Sulfonium and Sulfoxonium Ylids

Dimethyl sulfide is a useful precursor to sulfur ylids, reacting with iodomethane to give trimethylsulfonium iodide (609). When this salt was treated with n-butyllithium, deprotonation of the hydrogen on the a-carbon led to dimethylsulfonium methylid (610). Dimethyl sulfoxide also reacts with alkyl halides to give a sulfoxonium salt. When DMSO reacted with iodomethane, trimethylsulfoxonium iodide (611) was formed. As with the dimethyl sulfide derivative, treatment with a strong base such as -butyllithium generated the corresponding ylid, dimethylsulfoxonium methylid (612). 

Chapter 8. Nucleophilic Species That Form Carbon-Carhon Bonds 

It is known that dimethylsulfonium methylid (610) generally adds irreversibly to ketones and aldehydes. On the other hand, dimethylsulfoxonium methylid (612) adds reversibly to a carbonyl. This difference is reflected in their reactivity with cyclic ketones such as A-tert-huiyl cyclohexanone (617). Dimethylsulfonium methylid (610) reacted with 617 to give the oxirane with an axial exocyclic C—C bond (618), which is less stable than the product with an axial C—O bond (619).535 when 612 reacted with 617, however, the oxirane with the more stable equatorial exocyclic C—C bond (619) was formed.336 This is attributed to the reversibility of the addition of 612 to the ketone leading to the more stable oxirane, whereas 610 generated the intermediate salt irreversibly in what is essentially a kinetic process. 

Another difference in the reactivity of these two ylids is reflected in their reactions with a, 3-unsaturated ketones. The carboethoxy dimethylsulfonium methylid 620 reacted with cyclohexenone to give the usual alkoxy sulfonium salt 621. Intramolecular displacement of dimethyl sulfide led to the expected oxirane (622) analogous to the formation of 614. Dialkylsulfonium ylids react via 1,2-addition with displacement of the 

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Interactive mechanism for three-membered ring formation with sulfonium and sulfoxonium ylids... 

Sulfonium and sulfoxonium ylids from diazo compds. 

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