Imidazolium hydrogen carbonates versus imidazolium carboxylates as organic precatalysts for N-heterocyclic carbene catalyzed reactions
Résumé
Imidazolium-2-carboxylates (NHC-CO2 adducts, 3) and (benz)imidazolium hydrogen carbonates ([NHC(H)][HCO3], 4) were independently employed as organic precatalysts for various molecular N-heterocyclic carbene (NHC) catalyzed reactions. NHC-CO2 adducts were obtained by carboxylation in THF of related free NHCs (2), while the synthesis of [NHC(H)][HCO3] precursors was directly achieved by anion metathesis of imidazolium halides (1) using potassium hydrogen carbonate (KHCO3) in methanolic solution, without the need for the prior preparation of free carbenes. Thermogravimetric analysis (TGA) and TGA coupled with mass spectrometry (TGA-MS) of most [NHC(H)][HCO3] precursors 4 showed a degradation profile in stages, with either a concomitant or a stepwise release of H2O and CO2, between 108 and 280 °C, depending on the nature of the azolium and substituents. In solution, NHC generation from both [NHC(H)][HCO3] salts and NHC-CO2 adducts could be achieved at room temperature, most likely by a simple solvation effect. Both types of precursors proved efficient for organocatalyzed molecular reactions, including cyanosilylation, benzoin condensation, and transesterification reactions. The catalytic efficiencies of NHC-CO2 adducts 3 were found to be approximately 3 times higher than those of their [NHC(H)][HCO3] counterparts 4. © 2012 American Chemical Society.