Synthesis and characterization of new biphenolate and binaphtholate rare-earth-metal amido complexes: Catalysts for asymmetric olefin hydroamination/cyclization

Monomeric diolate amido yttrium complexes [Y{diolate}{N(SiHMe2)(2)}(thf)(2)] can be prepared in good yield by treating [Y{N(SiHMe2)(2)}(3)(thf)(2)] with either 3,3'-di-tert-butyl-5,5',6,6'-tetramethyl-1,1'-biphenyl-2,2'-diol (H-2(Bi-phen)), 3,3'-bis(2,4,6-triisopropylphenyl)-2,2'-dihydroxy-1,1'-dinaphthyl (H-2(Trip(2)BINO)) or 3,3'-bis(2,6-diisopropylphenyl)-2,2'-dihydroxy-1,1'-dinaphthyl (H-2(Dip(2)BINO)) in racemic and enantiopure form. The racemic complex [Y(biphen){N(SiHMe2)(2)}(thf)(2)] dimerizes upon heating to give the heterochiral complex (R,S)-[Y(bi-phen){N(SiHMe2)(2)}(thf)](2). The corresponding dimeric heterochiral lanthanum complex was the sole product in the reaction of H-2(Biphen) with [La{N(SiHMe2)(2)}(3)(thf)(2)]. Single-crystal X-ray diffraction of both dimeric complexes revealed that the two Ln(biphen){N(SiHMe2)(2)}(thf) fragments are connected through bridging phenolate groups of the biphenolate ligands. The two different phenolate groups undergo an intramolecular exchange process in solution leading to their equivalence on the NMR timescale. All complexes were active catalysts for the hydroamination/cyclization of aminoalkynes and aminoalkenes at elevated temperature, with [Y((R)-dip(2)bino){N(SiHMe2)(2)}(thf)(2)] being the most active one giving enantioselectivities of up to 57% ee. Kinetic resolution of 2-aminohex-5-ene proceeded with this catalyst with 6.4:1 trans selectivity to give 2,5-dimethylpyrrolidine with a k(rel) of 2.6.