Transformation of bio-derived levulinic acid to gamma-valerolactone by cyclopentadienone ruthenium(0) catalyst precursors bearing simple supporting ligands

Hydrogenation of bio-based levulinic acid (LA) to γ-valerolactone (GVL) was carried out using new cyclopentadienone ruthenium(0) complexes as catalyst precursors. These new complexes were obtained by reaction of cyclopentadione ruthenium(0) with widely available amino, pyridyl and phosphino ligands (L1–L6) using bridging N–N, P–N, or monodentate P or N donor frameworks. The hydrogenation reactions proceeded efficiently using formic acid as a hydrogen source and a low catalyst loading (0.2 mol%) in a benign solvent. At 120°C, up to 1,882 TON and >99% LA conversions and selectivities to GVL were seen, and the catalysts could be recycled up to four times with consistent activity and selectivity. In situ NMR studies show that hydrogen gas was generated from formic acid decomposition with simultaneous release of carbon dioxide. Ru-hydride species have been detected, by ¹H NMR spectroscopy, and the structure of the active catalyst is proposed along with a plausible reaction pathway.