Stereogenic atoms chiral

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... 

Chiral atoms are stereogenic, but not all stereogenic centres are chiral atoms. For example, in an alkene abC=Cab the double bond is a stereogenic element. 

A molecule that contains just one chiral (stereogenic) carbon atom (defined as a carbon atom connected to four different groups also called an asymmetric carbon atom) is always chiral, and hence optically active. As seen in Fig. 4.1, such a... 

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. 

We have previously shown that N,N -dimethyl-l,2-diphenylethanediamine 1 is a good ligand due to the fact that, in this case, the nitrogen atoms are stereogenic centres. It was first used in homogeneous catalysis [SJ, then as a monomer to prepare chiral polyureas on which rhodium was deposited [6]. Finally, it was used to prepare a diamine-rhodium complex (similar to the homogeneous one) that was then polymerised. 

Besides phosphines and phosphites preferentially used as ligands in hydro-formylation, phosphoramidites have also been proposed. Phosphoramidites (sometimes also called phosphoroamidites) are a class of organic phosphorus compounds derived from phosphites in which the P-OR groups have been replaced by P-NR2 groups (Figure 2.31). Three different P-substituents produce a chiral (stereogenic) phosphorus atom. Phosphoramidites play a crucial role in the synthesis of nucleic acids [1] and have also been frequently considered as mono- or bidentate ligands in transition-metal catalysis [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. 

Note In amino acids, the central C atom is stereogenic and the molecule is chiral, except for glycine. Glycine... 

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. 

The stereochemical analysis of chiral structures starts with the identification of stereogenic units [101], Those units consist of an atom or a skeleton with distinct ligands. By permutation of the ligands, stcrcoisomcric structures arc obtained. The three basic stereogenic units arc a center of chirality (c.g., a chiral tctravalcnt... 

In chemoinformatics, chirality is taken into account by many structural representation schemes, in order that a specific enantiomer can be imambiguously specified. A challenging task is the automatic detection of chirality in a molecular structure, which was solved for the case of chiral atoms, but not for chirality arising from other stereogenic units. Beyond labeling, quantitative descriptors of molecular chirahty are required for the prediction of chiral properties such as biological activity or enantioselectivity in chemical reactions) from the molecular structure. These descriptors, and how chemoinformatics can be used to automatically detect, specify, and represent molecular chirality, are described in more detail in Chapter 8. 

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. 

With a-alkyl-substituted chiral carbonyl compounds bearing an alkoxy group in the -position, the diastereoselectivity of nucleophilic addition reactions is influenced not only by steric factors, which can be described by the models of Cram and Felkin (see Section 1.3.1.1.), but also by a possible coordination of the nucleophile counterion with the /J-oxygen atom. Thus, coordination of the metal cation with the carbonyl oxygen and the /J-alkoxy substituent leads to a chelated transition state 1 which implies attack of the nucleophile from the least hindered side, opposite to the pseudoequatorial substituent R1. Therefore, the anb-diastereomer 2 should be formed in excess. With respect to the stereogenic center in the a-position, the predominant formation of the anft-diastereomer means that anti-Cram selectivity has occurred. 

Although the ion pairs of a-substituted benzyl anions and the corresponding cations are chiral species, which, in addition, often bear a pyramidal and hence stereogenic carbon atom, in most cases rapid racemization of the alkali and alkaline earth metal derivatives occurs in solution... 

Pitfalls are encountered when allowing chiral nonracemic aldehydes to react with chiral, but racemic, reagents having a stereogenic center at the metal-bearing carbon atom, since its chiral induction usually overrides that of the substrate leading to mixtures of two diastereomers in essentially equal amounts26,27 (Sections D.1.3.3.1.4.1., D.1.3.3.3.3.3.2. and D.1.3.3.3.8.2.3.1.). 

In most of these examples, the chiral auxiliary is introduced to the allylic reagent at a very late stage in the synthesis of the precursor, thus providing a facile access. It is obvious that in most examples, the central metal atom is kept from becoming stereogenic, and in addition, a C2-symmet-ric cation is desirable, in order to minimize the possible number of competing transition states. 

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