Stereogenic atoms Stereoisomers

For an alkene to exhibit cis-trans isomerism, the two groups on one end of the double bond must be different and the two groups on the other end of the double bond must be different. That is, in terms of the following structure, A must be different from B, and D must be different from E. When this is the case, both of the carbons of the double bond are said to be stereocenters. A stereocenter or stereogenic atom is defined as an atom at which the interchange of two groups produces a stereoisomer. 

Stcreocenter or stereogenic atom (Section 6.1) An atom at which the interchange of two groups produces a stereoisomer. 

A chirality center (or chiral center ) is one type of stereogenic center or, simply, stereocenter. A stereogenic atom is defined as one bonded to several groups of such nature that interchange of any two groups produces a stereoisomer. See E. L. Eliel and S. H. Wilen, Stereochemistry of Organic Compounds, Wiley-Interscience New York, 1994, p. 53. 

Stereogenic center (Section 5.4) When the exchange of two groups bonded to the same atom produces stereoisomers, the atom is said to be a stereogenic atom, or stereogenic center. 

Atropisomerism is significant because it introduces an element of chirality in the absence of stereogenic atoms. Axial chirality is observed with stereoisomers (or atropisomers) that result from hindered rotation about a single C—C or C—N bond. The barrier of rotation between atropisomers must be high enough to allow for their isolation. A minimum of three or/ho-substituents are generally required for an axially chiral biphenyl to have substantial stability toward racemization at room temperature. For general definitions and descriptions, see references [3-5]. 

FIGURE 4.37 The eight possible stereoisomers for a molecule containing three stereogenic atoms. 

However, the second potentially enantiomeric parr of molecules, F and G, are in fact identical species (Rg. 4.40) So in this example of a molecule containing two stereogenic atoms, we get only three stereoisomers, not the maximum number of 2 = 4. Clearly, we are one stereoisomer short because of the identity of F and G. This molecule, F = G, contains stereogenic carbons but is not chiral. Such a molecule is called a meso compound. 

You can expect more stereoisomers for structures that contain more stereogenic atoms, and the maximum number of possible stereoisomers is 2", where n is the number of chiral carbons. Each additional stereogenic atom doubles the number of stereoisomers except where meso compounds occur, or where a polycyclic ring system prohibits some configurations. Compound 3.1, tricyclo[4.4.0.0 ]decane, contains four stereogenic atoms, but there are only two stereoisomers. 

In most molecules, the tetrahedral stereogenic atoms are chirotopic as well. This is not always the case as exemplified by carbon 3 in the stereoisomers of 2,3,4-trichloropentane (Fig. 3.9). Isomers a and b are both meso, and carbon 3 is stereogenic but not chirotopic. Isomers c and d are racemic, and carbon 3 is chirotopic but not stereogenic. 

The reason that the third stereoisomer is achiral is that the substituents on the two asymmetric carbons are located with respect to each other in such a way that a molecular plane of symmetry exists. Compounds that incorporate asymmetric atoms but are nevertheless achiral are called meso forms. This situation occurs whenever pairs of stereogenic centers are disposed in the molecule in such a way as to create a plane of symmetry. A... 

The P-atom in sarin (9.84), soman (9.85), and tabun (9.87) is a stereogen-ic center, allowing for stereoselective enzymatic hydrolysis [162], This aspect has been extensively investigated for soman, which exists as four stereoisomers by virtue of the presence of a second stereogenic center (C-atom). These stereoisomers are usually designated as C(+)P(-), C(-)P(+), C(+)P(+), and C(-)P(-), where C(+/-) refers to the 1,2,2-trimethylpropyl moiety and P(+/ ) to the P-atom. Such a nomenclature may be convenient but has no implication for the absolute configuration. The C(+)P( ) and Cf-)P(-) epimers are the more active toward acetylcholinesterase and, hence, the more toxic ones. In contrast, the C(+)P(+) and C(-)P(+) epimers are preferentially hydrolyzed... 

Figure 1.11 Stereoisomers a carbon atom bonded to fonr different substituents in a chiral carbon or a stereogenic center. Such molecules cannot be superimposed upon their mirror image. A receptor will recognize one stereoisomer but not another. Such stereoisomers are designated as either R or S. Stereoisomerism may also occur around double bonds, producing cis or trans orientations of the substituents on either face of the double bond.

From a stereochemical point of view, the pinacol-type coupling is much more interesting, because two new stereogenic centers are established at the former carbonyl C atoms. When the coupling reactions described above were carried out with (R)-2 at 0 °C the pinacol product 19 (Fig. 1.5.4) was obtained as one out of three possible stereoisomers. 

When equivalent components of a molecule can be altered in turn to produce stereoisomers, that component which may be associated with an atom or symmetry planes or axes is termed stereogenic.117-119 A chemical process that is stereogenic can produce several stereoisomers, in which case it is said to be stereoselective if the products are not present in equimolar amounts, and stereospecific if only one stereoisomer is produced. A detailed mathematical model for the prediction of the extent of stereoselectivity in organic syntheses has been developed by Ruch and Ugi.120... 

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