Non-metallocene rare earth metal catalysts for the diastereoselective and enantioselective hydroamination of aminoalkenes

In this short review we summarize Our work on new cyclopentadienyl-free rare earth metal catalysts for the diastereoselective and enantioselective hydroarnination of aminoalkenes. Non-rnetallocene rare earth metal catalysts based on diamidoamine, biphenolate and binaphtholate ligands are readily available through alkane and amine elimination procedures. Diamidoamine yttriurn complexes are efficient catalysts for the highly diastercoselective cyclization of 1-methyl-pent-4-enylamine to yield trams-2,5-dimethyl-pyrroldine with trans to cis ratios of Lip to 23:1. The X-ray crystal structural analysis of [((2,6-Et2C6H3NCH2CH2)(2)NMe)Y{N(SiMe3)(2)}] is reported in which yttrium is coordinated in a highly distorted tetrahedral fashion. 3,3'Di-tert-butyl substituted biphenolate complexes tend to form phenolate-bridged hetero- and homochiral dimers. The low steric dernand of the tert-butyl substituents resulted also in low enantioselectivities in hydroamination/cyclization reactions. Binaphtholate complexes with sterically more demanding tris(aryl)silyl substituents were more efficient catalysts-, giving en anti oselectivities of up to 83% ee. These catalysts could also be applied in kinetic resolution of chiral aminoalkenes giving k(rel) values as high as 16. Catalyst activities strongly depend on the reactivity of the leaving group which is protolytically exchanged for the substrate uring the initiation step. Complexes with bis(dimethylsilyl)amido ligand initiate rather sluggishly because of the low basicity of this amido ligand and appreciable catalytic activity is only observed at elevated temperatures. Aryl and Alkyl complexes showed significant better rates comparable in magnitude to lanthanocene catalysts. (c) 2005 Elsevier B.V. All rights reserved.