Kinetic/thermodynamic templation thermodynamically controlled

The ability of a chemical to act as a template is frequently attributed to a combination of thermodynamic and kinetic factors. As has been defined by Busch [3] a thermodynamic template binds more strongly to one of the products present in an equilibrium (i.e. a mixture under thermodynamic control) shifting the reaction towards the formation of this specific product which is then obtained in higher yields. In contrast, kinetic templates operate under irreversible conditions by stabilising the transition state leading to the final product. 

DMA) binds reversibly to Cjq. A similar effect was observed for 2,6-dimethoxy-anthracene [60], Use of, for example, a ten-fold excess of DMA results in an equilibrium between the various Cgg DMA adducts, with e,e,e-C5oDMA3 as the main component. Hence, synergetic combination of kinetic and thermodynamic control results in the generation of templates such as with incomplete... 

Most of the systems described in Chapter 5 contain small- or medium-sized or multinuclear benzenoid and non-benzenoid arenes. In Chapter 6, Hoger gives an overview over the mastery of the synthesis of macro- and megacycles. He shows different approaches towards shape-persistent macrocycles and carefully examines and discusses selected examples that display the advantages and disadvantages of macrocycle synthesis under kinetic and thermodynamic control. The template approach (both supramolecular and covalent) towards functionalized rings is also discussed and introduces a strong motif of supramolecular chemistry, which is much further developed but in a more polymer-oriented topic, in the next chapter. 

Although it is generally accepted that the f-butylcalix[8]arene is the kinetically and the -calix[4]arene the thermodynamically controlled product while the formation of the -calix[6]arene seems to be due to a template effect, not much is really known about the mechanism of calixarene formation. The hypothesis that 2a is formed from 2e by an intramolecular step ( molecular mitosis ) could not be confirmed by isotopic labelling, which proves a more or less statistical fragmentation and recombination. ... 

Although the mechanism of the base-induced formation of calixarenes has been studied in some detail, the reaction pathways remain uncertain. The most intuitively reasonable proposal is that the immediate precursor of any particular calixarene, regardless of size, is the linear oligomer carrying the requisite number of aryl residues. Another proposal, however, postulates that calix[8]arenes, for example, arise from intermolecularly hydrogen-bonded dimers (hemicalixarenes) formed from a pair of crescent-shaped, intramolecularly hydrogen-bonded linear tetramers. Calix[4]arenes, formed under considerably more strenuous conditions, have been postulated to be the result not of direct cyclization of the linear tetramer but of reversion of the calix[8]arene. The cyclic octamer is viewed as the product of kinetic control, and the cyclic tetramer is viewed as the product of thermodynamic control. The particular efficacy of KOH and RbOH for the formation of calix[6]arenes suggests that the hexamer is the product of template control. 

Shea [13] and Shea and Sasaki [41,51] used ketals (entry f) for imprinting synthetic polymers. Diketones with various distances between the keto groups were treated with polymerizable diols, to obtain template monomers. After polymerization and removal of the template, these polymers showed considerable selectivity for the original template when the imprinted polymers were reloaded with a mixture of diketones. Unlike the reloading experiments mentioned earlier, this bond formation takes place slowly, and therefore it seems to be kinetically controlled in contrast to the bonding reactions described so far, which are thermodynamically controlled. If the arrangement of functional groups is the same, the size and shape of the rest of the... 

Templates can be classified as either thermodynamic or kinetic.[4] Thermodynamic templating occurs in reversible reactions under thermodynamic control the template, when added to a reaction mixture at equilibrium, binds to one of the products and shifts the equilibrium towards this species. Thermodynamic templating can result in very high yields because the template only has to stabilise tlie product. Kinetic templates... 

Therefore, chloroform does not act as a template for particular pillar[n]ar-ene homologs, and pillar[6]arene is obtained under kinetic control. Yields of the pillar[ ]arene homologs under kinetic control were much lower than those obtained under thermodynamic control. Therefore, we investigated several reaction solvents that fit into the pillar[6]arene cavity to act as a template solvent for the synthesis of pillar[6]arenes under thermodynamic control. 

Myriad polydentate aza-macrocycles have been reported 41. The extent of the subject forces limitation of this discussion to only macrocycles containing a pyridine or dipyridine subunit. Most of these coronands have been synthesized by a SchifF base condensation of an aldehyde or ketone with a hfc-primary amine in the presence of a metal ion. The metal ion acts as a template, resulting in dramatic increases in yield of the desired cyclic product over linear polymerization products42 46. Lindoy and Busch45 have described this effect in two ways, kinetic and thermodynamic. If the metal ion controls the steric course of a series of stepwise reactions, the template effect is considered to be kinetic. If the metal ion influences an equilibrium in an organic reaction sequence by coordination with one of the reactants, the template effect is termed thermodynamic. It is the kinetic effect that is believed to be operative in most metal ion-assisted (in situ) syntheses of... 

Metal ion template mediation in macrocyclic synthesis has been a part of the field since its inception, its importance having been realized early in the development of this area. Two specific roles for the metal ion in template reactions have been proposed. These are, in turn, kinetic and thermodynamic in origin." In the kinetic template effect, the arrangement of ligands already coordinated to the metal ion provides control in a subsequent condensation during which the macrocycle is formed. The thermodynamic effect serves to promote stabilization of a structure which would not be favored in the absence of a metal ion. Schiff base condensations tend to be dependent on this latter type of template effect. Some of the more routine and general synthetic procedures will be described here. A more in-depth treatment can be found in a review by Curtis, with particular emphasis on general methods as well as modifications of preformed macrocycles." ... 

Following the introduction of MCM-41 type materials [1], the synthesis of surfactant templated nanostructured materials has attracted the attention of the scientific community because it provides the possibility of tailoring pore size, geometry and surface chemistry through control of the synthesis conditions. Potential applications of these materials range from separations and catalysis [2] to the production of biomimetic materials [3] and devices for optical and electronic applications [4]. Several synthesis protocols have been developed in the last ten years and are the focus of many recent reviews [5]. Despite the enormous experimental effort to develop methods to control the structure and composition of templated nanoporous materials, modeling the different processes has remained elusive, mainly due to the overlapping kinetic and thermodynamic effects. The characterization of... 

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