The description of stereochemistry

The Fischer nomenclature for these two series of compounds, D for (1) and L for (2), which was based on their chemical correlation to the two enantiomers of glyceraldehyde, has been largely superseded by the Cahn-Ingold-Prelog system. 

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A complete description of stereochemistry of the carbon monoxide insertion and decarbonylation requires knowledge of configurational changes at the metal and a-carbon. Calderazzo and Noack (54) showed that the optical activity of the equilibrium mixture... 

However, there is evidence that reactions of aluminium hydride produced in situ involve single-electron-transfer (SET) processesThe reactions described by Trost and Ghadiri have most likely not been studied in sufficient detail to permit an adequate description of the reaction mechanism to be given at this stage. It is, however, quite likely that the Grignard reactions catalyzed by copper(II) and nickel(II) complexes , as developed by julia - and by Masaki , do involve SET processes, although, if this is so, the preservation of stereochemistry in some of the examples described by these workers is quite remarkable. (In this context, the reader s attention is drawn to Reference 196, end of this section.)... 

We have discussed in this chapter the thermal pyrolyses of a number of strained ring compounds. In most of the cases considered there is good evidence that the processes are unimolecular. Where possible we have tried to suggest plausible transition complexes, and reaction paths, based on a consideration of such factors as the kinetic parameters, stereochemistry of the reaction and effect of substituents. In reactions of this type, the description of the transition complex is fraught with difficulties, since the absence of such things as solvent effects (which can be so helpfrd in bimolecular reactions) limit the criteria on which such descriptions may be based. Often two types of transition complex may be equally good at accounting for the observed data. Sometimes one complex will explain some of the data while another is better able to account for the remainder. It is probable that in many cases our representation... 

It is the purpose of this section to provide the modern vocabulary required for the description of stereoselective reactions. This also implies the description of stereochemical aspects of starting materials and products, i.e., aspects of static stereochemistry. The material has been arranged in logical progression and, whenever possible, rules and directions for use or explicit definitions of important terms are given. 

The description of a stereoselective reaction primarily requires characterization of enantiomeric and/or diastereomeric products by their configuration (not their stereochemistry , see Introduction). Problems have not arisen with enantiomers but difficulties (see enumerations in refs 1 and 2) are. or perhaps were, apparent for diastereomers, a focal point having been acyclic compounds, in particular aldol addition products. These are pertinent examples to illustrate the problems and their various solutions very well ... 

The description of the general stereochemistry is the same as that used for [M(unidentate)5] and [M(unidentate A)(unidentate B)4] (Figure 6). Two ligands lie on the mirror plane with M—D = M—E = R, the remaining effective bond lengths being defined as unity. 

The description of ci-amino acids as D or L is a holdover from an older nomenclature system. In this system (5)-alanine is called L-alanine. The enantiomer would be D- or ( )-serine. The l (laevo, turned to the left D = dextro, turned to the right) designation refers to the ct-carbon in the essential amino acids. In alanine, there is a single a-carbon that is asymmetric. When two asymmetric centers are present as in L-threonine, the stereochemistry of both carbons must be considered. The common form of L-threonine is the 25,3R stereoisomer. 

There has been a decisive evolution in the treatment of steric effects in heteroaromatic chemistry. The quantitative estimation of the role of steric strain in reactivity was first made mostly with the help of linear free energy relationships. This method remains easy and helpful, but the basic observation is that the description of a substituent by only one parameter, whatever its empirical or geometrical origin, will describe the total bulk of the substituent and not its conformationally dependent shape. A better knowledge of static and dynamic stereochemistry has helped greatly in understanding not only intramolecular but also intermolecular steric effects associated with rates and equilibria. Quantum and molecular mechanics calculations will certainly be used in the future to a greater extent. 

This is because stereochemistry nomenclature is merely a formal description of the real molecular structure. The correct spatial shape of the pharmacophore is not represented at all. A correct description of stereochemistry is possible with descriptors based on the three-dimensional structure only. Molecules can show the same pharmacophore (i. e., same three-dimensional arrangement of pharmacophore centers) but different chirality according to chirality nomenclature. 

The terminology and notation that have been used to describe coordination compounds have been derived with one notable exception from the terms and symbols developed to describe the stereochemistry of carbon compounds. The terms ois, trans endo, exo dextro, d, D, (+) and leva, l, L (-) all have been used to describe the stereochemistry of coordination compounds in a close analogy with organic compounds (see Figure 1). As the descriptions of the chemistry and structures of coordination systems have become more varied and complex, the meanings of these terms have become less precise, as in the example of a ois or trans tricarbonyl octahedral compound (see Figure 2). The terms fao and mer were coined to indicate the facial and meridional disposition of substituted octahedral structures. 

The relative ranking of ligands for the description of the stereochemical properties of a molecule is the most utilized and accepted principle throughout stereochemical nomenclature. This is not yet the practice, however, in discussing the stereochemistry in coordination and inorganic chemistry. In the context of coordination and inorganic chemistry, stereochemical information is either presented in the more traditional terminology, or more often by means of a stereospecific structural representation. 

The chemistry of Xe is much the most extensive in this group and the known oxidation states of Xe range from -f-2 to -f-8. Details of some of the more important compounds are given in Table 18.2. There is clearly a rich variety of stereochemistries, though the description of these depends on whether only nearest-neighbour atoms are considered or whether the supposed disposition of lone-pairs of electrons is also included. Weaker secondary interactions in crystalline compounds also tend to increase the number of atoms surrounding a central Xe atom. For example, [XeFs] [AsF6] has 5 F at 179-182 pm and three further F at 265-281 pm, whereas [XeFsJ+iRuFe]" has 5 F at 179-184 pm and four further F at 255-292 pm. If only the most closely bonded atoms are counted, then Xe is known with all coordination numbers from 0 to 8 as shown schematically in Table 18.3. 

The aim ofthis chapter is to review the contribution of resonance Raman spectroscopy to our knowledge of the stereochemistry and electronic structure of carotenoid molecules involved in photosynthesis. However, a precise understanding of the information provided by resonance Raman about carotenoids requires the description of those relevant experiments, which were performed in vitro (usually in organic solvents), and which are at the foundation of our current interpretations ofthe Raman signals. In order to keep this review article as concise as possible, these will be presented after a short introduction on the method itself, and 1 will focus only on biologically relevant results. 

This evolution in QSAR was slow. As in many sciences, the evolution has been driven by discoveries of chemical behavior that could not be explained using conventional concepts and models. For example, Louis Pasteur recognized that optical activity (a phenomenon observed earlier ) was the result of the molecular dissymmetry later called chirality (from Greek cheir = hand). The concept of stereochemistry, however, was introduced by van t Hoff and Le Bel. It was V. Prelog" who pointed out that stereochemistry is not a branch of chemistry but a point of view. Part of this point of view is the description of structure that explains relevant behavior, which necessarily leads to additional levels of taxonomic analysis of chemicals. 

Discuss the aspects of stereochemistry that would have to be considered for complete description of the structure of molecules having the general structure A. How would the size of the (CH2) bridge affect conformational issues in these molecules ... 

A similar model for the representation of molecules is described in [165] and [166]. In [16], a and n electron systems are distinguished. Among other things, the representation of delocalized n electrons is intended to enable the description of aromatic systems. A comprehensive representation of compounds with non-covalent bonds can be found in [92], which also considers conhgurations, i.e. it can be applied to stereochemistry. 

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