Metal Ion Control of Ring Closure Reactions

The basic reactions which are involved in the synthesis of macrocyclic compounds are very simple and of a very few types. In this section we shall consider the basic organic chemistry of macrocyclic ring-closures and how a metal ion may activate or control these reactions. In most cases, these reactions involve the formation of heteroatom-carbon bonds. 

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Naturally, it is possible to extend the daisy-chain . The compounds formed from the interlinking of two rings are termed [2]-catenanes and we will conclude this section by showing one approach to a [3]-catenane. This illustrates another type of ring closure reaction, but is still dependent upon the conformational control of the metal ion in orienting the reactive ends of the precursors. 

It has been shown in previous studies by several fast reaction techniques that the complexation reaction between a metal ion and a bidentate ligand involves several discrete steps (5). As shown in figure 2, step 1 represents rapid (diffusion controlled) formation of an outer-sphere complex and step 2 the formation of a monodentate complex for which the forward rate constant can be identified with that for water exchange (kex) measured by n.m.r. methods. Since PADA is a bidentate ligand, a further ring closure step (3) takes place in which the final product is formed. The rate of ring closure is generally assumed to be rapid compared with the rate of dissociation of the monodentate complex (i.e. kj k 2) ... 

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