Azacrown decorated calixarenes

The use of the upper rim of calix 4 arcncs, blocked in the cone conformation, as convenient platforms for the introduction of metal ion binding sites is well documented [26e,291. Table 5.9 shows the catalytic performances of regioisomeric vicinal 21-Ba2 and distal 22-Ba2, and of the m-xylylene derivative 17-Ba2 in the basic ethanolysis of esters 14 and 23-25 [30]. 

The dinuclear catalyst 21-Ba2, in which the azacrown ether units are linked to vicinal positions of the calix 4 arcnc scaffold, is not only superior to its diagonal regioisomer 22-Ba2 in all cases, but is also superior to 17-Ba2 in the reactions of esters 14, 23, and 24. Modest levels of cooperation between metal ions are seen in the catalyzed reactions of the longest substrate 25, which indicates that dinuclear complexes cannot expand their intermetal distances to adapt to the long carboxylate-carbonyl distance in 25. 

There is no doubt that a close fit of ester size to intermetal distance is an important prerequisite for efficient catalysis. However, the superiority of 21-Ba2 to 22-Ba2 can hardly be ascribed to a more suitable intermetal distance in the former, which indicates that other factors, whose origin is still poorly understood, may come into play. 

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