Thiiranium intermediates

Chlorotrimethylsilane-induced Pummerer rearrangements effect the transformation of 4-ketothiane oxides into the corresponding a, /1-unsaturated thianes348, apparently via the formation and subsequent deprotonation of thiiranium intermediates rather than by the conventional sulfocarbonium mechanism depicted in equation 129. 

Neutral reactants form a dipolar activated complex, which reacts through a thiiranium intermediate [83, 515] to give the products. Similar solvent effects have been observed in the addition of chloro-phenylsulfane to alkenes [514],... 

In 2006, Ranu reported on an t/ fz-Markovnikov hydro-thiolation of styrene derivatives 205 with disulfides using indium(I) iodide and ZnCl2. Whereas the proposed mechanism invoked the formation of indium(III) thiolate species, the regiochemical outcome of the reaction was opposite to that of In(III)-cataIyzed hydrothiolation of alkenes. It is, thus, reasonable to propose the in situ generation of sulfenyl chloride from ZnCl2 and diaryl sulfides followed by the formation of a thiiranium intermediate 207 that is reductively opened by Ini to give 208. This explanation corresponds to no reaction observed in the absence of either Ini or ZnCL. 

Thiiranium intermediates were ruled unlikely in die addition of 1-methyl-l-thionia-2-thiacyclohexane 2,4,6-trinitrobenzenesulphonate to olefins and dienes and in the ene reaction of monothiobenzil with 2-methylpent-2-ene. ... 

Cyclodehydration of the ketone (51) with poly phosphoric acid gives only a small amount of the A -thiochromene (52). The major volatile product is the benzo[b]thiophen (53), probably formed from the thiochromene via a thiiranium intermediate. Irradiation of A -selenochromene at low temperature is reported to give the selenoketone (54). The reaction is reversed thermally."... 

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). 

S-Alkylthiiranium salts, e.g. (46), may be desulfurized by fluoride, chloride, bromide or iodide ions (Scheme 62) (78CC630). With chloride and bromide ion considerable dealkylation of (46) occurs. In salts less hindered than (46) nucleophilic attack on a ring carbon atom is common. When (46) is treated with bromide ion, only an 18% yield of alkene is obtained (compared to 100% with iodide ion), but the yield is quantitative if the methanesulfenyl bromide is removed by reaction with cyclohexene. Iodide ion has been used most generally. Sulfuranes may be intermediates, although in only one case was NMR evidence observed. Theoretical calculations favor a sulfurane structure (e.g. 17) in the gas phase, but polar solvents are likely to favor the thiiranium salt structure. 

S-Substituted thiiranium ions react with water and alcohols to give trans ring opening (Scheme 72). A report that oxygen nucleophiles attack sulfur as well as carbon has been shown to be incorrect (79ACR282). The intermediate thiiranium ion (57) in the presence of lithium perchlorate readily yields the carbenium ion which undergoes a transannular hydride... 

S-Substituted thiiranium ions react with secondary amines to give ring-opened products. Nitriles also react with thiiranium ions, probably via an open carbenium ion whose formation is favored by increasing the polarity of the medium by the addition of lithium perchlorate (Scheme 79) (79ACR282). An intramolecular displacement by an amide nitrogen atom on an intermediate thiiranium ion has been invoked (80JA1954). 

Homochiral thiiranium and aziridinium ion intermediates formed by Lewis acid-induced rearrangement of l-hetero-2, 3-epoxides 97SL11. 

Thiiranium and thiirenium ions as reaction intermediates and building blocks in organic synthesis 97G177. 

The reactivity of epoxides can be modified by various proximal functionality. For example, 2,3-epoxy sulfides 118 are converted to the corresponding TMS-thiiranium species 119 upon treatment with TMS triflate. This intermediate reacts with O-silyl amides regiospecifically to form l-substituted-3-hydroxy-2-thioethers (e.g., 120). Simple primary amines undergo polyalkylation, but imines can be used as an indirect amine equivalent <96TET3609>. 

Neighboring group participation effects appear to play a crucial role in the nucleophilic substitution of chlorine in Michael adducts of 1-R, 2-R, 3-X. Thus, this substitution proceeds very easily in any of the adducts formed with an electron rich nitrogen, sulfur and oxygen Michael donor. For the adducts of nitrogen nucleophiles, the facile substitution of the chlorine has been suggested to occur via formation of intermediate aziridinium ions 103 [8] (Scheme 32), and this postulate was later supported by isolation of azaspiropentane derivatives under appropriate conditions in several reactions (see Sect. 3.2.2) [11b, 53,56]. It is most likely that alkylthio substituents in adducts of type 85 participate in the same way to first form spirocyclopropane-annelated thiiranium ion intermediates which are subsequently opened by attack of the incoming nucleophile. 

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