Alcohols, oxidation with

One suggested mechanism is that the reaction may take place by a conjugate hydride-transfer mechanism, analogous to what occurs during alcohol oxidations with NAD+. Electrons on the enolate ion might expel a (3 hydride ion, which could add to the doubly bonded NS nitrogen on FAD. Protonation of the intermediate at N1 would give the product. 

Reaction of (S)-(+)-2-aminobutyrate hydrochloride with ethyl oxalyl chloride followed by replacing of the ethyl ester with amino alcohol, oxidation with Dess-Martin periodinate and cyclization using TFA/TFAA in acetic acid gave the cyclic product, which was further converted to the bromide. Sub-... 

S. Campestrini, M. Carraro, U. Tonellato, M. Pagliaro and R. Ciriminna, Alcohols oxidation with hydrogen peroxide promoted by TPAP-doped ormosils, Tetrahedron Lett., 2004, 45, 7283. 

The carbonyl group (and adjacent alcohol) oxidizes with excess phenyl hydrazine (PhNHNH2) to form an osazone (see Figure 16-15). Osazone formation is very important in determining the relationship between various monosaccharides. For example, both D-glucose and D-mannose produce the same osazone, so they re epimers. Epimers differ by only one chiral center, which osazone formation destroys. 

Scheme 17 illustrates the two-step mechanism for alcohol oxidation with Ar3BiX2-type oxidants, which was originally proposed by Barton and coworkers. 

Common alcohol oxidation methods employ stoichiometric amounts of toxic and reactive oxidants like Cr03, hypervalent iodine reagents (Dess-Martin) and peracids that pose severe safety and environmental hazards in large-scale industrial reactions. Therefore, a variety of catalytic methods for the oxidation of alcohols to aldehydes, ketones or carboxylic acids have been developed employing hydrogen peroxide or alkyl hydroperoxides as stoichiometric oxygen sources in the presence of catalytic amounts of a metal catalyst. The commonly used catalysts for alcohol oxidation are different MoAV(VI), Mn(II), Cr(VI), Re(Vn), Fe(II) and Ru complexes . A selection of published known alcohol oxidations with different catalysts will be presented here. 

SCHEME 115. Alcohol oxidation with H2O2 under biphasic conditions catalyzed by [(n-0ct)3NMe]+3 P04[W(0)(02)2l4 -... 

SCHEME 118. Mn/tmtacn/Na ascorbate catalyzed alcohol oxidation with H2O2... 

SCHEME 123. Heterogeneous Ru-catalyzed alcohol oxidation with the tmtacn ligand... 

The proposed mechanism is given in Scheme 15. Initially the dissociation of water, maybe trapped by the molecular sieve, initiates the catalytic cycle. The substrate binds to the palladium followed by intramolecular deprotonation of the alcohol. The alkoxide then reacts by /i-hydride elimination and sets the carbonyl product free. Reductive elimination of HOAc from the hydride species followed by reoxidation of the intermediate with dioxygen reforms the catalytically active species. The structure of 13 could be confirmed by a solid-state structure [90]. A similar system was used in the cyclization reaction of suitable phenols to dihydrobenzofuranes [92]. The mechanism of the aerobic alcohol oxidation with palladium catalyst systems was also studied theoretically [93-96]. 

It is important to note that the relative velocity of an uneventful oxidation of an alcohol with PCC versus a carbon-carbon bond breakage from a chromate ester, driven by the generation of a stable carbocation, is substantially substrate-dependent, and may change according to stereoelec-tronic factors, which may be difficult to predict. Thus, many alcohols are successfully oxidized to aldehydes and ketones, regardless of an apparently potential carbon-carbon bond breakage leading to stabilized carboca-tions.315 Consequently, failure to try an alcohol oxidation with PCC, because of fear of this side reaction is not recommended. 

The speed of alcohol oxidation with DDQ correlates with the following factors ... 

Campestrini S, Carraro M, Ciriminna R et al (2005) A mechanistic study on alcohol oxidations with oxygen catalysed by TPAP-doped ormosils in supercritical carbon dioxide. Adv Synth Catal 347(6) 825-832... 

Generally, the issue of whether a truly solid Cr catalyst has been created for the aforementioned reactions is unresolved. This point is illustrated most clearly by all the work that has been devoted, in vain, to Cr molecular sieves (55-57). Particularly the silicates Cr-silicalite-1 and Cr-sihcahte-2 and the aluminophosphate Cr-AlPO-5 have been investigated. These materials have been employed, among others, for alcohol oxidation with t-BuOOH, for allylic (aut)oxidation of olefins, for the autoxidation of ethylbenzene and cyclohexane, and even for the catalytic decomposition of cyclohexyl hydroperoxide to give mainly cyclohexanone ... 

Reaction parameters of alcohol oxidation with air in aqueous solutions... 

Metal species, including chromium,191 iron,192 cobalt,193 cerium194 and ruthenium195 compounds, have also been used to catalyse alcohol oxidation with peracetic acid. 

Fig. 4.73 Tungsten catalyzed alcohol oxidation with hydrogen peroxide.

The mechanisin of alcohol oxidation with NAD has several analoge in laboratory chemistry A base removes the O-TC proton from the alcohol and Keaerates an allcoxide ion, which expels a hydride ion leaving group as in the Cannizzaro reaction tSection 19.13>> The nucleophilic hydride ion dacn adds to the Cs=C-C=N part of NAD in a conjugate addition reaction, much the same as water adds to the C=C-C=0 part of the tt,p-unsaturated acyl CoA in step 2. 

Mechanism The mechanism of the alcohol oxidation with Cr(VI) is outlined in Scheme 7.1, and involves the formation of chromate ester. The base removes the proton and Cr species leaves in an intermolecular process (A) however, an intramolecular process (B) may also operate. The Cr(IV) ions in H2Cr03 or HCrOs" are converted back to Cr(III) ions. It is believed that part of alcohol molecules are oxidized by the free radical mechanism. 

Oxidation of a phenol to the corresponding ju-qninone nsing a copper catalyst takes place at room temperatnre nnder similar conditions as those nsed for alcohol oxidation, with O2 as oxidant. Likewise, hydroqninones (22) can be transformed to 3-alkoxy-/7-qninones (23) when reacted in the presence of an alcohol. In the case of 4-substituted phenols (24), polymer-based catalysts composed of ligands (e.g. PVBPy) that chelate copper have been used at elevated temperatures to selectively oxidize a benzylic carbon to yield 4-hydroxybenzaldehydes (25) in good yields. ... 

A modification of alcohol oxidation with dimethyl sulfoxide is the reaction of DMSO with alkyl chloroformates, which are formed from alcohols and phosgene (equations 220 and 221) [1021]. 

Cyclopropene underwent cycloaddition to tropylium perchlorate in aqueous dioxane at 20 °C to give an oily alcohol oxidation with chromic acid gave predominantly the ketone 4 derived from endo addition. ... 

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