Calixpyrroles

A new type of calixarene-capped calixpyrrole (9) has been generated (32 %) in one-step from p-fe/t-butylcalix[4]arene tetramethylketone as the template <96TL7881>. A cylindrical calix[4]-fois-cryptand, in which the central calix[4]arene possesses two 1,3-altemating diaza-tetraoxa macrocycles on each face, has been synthesized <96TL8747>. A series of substituted l,4-(2,6-pyridino)-bridged calix[6]arenes has prepared and studied <96LA1367>. 

The synthesis and biological evaluation of pyrrole macrocycles (i.e., porphyrins, expanded porphyrins, and calixpyrroles) and linear oligopyrroles comprise perhaps the largest body of work published each year that can be classified as pyrrole chemistry. Unfortunately, due to space limitations, the discussion of selected advances in this area could not be included. 

Calix[4]pyrroles offer an alternative receptor site for the recognition of anions. Miyaji et al. [400] prepared a series of calixpyrrole-anthracene receptors that possess conjugated (73) and unconjugated bond pathways of different lengths (74 and 75). The stability constants (103—105 M-1 determined by NMR titration) for the anion complexes of each member of the series follows the trend... 

Figure4.22 Separation of oligodeoxy thymidylate fragments containing 12-18 nucleotide sub-units (dT12 18) on calixpyrrole modified silica gel column. (Reproduced by permission of The Royal Society of Chemistry).

Sessler, J.L., Zimmerman, R.S., Bucher, C., Krai, V. and Andrioletti, B. (2001) Calixphyrins. Hybrid macrocycles at the structural crossroads between porphyrins and calixpyrroles, Pure Appl. Chem. 73, 1041-1057. 

A number of N-confused systems are known with macrocyclic conjugation interrupted by sp3 meso bridges. These systems, which include analogs of calixpyrroles [252] and calixphyrins [253], fall outside the scope of the present review. A number of fully conjugated N-confused expanded porphyrins have been obtained by use of predesigned N-confused substrates. The most representative examples of such structures are collected in Fig. 32. 

While the selective interactions of functionalised calixarenes with cations have been studied broadly for almost three decades, the application of cal-ixarene-based receptors for anion recognition is a relatively new research topic [2]. This review is focused on recent developments in the design and synthesis of calixarene-based anion receptors. Although the name calixarene was originally designated only for phenol-formaldehyde derivatives 1, recently many structural variations and mutations have been formed. Some of them, such as calixpyrroles [3], are widely used for anion recognition nevertheless, this review is restricted only to classical calixarenes 1 and newly discovered thiacalix-arenes 2 [4]. 

Other methods have been used, for example complexing a dye such as Methyl Red with a receptor for nitrate [135]. Addition of nitrate ion causes formation of a nitrate-receptor complex, leading to the release of the chromophore and a large absorbance change. Similar methods have been developed [136] using complexes between calixpyrrole and 4-nitrophenolate ion which are colourless, but on addition of a suitable anion, release the intensely yellow free 4-nitrophenolate anion. 

Other workers have used pyrilium cations [137] as sensors for anions such as ATP or sulphate. Charge-transfer complexes between calixpyrroles and chlo-... 

Calixpyrrole-Fluoride Interactions From Fundamental Research to Applications in the Environmental Field... 

The applications of calixpyrroles in the design of sensors and new materials with potential use as decontaminant agents for the removal of fluorides from water are discussed. 

Calixpyrroles, a more recent addition to the assortment of macrocycles are able to recognise anions selectively and these are discussed in the next section. 

Figure 1 shows the general structure of calixpyrroles. The basic ring structure resembles that of porphyrin. In the past, four pyrrole rings linked by methylene groups to form colourless macrocycles (that feature in the biosynthetic pathways to pyrrole pigments) were referred to as porphyrinogens [22], The term calix[4]pyrrole was later ascribed to these macrocycles and their synthetic derivatives because of their relation to calix[4]arenes [23],... 

Unfortunately, these issues have not been often considered and have led to some controversy in the anion complexation data involving calixpyrroles. The following sections will discuss 1H NMR, conductometric and thermodynamics studies involving calixpyrrole, its derivatives and their interactions with the fluoride anion in different media. 

Table 2. Thermodynamic parameters of complexation of calixpyrrole receptors with halides and dihydrogen phosphate anions (tetra-n-butylammonium as counterion) in acetonitrile, dichloromethane, A/,A/-dimethylformamide, dimethyl sulphoxide and propylene carbonate... 

As an illustrative example, receptor 1 is more selective for fluoride relative to chloride, bromide, iodide and dihydrogen phosphate by factors of 32, 363, 45,684 and 16, respectively. In acetonitrile receptors 8-aaftp, 22 and 24 show a higher or similar selectivity to dihydrogen phosphate relative to fluoride. On the other hand, the calixpyrrole derivative 2 exhibits a similar selectivity towards dihydrogen phosphate as for fluoride anion [S = KS(F )IKS(H2P04)) — 1]. 

Thus, fluoride and calixpyrrole receptors (1, 16, and 18) are more stable in acetonitrile than in dichloromethane (Table 4). The stability of the fluoride anion and 2 is greater in acetonitrile than in dimethyl sulphoxide and propylene carbonate by factors of 1349 and 8, respectively. The same analysis carried out for fluoride anion and receptors 1, 2 and 8-atx.pp shows that the stability of this anion and these receptors is greater in A/,A/-dimethylformamide than in acetonitrile (Table 4). 

The following section discusses some of the applications of calixpyrrole derivatives in aspects related to fluoride chemistry, which are relevant to environmental issues. 

The complexation of a calixpyrrole dimer with the p-nitrophenolate anion [49] has been used as a colorimeter sensor by displacing the chromogen anion upon the addition of targeted anions. In this case anions, such as fluoride, displace the p-nitrophenolate anion from the complex thus enhancing the absorbance of the p-nitrophenolate anion. This was observed as a colourless to yellow colour change. 

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