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{(R)-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 oxide-substituted 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.