Thiols desulfurization

Veralinine, a minor alkaloid from Veratrum album subsp. lobelianum, also has the rearranged 22,26-epiminocholestane skeleton (74). From chemical and spectroscopic evidence this Veratrum base is regarded as (22S,25S)-22,26-epimino-17/3-methyl-18-Jior-cholesta-5,12-dien-3 iS-ol (118). This structure was confirmed by correlation with veralkamine. The ketone 115 prepared from veralkamine was treated with ethanedi-thiol. Desulfurization of the resultant thioketal 119 with Raney nickel yielded the C-16 deoxo compound 120, which is identical with (22S,25S)-22,26-acetyl-epimino-17 3-methyl-18-7ior-5a,13a-cholestan-3j8-ol, also prepared from veralinine (118) via catalytic hydrogenation... 

Radical Chain Reactions. By radical thiol desulfurization in presence of an isocyanide, MTG leads to the electrophilic radical A that can be trapped by a nucleophilic double bond (eq 3). ... 

Thiol groups in the 2-, 3- and 6-positions have been desulfurized with Raney nickel under the usual conditions to give the corresponding unsubstituted compounds. 

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. 

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. 

Thiols and thioethers, both alkyl and aryl, can be desulfurized by hydrogenolysis... 

Can you think of reasons why substituted dibenzothiophenes are more difficult to desulfurize than thiophene or simple thiols (see Fig. 9.2) Depending on the choice of catalyst, hydrodesulfurization can be accompanied by hydrogenation to various extents. In which of the product streams in the refinery would you choose hydrogenative HDS and in which would you not ... 

In the context of diagenesis in recent anoxic sediments, reduced carotenoids, steroids, and hopanoids have been identified, and it has been suggested that reduction by sulhde, produced for example, by the reduction of sulfate could play an important part (Hebting et al. 2006). The partial reduction of carotenoids by sulfide has been observed as a result of the addition of sulfide to selected allylic double bonds, followed by reductive desulfurization. This is supported by the finding that the thiol in allylic thiols could be reductively removed by sulhde to produce unsaturated products from free-radical reactions (Hebting et al. 2003). 

Hebting Y, P Adam, P Albrecht (2003) Reductive desulfurization of allylic thiols by HS /HjS in water gives clue to chemical reactions widespread in natural environments. Org Lett 5 1571-1574. 

The progress on S-C bond activation, which covers the reduction of a C-S bond to a C-H bond, cross coupling reaction of sulfides with main group organometaUic nucleophiles, ring opening reactions of thietanes and thiiranes, and desulfurization of thiols, sulfides, and thiophenes has already been reviewed elsewhere [6-10], and... 

In particular, the thiols, like the thioethers, can efficiently be desulfurized over conventional catalysts under milder reaction conditions than those required to accomplish the HDS of the thiophene precursors.158,159... 

The overall degradation of (103) assisted by the cluster [(Cp )2 M o2Co2S3(CO)4] (Cp = CH3C5H4) is the model reaction that best resembles the heterogeneous counterparts, particularly those classified as Co/Mo/S phase,158 in terms of both structural motif and HDS activity.229 Morever, the Co/Mo/S cluster has successfully been employed to show that the C—S bond scission in the desulfurization of aromatic and aliphatic thiols occurs in homolytic fashion at 35 °C and that thiolate and sulfido groups can move over the face of the cluster as they are supposed to do over the surface of heterogeneous catalysts.230... 

With both the acidic and alkaline single time point assays, it remains unclear if acid labile H2S or desulfuration of proteins [40], respectively, creates artifactual overestimates of free H2S levels, as described earlier. Tissue H2S concentrations determined by thiol derivatization using bromobimane near neutral pH followed by HPLC also indicate... 

The hydrogenolysis of the thiol esters of carboxylic acids offers a delicate method for reducing an acid to the corresponding aldehyde or alcohol application of this method to the sugar series has been made by Wolfrom and Karabinos who reduced D-ribonyl chloride tetraacetate (LXXV) to aldehydo-D-ribose tetraacetate (LXXVII) in 22% yield via ethyl thiol-D-ribonate tetraacetate (LXXVI). The desulfurization of the thiol ester (LXXVI) was carried out with a somewhat aged sample of Raney nickel" which doubtless had less activity than the... 

Personal communication from Dr. J. V. Karabinos. Subsequent work on the reduction of steroid acids by Spero, McIntosh and Levin (ref. 12) has shown that Raney niokel partially deactivated by acetone is particularly suitable for the desulfurization of thiol esters to aldehydes. 

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]. 

Desulfurization by refluxing with Raney nickel in 70% ethanol for 6 hours converted thiol esters to aldehydes in 57-73% yields (exceptionally 22% yield) [1101] ( Procedure 6, p. 205). Desulfurization of a dithioester, methyl dithio-phenylacetate, by refluxing with Raney nickel in 80% ethanol for 1 hour afforded 65% yield of ethylbenzene [1102]. 

DESULFURIZATION WITH RANEY NICKEL Preparation of Aldehydes from Thiol Esters [1101]... 

Reductive amination of carbonyl compounds, 59, 1 71, 1 Reductive cyanation, 57, 3 Redutive desulfonylation, 72, 2 Reductive desulfurization of thiol esters,... 

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