Achirotopic atoms

The terms enantiotopic and diastereotopic describe the relationship between a pair of atoms or groups in a molecule. Sometimes it is also useful to describe the local environment of a single atom, group, or location in a molecule (even if it does not coincide with an atomic center) as chiral or not. A chirotopic atom or point in a molecule is one that resides in a chiral environment, whereas an achirotopic atom or point does not. All atoms and all points associated with a chiral molecule are chirotopic. In achiral molecules, achirotopic points are those that remain unchanged (are invariant) upon execution of an S that is a. symmetry operation of the molecule. For most situations, this means that the point either lies on a mirror plane or is coincident with the center of inversion of the molecule. Importantly, there will generally be chirotopic points even in achiral molecules. 

Tertiary carbon atoms along the chain have been defined as asymmetric (22-25, 34-37), pseudoasymmetric (6, 10, 38-40), stereoisomeric centers (30, 31), and diasteric centers (41). The first two terms put the accent on chirality and are linked to the use of models of finite and infinite length, respectively the last two consider only phenomena of stereoisomerism. Note the relationship between these last definitions and Mislow s and Siegel s recent discussion (42), where the two concepts—stereoisomerism (or stereogenicity) and chirality—are clearly distinguished. The tertiary carbon atoms of vinyl polymers are always stereogenic whether they are chinotopic or achirotopic (42) depends on stmctural features and also on the type of model chosen (43). 

A good illustration of the usefulness of the concepts local symmetry and chirotopicity is provided by the case of glycerol. The question as to whether the protons of the C,H20 groups are equivalent is problematic for most professional chemists. The answer is that they are not They are chirotopic (property ) and diastereotopic (relationship ). The issue that leads most chemists astray is the symmetry plane, which is the clue to that the molecule is achiral. However, only atoms within the symmetry plane are achirotopic. those outside are chirotopic. 

Chirotopic The property of any atom, and, by extension, any point or segment of the molecular model, whether occupied by an atomic nucleus or not, that resides in a chiral environment [83]. Achirotopic is the property of any atom or point that does not reside in a chiral environment (see also [84]). Chirotopic atoms located in chiral molecules are enantiotopic by external comparison between enantiomers. Chirotopic atoms located in achiral molecules are enantiotopic by internal and therefore also by external comparison. All enantiotopic atoms are chirotopic [83]. 

Table 15.1. Classification of Astereogenic/Stereogenic, Achirotopic/Chirotopic Atoms...

The eight examples in Figure 15.1 illustrate the four types of astereogenic/stereogenic, achirotopic/chirotopic atoms - o, o, s, and s. It turns out that (a) all four types of atoms - o, o, s and s - are present in achiral molecules, and (b) only o and s are found in chiral molecules. 

Every atom in a molecule is either chirotopic (types o and s ) or achirotopic (types o and s). The degree of chirotopicity of a given molecule, Cm, is equal to the sum of the degrees of chirotopicity of all chirotopic atoms of types o and s ... 

Chirotopicity is a local geometry that produces chirality. Not only are the atoms in a chiral environment said to be chirotopic, but the spaces around atoms in a chiral environment are also considered chirotopic. Atoms (or spaces) in an achiral environment are said to be achirotopic. 

Mislow and Siegel gave as an example the set of isomers of 2,3,4-trihydroxyglutaric acid, 107-110. Neither 107 nor 108 is chiral because both are meso structures. In both structures, C3 has been labeled "undoubtedly an asymmetric carbon atom." ° According to Mislow and Siegel, however, C3 is stereogenic and achirotopic in 107 and 108. On the other hand, C3 is nonstereogenic and chirotopic in compounds 109 and 110. 

These terms can be clarified by looking at some specific examples. In the following ro-tamers of ) cso-l,2-dichloro-l,2-dibromoethane, the only achirotopic site in rotamer A is the point of inversion in the middle of the structure. Every atom is in a locally chiral environment, and so is chirotopic. For rotamer B, all points in the mirror plane (a plane perpendicular to the page of fhe paper) are achirotopic. All other points in these conformers are chirotopic, existing at sites of no symmefry. In other words, all other points in these conformers feel a chiral environment, even though the molecule is achiral. 

---