Selective monohydrolysis of bridged and unbridged calix[4]arene esters and its inhibition by alkali ion. Evidence for hydronium ion complexation

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Abstract
Several bridged and unbridged calix[4]arene tetraester derivatives are hydrolysed by trifluoroacetic acid in chloroform with the loss of one, and only one, ester group to form triester monoacids. In bridged tetraethyl ester 11, the group that is hydrolysed is that on the aromatic moiety not carrying the methylene bridge. The hydrolysis process is inhibited by the presence of alkali metal cations, especially sodium. Evidence is presented in favour of a hydrolytic process involving hydronium ion complexation within the hydrophilic oxygenated cavity of the calixarene receptor. Crystals of the triester monoacid 5 were studied at 123 K and are monoclinic, space group P21/n, with four molecules in a cell of dimensions a= 13.554(6), b= 20.095(9), c= 20.658(17)Å, β= 102.27(5)°. The structure was solved by direct methods and refined by full-matrix least-squares calculations; R= 0.063 for 2017 observed reflections. The calix[4]arene has a distorted cone conformation similar to that found in other closely related derivatives. The hydroxy group of the carboxylic acid moiety is hydrogen bonded [O O 2.722(10)Å] to a proximal ethereal oxygen and not to carbonyl oxygen atoms.

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