An NMR study of the solution conformations of di(tertiary phosphine) complexes of orthopalladated 1-phenyl-1-(N,N-dimethylamino)ethane

Abstract

NOE intensities derived primarily from 2-D NMR ROESY spectra were used to study the solution conformations of the chelate rings of the (R)- and (S)-[1-phenyl-1-(N,N-dimethylamino)ethane][(2R,3R)-bis(diphenylphosphino)butane]palladium(II) cations, 3 and 4. In each, the diphosphine ring adopts a conformation with both N-methyl groups equatorial owing to potential unfavourable trans-annular methyl–phenyl interactions in the alternative conformation with axial methyl groups. For the orthometallated chelate ring the population of the conformer with C-methyl equatorial is less than 10% in 4, but is close to 55% in 3. This contrasting behaviour is attributed to inter-annular steric interactions between the N-methyl and the P-phenyl groups which also lead to a weakening of the Pd–N bond in 3 and can account for the enantioselectivity of the orthopalladated 1-phenyl-1-(N,N-dimethylamino)ethane moiety when used as a resolving agent.