Selective Conversion of 5‐Hydroxymethylfuraldehyde Using Cp*Ir Catalysts in Aqueous Formate Buffer Solution

The highly selective hydrogenation/hydrolytic ring‐opening reaction of 5‐hydroxymethylfuraldehyde (5‐HMF) was catalyzed by homogeneous Cp*Ir III half‐sandwich complexes to produce 1‐hydroxy‐2,5‐hexanedione (HHD). Adjustment of pH was found to regulate the distribution of products and reaction selectivity, and full conversion of 5‐HMF to HHD with 99 % selectivity was achieved at pH 2.5. A mechanistic study revealed that the hydrolysis/ring‐opening reaction of 2,5‐bis‐(hydroxymethyl)furan is the important intermediate reaction step. In addition, an isolated yield of 85 % for HHD was obtained in a 10 g‐scale experiment, and the reaction with fructose as the starting material also led to a 98 % GC yield (71.9 % to fructose) of HHD owing to the excellent tolerance of the catalyst under acidic conditions.