Sulfides desulfurization, stereochemistry

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). 

Desulfurization of sulfides (or thiols) by TBTH-AIBN tolerates nitriles, esters, ketones, ethers, lactones, 3-lactams and isolated alkenes. In contrast, halides are reduced and the stereochemistry of the starting sulfides is not conserved. Yields of these desulfurizations vary considerably, and 3 eliminations may occur with vicinally substituted compounds. Some examples of TBTH desulfurizations are given in Scheme 17. 

---