Aldol reaction Knoevenagel mechanism

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. 

The Knoevenagel reaction, the condensation of 1,3-dicarbonyl compounds with aldehydes to give unsaturated compounds, is catalyzed by 2° amines. A perfectly reasonable mechanism involving deprotonation of the 1,3-dicarbonyl compound by base, aldol reaction with the ketone, and Elcb elimination of H2O can be drawn. However, the Knoevenagel reaction does not proceed nearly so well using 3° amines, suggesting that the amine does not simply act as a base. 

Dicarbonyl compounds condense with aldehydes and ketones that do not undergo self-aldol reaction. The products are afi-unsaturated dicarbonyl compounds, and the process goes by the colorful name of Knoevenagel condensation, (a) An example of a Knoevenagel condensation is given below. Propose a mechanism. 

The Knoevenagel condensation is a base-catalyzed aldol-type reaction, and the exact mechanism depends on the substrates and the type of catalyst used. The first proposal for the mechanism was set forth by A.C.O. Hann and A. Lapworth Hann-Lapworth mechanism) In 1904." When tertiary amines are used as catalysts, the formation of a p-hydroxydlcarbonyl Intermediate is expected, which undergoes dehydration to afford the product. On the other hand, when secondary or primary amines are used as catalyst, the aldehyde and the amine condense to form an Imlnlum salt that then reacts with the enolate. Finally, a 1,2-ellmlnatlon gives rise to the desired a,p-unsaturated dicarbonyl or related compounds. The final product may undergo a Michael addition with the excess enolate to give a bis adduct. 

The Knoevenagel condensation is a variation of aldol condensation using a 3-dicarbonyl compound as the source of the enolate reaction partner. Its mechanism is the same as that of the aldol ... 

A Mannich-type condensation mechanism involving an iminium ion electrophile similar to the aminocatalytic Knoevenagel reaction has recently been proposed for the amine-catalyzed self-aldolization of propionaldehyde (Eq. (6)) [55]. Although this mechanism is not unreasonable it should be... 

The Knoevenagel reaction is a base-catalyzed aldol-type reaction that can occur through two possible mechanisms, depending on the type of base used. When Emil Knoevenagel made his initial discovery of this reaction, it was already known that benzaldehyde could condense with two equivalents of piperidine to provide the benzylidine bispiperidine aminal 1 Therefore, he proposed the intermediacy of an aminal (or imine) in the condensation. 

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