Reaction control calixarenes

This chapter focuses on reactivity control in stoichiometric and catalytic reactions taking place in the confines of supramolecnlar complexes of reactants with calixarene receptors. Earlier work on the subject was reviewed by us in 2000. Quite recently, Homden and Redshaw have published an extensive review on the use of calixarenes in metal-based catalysis. Because of space limitations and, even more importantly, to avoid extensive overlap with the already reviewed material, the scope of this chapter is restricted to works treated only marginally, or not treated at aU, in the above review article. The first section deals with examples in which reactivity control takes place via substrate inclusion into the calixarene cavity. The other section illustrates the use of the calix[4] aiene upper rim in the construction of di- and trimetalfic complexes capable of esterase and nuclease activity. 

In this chapter we have shown that calixarenes have undergone a limited, but highly significant amount of research on their ability to include into their cavities reactant guests or part-structures of reactants, thereby exerting a strong reactivity control of hosted species in stoichiometric and catalytic reactions. 

Although the mechanism of the base-induced formation of calixarenes has been studied in some detail, the reaction pathways remain uncertain. The most intuitively reasonable proposal is that the immediate precursor of any particular calixarene, regardless of size, is the linear oligomer carrying the requisite number of aryl residues. Another proposal, however, postulates that calix[8]arenes, for example, arise from intermolecularly hydrogen-bonded dimers (hemicalixarenes) formed from a pair of crescent-shaped, intramolecularly hydrogen-bonded linear tetramers. Calix[4]arenes, formed under considerably more strenuous conditions, have been postulated to be the result not of direct cyclization of the linear tetramer but of reversion of the calix[8]arene. The cyclic octamer is viewed as the product of kinetic control, and the cyclic tetramer is viewed as the product of thermodynamic control. The particular efficacy of KOH and RbOH for the formation of calix[6]arenes suggests that the hexamer is the product of template control. 

A DCL of short polymers has been generated from the condensation of resorcinol and 1,4-butanedial (Scheme 1.11) [41]. Resorcinol and 2,5-dimethoxytetrahydro-furan (3.3 equiv.) were combined in ethanol at 80 C in the presence of HCl. Acid-catalyzed decomposition of the 2,5-dimethoxytetrahydrofuran led to the formation of 1,4-butanedial. Each aldehyde then underwent two condensation reactions with resorcinol to form ladder-like polymers containing calixarene moieties under thermodynamic control. 

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