Cleavage alkyl aryl ethers

Cleavage of alkyl aryl ethers by hydrogen halides always proceeds so that the alkyl-oxygen bond is broken and yields an alkyl halide and a phenol as the final prod nets Either hydrogen bromide or hydrogen iodide is normally used... 

Section 24 12 The cleavage of alkyl aryl ethers by hydrogen halides yields a phenol and an alkyl halide... 

A A liquid soluble in pentane 65 Mostly aliphatic material with some mono-aromatic parts broken off the coal by C-C cleavage. Apart from combined-phenol it has negligible polar material It contains some free paraffinic material, but exists mostly as alkyl phenols and alkyl-aryl ethers... 

Under favourable circumstances, the initially formed /V-ylid reacts further through C-N cleavage. Thus, in the presence of a strong nucleophile, such as a phenoxide anion, the quaternary dichloromethylammonium cation forms an ion-pair with the phenoxide anion (Scheme 7.27), which decomposes to yield the alkyl aryl ether and the /V-formyl derivative of the secondary amine [22, 23]. Although no sound rationale is available, the reaction appears to be favoured by the presence of bulky groups at the 4-position of the aryl ring. In the absence of the bulky substituents, the Reimer-Tiemann reaction products are formed, either through the breakdown of the ion-pair, or by the more direct attack of dichlorocarbene upon the phenoxide anion [22,23],... 

Cleavage of alkyl aryl ethers with sodium or potassium metal shows excellent regio-selectivity [275]. Using limiting radical anion structures to analyze the results and noting the preference for such structures as dependent on the substituents, it is possible to formulate reaction pathways leading to the observed products. 

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 alkyl aryl ethers is more important as a preparative method for phenols than for alkyl halides (method 97). The procedure has been employed as the final step in a synthesis proposed as a means for increasing the carbon chain of an alkyl halide, viz., ... 

Cleavage of alkyl aryl ethers ArOR - ArOH. Alkyl aryl ethers are cleaved in high yield when treated with lithium iodide in dry 2,4,6-collidine. The reaction mixture becomes mildly basic as the cleavage proceeds but can be buffered with an acid such as benzoic acid. The reactions can also be carried out with Lil-3H20 at 180-200° in the absence of solvent. 

Proof of structure of a new ether would involve cleavage by hydriodic acid and identification of the products formed. Cleavage is used quantitatively in the Zeisel method to show the number of alkoxyl groups in an alkyl aryl ether. 

Cleavage of sp3-C-0 bond in alkyl aryl ethers coordinated to a cationic cyclop entadienyliron has been reported recently [80]. The alkyl-0 cleavage with KOR3 (R3=H or f-Bu) leads to an aryloxide-coordinated complex which can be converted into another ether-coordinated complex on treatment with alkyl halides, R4X (R4=alkyl, allyl, or benzyl), as shown in Scheme 9. 

Cyclic alkyl aryl ethers lead also to functionalized organolithium compounds by reductive carbon-oxygen bond cleavage in arene-catalyzed lithiation process. Thus, the treatment of 2,3-dihydrobenzofuran (47) with an excess of lithium in the presence of a catalytic amount of DTBB in THF at 0°C gives the dianion (48) which after reaction with different carbonyl compounds and final hydrolysis with water leads to... 

Aluminum bromide is superior to the chloride for cleavage of alkyl aryl ethers it is soluble in indifferent solvents such as benzene, carbon disulfide, and light petroleum. According to Pfeiffer and Haack,24 the reaction takes place according to the scheme ... 

C-0 bond in alkyl aryl ethers is more resistant toward cleavage than that in allyl or vinyl ethers. In alkyl aryl ethers such as anisole, activation of RO-Ar bond (type a in Scheme 3.41) is difficult in comparison with that of R-OAr bond (type b), probably because the relatively stronger bond dissociation energy of the aryl-0 bond than that of alkyl-0 bond due to aromatic conjugation as well as higher stability of aryloxo-metal products. 

The C-0 bond cleavage of dialkyl ethers is much more difficult than that of alkyl aryl ethers. Nevertheless some lanthanoid complexes are known to cleave C-0 bond of diethyl ether (Eq. 3.20) or dimethoxyethane (Eq. 3.21) to give ethoxide complexes [82,83]. 

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