Tishchenko reaction
The Tishchenko reaction or Claisen-Tishchenko reaction is named after the chemists Ludwig Claisen (1851–1930) and Vyacheslav Evgenyevich Tishchenko (1861–1941).
Normally, carboxylic acid esters are made from carboxylic acids and alcohols by acid catalysis . However, there is another way: If an aldehyde is available, can Ester by the Claisen Tishchenko reaction in a rearrangement reaction with aluminum alkoxide as a catalyst to be prepared. In the case of non- enolizable aldehydes, a sodium alcoholate can also be used instead of an aluminum alcoholate. In this way, on an industrial scale, ethyl acetate is produced from acetaldehyde using aluminum triethanolate as a catalyst.
If one starts from a mixture of several aldehydes instead of one aldehyde, complex mixtures of different esters usually result, which have to be separated in a laborious manner.
In contrast to the Cannizzaro reaction , in which the aldehyde is converted into carboxylic acid and alcohol, the reaction must take place in an anhydrous and alcohol-free medium.
A tandem reaction consisting of an aldol reaction and a Claisen-Tischtschenko reaction is referred to as an aldol-Tischtschenko reaction .
Possible side reaction
The aluminum alcoholate reacts one after the other with two molecules of the aldehyde and forms a cyclic transition state . In the first step, a (deprotonated) hemiacetal is formed by attack of an alcoholate ion on the carbonyl carbon of the aldehyde ( nucleophilic addition ) . The second step is a redox reaction in which the hemiacetal is raised from its oxidation state + I to the oxidation state + III of an ester. The reducing agent is the second aldehyde molecule, which is lowered from its oxidation state + I to that of a primary alcohol (-I). The reduction of the second aldehyde molecule to the alcoholate is mechanistically analogous to the central step of the Oppenauer oxidation .
The side reaction occurs especially when the catalyst concentration is increased.
Intramolecular variant
In the intramolecular Claisen-Tischtschenko reaction of dialdehydes that cannot be enolized, lactones can be formed.
Individual evidence
- ↑ Tsunetake Seki, Hiroto Tachikawa, Takashi Yamada, Hideshi Hattori: Synthesis of phthalide-skeleton using selective intramolecular Tishchenko reaction over solid base catalysts. In: Journal of Catalysis. 217, No. 1, 2003, pp. 117-126, doi : 10.1016 / S0021-9517 (02) 00187-2 .