Proton‐Promoted Nickel‐Catalyzed Asymmetric Hydrogenation of Aliphatic Ketoacids.

A robust and highly active homogeneous chiral nickel–phosphine complex for the asymmetric hydrogenation of aliphatic γ‐ and δ‐ketoacids has been discovered. The hydrogenation could proceed smoothly in the presence of 0.0133 mol% catalyst loading (S/C=7500). The coordination chemistry and catalytic behavior of Ni(OTf)2 with (S,S)‐Ph‐BPE were explored by 1H NMR and HRMS. The mechanistic studies revealed that a proton promoted the activation of the substrate C=O bond and controlled the stereoselectivity through hydrogen bonds. A series of chiral γ‐ and δ‐alkyl substituted lactones were obtained in high yields with excellent enantioselectivities (up to 98 % yield and 99 % ee). In addition, this catalytic system also demonstrated that levulinic acid produced from a biomass feedstock was converted into chiral γ‐valerolactone without loss of ee value. [ABSTRACT FROM AUTHOR]