Upper rim substitution

It has been shown that upper-rim substitution of some dithiocarbamoyl-derivatized calix[4]arenes affects selectivity of Ptr extraction.327,329 Some selectivity over Pb, Hgn, and Ptn has been achieved by the introduction of 2-pyridyl-A-oxide groups on the lower rim, but Sn11, Ag1, Pd11, and Au111 were still extracted to a greater extent.32... 

Both lower rim and upper rim substitutions have been used to modify the coordination strength of the calixarenes and the properties of the resulting edifices. We shall only give a few examples here, with an emphasis on the efforts aimed at designing efficient luminescent probes and extraction agents. 

Upper rim substitution is typically more synthetically challenging than the lower rim, but can confer very useful properties on the macrocycle. For example, water soluble / -sulfonatocalixarenes have been synthesized and found to be highly water soluble, unlike the hydrophobic p-t-butyl derivatives. The sulfonated calixarenes form a variety of metal complexes where the metal is usually bound to the sulfonato groups of the calixarene. 

Among about 200 structures deposited in CSD a large variation of calix[6]arenes derivatives is reported. The derivatization concerns the para-substitution on the upper rim, substitution of hydrogen atom of hydroxyl grout on the lower rim, or both. Other derivatives are also reported. 

Alternatively, the Sn2 nucleophilic substitution reaction between alcohols (phenols) and organic halides under basic conditions is the classical Williamson ether synthesis. Recently, it was found that water-soluble calix[n]arenes (n = 4, 6, 8) containing trimethylammonium groups on the upper rim (e.g., calix[4]arene 5.2) were inverse phase-transfer catalysts for alkylation of alcohols and phenols with alkyl halides in aqueous NaOH solution to give the corresponding alkylated products in good-to-high yields.56... 

A. Marra, M.-C. Schermann, A. Dondoni, A. Casnati, P. Minari, and R. Ungaro, Sugar calixarenes Preparation of calix[4]arenes substituted at the lower and upper rims with O-glycosyl groups, Angew. Chem. Int. Ed., 33 (1994) 2479-2481. 

We wanted to explore the limits of the self-assembly approach for the synthesis of well-defined structures with nanoscale dimensions by further increasing the size of the assemblies. Our group has shown that calix[4]arenes diametrically substituted with two melamine units at the upper rim are very good platforms for the formation of double rosette assemblies. 

Several works have been reported for selective substitution of nitro groups at the upper rim of the calix[4]arene [39,40], The selectively i/Avo-nilraled p-tert-butylcalix[4]arene was used as starting material for the preparation of bisphthaloni-trile 5. Nitro groups at the upper rims of 6 were coupled with nitro groups of four unsymmetrical Pcs 7 in order to obtain the hexanuclear zinc Pc 8, Fig. 2 [41],... 

Fig. 1 (a) Structure of a methylene-bridged cavitand 1 substituted with functional groups (G) at the upper rim. (b) General structure of cavitands with different bridging units (BU) and one example of an aryl-bridged cavitand [38], (c) Structure of calix[n]arenes 2 with functional groups (G) at the wider rim. (d) Conformations of calix[4]arenes... 

The introduction of nitro groups at the upper rim of calix[4]arenes is a quick and useful method for the preparation of functionalized calix[4]arenes. This reaction is quite general and can be conveniently carried out on a wide variety of calix[4]arenes substituted at the lower rim. When the starting calix-arene is partially substituted at the lower rim, the substitution takes place at the more reactive phenol rings. 

Some other capsules have been described in which both complimentary functions, carboxylic acids and nitrogen-containing aromatic rings, are respectively attached to the rims of two bowl-shaped molecules. Reinhoudt has prepared selfassembling, multi-hydrogen bonding molecular capsules in which a calix[4]arene substituted with four carboxylic functions at the upper rim interacts with a complimentary calix[4]arene with four pyridines attached to the lower rim. These capsules have been identified by H-NiMR, IR and VPO measurements but encapsulation properties have not been reported [106]. 

The first examples of urea-containing dimeric capsules were discovered independently by Rebek and Bohmer [165,166]. These are based on calix[4]arene ethers, in which the lower rim ether units help to fix the calixarenes in a cone conformation via intramolecular interactions. Figure 60 shows the generic structure of such systems. Synthetically, the urea calixarenes are readily prepared. The calixarenes are first nitrated, followed by a reduction of these groups into amines. The urea derivatives are fixed to the upper rim by reaction of the p-amino calixarenes with isocyanates. Many systems were studied using differently substituted ureas which contain either short alkyl chains or simple phenyl derivatives. Studies also involved the changing of the lower rim ether substituents. 

Finally, both upper and lower rim substitution have been used to produce a class of macrocycles referred to as calixarene-crown ethers, or calixcrowns. One simple example is shown here (103). These receptors combine characteristics of the crown ethers and calixarenes, and have been intensively studied for metal ion extraction, in particular, for the removal of cesium from nuclear waste. ... 

Three equivalents of a calix[4]arene derivative, diametrically substituted at its upper rim with two melamine units, have also been demonstrated to yield well-defined box-like assemblies on interaction with six equivalents of 5,5-diethylbar-bituric acid. The product, which is based on an extended rosette motif, consists of nine different components held together by 36 hydrogen bonds. It is stable in apolar solvents and remains so even at very low concentrations. Further, the structure remains intact in the solid state, as confirmed by an X-ray diffraction study. In an extension of this study, three enantio-pure calix[4]arene-dimelamine derivatives were employed for a parallel study. In this instance, the chiral information in the individual calixarene derivatives proved successful in controlling the conformation of the final assembly - a homochiral supramolecular entity was the result. 

Calix[5]arenes have also been demonstrated to form crystalline capsule-like dimers. Specifically, a tetra-substituted calix[5]arene with four methyl and iodo groups attached at opposite rings along the upper rim formed a dimer that encapsulated buckminsterfullerene, or C60 (Fig. 18).36 Raston has also described dimers based on tetrabenzyl-derivatized calix[3]- and calix[5]arenes that encapsulate C60 in the solid state.37,38 The components assembled via van der Waals forces. 

Synthesis of calix[4]arene library substituted with peptides at the upper rim, H. Hioki, Y. Ohnishi, M. Kubo, E. Nashimoto, Y. Kinoshita, M. Samejima and M. Kodama, Tetrahedron Lett., 2004, 45, 561. 

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