Rate Accelerations of Bromination Reactions with NaBr and H2O2 via the Addition of Catalytic Quantities of Diaryl Ditellurides

Diaryl ditellurides were oxidized in situ to give aryltellurinic acids, which catalyzed the oxidation of NaBr with H2O2 in buffered aqueous solutions. The aryltellurinic acids were slowly oxidized under the reaction conditions to the corresponding telluronic acids, which did not catalyze oxidation of NaBr with H2O2. Both 4-(methoxyphenyl)tellurinic acid and 4-(methoxyphenyl)telluronic acid were characterized in solution by 125Te NMR and for their effectiveness as catalysts in kinetics studies. The effectiveness of the tellurinic acids as catalysts was very sensitive to electron demand in the intermediates present during the course of the reaction. Electron-withdrawing substituents favor the deprotonated tellurinic acid (tellurinate) in solution, while electron-donating substituents favor the protonated tellurinic acid. Of the nine ditellurides screened for their ability to accelerate the oxidation of NaBr with H2O2, diphenyl ditelluride emerged as the most active. The addition of only 0.20 mol % of this ditelluride (relative to substrate) promoted a 240-fold increase in the rate of oxidation of NaBr with H2O2, as measured by the bromination of 4-pentenoic acid. The “Br+” species prepared in situ were trapped by a series of alkenoic acids and activated aryl compounds.