Prochiral centers, definition

The first aspect is the concept of the enantiofaces of the Michael acceptors. Fig. 2 shows how the re- and M-enantiofaces of the Michael acceptors can be defined. Here, the first priority is always given to the C= group irrespective of the other substituents. This definition is relatively insensitive to changes in the substituents compared to the conventional re/si-face definitions. Since the Michael acceptors possess two prochiral centers, the a-carbon and P-carbon, an enantio-face can be described as, for example, re(a),M(P),si(a)/si(P), or 2l(oc)/ (P). The enantiofaces si(a) or re(P) indicate the si-face with regard to the a-carbon atom and the re-face with regard to the P-carbon atom, respectively. 

The two protons at C(l) are topologically nonequivalent, since substitution of one produces a product that is stereoisomerically distinct from that produced by substitution of the other. Ligands of this type are termed heterotopic, and, because the products of substitution are enantiomers, the definition can be made more precise.Ligands that on substitution produce enantiomers are enantiotopic. If a chiral assembly is generated when a point ligand is replaced by a new point ligand, the original assembly is prochiral Both C(l) and C(3) of 1,3-propanediol are prochiral centers. 

Although the term prochirality is frequently used, especially by biochemists, it suffers from a limitation which arises from a corresponding limitation in the definition of chirality. Molecules may display purely stereochemical differences without being chiral cis-tram isomers of olefins and certain achiral cis-trans isomers of cyclanes are examples. Thus (Fig. 2) (Z)- and ( )-1,2-dichloroethylene (4, 5) are achiral diastereomers, as are cis- and /rtww-1,3-dibromocyclobutanes (6, 7) being devoid of chirality these compounds have no chiral centers (or other chiral elements). Thus it is inappropriate to associate stereoisomerism with the occurrence of chiral... 

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