Heterotopic groups

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. 

Constitutionally Heterotopic and Diastereotopic Groups Differ in all scalar properties and are distinguishable under any conditions, chiral or achiral. Asymmetric molecules cannot contain homotopic or enantiotopic groups, only diastereotopic or constitutionally heterotopic groups. 

Nuclear magnetic resonance chemical shift differences can serve as an indicator of molecular symmetry. If two groups have the same chemical shift, they are isochronous. Isochrony is a property of homotopic groups and of enantiotopic groups under achiral conditions. Diastereotopic or constitutionally heterotopic groups will have different chemical shifts (be anisochronous), except by accidental equivalence and/or lack of sufficient resolution. 

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. 

An element of prostereoisomerism is a partial structure that can be converted into an element of stereoisomerism not otherwise present, by considering one of a pair of homomorphic groups to be different from the other. The groups involved in this operation are necessarily heterotopic. Depending on the character of the element of stereoisomerism thus produced, one can divide the elements of prostereoisomerism into centers, lines, and planes and subdivide them, as appropriate, into those that are (fully) prochiral, only prographochiral, only... 

Analyze the stereochemical relationships between the methyl groups of hexamethyl (Dewar benzene). Identify at least one pair of methyl groups in which the methyl groups are homotopic, enantiotopic, diastereotopic, or heterotopic. Note If there are no pairs of a given kind, say so. 

Just as one divides stereoisomers into two sets, enantiomers (Greek enantios = opposite) and diastereomers, so it is convenient to divide heterotopic (non-equivalent) groups or faces into enantiotopic and diastereotopic moieties. Enantiotopic ligands are ligands which find themselves in mirror-image positions whereas diastereotopic ligands are in stereochemically distinct positions not related in mirror-image fashion similar considerations relate to planes of double bonds. 

---