Organophosphine oxide/sulfide-substituted lanthanide binaphtholate catalysts for enantioselective hydroamination/cyclization

A series of chiral organophosphine oxide/sulfide-substituted binaphtholate ligands have been prepared ({(R)-H₂BINOL-[P(O)R₂] ₂}^2-, R = Ph, Et, ^tBu, 3,5-xylyl, Bn, and OEt; {(R)-H₂BINOL-[P(S)Ph₂]₂}^2-). The X-ray crystal structures of (R)-H₂BINOL-[P(O)R₂] ₂ (R = Ph, Et, and ^tBu) evidence the varying binaphtholate dihedral angles and conformational flexibility possible for these ligands. Precatalysts for enantioselective intramolecular aminoalkene hydroamination/cyclization are conveniently generated in situ from the metal homoleptic dialkylamido precursors Ln[N(SiMe₃)₂] ₃ (Ln = La, Nd, Sm, Y, Lu, and Sc) and the above binaphthols. The crystal structure of La₂{(A)-BINOL-[P(O)Ph₂] ₂}₃ reveals strong coordination of lanthanide ion by the phosphine oxide moieties. Enantioselectivities as high as 65% ee for the hydroamination/cyclization of 2,2-dimethylpent-4-enylamine are obtained with {(R)-BINOL-[P(O)-Et₂]₂}NdN(SiMe₃)₂ at room temperature. Lanthanide catalysts having varying organophosphine oxidesubstituted binaphtholate ligands exhibit differing enantioselectivity trends with decreasing Ln³⁺ ionic radius, whereas Sc³⁺ catalysts afford enantioselectivities with opposite product configurations. Kinetic studies reveal that the hydroamination rate is zero-order in [aminoalkene], consistent with the generally accepted mechanism for organolanthanide-catalyzed hydroamination/cyclization.