Carbon dioxide utilization via carbonate-promoted C–H carboxylation

Molten salts at intermediate temperatures enable efficient carbonate-promoted carboxylation of very weakly acidic C–H bonds, revealing a new way to transform inedible biomass and carbon dioxide into valuable feedstock chemicals. The idea that the greenhouse gas carbon dioxide might be used as a source of feedstock chemicals is attractive but usually impractical — although it reacts readily with carbon-centred nucleophiles, generating the nucleophiles requires a high energy input. But now, inspired by the RuBisCO enzyme which catalyses carbon fixation in plants, Aanindeeta Banerjee et al. demonstrate that molten salts containing alkali metals at intermediate temperatures enable efficient carbonate-promoted carboxylation of very weakly acidic C–H bonds. The potential of this chemistry was illustrated by converting 2-furoic acid (readily made from inedible biomass) into the useful bio-based feedstock furan-2,5-dicarboxylic acid. Using carbon dioxide (CO2) as a feedstock for commodity synthesis is an attractive means of reducing greenhouse gas emissions and a possible stepping-stone towards renewable synthetic fuels1,2. A major impediment to synthesizing compounds from CO2 is the difficulty of forming carbon–carbon (C–C) bonds efficiently: although CO2 reacts readily with carbon-centred nucleophiles, generating these intermediates requires high-energy reagents (such as highly reducing metals or strong organic bases), carbon–heteroatom bonds or relatively acidic carbon–hydrogen (C–H) bonds3,4,5. These requirements negate the environmental benefit of using CO2 as a substrate and limit the chemistry to low-volume targets. Here we show that intermediate-temperature (200 to 350 degrees Celsius) molten salts containing caesium or potassium cations enable carbonate ions (CO32–) to deprotonate very weakly acidic C–H bonds (pKa > 40), generating carbon-centred nucleophiles that react with CO2 to form carboxylates. To illustrate a potential application, we use C–H carboxylation followed by protonation to convert 2-furoic acid into furan-2,5-dicarboxylic acid (FDCA)—a highly desirable bio-based feedstock6 with numerous applications, including the synthesis of polyethylene furandicarboxylate (PEF), which is a potential large-scale substitute for petroleum-derived polyethylene terephthalate (PET)7,8. Since 2-furoic acid can readily be made from lignocellulose9, CO32–-promoted C–H carboxylation thus reveals a way to transform inedible biomass and CO2 into a valuable feedstock chemical. Our results provide a new strategy for using CO2 in the synthesis of multi-carbon compounds.