Synthesis and Reactivity of Chiral, Wide‐Bite‐Angle, Hybrid Diphosphorus Ligands

Abstract

AbstractEffective and modular synthetic approaches toward phosphine–phosphite ligands and phosphine–phosphonite ligands featuring a diphenyl ether backbone have been developed. The phosphine–phosphite ligands are obtained by a two‐step protocol from 2‐bromo‐2′‐methoxydiphenyl ether. The phosphine–phosphonite ligands are prepared in a four‐step synthetic protocol that involves a novel, unsymmetrical diphenyl ether derived phosphine–phosphorusdiamide as key building block. Structural studies on PtII complexes with either phosphine–phosphite or phosphine–phosphonite ligands indicate strict cis coordination for these ligand systems. High‐pressure NMR spectroscopy studies of Rh complexes under syngas indicate the presence of two ea isomers for Rh(H)(CO)2(PP). The existence of this mixture is further supported by high‐pressure IR spectroscopy studies. In order to benchmark the activity and selectivity of these novel, wide‐bite‐angle, mixed‐donor ligands, they were screened in Pd‐catalyzed asymmetric allylic alkylation as well as Rh‐catalyzed hydrogenation and hydroformylation reactions. The ligands give 100 % conversion and low‐to‐moderate enantioselectivity in the allylic alkylation of 1,3‐diphenyl‐2‐propenyl acetate and cyclohexyl‐2‐enyl acetate with dimethyl malonate. In the hydroformylation of styrene, good conversion and regioselectivities are achieved but only moderate enantioselectivity. The ligands give good conversions in asymmetric hydrogenation of typical substrates, with good‐to‐excellent enantioselectivities of up to 97 % depending on the substrate.