Dioxiranes as oxidant

Diode array-UVD (DA-UVD) 953 1,3,2-Dioxaborins, formation of 683 Dioxanes—see 1,3-Benzodioxanes Dioxepens—see Dibenzodioxepens Dioxins—see Dibenzo-p-dioxins Dioxiranes, as oxidants 1255, 1257, 1258 Dioxocines—see Dibenzodioxocines Dioxygen-copper complexes, as oxidants 1194, 1198... 

This was also accomplished with BaRu(0)2(OH)3. The same type of conversion, with lower yields (20-30%), has been achieved with the Gif system There are several variations. One consists of pyridine-acetic acid, with H2O2 as oxidizing agent and tris(picolinato)iron(III) as catalyst. Other Gif systems use O2 as oxidizing agent and zinc as a reductant. The selectivity of the Gif systems toward alkyl carbons is CH2 > CH > CH3, which is unusual, and shows that a simple free-radical mechanism (see p. 899) is not involved. ° Another reagent that can oxidize the CH2 of an alkane is methyl(trifluoromethyl)dioxirane, but this produces CH—OH more often than C=0 (see 14-4). ... 

Efficient oxidation of imines into nitrones can be achieved by using methyl (trilluoromethyl)dioxirane as an oxidant. This method provides enantiopure nitrones derived from 2H -pyrrole 1-oxide (23, 24). 

Among many other methods for epoxidation of disubstituted E-alkenes, chiral dioxiranes generated in situ from potassium peroxomonosulfate and chiral ketones have appeared to be one of the most efficient. Recently, Wang et /. 2J reported a highly enantioselective epoxidation for disubstituted E-alkenes and trisubstituted alkenes using a d- or L-fructose derived ketone as catalyst and oxone as oxidant (Figure 6.3). 

These highly reactive 1,3-dipolar species readily isomerize and undergo 1,3-cycloaddition reactions in addition to their cyclization to the corresponding dioxirane. It is within this latter context that we describe the more recent theoretical studies on carbonyl oxides and their relationship to dioxiranes. As a result of the lability of carbonyl oxides much of the research on this class of compound has been of a theoretical nature ... 

A sometimes nagging aspect of dioxirane-based oxidations is the degradation of catalyst. In this regard, Camell and co-workers <99TL8029> have reported on the use of A//-dialkyl-alloxan 163 as a particularly robust dioxirane precursor, which can be recovered in high yield with no evidence of catalyst decomposition. Attempts thus far to parlay this catalyst into an asymmetric induction paradigm (e.g., via 164) have been unsuccessful. 

Dioxiranes constitute a new class of organic peroxides that possess great potential as oxidants with a variety of applications in synthetic organic chemistry.5 7 A new convenient route for the synthesis of silanol polymers has been developed by the selective oxidation of =Si—H bonds with dimethyldioxirane. A series of styrene-based silanol polymers and copolymers were synthesized (Scheme l).8 9 The precursor polymers and styrene copolymers containing =Si—H bond were first synthesized by free radical polymerization of the corresponding monomers or copolymerization of the... 

Numerous variations of this method were developed170-173. In general, 0.1-0.8 molar solutions of dioxiranes were prepared and used as oxidizing agents. These solutions can be stored in a refrigerator. Epoxidation of many ketones has been carried out in the above way, and the dioxiranes (7-14) thus prepared were characterized spectroscopically and also by chemical methods. 

AMI and PM3 calculations reveal that epoxidations by DMDO and TFDO involve peroxide-bond cr at a very early stage and that TFDO is the most reactive dioxirane as the CF3 group in it stabilizes this cr level. In accord with previous calculations a spiro transition state is predicted. Furthermore, allene is predicted to be less reactive than alkenes toward epoxidation by DMDO.192 DFT calculations on the oxidation of primary amines by dimethyldioxirane predict a late transition state with a barrier of 17.7 kcal mol-1 which is drastically lowered by hydrogen bonding to the O—O bond to just 1.3 kcal mol-1 in protic solvents.193... 

It should finally be pointed out that the mild reaction conditions typically employed in dioxirane-mediated oxidations enable the asymmetric epoxidation of enol ethers and enol esters. With the silyl ethers, work-up provides enantiomeri-cally enriched a-hydroxy ketones. As summarized in Table 10.1, quite significant enantiomeric excesses were achieved by use of catalyst 10 at loadings ranging from 30 [30] to 300 mol% [31]. Enol esters afford the intact acyloxyepoxides enantiomeric purities are, again, quite remarkable. 

A series of meso-dihydrobenzoins was also subjected to oxidative desymmetrization. Three equivalents of the chiral ketone 88 again provided the chiral dioxirane as the active species [138, 139]. As shown in Table 10.14, enantiomeric excesses up to 60% were achieved. In addition to the meso diols themselves, acetonides also proved suitable substrates in two instances (Table 10.14). 

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