Reactivity Control by Calixarenes

Luigi Mandolini, Roberta Cacciapaglia, and Stefano Di Stefano IMC CNR Sezione Meccanismi di Reazione, c/o Dipartimento di Chimica, Universita La Sapienza, 00185 Roma, Italy 

Calixarenes are oligomeric cyclophanes derived from the condensation of phenols with formaldehyde. They became popular in the late seventies and have played a very important role in Supramolecular Chemistry ever since, as witnessed by numerous books and review articles on the subject. Calixarenes are important bnilding blocks for the preparation of a large variety of supramolecular complexes and materials, thanks to their ability to act as hosts, and the possibility of introdncing a large nnmber of functions at either rim by means of selective derivatization. 

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. 

Molecular Encapsulation Organic Reactions in Constrained Systems 2010 John Wiley Sons, Ltd 

The p-sulfonatocalix[n]arenes 4, originally synthesized by Shinkai et al. as water soluble calixarenes for catalytic studies in water solution, were more recently used by Ueoka et alP as catalysts in the specific acid catalyzed methanolysis of A -Ac-L-amino acids (Phe, lyr, Trp, His, Lys, Arg). Rates of methanolysis in the presence of the calix[n] arene catalysts, normalized per sulfonic group, were compared with rates obtained in the presence of the noncyclic analogue p-hydroxybenzenesulfonic add. Rate enhancements - ranging from 12- to 86-fold - relative to control were recorded only in the methanolysis of basic amino acid derivatives (His, Lys, Arg), but neutral amino add derivatives (Phe, Tyr, Trp) responded virtually in the same way to the presence of cyclic and non-cyclic catalyst. Michaelis-Menten kinetics and H NMR spectral evidence pointed to the intermediacy of inclusion complexes of 4 with the protonated form of basic amino acid derivatives, as shown in 5 for the His-4 (n = 4) combination. 

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It is felt that calixarenes have not exhausted their potential yet, and that future research in the area of reactivity control and catalysis by calixarenes looks promising of exciting new results. 

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