Deprotonation process acetaldehyde

Many a,P-unsaturated carbonyl compounds (aldehydes and ketones as well as esters and amides [this chapter]) undergo aldol reactions on the a-carbon of the unsaturated partner with the carbonyl of a second partner. This interesting reaction (the Baylis-Hillman reaction) depends upon the temporary addition of a hindered base (l,4-diazabicyclo[2.2.2]octane, DAB(70, is commonly used) to the P-carbon of the a,P-unsaturated system rather than the deprotonation of the a-carbon. The enolate anion, a to the carbonyl of what was the a,P-unsaturated system, then adds to the other reactant and subsequent elimination provides the condensation product and the base is eliminated. The process is shown in Scheme 9.52 for the reaction between ethanal (acetaldehyde) and the a,P-unsaturated ester ethyl propenoate (ethyl acrylate). 

Recall that LDA causes irreversible enolate formation. If acetaldehyde is added dropwise to a solution of LDA, the result is a solution of enolate ions. Propionaldehyde can then be added dropwise to the mixture, resulting in a crossed aldol addition that produces one major product. This type of process is called a directed aldol addition, and its success is hmited by the rate at which enolate ions can equilibrate. In other words, it is possible for an enolate ion to function as a base (rather than a nucleophile) and deprotonate a molecule of propionaldehyde. If this process occurs too rapidly, then a mixture of products will result. 

Fig. 19.9). In order to understand this exchange process we need to consider the reversibility of the deprotonation reaction. In hydroxide (here deuteroxide), not all of the acetaldehyde will be transformed into the enolate. The pAT of acetalde-... 

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