Zincke Reaction - Reaction Mechanism

Reaction Mechanism

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 (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). This has been suggested to be the rate-determining step. 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 (A_N), ring opening and ring closing.