Ethers, aliphatic cleavage

The low reactivity of aliphatic ethers renders the problem of the preparation of suitable crystalline derivatives a somewhat difficult one. Increased importance is therefore attached to the physical properties (boding point, density and refractive index) as a means for providing preliminary information. There are, however, two reactions based upon the cleavage of the ethers which are useful for characterisation. 

Cleavage of ethers with hydriodic acid. Aliphatic ethers suflFer fission when boiled with constant boiling point hydriodic acid ... 

Fluorination of aliphatic ethers at gentle conditions with cobalt trifluoride or potassium tetrafluorocobaltate do not give perfluorinated products and cause only negligible cleavage of the ether bond. Complex mixtures are formed from ethyl methyl ether and from diethyl ether [9] (equations 16 and 17)... 

Furthermore, the mediator has been used for the bond cleavage of benzyl ethers, the oxidation of benzyl alcohol to benzaldehyde, the oxidation of toluene derivatives to benzoic acid esters, and the oxidation of aliphatic ethers [47]. 

The intensity of the molecular ion of aliphatic amines decreases regularly with increasing molecular weight. [33] Analogous general behavior has also early been noted for other compound types such as the aliphatic ethers [31], hydrocarbons [34] and aldehydes [24], and others. The reason for the low stability of amine molecular ions is their predisposition to a-cleavage. 

Open-chain aliphatic ethers are completely resistant to hydrogenolysis. Cyclic ethers (for epoxides, see p. 83) may undergo reductive cleavage under strenuous conditions. The tetrahydrofuran ring was cleaved in vigorous hydrogenations over Raney nickel [420] and copper chromite [420] to give, ultimately, alcohols. 

Aliphatic ethers are protonated in strong acids, at low temperatures the exchange rates of the acidic proton are slow enough to permit direct observation by NMR spectroscopy.133 Temperature-dependent NMR spectral studies allow us again to follow the kinetics of ether cleavage to form alkyl cations [Eq. (3.18)]. 

The molecular ion peak of ethers is weak or negligible. There are two fragmentation processes which are typical of ethers. A characteristic fragmentation is the cleavage of the carbon-oxygen bond. This often gives rise to the most abundant ion in the spectrum of aliphatic ethers. 

Protection of the peripheral phenolic groups as benzyl ethers was selected so that regioselective cleavage of these ethers in the presence of interior aliphatic ether linkages could be effected to liberate phenolic groups at some later stage. These functionalized dendrons were coupled to various anchor cores to produce... 

It is postulated that while iron catalysts may facilitate carbon-oxygen bond cleavage, as evidenced by the Increased reactivity of benzyl and aliphatic ethers, when phenoxy radicals are produced they are strongly adsorbed on the catalyst surface. They are not therefore readily available to propagate the radical chain reactions. 

Aryl ethers in the presence of a solvent can be preferentially halo-genated in the nucleus. Thus, anisole with phosphorus pentabromide or with iodine monochlotide yields p-bromoanisole (90%) and p-iodo-anisole (46%), respectively. Phosphorus pentachloride has also been used for the halogenation of the nucleus as in the preparation of 4-chloro-biphenyl ether (90%). The action of this reagent with aliphatic and aryl-aliphatic ethers is very complex, giving both cleavage and halogenation products. ... 

Similar to alcohols, aliphatic ethers [54], thiols [55], and sulfides are also protonated on oxygen or sulfur, respectively, at -60 °C in magic acid carbocations are subsequently formed upon raising the temperature. Promoted sulfides, excluding tertiary alkyl, are resistant to cleavage up to -i-70 °C [56]. 

By cleavage of suitable aliphatic ethers—e.g., diallyl ether with sodium (GH2=GH—CH2)20 4- 2 Na -h- GH2=GH—CHaNa -j- GH2=GH—GH20Na... 

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