La condensation de Claisen (à ne pas confondre avec le réarrangement de Claisen) est une réaction omniprésente en milieu biologique. The reaction proceeds when a strong base is present and the product of the reaction is a beta-keto ester or a beta-diketone. Claisen condensations between different ester reactants are called Crossed Claisen reactions. In a fashion similar to the aldol, one ester acts as a nucleophile while a second ester acts as the electrophile. Pour cette raison, on utilise en général l'alcoolate conjuguée de l'alcool formé par la réaction (par exemple l'éthanolate de sodium si on forme de l'éthanol), ce qui permet en plus de régénérer l'alcoolate. The mixed (or "crossed") Claisen condensation, where one enolizable ester or ketone and one nonenolizable ester are used. After completing this section, you should be able to. First, aldehydes are more reactive acceptor electrophiles than ketones, and formaldehyde is more reactive than other aldehydes. Step 5: The acid protonates enolate ion 2, Acid Dissociation Constant: Mastery Check. Crossed Claisen reactions in which both reactants can serve as donors and acceptors generally give complex mixtures. The Dieckmann condensation reaction, in which a molecule featuring two ester … Aldol condensations between different carbonyl reactants are called crossed or mixed reactions, and under certain conditions such crossed aldol condensations can be effective. Crossed Claisen condensation is a variation of Claisen condensation. The following abbreviated formulas illustrate the possible products in such a case, red letters representing the acceptor component and blue the donor. Step 2: Nucleophilic attack by the enolate. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. Homework: Write a mechanism for all of the steps in the Claisen condensation above between ethyl benzoate and ethyl acetate. Crossed Claisen Condensation; Contributors; Because esters can contain \(\alpha\) hydrogens they can undergo a condensation reaction similar to the aldol reaction called a Claisen Condensation. Because of this most Crossed Claisen reactions are usually not performed unless one reactant has no alpha hydrogens. A diester can undergo an intramolecular reaction called a Dieckmann condensation. Because of this most Crossed Claisen reactions are usually not performed unless one reactant has no alpha hydrogens. Watch the recordings here on Youtube! La dernière modification de cette page a été faite le 7 février 2020 à 19:01. For more information contact us at [email protected] or check out our status page at https://status.libretexts.org. A useful carbon-carbon bond-forming reaction known as the Aldol Reaction is yet another example of electrophilic substitution at the alpha carbon in enolate anions. Au moins l'un des réactifs doit être énolisable (c'est-à-dire disposer d'un atome d'hydrogène sur le carbone en alpha). In most cases two sets of \(\alpha\) hydrogens need to be considered. The success of these mixed aldol reactions is due to two factors. The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. Step 1: The alkoxide ion deprotanates the enolizable ester reversibly. The Dieckmann condensation, where a molecule with two ester groups reacts intramolecularly, forming a cyclic β-keto ester. Le LDA ne peut par contre pas être utilisé pour les condensations « classiques » ou les condensations de Dieckmann, car tous les esters peuvent être convertis en énolate d'ester et la condensation ne peut alors avoir lieu. Because esters can contain α hydrogens they can undergo a condensation reaction similar to the aldol reaction called a Claisen Condensation.
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