Prochirality: terminology

Possible elements of chirality in stereospecific polymerizations will be briefly recalled in order to indicate the used terminology. First of all, upon coordination, a prochiral olefin such as propene gives rise to not superpos-able si and re coordinations.22 According to the mechanism described, the isotactic polymer is generated by a large series of insertions of all si- or all re-coordinated monomers, while the syndiotactic polymer would be generated by alternate insertions of si - and re-coordinated monomers. 

Terms containing prochiral, such as prochiral unit or element, prochiral molecule or prochi-rality, must be abandoned. Perhaps in order to ridicule the pro-terminology", it was recently extended with terms such as pro-pro (CH3COOH), pro-pro-pro-stereogenic (CH4), and even pro-chirotopic6. There is but one comment - protest ... 

On the basis of such considerations, they have classified stereo-differentiating reactions into six types. While the terminology used is readily understandable, some comments on prochiral distinctions are in order. The thinking enzyme discussed earlier can use a sequence of binary congruence tests to examine the environmental relationships of materially identical groups within a molecule (topic analysis). The flow chart resembles that shown earlier for isomeric distinctions (see Scheme 2). 

Note that new terminology has been proposed. The concept of sphericity is used, and the terms homospheric, enantiospheric, and hemispheric have been coined to specify the nature of an orbit (an equivalent class) assigned to a coset representation. Using these terms, prochirality can be defined if a molecule has at least one enantiospheric orbit, the molecule is defined as being prochiral. ... 

Since an asymmetric synthesis is one that preferentially involves only one prochiral face of a planar substrate or reacts with only one prochiral group on a tetrahedral atom, some terminology is needed to unambiguously describe such species. 

A diisotactic polymer is an isotactic polymer that contains two chiral or prochiral atoms with defined stereochemistry in the main chain of the CRU. The relative configuration at two contiguous carbon atoms in the main chain bearing substituents a and b is designated by the prefix erythro or threo , as appropriate, by analogy with the terminology for carbohydrate systems. ... 

Fujita S. Prochirality revisited. An approach for restructuring stereochemistry by novel terminology. J. Org. Chem. 2002 67(17) 6055-6063. 

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