Nucleophilic oxidation

Electron deficient carbon-carbon double bonds are resistant to attack by the electrophilic reagents of Section 5.05.4.2.2(t), and are usually converted to oxiranes by nucleophilic oxidants. The most widely used of these is the hydroperoxide ion (Scheme 79). Since epoxidation by hydroperoxide ion proceeds through an intermediate ct-carbonyl anion, the reaction of acyclic alkenes is not necessarily stereospecific (Scheme 80) (unlike the case of epoxidation with electrophilic agents (Section 5.05.4.2.2(f)) the stereochemical aspects of this and other epoxidations are reviewed at length in (B-73MI50500)). 

The usual oxidizing agents transfer oxygen (or halogens and related species with subsequent hydrolysis) stepwise to the sulfur of thioethers Rates of step A compared with those of step B are faster with electrophilic oxidation agents (peroxy acids) inversely, rates of step B compared with those of step A are faster with nucleophilic oxidation agents (peroxy anions)339-341. 

In the cases of the selective oxidation reactions over metal oxide catalysts the so-called Mars-van Krevelen or redox mechanism [4], involving nucleophilic oxide ions 0 is widely accepted. A possible role of adsorbed electrophilic oxygen (molecularly adsorbed O2 and / or partially reduced oxygen species like C , or 0 ) in complete oxidation has been proposed by Haber (2]. However, Satterfield [1] queried whether surface chemisorbed oxygen plays any role in catalytic oxidation. 

The epoxidation of electon-defident olefins using a nucleophilic oxidant such as an alkyl hydroperoxide is generally nonstereospecific epoxidation of both cis- and /nmv- ,/3-unsatii rated ketones gives the trans-epoxide preferentially. However, the epoxidation of cis-ofi-unsaturated ketones catalyzed by Yb-(40) gives civ-epoxides preferentially, with high enantioselectivity, because the oxidation occurs in the coordination sphere of the ytterbium ion (Scheme 26).132... 

Rapid Nucleophilic/Oxidative Decontamination of Chemical Warfare Agents." Industrial ... 

In contrast to the reactions of pyrans with electrophiles and nucleophiles, oxidations of 2-amino-4H-pyrans are represented by very few examples. Oxidizing of 281 into 2-iminopyrans 296 with chloroanil or dichlor-odicyanoquinone (DDQ) (04JME6299) can be used to protect the amino group in Sandmeyer reaction (297), or to introduce a methyl group into position 4 of pyran 298. The latter is essential because the corresponding acetophenones do not give 298 with MN and aminophenols (Scheme 125). 

The oxidation of sulfides to sulfoxides (1 eq. of oxidant) and sulfones (2 eq. of oxidant) is possible in the absence of a catalyst by employing the perhydrate prepared from hexafluoroacetone or 2-hydroperoxy-l,l,l-trifluoropropan-2-ol as reported by Ganeshpure and Adam (Scheme 99 f°. The reaction is highly chemoselective and sulfoxidation occurs in the presence of double bonds and amine functions, which were not oxidized. With one equivalent of the a-hydroxyhydroperoxide, diphenyl sulfide was selectively transformed to the sulfoxide in quantitative yield and with two equivalents of oxidant the corresponding sulfone was quantitatively obtained. 2-Hydroperoxy-l,l,l-fluoropropan-2-ol as an electrophilic oxidant oxidizes thianthrene-5-oxide almost exclusively to the corresponding cw-disulfoxide, although low conversions were observed (15%) (Scheme 99). Deprotonation of this oxidant with sodium carbonate in methanol leads to a peroxo anion, which is a nucleophilic oxidant and oxidizes thianthrene-5-oxide preferentially to the sulfone. 

It was reported earher that the oxidation of a sulfoxide to a sulfone involves either an initial nucleophihc attack of the nucleophilic oxidant or an electrophihc attack by an electrophilic oxidant. It is noteworthy that the oxidation of p-tolyl methyl, phenyl methyl and p-chlorophenyl methyl sulfoxides to the sulfones using the sulfonylperoxy intermediate 51 appears to be electrophihc, namely the relative reactivity order was p-tolyl methyl > phenyl methyl > p-chlorophenyl methyl sulfoxide based on competitive oxidations. 

The reactivity of peroxo metal complexes as nucleophilic oxidants is a known process ". To visualize this type of reactivity one has to refer to peroxo metal complexes as a 1,3-dipolar reagent M+-0-0 interacting in a bimolecular fashion with electrophilic dipo-larophiles such as electron-poor olefins " (equation 15), to form peroxymetallacycle intermediates. 

Reports have appeared claiming that triperoxo vanadates behave as nucleophilic oxidants. In particular, triperoxo vanadium complexes, A[V(02)3]3H20 (A=Na or K), are proposed as efficient oxidants of a,-unsaturated ketones to the corresponding epoxide, benzonitrile to benzamide and benzil to benzoic acid, reactions which are usually carried out with alkaline hydrogen peroxide. Subsequent studies concerning the oxidation of cyclobutanone to 4-hydroxybutanoic acid, carried out with the above-cited triperoxo vanadium compound, in alcohol/water mixtures, clearly indicated that such a complex does not act as nucleophilic oxidant, but only as a source of HOO anion. 

Oxidation of a sulfide to sulfoxide is known to be an electrophilic reaction, in contrast with nucleophilic oxidation of sulfoxide to sulfone. Since 2-nitrobenzenesulhnyl ehloride/K02 oxidizes sulfides to sulfoxides selectively, intermediate 48 must be the actual active intermediate. Moreover, in the presence of l,4-diazabicyclo[2.2.2.]octane (DABCO), which is a radical capturing reagent, the oxidation of methyl phenyl sulfide to the sulfoxide was inhibited. In order to further detect the intermediate 48, pure 5,5-dimethyl-1-pyrroline-l-oxide (DMPO) was used as a trapping reagent and spin adduct was obtained223. The ESR spectrum of the DMPO spin adduct was obtained by the reaction of 02 with 2-nitrobenzenesulfinyl chloride (hyperfine coupling constants, aH = 10.0 G and aN = 12.8 G). 

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