Pyrrole-based receptors

In 2010 Kim and Sessler described three limiting ion pair association modes for a two site, ditopic receptor corresponding to (a) host-guest separated ion pair, (b) contact ion pair, and (c) solvent-separated ion pair, (cf. Fig. 12.4) [20]. This intellectual construct has facilitated the design of ion pair calix[4]pyrrole-based receptors in recent years. 

The use of extractants to transfer anionic species from water to a non-polar medium represents a ncmtrivial task. Calix[4]pyrrole-based receptors are very... 

Stated to be particularly useful in the treatment of obesity, a series of 1,5-diaryl-pyrrole-based compounds from AstraZeneca was claimed in a recent patent application [313], including compound (509). Although no specific biological data were presented, the CBi receptor affinities are claimed to be less than 1 pM and preferably less than 200 nM. 

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). 

Abstract This review article provides a broad overview to the area of anion coordination by synthetic organic receptors and includes examples of different functional groups used to bind anions. The first section examines neutral anion receptors containing amide-, sulfonamide-, urea- and thiourea-based receptors. Then aromatics such as pyrrole, car-bazole and indole are discussed before concluding the discussion of neutral systems with examples of hydroxy OH donors. A brief overview of charged systems is also provided. 

Fig. 9 Pyrrole and thiophene-based receptors containing separate redox-active reporter groups.

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)... 

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. 

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

H. Kusunose, Y Mihashi, Y Mizoguchi, T. BF2 Complexes of P-tetraethyl-substituted dipyrrolyldiketones as anion receptors Potential building subunits for oligomeric systems. J. Org. Chem. 2007, 72, 2612-2616. (c) Maeda, H. Haketa, Y Bando, Y Sakamoto, S. Synthesis, properties, and solid-state assemhlies of P-alkyl-substituted dipyrrolyldiketone BF2 complexes. Synth. Met. 2009,159, 792-796. (d) Maeda, H. Bando, Y Haketa, Y Honsho, Y Seki, S. Nakajima, H. Tohnai, N. Electronic and optical properties in the solid-state molecular assemhhes of anion-responsive pyrrole-based rr-conjugated systems. Chem. Eur. J. 2010, 16, 10994—11002. (e) Bando, Y Sakamoto, S. Yamada,... 

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). ... 

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