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 (k(2)) for the oxidation of a series of substituted pyridines (2a-g) by dimethyldioxirane were determined in dried acetone at 23degreesC. An excellent correlation with Hammett sigma values was found (rho = -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 (k(2) = 0.017 M-1 s(-1)), carbon tetrachloride/acetone (7:3; k(2) = 0.014 M-1 s(-1)), acetonitrile/acetone (7:3; k(2) = 0.047 M-1 s(-1)), and methanol/acetone (7:3; k(2) = 0.68 M-1 s(-1)). Kinetic studies of the oxidation of pyridine by 1 versus mole fraction of water in acetone [k(2) = 0.78 M-1 s(-1) (chi = 0) to k(2) = 11.1 M-1 s(-1) (chi=0.52)] were carried out. The results showed the reaction to be very sensitive to protic, polar solvents. (C) 2004 Elsevier Ltd. All rights reserved.