Chelate-Controlled Synthesis of <i>rac-</i> and <i>meso-</i>Me<sub>2</sub>Si(3-<sup>t</sup>Bu-C<sub>5</sub>H<sub>3</sub>)<sub>2</sub>ZrCl<sub>2</sub>
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The reaction of the chelated bis-amide complex Zr{PhN(CH2)3NPh}Cl2(THF)2 (2) with Li2[Me2Si(3-tBu-C5H3)2] yields meso-Me2Si(3-tBu-C5H3)2Zr{PhN(CH2)3NPh} (meso-3) in >98% yield. In contrast, the reaction of Zr{Me3SiN(CH2)3NSiMe3}Cl2(THF)2 (4) or the related mono-THF adduct Zr{Me3SiN(CH2)3NSiMe3}Cl2(THF) (5) with Li2[Me2Si(3-tBu-C5H3)2] yields rac-Me2Si(3-tBu-C5H3)2Zr{Me3SiN(CH2)3NSiMe3} (rac-6) in quantitative NMR yield and 89% isolated yield. X-ray crystallographic analyses show that the Zr{RN(CH2)3NR} chelate ring in rac-6 has a pronounced twist conformation, while that in meso-3 has a flatter, envelope conformation. It is proposed that the conformations of the Zr{RN(CH2)3NR} chelate rings in the stereodetermining transition states for addition of the second Cp- ring in these reactions are similar to those in the metallocene products and control the diastereoselectivity. meso-3 and rac-6 are converted to meso-Me2Si(3-tBu-C5H3)2ZrCl2 (meso-1) and rac-1, respectively, by reaction with HCl in Et2O.
