Pseudoasymmetric atom

A second problem that has repeatedly concerned us is the inability of the Sequence Rule to provide descriptors for some elements of stereoisomerism. When Cahn et al. (16) first encountered this problem with the all-cis and all-trans isomers of inositol, they attributed it to the fact that the symmetry has become so high that they have no asymmetric, nor even a pseudo-asymmetric atom. This interpretation, we believe, is incorrect. If the two ring ligands of any carbon atom of m-inositol were not heteromorphic, their exchange could not yield an isomer, as it clearly does. Each atom is a center of stereoisomerism with a pair of enantiomorphic ligands (Cg+g hi) and indistinguishable from the traditional pseudoasymmetric atom. The description of cu-inositol as all-5 could be accomplished by the same device that would allow one to specify the configurations of C(l) and C(4) of 4-methylcyclohexanol. 

Obviously, permutation and reflection do not, in general, give identical results. Unfortunately, the important classical terms asymmetric atom (center) and pseudoasymmetric atom (center)... 

For examples of reflection invariant stereogenic centers and faces, see dia-stereotopic, and pseudoasymmetric atom. 

The stereoisomerism of 3,4,5-trimethylheptane is complicated by the presence of a pseudoasymmetric atom see discussion in Reference 29. 

In most cases with more than two chiral centers, the number of isomers can be calculated from the formula 2", where n is the number of chiral centers, although in some cases the actual number is less than this, owing to meso forms.An interesting case is that of 2,3,4-pentanetriol (or any similar molecule). The middle carbon is not asymmetric when the 2- and 4-carbon atoms are both (/ ) [or both (5)] but is asymmetric when one of them is (/ ) and the other (5). Such a carbon is called a pseudoasymmetric carbon. In these cases, there are four isomers two meso forms and one dl pair. The student should satisfy himself or herself, remembering the rules... 

The two meso forms, although optically inactive differ in chemical properties. For example, on heating the meso form A, readily forms a lactone, whereas the meso form B does not. In such an example the central carbon atom is said to be pseudoasymmetric. But if one of the carboxyl groups is esterified so that the top and bottom parts of the molecule become structually different, then the central carbon atom becomes truly asymmetric and the molecule would have three true asymmetric atoms and it will exist in eight stereoisomeric forms. 

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 addition criterion may similarly be applied to recognize diastereotopic faces. Methyl a-phenethyl ketone, 58 in Fig. 19 has a chiral center addition clearly gives rise to diastereomers (59a, 59b) the faces of the carbonyl carbon are diastereotopic and the C = 0 group is prochiral. This case is of importance in conjunction with Cram s rule 10). Compounds 60, 62 and 64 also display diastereotopic faces even though the products 61, 63 and 65 are not chiral 60, 62 and 64 have prostereogenic rather than prochiral faces. The C=0 group in 60 is propseudoasymmetric, since C(3) in 61 is a pseudoasymmetric center. a-Phenethyl methyl sulfide (66) displays diastereotopic sides of a molecular plane not due to a double bond 5,24> and may alternatively be considered a case of diastereotopic phantom ligands (unshared pairs on sulfur). This case does involve chirality and the sulfur atom is prochiral. 

Vinyl polymers contain many pseudoasymmetric sites, and their properties are related to those of micromolecular compounds which contain more than one asymmetric carbon. Most polymers of this type are not optically active. The reason for this can be seen from structure 4-15. Any has four different substituents X, Y, and two sections of the main polymer chain that difl er in length. Optical activity is influenced, however, only by the first few atoms about such a center, and these will be identical regardless of the length of the whole polymer chain. This is why the carbons marked are not true asymmetric centers. Only those centers near the ends of macromolecules will be truly asymmetric, and there... 

Answer Optical activity is influenced only by the first few atoms around an asymmetric carbon (C ). For the two sections of the main chain, these will be identical regardless of the length of the whole polymer chain. The carbons marked C in (Xlll) are thus not truly asymmetric and are termed pseudoasymmetric or pseudochiral carbons. Only those C centers near the ends of a polymer molecule will be truly asymmetric, but since there are too few chain ends in a high molecular-weight polymer such centers do not confer any significant optical activity on the molecule as a whole. Polypropylene is thus optically inactive. 

If the observation is made from outside the system, i.e., observation of the relative configurations or diads, then, with isotactic molecules, all configurations about the central carbon atoms are equivalent. There is no configuration reversal about the pseudoasymmetric carbon atom as is observed when absolute configurations are being considered. 

Q has the useful property that = E, where E denotes the identity matrix. For a vinyl polymer with a nonarticulated side chain (such as a halogen atom), the -CHR—CH2- bond immediately following a pseudoasymmetric center has a D matrix that is either... 

Description of the Stereochemical Sequence. The stereochemical sequence is described using dl pseudoasymmetric centers, as defined on page 175 of Mattice and Suter [4]. The C-C bonds in the chain are indexed sequentially from 1 to . A local Cartesian coordinate system is associated with each bond. Axis x,- lies along bond i, with a positive projection on that bond. Axis yi is in the plane of bonds i — 1 and i, with a positive projection on bond i—l. Axis Zi completes a right-handed Cartesian coordinate system. The chain atom at the junction of bonds i — 1 and / is a if pseudoasymmetric center if it bears a methyl group with a positive coordinate along z, it is an I pseudoasymmetric center if this coordinate is negative. 

The consequences for the unperturbed dimensions are brought out in Table 3, which contains results for six distinguishable chains with equal numbers of / and d pseudoasymmetric centers, arranged in different repeating sequences. The first entry, taken from Table 1, is for the shortest such repeating sequence, where there is a strict alternation of / and d along the chain. The next two entries have a strict alternation of a pair of fs and a pair of if s. This sequence can be embedded in the chain in two distinct ways, which differ in whether two bonded carbon atoms with methyl substituents have the same or opposite chirality. The fourth entry has a repeating pattern of three / s followed by three if s. The fifth and sixth entries have a homopair (e.g.. It) followed by the opposite homopair (e.g., dd), and then a heteropair (e.g.. Id), which can be embedded in the chain in two ways. 

When copolymerizing vinyl type monomers, the number of possible diads, triads and higher sequences, which are distinguishable by NMR, depends on how many comonomers form asymmetric or pseudoasymmetric a-C-atoms during enchainment. For simplicity, in the following the term asymmetric a-C-atom is to include pseudoasymmetric a-C-atoms. Considering only the number of triads in the case of binary copolymers, there are 6 triads possible if none of the two comonomers forms an asymmetric a-C-atom, 10 triads if one of the monomers forms an asymmetric center, and 20 triads if both form an asymmetric a-C-atom. 

Such a site, labeled in 4-15, is termed a pseudoasymmetric or chiral carbon atom. 

In extended vinyl polymer chains, only relative chiralities can be distinguished. In a meso diad (c), two consecutive pseudoasymmetric carbon atoms (C2, C4) have the same configuration, whereas in syndiotactic diads (d) the configuration is opposite... 

The following two dicarboxylic acids represent achiral diastereomeric meso forms with a pseudostereogenic (formerly pseudoasymmetric) C-atom, C-5, which is given the descriptor r or s, respectively. 

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