Amidate ligand design effects in zirconium-catalyzed enantioselective hydroamination of aminoalkenes.

In situ generated axially chiral zirconium biphenyl amidate complexes were found to be efficient precatalysts for the enantioselective intramol. hydroamination of aminoalkenes, generating α-substituted pyrrolidines and piperidine with up to 74% ee. Five new chelating amide proligands and three new zirconium amidate complexes have been prepd. and fully characterized in this investigation of ligand structure/catalyst function. Solid-state mol. structures of the complexes suggest that the obsd. moderate and highly variable enantioselectivities are a consequence of the multiple isomers accessible to this family of complexes, including a κ2-(O,O)-bonding motif. Thermal stability studies of the complexes further revealed the tendency of these complexes to undergo diastereoselective dimerization to afford homochiral dimers. These dimeric precatalysts are less efficient when used for the cyclization of aminoalkenes in comparison to their monomeric precursors. These results illustrate the variable coordination modes accessible to amidate ligands and suggest steric factors that must be considered in advanced ligand design. [on SciFinder(R)]