Desulfurization mechanism

Rhodococcus sp. Strain T09 A Rhodococcus strain T09 was isolated by enrichment on media-containing BT. The desulfurization mechanism of this organism was reported to be similar to Gordonia sp. 213E due to the observation of similar intermediates however, the substrate specificity was different. The strain T09 could use 2-methyl, 3-methyl and 5-methyl BT apart from BT as sole source of sulfur for growth, but not 7-methyl or ethyl derivatives. Additionally, it could also use methyl thiobenzothiazole, marcaptobenzothiazole, as well as benzene sulfide, benzene sulfonate, biphenyl sulfinate, dimethyl sulfate, dimethyl sulfone, dimethyl sulfide, methane sulfonic acid, thiophene, and taurine as sole sulfur sources. However, it could not grow on DBT or DBT sulfone. 

Extrusion (or elimination) of sulfur from thiiranes and thiirenes is a facile process. Virtually all thiiranes and thiirenes, as well as their oxides and dioxides, undergo thermal extrusion of the sulfur moiety with increasing facility according to the trend S < < SO < S02. The thermolytic desulfurization mechanism is more complex than a simple cheletropic extrusion (85TL2789). 

Thermolysis of thiiranes causes extrusion of elemental sulfur, with the formation of alkenes <64HC(19/1)576, 66CRV297, 76RCR25) however, the desulfurization mechanism by thermolysis is not clear at present. It cannot be explained by a simple cheletropic extrusion, but involving a more complex reaction scheme <85TL2789>. 

Similarly, the condensation of 285 with N-chloroamidines in chloroform or acetonitrile readily yields 2-imidoyl-3-imino-5-methylthio-A4-l,2,4-thia-diazolines (289) in one stage.225,226 The intermediate formation of the linear precursors (287) is indicated by the isolation, and separate cyclization (to 290) of their oxygen analogs (288) in the corresponding condensations involving potassium alkoxythiocarbonylcyanamides (286). Here, the formation of the substituted triazines 292 via 291 (arising from 286 by a desulfurization mechanism) occurs as a side reaction in certain solvents, but becomes the exclusive reaction in acetonitrile.225... 

Carbon-sulfur bond cleavages are extensively studied not only for synthetic applications but also for interests in catalytic desulfurization mechanism of the industrial hydrodesulfurization (HDS) process of naphtha, petroleum and lubricants... 

In addition to the dimeric 1,2,4-trithiane derivative 58 (cf. Scheme 8), isolated as a side product in very low yield and characterized by X-ray crystallography (videsupra), 5,6-benzo-l,2,4-trithiin 239 was isolated from the continuous sulfur-carbon displacement reaction of benzopentathiepin 236 <1997H(44)187>. Upon treatment with the phosphorus ylides 237 in CH2CI2, mixtures of benzotetrathiepins 238 and benzotrithiins 239 were obtained in moderate yields (NaH was deemed the base of choice cf. Scheme 69). The new products 238 and 239 were separated easily by high-performance liquid chromatography (HPLC) and the structures characterized by H NMR spectroscopy, whereby the chemical shift of the methine proton was indicative of the cyclic structures that were formed (5.40-5.44 ppm for 238, but 5.96-6.07 ppm for 239). The present desulfurization mechanism is complex and is still unclear because likely intermediates have not been isolated or detected directly by spectroscopic techniques <1997H(44)187>. 

Active Raney nickel induces desulfurization of many sulfur-containing heterocycles thiazoles are fairly labile toward this ring cleavage agent. The reaction occurs apparently by two competing mechanisms (481) in the first, favored by alkaline conditions, ring fission occurs before desul-, furization, whereas in the second, favored by the use of neutral catalyst, the initial desulfurization is followed by fission of a C-N bond and formation of carbonyl derivatives by hydrolysis (Scheme 95). 

The flash roaster is flexible ia handling various flotation concentrates and reaching the degree of desulfurization desired, ie, 0.5—3.0% sulfate sulfur. Waste heat is easily recovered. However, grinding and rabbling must be done mechanically. 

The most important reaction with Lewis acids such as boron trifluoride etherate is polymerization (Scheme 30) (72MI50601). Other Lewis acids have been used SnCL, Bu 2A1C1, Bu sAl, Et2Zn, SO3, PFs, TiCU, AICI3, Pd(II) and Pt(II) salts. Trialkylaluminum, dialkylzinc and other alkyl metal initiators may partially hydrolyze to catalyze the polymerization by an anionic mechanism rather than the cationic one illustrated in Scheme 30. Cyclic dimers and trimers are often products of cationic polymerization reactions, and desulfurization of the monomer may occur. Polymerization of optically active thiiranes yields optically active polymers (75MI50600). 

