Rare-Earth-Metal Complexes Supported by New Chiral Tetra-Azane Chelating Ligands: Synthesis, Characterization, and Catalytic Properties for Intramolecular Asymmetric Hydroamination

A number of new chiral tetra-azane proligands (1R,2R)-N,N′-bis­(o-arylamino-benzylidene)-1,2-diaminocyclohexane
((1R,2R)-[(ArHN)­C6H4CHN]2C6H10, Ar =
2,6-Me2C6H3 (L1H2), 2,6-Et2C6H3 (L2H2), 2,6-iPr2C6H3 (L3H2)) have been synthesized
via a nucleophilic displacement of the two fluorine atoms in (o-C6H4FCHN)2C6H10 with the lithium salt of the corresponding
aniline derivative. Their rare-earth-metal complexes L1ScCl2Li­(THF)3 (1), L1YCl2Li­(THF)3 (2), L2YCl2Li­(THF)3 (3), and L3YCl2Li­(THF)2 (4) were synthesized in good yields via the salt metathesis
of MCl3 (M = Sc, Y) with the dilithium salts of the ligands L1Li2(THF)4, L2Li2(THF),4, and L3Li2(THF)4, respectively. Further more, the two diethylamido complexes L1Y­(NEt2)­ClLi­(THF)3 (5) and L3Y­(NEt2)­ClLi­(THF)2 (6) were also
synthesized from reactions of the corresponding chloride complexes 2 and 4 with diethylamidolithium. The new proligands L1H2–L3H2 and their rare-earth-metal
complexes 1–6 have been characterized
by elemental analyses and 1H and 13C NMR spectroscopy.
The structures of complexes 1, 2, and 4 have been further confirmed by single-crystal X-ray diffraction
analysis. The molecular structural analysis reveals that the metal
centers in complexes 1, 2, and 4 acquire a distorted-octahedral coordination environment in their
solid-state structures by sharing the chloride with a LiCl­(THF)n moiety. After in situ treatment with nBuLi or Me3SiCH2Li,
complexes 1–4 show reasonable catalytic
activity and good enantioselectivity (up to 90%) for intramolecular
asymmetric hydroamination reactions of terminal aminoalkenes. The
amido complexes 5 and 6 can catalyze the
intramolecular hydroamination reaction directly and show catalytic
activities and enantioselectivities similar to those of the in situ
formed alkyl complexes.