Dimer polyether

Similar formations of bisanthracenes have been studied with a variety of substituents. An interesting example of a cation-assisted intramolecular anthracene dimerization where the bisanthracene formation includes cycli-zation of a polyether chain to a crown ether is illustrated in (4.27). In the absence of Li+ the dimer reverts back to the open chein compound, but in the presence of Li+ the crown ether is stabilized so that the product itself also becomes more stable 430). 

Owing to their improved stability towards hydrolysis and oxidation, dimer diol polyethers (and dimer diol polycarbonates) are used as soft segments in the preparation of thermoplastic polyurethanes. Polyurethanes prepared from such oleo-chemical building blocks are very hydrophobic and show the expected stability. 

Most recently, we have investigated the use of iterative oxonium ylide [1,2]- or [2,31-shifts as a convenient approach to the polypyran domains often found in the marine polyether ladder toxins (Scheme 18.8) [21]. Initial studies indicated that [l,2]-shifts of O-benzyl oxonium ylides such as 19 a or 19 b were inefficient. Alternative metallocarbene processes including C-H insertion and dimerization were found to predominate in these cases, again suggesting that carbene-ylide equilibration may occur [21b]. On the rationale that concerted [2,3]-shifts of the corresponding O-allyl oxonium ylides might occur more readily, the allyl ethers 19 c, 19 d were then examined. These examples were much more effective, especially in conjunction with the optimized catalyst Cu(tfacac)2 [21a]. However, rhodium(II) triphenylacetate (Rh2(tpa)4) [22] was found to... 

Lasalocid is the more disruptive polyether antibiotic to biological systems. This is due to its high tendency to dimerize and form complexes with biologically important divalent ions such as ("a and Mg ". Lasalocid exhibits a wide range of complexation affinities and transport capabilities, encompassing not only inorganic polyvalent ions but also primary amines and catecholamines (22). 

The crystal structure of a barium picrate complex of (139) reveals a 2 2 dimeric structure of the form [(139)Ba(picrate)3Ba(139)]+(picrate) (148), 47 which is unique as three picrate anions are held in the space between the ligated cations. The cations are not sited within the macrocyclic cavities but are displaced towards the centre of the dimer. This situation is reminiscent of the Cs+-polyether dimer structure (Figure 15d). The outer surface of the (139)Ba(picrate)2 complex is highly lipophilic, and the bulky phenyl residues provide screening for the enclosed anions. 

The divalent polyether antibiotics have revealed the existence of dimeric molecules as monomeric complexes in their interaction with alkali and alkaline earth cations. Lasolocid (145) forms complexes of the type M+L and these may be monomeric when formed and recovered from methanol, but dimeric when recrystallized from the non-polar carbon tetrachloride. The shortened ligand backbone relative to the monovalent polyethers does not allow complete shielding of the cation from the solvent, and so in non-polar solvents this is overcome by dimerization. If the salicylate and tetrahydropyran moieties present in (145) are referred to as the head and the tail of the molecule then two forms of dimerization may be envisaged head-to-tail and head-to-head . 

A considerable number of NQR studies have been made on zinc and cadmium complexes, for example, the 35C1, 81Br and 127I NQR spectra of a number of CdX2-polyether complexes have been reported and indicate that the compounds are dimeric in solution, with symmetrical halogen bridges between the metal atoms.69 A number of amino acids and peptide complexes of cadmium(II) have been investigated by 14N NQR spectroscopy.70... 

Ionophores, or polyether (PET) antibiotics, produced by various species of Streptomyces, possess broad spectrum anticoccidial activities. They are chemically characterized by several cyclic esters, a single terminal carboxylic acid group, and several hydroxyl groups. Representative members of this class include salinomycin (SAL), monensin (MON), lasalocid (LAS), narasin (NAR), maduramicin (MAD), and semduramicin (SEM). The main chemical properties of interest in the extraction methodology are their low polarities and instability under acidic conditions. They are able to form stable complexes with alkaline cations. All of these compounds, with the exception of LAS, bind monovalent cations (e.g., Na+ and K+). Lasalocid has a tendency to form dimers and can form complexes with divalent cations such as Mg2+ and Ca2+. The formation of metal complexes results in all of these compounds adopting a quasi-cyclic formation consequent to head-to-tail hydrogen bonding. No MRLs have yet been set by the EU for any of the carboxylic acid PETs (98). 

In this system, however, intermolecular dimerization may take place competitively with intramolecular dimerization. To rule out this possibility, compound 5, in which two anthracenes are linked by two polyether chains, was synthesized.171 It was found that intramolecular photodimerization proceeds rapidly in the presence of Na+ as the template metal cation. Compound 6 was also synthesized.181 Although this compound has not been applied in a photoswitch system, it displays a remarkable fluorescence change upon binding with RbC104 or H3N+(CH2)7NHj.[81 Yama-shita et al.[9] also synthesized 7, in which intermolecular photodimerization of anthracene is completely suppressed. The photochemically produced cyclic form 8 displayed excellent Na+ selectivity. 

The kinetics of polycondensation hy nucleophilic aromatic substitution in highly polar solvents and solvent mixtures to yield linear, high molecular weight aromatic polyethers were measured. The basic reaction studied was between a di-phenoxide salt and a dihaloaromatic compound. The role of steric and inductive effects was elucidated on the basis of the kinetics determined for model compounds. The polymerization rate of the dipotassium salt of various bis-phenols with 4,4 -dichlorodiphenylsulfone in methyl sulfoxide solvent follows second-order kinetics. The rate constant at the monomer stage was found to be greater than the rate constant at the dimer and subsequent polymerization stages. 

The cholates 8.117-8.119 were designed for the preparation of dynamic hbraries with different binding affinities for alkah metal ions. The presence of a polyether chain in position 7 of 8.117 provided a recognition element for metal binding that was absent from the disubstituted p-methoxybenzyl substitution pattern of 8.118, while the 7-deoxy derivative 8.119 was even less prone to metal coordination. The three monomers were submitted to transesterification/cyclization protocols, either without metal templates or using different alkali metal salts as templates. The relative abundances of cyclic dimers, trimers, tetramers, and pentamers for each experiment are reported in Table 8.7. 

Ether bismuth halide adducts also exist. Structural work on bismuth chloride diethyl ether or THF complexes show that, at low temperature, polymeric chains of BiXs linked by hahde bridges exist and that the bismuth atoms may be coordinated by one or two ether molecules. Bismuth(ni) bromide coordinates three THF molecules. These solvent molecules are readily removed under vacuum. The polyethers diglyme and diethylcarbitol give dimeric adducts with Bids. In the presence of cyclic polyethers, simple coordination, or formation of polyether-coordinated bismuth cations... 

This polymerization process is a polycondensation in which the molecular weight builds up slowly as the small molecules of water are eliminated. Most step polymerization processes are polycondensations thus the terms step polymerization and condensation polymerization are often used synonymously. The stepwise reaction leads successively from monomers to dimers, trimers, and so on, until finally polymer molecules are formed. The polymers obtained are classified by taking into account the functional group of the repeating unit, for example, polyesters (— CO —O—), polyamides ( — CO— NH —), polyurethanes (—O — CO — NH —), polyethers ( — O —), and polycarbonates ( — O — CO —O —). 

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