- Intramolecular mobility of η5-ligands in chiral zirconocene complexes and the enantioselectivity of alkene functionalization by organoaluminum compounds
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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%.
- Parfenova, Lyudmila V.,Zakirova, Irina V.,Kovyazin, Pavel V.,Karchevsky, Stanislav G.,Istomina, Galina P.,Khalilov, Leonard M.,Dzhemilev, Usein M.
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p. 12814 - 12826
(2016/08/24)
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- Photoinduced rac/meso interconversions of bridged bis(indenyl) zirconium dichlorides
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Upon irradiation of a C2-symmetric rac-isomer of an ansa-zirconocene dichloride, an interconversion into the C(s)-symmetric meso-form is observed until a photostationary state between the two diastereomers is reached. The final rac/meso-proportion and the interconversion rate depend on the structure of the different metallocenes. Results of α-olefin polymerization catalyzed by [Me2Si(2-Me-4,6-(i)Pr2 Ind)2]ZrCl2 rac and meso, respectively, and methylalumoxane (MAO) as cocatalyst are reported. In propene polymerization the rac-diastereomer shows the higher activity whereas in pentene, hexene, and octene polymerization the meso-isomer is significantly more active. For all polymers a lower molecular weight is found using the meso-diastereomer as catalyst.
- Kaminsky, Walter,Schauwienold, Anne-Meike,Freidanck, Frank
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