Resonance 3-keto ester anions

Principal resonance structures of the anion of a 3 keto ester... 

A hydrogen attached to the a-carbon atom of a p-keto ester is relatively acidic. Typical P-keto esters have values of about 11. Because the a-carbon atom is flanked by two electron-withdrawing carbonyl groups, a carbanion formed at this site is highly stabilized. The electron delocalization in the anion of a p-keto ester is represented by the resonance structures... 

The Claisen condensation is one method of synthesizing (3-dicarbonyl compounds, specifically a (3-keto ester. This reaction begins with an ester and occurs in two steps. In the first step, a strong base, such as sodium ethoxide, removes a hydrogen ion from the carbon atom adjacent to the carbonyl group in the ester. (Resonance stabilizes the anion formed from the ester.) The anion can then attack a second molecule of the ester, which begins a series of mechanistic steps until the anion of the (3-dicarbonyl compound forms, which, in the second reaction step (acidification), gives the product. 

In the first step the base (usually an alkoxide, LDA, or NaH) deprotonates the a-proton of the ester to generate an ester enolate that will serve as the nucleophile in the reaction. Next, the enolate attacks the carbonyl group of the other ester (or acyl halide or anhydride) to form a tetrahedral intermediate, which breaks down in the third step by ejecting a leaving group (alkoxide or halide). Since it is adjacent to two carbonyls, the a-proton in the product p-keto ester is more acidic than in the precursor ester. Linder the basic reaction conditions this proton is removed to give rise to a resonance stabilized anion, which is much less reactive than the ester enolate generated in the first step. Therefore, the p-keto ester product does not react further. 

P-keto ester enolate (anion stabilised by resonance)... 

Step 3 drives the equilibrium forward. In this step, the /1-keto ester is converted to its enolate anion. The methylene (CH2) hydrogens in ethyl acetoacetate are a to two carbonyl groups and hence are appreciably more acidic than ordinary a-hydrogens. They have a of 12 and are easily removed by the base (ethoxide ion) to form a resonance-stabilized -keto enolate ion, with the negative charge delocalized to both carbonyl oxygen atoms. 

Michael Reaction (Section 19.8A) A Michael reaction involves the addition of a weakly basic nucleophile to a carbon-carbon double bond made electrophilic by conjugation with the carbonyl group of an aldehyde, a ketone, or an ester or with a nitro or cyano group. The mechanism of the Michael reaction involves initial formation of an enolate anion in base and attack of the enolate nucleophile at the )3-carbon of the Michael acceptor to create a new resonance-stabilized enolate anion intermediate that is protonated on oxygen to create an enol and regenerate the base then tautom-erization to the keto form completes the reaction. The base is catalytic in the Michael reaction. 

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