Pyrrole-based anion receptors

Gale and coworkers [60] have discovered a very simple displacement method for a calix[4]pyrrole-based anion receptor. Addition of calix[4]pyrrole 73 to a solution of tetrabutylammonium 4-nitrophenolate in CH2C12 caused a decrease in intensity of a UV band at 432 nm, and calix[4]pyrrole-4-nitrophe-nolate complex 74 was formed. Upon addition of F, 4-nitrophenolate was displaced from the complex, and the color of the solution changed from colorless to yellow due to the presence of the free nitrophenolate anion (Fig. 14). 

Pyrrole- and Poly pyrrole-Based Anion Receptors, p. 1176... 

Y. Haketa, S. Sakamoto, K. Chigusa, T. Nakanishi, H. Maeda, Synthesis, crystal structures, and supramolecular assemblies of pyrrole-based anion receptors bearing modified pyrrole p-substituents. J. Org. Chem. 76, 5177-5184 (2011)... 

FIGURE 4.7 (a) Anion-binding mode of pyrrole-based anion receptor 14a (b) anion receptor derivatives 14-17. 

Other macrocyclic pyrrole-based anion receptors inclnde the cyclo[n]pyrroles that consist solely of an array of pyrrole groups linked directly to each other via the 2- and 5-positions of the rings. For example, cyclo[8]pyrrole can be prepared by direct coupling of bipyrroles in the presence of iron(III) chloride in 1 M snlfuric acid (Scheme 4). ... 

Since the w-planes of the pyrrole unit also enable stacking, pyrrole rings can act as potential building blocks for nanoscale supramolecular structures. Thus, supramolecular assemblies built from planar pyrrole-based anion receptors would show anion-responsive sensing behavior in the forms of soft materials. In 2005, Maeda... 

The simplest pyrrole-based anion receptor is mesooctamethyl calix[4]pyrrole 57 which can be synthesized in one step by the acid-catalyzed condensation of pyrrole with acetone. In 1996, Sessler and co-workers demonstrated that 57 binds fluoride anion selectively in CD2CI2 solution [110]. Crystal stmcmre of 57 (Fig. 5.45a) showed that two of the NHs point up, and the other two point down. When coordinated with anion, the four NHs aU points at the anion. The selectivity of calix [4]pyrrole for anions depends on the nature of the solvent [111, 112], and fluoride selectivity could be lost under certain conditions. A detailed study by Sessler et al. with several chloride salts in solution by ITC and H NMR titrations and in the solid state by X-ray analysis showed that the stability was highly dependent on the solvent (Fig. 5.45b) [113], with Xa ranging from 10 to 10 In dichloro-methane, a strong dependence on the counter cation was also observed, with of the complexes with chloride varying within 10 -10" M . ... 

A more recent development has been the synthesis of bisamidopyrrolylmethane based anion receptor systems (Figure 19).15 These receptors might be regarded as containing half a calix[4]pyrrole combined with die 2-amido appendages in common with the pyrrolic amide cleft. Compounds 19 and 20 were synthesized by reaction of diethyl-5,5 -methylenebis(4-ethyl-3-methyl-2-pyrrole) carboxylate with aniline or n-butylamine in the presence of trimethylaluminium in dry dichloromethane at 35°C in 40 and 43% respective yields. 

Molecular Squares, Boxes, arid Cubes, p. 909 71-71 Stacking Theory and Scope, p. 1076 Preorganization and Complementarity, p. 1158 Pyrrole- and Polypyrrole-Based Anion Receptor-s. p. 1176 Rotaxanes arid Pseudorotaxanes, p. 1194 Self-Assernhling Catenanes, p. 1240 Siderophores, p. 1278 Spherands, p. 1344... 

Pyrrole- and Polypyrrole-Based Anion Receptors, p. 1176 Selectivity Thermodynamic and Kinetic, p. 1225 Stability Constants Definition and Determination, p. 1360 Strong Hydrogen Bonds, p. 1379... 

One of the simplest classes of pyrrole-based molecular receptors (2.68) binds oxo-anions in polar organic solvents. Compoimds 2.68 show a clear preference for oxo-anions over halides, for example, host 2.68a binds benzoate 18 times more strongly than chloride in acetonitrile, whereas host 2.68b binds benzoate 51 times more strongly than chloride in a 0.5 % DMSO/HjO solvent mixture. Dihydrogen phosphate also has some affinity for 2.68, with = 357 and 1450 for 2.68a and 2.68b, respectively. 

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