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Krapcho decarboxylation is a chemical reaction used to manipulate certain organic esters. [1] This reaction applies to esters with a beta electron-withdrawing group (EWG). The reaction proceeds by nucleophilic dealkylation of the ester by the halide followed by decarboxylation, followed by hydrolysis of the resulting stabilized carbanion. [2]
The Barton decarboxylation is an unusual radical reductive decarboxylation. As described above, most decarboxylations start with a carboxylic acid or its alkali metal salt, but the Krapcho decarboxylation starts with methyl esters. In this case, the reaction begins with halide-mediated cleavage of the ester, forming the carboxylate.
On heating, the di-ester undergoes thermal decarboxylation, yielding an acetic acid substituted by the appropriate R group. [1] Thus, the malonic ester can be thought of being equivalent to the − CH 2 COOH synthon. The esters chosen are usually the same as the base used, i.e. ethyl esters with sodium ethoxide.
Using a carboxylate-to-iodine ratio of 1:1 leads to an alkyl iodide product, in line with Borodin's findings and the modern understanding of the Hunsdiecker reaction. However, a 2:1 ratio favours the formation of an ester product that arises from decarboxylation of one carboxylate and coupling the resulting alkyl chain with the other. [9] [10]
Methyl esters are often susceptible to decarboxylation in the Krapcho decarboxylation. Phenyl esters react to hydroxyarylketones in the Fries rearrangement. Specific esters are functionalized with an α-hydroxyl group in the Chan rearrangement. Esters with β-hydrogen atoms can be converted to alkenes in ester pyrolysis.
Another example is the synthesis of 2,7-dimethyl-2,7-dinitrooctane from 4-methyl-4-nitrovaleric acid: [3] The Kolbe reaction has also been occasionally used in cross-coupling reactions . In 2022, it was discovered that the Kolbe electrolysis is enhanced if an alternating square wave current is used instead of a direct current .
The Barton decarboxylation is a radical reaction in which a carboxylic acid is converted to a thiohydroxamate ester (commonly referred to as a Barton ester). The product is then heated in the presence of a radical initiator and a suitable hydrogen donor to afford the decarboxylated product. [1] [2] This is
Diethyl malonate, also known as DEM, is the diethyl ester of malonic acid. It occurs naturally in grapes and strawberries as a colourless liquid with an apple-like odour, and is used in perfumes. It is also used to synthesize other compounds such as barbiturates, artificial flavourings, vitamin B 1, and vitamin B 6.