Cryptophanes structure

This review is dedicated to the synthesis of water-soluble cryptophanes and of the closely related hemicryptophane derivatives that were developed more recently. The study of their binding properties with different species and some peculiar properties related to their chiral structure are also described. A particular attention is given to xenon-cryptophane complexes since, as above mentioned, these complexes have played a major role in the development of water-soluble cryptophane derivatives. We describe in a concise manner the different approaches, which have been reported in the literature to introduce hydrophilic moieties onto the cryptophane structure. Finally, we report some physical properties of the water-soluble cryptophane complexes. This mainly concerns the study of their binding properties with neutral molecules or charged species. The preparation of enantiopure cryptophanes has also contributed to the development of this field. Indeed, it was stressed that cryptophanes exhibit remarkable chiroptical and binding properties in water [11]. These properties are also described. The last part of this review is devoted to hemicryptophane derivatives, which are closely related to the cryptophane structure and which allow the functionalization of the inner space of the molecular cavity. These show a renewed interest in their applications in chiral recognition and supramolecular catalysis. 

Hemicryptophanes are molecular containers, constructed on the cryptophane structure, but where one CTB unit is changed for another C3 symmetiy group. Compounds 60 [74], and 61 [75], (Fig. 21.20) are representative of this new class of host molecules. They have been designed to allow the endohedral functionaUzation of the inner molecular cavity. They were found to be efficient and selective receptors for cations, anions, ion pairs, zwitterions and carbohydrates. 

Other molecules include cryptophanes (e.g., 23), ° hemispherands (an example is 24 ° ), andpodands The lastnamed are host compounds in which two or more arms come out of a central structure. Examples are 2S °° and 26. ° Compound 26,... 

The spherically shaped cryptophanes are of much interest in particular for their ability to bind derivatives of methane, achieving for instance chiral discrimination of CHFClBr they allow the study of recognition between neutral receptors and substrates, namely the effect of molecular shape and volume complementarity on selectivity [4.39]. The efficient protection of included molecules by the carcerands [4.40] makes possible the generation of highly reactive species such as cyclobutadiene [4.41a] or orthoquinones [4.41b] inside the cavity. Numerous container molecules [A.38] capable of including a variety of guests have been described. A few representative examples of these various types of compounds are shown in structures 59 (cyclophane) 60 (cubic azacyclophane [4.34]), 61a, 61b ([4]- and [6]-calixa-renes), 62 (cavitand), 63 (cryptophane), 64 (carcerand). 

Figure 4.27 Crystal structure of [(Cp Ru)6(( )-cryptophane-E)]6+ 4.68, showing inclusion of a triflate anion within the cage structure.52...

Figure 6.43 X-ray crystal structure of the CHC13 complex of cryptophane-E.

Roesky, C. E. 0., Weber, E., Rambusch, T., Stephan, H., Gloe, K., Czugler, M., A new cryptophane receptor featuring three endo-carboxylic acid groups Synthesis, host behavior and structural study. Chem. Eur. J. 2003,9, 1104-1112. 

The name cryptophane [23], which designates molecules made of two cyclotriveratrylene units assembled by three bridges in the way shown in Fig. 3, recalls the [l.l.ljorthocyclophane structure of CTV and underlines the presence of an enforced cavity in these compounds allowing the inclusion of suitable guest species (Sect. 3). 

It is interesting to split AG° into its enthalpic and entropic components, which can be done by means of the usual van t Hoff plots (In K vs. 1/T). Relevant data for cryptophane-A (45), C (40), and E (38) are assembled in Table 4. Cryptophane-A is similar in structure to cryptophane-E, and has shorter bridges (0(CH2)20 in 45 instead of 0(CH2)30 in 38). Thus cryptophane-A and C should have cavities of the same size, slightly smaller than that of cryptophane-E. 

Moreover, the strong binding displayed by the tetramethyl ammonium cation led to consider that cryptophanes of the size of 38 might be suitable receptors for substrates of structure Me3N + — R in which R would be an aliphatic chain or a functionalized chain such as that of choline and acetylcholine. In fact, it is found that, in (CDC12)2, the binding constant drops rapidly as the length of an aliphatic... 

In water, isobutane is fifty times better bound than isobutene, and ten times better than n-butane. These results confirm the preference of the cryptophane cavity for tetrahedral structures, and show that branched hydrocarbons are much better substrates than linear ones. Separation processes based on such molecular... 

The crystal structure of [(38 + ) (PFg ) (CHC13)] depicted in Fig. 21 shows the 1 1 complex formed between the oxidized cryptophane 38 +, including a chloroform molecule in its cavity, and one PFg anion. EPR spectra of the crystals are consistent with the existence of quasi free delocalized it electrons. The persistence of the paramagnetic signal in solution indicates that 38 + is a stable species. The crystal packing is a closed packed three-dimensional fee array of the giant radical cations, with the small anions located in the octahedral sites. 

Loops correspond to compounds like ansa cyclophanes, but equally well they might induce the design of novel annelated benzenes. Simple and polycycles manifest the familiar classes of cyclophanes and cryptophanes commonly synthesized by practitioners of supramolecular chemistry [12,13]. The Platonic graphs are a subset of the polycyclic, but, because of their potential for high symmetry and structural simplicity, they form an especially provocative group. 

Figure 6.45 (a) conformational implosion of a cryptophane ester upon guest removal, (b) X-ray molecular structure of the imploded atropisomer and (c) TGA trace showing the facile loss of the three lattice THF molecules from the cryptophane (thf)4 complex (solid line) and the very reluctant loss of the intra-cavity thf molecule from cryptophane thf. The boiling point of pure thf is also marked. (Figures courtesy of Prof. K. T. Holman). 

The so-called cryptophanes which are structurally related to 10 have shown to be capable of complexing stable molecules like methane or various fluorocarbons [9] and the number of fullerenes with a content - helium, neon, numerous metal ions [10,11] is rapidly increasing. 

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