Homotopic ligands

The two criteria used to spot homotopic ligands and faces may also be used to detect those which are enantiotopic. 

Homotopic Ligands that are related by an n-fold rotation axis. Similarly, faces of a trigonal atom that are related by an n-fold rotation axis. Replacement of any of the ligands or addition to either of the faces gives an identical compound. See also heterotopic, enantiotopic, and diastereotopic. 

FIGURE 49 Synthesis of C2-symmetrical bis-tridentate homotopic ligands with various spacers (A) ONO-S-ONO donor groups (Lessmann and Horrocks, 2000) and (B) ONN-S-NNO donor groups (Ronson et al., 2007). 

Further, in the case of molecules vi and vii, nuclei a are homotopic in viii, they are enantiotopic and in ix and x, they are diastereotopic. And yet, the same pro-E/pro-Z notation is used to designate enantiotopic ligands a in viii, and diastereotopic ligands a in ix and x. Here, the designations utilize prostereotopicity descriptors. Finally, there is no descriptor for homotopic ligands in vi and vii. 

It is frequently necessary to distinguish between identical ligands, that, even though bonded to the same atom, may be topologically nonequivalent. Let us consider 1,3-propanediol as an example. If a process occurs in which a proton at C-2 is substituted by another ligand, say, deuterium, the two possible substitution modes generate identical products. The two protons at C-2 are therefore toj)olo ically equivalent and are termed homotopic ligands. 

The problem of tr-facial differentiation, i.e. diastereomer formation, encountered in the metal complexation of the above mentioned annulated cyclopentadienyl ligands is avoided when C2-symmetrical ligands [153] are utilized. Since in such ligands both sides of the five-membered rings are homotopic, only one isomer is... 

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 symmetry planes (a) in molecules 30, 32, 34, 36, 38, Fig. 13 should be readily evident. It is possible to have both homotopic and enantiotopic ligands in the same set, as exemplified by the case of cyclobutanone (34) HA and HD are homotopic as are HB and Hc, HA is enantiotopic with HB and Hc HD is similarly enantiotopic with Hc and HB. The sets HAjB and HC>D may be called equivalent (or homotopic) sets of enantiotopic hydrogen atoms. The unlabeled hydrogens at position 3, constitutionally distinct — see Section 3.4 — from those at C(2, 4), are homotopic with respect to each other. Enantiotopic ligands need not be attached to the same atom — viz. the case of mew-tartaric acid (32) and also the just-mentioned pair Ha, Hc [or Hb, Hd] in cyclobutanone. 

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