Wanzlick equilibrium

The Wanzlick equilibrium refers to a proposed chemical balance between a relatively stable carbene compound and its dimer. Initially, Hans-Werner Wanzlick and E. Schikora (1960) suggested that carbenes derived from dihydroimidazol-2-ylidene existed in an unfavorable equilibrium with their dimers (tetraaminoethylenes), based on reactivity studies showing free carbenes reacting with electrophiles, while dimers remained inactive. However, David M. Lemal later challenged this hypothesis by demonstrating that heating tetraaminoethylene derivatives did not produce mixed dimers, proposing instead that acid-catalyzed reactions explained the observed behavior.

Lemal suggested that electrophiles converted tetraaminoethylenes into cationic species, which dissociated into free carbenes and salts. This mechanism allowed for either carbene re-dimerization or reaction with electrophiles, leading to the same product: dihydroimidazolium salts. Further investigations by Michael K. Denk (1999) confirmed the equilibrium under specific conditions, such as deuterated tetrahydrofuran as a solvent, but not in other solvents or with added quenchers.

Subsequent studies by Böhm & Herrmann (2000) and Kirmse (2010) supported the existence of the Wanzlick equilibrium for certain systems, particularly when acid catalysis was involved. Acid-catalyzed dimerization was shown to occur for unhindered diaminocarbenes, while dissociation of triaminoolefins required acid catalysis. These findings underscored that the equilibrium depends on specific conditions and catalysts.