5-hydroxymethylfurfural (HMF) is a six-carbon furan ring obtained from the triple dehydration of hexose sugars. 5-hydroxymethylfurfural is primarily considered a starting material for other chemicals with important applications, such as monomers for plastics, solvents, or fuels. It has aldehyde and hydroxyl groups that allow it to be converted into a number of value added compounds via feasible transformations including reduction and oxidation. This compound can be gradually oxidized into 2,5-diformylfuran (DFF) and can be reduced to 2,5-dihydroxymethylfuran (DHMF). The two resulting compounds are considered versatile precursors for the production of functional polymers as well as other chemicals and pharmaceuticals.
Unfortunately, the ease of over-reduction and over-oxidation of 5-hydroxymethylfurfural, its selective conversion to 2,5-diformylfuran and 2,5-dihydroxymethylfuran, is still a challenging reaction that entails several by-products. In fact, only a few studies have focused on its conversion with external oxidants or reductants.
Meerwein–Ponndorf–Verley–Oppenauer reaction provides an effective way for selective reduction of aldehyde groups and selective oxidation of hydroxyl groups. Fortunately, both aldehyde and hydroxyl groups exist in 5-hydroxymethylfurfural. For this reason, the intermolecular hydrogen transfer between two 5-hydroxymethylfurfural molecules can occur via a Meerwein–Ponndorf–Verley–Oppenauer reaction. Thus, the conversion of 5-hydroxymethylfurfural to the two compounds is possible without the use of external redox reagent.
Fudan University researchers demonstrated for the first time the simultaneous synthesis of 2,5-diformylfuran and 2,5-dihydroxymethylfuran via a Meerwein–Ponndorf–Verley–Oppenauer reaction implementing 5-hydroxymethylfurfural as an oxidant and as a reductant. 5-hydroxymethylfurfural was selectively converted to the two compounds with 1:1 molar ratio through the special reaction, and a conversion efficiency of 44% was achieved under high initial 5-hydroxymethylfurfural concentration. Their work is published in peer-reviewed journal, ChemSuschem.
The conversion of 5-hydroxymethylfurfural was performed in a sealed vial under microwave irradiation. ALMe3, which is a conventional Lewis-acid catalyst, was selected as a catalyst to exhibit the feasibility of the proposed reaction. The reaction temperature was kept in the range of 60-120 °C for about 10-60 minutes. The researchers analyzed the products by using gas chromatography.
Quantifications of 5-hydroxymethylfurfural, 2,5-diformylfuran, and 2,5-dihydroxymethylfuran were extracted from the response factors and peak areas. They screened varying solvents to confirm the conversion of 5-hydroxymethylfurfural and the molar ratio of the resulting compounds reference to the fact that various solvents have varying properties that can affect the performance of the catalyst.
When the authors used dimethyl sulfoxide and N,N-dimethylformamide the two elements for dehydration of carbonates and 5-hydroxymethylfurfural oxidation, 5-hydroxymethylfurfural conversion was low. 2,5-diformylfuran was the only compound detected in the two systems. This was reference to the presence of an acyl group. This acyl group replaced the Lewis acid site coordinating to one of the 5-hydroxymethylfurfural molecules needed for the Meerwein–Ponndorf–Verley–Oppenauer reaction.
The use of tetrahydrofuran and dioxane as solvents led to low 5-hydroxymethylfurfural conversion. Fortunately, a higher conversion rate of 21.2% was recorded when the authors used acetonitrile that was caused by its high dielectric constant as opposed to the tetrahydrofuran and dioxane. The molar ratio of the resulting compounds was close to 1.0 in the acetonitrile. This implied that both 2,5-diformylfuran and 2,5-dihydroxymethylfuran were generated from the 5-hydroxymethylfurfural in almost equal proportions by using acetonitrile as a solvent and a Lewis-acid catalyst without using an external reductant and oxidant.
Gang Li, Zhen Sun, Yueer Yan, Yahong Zhang, and Yi Tang. Direct Transformation of HMF into 2,5-Diformylfuran and 2,5-Dihydroxymethylfuran without an External Oxidant or Reductant. ChemSusChem 2017, 10, 494 – 498.Go To ChemSusChem