Chiral ansa-metallocene

Enantioselective hydrozirconation using, for example, chiral ansa-metallocenes [242] remain largely unexplored. 

When a chiral ansa-type zirconocene/MAO system was used as the catalyst precursor for polymerization of 1,5-hexadiene, an main-chain optically active polymer (68% trans rings) was obtained84-86. The enantioselectivity for this cyclopolymerization can be explained by the fact that the same prochiral face of the olefins was selected by the chiral zirconium center (Eq. 12) [209-211]. Asymmetric hydrogenation, as well as C-C bond formation catalyzed by chiral ansa-metallocene 144, has recently been developed to achieve high enantioselectivity88-90. This parallels to the high stereoselectivity in the polymerization. 

Chiral active pharmaceutical ingredients, 18 725-726. See also Enantio- entries Chiral additives, 6 75—79 Chiral alcohols, synthesis of, 13 667-668 P-Chiral alcohols, synthesis of, 13 669 Chiral alkanes, synthesis of, 13 668-669 Chiral alkenes, synthesis of, 13 668—669 Chiral alkoxides, 26 929 Chiral alkynes, synthesis of, 13 668-669 Chiral ammonium ions, enantiomer recognition properties for, 16 790 Chiral ansa-metallocenes, 16 90 Chiral auxiliaries, in oxazolidinone formation, 17 738—739... 

The obtained chiral ansa-metallocenes, after activation with methylaluminox-ane, were indeed found, in independent studies by Ewen [22] and by Kaminsky and Kiilper [101], to polymerise propylene and higher y-oldins to produce highly isotactic polymers [30]. 

Cj-symmetric structures having one of the two Cp ligands endowed with bilateral symmetry (such as fluorenyl) have several synthetic advantages, the main being the absence of a meso-isomer (Figure 28). We recall here that the latter is instead formed as the undesired byproduct in the synthesis of the C2-symmetric chiral ansa-metallocenes. 

According to an early hypothesis21, stereoregular isotactic polymerization requires the presence of chirality within the catalyst. Thus, with achiral metallocenes mostly atactic polymers are obtained. Chiral ansa-metallocenes with substituted cyclopentadienyl ligands, or especially with indenyl and tetrahydroindenyl ligands, are effective in stereoselective polymer synthesis. 

Chiral ansa-Metallocenes of Type I-A. There have been a few but quite important experimental studies on substituted biscyclopentadienyl ligands, the earlier one having shed considerable light on the correlation between Cp substitution and i-PP isotac-ticity and molecular weight. " " Representative... 

Ewen, J. A. Elder, M. J. Jones, R. L. Rheingold, A. L. Liable-Sands, L. M. Sommer, R. D. Chiral ansa metallocenes with Cp ring-fused to thiophenes and pyrroles Syntheses, crystal structures, and isotactic polypropylene catalysts. J. Am. Chem. Soc. 2001,123, 4763-4773. 

In the same paper, Ewen reported that the chiral ansa-metallocenes first prepared by Brintzinger O were capable of producing isotactic polypropylene (Fig.4),ll Using a mixture of meso (II) and racemic (Ilia) ansa-titanocenes, Ewen produced a mixture of atactic and isotactic polymer chains. In 1985, Kaminsky and Brintzinger confirmed Ewen s proposal that the chiral metallocene isomer was responsible for the formation of the isotactic polypropylene by using an isomerically pure sample of a racemic zirconocene analogue (nib) to produce only isotactic polymer chains. 

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