Mechanistic concepts

Basic Mechanistic Concepts Kinetic versus Thermodynamic Control, Hammond s Postulate, the Curtin-Hammett Principle... 

SECTION 4.4. BASIC MECHANISTIC CONCEPTS KINETIC VERSUS THERMODYNAMIC CONTROL, HAMMOND S POSTULATE. THE CURTIN-HAMMETT PRINCIPLE... 

The fundamental mechanistic concept by which the stereochemical course of the aldol addition under conditions of kinetic control has been analyzed involves a cyclic transition state in which both the carbonyl and enolate oxygens are coordinated to a Lewis... 

Offer as complete as possible explanations, based on stmctural and mechanistic concepts, of the following observations. 

Kim and coworkers introduced silyl radical mediated addition of alkyl radical to silyloxy enamine 76. The silyloxy enamine moiety is readily accessible from a variety of functionalities. The mechanistic concept is illustrated in the Scheme 12 and involves the addition of R radical to 76 to give the radical adduct 77 and the subsequent homolytic cleavage of N-O bond to yield the desired product 78 and a silyloxy radical 79. The latter undergoes 1,2-phenyl migration to give the silyl radical 80 that abstracts halogen from the alkyl halide to regenerate the R radical. 

The basic mechanistic concepts and typical electrophilic aromatic substitution reactions are discussed in Sections 9.1 and 9.4 of Part A. In the present section, we expand on that material, with particular emphasis on synthetic methodology. 

Mechanistic insight is a key to both discovery of new reactions and to their successful utilization in specific applications. Use of reactions in a synthetic context often entails optimization of reaction conditions based on mechanistic interpretations. Part A of this text provides fundamental information about the reactions discussed here. Although these mechanistic concepts may be recapitulated briefly in Part B, the details may not be included where appropriate, reference is made to relevant sections in Part A. In addition to experimental mechanistic studies, many reactions of... 

Support for the mechanistic concept is a match between the experimental and calculated data using known k, and ku. 

Cyclopropanes in low yield were first noted in 1964 by Banks and Bailey (12) during the disproportionation of ethylene, but little significance was attached to that observation until recently, because such products had no obvious relevance to early mechanistic concepts based on pairwise rearrangements of bisolefin complexes. However, the subsequent adoption of carbenelike species as metathesis intermediates (4) provided a foundation for later development of cyclopropanation concepts. The notable results of Casey and Burkhardt (5) made an impact which seemed rather neatly to unify mechanistically the interconversion of cyclopropanes and metathesis olefins, although the reactions which they observed were stoichiometric rather than catalytic [see Eq. (4)]. Nevertheless, their work indicated a net redistribution of =CPh2 and =CH2 from (CO)5W=CPh2 and isobutylene, respectively, to form CH2=CPh2. Dissociation and transfer of CO yielded W(CO)6. Unfortunately, the fate of the isopropylidene moiety remained unknown. In 1976,... 

The lack of systematic studies of the two last-mentioned problems hinders elaboration of modern mechanistic concepts, which are very useful for the development of the chemistry of nitronates. 

Molecularity of a reaction the number of reacting partners in an elementary reaction uni-molecular (one), bimolecular (two), or termolecular (three) in the mechanism above, the first and third steps are unimolecular as written, and the remainder are bimolecular. Molecularity (a mechanistic concept) is to be distinguished from order (algebraic). 

According to this mechanistic conception, jr-donor substituted nitrosoarenes exhibit a strong correlation between sulfenamide and arylamine yield, respectively, and the thiol concentration employed38,56. In the case of nitrosobenzene and 4-chloronitrosobenzene, however, the 1-thioglycerol proportion (1 2 and 1 25, respectively) had virtually no effect on the sulfinamide/sulfenamide ratio at pH 6-938. As to our understanding, this effect lacks any reasonable explanation. 

The same two-step mechanism of metal dissolution has also been delivered for the anodic dissolution of nickel in acid solutions [Sato-Okamoto, 1964]. The mechanistic concepts for iron dissolution other than the two-step mechanism have also been presented in the literature [Heusler, 1958], 6ind the mechanism of metal dissolution is still a subject of research [Plonski, 1996]. 

The first mechanistic concepts of aromatization 16) originate from pregas-chromatography times. A direct alkane- cycloalkane reaction was proposed by Kazansky and co-workers 47). Several authors have interpreted the formation of six-membered rings over metal catalysts in terms of alkene-alkyl insertion (i.e., analogous to the Twigg mechanism) (7, 8, 14). 

However, it is instructive to consider radical chlorination of alkanes just a little further, to appreciate the mechanistic concepts. If we carry out light-induced chlorination of propane, then we obtain... 

The fundamental mechanistic concepts underlying these reactions were introduced in Chapter 8 of Part A. Here we will explore the scope and synthetic utility of these reactions. 

Entries 1—4 in Scheme 2.3 represent cases in which the nucleophilic component is converted to the enolate under kinetically controlled conditions by the methods discussed in Section 1.2. Such enolates are usually highly reactive toward aldehydes so that addition occurs rapidly when the aldehyde is added, even at low temperature. When the addition step is complete, the reaction is stopped by neutralization and the product is isolated. The guiding mechanistic concept for reactions carried out under these conditions is that they occur through a cychc transition state in which lithium or another metal cation is coordinated to both the enolate oxygen and the carbonyl oxygen. i. 4... 

This volume assumes a level of familiarity with structural and mechanistic concepts comparable to that in the companion volume, Part A, Structure and Mechanisms. Together, the two volumes are intended to provide the advanced undergraduate or beginning graduate student in chemistry a sufficient foundation to comprehend and use the research literature in organic chemistry. 

It is the intention of the authors to present a brief account on metal carbonato complexes which have a direct bearing on the reversible hydration of CO2 by the enzyme carbonic anhydrase. Emphasis is placed on the integration of the kinetic and mechanistic concepts derived from the studies on model systems with the available kinetic, chemical and structural information on the enzyme carbonic anhydrase. To start, the kinetics and equilibria of dissolved CO2, relevant to the present context, are presented. 

Bridge mediation mechanisms in heterogeneous outer sphere electrochemical reactions has also been theoretically treated using the pull—push and push-pull mechanistic concepts [84]. Schmidt [85] has considered theoretically homogeneous inner sphere bridge electron transfer reactions without atom or ion transfer. Bridge mediation in electron transfer reactions may also involve simultaneous atom or ion transfer. Heyrovsky [86] invoked mediation of electron transfer by formation of bridges to explain the enhancement of the rate of electroreduction of indium (III) ions in the presence of specifically adsorbed halide ions on mercury. 

An explanation could be achieved by the consistent application of the mechanistic concepts used to explain data at tetravalent phosphorus compounds (48). Most of the available data on nucleophilic displacements are usually explained by an apical entry of the nucleophile displacement of the leaving group occurs from an apical position (Scheme 3). The first invoked TBP intermediate is the most stable one on the basis of the relative apicophilicity of the groups attached to phosphorus the more electronegative ligands prefer apical positions. Since both inversion and retention can proceed, the retention pathway has been rationalized on the basis of intramolecular ligand exchange by pseudorotation (Scheme 3). 

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