(+)-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.

Graphical abstractThe synthesis of chiral terpenoid-substituted yttrocene complexes, in particular the yttrium analogue of Erker’s neomenthylindenyl zirconocene is reported. The application of these complexes in asymmetric hydroamination/cyclization of aminoalkenes was investigated.Figure optionsDownload full-size imageDownload as PowerPoint slide