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

Abstract

The chiral, terpenoid-substituted yttrocene [(η 5-neomenthylCp) 2Y{ o-C 6H 4CH 2NMe 2}] ( 1) can be prepared via facile arene elimination starting from [Y( o-C 6H 4CH 2NMe 2) 3]. Compound 1 retains a C 1-symmetric structure in solution on the NMR time scale, due to tight binding of the amine donor. The (−)-phenylmenthyl-substituted complexes [(η 5-(−)-phenylmenthylCp) 2Y(μ-Cl) 2Li(OEt 2) 2] ( 5) and [(η 5-(−)-phenylmenthylCp) 2YN(SiMe 3) 2] ( 6) were prepared via salt metathesis. Reaction of YCl 3 with the planar chiral (1-neomenthylindenyl)lithium predominantly produced a single, C 2-symmetric, racemic-like diastereomer. The X-ray crystal structure analysis confirmed that [(η 5-(+)-NMInd) 2Y(μ-Cl) 2Li(Et 2O) 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(SiMe 3) 2 to form [(η 5-(+)-NMInd) 2YN(SiMe 3) 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.