Stereochemical Nonequivalence of Protons

Stereochemical differences often result in different chemical shifts for protons on the same carbon atom. For example, the two protons on Cj of allyl bromide (3-bromo-propene) are not equivalent. is cis to the — CH2Br group, and H is trans. absorbs at 65.3 H absorbs at 85.1. There are four different (by NMR) types of protons in allyl bromide, as shown in the structure at left in the margin. 

To determine whether similar-appearing protons are equivalent, mentally substitute another atom for each of the protons in question. If the same product is formed by imaginary replacement cf either cf two protons, those protons are chemically equivalent. For example, replacing any of the three methyl protons in ethanol by an imaginary Z atom gives the same compound these hydrogens are chemically equivalent. 

Cyclobutanol shows these stereochemical relationships in a cyclic system. The hydroxyl proton H is clearly unique it absorbs between S3 and 65, depending on the solvent and concentration. H is also unique, absorbing between S3 and 64. Protons H and are diastereotopic (and absorb at different fields) because H is cis to the hydroxyl group, while is trans. 

To distinguish among the other four protons, notice that cyclobutanol has an internal mirror plane of symmetry. Protons are cis to the hydroxyl group, while protons H are trans. Therefore, protons are diastereotopic to protons H , and the two sets of protons absorb at different magnetic fields and are capable of splitting each other. 

Use the imaginary replacement technique to show that protons and H in cyclobutanol are diastereotopic. 

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

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. 

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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