Knoevenagel Condensation
Doebner Modification

The condensation of carbon acid compounds with aldehydes to afford α,β-unsaturated compounds.

The Doebner Modification, which is possible in the presence of carboxylic acid groups, includes a pyridine-induced decarboxylation.

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Mechanism

An enol intermediate is formed initially:

This enol reacts with the aldehyde, and the resulting aldol undergoes subsequent base-induced elimination:

A reasonable variation of the mechanism, in which piperidine acts as organocatalyst, involves the corresponding iminium intermediate as the acceptor:

The Doebner-Modification in refluxing pyridine effects concerted decarboxylation and elimination:

  

Recent Literature

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.

Reconstructed Hydrotalcite as a Highly Active Heterogeneous Base Catalyst for Carbon-Carbon Bond Formations in the Presence of Water
K. Ebitani, K. Motokura, K. Mori, T. Mizugaki, K. Kaneda, J. Org. Chem., 2006, 71, 5440-5447.

The ionic liquid 1-butyl-3-methylimidazonium tetrafluoroborate [bmim]BF₄ was used for ethylenediammonium diacetate (EDDA)-catalyzed Knoevenagel condensation between aldehydes or ketones with active methylene compounds. Catalyst and solvent can be recycled.
C. Su, Z.-C. Chen, Q.-G. Zhen, Synthesis, 2003, 555-559.

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.

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.