Ethers, allylic mechanism

So basically the Claisen rearrangement is a thermal [3, 3] sigmatropic rearrangement of allyl vinyl ether. The mechanism is probably as follows ... 

The intermolecular reaction of oxocarbenium ions with simple alkenes, less reactive carbon nucleophiles, is generally slow and inefficient. A recent paper has described that the (la)-promoted intermolecular reaction of acetals with vinylcyclopropane (22) gives 3,6-heptadienyl ethers (23) with high E selectivity [74]. The allylation mechanism involves the formation of a cyclopropylmethyl cation intermediate and its ring opening (Scheme 9.21). Homoallylstannane (24) also adds to the oxocarbenium ion generated from benzaldehyde dimethyl acetal and (la) [75]. 

Q The mechanism of the Claisen rearrangement of other allylic ethers of phenol is analogous to that of allyl phenyl ether What is the product of the Claisen rearrangement of C6H50CH2CH CHCH3 /... 

The mechanism of thermolysis and photolysis of ethers of 3-hydroxy-1,2-benzisoxazole has also been studied. Heating of the allyl ether (43) gave minor amounts of (44) and two benzoxazoles. Photolysis of (45) in methanol gave a benzisoxazole and an iminoester, via intermediate (46). Thermolysis at 600 °C gave a benzoxazole, a benzoxazolone and cyano-phenol (Scheme 16) (71DIS(D)4483). 

Mention may also be made of fixed diethers, some of which are unsaturated. These materials may be cured by a variety of mechanisms. An example is the allyl glycilyl mixed ether of bis-phenol A (Figure 26.15)... 

In certain cases the reaction may proceed by a concerted mechanism. With allyl ethers a concerted [2,3]-sigmatropic rearrangement via a five-membered six-electron transition state is possible " ... 

Ethers with a tertiary, benzylic, or allylic group cleave by an S l or FI mechanism because these substrates can produce stable intermediate carbocations. These reactions are often fast and take place at moderate temperatures. fcrf-Butyl ethers, for example, react by an El mechanism on treatment with trifluoroacetic acid at 0 °C. We ll see in Section 26.7 that the reaction is often used in the laboratory synthesis of peptides. 

Evidence for this mechanism comes from the observation that the rearrangement takes place with an inversion of the allyl group. That is, allyl phenyl ether containing a 14C label on the allyl ether carbon atom yields o-allylphenol in which the label is on the terminal vinylic carbon (green in Figure 18.1). It would be very difficult to explain this result by any mechanism other than a pericyclic one. We ll look at the reaction in more detail in Section 30.8. 

Lewis acids, particularly the boron trifluroride diethyl ether complex, are used to promote the reaction between allyl(trialkyl)- and allyl(triaryl)stannanes and aldehydes and ketones52-54. The mechanism of these Lewis acid promoted reactions may involve coordination of the Lewis acid to the carbonyl compound so increasing its reactivity towards nucleophilic attack, or in situ transmetalation of the allyl(trialkyl)stannane by the Lewis acid to generate a more reactive allylmetal reagent. Which pathway operates in any particular case depends on the order of mixing of the reagents, the Lewis acid, temperature, solvent etc.55- 58. 

In a related reaction, pyrolysis of allylic ethers that contain at least one a hydrogen gives alkenes and aldehydes or ketones. The mechanism is also pericyclic"" ... 

In these cases, of course, the final tautomerization does not take place even when R = H, since there is no aromaticity to restore, and ketones are more stable than enols.The use of water as solvent accelerates the reaction.The mechanism is similar to that with allylic aryl ethers. 

Intermolecular hydroalkoxylation of 1,1- and 1,3-di-substituted, tri-substituted and tetra-substituted allenes with a range of primary and secondary alcohols, methanol, phenol and propionic acid was catalysed by the system [AuCl(IPr)]/ AgOTf (1 1, 5 mol% each component) at room temperature in toluene, giving excellent conversions to the allylic ethers. Hydroalkoxylation of monosubstituted or trisubstituted allenes led to the selective addition of the alcohol to the less hindered allene terminus and the formation of allylic ethers. A plausible mechanism involves the reaction of the in situ formed cationic (IPr)Au" with the substituted allene to form the tt-allenyl complex 105, which after nucleophilic attack of the alcohol gives the o-alkenyl complex 106, which, in turn, is converted to the product by protonolysis and concomitant regeneration of the cationic active species (IPr)-Au" (Scheme 2.18) [86]. 

The reaction of carbenes with alcohols can proceed by various pathways, which are most readily distinguished if the divalent carbon is conjugated to a tt system (Scheme 5). Both the ylide mechanism (a) and concerted O-H insertion (b) introduce the alkoxy group at the originally divalent site. On the other hand, carbene protonation (c) gives rise to allylic cations, which will accept nucleophiles at C-l and C-3 to give mixtures of isomeric ethers. In the case of R1 = R2, deuterated alcohols will afford mixtures of isotopomers. 

Since nucleophilic addition to a metal-coordinated alkene generates a cr-metal species bonded to an -hybridized carbon, facile 3-H elimination may then ensue. An important example of pertinence to this mechanism is the Wacker reaction, in which alkenes are converted into carbonyl compounds by the oxidative addition of water (Equation (108)), typically in the presence of a Pd(n) catalyst and a stoichiometric reoxidant.399 When an alcohol is employed as the nucleophile instead, the reaction produces a vinyl or allylic ether as the product, thus accomplishing an etherification process. 

Scheme 3. Proposed mechanism for the protodesilylation of the polymer-bound metathesis products formed from allyl ethers or esters...

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