Ethers, crown interactions

Changes in the Inter- and intramolecular interaction and in the structure of carbanion pairs on adding cation-binding ligands (ethers, crown ethers, poly-amines) is briefly reviewed. 

Grootenhuis, P. D. J., Kollman, P. A. (1989), Molecular Mechanics and Dynamics Studies of Crown Ether-Cation Interactions Free Energy Calculations on the Cation Selectivity of Dibenzo-18-Crown-6 and Dibenzo-30-Crown-10, J. Am. Chem. Soc. Ill, 2152-2158. 

F. 9a-g. Types of binding interactions that can be exploited during templating a n-n interaction b hydrophobic or van der Waals interaction c covalent bonds d (transition) metal-ligand binding e hydrogen bonding f crown ether -ion interaction g ionic interaction... 

Mannich reaction of A, A -bis(methoxymethyl)diaza-18-crown-6 with 4-chloro-2-(l/f-pyrazol-3-yl)phenol gave the N-linked bis(3-(5-chloro-2-hydroxy)pyrazol-l-ylmethyl)-substituted diazacrown ether, which interacted with various metal ions and was evaluated by calorimetric titration <1999JOC8855>. Intramolecular nitrilimine cycloadditions were exploited in the preparation of a number of azacrown ethers having a medium or large ring annulated to pyrazole units <1997T3005>. 

P.D.J. Grootenhuis and P.A. Kollmann, Molecular mechanics and dynamics studies of crown ether-cation interactions free energy calculations on the cation selectivity of dibenzo-18-crown-6 and dibenzo-30-crown-10, J. Am. Chem. Soc., Ill (1989) 2152-2158. 

X-Ray crystallographic data on the interaction of alkali metal ions with mononuclear metal carbonyls are not very extensive. No data appear to be available for lithium salts of mononuclear carbonylates. The findings on sodium and potassium salts reveal interesting details in the variety of modes of interaction. The situation may be summarized as follows. An essentially end-on interaction (e.g., FeCO—Na) is seen in the majority of instances, but if the alkali metal ion is unencumbered with large ligands, such as crown ethers, an interaction also is seen with the carbonyl carbon atoms and with the metal. Thus the solid state species contain some of the same structural features which were discussed above for ion pairs in solution. Unfortunately, there are no detailed analyses of the vibrational spectra of the solids for which structures are available. This might provide one avenue to the refinement of our knowledge of the species present in solution. 

Although parent compound 47 (s. Scheme 5) and linear reference compounds 49 and (Table 12) which have two picolyl groups showed low extractabilit-ies toward Ag", doubly armed crownophane 48 efficiently extracted Ag", and with excellent selectivity. Ouchi and co-workers reported that a pair of polyether side chains attached at the same carbon atom of a crown ether can interact... 

Mechanics and Dynamics Studies of Crown Ether-Cation Interactions Free Energy Calculations on the Cation Selectivity of Dibenzo-18-crown-6 and Dibenzo-30-crown-10. 

The strong interaction of20 with dopamine 27 is noteworthy. [18]aneN6 3 H" alone binds with the catechol moiety of dopamine 27 with a Pi value of 1.1 x 10 M [16]. Addition of 10 eq. of benzo-15-crown-5 has not affected its polarographic behavior at all. However, when covalently attached as in 20, the crown ether moiety interacts complementarily with the primary ammonium cation part of dopamine 27, as depicted in 21, resulting in a j L value that is greater by a factor of ten. Since the... 

Applications of Crown Ethers.—Crown ethers have been found to form molecular complexes with bromine that can be used as reagents in bromination reactions, e.g. of alkenes. The crown-halogen interaction seems not to be encapsulation but rather to be the formation of a polymeric structure. The complexes of nitro-nium tetrafluoroborate (NO2BF4) with various crown ethers, for example the 1 1 complex with 18-crown-6, have been reported to be selective nitrating agents. A new rapid synthesis of alkylseleno- and alkyltelluro-arenes from arenediazon-ium tetrafluoroborates and dialkyl diselenides or ditellurides employs KOAc and 18-crown-6 as a phase-transfer catalyst a radical mechanism has been suggested. Fluorine-19 n.m.r. evidence has been presented to show that KF forms... 

The presence of fulleroN-methylpyrrolidine, attached to the crown ether, results in minima, where the cation is captured between fullerene and crown ether. Additional interactions that can further stabilize the complex are formed. 

The metal-ion complexmg properties of crown ethers are clearly evident m their effects on the solubility and reactivity of ionic compounds m nonpolar media Potassium fluoride (KF) is ionic and practically insoluble m benzene alone but dissolves m it when 18 crown 6 is present This happens because of the electron distribution of 18 crown 6 as shown m Figure 16 2a The electrostatic potential surface consists of essentially two regions an electron rich interior associated with the oxygens and a hydrocarbon like exterior associated with the CH2 groups When KF is added to a solution of 18 crown 6 m benzene potassium ion (K ) interacts with the oxygens of the crown ether to form a Lewis acid Lewis base complex As can be seen m the space filling model of this... 

The diffusion, location and interactions of guests in zeolite frameworks has been studied by in-situ Raman spectroscopy and Raman microscopy. For example, the location and orientation of crown ethers used as templates in the synthesis of faujasite polymorphs has been studied in the framework they helped to form [4.297]. Polarized Raman spectra of p-nitroaniline molecules adsorbed in the channels of AIPO4-5 molecular sieves revealed their physical state and orientation - molecules within the channels formed either a phase of head-to-tail chains similar to that in the solid crystalline substance, with a characteristic 0J3 band at 1282 cm , or a second phase, which is characterized by a similarly strong band around 1295 cm . This second phase consisted of weakly interacting molecules in a pseudo-quinonoid state similar to that of molten p-nitroaniline [4.298]. 

In the absence of die polyether, potassium fluoride is insoluble in benzene and unreactive toward alkyl halides. Similar enhancement of solubility and reactivity of other salts is observed in the presence of crown ethers The solubility and reactivity enhancement result because the ionic compound is dissociated to a tightly complexed cation and a naked anion. Figure 4.13 shows the tight coordination that can be achieved with a typical crown ether. The complexed cation, because it is surrounded by the nonpolar crown ether, has high solubility in the nonpolar media. To maintain electroneutrality, the anion is also transported into the solvent. The cation is shielded from interaction with the anion as a... 

In Pedersen s early experiments, the relative binding of cations by crown ethers was assessed by extraction of alkali metal picrates into an organic phase. In these experiments, the crown ether served to draw into the organic phase a colored molecule which was ordinarily insoluble in this medium. An extension and elaboration of this notion has been developed by Dix and Vdgtle and Nakamura, Takagi, and Ueno In efforts by both of these groups, crown ether molecules were appended to chromophoric or colored residues. Ion-selective extraction and interaction with the crown and/or chromophore could produce changes in the absorption spectrum. Examples of molecules so constructed are illustrated below as 7 7 and 18 from refs. 32 and 131, respectively. 

In this review, recent development of active transport of ions accross the liquid membranes using the synthetic ionophores such as crown ethers and other acyclic ligands, which selectively complex with cations based on the ion-dipole interaction, was surveyed,... 

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