Synthesis and characterization of 1, 3-disubstituted imidazol(in)ium-2-carboxylates

The air-and water-stable 1, 3-disubstituted imidazol(in)ium-2-carboxylates(NHC-CO₂) as important precursors of N-heterocyclic carbenes were prepared by three routes. Nine kinds of NHC-CO₂ with different groups on nitrogen atoms were obtained. The structure was characterized by ¹H NMR, elemental analysis and IR. Thermogravimetric analysis(TG/DTG) provided the evidence for decarboxylation in NHC-CO₂. The results show that CO₂ escapes from the molecule in the process of elevating temperature and free carbenes are released. Two processes(decarboxylation and decomposition) occur when the substituents on the nitrogen atoms are aryl group, and the imidazolium-2-carboxylates and the imidazolinium-2-carboxylates show similar performance of decarboxylation. The initial temperatures of mass loss are lower when the substituents on the nitrogen atoms are alkyl group, and no decomposition occurs.