Oxygen ethers

Typical carbon-oxygen bond distances m ethers are similar to those of alcohols (—142 pm) and are shorter than carbon-carbon bond distances m alkanes (—153 pm) An ether oxygen affects the conformation of a molecule m much the same way that a CH2 unit does The most stable conformation of diethyl ether is the all staggered anti conformation Tetrahydropyran is most stable m the chair conformation—a fact that has an important bearing on the structures of many carbohydrates... 

Poly(ethylene oxide). Although AH j is more than double that of polyethylene, the effect is offset by an even greater increase for AS j. The latter may be due to increased chain flexibility in the liquid caused by the regular insertion of ether oxygens along the chain backbone. 

Multiple ether oxygen atoms can be present in the molecule. Cleavage and coupling reactions occur with direct fluorination although to a lesser extent than with ECF. This allows the direct fluorination of acid-sensitive materials, such as the formal shown below, which would not survive ECF (8). 

Association Complexes. The unshared electron pairs of the ether oxygens, which give the polymer strong hydrogen bonding affinity, can also take part in association reactions with a variety of monomeric and polymeric electron acceptors (40,41). These include poly(acryhc acid), poly(methacryhc acid), copolymers of maleic and acryflc acids, tannic acid, naphthoHc and phenoHc compounds, as well as urea and thiourea (42—47). 

The avermectins also possess a number of aUyflc positions that are susceptible to oxidative modification. In particular the 8a-methylene group, which is both aUyflc and alpha to an ether oxygen, is susceptible to radical oxidation. The primary product is the 8a-hydroperoxide, which has been isolated occasionally as an impurity of an avermectin B reaction (such as the catalytic hydrogenation of avermectin B with Wilkinson s rhodium chloride-triphenylphosphine catalyst to obtain ivermectin). An 8a-hydroxy derivative can also be detected occasionally as a metaboUte (42) or as an impurity arising presumably by air oxidation. An 8a-oxo-derivative can be obtained by oxidizing 5-0-protected avermectins with pyridinium dichromate (43). This also can arise by treating the 8a-hydroperoxide with base. 

Another group of macrocyclic ligands that have been extensively studied are the cycHc polyethers, such as dibenzo-[18]-crown-6 (5), in which the donor atoms are ether oxygen functions separated by two or three carbon atoms. The name crown ethers has been proposed (2) for this class of compounds because of the resemblance of their molecular models to a crown. Sandwich stmctures are also known in which the metal atom is coordinated with the oxygen atoms of two crown molecules. 

The ether oxygen of tetrahydropyran (45 X = O) induces a large downfield shift of the a carbons, while the /3 and y carbons move slightly upfield, the y more noticeably. 

The resultant cross-linked polymer is not always entirely polyphenylene because of the presence of ether oxygen in many of the intermediates. Neither do the polymers have the heat resistance of the ultimate in polyphenylenes, graphite, which has a melting point of 3600°C. 

In this case the Ar and/or Ar group(s) will contain an ether oxygen. As before if Ar = A then basically identical products may be made by the two polysulphonylation routes (see e.g. Figure 21.4). Where a diether and a disulphonyl chloride are co-condensed there may be some reaction at o- and m-positions in addition to the desired reaction at the para-position. It has been found that o- and m-linked polymers are more brittle, at least in the above example. It is of interest to note that self-condensation of the monosulphonyl chloride of diphenyl ether, on the other hand, can proceed so as to give a virtually all-para and hence tough material. Whilst the self-condensation polysulphonylation route has been used commercially it is understood not to be in current commercial use. 

As with the other non-glycidyl ether resins the absence of the ether oxygen near to the epoxide group results in low reactivity with amine hardemers whereas activity with acid anhydride proceeds at reasonable rates. 

Transannirlar participation of ether oxygen has also been identified by kinetic studies of a series of cyclic ethers. The relative rates for compounds 10-13 show that there is a huge acceleration in the case of replacement of the 5-CH2 group by an ether oxygen. ... 

The C(ls) and 0(ls) spectra of polyphenylene ether (PPE) before and after evaporation of chromium onto the surface are shown in Fig. 23. C(ls) spectra of neat PPE consisted of two components, near 284.6 and 286.0 eV, that were assigned to carbon atoms in the benzene and methyl groups and in the ether groups, respectively. The 0(ls) spectrum consisted of a single peak, near 533.4 eV, that was assigned to the ether oxygen atoms. After evaporation of chromium, a new peak related to formation of Cr202 was observed near 531.0 eV. 

Oxygen chelates such as those of edta and polyphosphates are of importance in analytical chemistry and in removing Ca ions from hard water. There is no unique. sequence of stabilities since these depend sensitively on a variety of factors where geometrical considerations are not important the smaller ions tend to form the stronger complexes but in polydentate macrocycles steric factors can be crucial. Thus dicyclohexyl-18-crown-6 (p. 96) forms much stronger complexes with Sr and Ba than with Ca (or the alkali metals) as shown in Fig. 5.6. Structural data are also available and an example of a solvated 8-coordinate Ca complex [(benzo-l5-crown-5)-Ca(NCS)2-MeOH] is shown in Fig. 5.7. The coordination polyhedron is not regular Ca lies above the mean plane of the 5 ether oxygens... 

Condensation of prednisone, 40 with tetraethyl orthocarbonate leads to the cyelie ortho-carbonate 41 liydrolysis proceeds by protonation on the most accessible ether oxygen (that on carbon 21) to give the 17 mixed carbonate ester 42. Acylation with propionyl chloride proceeds on the remaining hydroxyl group to afford prednicarbate (43) [10],... 

Unlike other ethers, epoxide rings con be cleaved by base as well as by acid. Although an ether oxygen is normally a poor leaving group in an reaction (Section 11.3), the strain of the three-membered ring causes epoxides to react with hydroxide ion at elevated temperatures. 

Hydrogens on carbon next to an ether oxygen are shifted downfield from the normal alkane resonance and show U-f NMR absorptions in the region 3.4 to 4.5 8. This downfield shift is clearly seen in the spectrum of dipropyl ether shown in Figure 18.4. 

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