(+)-Neomenthyl- and (−)-phenylmenthyl-substituted cyclopentadienyl and indenyl yttrocenes as catalysts in asymmetric hydroamination/cyclization of aminoalkenes (AHA)

The chiral, terpenoid-substituted yttrocene (η⁵-neomenthylCp)₂Y{o-C₆H₄CH₂NMe₂}] (1) can be prepared via facile arene elimination starting from Y(o-C₆H₄CH₂NMe₂)₃]. Compound 1 retains a C₁-symmetric structure in solution on the NMR time scale, due to tight binding of the amine donor. The (−)-phenylmenthyl-substituted complexes (η⁵-(−)-phenylmenthylCp)₂Y(μ-Cl)₂Li(OEt₂)₂] (5) and (η⁵-(−)-phenylmenthylCp)₂YN(SiMe₃)₂] (6) were prepared via salt metathesis. Reaction of YCl₃ with the planar chiral (1-neomenthylindenyl)lithium predominantly produced a single, C₂-symmetric, racemic-like diastereomer. The X-ray crystal structure analysis confirmed that (η⁵-(+)-NMInd)₂Y(μ-Cl)₂Li(Et₂O)₂] (7) represents the same p-S, p-S metallocene diastereomer and adopts a very similar conformation as observed by Erker in his zirconocene complexes. Complex 7 reacts with LiN(SiMe₃)₂ to form (η⁵-(+)-NMInd)₂YN(SiMe₃)₂] (8) with retention of configuration. Complexes 1, 6 and 8 showed moderate to good catalytic activity in asymmetric hydroamination/cyclizations of aminoalkenes, but enantioselectivities were limited to a maximum of 38% ee for the sterically most hindered catalyst 8. The indenyl complex 8 is prone to protolytic loss of an indenyl ligand at low (?0.5%) catalyst loading, if sterically undemanding aminoalkene substrates are applied.