Chirality basic concepts

In principle, sites a, IT, and c need not be association sites as depicted by Ogston but could be steric sites that form obstructions such that the adsorbed molecule is chirally directed. Only one active site is actually required providing the remaining two sites (protuberances or cavities) are different from each other and from the active site that catalyzes the reaction. They could be identical providing they are not symmetrically oriented with respect to the active site (not an isosceles triangle). These are the basic concepts for a chiral environment on a surface and they lead to the three basic methods for creating chiral surfaces in heterogeneous catalysis. 

The basic concept behind all models of chiral self-assembly can be stated as follows. In general, chiral molecules do not pack parallel to their neighbors but rather pack at a slight angle with respect to their neighbors. This favored packing can be visualized by the packing of hard screws,6 as illustrated in... 

VIII. REACTIONS MEDIATED BY AN EXTERNAL CHIRAL LIGAND A. Basic Concept... 

The book provides systematic and detailed descriptions of the numerous approaches to chiral resolution. The first chapter is an introduction to basic concepts of molecular chirality and liquid chromatography. Chapters 2 through 9 discuss the chiral resolution of various classes of chiral stationary phases. Chapter 10 deals with chiral resolution using chiral mobile phase additives. These discussions elaborate the types, structures, and properties of the chiral phases,... 

The basic concepts of stereochemistry were independently developed by varft Hoff and Le Bel in 1874. Based on their findings Emil Fischer suggested that biological macromolecules are composed of chiral l amino acids and d sugars and in 1894 proposed his famous key and lock hypothesis that two chiral molecules interact through shape complementarity with each other and are therefore stereospecific [15]. 

Enantiomers also are referred to as chiral compounds, antipodes, or enantiomorphs. When introduced into an asymmetric, or chiral, environment, such as the human body, enantiomers will display different physicochemical properties, producing significant differences in their pharmacokinetic and pharmacodynamic behavior. Such differences can result in adverse side effects or toxicity, because one or more of the isomers may exhibit significant differences in absorption (especially active transport), serum protein binding, and metabolism. With the latter, one isomer may be converted into a toxic substance or may influence the metabolism of another drug. To discuss further the influence of stereochemistry on drug action, some of the basic concepts of stereochemistry need to be reviewed. 

Basic Concepts of Symmetry Breaking spontaneous, de facto, and de lege, as Related to the Geometric Example of Molecular Chirality... 

Stereochemistry is not primarily germane to the subject of this book, and it is not intended to discuss the subject in any detail. Nevertheless, the basic concepts, definitions and conventions, currently used in stereochemistry, will be considered to help those less familiar with the subject to understand the separation technology that will be introduced and described in the subsequent chapters. Stereochemistry is the study of the three-dimensional structure of chemical compounds. Isomers of the same substance, that only differ in the spatial arrangement of their atoms are called stereoisomers. Certain stereoisomers that diff er only in their capacity for rotating the plane of polarized light passed through them, are termed optically active, or chiral, and the isomers are called enantiomers. It follows, that as the subject of this book pertains to the separation of chiral substances, the method used for measuring optical activity needs to be briefly described. 

Although chiral calix[4]arenes can be generated by simply attaching chiral residues at the upper [40] or lower [41-43] rim of the calixarene skeleton, recent interest has been focused on the possibility of synthesizing inherently chiral calix[4]arenes, which are build up of nonchiral subunits and consequently owe their chirality to the fact that the calixarene molecule is nonplanar. Molecular asymmetry can arise from the substitution pattern at the lower rim and/or conformation. In this respect, Shinkai has recently reported a systematic classification of all possible chiral isomers derivable from calix[4]arene, and delineated some basic concepts for the design and synthesis of chiral derivatives [44]. 

Examples of dispersive VCD and IR spectra for three closely related chiral molecules are presented in Figure 3. These spectra are illustrative of a number of basic concepts. All three sets of spectra are recorded in the region of carbon-hydrogen... 

The brush-type (Pirkle-type) CSPs have been used predominantly under normal phase conditions in LC. The chiral selector typically incorporates tt-acidic and/or n-basic functionality, and the chiral interactions between the analyte and the CSP include dipole-dipole interactions, n-n interactions, hydrogen bonding, and steric hindrance. The concept of reciprocity has been used to facilitate the rational design of chiral selectors having the desired selectivity [45]. 

Extending the same concept of a planar chiral nucleophilic or basic heterocyclic Fe-sandwich complex, aza-ferrocenes 65 were prepared. The latter have also been successfully applied as bidentate ligands in transition metal catalysis [85]. 

The development of catalytic asymmetric reactions is one of the major areas of research in the field of organic chemistry. So far, a number of chiral catalysts have been reported, and some of them have exhibited a much higher catalytic efficiency than enzymes, which are natural catalysts.111 Most of the synthetic asymmetric catalysts, however, show limited activity in terms of either enantioselectivity or chemical yields. The major difference between synthetic asymmetric catalysts and enzymes is that the former activate only one side of the substrate in an intermolecular reaction, whereas the latter can not only activate both sides of the substrate but can also control the orientation of the substrate. If this kind of synergistic cooperation can be realized in synthetic asymmetric catalysis, the concept will open up a new field in asymmetric synthesis, and a wide range of applications may well ensure. In this review we would like to discuss two types of asymmetric two-center catalysis promoted by complexes showing Lewis acidity and Bronsted basicity and/or Lewis acidity and Lewis basicity.121... 

In order to understand this concept, we need to learn some basic stereochemical principles and notations (optical activity, chirality, retention, inversion, racemisation, etc.). 

During the early stages in the development of stereochemical concepts near the beginning of this century, chemists had to rely on rather indirect methods to deduce basic stereochemical information, such as the absolute configuration of a chirality center at carbon. In most cases, arguments were based on the chemical transformation of a molecule of interest to a molecule of known stereochemistry. Although optical rotation was a valuable tool in this process, absolute stereochemistry could not be deduced directly by spectroscopic techniques. The fine details of stereochemistry such as preferred conformational states and their rel-... 

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