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Control of ansa-Zirconocene Stereochemistry by Reversible Exchange of Cyclopentadienyl and Chloride Ligands

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journal contribution
posted on 28.03.2007 by Richard M. Buck, Nawaporn Vinayavekhin, Richard F. Jordan
The reaction of Li2[Me2Si(3-R-C5H3)2] salts with Zr{Me3SiN(CH2)3NSiMe3}Cl2(THF)2 in THF quantitatively afforded rac-Me2Si(3-R-C5H3)2Zr{Me3SiN(CH2)3NSiMe3} zirconocenes with 100% stereoselectivity. NMR monitoring studies showed that these reactions initially form rac/meso mixtures, which undergo isomerization to pure rac. The isomerization requires Cp−enantioface exchange and is catalyzed by chloride ion, which reversibly displaces Cp- from Me2Si(3-R-C5H3)2Zr{Me3SiN(CH2)3NSiMe3}. Kinetic analyses for selected complexes showed first-order dependence of the isomerization rate on both metallocene and chloride concentrations. Reversible Cp-/Cl- exchange is also facile for {ansa-bis-Cp}ZrCl2 and nonbridged Cp2ZrCl2 complexes.

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