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

Abstract

Solvent-free chemistry has been used to streamline synthesis, reduce waste, and access novel reactivity, but the physical nature of the reaction medium in the absence of solvent is often poorly understood. Here we reveal the phase behavior that enables the solvent-free carboxylation reaction in which carbonate, furan-2-carboxylate (furoate), and CO2 react to form furan-2,5-dicarboxylate (FDCA 2-). This transformation has no solution-phase analog and can be applied to convert lignocellulose into performance-advantaged plastics. Using operando powder X-ray diffraction and thermal analysis to elucidate the temperature-and conversion-dependent phase composition, we find that the reaction medium is a heterogeneous mixture of a ternary eutectic molten phase, solid Cs2CO3, and solid Cs2FDCA. During the reaction, the molten phase and solid Cs2CO3 diminish as solid Cs2FDCA accumulates. These insights are critical for increasing the scale of furoate carboxylation and provide a framework to guide the development of other solvent-free transformations. Main Text Solvent-free reactions have been investigated primarily as a means to improve the practicality of conventional solution-phase reactions by reducing waste, accelerating the rate, or simplifying purification. 1-9 The unusual properties of a reaction medium composed of neat reactants also raise the possibility of accessing reactivity that is very difficult or impossible to recapitulate in solution. To this end, we recently showed that carbonate (CO3 2-) can promote C-H carboxylation of very weakly acidic aryl C-H bonds (pKa > 35 in organic solvent) in carefully selected solvent-free alkali salts at elevated temperature. 10-13 By contrast, even under forcing conditions in organic solvent, CO3 2has been found to be incapable of promoting carboxylation of C-H bonds with pKa > 27. 14 The ability to use a simple, regenerable base such as CO3 2is essential if carboxylation is to be applied to scalable chemical synthesis. Solvent-free alkali salts unlock this possibility for some targets, but a better understanding of the reaction medium is needed to realize this potential.

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

“…Adding cheap and abundant K to completely or partially replace Cs can alleviate the dependence on Cs, but results in a lower yield of 2,5-FDCA due to insufficient C-H abstraction ability. 29 In order to improve the yield of 2,5-FDCA in the K-containing molten salt reaction system, the reaction conditions were first optimized by changing the reaction temperature, reaction time, CO 2 pressure and the K-FA/ Cs 2 CO 3 ratio. As shown in Fig.…”

“…Since M-FA conversion and 2,5-FDCA yield are highly sensitive to the melting temperature of the reaction mixture, 29 the effect of reaction temperature and time on FA conversion and 2,5-FDCA yield was studied for the K-FA/Cs 2 CO 3 reaction system in the presence of ZnCl 2 (Fig. 4c & d).…”

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

“…[29][30][31][32] This transformation is particularly useful for converting a monocarboxylate substrate into a dicarboxylate product, where the substrate enables the formation of a molten reaction medium. 33 More recently, we demonstrated that M 2 CO 3 dispersed into mesoporous TiO 2 (M 2 CO 3 /TiO 2 , Fig. 1a) promotes the carboxylation of benzene and other aromatic hydrocarbon C-H bonds in gas-solid reactions (Scheme 1b).…”

Improving Carbonate-Promoted C–H Carboxylation Using Mesoporous Carbon Supports

“…We previously showed that alkali carbonates can serve as base promoters for high-p K a C–H bonds when the reactions are performed in solvent-free alkali carboxylate media that form molten phases at intermediate temperatures. , This chemistry is particularly valuable for transforming monocarboxylates into dicarboxylates, as demonstrated by its use for the synthesis of furan-2,5-dicarboxylic acid . More recently, we showed that alkali carbonates dispersed in mesoporous support materials can perform C–H carboxylation of aromatic hydrocarbons and heteroarenes in gas–solid reactions without using an alkali carboxylate molten salt. , Dispersion of M 2 CO 3 in mesopores results in an amorphous, high surface area carbonate that deprotonates C–H bonds of gaseous reactants in the presence of CO 2 , generating putative carbanion intermediates that react with CO 2 to form carboxylates.…”