Diastereotopic protons, enantiotopization

But, you will recall that enantiomers are chemically indistinguishable unless they are in a chiral environment. Therefore we expect shifts of enantiotopic hydrogens to be identical, unless they are in a chiral environment. To summarize, enantiotopic protons normally will have the same chemical shifts, whereas diastereotopic protons normally will have different chemical shifts. 

For each compound given below (a-o), describe all spin systems (using Pople notation where appropriate), chemically shift equivalent protons, magnetic equivalent protons, enantiotopic protons, and diastereotopic protons. 

Enantiomerization of Tetrahedral Homochiral [(RNH3)4D Mg4(L12)6 ] Chelate Complexes Enantiotopization of Diastereotopic Protons via Enantiomerization. 145... 

What we have said so far explains to you why homotopic and enantiotopic groups appear identical in the NMR spectrum, but diastereotopic protons may not. Now we will give a quick guide to determining what sort of pair you are dealing with in a given molecule. 

Enantiotopic and diastereotopic protons and groups are discussed in Chapter 32, p. 000. 

Dicarbamate 189 bears enantiotopic branches each having diastereotopic protons. Deprotonation by s-BuLi/(-)-sparteine, followed by treatment with BF3 OEt2, results in the trans-cyclopropyl carbamate 191 in 76% ee [108]. From this result it is concluded that the preferential intermediate 190 stems from removal of the pro-S-H in the pro-S branch [Eq. (49)]. 

As an example of a molecule with diastereotopic ligands, consider the amino acid L-phenylalanine. The two protons at C-3 are diastereotopic, since substitution of either of them would generate a molecule with two chiral centers. Because the chiral center already present is 5, the two diastereomers would be the 2S,3R and the 25,35 stereoisomers. As in the case of enantiotopic protons, diastereotopic protons are designated pro-R or pro-S. The enzyme phenylalanine ammonia lyase catalyzes the conversion of phenylalanine to trans-cinnamic acid by a process involving anti elimination of the amino group and the 3-pro-S hydrogen. This stereochemical course has been demonstrated using deuterium-labeled L-phenyl-alanine as shown" ... 

One of the most common methods employed for analysis of chiral compounds is NMR spectroscopy [82, 83]. Enantiomers cannot be discriminated in an achiral medium because the resonances of enantiotopic nuclei are isochronous. However, diastereoisomers may be distinguished as the nuclei resonances are anisochronous. In NMR, nuclei can be classified as isochronous or anisochronous. Where diastereotopic protons show the same chemical shift, they are said to be equivalent or isochronous, and where they have different chemical shifts, the protons are described as anisochronous [84]. As long as there is a large enough... 

Identify the indicated sets of protons as unrelated, homotopic, enantiotopic, or diastereotopic ... 

A much more efficient procedure consists in the deprotonation of prochiral substrates 4 by chiral base 5 (equation 2). The removal of the enantiotopic protons in 4 proceeds through diastereotopic transition states having different energies AG and thus yielding the diastereomeric carbanions 6 and epi-6 in unequal amounts (equation 2). 

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