Complexes crown ether—

For crown ethers in which the H-nmr probes Ha and Hb are diastereotopic in the free crown ether too, the cation exchange process (50) becomes more complicated. For 1 1 complexes the measured free energy of activation also involves conformational inversion components (AG = AG + AG ), whereas exchange between free and complexed crown ether (2 1 ratio of crown ether to salt) only involves the free energy of activation for decomplexation (AG ). Bradshaw et al. (1979b) have recently reported AG -values for cation exchange in RNH3X complexes of crown ethers [257]. The AGf term was estimated to be 0.7, 0.5, and 1.2 kcal mol-1 for compounds [257 n = 1] with... 

By analogy with cation complexing crown ethers like 47-50, attachment of a defined number and type of Lewis acids to a rigidified molecular scaffold in such a way that their electron-efficient sites are... 

Upon addition of NaOTf, columnar hexagonal mesophases could be induced. For 68, 0.4 equiv. of salt were needed while for 67, only 0.2 equiv. of NaOTf were needed for mesophase induction. Upon complexation, the complexed crown ether moieties and the anions self-assemble within a supramolecular tube and the taper-shaped side groups stack over each other in the periphery. X-Ray scattering... 

The term metallomesogen is often used when a (transition) metal is complexed (by coordinative bonds between the crown heteroatoms and the metal center) by a crown ether. These compounds were discussed in the previous sections as complexed crown ethers (instead of metallomesogens ) together with their uncomplexed precursors. In this section, mesogens with a covalent bond between... 

Conformational analysis of free and complexed crown ethers in solution 91JPR817. 

Selected values of the ratio Dj/Q (ionic diameters of Ln(III) ions/ cavity diameter of complexed crown ether) versus coordination number. The cavity diameters of the complexed crown ethers are taken from crystallographic data (12C4 1.35 A, 15C5 1.9 A, 18C6 2.5 A). The best fit is given in bold characters. 

An interesting version of the rearrangement [76] is presented in Scheme 51. The base-induced reaction of 2,4-dinitrobenzenesulfonamide with various acyl chlorides provides AT-(2,4-dinitrobenzenesulfonyl) acylamide derivatives 159 that rearrange and fragment in situ to afford the respective nitriles 160. The reaction has been used for derivatization of complex crown ethers. 

Cerium tetrakis(acetylacetonate), 1114 Cesium complexes crown ethers, 40 Chevrel phases, 1321 Chlorella vulgaris nitrate reductase structure, 1438 Chlorine cations... 

Sodiuim bexakis(formato)molybdate, 1235 Sodium complexes crown ethers, 37 Sodium molybdate, 1230 Sodium peroxoborate, 101 Sodium/potassium ATPase vanadate inhibition, 567 Sodium pyroantimonate, 265 Sodium tetrahydroborate, 101... 

A high complexation constant between the ligand and the salt in the organic phase, together with a high partition ratio for the complexed crown ether in the organic phase, are both prerequisites to the successful use of crown ethers as catalysts in aqueous-organic biphasic systems [38,39]. 

Nucleophilic displacements under two-phase conditions in the presence of catalytic amounts of crown ether, 4, which is completely dissolved in the organic phase, follow the classical PTC mechanism and the observed pseudo first-order rate constants are linearly correlated with the amount of complexed crown ether in the organic phase. A narrow reactivity range was found for different anionic nucleophiles in the displacement of n-octyl methanesulfonate (Scheme 10). 

In fact, the actual concentration of the anion in the organic phase in the case of complexed crown ethers is strongly related to the nature of the anion through the stability constants of the complex. These are high for soft anions such as I and low for hard anions such as Cl [10]. As a consequence of low complexation, in the case of Cl, reaction with crown ether 4 is much slower than the onium salt-catalyzed reaction, whereas the opposite is true for iodide due to its high complexation. 

Once again, crown ethers behave differently from quaternary salts and cryptates. With the complexed crown ethers, removal of the hydration sphere of the anion in going... 

Finally, the hydration numbers of the anions bonded to the polymeric cations determined after equilibration with the aqueous and organic phases under reaction conditions, are very similar to those found for lipophilic quaternary salts, complexed crown ethers, and cryptates. This implies that the organic phase permanently surrounds the catalytic centers bonded to the polymer... 

Fig. 3.29 SALOTH-based PTh bearing a complexing crown ether bridge [120]...

A series of bis(organostannyl)methanes have been prepared, the halogen substituted derivatives (XPh2Sn)2CH2 showing anti-tumor activity, and complex with HMPA to give fluxional Isl complexes. Crown ethers form hydrated complexes with tri and diorganotin... 

Crystalline complexes of crown ethers and metal ions are obtained by mixing two components in a common solvent [182]. The complexes have higher melting points than the free crown ethers. Stoichiometry of the complexes (crown ether salt = 1 1, 2 1, or 1 2) is largely determined by the fit of crown ether cavity and... 

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