Solvent-promoted and acid- controlled selectivity oxidation of 5-hydroxy-methylfurfural over metal-free TEMPO catalyst

Achieving efficient metal-free synthesis of 2,5-diformylfuran and 2,5-furandicarboxylic acid is always a challenge. In this manuscript, the selective oxidation of 5-hydroxymethylfurfural was achieved by using TEMPO system in t-BuOH solvent with the suitable acid promoters. And t-BuOH was considered to be a suitable solvent due to its excellent stability, oxidation promotion and solubility for intermediates. Remarkably, 90% and 83% yield of DFF and FDCA can be obtained with acetic acid and phosphoric acid, respectively. Additionally, the role of acid additives was discussed in detail and the hydration degree of aldehyde intermediates was considered to be key to controlling the selectivity of this reaction. The isolated yield of 2,5-diformylfuran and 2,5-furandicarboxylic acid were 82% and 78% through the scale-up oxidation, respectively. This approach may be a promising alternative to traditional oxidations for the synthesis of carboxylic acids.