Aliphatic ether bonds, cleavage

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)... 

The second step involves coal activation. The relative ability of different media to split reactive crosslinkages of the coal is a crucial factor in obtaining conversion. The reactive crosslinks appear to be primarily ether bonds and aliphatic linkages, with suitably substituted neighboring aromatic centers (.5,6). Work in these laboratories has shown that ZnCl2 is an active catalyst for cleavage of these crosslinks (5,9). Addition of methanol may enhance this activity, whereas excessive solvent appears to dilute the catalyst. 

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]. 

Aliphatic 3-aza-4-oxa-Cope rearrangement of ester-amide dienolates increases the synthetic utility of anionic [3,3]-sigmatropic rearrangement initiated by N—O bond cleavage. Treatment of the enehydroxylamine 94 with KHMDS in the presence of TMSCl at —80°C provided a mixture of iV,0-disilylated 95 and 0,0-disilylated 96 derivatives (equation 30). Both of these would, on [3,3]-sigmatropic rearrangement, provide the corresponding silyl ethers 97 and 98, from which 99 and 100 are obtained on workup. 

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. 

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. 

Scheme 16.14 shows the reactions of two dimeric cyclic ether lignin models with O2 [64] Oxidation of the dimeric cyclic ether pinoresinol, 54 with O2 from NaOCl/ H2O2 at pH 7 underwent cleavage of the phenyl-aliphatic bond to produce meth-oxy-hydroquinone, 50 and 2,6-dihydroxy-3,7-dioxabicyclo [3.3.0] octane, 55. When treated under the same conditions as pinoresinol, 54, dihydrodehydrodiisoeugenol, 56 did not undergo any bond cleavage but rather an oxidation of the phenolic moiety to form the quinol, 57 (Scheme 16.14). 

Amines derived from p-alkoxybenzyl-type linkers, despite not being acid cleav-able, still have synthetic utility. Anilines anchored to Wang resin, once converted into carboxamides or sulfonamides by reaction with the appropriate electrophile, can be cleaved with TFA (Figure 14.6) [34]. Sulfonamides of aliphatic amines may also be cleaved with TFA [36]. Stronger acids are generally required for acyl derivatives of amines but this can cause cleavage of the linker benzylic ether bond, leading to formation of p-hydroxybenzylated by-products. 

Apart from problems of silicon-carbon bond cleavage, a second difficulty limiting the utility of this synthesis is that of obtaining the appropriate silylcarbinol. Whereas a-hydroxysilanes can be isolated from the reaction of silylmetallic reagents with aliphatic aldehydes 33,34), the initial adducts from reaction with aromatic aldehydes such as benzaldehyde rearrange to the isomeric silyl ethers too rapidly for any silylcarbinol to be isolated 33, 35). The mechanism of this reaction, summarized below, has been investigated in detail 30). 

The milling of poly(ethylene terephthalate) generally causes fracture of the heterobonds of the glycol chain units [20,21]. Simionescu and co-workers [21], in a complete study on poly(ethylene terephthalate), confirmed that homolytic bond cleavage occurs mainly at the weakest links, which were thought to be the heteroatomic bonds. Electron spin resonance has been widely employed for the study of rupture in heterochain polymers. If a chain has an aliphatic ether structure, for example, poly(ethylene oxide), rupture takes place mainly at the —C—C— bond [22]. The breakdown of aromatic polyesters and ethers occurs at the relatively weak —C—C— bonds around the aromatic nuclei [23]. In general, the bond most susceptible to rupture is beta-to-chain functionality as in polyesters and nylons. 

Reductive C-N bond cleavage has been demonstrated in more complex systems using ethyl chloroformate as a solvent. Chlorination of corticosteroid cyclic ethers has been observed. Tertiary aliphatic and alicyclic amines can be dealkylated. Phenyl chloroformate, however, is usually regarded as the reagent of choice. In summary, deamination, demethylation, debenzylation, and deal-lylation of tertiary amines can all occur on treatment with ethyl chloroformate but the regioselectivities of these reactions are difficult to predict. ... 

Both aliphatic and cycloaliphatic ethers 4, 11 and 13 react with a sulfur tetrafluoride/ hydrogen fluoride/chlorine mixture under mild conditions to give asymmetric chlorofluoro ethers in good yield, but under more rigorous conditions, cleavage of the C-O bond occurs and chlorofluoroalkanes arc the sole products.241... 

The principal type of bridging linkages between clusters are short aliphatic groups (CH2) (where n = 1—4), different types of ether linkages, and sulfide and biphenyl bonds. All except the latter may be considered scissible bonds in that they can readily undergo thermal and chemical cleavage reactions. 

---