Calix pyrrole, formation

Shao and coworkers [61] have demonstrated that a noncovalent charge-transfer complex of calix[4]pyrrole-chloranil can be used as a chromogenic sensor for F and H2POj. Upon addition of calix[4]pyrrole 75 to the solution of chloranil 76 in chloroform, the color changed from pale yellow to blue. This indicated the formation of a charge-transfer complex between calix[4] pyrrole and chloranil in chloroform. The Job s plot analysis showed a 1 1 complex with... 

Like calix[4]arenes, calix[4]pyrroles are versatile ligands to the extent that the composition of the anion receptor complex is solvent dependent. A representative example is that involving 8 and the fluoride anion. As shown in Fig. 4b, the well-defined change in curvature observed at 1 1 ligand fluoride mole ratio indicates that in acetonitrile one fluoride anion interacts per unit of receptor 8. However, in moving from acetonitrile to A/,A/-dimethylformamide, the noticeable changes in curvature observed at a ligand/anion mole ratio of 0.5 and 1, indicate respectively the formation of a 1 2 and 1 1 anion complexes, respectively, in this solvent. 

In the absence of suitable guests no self-assembly of 31 was observed, presumably because the calix [4]pyrrole core is not sufficiently well preorganized for dimerization and the entropic and enthalpic energy terms associated with the required conformational reorganization are not compensated by the formation of the 16 hydrogen bonds... 

Other interesting new developments in pyrrole chemistry encompass the synthesis of calix[6]pyrrole and calix[ ]furan[m]pyrroles, where m + n = 6 <02CEJ3148>, and the formation of cyclo[8]pyrroles from 2,2 -bipyrrole fragments <02AGE1422>. 

Chromogenic octamethyl calix[4]pyrrole-based sensors (e.g., 165-167) for antipyretic carboxylates such as naproxen, ibuprofen, and salicylate, without bias by bicarbonate or carboxy termini of blood plasma proteins, have been described <2005JA8270>. The formation of a sensor-anion complex results in partial charge transfer and a dramatic change in color. 

Kim et al. reported the formation of supramolecular complex 22 of the benzo-annulated TTF calix [4] pyrrole (TTF-C4P) as an electron donor with porphyrin as an electron acceptor in benzonitrile [78]. The TTF-C4P binds to the carboxylate moiety of the porphyrin through H-bonding with a 1 1 stoichiometiy and a binding constant of 6.3 x 10" IVT in this solvent at 298 K. Photoexcitation of complex 22 formed between these two components in PhCN at 298 K afforded the CS state, which was characterized by forward and backward intramolecular ET rate constants of 2.1 X 10" and 3.6 x 10 s, respectively. The triplet CS state produced upon photoirradiation of porphyrin was found to be 2.8 ms, one of the longest known lifetimes for a CS generated via a photoinduced electron transfer process within a noncovalentiy bound complex. It is interesting to see that free metal porphyrin acts as electron acceptor instead of donor in this case. 

Figure 24. h NMR spectrum of compound 18 recorded in dichloromethane-d2. The pyrrole-NH protons are deshielded and resonate at 11.22 ppm, a finding that is consistent with the formation of hydrogen bonds. (Reprinted from Tetrahedron Letters, Gale, P. A. Sessler,). L. Lynch, V. Sansom, P. I., "Synthesis of a New Cylindrical Calix(4]arene-Calix(4]pyrrole Pseudo Dimer," 1996, 37, 7881 -7884. Copyright 1996, with permission from Elsevier Science.)... 

One of the pioneers in the area of calix[4]pyrrole craitainer research is Ballester. In 2007, he and his coworkers reported the use of resorcinol-functionalized calix[4] pyrroles 39 to create hexameric cages (cf. Fig. 12.25) [80]. Solid-state analyses revealed that the formation of the hexameric molecular assembly was controlled by the relative stoichiometriy of the tetramethylammonium chloride (TMACl) salt and... 

Fig. 24.18 Left cytosine calix[4]pyrrole conjugates with a connection either at the fi- or meso-pyrrolic position (43 and 44) and complex formation of 44 with guanosine monophosphate [52], Right drug delivery vehicle calix[4]pyrrole-/rani -Pt(II) complex (45) and schematic representation of the interaction with adenosine monophosphate as a model compound [53]...

In this reaction, the corresponding cylic octamer 38 (11%) as well as hexamer 39 (2%) was also obtained. The formation of hexamer 39 was explained in terms of acid-catalyzed, reversible cleavage of the starting material during the reaction. The unsubstituted calix[l]furan[3]pyrrole 33 and calix[4]pyrrole 34 were isolated for the first time by Taniguchi et al in the similar BF3 catalyzed (3 + 1) condensation [39]. 

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