Chiral Wide-Bite-Angle Diphosphine Ligands: Synthesis, Coordination Chemistry, and Application in Pd-Catalyzed Allylic Alkylation

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 center.