Epoxides carbanion-based nucleophiles

Sulfonium salts react in several ways. They may behave as a leaving group, undergoing substitution by a nucleophile or fragmenting with the formation of an alkene. However, the most important reaction of sulfonium salts involves the formation of an ylide in the presence of a base. The carbanion of this sulfur ylide is stabilized by the adjacent positively charged sulfonium ion. The reaction of the carbanion with a carbonyl group parallels that of a phosphonium ylide in the Wittig reaction. However, the decomposition of the intermediate dipolar species is different and leads to the formation of an epoxide (oxirane) rather than an alkene. 

Selenoxides are even more reactive than amine oxides toward p elimination. In fact, many selenoxides react spontaneously when generated at room temperature. Synthetic procedures based on selenoxide eliminations usually involve synthesis of the corresponding selenide followed by oxidation and in situ elimination. We have already discussed examples of these procedures in Section 4.7, where the conversion of ketones and esters to their a,j5-unsaturated derivatives was considered. Selenides can also be prepared by electrophilic addition of selenenyl halides and related compounds to alkenes (see Section 4.5). Selenide anions are powerfiil nucleophiles that can displace halides or tosylates and open epoxides. Selenide substituents stabilize an adjacent carbanion so that a-selenenyl carbanions can be prepared. One versatile procedure involves conversion of a ketone to a bis-selenoketal which can then be cleaved by w-butyllithium. " The carbanions in turn add to ketones to give jS-hydroxyselenides. Elimination gives an allylic alcohol. 

An anionic synthon for cyclohexanone is inconvenient since a strong anionic base is needed for the generation of a carbanion, which acts like a competitive nucleophile and attacks epoxide. Neutral enamine is therefore the preferred nucleophile (Scheme 5.52). 

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