Knoevenagel condensation
The
Knoevenagel condensation reaction is an
organic reaction named after
Emil Knoevenagel. It is a modification of the
Aldol condensation.
A Knoevenagel condensation is a
nucleophilic addition of a
active hydrogen compound to a
carbonyl group followed by an
elimination reaction in which a molecule of water is lost (hence
condensation). The product is an alpha,beta
conjugated enone.
In this reaction the carbonyl group is an
aldehyde or a
ketone.The
catalyst is usually a weakly
basic amine. The active hydrogen component has the form
* Z-CH
2-Z or Z-CHR-Z for instance
diethyl malonate,
Meldrum's acid,
ethyl acetoacetate or
malonic acid.
* Z-CHR
1R
2 for instance
nitromethane.
where Z is an
electron withdrawing functional group. Z must be powerful enough to facilitate hydrogen abstraction to the
enolate ion even with a mild base. Using a strong base in this reaction would induce
self-condensation of the aldehyde or ketone.
With malonic compounds the reaction product can lose a molecule of
carbon dioxide in a subsequent step. The
Hantzsch pyridine synthesis, the
Gewald reaction and the
Feist-Benary furan synthesis all contain a Knoevenagel reaction step.
A Knoevenagel condensation is demonstrated in the reaction of
2-methoxybenzaldehyde 1 with the
barbituric acid 2 in
ethanol using
piperidine as a base . The resulting enone
3 is a
charge transfer complex molecule.
 |
center |
# E. Knoevenagel "Condensation von Malondiure mit Aromatiachen Aldehyden durch Ammoniak und Amine" Chemische Berichte 1898, volume 31, 2396-2619#
1,3-Diethyl-5-(2-methoxybenzylidene)-2-thioxodihydropyrimidine-4,6(1H,5H)-dione Abdullah Mohamed Asiria, Khaled Ahmed Alamrya Abraham F. Jalboutb, Suhong Zhang
Molbank 2004, M359 [
1] publication.
