True Nature of Ligands in Cu-Catalyzed Reactions with Al(alkyl)₃

Significance

It was revealed that BINOL-based phosphoramidite ligands 1 react with AlMe₃ in non-coordinating solvents to form dimethylaminophosphine 2, which is probably the active ligand in Cu(II)-catalyzed asymmetric transformations. A combination of ³¹P NMR studies, isolation of intermediates and independent synthesis of the suspected ligands was used to confirm the hypothesis. Importantly, the cleavage of the BINOL moiety of 1 was observed in CH₂Cl₂ and in toluene, but neither in THF or Et₂O, nor with zinc reagents. It was shown that aminophosphine 2 not only imparts the same enantioselectivity as 1, but also shows better reactivity. Modification of the chiral amine 2 to the 2-naphthyl analogue 3 enhanced the enantioselectivity (from 67% to 85% ee).