Overview of Ester Enolate reactions

Chapter 21: Ester Enolates

The most important reactions of ester enolates are the Claisen and Dieckmann condensations.
However, ester groups are common as part of active methylene enolates.
Remember that enolates are sources of nucleophilic C that react with electrophiles.
 
 

overview of ester enolate reactions Claisen condensation
Dieckmann condensation

  The Claisen Condensation
Claisen condensation of esters
Reaction type : Nucleophilic Acyl Substitution

Summary

Claisen condensation of ethyl ethanoate
Try to identify the enolate portion and the carbonyl portion in the different representations
Highlight enolate component Nu
Highlight carbonyl component E
Question
Methanoate esters can not undergo Claisen self condensations. Why ? there are no alpha-H to form an enolate with

STUDY TIPS:

taking a Claisen reaction apart to find the pieces
Related Reactions
MECHANISM OF THE CLAISEN CONDENSATION
Step 1:
First, an acid-base reaction. The alkoxide functions as a base and removes the acidic a-hydrogen giving the reactive ester enolate.
mechanism of the Claisen condensation
Step 2:
The nucleophilic ester enolate attacks the carbonyl C of another ester in a nucleophilic substitution process giving the tetrahedral intermediate.
Step 3:
The intermediate collapses, reforming the C=O, resulting in loss of the leaving group, the alkoxide,
leading to the b-ketoester product.

Note that the reaction is drawn to completion by deprotonation of the active methylene in the product by the ethoxide. The salt typically precipitates and is recovered after acid work-up.


 

The Dieckmann Condensation

Dieckmann condensation
Reaction type : Intramolecular Nucleophilic Acyl Substitution

Summary

Dieckmann condensation
How can you deduce this product ?
  1. Form the enolate
  2. There two possible equivalent enolates (at C2 or C5)
  3. The more favorable ring size will the 5-membered ring.
STUDY TIP:
When doing intramolecular reactions, it is a good idea to count carbon atoms to help avoid silly errors.

Related Reactions