Enantiospecific, definition

Summarising, in the chain-end control mechanism the last monomer inserted determines how the next molecule of 1-alkene will insert. Several Italian schools [7] have supported the latter mechanism. What do we know so far Firstly, there are catalysts not containing a stereogenic centre that do give stereoregular polymers. Thus, this must be chain-end controlled. Secondly, whatever site-control we try to induce, the chain that we are making will always contain, by definition, an asymmetric centre. As we have mentioned above, the nature of the solid catalysts has an enormous influence on the product, and this underpins the Cossee site-control mechanism. Thus both are operative and both are important. Occasionally, chain-end control alone suffices to ensure enantiospecifity. 

The formal view. The formal view is much simpler. The racemic catalysts have a twofold axis and therefore C2-symmetry. Both sites of the catalysts will therefore preferentially co-ordinate to the same face (be it re or si) of propene. Both sites will show the same enantiospecificity the twofold axis converts one site in the other one. Subsequently, insertion will lead to the same enantiomer. According to the definition of Natta, this means that isotactic polymer will be formed. If the chain would move from one site to the other without insertion of a next molecule of propene, it will continue making the same absolute configuration at the branched carbon atom. Hence, no mistake occurs when this happens. 

The final definition concerns formation of enantiomers, and the terms enantioselective and enantiospecific are used. If a reaction produces an unequal mixture of enantiomers it is enantioselective. If it generates only one enantiomer of two possibilities, it is enantiospecific. The baker s yeast reduction (see sec. 4.10.F) of 134 gave 135 with >99% ee (S). (Here % ee means percent of enantiomeric excess.) A 0% ee means a 50 50 mixture (racemic mixture), 50% ee means a 75 25 mixture and 90% ee means a 95 5 mixture. The predominance of the (S) enantiomer makes this reaction highly enantioselective. 

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