Sulfonium salts from alkyl halides

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). 

Arylmethyl(homobenzyl)ethylsulfonium salts are appropriate substrates for Suzuki-type coupling reactions. In this reaction, performed on a polymer-bound sulfonium tetrafluoroborate, the benzyl fragment on the sulfur atom was transferred to the boronic acid residue. The sulfonium salt was prepared from an al-kylthiol resin by alkylation with a substituted benzyl halide to give thioether 98 and subsequent alkylation with triethyloxonium tetrafluoroborate. Reaction with a boronic acid derivative yielded diaryl methanes 99 [94] (Scheme 6.1.22). 

Sulfur compounds are more nucleophilic than the corresponding oxygen compounds, because sulfur is larger and more polarizable and its electrons are less tightly held in orbitals that are farther from the nucleus. Although ethers are weak nucleophiles, sulfides are relatively strong nucleophiles. Sulfides attack unhindered alkyl halides to give sulfonium salts. 

Thiophene sulfonium salts have also been used for alkylation of phenols, thiophenols, and other nucleophiles (Equation 70) <2001T2871>. Ylides generated from THT and alkyl or allyl halides are known to react with aldehydes to form oxiranes. However, a modified procedure has been developed in which only a catalytic amount of THT is used for the preparation of vinyloxiranes from allyl bromides and aldehydes. In most of the cases, a m-/ra r-mixture of vinyloxiranes was obtained. Optically pure C2-symmetric /ra r-2,5-dimethyltetrahydrothiophene has also been used for the asymmetric version of this reaction, but the enantioselectivity was poor (25% ee) (Equation 71) <2003CC2074>. 

The proof given in support of dianion formation does not however seem to be sufficients Nevertheless the single step process is reasonably efficient when primary alkyl halides are used but none of the dialkylated compound is produced with isopropyl iodide and benzyl bromide. 2-(o>-Haloalkyl)-l,3-di-thianes have proved to be valuable precursors of 1,3-dithianes derived from cyclic ketoncs. Haloge-nated 1,3-dithianes have been in turn prepared from 2-lithio-l,3-dithiane and stoichiometric amounts of u-chloro- or bromo-alkyl iodides or with an excess of the corresponding dichloride (Scheme 64, entry c). Use of u-bromoalkyldithianes is often impractical because of the ease with which these compounds are transformed into cyclic sulfonium salts. ... 

Not all variations in isotope effects result from a shift in the rate-controlling step, for quite substantial changes have been noted in single-step reactions. A particularly good example is the E2 reaction of base with a substrate such as an alkyl halide or sulfonium salt to yield olefin [Eq. (32) J. This reaction can be... 

S-Alkylatlon. Methyl iodide alkylates thioalkoxides and sulfides to produce sulfides and sulfonium ions, respectively. For example, thioalkoxides produced from thiocarbonyl compounds are methylated with methyl iodide to generate the corresponding methyl thioether (eq 22). Sulfonium halides, derived from the reaction of methyl iodide with an alkyl sulfide, are sometimes labile in solution and may undergo further reaction (eq 23). Dimethyl Sulfoxide when refluxed with an excess of methyl iodide produces trimethyloxosulfonium iodide, which is collected as a white solid and recrystallized from water. Similarly, methylation of Dimethyl Sulfide produces trimethylsulfonium iodide. Treatment of trimethyloxosulfonium and trimethylsulfonium salts... 

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