Diastereomeric P,P*-mixed-donor phosphine-diamidophosphites with an oxalamide backbone as bridging ligands

Abstract

Two diastereomeric P,P*-phosphine-diamidophosphites featuring a 1,3,2-diazaphospholidine ring and an oxalamide scaffold were synthesized. These C₁-symmetric ligands are far more convenient for probing coordination by NMR than C₂-symmetric bisdiamidophosphite or bisphosphine analogs, because the chemical nonequivalence of their phosphorus nuclei gives rise to well-resolved spin-spin couplings in the NMR spectra of the resulting complexes. The NMR spectral data, in combination with DFT calculations, demonstrate the bridging, rather than chelating, mode of the “unnatural” diastereomer in Pd(II) and Rh(I) complexes. Both ligands were evaluated in Pd-catalyzed asymmetric allylic substitution, and in Rh-catalyzed asymmetric hydrogenation. The influence of ligands' chiral constituents on enantioselectivity was assessed and the effects of the ligand-to-metal ratio on complexation and catalytic performance were investigated.