Solvent-driven isomerization of muconates in DMSO: reaction mechanism and process sustainability

Abstract

The production of renewable chemicals and monomers is fundamental for transitioning to a future circular economy. Currently, cis,cis-muconic acid (ccMA) is a bio-sourced platform chemical with great potential for added-value chemicals, monomers, and specialty polymers. Among the three isomers, the trans,trans (tt-isomer) stands out due to its reactivity for polymerization and unique ability as a substrate for the Diels–Alder cycloaddition reaction. Whereas earlier research has focused on producing this isomer, the most promising solvent-driven isomerization in DMSO-containing water yields moderate ttMA due to a competitive ring-closing lactonization reaction, especially in highly concentrated systems. This work highlights the unique ability of DMSO, among several other solvents, to produce ttMA. In addition, we report the effect of the acidity of the initial MA concentration and the amount of water on the lactonization reaction. Control of reaction conditions and use of muconates (diethyl muconates = DEM) countered the competitive lactonization, reaching >90% tt-isomer selectivity. The involvement of water and DMSO in the isomerization mechanism was investigated in detail by probing the reaction mechanism with in situ NMR. Identifying the reaction products and several intermediates led us to propose a plausible mechanism. Based on this knowledge, condition optimization led to a significant thirty-fold ttDEM productivity improvement, viz. from 10 to 328 mM h⁻¹. The DEM can be isolated almost quantitatively from the DMSO solvent system by extraction.