Metallacrown Ether Catalysts Containing Phosphine–Phosphite Polyether Ligands for Rh‐Catalyzed Asymmetric Hydrogenation – Enhancements in Activity and Enantioselectivity

Abstract

AbstractA new class of tunable metallacrown ether rhodium catalysts based on α,ω‐(phosphine–phosphite) polyether ligands were prepared either by a template‐induced method or by a nontemplate procedure. For the asymmetric hydrogenation of α‐arylenamides with the addition of K+ cations, the ortho‐diphenylphosphine‐substituted metallacrown ether catalyst showed high enantioselectivities (up to 99 % ee), which are comparable to or better than the results obtained for phosphine–phosphite ligands with two or more chiral elements. Remarkable enhancements in enantioselectivity and noticeably increased catalytic activity were achieved through the supramolecular recognition between K+ cations and the metallacrown ether catalyst.