Alkenes, reaction with dioxiranes

FIGURE 3. Preferred transition structures for the oxygen transfer in the reaction of alkenes, sulfides and alkanes with dioxiranes... 

The epoxidation of unsaturated materials is the most commonly used reaction of dioxiranes. The reaction, whether carried out in situ or using the solution method, gives high yields of epoxides, usually in rapid reactions. The isolated dioxirane method is particularly useful in cases where oxidation products are sensitive to the acidic conditions of the most commonly used epoxidation reagents or the basic conditions of the in situ method. Use of the isolated dioxirane permits reaction under neutral conditions so that labile epoxides are conveniently prepared. When the desired epoxide is sensitive to moisture, the dioxirane solution can be dried with molecular sieves for use in such epoxidations. The epoxidation of water-insoluble alkenes with the in situ method requires the use of a phase-transfer catalyst, but the reaction still proceeds in high yields. Dioxiranes, in situ generated... 

All carbonyl oxides proved to be highly photolabile, and on photolysis yield dioxiranes 3 or split off oxygen atoms to produce ketones. Oxygen atoms are also formed thermally from vibrationally excited 1. Thus, if the large exothermicity of the ozonolysis reaction is taken into account, 1 might be a source of O atoms and OH radicals in the troposphere. The role of dioxiranes has not yet been discussed in context with atmospheric chemistry, although the formation of these species in contrast to the isomeric carbonyl oxides - in ozone/alkene reactions has been unequivocally demonstrated [13]. 

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... 

Many other reagents for converting alkenes to epoxides,including H2O2 and Oxone , VO(0-isopropyl)3 in liquid C02, ° polymer-supported cobalt (II) acetate and 02, ° and dimethyl dioxirane.This reagent is rather versatile, and converts methylene oxiranes to spiro-epoxides. ° ° One problem with dimethyloxirane is C—H insertion reactions rather than epoxidation. Magnesium monoperoxyphthalate is commercially available, and has been shown to be a good substitute for m-chloroperoxybenzoic acid in a number of reactions. 

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. 

Asymmetric epoxidation of olefins is an effective approach for the synthesis of enan-tiomerically enriched epoxides. A variety of efficient methods have been developed [1, 2], including Sharpless epoxidation of allylic alcohols [3, 4], metal-catalyzed epoxidation of unfunctionalized olefins [5-10], and nucleophilic epoxidation of electron-deficient olefins [11-14], Dioxiranes and oxazirdinium salts have been proven to be effective oxidation reagents [15-21], Chiral dioxiranes [22-28] and oxaziridinium salts [19] generated in situ with Oxone from ketones and iminium salts, respectively, have been extensively investigated in numerous laboratories and have been shown to be useful toward the asymmetric epoxidation of alkenes. In these epoxidation reactions, only a catalytic amount of ketone or iminium salt is required since they are regenerated upon epoxidation of alkenes (Scheme 1). 

The breakthrough came already in 1996, one year after Curd s prediction, when Yang and coworkers reported the C2-symmetric binaphthalene-derived ketone catalyst 6, with which ee values of up to 87% were achieved. A few months later, Shi and coworkers reported the fructose-derived ketone 7, which is to date still one of the best and most widely employed chiral ketone catalysts for the asymmetric epoxidation of nonactivated alkenes. Routinely, epoxide products with ee values of over 90% may be obtained for trans- and trisubstituted alkenes. Later on, a catalytic version of this oxygen-transfer reaction was developed by increasing the pH value of the buffer. The shortcoming of such fructose-based dioxirane precursors is that they are prone to undergo oxidative decomposition, which curtails their catalytic activity. 

A kinetic study of the epoxidation showed the reaction to be of the first order with respect to both alkene and dioxirane174. A large steric effect was observed in the epoxidation of certain m/fnmv-dial kyIal kenes the cis compounds were found to exhibit reactivities one order of magnitude higher than the corresponding trans isomers. This large effect reflects a repulsive interaction between the substituents of the olefin and the dioxirane in the transition state (equation 25)9. 

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