Calixcrowns

A first example of nucleophilic catalyst with transacylase activity is given by an equimolar mixture of a Ba + salt (either bromide or perchlorate) and p-tert-butylcalix [4]arene-crown-5 (1), which catalyzes the methanolysis of aryl acetates in MeCN— MeOH (1 1, v/v) under slightly basic conditions (3 1 diisopropylethylamine-per-chlorate salt buffer) at 25°C [19,20]. 

Calixcrown 1 remains in its unionized form in the given buffer solution, but upon addition of 1 molar equivalent of Ba salt it is transformed into a mixture of the Ba + complexes of the singly and doubly ionized forms. There is evidence that the Ba complex of the doubly ionized form 4-Ba is the active form of the catalyst. 

Vis) in the presence of buffer alone (curve a) and of buffer plus 

The time-dependent concentration ofpNPOH was analyzed on the basis of Eq. (6), which is the sum of an exponential (pre-steady state) phase, characterized by a first-order rate constant k, and a linear (steady state) phase obtained when the exponential term dies out (kt 5). The equation reduces to the form of Eq. (7). The intercept ofthe linear portion defines the burst ji of ArOH liberation.  

A pure sample of 2 undergoes methanolysis under conditions identical to those of the catalytic experiment with a first-order rate constant of 7.7 x 10 min, in good agreement with the value obtained from the two-parameter treatment of the kinetics 

Regnouf-de-Vains, J.-B. Lamartine, R. Helv. Chim. Acta 1994, 77, 1817. 

Danil de Namor, A. F. Sueros Velarde, F. J. Cabaleiro, M. C. J. Chem. Soc., Faraday Trans. 

Sabbatini, N. Casnati, A. Fischer, C. Girardini, R. Guardigli, M. Manet, I. Sarti, G. Ungaro, 

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Amalgamation of stmctural units typical of crowns and calixarenes has led to the development of calixpodands, calixcrowns, and calixspherands (55). Naturally they behave as cation complexants rather than iaclusion hosts for uncharged molecules. 

High quantum yields and long lifetimes have been reported for a series of calixcrowns with appended bipyridyl chromophores, such as L48 (82). Photophysical studies in acetonitrile gave lifetimes of 0.95-1.38 and 1.83-1.93 ms, and quantum yields of 0.18 0.32 and 0.32-0.39 for europium and terbium complexes, respectively. 

Calixcrown 5, featuring two diethylaminomethyl side-arms at the polyether bridge, testifies an attempt at a higher order multifunctional catalysis of ester cleavage, namely, from nucleophilic-electrophilic to nucleophilic-electrophilic-general acid catalysis [20]. 

With the idea that crown ethers based on the p-tert-butylcalix[5]arene platform could provide an interesting extension of our catalytic studies, several calix[5]-crown-ethers were investigated as potential catalysts of ester methanolysis in the presence of a Ba + salt [22]. Of the various structures investigated, the calixcrown-5 derivative 7 gave the... 

Additional examples of type d (Scheme 5.1) bifunctional reactants are provided by the alkaline-earth metal ion complexes of lariat ethers 8-10, bearing a sulfhydryl side arm, instead ofthe phenolic hydroxyl of a calixcrown [23,24]. Here the acyl-receiving and acyl-releasing unit, like in papain and ficin, is a sulfhydryl group. 

Baldini, L., Bracchini, C., Cacciapaglia, R., Casnati, A., Mandolini, L. and Ungaro, R. (2000) Catalysis of acyl group transfer by a double-disiplacement mechanism the cleavage of aryl esters catalyzed by calixcrown-Ba complexes. Chem.-Eur. J., 6, 1322. 

Cacciapaglia, R., Mandolini, L., Arnecke, R., Rohmer, V. and Vogt, W. (1998) Ba(II) complexes of calixcrowns derived from p-tert-butylcalix[5]arene as potential transacylation catalysts. Regio- and stereoselective monoacylation of the calixcrown. J. Chem. Soc., Perkin Trans., 2, 419. 

