Imidazolium cations, tautomerism

Up to the present the principal interest in heteroaromatic tautomeric systems has been the determination of the position of equilibrium, although methods for studying fast proton-transfer reactions (e.g., fluorescence spectroscopy and proton resonance ) are now becoming available, and more interest is being shown in reactions of this type (see, e.g., references 21 and 22 and the references therein). Thus, the reactions of the imidazolium cation and imidazole with hydroxyl and hydrogen ions, respectively, have recently been demonstrated to be diffusion controlled. ... 

Resonance and tautomerism are closely related. Thus the acidity of carbon-bound hydrogen in ketones, which allows formation of enol tautomers, is a direct result of the fact that the enolate anion produced by dissociation of one of these hydrogen atoms is stabilized by resonance. Similarly, tautomerism in the imidazole group of histidines is related to resonance in the imidazolium cations. 

Several examples of imino-functionalized carbene complexes of palladium have been described. " The imidazolium salt 97 was prepared and coordinated to Pd by an Ag-transfer reaction. Tautomerization of the carbene-imine bond occurs during the transfer reaction to give the the enamine complex 98. Neutral and cationic Pd complexes of a second closely related chelating iminocarbene ligand, 99, were also prepared via an Ag-transfer route. " In this case, no tautomerization was reported. However, evidence of solvent-dependent hemilability was observed. 

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