5,5'(Oxy-bis(methylene)bis-2-furfural (OBMF) from 5-hydroxymethyl-2-furfural (HMF): a systematic study for the synthesis of a new platform molecule from renewable substances

The continued exploitation and depletion of fossil fuels has prompted the scientific community to search for more sustainable and environmentally friendly alternatives. In the last decade, the synthesis of biomass-derived chemicals has become a priority to boost the transition from refinery to biorefinery. Sugars are an extremely abundant bio-resource in nature; even today, one of the most studied reactions is the synthesis of 5-hydroxymethyl-2-furfural (HMF). This compound is considered extremely important for biorefinery because of its wide range of possible applications (pharmaceutical, biofuels, polymer precursors, surfactants). However, it has been observed, during the spontaneous degenerative process of HMF, the formation of a compound that could be equally important 5,5'-[oxybis(methylene)]bis-2-furfural (OBMF). The synthesis of OBMF is scarcely reported in the literature, only in recent years interest in this dimer of HMF has emerged for its possible applications in industry. Good yield values of OBMF are reported in the literature from HMF (Figure 1) in the presence of an acid catalyst; however, the solvents used are the most common halogenated and/or aromatic solvents, known to be toxic. The objective of this work was to find a viable synthetic route to access OBMF without having to resort to the use of such solvents and, in addition, utilize already commercially available and inexpensive acid catalysts. Through smallscale optimizations, the best solvent was found to be dimethyl carbonate;4 In addition, two heterogeneous acid catalysts - Purolite 269 and ferric sulfate (Fe2(SO4)3) - showed excellent efficiency in promoting the HMF etherification reaction with quantitative yields (> 90%). Subsequently, a scale-up of the reaction was carried out, obtaining OBMF with an isolated yield of 81%. Given the excellent results obtained, this work can be a starting point to undertake the study of new synthetic methodologies for this molecule such as continuous flow reactions of which the literature is lacking.