Asymmetric Catalytic Intramolecular Hydroamination of Aminoallenes by Tantalum Amidoalkoxide Complexes

A series of tantalum complexes of chiral bidentate amidoalkoxide ligands was prepared. The crystal structure of one complex, Ta(NMe2)3[-OCPh2CH(CHMe2)N(c-C6H11)-] was obtained. Unlike previously described titanium complexes with these ligands, which are dimeric with bridging oxygen atoms, this tantalum complex is monomeric with an approximate trigonal-bipyramidal geometry. The resulting complexes were examined for their in situ activity for catalytic asymmetric hydroamination/cyclization of an aminoallene. Enantioselectivities up to 80% ee were observed for the cyclization of 6-methylhepta-4,5-dienylamine to 2-(2-methylpropenyl)pyrrolidine at 135 °C with 5 mol % catalyst loading.