Disulfides desulfurization

Partial desulfurization. Disulfides (cyclic, benzylic, aralkyl, and dialkyl) are converted into sulfides by reaction with the reagent at 25-80°.3 For example, when dibenzyl disulfide is treated with a 10% excess of the phosphine in refluxing benzene... 

Polymeric aminophosphines (Fig. 12-4) have been used to desulfurize disulfides, a thiosulfinate ester, and a thioimide (configurational inversion) (Harpp et aL, 1978). 

Diallylsulfonium salts undergo intramolecular allylic rearrangement with strong bases to yield 1,5-dienes after reductive desulfurization. The straight-chain 1,5-dienes may be obtained by double sulfur extrusion with concomitant allylic rearrangements from diallyl disulfides. The first step is achieved with phosphines or phosphites, the second with benzyne. This procedure is especially suitable for the synthesis of acid sensitive olefins and has been used in oligoisoprene synthesis (G.M. Blackburn, 1969). 

Since the original cmde oils contain some sulfur compounds, the resulting products and gasolines also contain sulfur compounds, including hydrogen sulfide, mercaptans, sulfides, disulfides, and thiophenes. The processes used to sweeten, ie, desulfurize, the products depend on the type and amount of the sulfur compounds present and the specifications of the finished gasoline or other stocks. 

Sulfur in cmde oil is mainly present in organic compounds such as mercaptans (R-SH), sulfides (R-S-R ) and disulfides (R-S-S-R ), which are all relatively easy to desulfurize, and thiophene and its derivatives (Fig. 9.2). The latter require more severe conditions for desulfurization, particularly the substituted dibenzothiophenes, such as that shown in Fig. 9.2. Sulfur cannot be tolerated because it produces sulfuric add upon combustion, and it also poisons reforming catalysts in the refinery and automotive exhaust converters (particularly those for diesel-fueled cars). Moreover, sulfur compounds in fuels cause corrosion and have an unpleasant smell. 

Rhodococcus strain SY1 was reported to desulfurize dimethyl sulfide, dimethyl sulfoxide, and several alkyl sulfonates [41] in addition to DBT [78], Barium chloride has been used to precipitate sulfate and shown to alleviate sulfate repression partially. The authors proposed a tentative pathway for oxidative removal of sulfur from DBT and other organosulfur compounds. It should be noted that phenyl disulfide and thianaph-thene were not desulfurized by any of the Rhodococcus strains, but have been reported to be substrates of Gordonia CYKS2. 

Chang and co-workers isolated strain Nocardia sp. CYKS2 from a dyeing industry wastewater using DBT as the sole sulfur source [27]. This strain also desulfurized DBT to the same product 2-HBP however, it had broader substrate specificity and was reported to desulfurize thiophenes, sulfides, and disulfides (Table 3) in addition to DBT. However, it did not desulfurize trithiane, thianthrene and 4,4 -thiodiphenol. The desulfurization experiments were conducted in batch with the rate reported as 0.279 mg-sulfur/L dispersion/h for DBT conversion. 

Novel example of this reaction type are given by the copper-catalyzed decomposition of ethyl diazoacetate in the presence of bis(dialkoxyphosphoryl)disulfides 374 350 where P/S insertion sometimes accompanies the S/S insertion, and of bis(dialkoxy-thiophosphoryl)trisulfides 375 351 where desulfurization to give the disulfide derived product occurs during the reaction. Only P/S insertion product was obtained from bis(dialkoxyphosphoryl)trisulfide or -tetrasulfide 376 the copper-catalyst is dispensable in this case351K... 

Enantioselective desymmetrization of the seven-membered meso-cyclic disulfide 128 (n = 3, R = Me) by desulfurization with a chiral rert-aminophospine, gave 129 (n = 3, R = Me) in modest yields with up to 30% ee <00JCS(P1)1595>. 

The present procedure2 describes the conversion of resin-bound, primary aliphatic amines into isothiocyanates and the conversion of the latter into 3-aminothiophenes. The generation of isothiocyanates is related to known procedures,3 in which amines are first treated with carbon disulfide and the resulting dithiocarba-mates are desulfurized by treatment with a condensing agent (alkyl chloroformates, carbodiimides, lead or mercury salts, etc.). The presence of resin-bound isothiocyanates on the polystyrene support could be qualitatively ascertained by infrared spectroscopy (KBr-pellet strong absorption at 2091 cm-1). 

Disulfides can be either reduced to two thiols or desulfurized. The former reaction was achieved in high yields using lithium aluminium hydride [680, 681], lithium triethylborohydride [100] and sodium borohydride [682]. 

Partial desulfurization of disulfides to sulfides was accomplished by treatment with tris(diethylamino)phosphine in good yields [303]. 1,2-Dithiacy-clohexane was thus quantitatively converted to thiophane (tetrahydrothio-phene) at room temperature [303]. Complete desulfurization to hydrocarbons resulted when disulfides were refiuxed in ethanol with Raney nickel or nickel boride (yields 86 and 72%, respectively) [673]. 

An interesting synthetic approach to thietanes is the selective desulfurization of cyclic disulfides.The treatment of dithiolanes with a diethyl-aminophosphine results in a ring contraction to thietanes, (Eq. 19). This has been demonstrated with a-lipoic acid, a coenzyme with a dithiolane structure involved in the biological oxidation of pyruvic acid. The reaction is proposed to be initiated by the electrophilic attack of the phosphorus on the ring sulfur atom, resulting in the formation of an acyclic internal phosphonium salt, which by subsequent elimination of a phosphine sulfide, closes to the four-membered ring. °... 

Critical appraisal of the method, 23,26 using attempts to synthesize nonsymmetrically substituted lanthionines, resulted in rearrangement of the products, presumably due to phosphine-catalyzed disproportionation of the unsymmetrical disulfides. This reaction should proceed more rapidly than the desulfurization process and is thought to occur because sulfur extrusion takes place via a reversible reaction by recombination of ionic intermediates (Scheme 3). 21-22-24 Thus, the reaction of nonsymmetrical cystine derivatives results in the formation of a mixture of three different lanthionines. 

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