Biogenic zwitterionic ionic liquids of carnitine metabolites in catalytic fixation of carbon dioxide

The cycloaddition of carbon dioxide (CO₂) to epoxides (CCE) reaction is one of the few industrially viable routes to convert carbon dioxide into value-added chemicals. CCE reactions were indispensable of catalyst in which halide was dominant as the nucleophilic co-catalytic species, nevertheless, halide was a risk to steel process equipments and an environmental burden, thus the development of halide-free catalyst was highly desirable. We propose an array of biogenic zwitterionic ionic liquids of carnitine metabolites, a series of ω-trimethylammonio carboxylate type biomolecules, by exploiting their cationic quaternary ammonium and anionic carboxylate functional groups as the two catalytic active sites for CCE reactions. Among these catalysts, the natural product acetyl-carnitine (acCAR) exhibits optimal catalytic performance that transformed terminal epoxides to the corresponding cyclic carbonates by 80 to 99 % conversions with 99 % selectivity at 120 °C, CO₂ pressure of 1 MPa, and acCAR catalyst loading of 1 mol% in 6 h. Zwitterionic bifunctional activation mechanism was proposed and validated by ¹H and ¹³C NMR titrations. The methyl α–H of the quaternary ammonium acted as hydrogen bond donor to activate the epoxide, and the carboxylate interacted with CO₂ promoting the formation of acyl carbonate intermediate that attacked the epoxide. Zwitterionic ionic liquids of carnitine metabolites exemplified halide-free and biogenic organocatalyst for CO₂ fixation and wide range of bifunctional catalytic transformations.