Fluorinated dioxiranes

The synthetically most useful method for the preparation of dioxiranes is the reaction of appropriate ketones (acetone, trill uoroacetone, 2-butanone, cyclohexanone etc.) with Caroate, commercially available as the triple salt of potassium monoperoxysul-fate (KHSOs). The catalytic cycle of the dioxirane formation and oxidation is shown in Scheme 1 in general form. For acetone as the ketone, by simple distillation at a slightly reduced pressure ca 100 torr) at room temperature ca 20 °C), Jeyaraman and Murray successfully isolated dimethyldioxirane (DMD) as a pale yellow solution in acetone (maximally ca 0.1 M). This pivotal achievement in 1985 fomented the subsequent intensive research activity in dioxirane chemistry, mainly the synthetic applications but also the mechanistic and theoretical aspects. The more reactive (up to a thousandfold ) fluorinated dioxirane, methyl(trifluoromethyl)dioxirane (TFD), was later isolated in a similar manner by Curd, Mello and coworkers". For dioxirane derived from less volatile ketones, e.g. cyclohexanone, the salting-out technique has been developed by Murray and coworkers to obtain the corresponding dioxirane solution. 

The general features of reactivity and selectivity of this novel oxidant are displayed in Table 3. It is significant that the fluoro derivative, i.e. methyl-(trifluormethyl)dioxirane [6], is at least 1000-fold more reactive than dimethyldioxirane. As a consequence, the fluorinated dioxirane oxidizes alkanes to the corresponding alcohols and/or ketones within minutes even at subambient temperatures [7]. 

Halogen substitution is expected to increase the electrophilicity of the carbenes, and in particular lh with four fluorine substituents is expected to be highly electrophilic and of unusual reactivity. All the carbenes le-g could be matrix-isolated by irradiation of their corresponding quinone diazides 2 in argon at 8-10 K.24 68,62 Again, the thermal reaction in (Vdoped matrices results in the formation of quinone oxides 7, which show the expected photochemical rearrangement to the spiro dioxiranes 8 and finally lactones 9. 

From this series of calculations it is noted that the gas-phase reactivity of TFDO is substantially greater than that of DMDO. This rate difference has been ascribed largely to the inductive effect of the CF3 group. Fluoro-substituted dioxiranes have also played a unique role in the chiral epoxidation of alkenes. Houk and coworkers96 have identified a novel stereoelectronic effect that increases the rate of epoxidation when the fluorine substituent is anti to the oxygen of the developing C=0 group in the TS for epoxidation. 

Highly fluorinated ketones such as hexafluoroacetone are oxidation catalysts even when H2O2 is used as an oxygen source. In this case, the reaction probably involves an a-hydroxyhydroperoxide, rather than a dioxirane, and the reactions require more drastic conditions (402, 403). Nevertheless, the observations demonstrate that fluorinated ketones are useful catalysts for activation of oxidants. 

Less regularly used reagents are tert-hu y hydroperoxide, tf/Y-butyllithium, ozone. dioxiranes, fluorine/water/acetonitrile, or A, A -diethylhydroxylamine. Alkenes carrying a donor substituent can also be epoxidized with pcracids. Fluorinated allylic alcohols give, under Sharpless conditions, epoxides in good yield and enantiosclcclivity. ... 

The epoxidation of alkenes is accelerated in fluorinated alcohol solvents the factors responsible for this rate acceleration have been examined <2006JA8421, 2006JA13412>. Chiral dioxiranes can now be used in a catalytic sense for the synthesis of vinyl m-epoxides <2006AGE4475>. 

NMR spectra of dioxiranes derived from fluorinated acetophenones 29-36 using Oxone have also been reported <20030L2853>. The fluorine nuclei of CF3 group bonded to dioxirane ring typically resonates at —85 0.5 ppm. 

Fluorinated alcohols as a new medium for selective epoxidation with dioxirane or H202 04SL18. 

Based on its UV absorption, bis(trifluoromethyl)dioxirane (79) appears to have been generated by treatment of the mono- or dilithium salt of hexafluoroacetone hydrate (78) with elemental fluorine. ... 

D Accolti, L. Fusco, C. Rella, M.R. Curci, R. Concerning synthesis of ring-A fluorinated anthracychnes. The dioxirane shunt. Synth. Commun. 2003, 33, 3009. 

Fluorinated alcohols, and in particular HFIP, have also proven beneficial for epoxidations of alkenes using persulfate (Oxone) as the terminal oxidant and fluoroketones as catalysts [9, 10]. Under these conditions, the fluorinated ketones are converted to dioxiranes, which are the active epoxidizing spedes. Typical catalysts... 

Fluorine-containing ketones have proven to be one of the most successful types of catalyst for dioxirane-mediated epoxidation [27]. Demnark has shown that good to excellent enantioselectivities can be achieved with catalyst 9 for tra 5-olefins (Scheme 1.10). However, catalyst loadings are high (30 mol%) [28]. 

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