Intramolecular mobility of η(5)-ligands in chiral zirconocene complexes and the enantioselectivity of alkene functionalization by organoaluminum compounds.

The effect of solvent nature (CD2Cl2, d8-toluene, d8-THF) on the conformational behavior of neomenthyl-substituted zirconocenes CpInd*ZrCl2 (Cp = η(5)-C5H5, Ind* = η(5)-neomenthylindenyl), CpCp'ZrCl2 (Cp = η(5)-C5H5, Cp' = η(5)-neomenthyl-4,5,6,7-tetrahydroindenyl), and Ind*2ZrCl2 (Ind* = η(5)-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(η(5)-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%.