Alkyl aryl ethers protonation

The first step in the reaction of an alkyl aryl ether with a hydrogen halide is protonation of oxygen to form an alkylaryloxonium ion ... 

Alkyl aryl ethers have a long history in lithiation reactions, and there are detailed reports on the mechanism (see section 2.3.1) and relative efficiency135 of the lithiation of anisole and its derivatives. Methyl aryl ethers are often lithiated in Et20, but THF can be just as effective. The stabilisation afforded to an ortholithiated anisole relative to a /rara-lithiated anisole can be judged from their relative heats of protonation by s-BuOH.233... 

Protonation of the anion [SN2] by acetic acid in diethyl ether produces the thermally unstable sulfur diimide S(NH)2. Like all sulfur diimides, the parent compound S(NH)2 can exist as three isomers (Scheme 5.5). Ab initio molecular orbital calculations indicate that the (cis,cis) configuration is somewhat more stable than the (cis,trans) isomer, while the (trans,trans) isomer is expected to possess considerably higher energy. The alternative syn,anti or E,Z nomenclatures may also be used to describe these isomers. The structures of organic derivatives S(NR)2 (R = alkyl, aryl) are discussed in Section 10.4.2. 

Fig. 7-12. Reactions of phenolic /8-aryl ether and a-ether structures (1) during neutral sulfite pulping (Gierer, 1970). R = H, alkyl, or aryl group. The quinone methide intermediate (2) is sulfonated to structure (3). The negative charge of the a-sulfonic acid group facilitates the nucleophilic attack of the sulfite ion, resulting in /8-aryl ether bond cleavage and sulfonation. Structure (4) reacts further with elimination of the sulfonic acid group from a-position to form intermediate (5) which finally after abstraction of proton from /8-position is stabilized to a styrene-/8-sulfonic acid structure (6). Note that only the free phenolic structures are cleaved, whereas the nonphenolic units remain essentially unaffected.

Fig. 7-25. Main reactions of the phenolic /8-aryl ether structures during alkali (soda) and kraft pulping (Gierer, 1970). R = H, alkyl, or aryl group. The first step involves formation of a quinone methide intermediate (2). In alkali pulping intermediate (2) undergoes proton or formaldehyde elimination and is converted to styryl aryl ether structure (3a). During kraft pulping intermediate (2) is instead attacked by the nucleophilic hydrosulfide ions with formation of a thiirane structure (4) and simultaneous cleavage of the /3-aryl ether bond. Intermediate (5) reacts further either via a 1,4-dithiane dimer or directly to compounds of styrene type (6) and to complicated polymeric products (P). During these reactions most of the organically bound sulfur is eliminated as elemental sulfur.

Reactions that proceed under neutral conditions are highly desirable. An important event in TT-allylpalladium chemistry is the introduction of highly reactive allylic carbonates (Sect. V.2.1.3), Their reactions can be carried out under mild neutral conditions. " Also, reactions of allylic carbamates, " allyl aryl ethers, and vinyl epoxides proceed without addition of bases. As shown by the mechanism in Scheme 6, the oxidative addition of allyl methyl carbonates is followed by decarboxylation as an irreversible process to afford TT-allylpalladium methoxide, and the generated methoxide picks up a proton from pronucleophiles (NuH), such as active methylene compounds. This in situ formation of the alkoxide is the reason why the reaction of aUyl carbonates can be carried out without addition of bases from outside. Alkoxides are rather poor nucleophiles, and alkyl allyl ethers are not formed from them. In addition, formation of TT-allylpalladium complexes from allylic carbonates involving decarboxylation is irreversible. In contrast, the formation of TT-allylpalladium acetate from allyl acetate is reversible. 

The Birch reduction has been used by several generations of synthetic organic chemists for the conversion of readily available aromatic compounds to alicyclic synthetic intermediates. Birch reductions are carried out with an alkali metal in liquid NH3 solution usually with a co-solvent such as THF and always with an alcohol or related acid to protonate intermediate radical anions or related species. One of the most important applications of the Birch reduction is the conversion of aryl alkyl ethers to l-alkoxycyclohexa-l,4-dienes. These extremely valuable dienol ethers provide cyclohex-3-en-l-ones by mild acid hydrolysis or cyclohex-2-en-l-ones when stronger acids are used (Scheme 1). 

Protonic acids and some other electrophiles cause the aromatization of naphthalen-l,4-imines and of derivatives of the related 1,4-epoxy-1,4-dihydronaphthalene ring system (126) to naphthalene derivatives (see Section III,F), and simple electrophilic addition to the 2,3-double bond has not been observed. Ring-opening of the ether (126) also occurs on addition of alkyl or aryl lithium reagents as a result of exo attack by the nucleophile at the 2-position. ... 

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