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

Abstract

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.