Intricacies of Cation–Anion Combinations in Imidazolium Salt-Catalyzed Cycloaddition of CO₂ Into Epoxides

Abstract

The cycloaddition of CO 2 into epoxides catalyzed by imidazolium and related salts continues to attract attention due to the industrial importance of the cyclic carbonate products. The mechanism of the imidazolium-catalyzed transformation has been proposed to require the participation of the acidic C2 proton. However, other simple salts without acidic protons, such as N,N,N,N-tetrabutylammonium chloride, are also efficient catalysts for the reaction. Hence, we decided to investigate the role of the ring protons of imidazolium salts in this reaction. To this end, we systematically studied the catalytic activity of a series of methylsubstituted imidazolium cations, in the presence of various halide anions, both by experiment and in silico. Our results demonstrate that, while stabilization of intermediates by C2, C4, or C5 protons in imidazolium salts takes place, it is the nucleophilicity of the anion that governs the overall activity, which is intimately related to the strength of the interactions between the cation and anion. Consequently, the reactivity of the halide anion strongly depends on the nature of the cation and cosolvents. This study completes the (known) mechanism and should facilitate the development of highly efficient catalysts.

Efficient transformation of CO2 to cyclic carbonates using bifunctional protic ionic liquids under mild conditions

“…11 One of the typical feasible routes is the synthesis of five-membered cyclic carbonates by cycloaddition of CO2 with epoxides. [12][13][14] As one of a few successfully industrial products that efficiently utilize CO2 as a carbon feedstock, cyclic carbonate can be wildly used as solvents in chemical processes, [15][16][17] electrolyte components in lithium batteries, 18 useful monomers for acyclic carbamates 19,20 and carbonates, 21 and intermediates in the production of fine chemicals, etc. 22,23 To date, various catalysts have been developed for the synthesis of useful cyclic carbonates from CO2 and epoxides, such as metal-based catalysts, [24][25][26][27][28][29][30][31][32][33][34] organocatalysts, [35][36][37][38][39][40][41][42][43][44] and ionic liquids etc.…”

Imidazolium‐Functionalized Ionic Hypercrosslinked Porous Polymers for Efficient Synthesis of Cyclic Carbonates from Simulated Flue Gas

“…Based on the structural character of the ionic HCPs present and previous mechanistic studies related to IL‐catalyzed cycloadditions of CO 2 with epoxides, the most probable reaction route is proposed in Figure S18 in the Supporting Information. First, the epoxide was activated through the interaction of the C2 proton in the imidazolium cation with the oxygen of the epoxide through a hydrogen bond, resulting in the polarization of the C−O bond.…”

Delineation of the Critical Parameters of Salt Catalysts in the N‐Formylation of Amines with CO2

“…[26,27] Hydrogen bonds play a key role in an umber of salt catalyzed CO 2 reactions such as the cycloaddition of CO 2 to epoxides, [28,29] which like the N-formylation reaction, is also predominantly driven by the anion. [30] In addition, NHCs were shown to be excellent catalysts for the N-formylation of amines with CO 2 and hydrosilanes. [31] However,n either H-bonding interactions nor NHCs seem to facilitate the reaction.…”