Concepts of Elimination Reactions

Bruckner R (author), Harmata M (editor) In Organic Mechanisms - Reactions, Stereochemistry and Synthesis Chapter DOI 10.1007/978-3-642-03651-4 4, Springer-Verlag Berlin Heidelberg 2010 

Some a-eliminations have already been discussed, like the formation of dichlorocarbene from chloroform and base. Others will be presented in certain contexts later. 1,3-Eliminations are mentioned in the preparation of 1,3-dipoles such as diazoalkanes or a-diazoketones and nitrile oxides (Chapter 15). Chapter 4 is limited to a discussion of the most important eliminations, which are the alkene-forming, /3-eliminations. Note that /3-Eliminations in which at least one of the leaving groups is removed from a heteroatom are considered to he oxidations. Eliminations of this type are therefore not treated here hut in the redox chapter (mainly in Section 17.3.1). 

If we consider what has just been said from a different point of view, we arrive at the following conclusion  

Eliminations of H/Het from substrates in which the leaving group (Het) is bound to a stereocenter and the H atom is not, may be suited for the stereoselective synthesis of trans- or of E alkenes. However, even this kind of stereocontrol is limited to alkenes that are considerably more stable than their cis- or Z isomers, respectively. On the other hand, eliminations of this type are never suitable for the synthesis of cis- or Z alkenes. 

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Thus, the well-known concept of stationary reaction rates limitation by "narrow places" or "limiting steps" (slowest reaction) should be complemented by the ergodicity boundary limitation of relaxation time. It should be stressed that the relaxation process is limited not by the classical limiting steps (narrow places), but by reactions that may be absolutely different. The simplest example of this kind is an irreversible catalytic cycle the stationary rate is limited by the slowest reaction (the smallest constant), but the relaxation time is limited by the reaction constant with the second lowest value (in order to break the weak ergodicity of a cycle two reactions must be eliminated). 

In Chapter 6, elimination reactions were presented. In the context of elimination reactions, the formation of double bonds was noted regardless of the elimination mechanism discussed. Continuing from the concept of using elimination reactions to form sites of unsaturation, one may reason that addition reactions can be used to remove sites of unsaturation. Thus, elaborating upon addition reactions, this chapter provides an introduction to relevant mechanisms applied to both carbon-carbon double bonds (olefins) and carbon-oxygen double bonds (carbonyls). 

Reaction progress kinetic analysis offers a reliable alternative method to assess the stability of the active catalyst concentration, again based on our concept of excess [e]. In contrast to our different excess experiments described above, now we carry out a set of experiments at the same value of excess [ej. We consider again the proline-mediated aldol reaction shown in Scheme 50.1. Under reaction conditions, the proline catalyst can undergo side reactions with aldehydes to form inactive cyclic species called oxazolidinones, effectively decreasing the active catalyst concentration. It has recently been shown that addition of small amounts of water to the reaction mixture can eliminate this catalyst deactivation. Reaction progress kinetic analysis of experiments carried out at the same excess [e] can be used to confirm the deactivation of proline in the absence of added water as well to demonstrate that the proline concentration remains constant when water is present. 

Finally, although sulfurane intermediates have been proposed in many cases, they have not been isolated from nucleophilic substitution reactions. However, the concept of an addition-elimination mechanism is supported by the independent syntheses of a number of stable sulfuranes these compounds have a trigonal-bipyramidal structure and in some cases the ligand reorganization was found to occur very easily (189-191). 

If we apply this concept of samration to drug elimination we get a similar picture. The anticonvulsant phenytoin depends critically for its elimination on one enzyme reaction (to produce the p-hydroxy-phenyl metabolite) and this, like the turnstile, can exceed its capacity to metabolize the drug. Phenytoin is then eliminated at a constant amount (not a constant proportion) per unit time. If input then exceeds this elimination capacity (and volume of distribution does not change), plasma concentration will rise rapidly into the toxic range. 

Within the Horiuti s approach, the physical meaning of the molecularity is clear. Horiuti introduced the concept of stoichiometric numbers (Horiuti numbers, v) Horiuti numbers are the numbers such that, after multiplying the chemical equation for every reaction step by the appropriate Horiuti number v, and subsequent adding, all reaction intermediates are cancelled. The equation obtained is the overall reaction. In the general case, the Horiuti numbers form a matrix. Each set of Horiuti numbers (i.e. matrix column) leading to elimination of intermediates corresponds to the specific reaction route. ... 

Nucleophilic Displacement of Halogens at Saturated Carbon Atoms Box 13.1 The Concept of Hard and Soft Lewis Acids and Bases (HSAB) Illustrative Example 13.2 Some More Reactions Involving Methyl Bromide Illustrative Example 13.3 1,2-Dibromoethane in the Hypolimnion of the Lower Mystic Lake, Massachusetts Polyhalogenated Alkanes — Elimination Mechanisms... 

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