Oxidation of substituted pyridines by dimethyldioxirane: kinetics and solvent effects

The oxidation of a series of substituted pyridines by dimethyldioxirane (1) produced the expected N-oxides in quantitative yields. The second order rate constants (k2) for the oxidation of a series of substituted pyridines (2a–g) by dimethyldioxirane were determined in dried acetone at 23 °C. An excellent correlation with Hammett sigma values was found (ρ = −2.91, r = 0.995). Kinetic studies for the oxidation of 4-trifluoromethylpyridine by 1 were carried out in the following dried solvent systems: acetone (k2 = 0.017 M−1 s−1), carbon tetrachloride/acetone (7:3; k2 = 0.014 M−1 s−1), acetonitrile/acetone (7:3; k2 = 0.047 M−1 s−1), and methanol/acetone (7:3; k2 = 0.68 M−1 s−1). Kinetic studies of the oxidation of pyridine by 1 versus mole fraction of water in acetone [k2 = 0.78 M−1 s−1 (χ = 0) to k2 = 11.1 M−1 s−1 (χ = 0.52)] were carried out. The results showed the reaction to be very sensitive to protic, polar solvents.