Bromination of Tocopherols: Oxidative Halogenations and Rearrangements

Abstract

AbstractThe bromination behaviour of all four tocopherols and the corresponding model compounds in acidic and basic aqueous media was studied. Acidic conditions resulted in quinones and brominated quinones, with the bromination of the tocopherol preceeding the oxidation to the quinone. Buffer type and concentration did not influence the reaction results, whereas pH and the ratio of buffer and ethanol used as a co‐solvent strongly affected the reaction rates. In alkaline media, two or three major products and a multitude of minor byproducts were obtained, because the combination of oxidizing hypobromite and alkaline conditions allowed for more complex reaction pathways (oxidation, substitution, condensation, elimination) than the acidic conditions did. The para‐quinones were major reaction products observed for all four tocopherols. Interestingly, if the quinone was substituted at C‐5, it rearranged in a Michael‐type addition process followed by substitution with a bromonium ion. The absence of a substituent at C‐5, the presence of a bromine atom at C‐7, or the absence of bromine in the reaction medium were all able to prevent this reaction. The rearrangement products could react further by substitution of the newly acquired bromine group for a hydroxy group. In the cases of γ‐ and δ‐tocopherol, further rearrangement led to the formation of trioxo compounds.