Benzylic oxidations

Fremy s salt, which is frequently employed in a wide range of oxidation reactions [l], has been shown to selectively oxidize benzylic alcohols in preference to saturated and allylic alcohols [2], Yields are generally >95%, except for 4-nitrobenzyl alcohol. Under the normal reaction conditions, saturated primary alcohols are not... 

Reaction (3.147) is the dominant means of oxidizing benzyl radicals. It is a slow step, so the oxidation of toluene is overall slower than that of benzene,... 

Starting from glucoside 3, the 4,6-O-benzylidene 25 could be produced by the same method, then 4,6-0-benzylidene-3-0-benzyl glucoside 11 was obtained through the same as above mentioned tin oxide benzylation method (Scheme 3). The lower yield was caused by the similar reactivity between the 2- and 3-positions of 25. 

Like teUnrones, and in contrast to teUnroxides, tellnrinic anhydrides oxidize benzylic alcohol to carbonyl componnds. 

Osmium(VI) hydrazido complexes can be generated by electrochemical oxidation of the corresponding osmium(V) hydrazido complexes (Section 5.6.5.3.1). The complex trans-[0s (tpy)(Cl)2(NN(CH2)40)] " (83) is able to oxidize benzyl alcohol to benzaldehyde. It also oxidizes PPhs to PPh30, and R2S to give R2SO the source of O atoms is presumably H2O in the solvent. 

The reactivities of [Ru "(0)(14-TMC)(X)]"+ and its related 15-TMC, 16-TMC, and CRMes coi lexes with organic substrates have also been examined. " " In contrast to polypyridyl Ru =0 species, these macrocyclic Ru =0 complexes are weak oxidants. They oxidize benzyl alcohol to benzaldehyde but do not react with alkenes at room temperature. The lower oxidizing ability of these systems than the polypyridyl systems is due to their lower values. However, [Ru (0)(H20)(N202)](C104)2, which has a higher H value, is able to catalyze the oxidation of norbornylene, styrene, and cyclooctene by PhlO. " ... 

There are very few osmium(IV) 0x0 species. There is evidence that the Os =0 species is more reactive than the corresponding Os (0)2 species. [Os (0)2(phenba)] (see Figure 12 for structure of ligand) is able to oxidize benzyl alcohol in the presence of one equivalent of PPhs, the active... 

This heme-dependent enzyme [EC 1.11.1.14], also known as diarylpropane peroxidase, diarylpropane oxygenase, and ligninase I, catalyzes the reaction of 1,2-bis(3,4-dimethoxyphenyl)propane-l,3-diol with hydrogen peroxide to produce veratraldehyde, l-(3,4-dimeth-ylphenyl)ethane-l,2-diol, and four water molecules. The enzyme brings about the oxidative cleavage of C—C bonds in a number of model compounds and also oxidizes benzyl alcohols to aldehydes or ketones. 

MoO(02)2(dmpz)2, 120, containing 3,5-dimethylpyrazole (dmpz) in the coordination sphere, in the presence of H2O2, selectively oxidizes benzylic C—H bonds of several alkylbenzenes to the corresponding alcohols and ketones (see, e.g., equation 82). 

The oxidation of benzaldoxime with perchloryl fluoride (FClOj) has been reported [29 a) to give a complex mixture in which benzaldoxime benzoate and diphenyl oxadiazole are the main products. Sodium nitrohydroxamate [Na2(02NN0)] has been reported [99b) to oxidize benzyl chloride to a mixture of compounds from which benzyl alcohol, benzaldehyde, benzoic acid, 3,4,5-triphenylisoxazole, benzyl-ethyl-ether, phenylnitromethane and diphenyloxadiazole have been isolated. 

Oxidation. Benzylic and allylic alcohols are oxidized to the corresponding ketones or aldehydes in 80 90% yield. Unactivated alcohols resist attack by this oxidant. 

It is noteworthy that Pt02(PPh3)2 can oxidize benzyl to benzoic anhydride with 55% yield by inserting one oxygen atom between the two carbonyl groups.195... 

Bis(p-methoxyphenyl) telluroxide (118) and the corresponding tellurone (119) have recently been shown to exhibit mild oxidizing properties towards easily oxidizable substrates. Thus the telluroxide (118) oxidizes thiocarbonyl compounds RR C=S to the corresponding ketone RR C—O364,365 and 3,4-di-f-butylpyrocatechol to 3,4-di-f-butyl-o-quinone.364 The tellurone (119) oxidizes benzyl alcohols to the corresponding carbonyl compounds, a reaction. .which is not observed with (118).366... 

