. 3.7) has been used with thioethers [108], sulfoxides [109], sulfones [110], sulfonic acids and their corresponding derivatives [111]. Because carbon-sulfur bonds can be cleaved under very mild conditions, some Hnkers have been based on this effect. They can be cleaved under reductive conditions ]112, 113], photolytic conditions [114, 115] or with strong bases [116]. Various safety catch Hnkers have been developed based on the fact that thiols can be oxidized to sulfoxides and sulfones [112, 113]. 

Among the electron-rich alkenes, vinylsulfides are especially amenable to cation-radical reduction an important feature is the absence of hydrogenolysis of carbon-sulfur bonds. The reduction of [(phenylthio)methylene]cyclohexane is efficient (88%), and the retention of the phenylthio group clearly contrasts with catalytic hydrogenation (Mirafzal et al. 1993). This provides versatile functionality for further synthetic operations. 

In addition to deoxygenation sulfoxides undergo reductive cleavage at the carbon-sulfur bond when heated in tetrahydrofuran with aluminum amalgam. Keto sulfoxides were thus converted to ketones in usually quantitative yields (the keto group remained intact) [141]. 

Sulfur analogs, 2-tetrahydrofuranyl and 2-tetrahydropyranyl thioethers, were reduced by alone to alkyl 4- or alkyl 5-hydroxyalkyl thioethers resulting from the preferential reductive cleavage of the carbon-oxygen (rather than the carbon-sulfur) bond. Thus refluxing for 2 hours with alane in ether converted 2-alkylthiotetrahydrofurans to alkyl 4-hydroxybutyl thioethers in 63-72% yields, and 2-alkylthiotetrahydropyrans to alkyl 5-hydroxypentyl thioethers in 58-82% yields [794]. 

The carbon-sulfur bond of thiazines can be reductively cleaved (Sections 7.06.5.7 and 7.06.6.7) and 2//-thiazines, being imines, can be hydrolyzed (Section 7.06.5.5). Saturated thiazines (thiomorpholines) are stable toward alkaline hydrolysis <1981CPB1554> and Lewis-acidic boron trifluoride <1980JHC449>. 

Tetrahydrothiazin-3-ones are lactams that have been reduced to tetrahydrothiazines with borane <1980JHC449>, sodium borohydride <1992JOC4215>, or LAH <1987H(26)1503>, without cleavage of carbon-sulfur bond. In one case, incomplete reduction occured with LAH the intermediate lactol was dehydrated to give a dihydrothiazine as main product <1989JPS937>. 

Reductive Cleavage of Carbon-Phosphorus and Carbon-Sulfur Bonds... 

The reduction of sulfones with Sml2 can result in deoxygenation, to give the parent sulfide,59 or carbon-sulfur bond cleavage, depending on the substrate and the reaction conditions employed.60 For example, alkyl and alkenyl phenyl sulfones are reductively cleaved by Sml2 in the presence of HMPA. In the reduction of alkenyl sulfones, competing reduction of the electron-deficient alkene is not observed (Scheme 4.50).60... 

Electrochemical reduction of specific oriented chemisorbed intermediates is also very selective [77-79]. For example, reduction of chemisorbed thioph-enols (and mercaptans) result in selective scission of the carbon-sulfur bond to yield an unadsorbed hydrocarbon and an adsorbed sulfur atom. 

Cohen and coworkers reported on dilithio species of ketone ,/ -dianions, in which cyclopropyllithium constitutes the / -carbanion portion10. In the examples given in Scheme 9, the first lithium enolate was produced by proton-abstraction with lithium 2,2,5,5-tetramethylpiperidide (LTMP), and the second involved the reductive cleavage of the carbon-sulfur bond by lithium 4,4 -di-tczt-butylbiphenylide (LDBB). 

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