Selective Oxidation of 5-(Hydroxymethyl)furfural to Furan-2,5-dicarbaldehyde with Sodium Nitrite in Phosphoric Acid

5-(Hydroxymethyl)furfural is readily oxidized to furan-2,5-dicarbaldehyde with sodium nitrite in phosphoric acid at room temperature (25°C). The best selectivity for furan-2,5-dicarbaldehyde is achieved using 2.5 equiv of sodium nitrite with respect to the substrate. In this case, nearly quantitative yield of furan- 2,5-dicarbaldehyde is attained in 1 h, the substrate conversion being complete. Due to high selectivity in combination with low cost and toxicity of phosphoric acid, the system NaNO 2 –H 3 PO 4 is more convenient than NaNO 2 –CF 3 COOH for the preparative synthesis of furan-2,5-dicarbaldehyde.