Dioxiranes may either be prepared in advance or generated in situ for epoxidation reactions. In most cases, a two-phase system must be set up for in situ epoxidations, as KHSO is not soluble in organic solvents. Thus, substrates or products sensitive to hydrolysis will not survive in situ epoxidations. This section describes epoxidation conditions for alkenes with electron-donating or -withdrawing substituents, both of which may be epoxidized with dioxiranes in either the stoichiometric or catalytic mode.
Although dioxiranes are highly electrophilic, they epoxidize both electron-rich and electron-poor alkenes in good yield (although the latter react much more slowly). Electron-poor epoxide products also exhibit enhanced hydrolytic stability, meaning that they can often survive in situ conditions. Epoxidations of electron-rich double bonds have yielded intermediates of Rubottom oxidation. Upon hydrolysis, these siloxyepoxides yield α-hydroxyketones.
Electron-poor double bonds take much longer to epoxidize. Heating may be used to encourage oxidation, although the reaction temperature should never exceed 50 °C, to avoid decomposition of the dioxirane.
Alkenes bound to both electron-withdrawing and -donating groups tend to behave like the former, requiring long oxidation times and occasionally some heating. Like electron-poor epoxides, epoxide products from this class of substrates are often stable with respect to hydrolysis.
In substrates containing multiple double bonds, the most electron-rich double bond can usually be selectively epoxidized.
Epoxidations employing aqueous Oxone and a catalytic amount of ketone are convenient if a specialized dioxirane must be used (as in asymmetric applications) or if isolation of the dioxirane is inconvenient. Hydrolytic decomposition of the epoxidation product may be used to good advantage.