Crown ethers, from

Perfluoro crown ethers from the hydrocarbon dibenzo crown ethers have also been synthesized (58) and the first perfluorocryptand molecule [2.2.2] has been reported (59). The perfluorocryptand is a stable, inert, high boiling clear oil. 

The recent theory of Penzcek, which involves co-ordinated oxy-carbenium ions, is also shown not to be applicable to the systems considered here. The heuristic value of the ring-expansion theory is illustrated briefly by reference to a new method of synthesising crown ethers from 1,3-dioxacycloalkanes, which arose from it. 

We shall now consider the synthesis of chiral crown ethers from the viewpoints of strategies and some recent accomplishments. For a detailed discussion of the earlier work on the synthesis of chiral crown ethers from natural product precursors, the reader is referred to reviews (91, 92) that appeared in the literature in 1981 and 1982. 

A Monte-Carlo study is conducted of the formation of benzo-crown ethers from an acyclic precursor. The major features of the computed cyclization constants are in harmony with trends observed in kinetic constants reported. 

The macrocyclic polyethers, termed crown ethers from their structural resemblance to crowns, were first synthesized by C. J. Pedersen in 1967 by reactions such as the following ... 

Synthesis of monoaza crown ethers from A, A -di[oligo(oxyalkylene)]amines and oligoethylene glycol di(p-toluenesulfonates) or corresponding dichlorides, H. Maeda, S. Furuyoshi, Y. Nakatsuji and M. Okahara, Bull. Chem. Soc. Jpn., 1983, 56, 212. 

The formation of macrocyclics in the cationic polymerization of certain cyclic ethers has been known for some time ( 2). McKenna et al. ( 3) have reported on the macrocyclics from THF. Dale (4j has studied the formation of cyclic oligomers from EO, and Hammond ( 5) has identified some of the macrocyclics produced in the copolymerization of propylene oxide and THF. We have recently reported (6) on the formation of crown ethers from EO and THF using CF3SO3H as a catalyst. 

Most recently HPLC separations of metal ions have been carried out utilising the formation of stable complexes of metal cations with large cyclic ethers called crown ethers (from the geometry of their appearance), though the theoretical plate numbers usually characteristic of HPLC have not yet been achieved with the development stage attained by this complexing technique. 

The effects of added water on the rates of displacement of benzyl bromide and benzyl chloride with KCN salt in toluene catalyzed by 18-crown-6 were reported [145], It was observed that a small amount of water considerably increased the reaction rates compared to the anhydrous conditions and that the rate increased sharply to a maximum value in the presence of an optimum amount of added water. An important observation was that under anhydrous conditions, the reaction followed zero-order kinetics while in the presence of added water it followed first-order kinetics. It was suggested that the initial small amounts of added water coated the surface of the salt particle, which extracted the crown ether from the organic phase to form a new interfacial region called the omega (cd) phase. It was believed that the catalytic reaction took place mainly in the omega phase, since the quantity of added water corresponding to the maximum quantity of crown ether on the surface of the salt particles correlated well with the optimum quantity of added water. 

The earliest examples of synthetic allosteric systems involved modification of binding properties of crown ethers by changing their conformations as a result of complexation with another metal cation at an adjacent binding site. For example, complexation of a bipyridine moiety of 1 (Fig. 2) with W(CO)4 changes the selectivity of the crown ether from preferable binding of over Na to preferable binding of Na over In a more recent... 

Liquid clathrates as crystallization media have resulted in many new structures. Crystals of crown ether, cryptand, and tetramethylethylenediammonium and triphenylphosphonium complexes were prepared. Presented in Table 3 is a short list of some oxonium ion crown-ether crystal complexes which deposited from the liquid clathrate mixture. Oxonium ion complexes with crown ethers from a liquid clathrate media were formed of a suitable size and shape to crystallize lanthanides and actinides. 

Hassaballa. H. Steed. J.W. Junk. P.C. Elsegood, M.R.J. Formation of lanthanide and actinide oxonium ion complexes with crown ethers from a liquid clathrate medium. Inorg. Chern. 1998. 37. 4666-4671. 

Roncali, J., R. Garreau, and M. Lemaire. 1990. Electrosynthesis of conducting poly-pseudo-crown ethers from substituted thiophenes. J Electroanal Chem 278 373-378. 

