Stoichiometry - or Other

The formation of cyclic ligands by template reactions has another associated complexity which we encountered in Section 6.3.3. This concerns the number of reacting molecules involved in the formation of the cyclic products and the overall stoichiometry of the reaction. We have not yet considered the control of stoichiometry of the cyclisation reactions in any great detail. 

The formation of macrocyclic ligands by template reactions frequently involves the reaction of two difunctionalised precursors, and we have tacitly assumed that they react in a 1 1 stoichiometry to form cyclic products, or other stoichiometries to yield polymeric open-chain products. This is certainly the case in the reactions that we have presented in Figs 6-8, 6-9, 6-10, 6-12 and 6-13. However, it is also possible for the difunctionalised species to react in other stoichiometries to yield discrete cyclic products, and it is not necessary to limit the cyclisation to the formal reaction of just one or two components. This is represented schematically in Fig. 6-19 and we have already observed chemical examples in Figs 6-4, 6-11 and 6-18. We have already noted the condensation of two molecules of 1,2-diaminoethane with four molecules of acetone in the presence of nickel(n) to give a tetraaza-macrocycle. Why does this particular combination of reagents work Again, why are cyclic products obtained in relatively good yield from these multi-component reactions, rather than the (perhaps) expected acyclic complexes We will try to answer these questions shortly. 

Many examples are known in which multiple components are brought together about a metal ion to form macrocyclic complexes. Typical examples include the formation of meso-tetraphenylporphyrin (6.21) from benzaldehyde and pyrrole (Fig. 6-20, [4+4]), or phthalocyanine (6.6) from phthalonitrile (Fig. 6-21). The formation of the tetraphenyl-porphyrin is catalysed by a range of Lewis acids, and the facile preparation from aldehydes and pyrroles has obvious implications for the bioevolution of porphyrin pigments. Virtually any benzene derivative with ortho carbon-bearing substituents can be converted to a phthalocyanine complex on heating with a metal or metal salt in the presence of ammonia or some other nitrogen source. 

Another spectacular example is seen in the formation of 6.22 from the [4+4] condensation of 2,6-diacetylpyridine with 2-hydroxy-1,3-diaminopropane in the presence of a 

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Reaction SMILES is an extension to standard SMILES used to represent a specific reaction. It uses punctuation to distinguish reactants from products. For example, the reaction SMILES CC(=0)0.CN CC(=0)NC.0 represents the reaction of acetic acid with methylamine to form N-methylacetamide plus water. As with standard SMILES, explicit H atoms are typically not shown, although they may be. For example, the same reaction can represented as [CH3]C(=0)[0FI].[CH3][NH2] [CH3]C(=0)[NH][CH3].[H]0[H]. The punctuation is used to separate reactants from products, and the period is used to separate reactants or products from each other. There are no rules in reaction SMILES that enforce correct reaction stoichiometry or other aspects of actual chemical reactions. 

The characteristics of various substrate materials have been summarized in previous sections. However, there may be substantial variations in the parameters due to processing, composition, stoichiometry, or other factors. This section covers the materials in more detail and also describes some common applications. 

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