Jankowski, C.K., Dozol, J.F., Attain, F. et at. 2002. Nitration of calixcrown 6 influence on extracting abilities. Use of cesium salts for detection of crown ether macrocycles with the electrospray ionization mass spectrometry technique. Polish J. Chem. 76 701-709. 

Moyer, B.A., Birdwell, J.F. Jr., Bonnesen, P.V., Delmau, L.H. 2005. Use of macrocycles in nuclear-waste cleanup A real-world application of a calixcrown in technology for the separation of cesium. In Macrocyclic Chemistry - Current Trends and Future Prospectives. Gloe K. Ed., Springer, Dordrecht, pp. 383-405. 

Hill, C., Dozol, J.-F., Lamare, V., Rouquette, H., Eymard, S., Tournois, B., Vicens, J., Asfari, Z., Bressot, C., Ungaro, R., Casnati, A. Nuclear waste treatment by means of supported liquid membranes containing calixcrown compounds. J. Incl. Phenom. Maerocyclic Chem. (1994), 19 (1—4), 399 -08. 

USE OF MACROCYCLES IN NUCLEAR-WASTE CLEANUP A REAL-WORLD APPLICATION OF A CALIXCROWN IN CESIUM SEPARATION TECHNOLOGY... 

Figure 1. Calixcrown extractant adopted for CSSX, as complexed with Cs+ ion. Left chemical drawing of the complex. Right a space-filling view of part of a crystal structure of the model complex Cs2Calix[4]arene-bis(benzo-crown-6)(N03)2 3CHCl3 [69] showing the good fit of the Cs+ ion inside the calixarene cavity the crown ether atoms have been removed for clarity.

Figure 2. Chemistry occurring in three sections of a simple flowsheet in solvent extraction using CSSX as an example. In the extraction step, essentially all of the Cs+ and a minor fraction of the K+ ions in the waste are extracted (i.e., M+ = Cs+ or K+). The K+ ions are removed in scrubbing as shown, while the calixcrown-Cs+ complex effectively remains in the solvent so that a pure cesium nitrate product is obtained on stripping.

Largest quantum yields reported for Eu(III) and Tb(III) complexes with calixcrowns and calixcryptands. From data reported by N. Sabbatini et al in Calixarenes 2001, Z. Asfari, V. BBhmer, J.M. Harrowfield, J. Vicens, eds, Dordrecht Kluwer Academic Publishers, 2001, Ch. 31. 

During nearly four decades of study, it has become clear that extremely large heteromacrocyclic rings can be prepared and that nearly any combination of O, N, and/or S can be included within the cycle. Numerous subcyclic units including hydrocarbons (cyclohexane, naphthalene, etc.) and heterocycles (furan, pyridine, imidazole, quinoline, etc.) have also been incorporated. Heteromacrocycles have also been fused with other receptors such as calixarenes to make calixcrowns. In recent years, the fusion of receptors and variations in the attached sidearms has been a dominant theme. 

Two calix[4](diseleno)crown ethers were synthesized by reaction of the disodium salt of 1,3-propanediselanol 188 with the preorganized l,3-dibromoethoxycalix[4]arenes. These potentially ionophoric calixcrown selenoethers form interesting infinite aggregate sheets via self-inclusion and intermolecular Se- Se interactions in the solid state <2002TL131>. 

Finally, both upper and lower rim substitution have been used to produce a class of macrocycles referred to as calixarene-crown ethers, or calixcrowns. One simple example is shown here (103). These receptors combine characteristics of the crown ethers and calixarenes, and have been intensively studied for metal ion extraction, in particular, for the removal of cesium from nuclear waste. ... 

Gong S-L, Zhong Z-L, and Chen Y-Y. Calixcrown obgomers as cation carriers in a bulk hquid membrane. React Funct Polym, 2002 51 (2-3) 111-116. 

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