Calixarenes nitration

A novel, mild system for the direct nitration of calixarenes has been developed using potassium nitrate and aluminum chloride at low temperature. The side products of decomposition formed under conventional conditions are not observed in this system, and the p-nitro-calixarenes are isolated in 75-89% yields.17 Such Friedel-Crafts-type nitration using nitryl chloride and aluminum chloride affords a convenient system for aromatic nitration.18 Nitryl chloride was previously prepared either by the oxidation of nitrosyl chloride or by the reaction of chlorosulfonic acid with nitric acid. However, these procedures are inconvenient and dangerous. Recently, a mixture of sodium nitrate and trimethysilyl chloride (TMSC1) has been developed as a convenient method for the in situ generation of nitryl chloride (Eq. 2.6). 

Following the recommendation of Reinhoudt,23 in almost the majority of cases, nitrophenyl alkyl ethers were used as diluents for the extraction tests by the Cadarache group, because they are able to dissolve calixarenes at relatively high concentration. Moreover, the basicity as well the dielectric constant of these diluents improves cation extraction by better solvation of the associated nitrate anions (Table 4.3). 

Calixarene crown-6 compounds, which are neutral extractants like crown ethers, are able to coextract technetium with cesium. Tests carried out with several calix-arene-crown ethers (MC7, MC8, MC14, BC2, BC5, BC8, and BC10) show that the extraction of technetium, present in the aqueous phase at a concentration 10 5 M, is enhanced as the cesium concentration in the aqueous phase increases from 10 5 to 10-2 M. As expected, an increase of nitrate concentration prevents pertechnetate extraction in competition with nitrate anion. The extraction of technetium is only appreciable when the nitric acid does not exceed 1 M. Distribution ratios DCs (close to 8) are comparable for the various calixarenes. However, a decrease of extraction is observed for naphtho derivatives.88 89... 

Extraction of thorium nitrate and europium nitrate 0 0 4 M) from 1 M HN03 into dichloromethane was carried out for 19 calixarenes totally or partially substituted on the lower rim by phosphine oxide moieties (CPol-CPol7-CPol9-CPo20) and for one calixarene substituted by phosphinate (CP0I8) (see Section 4.7).IM 152 154... 

Percentage Extraction, % , and, in Brackets, Distribution Ratios, D, for Lanthanides and Thorium Nitrates by Narrow-Rim Calixarenes CMPO from 1 M HN03 into Dichloromethane T = 20°C, CM = 10 4 M... 

To increase the distribution ratios, a solution of lithium nitrate 1M was used. This salt, which has a common anion with europium and americium to be extracted but a cation which is usually negligibly extracted by other calixarenes, should increase the distribution ratios according to the relation Du = A (JU "[N03- ". It seems that these calixarenes, as several nitrogen ligands do, present a certain affinity for this lithium cation. The lipophilic dicarbollide anion (BrCosan), which is known to facilitate cation extraction, was implemented and led to a strong increase of the extraction of cations from 10 3 M HN03 solutions. Under these conditions, only thiopicolinamide was not able to significantly extract trivalent actinides.187... 

The effect of radiolysis on complexed solutions proposed for the FPEX process was investigated. The calixarene BOBCalixC6 was present with substi-tuted-18-crown-6, aromatic fluoro-propanol as modifier, and trioctylamine in aliphatic diluents (35). The effect of gamma irradiation was negligible up to 0.02 MGy. An important change of coloration and the appearance of a third phase was observed, but due to the nitration of the modifier. Instead of the BC-6 and oct-MC6 calixarenes, which presented a decrease of Cs extraction after radiolysis, the presence of alkylated phenyl moieties provided protection for the Cs site. 

The related monodentate ligands 5.33 and 5.34 form MT4 type complexes with the relatively inert platinum(II). The complex [Pt(5.33)4]2+ adopts a 1,2-alternate conformation (by analogy with the calixarenes, Section 3.14) giving two anion-binding pockets that can bind planar anions such as nitrate and acetate in a 1 2 ratio. Acetate displays a positive allosteric effect with Kn being more than twice Ku. Tetrahedral anions are bound in a 1 1 ratio, however, and it is likely the complex can adopt a variety of conformations in solution. Related to [Pt(5.33)4]2+ is [Pt(5.34)4]2+. Like the complex with 5.33, X-ray diffraction results show that the chloride complex adopts a 1,2-alternate conformation with a... 

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. 

The bromination and acetylation reactions show that the calixarenes are capable of undergoing conventional electrophilic substitution reactions without rupture of the macrocyclic ring. Zinke et al.18) reported the nitration of p-tert-butylcalixarene (ring size uncertain) and obtained a material that failed to melt up to 400 °C, exploded when heated more strongly, and had a nitrogen analysis in agreement with a tetranitro-calixarene. Attempts to obtain a nitration product from calix[4]arene 59,... 

The fact that the one-pot synthesis of caUxarenes works best with p-f-butylphenoP was beneficial for the whole area, since the f-butyl groups are easUy removed by trans-butylation with AICI3 in toluene (as solvent and acceptor), converting calixarenes 2a-e into the p-unsubstituted calixarenes 2na-e. Thus, the p-positions at the wide rim are available for virtually all kinds of electrophilic substitution reactions which are possible with phenols . Sulphonation, nitration, bromination (or iodination), bromomethyla-tion, aminomethylation, formylation, acylation and coupling with diazonium salts are examples. 

Partial (ipso) substitutions at the wide rim of calixarenes are known, and in some cases are of preparative importance. The ipio-nitration of cahx[4]arene tetraethers, for instance. 

Examples of the selective substitution of phenol units in partially O-alkylated (or 0-acylated) calixarenes (reaction c)) are the bromination , iodination, nitration , formylation , chloromethylation , alkylation and coupling with diazonium salts. ... 

The two calix[4]arenes, 24 and 26, are examples of useful derivatives the former binds lanthanides to form luminescent complexes, the latter, prepared from compound 25, is water soluble and crystallizes in a variety of forms, from laminates to nanospheres. Other derivatives are to be found in the literature. Simple derivatives, such as the O-alkyl compounds, are used as a basis for upper rim functionalization treatment of these compounds with nitric acid replaces the upper f-butyl groups with nitrates. Nitrocalixarenes can be reduced to the corresponding amines to generate a platform for further extension of the cavity and have been used to bind metals or small molecules. Oxa- and azacalix[3]arenes represent crown ether-calixarene hybrids and have binding modes reminiscent of both classes of molecules. Many alternative upper rim substituents can be introduced and, in the case of the azacalixarenes, the /V-substituent can easily be varied. 

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