Persistent N-Chirality as the Only Source of Asymmetry in Nonracemic N2PdCl2 Complexes

Abstract

The combination of prochiral 1,2-diamines, Pd(OAc)2, and (R)-K2-3,3‘-Me2BINOL leads to enantio- and diastereopure N2Pd(3,3‘-Me2BINOL) complexes. HCl removes the Me2BINOL resolving ligand and provides a family of enantiopure N2PdCl2 complexes whose only stereochemistry resides on the stereogenic nitrogen centers. In effect, nitrogen inversion (>105 s-1) is halted by metal coordination and utilized to generate enantiopure complexes. When the diamine substituents are relatively small, the N-chirality is stable; however, large substituents accelerate N-dissociation processes and concomitant racemization. Enantiopure N2Pd2+-Lewis acid catalysts can be generated for the Diels−Alder reaction, and although enantioselectivities are low (<25% ee), this is due to inefficient stereochemical transfer and not a degradation of the catalyst's chirality.