Nonequivalent stereogenic centers

Chirality center (Section 7.2) An atom that has four nonequivalent atoms or groups attached to it. At various times chirality centers have been called asymmetric centers or stereogenic centers. 

The asymmetric hydrogenation of yS,yS-disubstituted a-dehydroamino acids, in which the yS-substituents are nonequivalent, provides the opportunity to selectively construct two stereogenic centers. The Me-DuPhos or Me-BPE ligands facilitate the rhodium-catalyzed hydrogenation of the E- and Z-isomers of yS,yS-disubstituted a-dehydroamino acid... 

In addition to this nonequivalence, the imprecise definition of the chirality axis and difficulties in dealing with stereogenic centers in polycyclic structures led to confusion in the case of certain adamantanes, cyclohexanes and spiranes (see Section 1.1.5.3.3.). These problems were solved in the late 1970s and it was recommended1 that the PjM rather than the obsolete aR/aS description be used. This use is recommended as it leads to a considerable simplification of static Stereochemistry. 

In 10, there are two remote stereogenic centers at phosphorus and it was isolated as an almost equimolecular mixture of the two sets of diastereoisomers, that is, (RR/SS) and (RS/SR). In each chiral diastereoisomer, the phosphorus atoms are nonequivalent and four distinct 31P resonances were obtained almost in a 1 1 1 1 ratio <1997J(P2)1445>. The kinetics of the thermal decomposition of 11 in dibutyl phthalate was studied. The high rate of decomposition was probably determined by mutual steric influence of the bulky dinitromethylene moieties <2006RJC499>. 

Molecule 4-11 contains a chiral or stereogenic center in place of the methyl group, so that the three rotamers are now distinct (4-lla-c). Moreover, no symmetry operation in any of them relates to Hg. Consequently, even with rapid C-C rotation, and Hg have different chemical shifts and exhibit a mutual coupling constant. The spin system is ABX (AMX if the chemical-shift differences are large). The AB protons in 4-11 exemplify chemically nonequivalent nuclei that are termed diastereotopic. Diastereoisomers are stereoisomers other than enantiomers. Replacing by deuterium gives 4-1 Id, a diastereoisomer of 4-lle, which is formed when Hr is replaced by deuterium. The deuterated derivative has... 

This reveals that the original two structures in the problem are equivalent. ip) These two structures are nonsuperposable mirror images of a molecule with two nonequivalent stereogenic centers they are enantiomers. 

If a molecule contains two or more stereogenic centers, and if they are not bonded to identical groups, the stereogenic centers are nonequivalent. For n nonequivalent centers, the number of stereoisomers equals 2 . The following example, 2,3,4-trihydroxybutanal, illustrates the general principle. 

FIGURE 1.20. Chiral compounds with two nonequivalent stereogenic centers (erythrose and eryth-rose ephedrine and pseudoefedrine), and mes o-tartaric acid, as an example of a compound with two equivalent stereogenic centers. 

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