Heterotopic hydrogen

Although in this volume only the Re/Si system is applied, transformation from one system into the other is frequently necessary. Fortunately, for the most common case of heterotopic hydrogen atoms (see Table 3). always last in ligand rank, the equivalence pro-R or HR = 1IR( holds. This is also true when the heterotopic ligands possess the highest rank. However, for heterotopic groups of intermediate rank, as is typical for the methyls of isopropyl groups, e.g.. in isopropyl alcohol and valine, there is no equivalence. 

All the examples given so far in this Section involve heterotopic hydrogen atoms and elucidation of stereochemical reaction course by use of deuterium or tritium. We shall conclude by providing two examples involving other heterotopic atoms, one concerned with carbon (12C and 13C) and one involving oxygen (160 and 180). 

The term equivalent is overly general and therefore bland and of equivocal meaning. Thus the methylene hydrogen atoms in propionic acid (Fig. 1) are equivalent when detached (i.e. they are homomorphic), but, as already explained, they are not equivalent in the CH3CH2C02H molecules because of their placement — i.e. they are heterotopic. Ligands that are equivalent by the criteria to be described in the sequel are called homotopic from Greek homos = same and topos = place 6>, those that are not are called heterotopic . 

Two ligands are enantiotopic if replacement of either one of them by a different achiral ligand7 (see also footnote 5 on p. 9) gives rise to enantiomeric products. Examples are shown in Fig. 13. The marked hydrogens (HA, HB) in CH2ClBr (30), meso-tartaric acid (32), cyclobutanone (34) [at C(2) and C(4) but not C(3)] and chloroallene (36) [at C(3)] are enantiotopic, as are the methyl carbons in isopropyl alcohol (55). meso-Tartaric acid, incidentally, exemplifies one of the rare instances of a molecule with heterotopic ligands but no discernible prochiral center or other element of prochirality. 

Any one of the methyl protons and any one of the methylene protons of butane (91) are constitutionally heterotopic because the replacements produce the constitutional isomers 92 and 93, respectively (Figure 2.37). The two protons on either one of the methylene groups of butane are enantiotopic, however, since the product of replacing one methylene hydrogen by X (94a) is the enantiomer of the product formed by replacing the other hydrogen (on the same methylene) by X (94b, Figure 2.38). Similarly, the two methylene... 

If replacing hydrogens by a different atom gives different compounds, the hydrogens are said to be heterotopic. 

Chloroethane, therefore, has two sets of hydrogens that are heterotopic with respect to each other, the CH3 hydrogens and the CH2 hydrogens. The hydrogens of these two sets are not chemical shift equivalent, and chloroethane gives two NMR signals. 

Vicinal coupling between heterotopic protons generally follows the n + rule (Section 9.2C). Exceptions to the + 1 rule can occur when diastereotopic hydrogens or conformationally restricted systems are involved. 

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