New chiral tetradentate oxazolinylphosphine ligands for nickel and palladium.: Coordination behavior and catalytic activity in allylic alkylations

Polydentate NPPN ligands containing nitrogen and chiral phosphorus atoms have been synthesized (A-D), starting from diphosphines (dppe and dppp) and 2-(3',4'-dihydro-4'-R-2'-oxazolyl)-1-chlorobenzene (R = ethyl, isopropyl). Their coordination behavior has been studied with nickel and palladium precursors, giving mono- and bimetallic complexes depending on the reaction conditions. For the monometallic compounds, the NPPN ligands act as tetra- or tricoordinate groups (Ni-a-d and Pd-a,b, respectively), while for the bimetallic palladium complexes, the ligand bridges two metallic atoms (1a, 3a-c, 4a-c, 5a-c, 6a-d, 7a, 8d) in an N,P bis-bidentate coordination. The X-ray crystal structure of bis(eta(3)-2-methylallyl) [mu-(4'R)-phenyl(2-(3',4'-dihydro-4'-ethyl-2'-oxazolyl))phenylphospshinoethane] dipalladium(II) hexafluorophosphate (RSRR-7a) is described. Acyl complexes 4a-c were obtained from the methyl compounds 3a-c by insertion of carbon monoxide into Pd-CH3 bond, at high pressure and room temperature. However, these compounds did not undergo insertion of norbornene or norbornadiene into the Pd-COCH3 bonds. Ionic methyl complexes 5a-c reacted faster than the neutral 3a-c toward the insertion of CO, but these ionic acetyl compounds decarbonylated easily, even in the solid state. The activity of the allylic complexes (6a-d) in palladium-catalyzed allylic alkylation of rac-3-acetoxy-1,3-diphenyl-1-propene with dimethyl malonate was tested, affording enantiomeric excesses up to 90%. The enantioselectivity of these catalytic systems exhibits a strong dependence on the Pd/NPPN ratio, because of the presence of different coordination modes.