Stereoselectivity mechanisms

Stereoselectivity Mechanism of Chiral Orientation of Growing Chain... 

A large part of the stereospecific behavior of polymerization catalysts presented in this review can be rationalized in the framework of a stereoselectivity mechanism involving a chiral orientation of the growing chain. The discovery... 

According to analogous molecular mechanics analyses,38 this stereoselectivity mechanism would also operate for catalytic systems with oscillating stereocontrol, leading to atactic-isotactic stereoblock polymers,39,40 like those based on two unbridged 2-phenyl-indenyl ligands.40... 

The examination of models for stereospecific Ziegler-Natta polymerizations indicate the importance of the nonbonded interactions for the stereoselectivity mechanisms. 

The stereoselectivity mechanisms for polymerizations of dienes present several peculiar aspects mainly related to the nature of the bond between the transition metal of the catalytic system and the growing chain, which is of allylic type rather than of o type, as for the monoalkene polymerizations. There is experimental evidence, also supported by molecular modeling studies, that a relevant role for chemoselectivity and stereoselectivity is also played by the chirality of the back-biting coordination to the metal of the double bond of the polydienyl chain closest to the coordinated allyl group. 

The neighboring group participation mechanism requires two conditions a neighboring ester group and traas-configmation. For example, in the course of 3- and 4-thioglycoside synthesis, a solvent-dependent kinetically controlled stereoselective mechanism was found (Figure 8). 

Figure 8. Solvent-dependent kinetically controlled stereoselective mechanism a) kinetic control in toluene b) neighboring group participation in DMF.

One of the most versatile methodologies of EPC synthesis of allenes is the chirality transfer reaction which involves highly stereoselective, mechanism-controlled, metal-mediated propyn-yl-allenyl transpositions. Of these, the organocopper(l)-mediated reactions are especially useful and provide a relatively straightforward route to a broad variety of allenes of high enantiomeric purity. 

Kemmler, M., Herdtweck, E., and Bach, T. (2004) Inter- and intramolecular [2 + 2]-photocycloaddition of tetronates -stereoselectivity, mechanism, scope and synthetic applications. European Journal of Organic Chemistry, 4582-4595. 

The outlined mechanism is in a complete accord with the stereoselection mechanism suggested previously for fhe Rh-diphospine catalyzed hydrogenations vide supra). 

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