A Systematic Investigation of Factors Influencing the Decarboxylation of Imidazolium Carboxylates

Abstract

A series of 1,3-disubstituted-2-imidazolium carboxylates, an adduct of CO(2) and N-heterocyclic carbenes, were synthesized and characterized using single crystal X-ray, thermogravimetric, IR, and NMR analysis. The TGA analysis of the NHC-CO(2)'s shows that as steric bulk on the N-substituent increases, the ability of the NHC-CO(2) to decarboxylate increases. The comparison of NHC-CO(2)'s with and without methyls at the 4,5-position indicate that extra electron density in the imidazolium ring enhances the stability of an NHC-CO(2) thereby making it less prone to decarboxylation. Single crystal X-ray analysis shows that the torsional angle of the carboxylate group and the C-CO(2) 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 I(t)BuPrCO(2) (2f) indicate that the C-CO(2) 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-CO(2) bond may be involved in the bond breaking event to release CO(2).

Organic reactions for the electrochemical and photochemical production of chemical fuels from CO2 – The reduction chemistry of carboxylic acids and derivatives as bent CO2 surrogates

“…Louie and coworkers have now shown that the strength of binding CO 2 to the NHC fragment is tunable in a series of decarboxylation and transcarboxylation studies [33,34]. Thermal stability studies have yielded a series of data shown in Fig.…”

“…In Scheme 7, such binding is demonstrated to occur with a phosphine Lewis Base and a borane Lewis Acid. Catalytic reductions of CO 2 to methanolic species have been shown with the help of phosphine-borane (33) binding and catalysis [60,61] with a borane terminal reducing agent (32).…”

Heterogenously catalyzed one-step alkylation and carboxylation of N-methylimidazole with dimethyl carbonate in continuous flow

“…For the catalytic reaction, a different reaction pathway can be assumed similar to the proposed mechanism in literature for the formation of isolable NHC [22,29,30] . For a catalyst with much lower basicity, e.g., as for the Al 2 O 3 used here, carbene formation was not observed.…”

“…The alkaline catalyst forms a stable N-heterocyclic carbene (NHC) ( 4 ) which can be isolated and purifi ed for further use (compare Scheme 2 ) [28,29] .…”

Self-Assembly Fabrication of Hollow Mesoporous Silica@Co–Al Layered Double Hydroxide@Graphene and Application in Toxic Effluents Elimination

“…1630 to 1690 cm −1 depending on the N-substituents. 26,27 The 13 C MAS NMR spectrum of 2 shows a single set of resonances assignable to the ligand resonances. Most notably, two resonances at 156.1 and 145.0 ppm are assigned to the carbons of the NHC-coordinated CO 2 and the azolium C 2 respectively.…”