Oxidation of secondary alcohols and ethers by dimethyldioxirane

Article abstract of DOI:10.1139/v99-011

The oxidation of several series of secondary alcohols 2-9, ethers 10- 17, and related derivatives 18 and 19, by dimethyldioxirane, 1, in acetone at 25°C produced the corresponding ketones in good to excellent yields for all but two cases. (The exceptions: oxidation of 1-methoxy-2-methyl-1- phenylpropane (48%) and 1-methoxy-2,2-dimethyl-1-phenylpropane (24%).) The oxidation of the secondary alcohols was found to yield k₂ values that were roughly 10-fold greater than those of the corresponding methyl ethers. The rate constant for oxidation of a silyl ether was slightly lower than that for the corresponding methyl ether while that for the ester derivative was roughly half the value. For oxidation of alcohols and methyl ethers, the k₂ values became smaller as the R'' series (Me, Et, nPr, iPr, and tBu) increased in steric bulk (p* = 1.7; r = 0.998 and p* = 3.2; r = 0.95, respectively). The Hammett study for the oxidation of the methyl ethers of α-methyl-p- benzyl alcohols (10, 20-25) yielded a ρ value of -0.74. The activation parameters for oxidation of the parent compound of the ether series (1- methoxy-1-phenylethane) were ΔH(+) = 14.8 ± 0.5 kcal/mol, Δ(+) = -21.9 eu, ΔG(+) = 21.3 kcal/mol, k₂ (25°C) = 1.6 x 10^-3 M^-1 s^-1. The mechanistic aspects of the oxidation are discussed in relation to two mechanistic extremes: (a) direct insertion of the oxygen atom into the CH bond and (b) direct abstraction of the H by dimethyldioxirane to yield a caged-radical pair, with subsequent coupling to hemi-ketal intermediates that fragment to yield acetone, alcohol or water, and ketone as the final products.