N-Heterocyclic Olefin-Carbon Dioxide and -Sulfur Dioxide Adducts: Structures and Interesting Reactivity Patterns

Depending on the amount of methanol present in solution, CO₂ adducts of N-heterocyclic carbenes (NHCs) and N-heterocyclic olefins (NHOs) have been found to be in fully reversible equilibrium with the corresponding methyl carbonate salts [EMIm][OCO₂ Me] and [EMMIm][OCO₂ Me]. The reactivity pattern of representative 1-ethyl-3-methyl-NHO-CO₂ adduct 4 has been investigated and compared with the corresponding NHC-CO₂ zwitterion: The protonation of 4 with HX led to the imidazolium salts [NHO-CO₂ H][X], which underwent decarboxylation to [EMMIm][X] in the presence of nucleophilic catalysts. NHO-CO₂ zwitterion 4 can act as an efficient carboxylating agent towards CH acids such as acetonitrile. The [EMMIm] cyanoacetate and [EMMIm]₂ cyanomalonate salts formed exemplify the first C-C bond-forming carboxylation reactions with NHO-activated CO₂ . The reaction of the free NHO with dimethyl carbonate selectively led to methoxycarbonylated NHO, which is a perfect precursor for the synthesis of functionalized ILs [NHO-CO₂ Me][X]. The first NHO-SO₂ adduct was synthesized and structurally characterized; it showed a similar reactivity pattern, which allowed the synthesis of imidazolium methyl sulfites upon reaction with methanol.