Phase Behavior That Enables Solvent-Free Carbonate-Promoted Furoate Carboxylation

Enhanced carboxylation of furoic salt with CO2 by ZnCl2 coordination for efficient production of 2,5-furandicarboxylic acid

C-H carboxylation of furoic acid (FA) with CO2 is an atom-efficient strategy to produce 2,5-furandicarboxylic acid (2,5-FDCA) from lignocellulose. The existing carbonate-promoted CO2 carboxylation processes rely on the use of large amounts of expensive Cs2CO3 as a deprotonating reagent and molten salt. Substitution of Cs with other cheap and abundant alkali ions (such as K and Na) can reduce the use of Cs, but it faces the problem of a low yield of 2,5-FDCA. This study found that the addition of catalytic amounts of ZnCl2 as a Lewis acid can increase the yield of 2,5-FDCA in the CO2 carboxylation reaction of Na/K-FA in a molten salt reaction system. 1H NMR analysis and DFT calculations confirmed that ZnCl2 coordinates with the furan ring through electron transfer from the conjugated furan ring to Zn2+, thereby activating the H at the C5 position of Na/K-FA. This coordination lengthened the C5-H bond and lowered its heterolytic dissociation energy, making it more susceptible to being deprotonated by CO32- and subsequently carboxylated by CO2. The developed Lewis acid coordination strategy provides a new idea for the efficient construction of C-C bonds between CO2 and aromatics through carbonate-promoted C-H carboxylation.

Carbonate-promoted C-H carboxylation of electron-rich heteroarenes

C-H carboxylation is an attractive transformation for both streamlining synthesis and valorizing CO₂. The high bond strength and very low acidity of most C-H bonds, as well as the low reactivity of CO₂, present fundamental challenges for this chemistry. Conventional methods for carboxylation of electron-rich heteroarenes require very strong organic bases to effect C-H deprotonation. Here we show that alkali carbonates (M₂CO₃) dispersed in mesoporous TiO₂ supports (M₂CO₃/TiO₂) effect CO₃ ^2--promoted C-H carboxylation of thiophene- and indole-based heteroarenes in gas-solid reactions at 200-320 °C. M₂CO₃/TiO₂ materials are strong bases in this temperature regime, which enables deprotonation of very weakly acidic bonds in these substrates to generate reactive carbanions. In addition, we show that M₂CO₃/TiO₂ enables C3 carboxylation of indole substrates via an apparent electrophilic aromatic substitution mechanism. No carboxylations take place when M₂CO₃/TiO₂ is replaced with un-supported M₂CO₃, demonstrating the critical role of carbonate dispersion and disruption of the M₂CO₃ lattice. After carboxylation, treatment of the support-bound carboxylate products with dimethyl carbonate affords isolable esters and the M₂CO₃/TiO₂ material can be regenerated upon heating under vacuum. Our results provide the basis for a closed cycle for the esterification of heteroarenes with CO₂ and dimethyl carbonate.