Doebner modification

The term Knoevenagel reaction however is used also for analogous reactions of aldehydes and ketones with various types of CH-acidic methylene compounds. The reaction belongs to a class of carbonyl reactions, that are related to the aldol reaction. The mechanism is formulated by analogy to the latter. The initial step is the deprotonation of the CH-acidic methylene compound 2. Organic bases like amines can be used for this purpose a catalytic amount of amine usually suffices. A common procedure, that uses pyridine as base as well as solvent, together with a catalytic amount of piperidine, is called the Doebner modification of the Knoevenagel reaction. 

When the reactant is of the form ZCH2Z, aldehydes react much better than ketones and few successful reactions with ketones have been reported. However, it is possible to get good yields of alkene from the condensation of diethyl malonate, CH2(COOEt)2, with ketones, as well as with aldehydes, if the reaction is run with TiCU and pyridine in THF. In reactions with ZCH2Z, the catalyst is most often a secondary amine (piperidine is the most common), though many other catalysts have been used. When the catalyst is pyridine (to which piperidine may or may not be added) the reaction is known as the Doebner modification of the Knoevenagel reaction. Alkoxides are also common catalysts. 

When the amine catalyst is specifically pyridine, the reaction is known as the Doebner Modification of the Knoevenagel Reaction ... 

Doebner modification of, 463, 465, 710, 711J, 719 Kofler hot bench, 82 Knorr pyrrole synthesis, 839j, 840 Kolbe-Schmitt reaction, 754, 755J, 774-776... 

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

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

Knoevenagel condensation Doebner modification. Condensation of aldehydes or ketones with active methylene compounds (specifically malonic ester) in the presence of ammonia or amines the use of malonic acid and pyridine is known as the Doebner modification. 

Malonic acid undergoes Knoevenagel condensations with nearly every type of aldehyde and with very reactive ketones. If condensations with malonic acid are performed in ethanolic ammonia below 70 C, the methylenemalonic acids are usually obtained. If, however, the condensations are performed in pyridine (Doebner modification), decarboxylation normally takes place and the acrylic or cinnamic acid is... 

Simple aliphatic aldehydes and ketones cannot be used as the carbonyl component in the Perkin reaction. Knoevenagel5 reported the successful condensation of aldehydes and ketones with malonic acid in the presence of ammonia or amines. The most satisfactory method uses pyridine as a catalyst and is known as the Doebner modification.6 Thus, acetaldehyde 18 reacts with malonic acid (19) in the presence of a pyridine catalyst to afford the acid 20 in 60% yield. 

In this particular instance, the condensation of aldehydes or ketones normally take place with active methylene compoimds in the presence of either amines or ammonia however, the usage of malonic acid and P3rridine is commonly known as the Doebner modification. Thus, we have ... 

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