Knoevenagel condensation

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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-CH₂-Z or Z-CHR-Z for instance diethyl malonate, Meldrum's acid, ethyl acetoacetate or malonic acid.
Z-CHR₁R₂ 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.

Example reactions

A Knoevenagel condensation is demonstrated in the reaction of 2-methoxybenzaldehyde 1 with the barbituric acid 2 in ethanol using piperidine as a base ^{[#endnote_Asiria]}. The resulting enone 3 is a charge transfer complex molecule.

References

↑ 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 [link] publication.

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