Question 1

What is a crossed Claisen condensation and how does it differ from regular Claisen condensation?

Accepted Answer

A crossed Claisen condensation is a base-promoted reaction between two different ester molecules that produces β-dicarbonyl compounds. Unlike regular Claisen condensation between identical esters, crossed condensation involves distinct ester partners. This approach solves the problem of multiple product formation that occurs when both esters contain α hydrogens, making the reaction more synthetically useful and easier to isolate desired products.

Question 2

Why do aryl esters work well in crossed Claisen condensations?

Accepted Answer

Aryl esters are effective in crossed Claisen condensations because they contain no α hydrogen atoms and therefore cannot form enolates. This property makes them excellent electrophilic partners that react with enolate-forming ester molecules. Since aryl esters cannot self-condense, they selectively produce crossed-condensation products rather than unwanted self-condensation byproducts.

Question 3

What role does LDA play in directed crossed Claisen condensations?

Accepted Answer

LDA (lithium diisopropylamide) is a strong, sterically hindered base that facilitates directed crossed Claisen condensations when both esters contain α hydrogens. LDA irreversibly deprotonates one ester to form an enolate, while the other ester acts as the electrophile. This controlled approach prevents formation of multiple products and ensures selective formation of the desired β-dicarbonyl compound.

Question 4

How do formate esters contribute to successful crossed Claisen condensations?

Accepted Answer

Formate esters are highly reactive molecules that lack α hydrogen atoms, making them ideal electrophilic partners in crossed Claisen condensations. Their enhanced reactivity compared to other esters allows them to readily accept the nucleophilic attack from enolate-forming ester molecules. This combination of high reactivity and absence of α hydrogens ensures selective product formation.

Question 5

What happens when you use a less reactive ester with no α hydrogen in crossed Claisen condensation?

Accepted Answer

When a less reactive ester lacking α hydrogens is used, an excess quantity of that ester is required to make the reaction feasible. The excess drives the equilibrium toward product formation by increasing the concentration of the electrophilic partner. This strategy allows less reactive esters to participate effectively in crossed Claisen condensations despite their lower reactivity.

Question 6

Can ketones react with esters in crossed Claisen condensations?

Accepted Answer

Yes, ketones can react with esters in a variation of crossed Claisen condensation. The enolate of the ketone acts as the nucleophile and attacks the carbonyl carbon of the ester, producing β-dicarbonyl compounds. This ketone-ester variant expands the scope of crossed Claisen condensations beyond ester-ester reactions.

Question 7

Why do both esters containing α hydrogens create synthetic problems in crossed Claisen condensation?

Accepted Answer

When both esters contain α hydrogens, four different β-ketoester products form: two self-condensation products and two crossed-condensation products. This mixture is difficult to isolate and reduces synthetic utility. Using esters without α hydrogens or employing LDA eliminates this problem by preventing self-condensation and ensuring selective formation of the desired crossed-condensation product.

β-Dicarbonyl Compounds via Crossed Claisen Condensations

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