Oxidative NHC Catalysis: An Unconventional Tool for Macrocyclic Oligoesters Synthesis

The application of N-heterocyclic carbene (NHC) catalysis under highly diluted oxidative condition to the polycondensation of dialdehydes and diols is herein presented as an alternative, atom-economical synthetic route to macrocyclic oligoesters (MCOs). The disclosed protocol paves the way to the straightforward access to MCOs, starting from commercial dialdehydes, avoiding the use of toxic diacyl chlorides, commonly employed in traditional MCOs synthetic processes. The method is totally metal-free, takes place in the green Me-THF solvent and requires the use of a fully recyclable quinone oxidant. The protocol versatility is confirmed by the employment of fossil-based and bio-based monomers such as 2,5-diformylfuran (DFF), 2,5-bis(hydroxymethyl)furan (BHMF), and isomannide, synthesizing a series of novel and known synthetically relevant macrocyclic oligoesters, fully characterized by NMR and MALDI-TOF MS analysis, with product yields (51-86 %) comparable to those obtained by traditional synthetic routes. Finally, to emphasize the synthetic relevance of the target macrocycles, an entropically-driven ring opening polymerization (ED-ROP) key study has been performed, optimizing the organocatalyzed synthesis of poly(2,5-furan-dimethylene 2,5 furandicarboxylate) (PBHMF) with number-average molecular weight up to 8200 g mol^-1 and 66 % isolated yield.