Intramolecular mobility of η5-ligands in chiral zirconocene complexes and the enantioselectivity of alkene functionalization by organoaluminum compounds

Abstract

The effect of solvent nature (CD₂Cl₂, d₈-toluene, d₈-THF) on the conformational behavior of neomenthyl-substituted zirconocenes CpInd*ZrCl₂ (Cp = η⁵-C₅H₅, Ind* = η⁵-neomenthylindenyl), CpCp′ZrCl₂ (Cp = η⁵-C₅H₅, Cp′ = η⁵-neomenthyl-4,5,6,7-tetrahydroindenyl), and Ind*₂ZrCl₂ (Ind* = η⁵-neomenthylindenyl) was shown by means of dynamic NMR spectroscopy, and the constants and thermodynamic parameters of conformer exchange were determined. The experimental conformational composition of the complexes was compared with structures obtained by quantum chemical modeling using the DFT methods PBE/3ζ and M06-2X/cc-pVDZ(H, C, Cl)/cc-pVDZ-PP(Zr), which predicted three rotamers in the case of both CpInd*ZrCl₂ and CpCp′ZrCl₂, and seven rotational isomers for Ind*₂ZrCl₂, three of these being C₂-symmetric and the others being asymmetric. The enantioselectivity of the conformationally mobile complex Ind*₂ZrCl₂ in the reactions of terminal alkenes with AlR₃ (R = Me, Et) was compared with that of rigid ansa-complexes, rac-p-S, p-S-[Y(η⁵-C₉H₁₀)₂]ZrX₂ (Y = SiMe₂, C₂H₄; X = S-binaphtholate). Faster exchange between the conformers of Ind*₂ZrCl₂ in a chlorinated solvent gives the structural isomer of catalytically active sites, which affords higher substrate conversion and reaction enantioselectivity. Binding of the ligands to ansa-zirconocenes prevents the rotational isomerism of the complexes, providing the same configuration of the β-stereogenic center in the methyl- and ethylalumination products (unlike the conformationally mobile complex Ind*₂ZrCl₂) with an enantiomeric purity of 50–65%.