Aromatic lithium enolate, stability

The aldol reaction is an addition of metal enolates to aldehydes or ketones to form P-hydroxy carbonyl compounds.1 The simplest aldol reaction would be the reaction of acetaldehyde lithium enolate with formaldehyde (Scheme 2.1). As the transition state of this reaction involves six atoms, the aldol reaction is another example where a six-membered transition state is presumed to be operating. The transition state of the aldol reaction is very similar to those of Claisen and Cope rearrangements, and therefore the remarkable facility of the lithium enolate reaction is attributed to the stability of an aromatic transition state.2... 

A few comments concerning the crystallization of carbanions are in order. These comments are based upon the personal experience developed in our own laboratory and also upon observations noted in the literature in the course of crystallizing enolate anions. Although alkali metal enolate anions are relatively unstable compounds, they have been prepared in the solid state, isolated, and characterized by IR and UV spectroscopy in the 1970s. Thus the ot-lithiated esters of a number of simple esters of isobutyric acid are prepared by metallation of the esters with lithium diisopropylamide in benzene or toluene solution. The soluble lithiated esters are quite stable at room temperature in aliphatic or aromatic hydrocarbon solvents and are crystallized out of solution at low temperature (e.g. -70 °C.). Alternatively the less soluble enolates tend to precipitate out of solution and are isolated by centrifugation and subsequent removal of the solvent. Recrystallization from a suitable solvent can then be attempted. The thermal stability of the lithiated ester enolates is dramatically decreased in the presence of a solvent with a donor atom such as tetrahydrofuran. 

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