Asymmetric atom 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... 

A molecule that contains just one chiral carbon atom (defined as a carbon atom connected to four different groups also called an asymmetric or stereogenic carbon atom) is always chiral, and hence optically active. As seen in Figure 4.1, such a molecule cannot have a plane of symmetry, whatever the identity of W, X, Y, and Z, as long as they are all different. However, the presence of a chiral carbon is neither a necessary nor a sufficient condition for optical activity, since optical activity may be present in molecules with no chiral atom and since some molecules with two or more chiral carbon atoms are superimposable on their mirror images, and hence inactive. Examples of such compounds will be discussed subsequently. 

The great majority of known chiral compounds are naturally occurring organic substances, their molecules having one or more asymmetrically substituted carbon atoms (stereogenic atoms). Chirality is present when a tetrahedrally coordinated atom has... 

Asymmetric Atom (Center) - This is an outdated term that usually should be replaced by stereogenic center (unit). 

A stereogenic carbon atom (chiral center, chiral atom, asymmetric atom) is bound to four unlike groups and thus generates chirality. Note that a molecule may possess a molecular chirality without having a stereogenic center. 

The cobalt atoms in Fig. 14.1 are stereogenic and the site symmetries are C2v and D4h, for (A) and (B), respectively. The term stereogenic center is at present used to designate a coordinated atom that was formerly called an asymmetric atom [2],... 

Traditionally such atoms have been called asymmetric atoms, or stereogenic atoms, or stereocenters. In 1996, however, the lUPAC recommended that such atoms be called chirality centers, and this is the usage that we shall follow in this text. It is also important to state that chirality is a property of the molecule as a whole, and that a chirality center is a structural feature that can cause a molecule to be chiral. 

Substructure, usually a central atom (stereogenic center) or bond (stereogenic bond) and its ligands, that could give rise to stereoisomerism, given sufficiently asymmetric substitution patterns. This definition is different from current textbook usage which restricts the term to those atoms or bonds that are sufficiently asymmetrically substituted. ... 

There are also classes of organic compounds that are chiral and cannot be superimposed on their mirror images, but which do not contain any classical asymmetric atoms. The stereogenic unit in these molecules is essentially a twisted structure, with either a right- or left-handed twist. The first category comprises the allenes. If you look back to Problem 3.8(b), you should remember that allenes are not planar. If one double bond is formed from p atomic orbitals, then the other must be formed from p orbitals (7.84). So the shape of 2,3-pentadiene, 7.85, is twisted, and the two mirror images cannot be superimposed. 

Compounds in which one or more carbon atoms have four nonidentical substituents are the largest class of chiral molecules. Carbon atoms with four nonidentical ligands are referred to as asymmetric carbon atoms because the molecular environment at such a carbon atom possesses no element of symmetry. Asymmetric carbons are a specific example of a stereogenic center. A stereogenic center is any structural feature that gives rise to chirality in a molecule. 2-Butanol is an example of a chiral molecule and exists as two nonsuperimposable mirror images. Carbon-2 is a stereogenic center. 

There are a number of important kinds of stereogenic centers besides asymmetric carbon atoms. One example is furnished by sulfoxides with nonidentical substituents on sulfur. Sulfoxides are pyramidal and maintain dieir configuration at room temperature. Unsymmetrical sulfoxides are therefore chiral and exist as enantiomers. Sulfonium salts with three nonidentical ligands are also chiral as a result of their pyramidal shape. Some examples of chiral derivatives of sulfur are given in Scheme 2.1. 

Chirality center (Section 7.2) An atom that has four nonequivalent atoms or groups attached to it. At various times chirality centers have been called asymmetric centers or stereogenic centers. 

The most common, although not the only, cause of chirality in an organic molecule is the presence of a carbon atom bonded to four different groups—for example, the central carbon atom in lactic acid. Such carbons are now referred to as chirality centers, although other terms such as stereocenter asymmetric center, and stereogenic center have also been used formerly. Note that chirality is a property of the entire molecule, whereas a chirality center is the cause of chirality. 

The retrosynthetic operations that we have addressed thus far have not resulted in significant structural simplification. After all, intermediate 6 still possesses a linear fusion of four rings and six contiguous asymmetric carbon atoms. But, nevertheless, intermediate 6 could potentially be derived in one step from intermediate 8, a polyunsaturated monocyclic compound containing only one stereogenic center. Under conditions that would be conducive to a heterolytic cleavage of the C-OH bond in 8, it is conceivable that the resultant tertiary allylic carbonium ion 7 would participate in a... 

Associated to copper(II) pre-catalysts, bis(oxazolines) also allowed the asymmetric Diels-Alder and hetero Diels-Alder transformations to be achieved in nearly quantitative yield and high diastereo- and enantioselectivities. Optically active sulfoximines, with their nitrogen-coordinating site located at close proximity to the stereogenic sulfur atom, have also proven their efficiency as copper ligands for these asymmetric cycloadditions. Other precursors for this Lewis acid-catalyzed transformation have been described (e.g., zinc salts, ruthenium derivatives, or rare earth complexes) which, when associated to bis(oxazolines), pyridine-oxazolines or pyridine-bis(oxazolines), led to efficient catalysts. 

Bolm et al. [108] prepared a C2-symmetric bis (sulfoximine) as ligand for the copper-catalyzed hetero-Diels-Alder reaction. The stereogenic sulfur atom being located near the AT-coordinating atom, these structures were assumed to be promising for asymmetric catalysis. Their Hgand (79 in Scheme 43) was synthesized by palladium-catalyzed N-aryl imination from 1,2-dibromobenzene and (S)-S-methyl-S-phenylsulfoximine with Pd2dba3 in 70% yield. 

From all these results, optically active sulfoximines, with their nitrogencoordinating site located at the close proximity to the stereogenic sulfur atom, have thus proven their efficiency as copper-ligands for asymmetric Diels-Alder and hetero Diels-Alder reactions. 

This chapter, however, does not deal with above-mentioned reactions of sulfoxides. Rather it is limited to asymmetric synthesis using a-sulfinyl carbanions and -unsaturated sulfoxides, specifically in which the stereogenic sulfoxide sulfur atom is enantiomerically pure. Therefore reactions of racemic sulfoxides are for the most part excluded from this review. For more general discussions, the reader is referred to other chapters in this volume and to other reviews on the chemistry of sulfoxides. Especially useful are the reviews by Johnson and Sharp and by Mislow in the late 1960s and by Oae and by Nudelman as well as a book by Block . A review by Cinquini, Cozzi and Montanari" through mid-1983 summarizes the chemistry and stereochemistry of optically active sulfoxides. This chapter emphasizes results reported from 1984 through mid-1986. 

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