Metal-ion complexes of ethers

Additional examples of type d (Scheme 5.1) bifunctional reactants are provided by the alkaline-earth metal ion complexes of lariat ethers 8-10, bearing a sulfhydryl side arm, instead ofthe phenolic hydroxyl of a calixcrown [23,24]. Here the acyl-receiving and acyl-releasing unit, like in papain and ficin, is a sulfhydryl group. 

The electrochemical oxidation of tyramine in NaOH/MeOH media gives films of polytyramine (25). The film, on a platinum electrode, can complex copper(II) ions from aqueous media and cobalt(II), iron(II), manganese(II) and zinc(II) from organic media. X-ray photoelectron spectroscopy established that coordination of the metal ions had occurred. For cobalt, evidence of coordination to both ether and amine functions is obtained, but for the other metal ions evidence of ether coordination is less definitive. 

Chemically modified crowned spirobenzopyran 112, containing a pyrenyl fluorophore attached at the nitrogen atom, can function as a fluorescence emission switch <2004T6029>. This sensor displayed a quenching of the PET fluorescence emission of the fluorophore in the absence of metal ions (the merocyanine form was not produced). When, however, the spiro form of 112 was converted into the merocyanine form by metal ion complexation of the crown ether portion of the molecule, a fluorescence resonance energy transfer (FRET) from the pyrene to the merocyanine moiety took place, producing fluorescence emission. 

Becher <90JA3709> has studied by californium-252 plasma desorption mass spectrometry the alkali metal ion complexation of a crown ether containing two pyrazole subunits. Rebek <93RTC330> prepared one of his famous molecular clefts with two 3-/-butylpyrazol-l-yl substituents. 

The metal-ion complexing properties of crown ethers are clearly evident in their-effects on the solubility and reactivity of ionic compounds in nonpolar- media. Potassium fluoride (KF) is ionic and practically insoluble in benzene alone, but dissolves in it when 18-crown-6 is present. This happens because of the electron distribution of 18-crown-6 as shown in 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 in benzene, potassium ion (K ) interacts with the oxygens of the crown ether to for-m a Lewis acid-Lewis base complex. As can be seen in the space-filling model of this... 

In this particular case the binding of an alkaline or alkaline earth metal ion to the crown ether decreases the stability of the host with u-allose (negative allosterism), since the conformation of the crown-ether changes due to the metal ion complexation [264-268]. 

The mechanism of ion transport in the MEEP/metal salt complexes has been modelled on the PEO transport mechanism, that is to say in terms of jumps of the metal ion between the ether oxygen nuclei of the side groups, the nitrogen atoms of the backbone being not involved in the coordination [599]. 

Earlier work in this field has been thoroughly reviewed [1,2]. However, to illustrate in a sensible and logical way the evolution from simple metal ion promotion of acyl transfer in supramolecular complexes to supramolecular catalysts capable of turnover catalysis, an account of earlier work is appropriate. The following sections present a brief overview of our earlier observations related to the influence of alkaline-earth metal ions and their complexes with crown ethers on the alcoholysis of esters and of activated amides under basic conditions. 

Crown-ether containing triarylphosphines have been studied in the Shell laboratories in an attempt to find a favorable effect of metal ion complexation in rhodium-catalyzed hydroformylation [68] no effect was found, although precedents exist in which the metal ion acts as a Lewis add that activates coordinated carbon monoxide towards methyl migration [69]. 

For example, the formation constants of 1 1 complexes of alkali metal ions with crown ethers or cryptands have been determined by Procedures 1 or 2 below [24] ... 

Crown ethers continue to be one of the most useful parts of supramolecular chemistry/91 From the beginning computations of metal ions complexes with synthetic ionophores/101 which have been aptly reviewed/111 emphasized the importance of including explicitly solvation in free energy calculations, also with ab initio calculations on calixarene complexes/121 Molecular dynamics simulations of 18-crown-6 ether complexes in aqueous solutions predict too low affinities, but at least correctly reproduce the sequence trend K+ > Rb+ > Cs+ > Na+. However, only the selection of K+ over Rb+ and Cs+ is ascribed to the cation size relative to that of the crown cavity, whereas K+ appears in these calculations to be selected over Na+ as consequence of the greater free energy penalty involved in displacing water molecules ftomNa/1131... 

The chiral selectors most commonly used as additives in the buffer can be divided into three main categories inclusion systems [e.g., cyclodextrins (CDs) or crown ethers], enantioselective metal-ion complexes [e.g. cop-per(II)-L-histidine or copper(II)-aspartame], and optically active surfactants (e.g., chiral mixed micelles or bile acids). Cyclodextrins are the most widely reported, and they are used in low-pH buffers for the resolution of... 

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