Zirconocenes achiral

The application of achiral cationic zirconocene compounds for methyl methacrylate polymerisation, e.g. a mixture of [Cp 2ZrMe(THF)]+[BPh4] and Cp 2ZrMe2 in methylene chloride solution, leads to the formation of syndio-tactic poly(methyl methacrylate). The species responsible for propagation are believed to be the bimetallic ones, involving cationic zirconium enolate and neutral zirconocene, which facilitates the process. Propagation is postulated to occur via the Michael reaction between the coordinating monomer and the cationic enolate [537] ... 

Atactic polypropylenes are produced in catalysis by C2v-symmetric metallocenes that are achiral, such as Cp2MCl2 or (Me2Si(FLu)2)ZrCl2. The only stereocontrol observed is both of the chain-end type and low because the chiral center of the terminal monomer unit of the growing chain is in the P position as a consequence of the 1,2 insertion of the monomers. A significant influence on the tacticity is observed only at low temperatures, being much more pronounced for titanocenes and hafnocenes than zirconocenes as a consequence of their shorter M-Ca bonds, bringing the chiral p-carbon closer to the active center (147,148). 

The first chiral bridged zirconocene synthesized in 1984 by Brintzinger and used as an isospecific polymerization catalyst was racemic ethylenebis-(4,5,6,7-tetrahydro-l-indenyl)zirconium dichloride (see Structure 9) [45]. Ewen showed that the analogous ethylenebis(l-indenyl)titanium dichloride (a mixture of the meso form and the racemate) produces a mixture of isotactic and atactic polypropylene [46]. The chiral titanocene as well as the zirconocene were shown to work by enantiomorphic site control in the case of the titanocene, the achiral meso structure causes the formation of atactic polymer. 

The polymers with trans-fused five-membered rings linked with a diisotactic head-to-tail sequence have chirality, although the polymers composed of the cis-fused ring are achiral. Scheme 10 summarizes the structures of the stereoisomeric polymers. The optically active zirconocene complex with a C2 symmetric structure catalyzes the enantioselective cyclopolymerization of 1,5-hexadiene (Eq. 20) [98, 99]. Although the polymer contains not only trans-fused ring but also cis-fused ring units (ca. 68 32), it shows optical rotation due to the main chain chirality. 

As noted above, even if monomer addition to the growing species is highly enantioface-selective in an isotactic-spedfic polymerization, the resulting isotaaic polymer is configurationally achiral except for the chirality of the stereocenters in the vicinity of the chain terminal. Related examples have been reported for coordination oligomerization and polymerization of propylene, f-pentene, and 4-methyl-f-pentene with optically active zirconocene catalysts and for radical... 

The asymmetric addition of organozinc reagents to aldehydes is one of the most thoroughly studied and successful enantioselective processes. While chiral ligand selection is much more delicate for the Zr-Zn system, due to the fast background addition reaction mediated by the achiral zirconocene that is present in stoichiometric amounts in the reaction mixture, chiral allylic alcohols can be obtained in >92% when amino thiols are used as chiral inducers (Scheme 12.7)... 

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