Fig. 22. Structural formulae of crown ethers. From left to right and top to bottom benzo[15]crown-5, [18]crown-6 dibenzo[18]crown-6, dicyclohexano[18]crown-6 tetramethyldibenzo[18]crown-6, di-benzo[24]crown-8 dibenzo[30]crown-10, spiro-bis[19]crown-6...

Salts such as KF, KCN, and potassium acetate can be transferred into aprotic solvents using catalytic amounts of 18-crown-6. Use of a crown ether with a nonpolar solvent can be very favorable for an 5 2 reaction because the nucleophile (such as F , CN , or acetate from the compounds just listed) is unencumbered by solvent in an aprotic solvent, while at the same time the cation is prevented by the crown ether from associating with the nucleophile. Dicyclohexano-18-crown-6 is another example of a phase transfer catalyst. It is even more soluble in nonpolar solvents than 18-crown-6 due to its additional hydrocarbon groups. Phase transfer catalysts can also be used for reactions such as oxidations. (There are phase transfer catalysts that are not crown ethers, as well.)... 

Considerable developments are taking place in this area as evidenced by the wide range of syntheses reported below. An important review of progress to date Carbohydrate derivatives in the asymmetric synthesis of natural products has been provided by Fraser-Reid and Anderson. A further review covers the preparation of asymmetric crown ethers from carbohydrates. Highly abbreviated Schemes are used in this report because space does not permit the reproduction of the lengthy routes used in many of the syntheses. 

Abstract. Crown ethers derived from tartaric acid present a number of interesting features as receptor frameworks and offer a possibility of enhanced metal cation binding due to favorable electrostatic interactions. The synthesis of polycarboxylate crown ethers from tartaric acid is achieved by simple Williamson ether synthesis using thallous ethoxide or sodium hydride as base. Stability constants for the complexation of alkali metal and alkaline earth cations were determined by potentiometric titration. Complexation is dominated by electrostatic interactions but cooperative coordination of the cation by both the crown ether and a carboxylate group is essential to complex stability. Complexes are stable to pH 3 and the ligands can be used as simultaneous proton and metal ion buffers. The low extractibility of the complexes was applied in a membrane transport system which is a formal model of primary active transport. 

Procedure Generate ions by positive-ESI accumulate a sufficiently high number of pseudorotaxane ions in hexapole introduce ion package into the FTICR analyzer cell mass-select the desired pseudorotaxane ions irradiate with IR laser (10.6 Jm wavelength 500-1000ms) -> detect product ions -> repeat experiment at different laser power settings -> determine sequence of crown ethers from fragmentation sequence. 

Table 1 Templated synthesis of crown ethers from ethylene oxide.

Indeed, if one plots the Na linewidth, reduced to unit viscosity, as a function of the molar ratio of sodium complexant C to the E Na salt, a plateau is also reached with 5-10 equivalents when C is taken as 15-crown-5. However, rather than the expected monotonic decrease of the linewidth, when the cation is pulled away by the crown ether from the anion, one witness a sharp initial increase upon addition of minute quantities of the polyether, followed by a more gradual decrease to a value of the order of 500 Hz (cP) 1. ... 

HPAs were used as an effective catalyst for the synthesis of thioxanthone crown ethers from the reaction of thiosahcylic acid and benzocrown ethers. This reaction was carried out subsequently via intramolecular electrophilic cyclization. The reaction was in mild and clean conditions, and has high selectivity with good yields (Scheme 3.21) [61],... 

A novel strategy for forming controlled-size pseudo-crown ethers from acyclic polyethers has recently been proposed by Fabre et al. [258]. As conceptualized in Fig. 10, it consisted of the electrosynthesis of electroactive copolymers from compounds possessing two different electropolymerizable aromatic groups (pyr-role/thiophene (10), pyrrole/dimethoxybenzene (11), and thio-phene/dimethoxybenzene (12)) linked together by a polyether chain. 

Some thia-crown ethers have been prepared from the reaction between caesium thiolates of appropriate l,w-dithiols and polyethylene glycol dibromides.The synthesis of mono-aza-crown ethers from dialkanolamines [equation (17)] can be performed without the need for a protecting group on... 

Potassium fluoride coated on alumina has been shown to be an effective and practical base for the usual Williamson-type synthesis of crown ethers from polyethylene glycols and polyethylene glycol ditosylates [equation (23)],giving yields comparable to standard methods. 

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