Template mechanistic aspects

In a recent study, some mechanistic aspects of this templated process have been determined quantitatively [28]. Using UV-Vis spectroscopy to monitor the kinetics of the macrocyclization reaction, it has been established that the rate of ring closure of the cationic precursor to the [ljimidazoliophane (4) is increased up to ten times in the presence of 0.04 mol/1 solution of a chloride source. The chloride stabilizes the transition state (i.e. a kinetic template) favouring the macrocyclization through hydrogen bonding. 

An essential feature of template reactions of both the thermodynamic and kinetic type is the formation of a new chelate ring. One of the main difficulties in a discussion of mechanistic aspects of template reactions is the inevitable mixture and overlap of mechanistic effects. Those reactions which clearly exhibit a kinetic template effect could also depend to some extent on a thermodynamic one. Also, in some multistep macrocyclization processes, for example, both effects could be involved. Despite the inherent difficulties, the following discussion in this section will be subdivided into two, to cope predominantly with the thermodynamic and kinetic template effects. A further subdivision of thermodynamic and equilibrium effects will not be made. 

The most detailed picture of zeolite synthesis available to us is for siliceous silicalite. We will discuss this here in an attempt to illustrate molecular recognition of template, evolutionary recombination of reaction intermediate oligomers and self replication. Detailed information became possible once homogeneous conditions for zeolite formation in silicate synthesis were found in the absence of gel formation. For a concise and excellent review on general mechanistic aspects of zeolite synthesis we refer to Cundy an Cox l... 

Mechanistic Aspects of Template-Assisted Polyaniline Synthesis. 82... 

Currently the main interest in template reactions lies in their key role in the controlled synthesis or the self-assembly of a variety of supramole-cular entities (449). One needs a combination of intuition, conjecture, and serendipity (450) a recent example of successfully combining serendipity and rational design is provided by the silver(I)-promoted assembly of one-dimensional stranded chains (451). One also needs an understanding of mechanism in order to optimize the selection and design of building blocks and templates for the generation of yet more sophisticated supramolecular structures references cited in this present review contain at least some kinetic or mechanistic information or speculation. Template routes to interlocked molecular structures have been reviewed (452), while a discussion of switching by transition metal contains a little about the kinetics and mechanisms of this aspect of template... 

Template reactions are now widely claimed but seldom proved. Detailed mechanistic work has not been carried out on many of the more complicated processes, largely because of the level of complexity and the number of steps involved. However, the concept has proved to be useful, especially as a stimulus for the synthesis of very complex, polyfunctional molecules. This aspect of synthetic utility will be covered in Chapter 61.1. 

More recently imidazoiium ILs have been used as a template for the synthesis of a plethora of stable transition-metal NPs with small size and narrow size distributions [23, 24]. These particles immobilized in ILs constitute highly active multiphase catalytic systems for various reactions. The main goal of this chapter is to disclose the mechanistic and structural aspects of the formation and stabilization of transition-metal NPs in imidazoiium ILs, especially those that have been used in catalytic reactions. 

The key aspect of the LCT mechanism is that the liquid crystalline mesophases or micelles act as templates rather than individual single molecules or ions. Accordingly, the final product is an inorganic (e.g., silicate) skeleton that contains voids, mimicking the shape of the surfactant mesophases. This mechanism, first proposed by researchers at the Mobil Corporation for the synthesis of M41S materials, is based on similarities between liquid-crystalline surfactant assemblies and the resulting mesoporous solid product. The whole process can be described by two possible mechanistic pathways, which are represented schematically in Fig. 4. 

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