Oxidations by methyl(trifluoromethyl)dioxirane. 5. Conversion of alcohols into carbonyl compounds

The oxidation of a number of secondary alcohols (i.e., 2-propanol, 1-phenylethanol, 3-octanol, cyclobutanol, exo- and endo-2-norborneol) by methyl(trifluoromethyl)dioxirane (1a) affords the corresponding ketones in high yield (92-99%), under mild conditions and within short reaction times (2-20 min). Primary alcohols 1-butanol and benzyl alcohol are converted by 1a into butyric acid and into PhCHO/PhCO2H mixtures, respectively, while 2-methyl-2-propanol is not oxidized. Functional group selectivity is illustrated by the clean conversion of two epoxy alcohols, namely 3,4-epoxy-2-butanol (8) and (+)-1,2-epoxy-3-pentanol (9), into the corresponding epoxy ketones, leaving the epoxy functionality untouched. The oxidation of cyclohexanol by 1a follows a second-order rate law, and a kinetic isotope effect (k(H)/k(D)) = 1.6 +/- 0.15 was measured by using cyclohexanol-d11. Remarkable stereoselectivity was recorded in the oxidation of 2-norborneol, since the endo-alcohol was found to be ca. 40 times more reactive than its exo stereomer. The available evidence suggests that a radical-chain mechanism is unlikely for the title transformation