Solvent-dependent decarboxylation of 1,3-dimethylimdazolium-2-carboxylate.

Abstract

1,3-Dimethylimidazolium-2-carboxylate 1 is stable in both pure water and pure acetonitrile. However, in mixtures of the two solvents, this compound suffers a rapid decarboxylation/protonaton reaction, forming 1,3-dimethylimidazolium cation 2. A series of kinetic and mechanistic experiments, along with DFT calculations, were carried out to understand the mechanism of this process and to elucidate the role of solvation on the stability of 1. These findings demonstrate that the decomposition process is a reversible decarboxylation forming the corresponding N-heterocyclic carbene (1,3-dimethylimidazolylidene, 3), followed by a rapid protonation of 3 by water or other protic species. The length and strength of the C-C bond between in the imidazolium ring and the carboxylate group (denoted C(NHC)-CO2) of 1 is remarkably dependent on the polarity of the solvent. Density functional theory (DFT) calculations predict a ca. 20 kcal/mol change in the barrier to decarboxylation in going from the gas phase to (SMD-simulated) water. Thus, addition of water has two effects on the stability of 1. At low concentrations, it provides a proton source for the trapping of the carbene 3 and accelerates decomposition. At higher concentrations, it increases the polarity of the medium. slowing the decarboxylation process and likewise the overall decomposition rate.