Achirotopic

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). 

The terms stereocenter, chirotopic, and achirotopic will be used in this text in line with the most recent terminology. However, other terms are found in the older literature. C was previously referred to as a chiral or pseudochiral center. The term pseudochiral center is based on the same convention used to classify C as achirotopic instead of chirotopic. The terms asymmetric and pseudoasymmetric center are found in much older literature. 

The stereocenters in all three stereoregular polymers are achirotopic. The polymers are achiral and do not possess optical activity. The diisotactic polymers contain mirror planes perpendicular to the polymer chain axis. The disyndiotactic polymer has a mirror glide plane of symmetry. The latter refers to superposition of the disyndiotactic structure with its mirror image after one performs a glide operation. A glide operation involves movement of one structure relative to the other by sliding one polymer chain axis parallel to the other chain axis. 

Polyacetaldehyde [IUPAC Poly(oxy[methylmethylene])], like the systems described previously, contains stereocenters that are achirotopic. Both the isotactic and syndiotactic polymers are achiral and do not possess optical activity. 

The two coordination (active) sites of a meso Cs metallocene are diastereotopic and nonequivalent, but achirotopic. Each site resides in an achiral environment and polymerization produces a highly atactic polymer, although the regioselectivity is very high, even higher than the best C2 metallocenes. Unlike some C2v metallocenes, there are no reported cases of even modest stereoselective polymerization, either syndioselective or isoselective, due to chain end control. 

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. 

Finally, special descriptors, re and si, are logically required for achirotopic diastereotopic groups as in X(AAFF). The reason has already been explained in conjunction with two-dimensional "pscudoasymmetric figures in Section 1.1.2.2.2. 

X(ABCD) and X(FFCD) the centers X are clearly chirotopic and achirotopic, respectively (see Section 1.1.3.5.). However, this possibility is barred, as the permutation characteristics of these units are preserved in chiral models such as X(AFFG), with three chiral ligands. Here we have a pseudoasymmetric center and X is chirotopic. This observation emphasizes again the necessity to separate the concepts of stereogenicity and symmetry relationships. 

The number of classes, equal to 5, is derived considering all the possible conditions for chirotopicity of the catalytic sites corresponding to L. If they are not chirotopic, i.e. if they are achirotopic (e.g. they are bisected by a horizontal mirror plane), there are two possibilities only the two sites are equal (class I catalysts) or different from each other (class II catalysts). If, on the contrary, they are chirotopic, three possibilities exist the two catalytic sites are homotopic (equal) - related by a twofold symmetry axis (class III catalysts), enan-tiotopic - related by a vertical mirror plane (class IV catalysts) or diastereotopic (different from each other) - no symmetry element is present (class V catalysts). As a consequence, only five classes of metallocene catalysts may exist if interconversion among stereoisomers is not taken into account [122]. 

Class I catalysts obtained from achirotopic metallocene precursors of the Cp2MtX2 type, which produce atactic polypropylene at elevated temperature in the range of ca 50-70 °C (characteristic of propylene polymerisation in the presence of heterogeneous catalysts), can yield stereoblock isotactic polypropylene at lowered temperature, e.g. — 45 °C (Table 3.1) [22]. 

Stereocontrol of Isospecific Propagation with Achirotopic Catalysts... 

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]. 

Stereotopicity and chirotopicity are deemed independent attributes of molecular sites - be it ligands, bonds, molecular faces or molecular segments. The former attribute is defined by a specific topic relationship between two given sites, whereas the latter attribute describes the chirality/achirahty of the molecular field. Indeed, two molecular sites are, with respect to one another, either stereotopic or nonstereotopic, irrespective of the achirality/chirality of the molecule in which they are situated. Conversely, a molecular site is either achirotopic or chirotopic, regardless of any stereotopic or nonstereotopic relationship(s) it may bear with respect to another (or other) intramolecular site(s). 

The proposed descriptors of homotopic/diastereotopic faces/ligands establish a common basis for the discussion of their reactivities and selectivities (vide infra). These descriptors are not intended to replace either the pro-R, pro-S, pro-r, pros descriptors for ligands, or the Re/Si/re/si descriptors for faces. The proposed specifications are short, concise, and universal they emphasize the similarities in reactivity for ligands and faces further, they reveal the stereotopic nature of the ligands (homotopic vs. enantiotopic vs. diastereotopic), as well as the chirotopicity vs. achirotopicity of the molecular environment. 

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. 

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