Chiral wide bite angle diphosphine ligands : synthesis, coordination chemistry and application in Pd-catalyzed allylic alkylation

Abstract

A series of diphosphine ligands bearing ester- and ether-modified diphenylether backbones have been prepared. The introduction of carboxylic acid or ether auxiliaries in the ortho-positions relative to the diphenylphosphine groups was achieved via straightforward four-step synthetic protocols, prior to introduction of the phosphines. The electronic properties of these backbone-modified DPEPhos ligands were evaluated by probing the relevant carbonyl stretching frequencies (νCO) of Ni(CO)2(PP) species (PP = diphosphine) using IR spectroscopy and by determining the phosphorus-selenium coupling constant JSe-P of phosphine selenide derivatives using 31P{1H} NMR spectroscopy. Also the X-ray structure for the bis(carbonyl)nickel(0) species with one of the ligands is reported. The [Pd(η3-allyl)(PP)]-complexes were characterized by multinuclear NMR spectroscopy and applied in the asymmetric allylic alkylation of l,3-diphenyl-2-propenyl acetate and cyclohex-2-enyl acetate with dimethyl malonate in order to benchmark their catalytic potential. The enantioselectivity (ranging from 3 to 70 %) was found to depend on the size of the chiral auxiliary introduced within the diphenyl ether backbone and its proximity to the phosphorus donor groups and hence to the active metal centre. These studies revealed that substituents on the backbone have only a minor effect on the electronic character of the diphenylphosphine groups.