Stereogenic centre, 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 answer is most definitely yes— what is needed is an enantiomerically pure molecule or part of a molecule that will be present during the reaction and will interact with the transition state of the reaction in such a way that it controls the formation of the new stereogenic centre. This molecule might be a reagent or a catalyst, or it might be covalently attached to the starting material. We will consider all of these possibilities, the last first, and you will see that they really are the most powerful and versatile ways of making enantiomerically pure compounds. 

It can be seen that the stereoisomers with configuration (17 ,27 ) and (15,25) are related as enantiomers, and the (17 ,25) and (15,27 ) stereoisomers are also related as enantiomers. What then is the relationship between the stereoisomers of 1 with configurations (17 ,25) and (17 ,27 ), for example With respect to each other, these have one stereogenic centre of common, (17 ), configuration, and the other of opposite, (25) and (27 ), configuration. Clearly these stereoisomers are not enantiomers. Instead, they are related as diastereoisomers and the same can be said for the (15,27 ) and (15,25) stereoisomers. The most concise definition, given in the plural, is diastereoisomers are stereoisomers that are not enantiomers . 

A second area concerns the use of the word chiral. This has a specific definition and can only be properly applied to three-dimensional objects. Thus it is incorrect to refer to a chiral centre/axis or a chiral synthesis . The former can be a chirality centre/axis or a centre/axis of chirality although for most purposes we prefer to replace these terms by stereogenic centre/axis (which is, however, not equivalent). [51 As a collective term for enantio- and diastereoselective... 

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