Cryptophanes methane

The cryptophane is typical of the chiral resolution of methane derivatives (eg, CHFClBr) (146) and the basket-shaped host of Figure 23d exhibits extremely high enantioselectivity for various peptides (144). 

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). 

The binding constant of cryptophane-A for methane is 130 M 1 in (CHCl, a solvent that is not bound by the cavity. Calculate the binding constant in CHC13 if Ks for this solvent is 10 M 1 and the density of liquid CHC13 is 1.48 g cur3 at 25 °C. 

Collet and coworkers have reported studies with a cryptophane (Figure 68) which binds methane and chlorofluorocarbons in 1,1,2,2-tetrachloroethane [93]. The association constants were of the order of 100 M , and the authors... 

Figure 68 A cryptophane host for binding methane and chlorofluorocarbons 93)...

K, the temperature and pressure at which there is no longer any distinction between liquid and gas), p rises to 0.17, while solid methane at 0 K exhibits a p value of 0.67. In occupancy terms, then, the intracavity environment is akin to a supercritical fluid. Translated to the macroscopic scale, one molecule of methane in the cavity of cryptophane-A is equivalent to one mole of methane in 49 mL, exerting a pressure of 610 atm at 298 K. In contrast, for CHCI3 the occupancy factor of 0.89 is akin to a very densely packed crystal, and indeed the entropic and enthalpic changes on binding are similar to those that occur in the formation of crystalline organic compounds. 

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. 

Garel, L. Dutasta, J.P. Collet. A. Complexation of methane and chlorofluorocarbons by Cryptophane-A in organic solution. Angew. Chem., Int. Ed. Engl. 1993. 32. 1169-1171. 

Unlike CTVs, cryptophanes are able to bind small guest molecules in solution as weU as within solid-state complexes. In solution, cryptophanes show particularly strong affinity for small molecules including methane, chloroform, dichloromethane, and xenon. 

Petrov et al. have presented a variable temperature solid-state NMR investigation of cryptophane-Exhloroform and cryptophane-Erdichlorometh-ane inclusion complexes.The line shapes and nuclear spin relaxation rates were analysed in terms of the distribution of C-D bond orientations and the time scale of the guest dynamics. It was found that encaged chloroform produces broad spectra, and that its reorientation is relatively slow with a correlation time of 0.17 ps at 292 K. In contrast, the line shapes of encaged dichloromethane were narrow and the motion of this guest molecule was fast with a correlation time of 1.4 ps at 283 K. The NMR data were complemented by an X-ray diffraction study of the cryptophane-E dichloro-methane structure, which was utihsed in the analysis of the NMR parameters. 

Optical resolution of bromochlorofluoro-methane was performed in [131] using its encapsulation by Scheme 2.116 within the cavity of a chiral caging host 184, cryptophane-C, designed to fit substrates of a given size. The diastereo-meric 1 1 cage complex formed by the (-)-mac-robicyclic ligand and the (-)-guest molecule is more stable than those formed by hosts and... 

Figure 22.5 Qiemical structure of cryptophane A showing methane within its molecular pocket Because methane occupies just 35% of the cavity volume, it may readily disorder within the host [130].

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