Upper Rim of Calixarenes

The fragment condensation method for the synthesis of calixarenes (see Section 

Small amounts of phenol are often added to the de-tert-butylating mixture to increase the rate of reaction, possibly because phenol is a good acceptor molecule but also because, for steric reasons, it may be more effective than the calixarene in generating the H necessary to initiate the reaction. Thus, 4 affords 4 in 

Kammerer, H. Happel, G. Bohmer, V. Rathay, D. Monatsh. Chem. 1978, 109, 767. 

This suggestion is supported by the fact that the de-rm-butylation of the more weakly intra-molecularly hydrogen-bonded p-rt rt-butylcalix[5]arene is very rapid even in the absence of phenol, while that of the more strongly intramolecularly hydrogen-bonded p-rert-butyl-calix[4]arene is very sluggish Gibbs, C. G. Gutsche, C. D. unpublished observation. 

With the p-positions of the calixarenes made available by de-tert-butylation, a wide variety of p-functionalization procedures have been explored. A number of these were developed in the 1980s (see ref. 1, pp. 135-144), including the electrophilic substitution route 4 , 5 , 6 , etc. 169 (E = NO2, SO3H, COR, COAr, CH2CI), the p-Claisen rearrangement route 4 , 5 , 6 , etc. - 173 174, and the 

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Contrary to derivative 3, calixarene 5 was found to create a 1 2 complex with chloride anion in the solid state. Both amidic functions introduced into the upper rim of calixarene operated independently of each other and held one chloride anion with a Cl... HN distance of approx. 2.37 A and two CH... Cl con-... 

The introduction of -S03H groups at the upper rim of calixarenes can be easily obtained by sulfonation with concentrated H2S04 (Scheme 7.11).25 This method represents a rapid and efficient way of preparing water-soluble calixarenes. 

Conjugates of pyridine at the upper rim of calixarenes are relatively scarce. Except for one case of direct attachment of pyridine N, mostly... 

Fig. 1 Molecular structures of the calixarenes, where, commonly, n = 4. 6. and 8 and the calixresorcinarene (A). The strategy for cation complexation by the introduction of ligand groups at either the lower or upper rims of calixarene derivatives (B). Metal complex with...

Another outstanding study on the usage of an effective calixarene derivative as a biomimetic catalyst for the cleavage of the RNA model compound, 2-hydroxypropyl p-nitrophenyl phosphate was reported by Baldini et al. [35]. They prepared four calix[4]arene derivatives substituted with two to four guadinium moieties at the upper rim of calixarenes, and investigated their catalytic affinities in the cleavage of the RNA model compound 2-hydroxypropyl p-nitrophenyl phosphate in water (see Figs. 27.17 and 27.18). They found that the... 

Fig. 27.17 Prepared four calix[4]arene derivatives that substituted with two to four guadinium moieties at the upper rim of calixarenes [35]...

The facile removal of the p-tert-bntyl groups from the upper rim of calixarenes by treatment with aluminum chloride as Lewis acid is typically carried out before the insertion of the Uphophilic moieties either at the lower (phenolic) or the upper rim of the macrocycle (para- position). 

Changing a functional group at the upper rim of calixarene molecule is generally attempted since functionalization at the lower rim often induces changes in the cone/bowl shape of macrocycle. Sorption isotherms of 8 show that it has more binding affinity toward selected guest vapors compared to 2 [31], Material 8 absorb twice as much vapor of benzene and toluene and four times as much cyclohexane and carbon tetrachloride than 2 at similar experimental conditions. 

The smaller p -tert-butyl-calix [4] arenes have a rich coordination chemistry as well (65). Of these, however, only the upper-rim modified calixarenes seem to support metal complexes with confined binding sites (66), except in those cases where the lower-rim substituents form an appended cavity. Thus, Matt and coworkers have reported (67) a pocket-shaped calix[4]arene ligand L2 bearing two lower-rim [([Pg.410]

Calixarene derivatives involving crown ethers like 233 exhibit complexation ofalkali or ammonium cations typical of crown ethers that is especially effective in the partial cone conformation [26]. By a proper functionalization selective synthetic receptors for specific hosts have been developed. For instance, two 2,4-diaminotriazine groups at diametrical positions of the upper rim of a... 

As with cation binding hosts, it is also possible to use calixarene frameworks (Section 3.14) to organise anion binding moieties. Addition of a cobaltocinium bis(amide) moiety to the upper rim of a calix[4]arene with p-toluenesulfonate functionalities to enhance solubility results in a rigid host 4.64... 

The shape of a cyclodextrin is often represented as a tapering torus or truncated funnel and, like the upper and lower rims of calixarenes, there are two different faces to the cyclodextrins, referred to as the primary and secondary faces. The primary face is the narrow end of the torus, and comprises the... 

The most readily available calixarenes are compounds derived from p-t-butylphenol, 1-3. In order to introduce functional groups at the upper rim of the calix, the f-butyl group has to be removed, and this can be performed in 70-90% yield by reacting the p-f-butylcalixarenes with an excess of anhydrous aluminium trichloride in toluene in the presence of phenol at room temperature (Scheme 7.10). The unsubstituted p-//calixarenes thus obtained are versatile intermediates which give access to hosts having a large variety of properties. 

Both the lower (phenolic) rim and the upper rim of the calixarene can be functionalized. A huge range of calixarene derivatives have been synthesized in this way, and only a few can be mentioned here. In addition, the linking methylene groups can be modified to give another group of useful ligands. ... 

The only way to functionalize the upper rim of 4-f-butylcalix[4]arene is to remove the f-butyl group and replace it with something more useful. Two methods have been reported by which this de-ferf-butylation, sometimes referred to by the more evocative term of neutering , may be achieved. The first method is probably the more conventional of the two and involves the use of aluminium trichloride and phenol. It is in essence a Friedel-Crafts reaction where a f-butyl group is attached to either benzene or phenol the innovation is that the source of the f-butyl substituent comes from the calixarene [1]. This retro-Friedel-Crafts reaction is thus an effective method for transalkylation that removes an unwanted alkyl group from calix[4]arenes as shown in Figures 3.10 and 3.11. Moreover, it is possible to subtly alter the reaction conditions and partially de-ferf-butylate... 

Embroidering the Baskets Modifying the Upper and Lower Rims of Calixarenes... 

Considerable attention was devoted in the 1980s to functionalizing the upper and lower rims of calixarenes (see ref. 1, pp. 127-148), and this has continued unabated in the 1990s, as witness the relative lengths of the Chapters 5 in the two volumes. This comes as no surprise, because the utility of the calixarenes for the majority of potential applications depends upon suitable modification of the parent compounds. Organic synthesis in its many guises remains essential for a large fraction of chemical research. 

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