Synthesis and Reactivity of Zirconaaziridines

Abstract

η²-Imine complexes of zirconium, or zirconaaziridines, have attracted attention as amino carbanion equivalents. They can be prepared from an amine via C–H activation within a zirconium methyl amide complex, from an imine by addition to a zirconocene equivalent, and from an isocyanide by rearrangement of the product of isocyanide insertion into a Zr–H, Zr–C, or Zr–Si bond. Zirconaaziridines contain a chiral center if their ring carbon bears two unequal substituents. Insertion of unsaturated organic compounds into the polar Zr–C bonds of zirconaaziridines leads to amines, allylic amines, heterocycles, diamines, amino alcohols, amino amides, amino amidines, and amino acid esters. These reactions can be carried out asymmetrically if the zirconaaziridines are prepared from enantiomerically pure zirconium complexes, or if racemic zirconaaziridines are treated with enantiomerically pure insertion reagents. Enantioenriched allylic amines and α-amino acid esters have been prepared. The lability of their chiral centers distinguishes zirconaaziridines from other organometallic reagents and makes it possible for them to undergo “dynamic kinetic asymmetric transformations”. These chiral centers can invert through isomerization of the zirconaaziridine to (a) (when possible) an azaallyl hydride complex, or (b) (if necessary) an η²-imine complex. The relative rates of inversion and insertion determine the stereochemical outcome of insertion reactions.