New syntheses of ansa-metallocenes or unbridged substituted metallocenes by the respective reductive dimerization of fulvenes with Group 4 metal divalent halides or with Group 4 metal dichloride dihydrides

摘要

Two unprecedented syntheses of Group 4 metallocenes from 6-substituted fulvenes have been discovered and developed into high-yielding processes. In the first route the di-n-butylmetal dichlorides of Ti, Zr and Hf are generated in toluene suspensions of LiCl at -78 degrees C from the metal tetrachlorides and 2 equiv. of n-butyllithium. Bringing the (Bu2MCl2)-M-n to 25 degrees C and then heating at reflux for several hours gave complete conversion to slurries of MCl2 (M = Ti, Zr, Ho. Heating such slurries Of MCl2 with 2 equiv. of 6-substituted or 6,6-disubstituted fulvenes gave high yields of ansa-metallocenes or substituted ethylene-bis(cyclopentadienyl)metallocene dichlorides (fulvenes: 6,6-dimethyl-, 6-phenyl-, 6-(1-naphthyl)-, 6-(9-anthryl)-). For 6-substituted fulvenes, both racemic- and meso-1,2-disubstituted ethylene-ansa-metallocene dichlorides are expected to form, but with M = Zr (or Ti), the actual racemic- to meso-ansa-metallocene dichloride ratios observed were: phenyl, 50:50; 1-naphthyl, 83:17; 9-anthryl, 100:0. Apparently for steric reasons 6,6-diphenylfulvene underwent no ansa-metallocene dichloride formation with ZrCl2 but rather produced bis(diphenylmethyl(cyclopentadienyl))zirconium dichloride. The second route to novel metallocenes involves generating (Bu2MCl2)-M-n at -78 degrees C in toluene slurry, as in the foregoing method, but then adding 2 equiv. of the 6-substituted or 6,6-disubstituted fulvene immediately thereafter at -78 degrees C. Except with (Bu2TiCl2)-Ti-n, warming the reaction mixture to 25 degrees C and then further heating at 65 degrees C cause a smooth bis-hydrometallation by transfer to occur, giving good to very good yields of bis(substituted cyclopentadienyl)metal dichlorides (M = Zn, Hf). The instability of (Bu2Cl2)-Cl-n, even at -78 degrees C, rapidly led to a mixture of TiCl2 and (Bu2TiCl2)-Ti-n and hence to a mixture of ansa-titanocene dichlorides and unbridged, bis(substituted cyclopentadienyl)titanocene dichlorides.With a detailed study of the attainment and the stereochemistry of the formation of ansa-bridged complexes or metallocenes with acetophenone, benzyfideneaniline and 6-arylfulvenes, a mechanistic model is developed involving either a three-membered metallocycle formed from MCl2 or an "open-face sandwich" complex of the fulvene and MCl2. Such intermediates offer a rational steric explanation for the observed stereochemistry of ansa-bridge C-C bond formation.Finally, in comparative polymerizations of ethylene by such metallocenes, cocatalyzed by MAO, the superior catalytic activity of ansa-metallocenes in the order, Ti > Zr > Hf and of ansa-metallocenes over unbridged substituted metallocenes is attributed to the hyperconjugative stabilization afforded by the ansa sigma C-C bond to the metallocenium cation at the active olefin-polymerization site. (c) 2005 Elsevier Ltd. All rights reserved.