Imidazolium Hydrogen Carbonates versus Imidazolium Carboxylates as Organic Precatalysts for N?Heterocyclic Carbene Catalyzed Reactions

摘要

Imidazolium-2-carboxylates (NHC−CO2 adducts, 3) andn(benz)imidazolium hydrogen carbonates ([NHC(H)][HCO3], 4) werenindependently employed as organic precatalysts for various molecular Nheterocyclicncarbene (NHC) catalyzed reactions. NHC−CO2 adducts werenobtained by carboxylation in THF of related free NHCs (2), while thensynthesis of [NHC(H)][HCO3] precursors was directly achieved by anionnmetathesis of imidazolium halides (1) using potassium hydrogen carbonaten(KHCO3) in methanolic solution, without the need for the prior preparation ofnfree carbenes. Thermogravimetric analysis (TGA) and TGA coupled with massnspectrometry (TGA-MS) of most [NHC(H)][HCO3] precursors 4 showed a degradation profile in stages, with either anconcomitant or a stepwise release of H2O and CO2, between 108 and 280 °C, depending on the nature of the azolium andnsubstituents. In solution, NHC generation from both [NHC(H)][HCO3] salts and NHC−CO2 adducts could be achieved atnroom temperature, most likely by a simple solvation effect. Both types of precursors proved efficient for organocatalyzednmolecular reactions, including cyanosilylation, benzoin condensation, and transesterification reactions. The catalytic efficienciesnof NHC−CO2 adducts 3 were found to be approximately 3 times higher than those of their [NHC(H)][HCO3] counterparts 4.