What is a Knoevenagel reaction
Organic Chemistry Portal
Organic Chemistry Portal: Knoevenagel Condensation
The Knoevenagel reaction is a condensation of C-acidic methylene compounds with aldehydes or ketones to form unsaturated compounds.
The reaction with pyridine, which leads to a decarboxylation, is also called a Doebner modification.
The reaction takes place via the enol form:
As a nucleophile, the enol attacks an aldehyde (or a ketone). The addition step is followed by a condensation:
There are indications that, depending on the starting material, an imine takes part in the reaction instead of the carbonyl, and that piperidine would also be an organocatalyst here:
Malonic acid esters can be decarboxylated after saponification. If the Knoevenagel condensation is carried out in pyridine under reflux (Doebner modification), the decarboxylation often takes place directly. After the saponification, the following process is assumed:
Instead of piperidine, other (organo) catalysts can also be used for the Knoevenagel condensation. In the examples from the current literature, e.g. triphenylphosphine or an application-specific ionic solvent is used.
Phosphane-Catalyzed Knoevenagel Condensation: A Facile Synthesis of α-Cyanoacrylates and α-Cyanoacrylonitriles
J. S. Yadav, B. S. S. Reddy, A. K. Basak, B. Visali, A. V. Narsaiah, K. Nagaiah, Eur. J. Org. Chem., 2004, 546-551.
Ionic Liquid as Catalyst and Reaction Medium - A Simple, Efficient and Green Procedure for Knoevenagel Condensation of Aliphatic and Aromatic Carbonyl Compounds Using a Task-Specific Basic Ionic Liquid
B. C. Ranu, R. Jana, Eur. J. Org. Chem., 2006, 3767-3770.
Further reaction systems can be found on the English-language site.
Prof. Emil Knoevenagel (1865-1921)
The German chemist Knoevenagel studied chemistry at the Technical University of Hanover, did his doctorate in Göttingen on the subject of radicals and completed his habilitation in Heidelberg on asymmetrically substituted carbon atoms. As a professor, he was mainly concerned with N-Heterocycles obtained by condensation reactions of 1,5-diketones with amines. The reaction named after him was published in 1896.
Prof. Oskar Doebner (1850-1907)
After initial studies in the botany department, Doebner moved to the chemistry department in Leipzig. After completing his doctoral thesis on diphenyl in Tübingen and further positions as an assistant, Doebner became Professor of Chemistry and Pharmacy at the University of Halle. He drew attention mainly with the synthesis of malachite green and choline derivatives.
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