Zinc dialkylamide

Zinc ester enolates may also be obtained by the addition of ZnX2 to lithium or sodium enolates as first described by Hauser and Puterbaugh (equation 6)P This approach has so far received little attention but similar reactions have been used to obtain zinc ketone enolates. In this regard, it should be noted that Heathcock and coworkers have shown that deprotonation reactions of ketones with zinc dialkylamide bases reach equilibrium at only about 50% conversion (equation 7). This result implies that attempts to prepare zinc enolates from solutions of amide-generated lithium enolates will be successful only when the lithium enolate is made amine-free. 

Trimethylsilylacetate esters may he converted to the enolate by treatment with lithium dialkylamide bases (LDA in Eq. 7.28) in THF at -78°C. These will add to ketones or aldehydes quickly at -78°C, followed by elimination of MOjSiOLi and formation of a,p-unsaturated esters in high yields, uncontaminated by p,y-unsaturated isomers [47]. This is known as the Peterson reaction [48, 49]. The requisite ethyl trimethylsilylacetate is made by the reaction of cldorotrimethylsilane, ethyl bromoacetate, and zinc [50]. Esters of longer-chain acids give mostly 0-silylation under these conditions, but diphenylmethylchlorosilane gives C-silylation selectively. These diphenyl-methylsilylated esters also give the Peterson reaction (Eq. 7.29) [51]. 

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