Stereocontrol enantiotopic catalysts

Stereocontrol of Syndiospecific Propagation with Chirotopic (Enantiotopic) Catalysts... 

The invention of syndiospecific Cj-symmetric metallocenes has marked the turning point in the understanding of the mechanism of stereocontrol with metallocene catalysts. Again, the presence of isolated insertion errors of the type rrrrmmrrr is consistent with site control (Scheme 27). In the case of the syndiospecific Me2C(Cp)(9-Flu)ZrCl2 catalyst, in which the two sites are enantiotopic, occasional skipped insertions produce a minor amount of insertion errors of the type rrrrmrrrr, which are identical to those produced by chain-end control. In the case of isospecific C2-symmetric metallocenes, skipped insertions would not be observable due to the presence of two homotopic sites. 

For achiral metallocene-based catalysts Czv and achiral Q metallocenes in Chart 2) the chain-end control is present as the only stereocontrol mechanism. It derives from the presence of an asymmetric carbon atom on the last inserted monomer. The chirality R or 5) of this atom is related to the enantiotopic face of the olefin where the insertion took place (Scheme 34). In the NMR spectrum of the polymer we lose this kind of information, as two successive insertions of the re olefin face and two successive insertions of the si face produce the same m diad (see section II.G). As a consequence, we can observe only the relative chirality between consecutive inserted monomer units (5,5 or R,R as m diads and S,R or R,S as r diads) disregarding the absolute configuration of tertiary atoms. We prefer to use the re and si nomenclature indicating the stereochemistry of the methines in the polymer chain (Scheme 35), bearing in mind that the insertion of the re propene enantioface will produce an 5 configuration on the methine. 

The Hashimoto catalyst (9.101) has been used to give very high stereocontrol in the selective insertion into the enantiotopic aryl C-H bonds of substrate (9.102), to give the product (9.103) with a quaternary chiral centre in up to 98% ee. ... 

Busico, V. CipuUo, R. Cutillo, R Talarico, G Razavi, A. Syndiotactic poly(propylene) from [Me2Si(3,6-di-fert-butyl-9-fluorenyl)(N-fert-butyl)]TiCl2-based catalysts Chain-end or enantiotopic-sites stereocontrol Macrc>/nc>/. Chem. Phys. 2003, 204, 1269-1274. 

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