Heterotopic atoms

The concept of heterotopic atoms, groups, and faces can be extended from enantiotopic to diastereotopic types. If each of two nominally equivalent ligands in a molecule is replaced by a test group and the molecules that are generated are diaster-eomeric, then the ligands are diastereotopic. Similarly, if reaction at one face of a trigonal atom generates a molecule diastereomeric with that produced at the alternate face, the faces are diastereotopic. 

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). 

Heterotopic atoms have different chemical shifts and are not chemical shift equivalent. 

Heterotopic (chemically nonequivalent atoms) (Section 9.8A) Atoms in a molecule where replacement of one or the other leads to a new compound. Heterotopic atoms are not chemical shift equivalent in NMR spectroscopy. 

Diastereotopic are those heterotopic atoms or groups that during substitution result in a pair of diastereomers. When substimtion of two H-atoms with deuterium or another atom/ group (i.e., desymmetrization) forms diastereomers, those two H-atoms are not equivalent, and give two signals in the NMR spectmm. Several examples of organic compounds with diastereotopic atoms are provided in Figure 1.33. 

In this way hosts 141-145 with both a cr-bonded Lewis acidic boron atom for complexation of anions and a conventional multidentate ligand for cations are generated. Complexation experiments of the 21-membered crown [6] boronate 142 with different potassium salts KX (X = F, Cl, Br, I, SCN, CN, OMe) indicate that there is a high specificity for the incorporation of KF, whereby F is bound covalently to the boron atom and is complexed by the crown ether (146, Fig. 39). An X-ray study has shown that the complexation of KF is heterotopic, i.e., both ions are complexed inside the same host. Some of the salts can only be bound in a monotopic way (KI and KSCN) [237]. 

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. 

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 . 

The constitution of the angular syn-bisdienophile 74 (see Scheme 20) maintains the bicyclic nature of the two dienophilic units. However, the [a,c]-fusion present in the phenanthrene constitution reduces the end-to-end symmetry of the rc-systems, rendering the two olefinic carbon atoms in each dienophilic unit constitutionally-heterotopic. The effect of this reduction in the symmetry of the 7t-systems in 74 upon the diastereoselectivities exhibited in its cycloadditions was of considerable interest to us. Furthermore, the incorporation of two phenanthrene units into a macropolycyclic belt could ultimately lead to the preparation of an angular isomer 70 of [12]cyclacene 50 (Fig. 20). Compared with [12]cyclacene 50, the angular isomer 70 has some appealing electronic features associated with it. 

A scheme analogous to the upper part of Fig. 2 has been presented for topic and morphic relationships [18,20], Thus, fragments of the same atomic composition may be homotopic or heterotopic, depending on whether they are superimposable or not. If the latter have the same constitution, they are stereoheterotopic, in the other case they are constitutionally heterotopic. Stereoheterotopic fragments are enantiotopic or diastereotopic. Morphic analysis yields the corresponding classification (see [19]). 

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