Journal of the American Chemical Society · 1996 · 188 citations · 94 references
The amine elimination reaction of Zr(NMe2)4 (2) and (EBI)H2 (1,2-bis(3-indenyl)ethane, 3) in toluene at 100 °C affords pure rac-(EBI)Zr(NMe2)2 (rac-4, EBI = ethylene-1,2-bis(1-indenyl)) in 68% isolated yield. This reaction proceeds via the rapidly formed mono-indenyl intermediate (η5-C9H6CH2CH2C9H7)Zr(NMe2)3 (6) which undergoes reversible intermolecular amine elimination with a second equivalent of 2 to give the binuclear species (μ-η5,η5-EBI){Zr(NMe2)3}2 (5, rac and meso isomers) or reversible intramolecular amine elimination to give either rac-4 or meso-4. The kinetic metallocene product is a 1/1 mixture of rac-4 and meso-4, the thermodynamic product is rac-4, and the meso-4 to rac-4 isomerization is catalyzed by the NMe2H co-product. The rac-4/meso-4 product ratio can be controlled by adjusting the rate of NMe2H removal from the reaction vessel and the steady state concentration of amine in the reaction mixture. The molecular structure of rac-4 has been determined by X-ray crystallography. rac-4 is converted to rac-(EBI)ZrCl2 (rac-1) in high yield by reaction with NMe2H·HCl (92% isolated) or Me3SiCl (quantitative). The syntheses of 2, rac-4, and rac-1 can be combined in a “one pot” synthesis of rac-1 from ZrCl4 in 68% overall yield. Alkylation of rac-4 with AlMe3 affords rac-(EBI)ZrMe2 (rac-7) in 90% isolated yield. rac-4 can be used directly as a catalyst precursor for the isospecific polymerization of propylene.
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