The Role of Iodine and Pyridine Bases in the Alcohol Electrocatalytic Oxidation Mediated by 4-AcNH-TEMPO

Abstract

The role of iodine, pyridine bases, and the nitroxyl radical 4-AcNH-TEMPO in the electrooxidative conversion of alcohols into carbonyl compounds in a two-phase medium CH₂Cl₂/NaHCO₃(aq) is studied. Using cyclic voltammetry, it was established that in a weakly alkaline medium (pH 8.6) the iodide ion is oxidized to active forms of iodine (I₂ and I⁺), which are terminal oxidizing agents converting the nitroxyl radical into oxoammonium cations necessary for the oxidation of alcohol. It has been established spectrophotometrically that the pyridine bases are capable of stabilizing I₂ and I⁺ as [PyI₂], [PyI]⁺ complexes whose formation occurs predominantly in the organic phase. The stabilized forms of iodine effectively convert the nitroxyl radical into oxoammonium cations at the electrode interface. The formation of a catalytic complex between the oxoammonium cations and the pyridine base occurs in the aqueous phase. The cyclic-voltammetry studies showed that the rate of nitroxyl-radical-mediated alcohol oxidation increased up to fourfold in the presence of the pyridine base, as compared to the oxidative transformation in the absence of the pyridine base. This proves the advantages of the nitroxyl radical/pyridine base catalytic system and specifies the role of the pyridine base as a promoter in the alcohol indirect electrooxidation.