Effet de cation sur la réactivité ambidente des anions: la nette tendance à la O-alkylation manifestée par les énolates de lithium de composés β-dicarbonylés

Abstract

The study of the specific reactivity of the ion pairs of alkaline enolates of β-dicarbonyl compounds shows that the lithium enolates, in spite of their characteristically strong enolate–cation interactions display, especially when compared to sodium enolates, a marked tendency towards O-alkylation. This strong O-/C-nucleophilicity of the lithium ion pairs is not very sensitive to changes in medium polarity (from THF or DME to DMF) but depends on the nature of the alkylating agent, being enhanced with moderately electrophilic alkyl sulfonates or sulfate but lessened with alkyl halides or with very electrophilic sulfonates (triflate). It is suggested that the lithium enolate tendency towards O-alkylation is partly the result of a Li+ – leaving group interaction (especially in the case of the sulfonates) and partly an intrinsic property of the enolate–lithium ion pair; this latter property is discussed in terms of the respective solvation abilities towards cations of the C- and O-alkylation transition states.Some of our results about the nucleophilic reactivity of ion pairs in DMF are at variance with previous reports in the literature. The origin of the discrepancy is discussed.