Linked bis(β-diketiminato) yttrium and lanthanum complexes as catalysts in asymmetric hydroamination/cyclization of aminoalkenes (AHA)

Abstract

Linked bis(β-diketiminato) rare-earth metal complexes based on the ethylene-bridged ligand [C 2H 4(BDI DClP) 2]H 2 [DClP = 2,6-Cl 2C 6H 3] and the cyclohexyl-bridged ligands [Cy(BDI Ar) 2]H 2 [Ar = PMP (= p-MeOC 6H 4), Mes (= 2,4,6-Me 3C 6H 2), DIPP (= 2,6- iPr 2C 6H 3)] were prepared via amine elimination starting from [Ln{N(SiMe 3) 2} 3] (Ln = La, Y). The three cyclohexyl-bridged complexes [{( R, R)-Cy(BDI Mes) 2}YN(SiMe 3) 2] (( R, R)- 3), [{( R, R)-Cy(BDI Mes) 2}LaN(SiMe 3) 2] (( R, R)- 4), and [{( R, R)-Cy(BDI DIPP) 2}LaN(SiMe 3) 2] (( R, R)- 5) were obtained enantiomerically pure. The X-ray crystal structure analysis of the racemic variants of 3 and 4 revealed a distorted square pyramidal coordination geometry around the rare-earth metal, in which the amido ligand occupies the apical position and the two linked β-diketiminato moieties form the basis. The two aromatic substituents adopt a transoid arrangement and both β-diketiminato moieties are bound in a η 5 coordination mode with close Ln⋯C contacts. Due to the smaller ionic radius of yttrium vs. lanthanum, the front side of the yttrium complex 3 is sterically more hindered than in the lanthanum complex 4, but there is much more empty coordination space on the rear side, which may rationalize the observed differences in selectivity of 3 in comparison to 4. The catalytic efficiency of the β-diketiminato complexes was strongly affected by steric factors such as ionic radius of the metal and the steric bulk of the aryl substituents, which is an indication for highly steric encumbered catalytic species. The complexes displayed good to moderate catalytic activity in the hydroamination/cyclization of aminoalkenes depending on the steric hindrance around the metal center. The sterically most demanding diisopropylphenyl-substituted complex ( R, R)- 5 displayed significantly higher enantioselectivities (up to 76% ee), but lower catalytic activity in comparison to the sterically more open mesityl-substituted complex ( R, R)- 4. The smaller yttrium metal center in complex ( R, R)- 3 led to reduced activity as well as a reversal in enantioselectivity, which may be rationalized by a change of the approach of the alkene moiety to the Ln-amido bond in the cyclization transition state.