Chiral molecules basic principles

The basic principles of the mechanism of this Lewis-base-catalyzed aldol reaction have already been described in Section 6.2.1.1. With regard to the course of the enantio- and diastereoselective formation of aldol adducts with two stereogenic centers, it is proposed that synthesis of anti-products proceeds via a chair-like transition structure. A distinctive feature of the cationic transition state complex is a hexacoordinated silicon atom bearing two chiral phosphoramide molecules as ligands (Scheme 6.30). 

The basic principles of asymmetric synthesis have been discussed in a number of recent reviews (2,3). The heart of an asymmetric synthesis is a reaction in which an achiral unit in an ensemble of substrate molecules is converted into a chiral unit in such a manner that the stereoisomeric products are obtained in unequal amounts. For almost all cases this is equivalent to the statement that in an asymmetric reaction a prochiral unit is converted into a chiral unit. 

Chiral separation or sorption is another important technique in chirotechnology. In fact, due to the high cost of chiral catalysts, industries generally prefer chiral separation over asymmetric catalysis to obtain optically pure compounds. As in asymmetric heterogeneous catalysis, a chiral selector (a chiral molecule in optically pure form) can be immobilized on a solid support to make a chiral stationary phase (CSP) of use in direct chiral separation. The basic principle of chiral separation is that the chiral selector interacts differently with the enantiomers of a racemic or enantioenriched mixture to form transient diastereoisomeric species of different stability, and this fine distinction leads to the separation of enantiomers during elution. This topic has also produced a huge number of papers and the readers are referred to the previous reviews for more knowledge on this field [70-73]. 

Contents B. T Sutcliffe The Concept of Molecular Structure. - 0. E. Polansky Topology and Properties of Molecules. -J.RDahl Symmetry in Molecules. -L D. Barron Chirality of Molecular Structures - Basic Principles and their Consequences. - J. E. Boggs Interplay of Experiment and Theory in Determining Molecular Geometries. A. The Experiments. -... 

In the earlier chapters, selected chiral molecules and chiral ligands were mentioned (see Sections 1.3 and 2.1.7). In this section we discuss the basic principles behind asymmetric catalysis. In other words, we discuss the structural requirements and the physical principles behind enantioselective catalysis. 

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 reader unfamiliar with basic chemistry and the way that chemists draw molecules should not be waylaid by all of the chemical drawings that are given in early chapters. Certainly, the ideas and principles may be understood without appreciation for the three-dimensional structures. The reader is especially encouraged to continue forward to Chapters 6-8, where photographs and diagrams of familiar objects are used to illustrate how chirality is present in the macroscopic world. Many readers who are far removed from their chemistry studies may benefit from the quick review of chemical structural drawings given in the short appendix at the end of the book. 

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