Integration of a chiral phosphine ligand and ionic liquids: sustainable and functionally enhanced BINAP-based chiral Ru(ii) catalysts for enantioselective hydrogenation of β-keto esters

Abstract

Herein, a novel class of room temperature chiral phosphine-functionalized polyether imidazolium ionic liquids (RTCP-PolyIMILs) was synthesized for the first time by an ion exchange reaction between the 5,5′-disulfonato-(S)-BINAP ligand and polyether imidazolium ILs. The interactions with the polyether imidazolium transformed the solid sulfonated BINAP with a high melting point into a room temperature IL, which can act as a phosphine ligand and a carrier. Using the attractive dual functionality of RTCP-PolyIMILs, an efficient and sustainable homogeneous catalysis and biphasic separation system (HCBS) was constructed for Ru(II)-catalyzed enantioselective hydrogenation of β-keto esters to optically active β-hydroxy esters. Only a catalytic amount of RTCP-PolyIMIL was needed in the absence of any external carrier ILs, allowing the easy recovery and recycling of the chiral Ru catalyst. The Ru(II)-RTCP-PolyIMIL catalyst showed higher activity and better adaptability to a wide range of solvents than the traditional Ru(II)-BINAP catalyst. Specifically, a TOF value of up to 3900 h⁻¹, an enantioselectivity of up to 99%, long-term stability up to a total TON value of nearly 17 000, an extremely low Ru loss rate of only 0.08–0.09%, and good universality for structurally diverse β-keto esters with different electronic and steric properties were demonstrated. These dramatic enhancing effects can be attributed to the structure of RTCP-PolyIMILs integrated with a phosphine ligand and polyether ILs, where polyether-induced clustering blocking-up and polarity enhancement effects played critical roles.