Reduction of pyridine N-oxides by titanium(III)

Abstract

The reactions between Ti^III and a number of substituted pyridine N-oxides have been studied in a number of acids and have been found to exhibit the stoicheiometry. 2Ti^III+ X-pyo + H₂O → 2[TiO]²⁺+ X-Hpy⁺+ H⁺ and to obey the rate law (i). The reactions are suggested to proceed by the stepwise reduction of the pyridine N-oxide by, –d[Ti^III]//_2dt=k[Ti^III][X-pyo]//_[H⁺](i)[Ti(OH)]²⁺via a free-radical path. The rate constants, which range from 5.6 × 10^–4 to 0.75 s^–1, are influenced more by the steric and complexation effects of the ring substituents (Br, Cl, Me, OMe, CN, or CO₂H) than by their electron-donating or -withdrawing properties.