Protons stereochemical nonequivalence

Five- and six-membered rings formed by coordination of diamines with a metal ion have the stereochemical characteristics of cyclopentane and cyclohexane. The ethylenediamine complexes have puckered rings and the trimethylenediamine complexes have chair conformations. The methylene protons are nonequivalent in these nonplanar conformations, taking on the character of equatorial and axial substituents. They are made equivalent as the result of rapid conformational inversion at room temperature, just as in the alicyclic compounds (Fig. 7.1). This has been observed in nmr studies of planar and octahedral complexes of ethylenediamine-type ligands with a number of metals. 

Stereochemical Nonequivalence of Protons 591 13-11 Time Dependence of NMR Spectroscopy 594 Problem-Solving Strategy Interpreting Proton NMR Sectra 597... 

Chemical shift nonequivalence can occur when two environments are stereochem ically different The two vinyl protons of 2 bromopropene have different chemical shifts... 

The two protons at C-1 are topologically nonequivalent, since substitution of one produces a product tiiat is stereochemically distinct fiom that produced by substitution of the other. Ligands of this type are termed heterotopic, and, because the products of substitution are enantiomers, the more precise term enantiotopic also applies. If a chiral assembly is generated when a particular ligand is replaced by a new ligand, the original assembly is prochiral. Both C-1 and C-3 of 1,3-propanediol are prochiral centers. 

Carbanions derived from sulfoxides have an interesting stereochemical feature. Because of the chiral nature of the adjacent sulfur atom, the two faces of the carbanion are nonequivalent. Conversely, there is a preference for removal from the sulfoxide of one of the two diastereotopic protons on an adjacent methylene... 

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