Crown ether system

So many different crown ether systems have been prepared over the recent decade that it sometimes seems that any of them could be placed in a miscellaneous category. On the other hand, each has its interesting features and probably merits a separate section for adequate discussion. Because both of these criteria cannot be met simultaneously, we have placed a number of compounds in this section which are fully deserving of detailed discussion, but not enough examples are yet available to group them separately. 

Okahara and his coworkers have made a number of contributions to the synthesis of crown ethers using a one-pot method (see Sect. 3.13). These methods have been applied largely to the preparation of simple aliphatic crown ether systems. In addition, this group has prepared macrocyclic ester compounds using a one-pot procedure. Although... 

A similar study was undertaken on the related crown ether systems 201 <2001PS29>. They all showed moderate extraction of both Ag(l) and Hg(ll) ions and so were less selective than compounds 184a and 184b from the previous study. However, the presence of the benzo-15-crown-5 substituent offered the simultaneous complexation of the hard alkali cation Na(l) as well as the thiophilic metals Ag(l) and Hg(n) by the thieno sulfur. Interestingly, this second extraction was not influenced by the presence of the other metal. 

The few examples discussed so far illustrate the varied behaviour that may be exhibited by alkali metal/crown ether systems. Unfortunately, as yet, the reason for much of this variation is little understood. 

The introduction of substituent groups into the benzo rings on dibenzo-18-crown-6 ethers leads to marked effects on their selectivity. A reversal of the usual selectivity sequences occurs when strong, electron-withdrawing substituents are present.490 Similar effects are found for benzo-15-crown-5, and a good correlation is found with the Hammett functions of the substituents.491 In contrast for substituted benzo-18-crown-6 systems, no such correlation was found, indicating that caution must be exercised on extrapolating from one crown ether system to another. 

This work was later expanded to include a larger crown ether system functionalized with either the above-mentioned cyclene macrocycle (2), two cyclene macrocycles (4), or a triazacyclononane macrocycle (3) [32]. Unfortunately, the Cu-32+ complex did not display any appreciable affinity for the phosphate species (NaH2P04, KH2P04, sodium glycerophosphate, or disodium 4-nitrophenylphosphate). 

Signal Input/Output in Crown Ether Systems... 

The influence of alkali metal perchlorates on the photocyclisation of (34) has also been studied. The quantum yield for cyclization is reduced from 0.21 for the free system to 0.17 for irradiations in the presence of sodium perchlorate. The effect is even more dramatic with potassium and rubidium perchlorates when (j) is reduced to 0.02. The crown ether systems are obviously important since the overall shape can be controlled by the photochemical ring closure. 

Macrocycles Other than Crown Ethers Systems containing Nitrogen as the only Heteroatom. - The aza[18]annulene (35) has been synthesized by photolysis of (36) at — 80°C. Aza[14]-annulenes have been prepared by the same route. An efficient route to... 

The incorporation of two aryl bridgeheads, both linked in turn through the 2,6-substitution of a pyridine nucleus has provided an intermediate which has been converted to a crown ether. Thus, 2,6-bis(2, 6 -dimethoxyphenyl)pyridine after demethylation with boron tribromide or hydrogen bromide in 65% yield was reacted in dimethylsulphoxide containing potassium carbonate with the a,o)-diiodo derivative of triethylene glycol to afford a crown ether system in 22% yield (ref. 199). 

Reissert reaction. The alkylation of Reissert compounds with bcnzylic halides was originally conducted with NaH as base in DMF. Skiles and Cava have examined three more modern systems LDA and HMPT in THF, KOH and a crown ether, and phase-transfer conditions. Of these, the two last systems are clearly superior to the first the phase-transfer system is somewhat superior to the crown ether system, both in yields and economic use of reagents. Cetyltrimethyl-ammonium bromide served as catalyst. 

The first concept to be considered is cation displacement of a complexed quencher of fluorescence. Scheme I illustrates the concq>t. To be operable the scheme requires a quencher that is complexed by a crown ether, a metal ion of interest that is not a quencher but is complexed effectively, and a crown ether ring that orients the complexed quencher so it will effectively quench the chromophore fluorescence. Scheme II shows a relatively simple crown ether system that we hoped would fulfill these requirements(22,2i). The l,5-naphtho-22-crown-6 compound was selected because of the ability of its crown ether band to hold a quencher against the face of the pi system of the naphthalene chromophore. The heavy atom ion Cs+ was selected as a quencher based on its propensity to increase inter-system crossing from the fluorescent Sj state to the nonfluorescent T state(i,2). It was likely, based on previous results(2), that potassium ion would be complexed by the crown, but not quench naphthalene fluorescence appreciably. 

In our work we have developed methods to build heavily modified crown-like systems in which the chiral components are amino acids. The general structural type is illustrated by (24a, b). In this case the macrocyclic crown ether system has been badly broken by extra substituents and heteroatoms (amide nitrogen, ester ether oxygen) of lowered basicity. 

Recently, l iriat ethers" and gther single armed crown ethers have extensively been investigated. They exhibited interesting cation binding and extracting profiles, based on their three-dimensional coordination geometry. Similar principles should be involved in the present "double armed crown ether" systems. 

The trivalent crown ether systems of Stoddart bind to ligands that display three cations with binding affinities that are enhanced relative to the monovalent interactions the degree of enhancement is similar to that of the zinc porphyrin-pyridyl systems. Smdies with these crown ethers reveal the mechanism of dissociation for each noncovalent interaction in the context of a multivalent complex. The dimer of a trivalent crown ether and a tris(benzylammonium) tri-cation displays a stepwise mechanism of dissociation (as opposed to a concerted process) in acetonitrile-dimethylsulfoxide mixtures loss of one noncovalent interaction provides a divalent complex, loss of a second interaction results in the monovalent entity, and loss of the third interaction provides the fully dissociated species (Figure 4). 

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