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 )-H 2 BINOL-[P(O)R 2 ] 2 } 2-, R = Ph, Et, t Bu, 3,5-xylyl, Bn, and OEt; {( R )-H 2 BINOL-[P(S)Ph 2 ] 2 } 2- ). The X-ray crystal structures of ( R )-H 2 BINOL-[P(O)R 2 ] 2 (R = Ph, Et, and t Bu) 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(SiMe 3 ) 2 ] 3 (Ln = La, Nd, Sm, Y, Lu, and Sc) and the above binaphthols. The crystal structure of La 2 {( R )-BINOL-[P(O)Ph 2 ] 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)Et 2 ] 2 }NdN(SiMe 3 ) 2 at room temperature. Lanthanide catalysts having varying organophosphine oxide-substituted binaphtholate ligands exhibit differing enantioselectivity trends with decreasing Ln 3+ ionic radius, whereas Sc 3+ 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.