Ethers, dialkyl cleavage

Cleavage of ethers, Dialkyl and alkyl aryl ethers are cleaved by 47% hydro-bromic acid in an aqueous organic solvent in the presence of an onium salt (hexadecyltributylphosphonium bromide, tetraoctylammonium bromide) that is completely soluble in the organic phase. Dialkyl ethers are converted into alkyl bromides (65-90% yield),whereas a phenol and an alkyl bromide are obtained from alkyl aryl ethers (85-90% yields). 

The cleavage of dialkyl ethers by hydrogen halides was discussed in Section 16 8 where It was noted that the same pair of alkyl halides results irrespective of the order in which the carbon-oxygen bonds of the ether are broken... 

Attack by the halide nucleophile at the sp hybridized carbon of the alkyl group is anal ogous to what takes place in the cleavage of dialkyl ethers Attack at the sp hybridized carbon of the aromatic nng is much slower Indeed nucleophilic aromatic substitution does not occur at all under these conditions... 

A commonly used and important reaction of sulfonic acids, or sulfonates, is their conversion to sulfonyl chlorides by treatment with phosphorus halides, or sometimes with thionyl chloride. Although it is easy to postulate mechanisms for this conversion, the exact path followed has never been determined. Similarly, although mechanisms can be suggested for other known reactions involving sulfonic acids, such as the cleavage of dialkyl ethers by anhydrous sulfonic acids (Klamann and Weyerstahl, 1965), or the formation of sulfones by treatment of an aromatic hydrocarbon with a mixture of sulfonic acid plus polyphosphoric acid (Graybill, 1967), nothing truly definitive is known about the details of the actual mechanisms of these reactions. 

Ethers are the least reactive of the functional groups. The cleavage of C-O bond in ethers takes place under drastic conditions with excess of hydrogen halides. The reaction of dialkyl ether gives two alkyl halide molecules. 

B.2.2. Dialkyl Ether Cleavage. Varying the mole fraction of AICI3 in an ionic liquid can optimize reactions that are sensitive to the strength of Lewis acidity. When the ionic liquids [EMIM]C1/A1C13 were applied for the acylative cleavage of dialkyl ethers (Scheme 6), the reaction was found to be sensitive to the bulk Lewis acidity of the ionic liquid solvent 138). 

R" may be alkyl or aryl. For dialkyl ethers, the reaction does not end as indicated above, since R OH is rapidly converted to R OR by the sulfonic acid (reaction 0-16), which in turn is further cleaved to R 0S02R" so that the product is a mixture of the two sulfonates. For aryl alkyl ethers, cleavage always takes place to give the phenol, which is not converted to the aryl ether under these conditions. Ethers can also be cleaved in a similar manner by mixed anhydrides of sulfonic and carboxylic acids733 (prepared as in 0-33). p-Hydroxy alkyl perchlorates734 and sulfonates can be obtained from epoxides.735 Epoxides and oxetanes give dinitrates when treated with N2Os,736 e.g.,... 

Cleavage of dialkyl ethers.1 Methyl, allyl, and benzyl alkyl ethers are cleaved by this combination to the alcohols and methyl, allyl, and benzyl iodide (75-90% yield). The reagent also cleaves epoxides to frans-2-iodoalkanols (—70% yield). 

Whereas the torsion angle across the central arene C=C bond is small (< 6°) for 5,11-dialkyl substitution, it increases with diaryl substitution and terminal Cr(CO)3 coordination to > 30° for 5,ll-diphenylchrysene-6,12-dione, obtained from 98a upon careful ether cleavage and oxidation. 

Cleavage of ethers. All, cleaves alkyl phenyl ethers in refluxing CH,CN or CS, selectively to phenols and alkyl iodides. The regioselectivity is the same as that shown by AICI but the reagent is more reactive. Cleavage of dialkyl ethers requires elevated temperatures and long reaction times. 

Another approach to bis(cyclopentadienyl)lanthanide alkoxides is the cleavage of ethers by bis(cyclopentadienyl) lanthanide hydrides. DiaUcyl ethers, ROR, are cleaved by (Cp 2LnH)2 (Ln = Y, La, Ce) to form Cp 2LnOR, Cp 2LnOR, R H, and RH. The extent to which either of the C-0 bonds of asymmetrically substituted dialkyl ethers ROR ... 

The cleavage of mixed dialkyl ethers occurs at the more substituted carbon-oxygen bond. Methyl ethers of secondary or tertiary alcohols give methanol and secondary or tertiary alkyl bromides selectively by reacting with BBra [17], although the addition of Nal and 15-crown-5 ether can change this selectivity (Eq. 9) [18]. In contrast, methyl ethers of primary alcohols are generally cleaved at the Me-O bond [19]. 

The Te—Te bond in diorgano ditellurium compounds is rather weak with a tendency to homolytic cleavage. Species that act as diradicals may insert into the Te —Te bond. Dialkyl or diaryl ditellurium compounds and diazomethane in diethyl ether solution produce bis[organotelluro]methanes ... 

Cleavage of dialkyl ethers with MeaSiBr is strongly catalyzed by 0.1 equiv. of IBr (equation 22). This makes possible synthetic applications which were previously ruled out by low reactivity. 

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