Oxidation benzyl ethers

Peroxides. These are formed by aerial oxidation or by autoxidation of a wide range of organic compounds, including diethyl ether, allyl ethyl ether, allyl phenyl ether, dibenzyl ether, benzyl butyl ether, n-butyl ether, iso-butyl ether, r-butyl ether, dioxane, tetrahydrofuran, olefins, and aromatic and saturated aliphatic hydrocarbons. They accumulate during distillation and can detonate violently on evaporation or distillation when their concentration becomes high. If peroxides are likely to be present materials should be tested for peroxides before distillation (for tests see entry under "Ethers", in Chapter 2). Also, distillation should be discontinued when at least one quarter of the residue is left in the distilling flask. 

Bohme77 employed excess monoperphthalic acid in diethyl ether to oxidize dibenzyl and benzyl ethyl sulphoxides. Reaction time was 24 h at - 15 to + 10 °C, after which he added potassium iodide and water and titrated the iodine set free with thiosulphate. Dickenson78 oxidized dimethyl sulphoxide in malt, wort or beer with Na2S2Os. In... 

Protecting diisopropyl ether against oxidation requires 16 ppm of N-benzyl-4-aminophenol or 50 ppm of diethylenetriamine. 

Ruthenium tetroxide is a potent oxidant, however, and it readily attacks carbon-carbon double bonds.19 Primary alcohols are oxidized to carboxylic acids, methyl ethers give methyl esters, and benzyl ethers are oxidized to benzoate esters. 

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]. 

At least three methods have been found to be applicable to the solubilization of chemically modified wood. The first experiment (4) (Direct method) employed severe dissolution conditions. For example, in 20-150 min at 200-250°C, wood samples esterified by a series of aliphatic acids could be dissolved in benzyl ether, styrene oxide, phenol, resorcinol, ben-zaldehyde, aqueous phenol solutions, etc. For carboxymethylated, ally-lated and hydroxyethylated woods, the conditions provided for dissolution in phenol, resorcinol or their aqueous solutions, formalin, etc., by standing or stirring at 170°C for 30 to 60 min (5). 

Later on, all the protecting groups will be removed the silyl groups with fluoride and the p-methoxybenzyl ether by oxidation with Ce(IV). In Ley s recently completed synthesis9 of azadirachtin 64 after 22 years of hard labour the key intermediate was 65. You will notice benzyl ethers, acetals and a silyl ether. This is a more modem, one might almost say minimalist, use of protection. In an ideal world no protecting groups would be necessary but in a real synthesis they will almost certainly be required as we shall see in the rest of the book. But our aim should be to keep them to a minimum. 

Radical oxidation of silyl ethers.1 Primary benzylic silyl ethers are oxidized by NBS catalyzed by AIBN to the corresponding aldehydes in 70-87% yield. Yields are lower in a similar oxidation of secondary benzylic silyl ethers because the corresponding a-bromo ketone is a by-product. 

Oxidation of allyl (or benzyl) methyl ethers.1 These ethers are oxidized by DDQ in refluxing toluene to carbonyl compounds. 

Nitroaromatics also sensitize the oxidation of methylarenes and it has been found that silica-grafted 2,4,6-trinitrobenzene is a convenient heterogeneous sensitizer, giving the aldehydes in 79-90% yields with 100% selectivity [229]. Bibenzyls, pinacols and pinacol ethers are likewise oxidized to ketones or respectively esters through carbon-carbon bond fragmentation upon dicyanonaphthalene sensitization [230]. A good method for benzylic oxidation is based on titanium dioxide photocatalysis [231-233]. 

The first trial of the liquefaction of wood was accomplished by using very severe dissolving conditions [10]. One example used wood samples esterified with a series of aliphatic acids, which could be liquified in benzyl ether, styrene oxide, phenol, resorcinol, benzaldehyde, aqueous phenols, a chloro-form-dioxane mixture, or a benzene-acetone mixture after treating at 200-270°C for 20-150 min. 

Protection of hydroxyl groups (11, 166).- 3.4-Dimethoxybcnzyl ethers are oxidized by DDQ more readily thanp-methoxybenzyl ethers. Moreover, the dimethoxybenzyl ethers of secondary alcohols can be selectively oxidized in the presence of the corresponding ethers of primary alcohols. Benzyl, p-methoxybenzyl, and 3,4-dimethoxybenzyl ethers all undergo hydrogenolysis catalyzed by Pt/C or Pd/C, but selective hydrogenolysis of benzyl ethers is possible with W-2 Raney Ni. 

Oxidation of benzyl ethers. Benzyl ethers are oxidized to benzoate esters in yields of 54-96% by ruthenium tetroxide using the Sharpless conditions (11, 462). This reaction provides a useful method for cleavage of benzyl ethers, which is usually effected by hydrogenation. 

The rp.action described is of considerable general utility for the preparation of benzoyloxy derivatives of unsaturated hydrocarbons. Reactions of /-butyl perbenzoate with various other classes of compounds in the presence of catalytic amounts of copper ions produce benzoyloxy derivatives. Thus this reaction can also be used to effect one-step oxidation of saturated hydrocarbons, - esters, dialkyl and aryl alkyl ethers, benzylic ethers, cyclic ethers, straight-chain and benzylic sul-fides, cyclic sulfides, amides, and certain organo-silicon compounds. ... 

In nonsymmetrical ethers, the oxidation of various groups takes place preferentially in the following sequence benzyl > phenylalkyl > primary alkyl > secondary alkyl > methyl [902] (equation 326). 

The resonances in the butyl ether region occur in three distinct bands. Chemical shift data for the a carbon atom resonances in about 20 ethers indicate that the resonances centered about 872.9 may result from hindered aryl ethers, for example, butyl 2,6-dimethylphenyl ether, butyl benzyl ethers, or butyl n-alkyl ethers, for example, dibutyl ether. The resonances in this region could arise from tetrahydrofuran residues in the coal product. However, the results obtained in this laboratory and in Larsen s laboratory are much more compatible with interpretations that exclude the involvement of tetrahydrofuran and focus on the reactions of the labeled butylation reagent with 2,6-disubstituted phenoxides, benzylic oxides, and primary alkoxides liberated in the formation of the coal polyanion. The most intense resonance centered at... 

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