The Unexpected Role of Se(VI) Species in Epoxidations with Benzeneseleninic Acid and Hydrogen Peroxide

Benzeneperoxyseleninic acid has been proposed as the key intermediate in the widely used epoxidation of alkenes with benzeneseleninic acid and hydrogen peroxide. However, it reacts sluggishly with cyclooctene and instead rapidly decomposes in solution to a mixed selenonium-selenonate salt that was identified by x-ray absorption and 77 Se NMR spectroscopy, as well as by single crystal x-ray diffraction. This process includes a selenoxide elimination of the peroxyseleninic acid with liberation of oxygen and additional redox steps. The salt is relatively stable in the solid state, but generates the corresponding selenonic acid in the presence of hydrogen peroxide. The selenonic acid is inert towards cyclooctene on its own; however, rapid epoxidation occurs when hydrogen peroxide is added. This shows that the selenonic acid must first be activated through further oxidation, presumably to the heretofore unknown benzeneperoxyselenonic acid. The latter is the principal oxidant in this epoxidation. Peroxyseleninic or peroxyselenonic? What a difference a vowel makes! The main active intermediate in the epoxidation of cyclooctene with benzeneseleninic acid and hydrogen peroxide is not benzeneperoxyseleninic acid, but surprisingly, the corresponding postulated peroxyselenonic acid.