Thiiranium salts

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. 

Halide ions may attack 5-substituted thiiranium ions at three sites the sulfur atom (Section 5.06.3.4.5), a ring carbon atom or an 5-alkyl carbon atom. In the highly sterically hindered salt (46) attack occurs only on sulfur (Scheme 62) or the S-methyl group (Scheme 89). The demethylation of (46) by bromide and chloride ion is the only example of attack on the carbon atom of the sulfur substituent in any thiiranium salt (78CC630). Iodide and fluoride ion (the latter in the presence of a crown ether) prefer to attack the sulfur atom of (46). cis-l-Methyl-2,3-di-t-butylthiiranium fluorosulfonate, despite being somewhat hindered, nevertheless is attacked at a ring carbon atom by chloride and bromide ions. The trans isomer could not be prepared its behavior to nucleophiles is therefore unknown (74JA3146). 

Alkylation of thiiranes is usually unsuccessful in the preparation of thiiranium salts, but see Scheme 34 for an exception. 

Thiiranium chloride, 1,2,2,3,3-pentamethyl-formation, 7, 174 Thiiranium salts isomerization, 7, 163 reactions... 

Thiiranium salts, l-methyI-2-methylene-1-oxide, 7, 134 Thiiranium salts, I-phenyl-collisional activation spectra, 7, 135 Thiiranium tetrafluoroborate, 2,3-di-t-butyl-l-methyl-inversion barriers, 7, 134 Thiiranium ylides formation, 7, 174, 175 Thiiran-2-ones... 

Structures of thietanium tetrafluoroborate and hexachloroantimonate salts of 107, formed by anionotropic rearrangement of corresponding thiiranium salts, were obtained by X-ray crystallography and by calculation at the RHF/6-31G //RHF/6-31G level <2001HCA860>. 

The /ra t-thiiranium ion, in separate experiments, reacted with water to afford /ra t-di-/-butylethene, in amounts depending on the concentration of water and the thietanium ion. The authors estimated rate constants for the reactions of both the as- and the /ra t-thiiranium ions with water at 25 °C and pointed out that the thiiranium salts are models for transition states for addition of electrophilic sulfur compounds to alkenes. A summary of the findings and their interpretation was published in 1997 <1997G177>. 

While most nucleophiles react with the fused thiiranium salt at carbon (Section 1.06.6.5), certain soft nucleophiles including anion of malonate ester, iodide, thiourea, phosphines, and tertiary amines promote elimination of the sulfur atom by nucleophilic attack on sulfur (Scheme 36) <1980T1361>. 

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