An expedient preparation of amine-free lithium enolates using immobilized amide reagents

Amine-free lithium enolates can be prepared by a simple procedure using polymer-immobilized lithium amides derived from known, easily accessible immobilized secondary amines, or from a new bidentate amine [lithium diisopropylamide (LDA) or lithium cyclohexylisopropylamide (LICA) analogues]. The procedure involves physical separation of the polymeric amine reagent and the enolate formed by simple washing with solvents. The amine-free enolates were identified by their characteristic NMR signals in THF solution. The enolates (and an azaenolate) obtained were used in selected reactions with electrophiles (PhCHO, MeCHO, PhCH = CHCOCN, BnBr) giving improved results. The higher reactivity of the lithium enolates toward ‘difficult electrophiles’ is demonstrated by quantitative levels of deuterium incorporation on D2O quenching, where the standard LDA-based method, that is, in the presence of amine gives a ca. 50% yield. Recycling of used polymers via the ‘filtration procedure’ is also easier and uncomplicated by possible side reactions with electrophiles and their possible effects on the polymeric regents.