Iodosobenzene diacetate [IBD, PhI(OAc)2] is able to oxidize benzylic alcohols to benzaldehydes when a solid mixture of iodosobenzene diacetate and the alcohol is irradiated with microwaves. Best results are obtained when iodosobenzene diacetate is supported on alumina.118 The use of polymer supported iodosobenzene diacetate (PSDIB) simplifies the work-up in the oxidation of benzylic alcohols to benzaldehydes.119 PSDIB can be employed in the presence of KBr and using water as solvent, resulting in the transformation of secondary alcohols into ketones and primary alcohols into carboxylic acids.117... 

Zinc and copper nitrates on silica gel are able to oxidize benzylic and saturated secondary alcohols but not aliphatic primary alcohols.157 On the other hand, ZrO(OAc)2 is able to catalyze, under the action of f-BuOOH, the oxidation of benzylic alcohols—both primary and secondary—and primary saturated alcohols to aldehydes and ketones, while secondary saturated alcohols are very unreactive.158... 

The kinetics of oxidation of allyl alcohol with quinolinium fluorochromate have been studied.11 This reagent has also been used to oxidize benzyl alcohols the activation enthalpies and entropies obtained for the substituted derivatives are linearly related, and a Hammett relationship (p = —1.04 0.08) is obeyed.12... 

The reagent oxidizes benzyl and allyl alcohols to carbonyl compounds in high yield. Yields are low for primary alkanols and cycloalkanols ( 30%).1... 

Oxidation of aldoses by hypoiodite can also be used for preparative purposes.7 Other types of halogen oxidants, especially A-halo compounds, are useful. For example, A -bromocarbamide was recommended by Kiss8 as a selective and convenient reagent for oxidizing benzylated sugars to their corresponding aldonolactones in yields exceeding 90%. Another example is the use of A-iodosuccinimide and tetrabutylammonium iodide.9... 

Oxidation. Benzylic secondary amines are oxidized in high yield by dimethyldioxirane to nitrones,1 probably via hydroxylamines.2... 

Although a suitable acceptor for the transfer dehydrogenation of benzylic alcohols has not yet been found, under the present conditions the low conversion of benzylic alcohols is only an apparent drawback. Indeed, it has a positive side as it allows us to fine-tune the system s selectivity. This makes the catalytic system unique among all the others known, operating both under aerobic and anaerobic conditions, that preferentially oxidize benzylic alcohols with respect to nonacti-vated secondary ones. 

Immobilization of 133 on a polymeric resin shows several advantages over the corresponding monomeric species.199 Oxidation of 133 with sodium periodate affords the corresponding bis-(p ra-methoxyphenyl) tellurone (An2Te02) 134, which presents a peculiar reactivity.198 For example, the tellurone 134 oxidizes benzyl alcohols to benzaldehydes in good yields (Scheme 72), and also converts hydrobenzoin in benzaldehyde in good yield. 

Benzyl-2-pyrazinecarboxamide 5-Benzyl-2-pyrazinecarboxamide 4-oxide Benzyl 2-pyrazinecarboxylate... 

Alkaline hydrogen peroxide does not attack alkyl side chains directly, but it can be very useful for the conversion of the aromatic aldehyde to the carboxylic acid in the side chain oxidation sequence. The chemistry is covered in Section 6.1. Peracids usually do not attack alkyl side chains, but can oxidize benzylic alcohols and aldehydes to the carboxylic acids and benzylic ketones to phenyl ester. These reactions are covered in sections on alcohol (5), aldehyde (6.1) and ketone (6.2) oxidation. 

A subsequent study ° from the Arnold group showed an intriguing stereoelectronic effect in oxidative benzylic carbon-hydrogen bond cleavage reactions of substrates 8 and 9 (Scheme 3.7). In this study, electron transfer reactions were conducted in the presence of a nonnucleophilic base. Radical cation formation also weakens benzylic carbon-hydrogen bonds, thereby enhancing their acidity. Deprotonation of benzylic hydrogens yields benzylic radicals that can be reduced by the radical anion of dicyanobenzene to form benzylic anions that will be protonated by solvent. This sequence of oxidation, deprotonation, reduction, and protonation provides a sequence by which epimerization can be effected at the benzylic center. In this study, tram isomer 10 showed no propensity to isomerize to cis isomer 11 (equation 1 in Scheme 3.7), but 11 readily converted to 10 (equation 2 in Scheme 3.7). The reactions were repeated in deuterated solvents to assure that these observations resulted from kinetic rather than thermodynamic factors. Trans isomer 9 showed no incorporation of deuterium (equation 3 in Scheme 3.7) whereas cis isomer 11 showed complete deuterium incorporation. The authors attributed this difference in reactivity to... 

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