Chemical reactions stereochemistry and

How important is stereochemistry in chemical reactions The answer depends on the nature of the reactants. First, consider the formation of a chiral product from achiral reactants for example, the addition of hydrogen bromide to 1-butene to give 2-bromobutane in accord with Markovnikov s Rule. 

The product has one stereogenic center, marked with an asterisk, but both enantiomers are formed in exactly equal amounts. The product is a racemic mixture. Why Although this result will be obtained regardless of the reaction mechanism, let us consider the generally accepted mechanism. 

The intermediate 2-butyl cation obtained by adding a proton to the end carbon is planar, and bromide ion can combine with it from the top or bottom side with exactly equal probability. 

Therefore, the product is a racemic mixture, an optically inactive 50 50 mixture of the two enantiomers. 

We can generalize this result. When chiral products are obtained from achiral reactants, both enantiomers are formed at the same rates, in equal amounts. 

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There are several specialized reviews8-12,21 pertaining to the natural quinuclidine derivatives, in particular to the history of their discovery, practical uses, natural sources, methods of analysis, extraction and isolation, and structural determination, as well as their stereochemistry and chemical reactions. Such material is therefore excluded from this chapter. 

Aldoses exist almost exclusively as their cyclic hemiacetals very little of the open-chain form is present at equilibrium. To understand their strorctures and chemical reactions, we need to be able to translate Fischer projections of car bohydrates into their cyclic hemiacetal forms. Consider first cyclic hemiacetal formation in D-erythrose. To visualize furanose ring formation more clearly, redraw the Fischer projection in a form more suited to cyclization, being careful to maintain the stereochemistry at each chirality center. 

Chapters 34 and 35 cover the structure, stereochemistry, nomenclature, and chemical reactions of carbohydrates. 

All in all, liquid-crystalline media are not generally useful solvents for controlling the rates and stereochemistries of chemical reactions. In each case, careful consideration of the fine details regarding the structure of educts and activated complex, their preferred orientations in a liquid-crystalline solvent matrix, and the disruptive effects that each solute has on the solvent order has to be made. A mesophase effect can only be expected when substantial changes in the overall shape of the reactant molecule(s) occur during the activation process [734],... 

This paper calls attention to the need for new ions in coordination chemistry—ions that would permit more detailed physico-chemical studies to be made, ions that would facilitate studies of less familiar metals and of less familiar coordination numbers, and ions that would help studies of chemical bonding and reaction mechanisms. Organometallic ions of the type RmM+ are such ions, and these form metal-chelate compounds of the type RmM Ch) . Three aspects of the chemistry of organometallic-chelate compounds are described (1) equilibria of compound formation ( ) kinetic and mechanistic studies of three types of reactions (a) reactions of the coordinated ligand, (b) substitution at the 4-, 5-, or 6-coordinate metal atom, and (c) reactions of the organic moiety and (3) studies of stereochemistry and chemical bonding. 

As noted earlier, the substitution of organometallic ions or pseudometal ions for a central metal ion has applications which are of interest to the physical-inorganic chemist. These applications are best indicated by looking at examples from three representative areas (1) the equilibria of complex formation (2) studies of reaction mechanisms and (3) problems of stereochemistry and chemical bonding. 

Fig. 14.1-4)[411. This compound is an important intermediate in the shikimate pathway for the biosynthesis of aromatic amino acids in plants. The RAMA reaction produced the desired d-threo stereochemistry, and chemical reduction of the keto group gave the desired (6R)-stereoisomer in 60% diastereomeric excess. Other analogs of DAHP are also potentially available by this route, due to the broad substrate specificity of RAMA.

Many chemical reactions proceed with a clearly defined stereochemistry, requiring the bonds to be broken and made in the reaction to have a specific geometrical arrangement. This is particularly true for reactions that are controlled by enzymes. 

Note that the stereochemistry comes out right. H s a and b are cis because they were cis in the starting quinone and the Diels-Alder reaction is stereospecific in this respect. H is also cis to and H " because the Diels-Alder reaction is stereoselectively endo. These points are described in more detail in Norman p.284-6 and explained in Ian Fleming Frontier Orbitals and Organic Chemical Reactions, Wiley 1976, p. 106-109. How would you make diene A ... 

This chapter is divided into two parts The first and major portion is devoted to carbohydrate structure You will see how the principles of stereochemistry and confer matronal analysis combine to aid our understanding of this complex subject The remain der of the chapter describes chemical reactions of carbohydrates Most of these reactions are simply extensions of what you have already learned concerning alcohols aldehydes ketones and acetals... 

In this chapter, configurational relationships will be emphasized. Both structural and dynamic aspects of stereochemical relationships will be considered. We will be concerned both with the fimdamental principles of stereochemistry and the conventions which have been adopted to describe the spatial arrangements of molecules. We will consider the stereochemical consequences of chemical reactions so as to provide a basis for understanding the relationships between stereochemistry and reaction mechanism that will be encountered later in the book. 

Up to this point, we have emphasized the stereochemical properties of molecules as objects, without concern for processes which affect the molecular shape. The term dynamic stereochemistry applies to die topology of processes which effect a structural change. The cases that are most important in organic chemistry are chemical reactions, conformational changes, and noncovalent complex formation. In order to understand the stereochemical aspects of a dynamic process, it is essential not only that the stereochemical relationship between starting and product states be established, but also that the spatial features of proposed intermediates and transition states must account for the observed stereochemical transformations. 

Enantioselective processes involving chiral catalysts or reagents can provide sufficient spatial bias and transition state organization to obviate the need for control by substrate stereochemistry. Since such reactions do not require substrate spatial control, the corresponding transforms are easier to apply antithetically. The stereochemical information in the retron is used to determine which of the enantiomeric catalysts or reagents are appropriate and the transform is finally evaluated for chemical feasibility. Of course, such transforms are powerful because of their predictability and effectiveness in removing stereocenters from a target. 

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