Structure and reactivity of chiral fenchone based organozinc catalysts

The propensity of zinc alkoxides to dimerize arises from the polarity of Zn(delta+)-O(delta-) units, which can be visualized by electrostatic potential plots, e.g. of the zinc chelate complex {methylzinc(1R,2R,4S)-2-endo-oxido-2-exo-(2-methoxyphenyl)-1,3,3-trimethylbicyclo[2.2.1]heptane}. Monomer-dimer equilibria of this fenchone-based methylzinc chelate complex and its derivatives determine catalyst reactivity in dialkylzinc additions to aldehydes and were computed [ONIOM (RHF/LanL2DZ:UFF)] to assess the relative reactivities of the catalysts. Increased monomer formation, i.e. increased catalyst reactivity, is predicted in ligand systems with bulky t-butyl and Si(CH3)(3) ortho-substituents but not for the methyl derivative. Geometrical aspects of dimeric zinc chelate complexes, such as interring C-aryl-C-aryl distances, the dimer forming and internal Zn-O bond distances and the (H3C)-O-C-aryl-C-aryl dihedral angles were found to correspond with the relative stabilities of the dimeric complexes. These geometrical criteria are promising structural probes to assess catalyst reactivity and hence are helpful tools for a rational catalyst design.