Zincke reaction

The Zincke reaction is a reaction name in the field of organic chemistry. It is used in the synthesis of pyridinium compounds of pyridines and is named after its discoverer Theodor Zincke. It is not to be confused with the Zincke - Suhl reaction or the Zincke nitration.

Mechanism

In a first step, a Zincke salt (2) of the pyridine derivative used is prepared. This is done by reacting the pyridine with an electron-deficient aromatics, frequently from 1 -chloro- 2 ,4-dinitrobenzene, thereby forming a pyridinium salt. The salt is then reacted with two equivalents of the primary amine, which bears those residue which after the end of the reaction is to remain on the pyridine. First, the amine is added in the 2- position on the Pyridinumsalz (3), leading to the deprotonation of the opening of the ring ( 4). By substitution of the amine from the previous pyridine with a third equivalent of the desired amine, the desired rest is brought to the correct position (6a). The reaction proceeds further as the reverse of the previous reaction. The ring is closed ( 7) located in the 2- position amine protonated (8) and releasing the desired pyridinium compound (9) with the elimination of the amine.

If instead of primary amines, secondary amines are used, are obtained from (6 ) in the presence of water open-chain aldehydes, known as Zincke aldehydes.

Errata

Attention in the recent literature learned the publication of an alleged one-pot synthesis of Diazaannulens ( 1), which had been published independently by two research groups. Then wrote the German professor Manfred Christl, a former student of R. Huisgen and S. Huenig, a reader's letter in which he doubted valid structure and synthesis of the postulated Diazaannulens. Furthermore, he pointed out that it is actually the ongoing reaction to the well-known for over a hundred years Zincke reaction would RELATES and formed the corresponding product of the Zincke reaction (2). Both groups then moved back their publications. About the case of modern sloppy science research has also been reported in the mirror and the Nature. It was found that both working groups the necessary experience in the evaluation of mass and NMR spectra lacked techniques so that did not exist at the time of Zincke.

Application in the synthesis planning

In addition to the variable synthesis possibility alkyl or aryl -substituted pyridinium the Zincke reaction is also used in newer Publications on current problems. For example, a new indole synthesis based on the Zincke reaction has been developed.