Intermediates transient ionic

Detection of transient ionic intermediates in the [241] Wittig, Mitsunobu, and Staudinger reactions directly from solution... 

The corresponding reactions of transient Co(OEP)H with alkyl halides and epoxides in DMF has been proposed to proceed by an ionic rather than a radical mechanism, with loss of from Co(OEP)H to give [Co(TAP), and products arising from nucleophilic attack on the substrates. " " Overall, a general kinetic model for the reaction of cobalt porphyrins with alkenes under free radical conditions has been developed." Cobalt porphyrin hydride complexes are also important as intermediates in the cobalt porphyrin-catalyzed chain transfer polymerization of alkenes (see below). 

Such radicals or ion pairs are formed transiently as reactive intermediates in a very wide variety of organic reactions, as will be shown below. Reactions involving radicals tend to occur in the gas phase and in solution in non-polar solvents, and to be catalysed by light and by the addition of other radicals (p. 300). Reactions involving ionic intermediates take place more readily in solution in polar... 

A. H. Zewail The relationship to real-life DNA structures is certainly not known to us. However, as you can see from the paper in Nature [1], the transient structures have never been isolated and there is another important point. The presence of an ionic intermediate is possibly significant for recognition by enzymes of mutation by this mechanism, if operative. 

Aqueous surfactants are another class of catalysts. Substantial rate enhancement is seen in the reaction occurring at the micellar hydrocarbon-water interface, which is ascribed to a concentration of the reactant in the micellar pseudo-phase. Chiral p-nitrophenyl esters derived from phenylalanine are hydrolyzed by a histidine-containing dipeptide at a micellar interphase, at which a very high enantiomer discrimination, kR/ks up to 30.4 at 0°C, is observed (49). As shown in Scheme 20, the enantioselectivity is expressed at the stage at which a transient, zwitter-ionic tetrahedral intermediate leading to the acylimidazole is formed,... 

The pH dependence of Ks/Km is similar for step 1 and step 2 reactions as shown in Fig. 26b, but this similarity in the pH curves indicate only that the same titratable groups on the free enzyme and/or free substrate are involved in the two steps. As discussed explicitly by Usher et al. (522) the roles of the two histidines could be reversed and this would make no difference since the ratio of HE EH where these are the two singly protonated species is independent of pH. Similar ks and Ka curves for the two steps would also fail to prove identical roles for the two histidines. Since a pentacovalent species—whether it is a transient activated complex or a more stable intermediate—is common to the various alternatives, pK shifts deduced from ka curves could be the same. Both substrates are monovalent anions with low pK values so that 1 /Km, whether interpreted as an equilibrium binding value or as a function of the kinetic parameters mirroring the total occupancy of all the stable intermediates, could also be the same for both steps. The values for the reverse of step 2 would behave differently since the pj of 3 -CMP, for example, is 5.9. It should also be noted that ks/Km curves should be and are ionic strength dependent (508) in the same way that the His 12 and His 119 pK values are as observed by NMR (280). 

Quenching of singlet cyanoanthracenes by t-1 in acetonitrile solution results in the formation of the cyanoanthracene anion radical and t-1 cation radical. The formation and decay of these radical ion intermediates have been investigated by transient absorption (143) and Raman (TR3) spectroscopy (89). The yield of ionic photodissociation from the dicyanoanthracene-t-1... 

Although the heterolytic process here is formally a concerted ionic splitting of H2 as often illustrated by a four-center intermediate with partial charges, the mechanism does not have to involve such charge localization. In other words, the two electrons originally present in the H H bond do not necessarily both go into the newly-formed M H bond while a bare proton transfers onto L or, at the opposite extreme, an external base. The term a-bond metathesis is thus actually a better description and may comprise more transition states than the simple four-center intermediate shown above, e.g., initial transient coordination of H2 to the metal cis to L and dissociation of transiently bound H- L as the final step. Examples of this type of activation will be given in this Section. 

Equation (7.12) can be extended to more complex rate equations. Some of the advantages of the stirred flow method are (i) complicated rate expressions can be handled without integration, (ii) reaction can be carried out under constant conditions of solvent composition, ionic strength and transient intermediate can be built up in concentration, detected and measured. 

The mechanism of a chemical reaction consists of a series of chemical equations describing the individual elementary steps that lead to the formation of products from reactants. Much of what chemists know about mechanisms has been gained from studies in which the rate at which reactants are consumed or products are formed is measured as a function of such variables as reactant and product concentration, temperature, pressure, pH, and ionic stiength. Such studies lead to an empirical rate law that relates the reaction rate to the concentrations of reactants, products, and intermediates at any instant. Mechanisms are derived by postulating a series of elementary steps that are chemically reasonable and consistent with the empirical rate law. Often, such mechanisms are further tested by doing studies designed to discover or monitor any transient intermediate species predicted by the mechanism. 

Palladium-catalysed processes typically utilise only 1-5 mol% of the catalyst and proceed through small concentrations of transient palladium species there is a sequence of steps, each with an organopalladium intermediate, and it is important to become familiar with these basic organopalladium processes in order to rationalise the overall conversion. Concerted, rather than ionic, mechanisms are the rule, so it is misleading to compare them too closely with apparently similar classical organic mechanisms, however curly arrows can be used as a memory aid (in the same way as one may use them for cycloaddition reactions), and this is the way in which palladium-catalysed reactions are explained in the following discussion. (For convenience, an organometallic component can be referred to as the nucleophilic partner and the halide as the electrophilic partner, but this should not necessarily be taken to imply reactivity as defined in classical chemistry. Also, references to the halide should be understood to include all related substrates, such as triflates.)... 

Crystal structures of transient intermediates are accessible to study, provided that substantial concentrations of the desired intermediates can be attained, that their lifetimes are long with respect to the minimum x-ray exposure necessary to obtain a quantifiable diffraction pattern, and that crystallographic isomorphism is maintained throughout the progress of the reaction. Concentrations and lifetimes may be manipulated by adjustment of the chemical nature of the reactants, the pH, ionic strength and nature of the solvent, the presence or absence of activators and inhibitors, by pressure, and of course by temperature. Different intermediates may accumulate to high concentrations, with extended lifetimes, under different experimental conditions but it may be true that some key intermediates may not accumulate under any conditions. 

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