Zincke reaction
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| Zincke reaction | |
|---|---|
| Named after | Theodor Zincke |
| Reaction type | Coupling reaction |
The Zincke reaction is an organic reaction, named after Theodor Zincke, in which a pyridine is transformed into a pyridinium salt by reaction with 2,4-dinitro-chlorobenzene and a primary amine.[1][2][3][4]
The Zincke reaction should not be confused with the Zincke-Suhl reaction or the Zincke nitration. Furthermore, the Zincke reaction has nothing to do with the chemical element zinc.
The first reaction is the formation of the N-2,4-dinitrophenyl-pyridinium salt (2). This salt is typically isolated and purified by recrystallization.
Upon heating a primary amine with the N-2,4-dinitrophenyl-pyridinium salt (2), the addition of the amine leads to the opening of the pyridinium ring. A second addition of amine leads to the displacement of 2,4-dinitroaniline (5) and formation of the König salt[5] (6a and 6b). The trans-cis-trans isomer of the König salt (6a) can react by either sigmatropic rearrangement or nucleophilic addition of a zwitterionic intermediate to give cyclized intermediate (7).[6] This has been suggested to be the rate-determining step.[7][8] After proton transfer and amine elimination, the desired pyridinium ion (9) is formed.
This mechanism can be referred to as an instance of the ANRORC mechanism: nucleophilic addition (AN), ring opening and ring closing.
Applications
In one solid-phase synthesis application, the amine is covalently attached to Wang resin.[9]
Another example is the synthesis of a chiral isoquinolinium salt.[10]
