Metal-Directed Rearrangement to Produce a More Suitable Cavity

5 Metal-Directed Rearrangement to Produce a More Suitable Cavity 

We have now seen that the relative sizes of the metal ion and the bonding cavity of the macrocyclic ligand may have profound effects upon the course of template syntheses of macrocycles. These have been expressed in the stoichiometry of the reaction and in the conformation of the macrocyclic products. However, it is also possible to observe changes in the reactivity of the macrocyclic ligand itself as a result of interaction with metals of the wrong size. 

Let us consider the [2+2] macrocyclic ligand 6.39, which is prepared by the non-template condensation of 1,2-diaminobenzene with 2,6-diformylpyridine. The hole size of this ligand is about 1.3 A, so we would expect it to be too large to bind first-row transition metal dications. As a matter of interest, the ligand binds K+, with an ionic radius of 1.38 A, more strongly than does the crown ether, 18-crown-6. In the absence of any che- 

In earlier chapters we noted that metal ions could either activate or deactivate an imine with respect to addition of a nucleophile. We will now see an example of metal-ion activation in action. In fact, the complexes that are formed from 6.39 arise as a result of metal-initiated nucleophilic attack at the imine groups. The reaction of the free ligand 6.39 with methanolic cobalt(n) acetate results in the attack of methanol upon one of the imine bonds of the initially formed complex (Fig. 6-39). 

A similar complexity of rearrangements is observed in the related compound 6.42, which is derived from the metal-free reaction of 2,6-diacetylpyridine with 1,2-diamino-benzene (Fig. 6-41). In this case, the steric interactions between the methyl groups and the 

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