Oxidation of benzyl ethers

When the C—H bond to be oxidized is proximate to a functional group, as we have stated already, its reactivity depends on the type of functional group. In the case of the hydroxy group, especially in secondary alcohols, these are more prone to dioxirane oxidation than their alkane precursors and, consequently, usually carbonyl products are obtained as the final product. Primary alcohols are less reactive, but may still be converted slowly to the corresponding aldehydes or carboxylic acids (due to the facile further oxidation of aldehydes)The functional-group transformation of the alcohols to ethers or acetals reduces the oxidative reactivity " but these C—H bonds are still more reactive than unfunctionalized ones. Thus, dioxirane oxidation of benzyl ether or acetal may... 

Collins reagent is able to transform benzyl ethers into ketones and benzoates.110 Normally, this causes no interference with the oxidation of alcohols, because the oxidation of benzyl ethers demands more drastic conditions. 

Related oxidants that have been exploited to similar ends include l-(tert-butoxy)-l,2-benzoiodoxol-3(l//)-one292 and sodium bromate.293 Oxidation of benzyl ethers by l-(/erf-butoxy)-l,2-benzoiodoxol-3(l/f)-one followed by easy basic hydrolysis of the resultant benzoate ester provides a convenient alternative to the usual reductive deprotection. The reaction is carried out in the presence of alkali metal carbonates and the conditions are mild enough to be compatible with other hydroxyl protecting groups such as methoxymethyl, tetrahydropyranyl, TBS and acetate. 

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. 

Oxidation of benzyl ethers with trityl fluoroborate can be used either to prepare aromatic aldehydes or to cleave benzyl ethers of alcohols. 

Oxidations. Oxidation of benzyl ethers with KMn04 in the presence of benzyltriethylammonium chloride gives alkyl benzoates in refluxing dichloromethane," and under similar conditions, (V-phenyl azacycles afford lactams. ... 

A-bromonicotinamide (NBN) oxidation of aliphatic alcohols to corresponding aldehydes and ketones in 60% ACOH-H2O exhibited first-order dependence on NBN and alcohols. The rate increased with increasing dielectric constant of the medium.The similar oxidation of benzyl ethers was first order in NBN and zero order in substrate. The effect of dielectric constants and ionic strength of the medium was studied. ... 

Li achieved metal-free CDC reaction between simple ketones and ben l ethers by the simple addition of DDQ (Scheme 8.31). The oxidation of benzyl ether to a benzoiq cation by DDQ followed by the enolization of the ketone and its nucleophilic attack is proposed as the reaction mechanism to afford the p-alko q -p-arylketone (76). 

The main oxidation product from dibenzyl ether is benzaldehyde (up to 80% yield) with smaller amounts of benzyl alcohol and benzoic acid. The rates of oxidation are only slightly affected by major stereochemical changes, and it is considered that an outer-sphere oxidation of the ether is followed by radical breakdown, viz. 

Production of considerable amounts of cyclohexanol and cyclohexanone as well as benzaldehyde and benzoic acid in the oxidation of benzyl cyclohexyl ether shows the primary radical to be CgHjCHOCeHjj. Abstraction from aliphatic C-H bonds cannot occur in the case of diphenyl ether which is oxidised rapidly, and removal of a 7t-electron is likely. 

Izumi and Urabe [105] found first that POM compounds could be entrapped strongly on active carbons. The supported POMs catalyzed etherization of ferf-butanol and n-butanol, esterification of acetic acid with ethanol, alkylation of benzene, and dehydration of 2-propanol [105], In 1991, Neumann and Levin [108] reported the oxidation of benzylic alcohols and amines catalyzed by the neutral salt of Na5[PV2Mo10O40] impregnated on active carbon. Benzyl alcohols were oxidized efficiently to the corresponding benzaldehydes without overoxidation ... 

Ph,P)4Pd(0) and quaternary ammonium salts catalyse the oxidation of benzylic halides by nitrous oxide in a basic medium into O-benzylic ethers of the corresponding aryl aldoximes, ArCH=NOCH2Ar (20-40%) [21 ]. The procedure does not have a great synthetic appeal. 

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]. 

A number of substrates having a benzylic ether moiety were reacted with 51 to afford the corresponding benzylic esters in good yields (equation 84). For evaluating the effects of p-substiments on the oxidation of a series of benzylic ethers, a competitive oxidation of p-substimted benzylic propyl ethers with 51 was carried out. The Hammett correlation plot for the oxidation reaction gave a better correlation of the relative ratio factors with the a rather than with the a+ substituent constants and afforded a reaction constant p+ = —0.57 (r = 0.99). This p+ value shows that 51 is an electrophilic species and appears to be comparable to the p+ value of —0.65 for benzylic hydrogen abstraction from dibenzyl ethers by the benzoyloxy radicaP . 

In several cases A-hydroxyphthalimide has been used as an organic mediator for the oxidation of alcohols to ketones, of benzyl ethers to benzoates , of alkyl aromatics to aryl ketones , and of 4-phenyl-l,3-dioxolanes to unprotected ketones... 

The unique chemical behavior of K02 is a result of its dual character as a radical anion and a strong oxidizing agent (68). The reactivity and solubility of K02 is gready enhanced by a crown ether (69). Its usefiilness in furnishing oxygen anions is demonstrated by its applications in SN2-type reactions to displace methanesulfonate and bromine groups (70,71), the oxidation of benzylic methylene compounds to ketones (72), and the syntheses of a-hydroxyketones from ketones (73). 

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. 

The aerobic oxidation of benzylic and allylic alcohols to their corresponding carbonyl compounds in [C4Ciim][PF6] with TEMPO-CuCl (TEMPO = 2,2,6,6-tetramethylpiperidinyl-l-oxy) as catalyst was found to proceed at higher rates than that of aliphatic alcohols, which is in agreement with results in classical solvents.[75] After product extraction with diethyl ether, the ionic liquid was washed with water and dried at 70°C, prior to the next run. In that manner catalyst activity remained relatively stable for 8 cycles. 

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