Hydrogenation of biomass-derived levulinic acid to γ-valerolactone catalyzed by PNP-Ir pincer complexes: A computational study

Density functional theory (DFT) calculations were used to study the mechanisms for hydrogenation of biomass-derived levulinic acid (LA) to g-valerolactone (GVL) catalyzed by PNP-ligated (PNP = 2,6-bis(diter-butylphosphinomethyl) pyridine) iridium complexes, ((BuPNP)-Bu-t)IrH3, 1. The transformation proceeds dearomatization/rearomatization including four stages: hydride transfer to give LA-H- and dihydride iridium specie ((BuPNP)-Bu-t)IrH2+, methylene proton in the PNP ligand transfer to substrate to form gamma-hydroxyvaleric acid (HVA), catalyst regeneration via H-2 splitting, cyclization of HVA to GVL. Compared with the hydrogenation of CO2 catalyzed by the same complex, herein the rate-limiting step is the hydride transfer step rather than the methylene proton transfer. The calculations show that the hydride transfer step has a significantly higher barrier (21.1 vs 5.3 kcal mol(-1) for the hydrogenation of CO2). Besides, the cyclization is very easily achieved due to the low barrier. As an alternative pathway, different from the above dearomatization/rearomatization process, a base abstracts a proton from the dihydrogen ligand to form the trihydride iridium with a free energy barrier of 24.6 kcal mol(-1). Our results will help in the understanding of the mechanisms of hydrogenation of LA to GVL catalyzed by metal complexes. (C) 2015 Elsevier B.V. All rights reserved.