Aldol-like Reactions
Many variations of aldol reaction were established. These work on the same principle as classical aldol reaction does, such as ...
- ... intramolecular aldol reaction, which yields cyclic compounds.
- ... crossed aldol reaction, in which two different carbonyl compounds are applied together, resulting in a variety of products.
- ... Knoevenagel reaction, in which particularly strongly CH-acidic compounds, such as 1,3-dicarbonyl compounds or nitroalkanes, are applied instead of a "simple" enolate.
- ... Michael reaction, in which the conjugated base (anion) of a CH-acidic compound is added to the β carbon of an α,β-unsaturated carbonyl compound. If the CH-acidic compound is an aldehyde or a ketone, the Michael reaction yields a 1,5-dicarbonyl compound.
- ... Robinson annulation, which is a combination of a Michael reaction and a subsequent intramolecular aldol condensation. Robinson annulation is useful for the construction of complex aliphatic ring skeletons, such as that of steroids and terpenes.
Knoevenagel condensation:InKnoevenagelcondensation, particularly strongly CH-acidic compounds, such as 1,3-dicarbonyl compounds or nitroalkanes, are applied instead of a "simple" enolate.
Knoevenagel Condensation
In a variation of the aldol reaction, other CH-acidic compounds are applied instead of the classical enol. Theoretically, every carbanion is capable of nucleophilically attacking the electrophilic carbonyl group of aldehydes and ketones.
- KNOEVENAGEL CONDENSATION
- The condensation of aldehydes or ketones with a variety of CH-acidic compounds in the presence of a base, such as a tertiary amine or KOH, is called Knoevenagel condensation.
The reaction is named after Emil Knoevenagel (1865-1921). In comparison to aldehydes and ketones (pKa values: 14-20), 1,3-diketones or β-keto carbonyl compounds are much stronger acids (pKa values: 5-13). Thus, they can relatively easily be deprotonated to the corresponding carbanion or enolate. Nitroalkanes, depicted in the second example in the illustration below, are considerably CH-acidic compounds (pKa values about 10). As a result, they may be successfully applied in the Knoevenagel condensation. This reaction is also often called the Henry reaction. Due to the relatively high acidity of the CH-acidic compounds applied in the Knoevenagel condensation, only a relatively weak base, such as an amine or KOH, is required. Much like in aldol reactions, the addition of the carbanion to the carbonyl compound is frequently followed by dehydration.
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