| Description |
A series of 1,3-disubstituted-2-imidazolium carboxylates, an adduct of CO2 and N-heterocyclic carbenes, was synthesized and characterized using single crystal X-ray, thermogravimetric, IR, and NMR analysis. The TGA analysis of the imidazolium carboxylates shows that as steric bulk on the N-substituent increases, the ability of the NHC-CO2 to decarboxylate increases. Single crystal X-ray analysis shows that the torsional angle of the carboxylate group and the C-CO2 bond length with respect to the imidazolium ring is dependent on the steric bulk of the N-substituent. Rotamers in the unit cell of a single crystal of ItBuPrCO2 (2f) indicate that the C-CO2 bond length increases as the N-substituents rotate toward the carboxylate moiety, which suggests that rotation of the N-substituents through the plane of the C-CO2 bond may be involved in the bond breaking event to release CO2. Combination of N,N-bis(2,6-diisopropylphenyl)imidazolum-2-carboxylate (IPrCO2) with the Lewis acids MBPh4, where M=Li or Na, provided two separate complexes, a monomer and dimer, respectively. Combination of N,N-bis(2,4,6- trimethyllphenyl)imidazolum-2-carboxylate (IMesCO2) with LiBPh4 afforded a dimeric species that was similar in global structure to that of the IPrCO2+NaBPh4 dimer. Thermogravimetric analysis of the crystals demonstrated that decarboxylation occurred at lower temperatures than the decarboxylation temperature of the parent NHC·CO2. Kinetic analysis of the transcarboxylation of IPrCO2 to acetophenone with NaBPh4 to yield sodium benzoylacetate was performed. First order dependencies were observed for IPrCO2 and acetophenone while zero order dependence was observed for NaBPh4. Direct dicarboxylation of MeCN was observed after combination with ItBuCO2 in the absence of salts. An imidazolium salt, 1-2,4,6-trimethylphenyl-3-lactamidebenzylimidazolium tosylate was synthesized and characterized by 1H NMR, 13C NMR, and single crystal xray analysis. The downfield shift of the amide protons in the 1H NMR spectrum salt relative to the starting material lactamide-OTs indicate that hydrogen bonding was present in solution. The crystal structure of imidazolium tosylate showed a series of intermolecular hydrogen bonds between amide groups and tosylate anions. The amount of hydrogen bonding present in the carbene precursor is promising for attempts to observe intramolecular hydrogen bonding with a carbene and the carboxylate. |
