Control Over the Ring Size

The operation of the mismatch effects may be seen to best advantage when a range of products is possible from a single reactant or set of reactants. The reaction of ethylene oxide with metal salts results in the formation of crown ethers (Fig. 6-30). Obviously, a whole range of different cyclic oligomers and acyclic polymers could be formed from ethylene oxide. If we specifically wanted to obtain 18-crown-6, with a hole size of about 1.4 A, we would expect to use a potassium ion as template (r = 1.38 A). In fact, 18-crown-6 is obtained in good yield from the reaction of ethylene oxide with potassium tetra-fluoroboratc. In contrast, if we wanted 12-crown-4, with a hole size of about 0.8 A, it 

In many cases it is possible to utilise the hole size effects for the synthesis of specific types of macrocycle. Thus, a tetradentate macrocycle (6.33) is expected to be obtained from a template condensation of 2,6-diacetylpyridine with 1,5,9-triazanonane in the presence of small, first-row transition metal dications. The hole size of 6.33 closely matches the size of these metal ions. This is indeed what happens when Ni2+ (r = 0.8 A) is used as a template for the condensation and the nickel(n) complex of 6.33 is obtained in good yield (Fig. 6-32). However, when Ag+ (r = 1.0 A) is used as a template, the metal ion 

Clearly it is possible to exert relatively subtle control over the direction of cyclisation reactions by a judicious choice of template ion, even if the effects may not be fully quantified or understood. 

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