Ethers and Polyethers

Many classes of perfluoroethers such as acyclic [63], cyclic [64, 65], glymes [66,67], highly branched [63],macrocyclic [68-73], orthoformates [74-76] and poly-ethers [15, 77], have been succesfully prepared in high yield by direct fluorination techniques (Figs. 19 and 20). Carbon-oxygen bond cleavage is minimised in these processes by the addition of an HF scavenger, such as sodium fluoride, to the reaction mixture [14,45,46]. 

Carbon-oxygen bond cleavage may occur by /1-scission (Fig. 21) but only significantly in cases where relatively stable radicals are the resulting intermediates, such as in the fluorination of f-butylmethyl ether (Fig. 22) [78]. 

Orthocarbonates are particularly susceptible to /1-scission, as indicated by the products that are obtained upon fluorination of tetramethylorthocarbonate (Fig. 23) [74]. 

Perfluorinations of many partially fluorinated ethers have been carried out (Figs. 24-26) [39,55,65,79,80]. 

Perfluoropolyethers have found widespread use as high-performance lubricants and several companies manufacture a range of these materials (Krytox , Du Pont Fomblin , Montefluos Demnum , Daikin). The Fomblin fluids and Krytox require the use of fluorine in the finishing stages [81] whilst Demnum is synthesised by polymerisation of the fluorooxetane 16 followed by per-fluorination using fluorine (Fig. 27) [82]. 

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Solvent for Displacement Reactions. As the most polar of the common aprotic solvents, DMSO is a favored solvent for displacement reactions because of its high dielectric constant and because anions are less solvated in it (87). Rates for these reactions are sometimes a thousand times faster in DMSO than in alcohols. Suitable nucleophiles include acetyUde ion, alkoxide ion, hydroxide ion, azide ion, carbanions, carboxylate ions, cyanide ion, hahde ions, mercaptide ions, phenoxide ions, nitrite ions, and thiocyanate ions (31). Rates of displacement by amides or amines are also greater in DMSO than in alcohol or aqueous solutions. Dimethyl sulfoxide is used as the reaction solvent in the manufacture of high performance, polyaryl ether polymers by reaction of bis(4,4 -chlorophenyl) sulfone with the disodium salts of dihydroxyphenols, eg, bisphenol A or 4,4 -sulfonylbisphenol (88). These and related reactions are made more economical by efficient recycling of DMSO (89). Nucleophilic displacement of activated aromatic nitro groups with aryloxy anion in DMSO is a versatile and useful reaction for the synthesis of aromatic ethers and polyethers (90). 

In general, the best acetaldehyde rates and selectivities, typically 3-5 M/hr and 80-90%, are obtained in ethers and polyethers. Relative to the solvent-free case (methanol only) the rate is somewhat lower and the selectivity is normally 10% higher. 

Fig. 12 Selected examples of taper shaped crown ethers and polyethers with three fluorinated chains (77°C) data refer to compounds 30, 32 and 34 [132-135]...

Equilibrium constants of complexation of Na by ethers and polyether solvents at 25° (30). 

Some of the following work was performed with a view to synthesis of perfluoro—ethers and —polyethers, which is not the subject of this present discussion, but we were interested in the relative reactivities of various sites in ether systems. Hence the following results are revealing. 

Therefore, we may conclude that the relative order of reactivity of various sites in ethers and polyethers, towards fluorinated alkenes in free—radical processes will be ... 

Fig 20.22 Catalysts, ionic liquids, polydimethylsiloxanes, ethers, and polyethers which were... 

TBA][L-Met], etc. with high yield and purity. Recently Morrissey et al. [91] synthesized oxygen-functionalized and amide ILs containing a wide range of ether and polyether esters in two steps followed by anion exchange (Scheme 20.2). 

We shall, of course, also cover directed selectivity in ether- and polyether synthesis, but we look at them not mainly as building blocks for very complicated molecules but rather as tools in stereoselective synthesis in general. Since selectivity, particularly diastereoselectivity, is very often unsatisfactory in acyclic systems, cyclic, conformational less mobile equivalents of the desired molecules could come to the rescue. 

First, it plays a decisive role in ether and polyether biosynthesis and there are indeed two routes available to transform epoxides into cyclic ethers. With both, unfortunately, we have to worry about regioselectivity [95]. 

Cyclic Halogenated Cns Ethers and Polyethers of Laurenda and Chondria... 

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