Template reactions/synthesis benzo crown

A number of reports [10, 17, 22, 25-27] suggest that, in the case of condensation between aromatic diols and polyethylene glycols, the nature of the template exerts an influence on the rate of macrocyclisation. The templates form various series depending on the size of the synthesised crown ether, but lithium ion is an inhibitor in all cases. This must be attributed to the fact that Li+ forms the most stable ion pair with phenolate and simultaneously gives the least stable complexes with benzo crown ethers. It should be noted that alkaline earth metal ions, even in small concentrations, promote these reaction more effectively than alkali metal ions. In addition, it has been emphasised [12] that there is a definite correlation between the basicity of the substance used in the template synthesis of macrocyclic polyethers and the yield of final product. 

The use of metals for prearranging reaction centers as neighboring groups has a special value in the production of macrocycles (template effect). Although these ligands can be sometimes prepared directly, the addition of metal ion during the synthesis will often increase the yield, modify the stereochemical nature of the product, or even be essential in the buildup of the macrocycle. There have been few mechanistic studies of these processes. The alkali and alkaline-earth metal ions can promote the formation of benzo[18]crown-6 in methanol ... 

Figure 3.45 shows the results of kinetic studies of the template synthesis of benzo [18] crown-6 according to the reaction shown, as a function of the templating cation. Clear rate enhancements... 

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