Catalytic transformation of bio-derived furans to valuable ketoacids and diketones by water-soluble ruthenium catalysts†
Bio-derived furans such as 2-furfural (furfural), 5-hydroxymethyl-2-furfural (5-HMF) and 5-methyl-2-furfural (5-MF) were successfully transformed to a ketoacid, levulinic acid (LA), and diketones, 1-hydroxyhexane-2,5-dione (1-HHD), 3-hydroxyhexane-2,5-dione (3-HHD) and hexane-2,5-dione (HD), under moderate reaction conditions using water soluble and recyclable 8-aminoquinoline coordinated arene–ruthenium(II) complexes. Under the optimized reaction conditions using 1 mol% catalyst in the presence of 12 equivalents of formic acid at 80–100 °C, complete conversion of furfural to LA with high selectivity was achieved. Several experiments along with 1H NMR spectral studies are described which provide more insights into the mechanism underlying the transformation of furans to open ring components. Experiments performed using structural analogues of the active catalyst inferred a structure–activity relationship for the observed superior catalytic activity of the 8-aminoquinoline coordinated arene–ruthenium(II) complex. Furthermore, due to the high aqueous solubility of the studied complexes, high recyclability, up to 4 catalytic runs, was achieved without any significant loss of activity. Molecular identities of the studied 8-aminoquinoline coordinated arene–ruthenium(II) complex were also confirmed using single-crystal X-ray diffraction studies.