Alkenes by dioxiranes

The epoxidation of unfunctionalized alkenes by dioxiranes was investigated mainly for mechanistic purposes P . Some representative cases are collected in Scheme 3. Although such unfunctionalized alkenes have not been studied as intensively as the other olefin types, the recent asymmetric epoxidations by dioxirane were performed mainly on this substrate class (vide infra) J P. For this purpose, in-situ-generated dioxiranes in carefully buffered aqueous solutions had to be used, since the chiral dioxiranes cannot be readily isolated. Fortunately, the epoxides of unfunctionalized alkenes are more resistant to... 

Curd, R., Fiorentino, M. and Serio, M. R. Asymmetric Epoxidation of Unfunctionahzed Alkenes by Dioxirane Intermediates Generated from Potassium Peroxomonosulphate and Chiral Ketones. J. Chem. Soc., Chem. Commun. 1984, 155-156. 

Two transition states have been proposed for the epoxidation of alkenes by dioxiranes and oxaziridines, the spiro and the planar (Fig. 5.8). In the spiro transition state, the alkene approaches the oxaziridinium moiety in such a way that the axis of the carbon-carbon double bond is perpendicular to the carbon-nitrogen bond axis. In the planar transition state, the two components approach one another so that their axes are parallel to one another, and they and the oxygen atom are in the same plane. 

Tetra-n-butylammonium hydrogen sulphate facilitates the enantiomeric epoxida-tion of alkenes by persulphates in the presence of chiral ketones (10.6.8). The reaction proceeds via the initial formation of chiral dioxiranes [23]. 

Of the organohahdes, only the iodides are prone to oxidation by dioxirane for example, iodobenzene is oxidized by DMD to a mixture of iodosobenzene and iodylbenzene. In contrast, alkyl iodides afford labile primary oxidation products, which eliminate the oxidized iodine functionality to result in aUcenes (equation 23). In such a dioxirane oxidation, the subsequent in-situ reaction of the alkene affords the corresponding epoxides . 

Electronic and steric effects in the epoxidation of alkenes by dimethyldioxirane have been investigated313. Both mechanistic and synthetic aspects of the chemistry of dioxiranes have been reviewed314,315. 

Simple alkenes can be epoxidized by dioxiranes in an asymmetric manner. This chemistry is discussed in detail in Chapter 10. 

Asymmetric epoxidation The catalytic asymmetric epoxidation of alkenes has been the focus of many research efforts over the past two decades. The non-racemic epoxides are prepared either by enantioselective oxidation of a prochiral carbon-carbon double bond or by enantioselective alkylidenation of a prochiral C=0 bond (e.g. via a ylide, carbene or the Darzen reaction). The Sharpless asymmetric epoxidation (SAE) requires allylic alcohols. The Jacobsen epoxidation (using manganese-salen complex and NaOCl) works well with ds-alkenes and dioxirane method is good for some trans-alkenes (see Chapter 1, section 1.5.3). 

Denmark et al. reported a general protocol for the catalytic epoxidation of alkenes by in r// -generated reactive dioxiranes capable of epoxidizing a variety of alkenes under biphasic conditions <1995JOC1391>. The epoxide diastereoselectivity (Scheme 4) showed pronounced dependence on the solvent used since the ratio of diastereo-mers, as well as the distribution between epoxide and enone products, is dependent on the solvent <1995TL2437, 1999TL8023>. Selected examples are given in Table 2. 

Alkenes can be epoxidised by dioxiranes, and this process can be achieved enan-tioselectively if an enantiomerically pure dioxirane is employed. Since... 

The oxidation of organic compounds by dioxiranes has attracted considerable interest in recent years. Bravo et al. have reported that the oxidation of a variety of organic compounds (alkanes, alcohols, ethers, aldehydes, and alkenes) by dimethyl-dioxirane may be explained via a radical mechanism. The proposed molecule-induced homolysis of dimethyldioxirane is supported by radical trapping with CBrCls or protonated quinolines. The presence of oxygen has also been shown to have a significant effect on these reactions and supports a radical mechanism. The... 

In general, peroxomonosulfates have fewer uses in organic chemistry than peroxodisulfates. However, the triple salt is used for oxidizing ketones (qv) to dioxiranes (7) (71,72), which in turn are useful oxidants in organic chemistry. Acetone in water is oxidized by triple salt to dimethyldioxirane, which in turn oxidizes alkenes to epoxides, polycycHc aromatic hydrocarbons to oxides and diones, amines to nitro compounds, sulfides to sulfoxides, phosphines to phosphine oxides, and alkanes to alcohols or carbonyl compounds. 

The epoxidation of nonfunctionalized alkenes may also be effected by chiral dioxiranes. These species, formed in situ using the appropriate ketone and potassium caroate (Oxone), can be formed from C-2 symmetric chiral ketones (29)[93], functionalized carbohydrates (30)[94] or alkaloid derivatives (31)[95]. One example from the laboratories of Shi and co-workers is given in Scheme 19. 

Epoxidation of alkenes can be effected by potassium persulphate. When the oxidation is conducted in the presence of chiral trifluoroketones, chiral oxiranes (ee 12-22%) are produced [14]. The chirality appears to be achieved via the initial reaction of the persulphate with the ketone to generate chiral dioxiranes, which then interact with the alkenes. 

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