Wanzlick equilibrium

The Wanzlick equilibrium is a chemical equilibrium between a relatively stable carbene compound and its dimer. The equilibrium was proposed to apply to certain electron-rich alkenes, such as tetraminoethylenes, which have been called "carbene dimers." Such equilibria occur, but the mechanism does not proceed simply, but requires catalysts.

In 1960, H.-W. Wanzlick and E. Schikora proposed that carbenes derived from dihydroimidazol-2-ylidene were generated by vacuum pyrolysis of 2-trichloromethyl dihydroimidazole derivatives, with the loss of chloroform.

Wanzlick and Schikora believed that once prepared these carbenes existed in an unfavourable equilibrium with their corresponding dimers. This assertion was based on reactivity studies which they believed showed that the free carbene reacted with electrophiles (E-X). The dimer (a substituted tetraaminoethylene) was believed to be inactive to the electrophiles (E-X), and thought to merely act as a stable carbene reservoir.

Wanzlick’s hypothesis of a carbene-dimer equilibrium was tested by D. Lemal and others. Heating mixtures of tetraaminoethylene derivatives did not produce mixed dimers:

This result indicates that a 'carbene-dimer equilibrium' does not occur for these dihydroimidazol-2-ylidene derivatives.

Lemal proposed that Wanzlick's observations could be explained by acid-catalyzed reactions.

Lemal proposed that the electrophile converts the tetraaminoethylenes into cationic species. He proposed that this cation then dissociated into the free carbene plus the resultant salt. The free carbene could then either re-dimerise, regenerating the tetraaminoethylene starting material, or react with E-X (as Wanzlick originally predicted), with either route eventually giving the same reaction product, the dihydroimidazolium salt.

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