Wanzlick equilibrium

Under a Wanzlick balance refers to a chemical equilibrium reaction between a stable carbene and its dimer.

In the 1960s suspected Hans -Werner Wanzlick and Eberhard Schikora that the production of a carbene should be possible with the dihydroimidazol -2 -ylidene structure by vacuum pyrolysis of 2- Trichlormethyldihydroimidazol derivative with elimination of chloroform.

Both believed that such a carbene is produced to a small extent in equilibrium with its dimer. This, they concluded from reactivity, as was obtained in the reaction with an electrophile, the product of electrophilic substitution. The dimer would be unable to this reaction.

The hypothesis of the carbene dimer equilibrium has been studied by David M. Lemal and HE Winberg. They heated the mixture of two differently substituted Tetraaminoethylenen.

In these experiments, no crossed dimers could be found, but which according to the hypothesis of Wanzlick the case should have been. They point rather to the fact that there is no carbene dimer equilibrium in the case of dihydroimidazol -2 -ylidene derivatives.

Lemal suggested an alternate mechanism for the results of Wanzlick, taking into account the reactivity of the electron-rich Tetraaminoethylene and not stable carbenes which exist at this point only as an hypothesis.

Lemal believed that a Tetraaminomethylen and not the carbene reacts with the electrophile EX through a cationic intermediate. He suggested that the cation dissociates to a carbene and a salt. The free carbene can then dimerize to the starting compound or react with the electrophile. Both paths lead to the same reaction products, the dihydroimidazolium salt.

In 1999, M. thinking again examined the cross - experiment and was able to observe the crossed dimer. These results prompted Lemal to repeat his experiments of 1964. It was possible to reproduce the results of thought, but only in Deutero -THF as a solvent. In toluene and an electrophile scavengers again managed not to prove the crossed product.

In 1999 and 2000, respectively Lemal and Hahn were able to observe a balance between Dibenzotetraazafulvalen derivatives and its carbenes. This observation Böhm and Herrmann 2000 come to the conclusion that a Wanzlick balance between Tetraaminoethylen and the corresponding carbene exists, which was confirmed in 2010 by Kirmse.

In further studies, it was shown by Alder that unimpeded diaminocarbenes under acid catalysis as described by Lemal observed a dimer form. Excesses of acid form a dimeric salt.