Electrophilic attack on the sulfur atom of thiiranes by alkyl halides does not give thiiranium salts but rather products derived from attack of the halide ion on the intermediate cyclic salt (B-81MI50602). Treatment of a s-2,3-dimethylthiirane with methyl iodide yields cis-2-butene by two possible mechanisms (Scheme 31). A stereoselective isomerization of alkenes is accomplished by conversion to a thiirane of opposite stereochemistry followed by desulfurization by methyl iodide (75TL2709). Treatment of thiiranes with alkyl chlorides and bromides gives 2-chloro- or 2-bromo-ethyl sulfides (Scheme 32). Intramolecular alkylation of the sulfur atom of a thiirane may occur if the geometry is favorable the intermediate sulfonium ions are unstable to nucleophilic attack and rearrangement may occur (Scheme 33). 

Oxygen nucleophiles usually attack a ring carbon atom rather than the sulfur atom of a thiirane, and those cases in which desulfurization is observed on treatment of a thiirane with oxygen bases probably involve the extrusion of sulfur by mechanisms other than a nucleophilic attack on sulfur, e.g. thermal. Desulfurization of thiirane intermediate (43)... 

In 1974, Gassman et al. reported a general method for the synthesis of indoles. For example, aniline 5 was reacted sequentially with r-BuOCl, methylthio-2-propanone 6 and triethylamine to yield methylthioindole 7 in 69% yield. The Raney-nickel mediated desulfurization of 7 then provided 2-methylindole 8 in 79% yield. The scope and mechanism of the process were discussed in the same report by Gassman and coworkers as well. 

In the desulfurization of 3-substituted thiophenes several stereoisomers may be formed in certain cases. Both meso and racemic compounds have been obtained from the desulfurization of 3,4-diaryl-substituted thiophenes. It is claimed, however, that only meso, -diphenyladipic acid is obtained upon desulfurization of 3,4-di-phenyl-2,5-thiophenedicarboxylic acid and only di-isoleucin from 3-thienylglycine. The formation of small amounts of dimeric products in the desulfurization has been discussed with reference to the mechanism of this reaction. ... 

The low reactivity of alkyl and/or phenyl substituted organosilanes in reduction processes can be ameliorated in the presence of a catalytic amount of alkanethiols. The reaction mechanism is reported in Scheme 5 and shows that alkyl radicals abstract hydrogen from thiols and the resulting thiyl radical abstracts hydrogen from the silane. This procedure, which was coined polarity-reversal catalysis, has been applied to dehalogenation, deoxygenation, and desulfurization reactions.For example, 1-bromoadamantane is quantitatively reduced with 2 equiv of triethylsilane in the presence of a catalytic amount of ferf-dodecanethiol. 

IMP-S02 [49], and ECRD-1 [50], Many of these strains are similar in the mechanism of DBT desulfurization, but vary in terms of the desulfurization rate as well as physiological characteristics. 

The sulfur-specific pathway for desulfurization of benzothiophene (BT) has been reported in Gordonia sp. Strain 213E. The metabolites of BT conversion were determined by ethylacetate extraction of the culture medium followed by GC-MS analysis [33,34], The reaction mechanism was proposed to be very similar to that of DBT for the first two steps (Fig. 4) however, the third step was quite different. 

A thermophilic strain M. goodii X7B was reported to carry out desulfurization of BT and DBT [38], The growth of the strain on various sulfur substrates including DBT, BT, 4,6-dimethyl DBT, 5-methyl BT, 2-thiophene carboxylic acid and propylmercaptan was studied at 45°C and varying degree of growth was observed. A mechanism of sulfur removal was proposed for BT based on metabolites identified by GC-MS (Fig. 7 [125]). 

Desulfurization using cell-free extracts The first report of desulfurization by cell-free extract of R. erythropolis was by Ohshiro et al. [180], This report showed stoichiometric desulfurization of DBT by a cell-free system and identified NADH as a necessary co-factor for desulfurization. Subsequently, the enzyme activity of cell-free extracts of the strain R. erythropolis D-l was found to be inhibited by a 2-HBP, and its analog 2,2 -dihydroxybiphenyl (DBHP). Sulfate did not inhibit enzyme activity [90], further proving that its role is not in controlling enzyme activity directly but via a genetic repression mechanism as indicated above. 

The second important issue related to commercial use of desulfurization biocatalysts is their inhibition by sulfate. The sulfur repression mechanism in most Rhodococcus species limits their use or activity in presence of sulfate- and sulfur-containing amino-acids such as cysteine, methionine, etc. To alleviate this problem, expression of the dsz genes under the control of alternate promoters has been investigated. 

The alkane rc-tetradecane was found to have significant effect on desulfurization ability, with the rate being 10 times more than that obtained when using glucose for biocatalyst growth. This effect was associated with production of rhamnolipids by the strain. However, the mechanism by which alkane actually enhances desulfurization activity, whether it is by assisting in biosurfactant production or by some other mechanism was not reported. However, this biocatalyst was found to be active for only a short period (4h) during its desulfurization test with oils. 

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