Calixspherands

A particularly good example of the power of preorganization combined with conformational adaptation is seen in the calix[4]spherands (142a), compounds in which a hemispherand moiety is affixed to the lower rim of a calix[4]arene. Although the hemispherand part of the molecule is 

Iwema-Bakker, W. I. Reinhoudt, D. N. New Separation Chemistry Techniques for Radioactive IVasteand Other Specific Applications, Elsevier Applied Science London 1991, p. 142. 

Iwema-Bakker, W. I. Haas, M. Khoo-Beattie, C. Ostaszewski, R. Franken, S. M. den Hertog, 

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Calix-cavitates Calixciowns Calixin Calixpodands Calixspherands C. alkanolyticum... 

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. 

Numerous modifications of calixarenes, spherands and similar systems have been synthesized 241 [40], 242 [41], calixspherands like 243 [42] and torands like 244 [43] to name but a few pertaining interesting ligands. 

Calix[4]arenes can be functionalized both at the phenolic OH groups (lower rim) and at the para positions of the phenol rings (upper rim).21 A special class of selectively functionalized calix[4]arenes comprises bridged calix[4]arenes in which two phenol rings are connected by a cap.22 An important subgroup of this class are the calixspherands in which a calix[4]arene is diametrically bridged with a rigid terphenyl moiety. 

The calixspherands form kinetically stable complexes with Na+, K+ and Rb+. The kinetic stability of the different complexes, which can be determined by H NMR spectroscopy or by a new method25 based on the exchange of radioactive rubidium or sodium in the complexes for non-radioactive sodium in different solvents, is strongly increased when the size of the group on the central aromatic ring of the m-terphenyl is increased. Due to the highly shielded cavity, calixspherands 22b,c also form kinetically stable complexes with Ag+.26... 

Synthesis of calixspherand diols from p-f-butylcalix[4]arene and 3,3"-bis(bromomethyl)-1,1 3, 1"-terphenyls9 (Scheme 8.6)... 

Put the calixspherand diol and 0.25 mL of iodomethane in the flask, add 80 mL of dry THF and keep the flask under a slightly positive pressure of nitrogen. 

Dissolve the potassium t-butoxide in 20 mL of dry THF in the dropping funnel and add this solution to the solution of the calixspherand diol at room temperature. 

Recrystallize the residue by dissolving it in a minimum amount of cold dichloromethane and carefully add diisopropyl ether until crystallization starts. Collect the solid and dry under vacuum to give the potassium iodide complex of the calixspherand as a white solid. 

Redissolve the residue in chloroform (25 mL) and remove undissolved material by filtration. Concentration of the filtrate under vacuum gives the potassium picrate complex of the calixspherand as a yellow solid (1.1 g, 81% yield). 

Put the calixpherand complex in the flask and add methanol (4 mL). Dissolve the calixspherand complex by stirring. 

Reinhoudt DN, Dijkstra PJ, in t Veld PJA, Bugge KE, Harkema S, Ungaro R, Ghidini E, Kinetically stable complexes of alkali cations with rigidifiedcalix[4]arenes- X-ray structure ofa calixspherand sodium picrate complex. J. Am. Chem. Soc. 1987 109, 4761. 

Figure 6.3 X-Ray crystallographic structure of the complex of calixspherand 142a [R = Me) Taken from Bakker et...

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