Ethers, crown bicyclic

Lariat ethers crown ethers with pendant chains Sepulchrates bicyclic caged macrocycles Spherands macrocycles with phenyl backbones... 

Fig. 8.5. Slope a as a measure of conformational changes and intercept 7AS°o as a measure of desolvation upon complexation, initially proposed for cation binding by acyclic glyme, cyclic crown ether, and bicyclic cryptand, where D denotes a donor atom such as oxygen and nitrogen.

In the early years of supramolecular chemistry, a number of studies showed that in comparison to the cyclic crown ethers and bicyclic cryptands, alkali metal cation binding by podands of the oligoethylene glycol type is relatively weak in polar solvents such as water and methanol. Thus [18]crown-6 binds K+ some four orders of magnitude more strongly in methanol solution compared to its open chain podand analog, 2. Representative binding constants for cyclic and acyclic species are shown... 

Ma, Li, Jiang and Wen and co-workers synthesized a pillar[5]arene and crown ether-fused bicyclic host (H5.23 Figure 5.23). ... 

A number of bridged crown ethers have been prepared. Although the Simmons-Park in-out bicyclic amines (see Sect. 1.3.3) are the prototype, Lehn s cryptands (see Chap. 8) are probably better known. Intermediates between the cryptands (which Pedersen referred to as lanterns ) and the simple monoazacrowns are monoazacrowns bridged by a single hydrocarbon strand. Pedersen reports the synthesis of such a structure (see 7, below) which he referred to as a clam compound for the obvious reason . Although Pedersen appears not to have explored the binding properties of his clam in any detail, he did attempt to complex Na and Cs ions. A 0.0001 molar solution of the clam compound is prepared in ethanol. The metal ions Na and Cs are added to the clam-ethanol solutions as salts. Ultraviolet spectra of these solutions indicate that a small amount of the Na is complexed by the clam compound but none of the Cs . 

Attracted by the benefits of the cryptand effect, Coxon and Stoddart (1975, 1977) prepared the bicyclic compound [39]. Compared with the monocyclic crown ether [40], the bicyclic compound showed no increased ion binding... 

Fig. 5.21 Coordination of cations by (a) crown ethers and (b) bicyclic cryptands indicating that the cation is better shielded from the anion in (h).

As stated by Pedersen [1] at the very beginning, systematic names of polyethers are too cumbersome for repeated use. Thus, he coined an efficient system of their trivial names that is in common use. The polyethers were dubbed crown ethers or crowns . The total number of atoms in the macrocycle preceded this term whilst the number ofoxygen atoms followed it. The presence ofaromatic 44 or saturated 60 rings in the systems is marked at the beginning. In Lehn s system for bicyclic molecules called cryptands the number of OCH2CH2 units in each branch ofthe bicyclic hosts 61-63 ([1.1.1]-, [2.1.1]- and [2.2.2]cryptands... 

Macropolycyclic ligands containing intramolecular cavities of a three-dimensional nature are referred to as cryptands. The bicyclic cryptands (73) exist in three conformations with respect to the terminal nitrogen atoms, exo-exo, endo-exo and endo-endo 6 these forms can rapidly interconvert via nitrogen inversion but only the endo-endo form has been found in the crystal structures of a variety of complexes372 and for the free ligand ([2.2.2], 73, m = n = / = l).449 In their complexes with alkali and alkaline earth cations, the cryptands exhibit an enhanced stability over the crown ethers and coronands dufe to the macrobicyclic, or cryptate, effect.33 202... 

Sodium or potassium ions can also participate in the phase-transfer process when they are converted to lipophilic cations by complexation or by strong specific solvation. A variety of neutral organic compounds are able to form reasonably stable complexes with K+ or Na + and can act as catalysts in typical phase-transfer processes. Such compounds include monocyclic polyethers, or crown ethers (1), and bicyclic aminopolyethers (cryptates) (2). They can solubilize inorganic salts in nonpolar solvents and are particularly recommended for reactions of naked anions. Applications of these compounds have been studied.12,21-31... 

The late 60 s and the early 70 s saw the advent of the first artificial molecules where these concepts were fully utilised, the crown ethers and their sequels the cryptands. Crown ethers or coronands are hetero-cyclic structures where the hetero-atoms are separated with C2-units cryptands are bicyclic analogues... 

Another interesting development is the use of bicyclic guanidinium receptors as heteroditopic systems for the enantioselective recognition of amino acids. In this case the guanidinium fragment was combined via a flexible linker with a crown ether moiety in order to promote the simultaneous binding of the carboxylate anion and the ammonium cation belonging to a zwitterionic amino acid [45]. 

Usually, the ion exchange rate between a solvated metal ion and the ion fixed in the complex compoimd is very fast. This is the reason why most of the cation complexes with cyclic polyethers have high exchange rates which are in the order of 10 s >20) Although the exchange rate for metal ions with a bicyclic cryptand is significantly lower than with monocyclic crown ethers the kinetic isotopic... 

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