Oxidations by methyl(trifluoromethyl)dioxirane. 2. Oxyfunctionalization of saturated hydrocarbons

The reaction of methyl(trifluoromethy1)dioxirane (lb), a novel dioxirane species, with two open-chain, four cyclic, and five polycyclic saturated hydrocarbons and two aralkyl hydrocarbons in CH,C12/ 1,l,I-trifluoropropanone has been studied; under mild conditions (-22 to 0 "C), it gives alcohols and/or ketones (deriving from further oxidation of secondary alcohols) in high yields and within very short reaction times. Primary C-H bonds are not appreciably oxidized and high regioselectivities were determined for attack at tertiary over secondary C-H bonds, with the exception of norbornane, which showed opposite regioselectivity. The reaction is also highly stereoselective, since hydroxylations of cis- and trans-decalin and of cis- and trans-1,2-dimethylcyclohexanewere found to be in each case stereospecific with retention. From kinetic data, E, = 14.3 kcal mol-' and log A = 9.9 were estimated for cyclohexane oxidation. Relative rates change in the order cyclohexane (0.78) < norbornane (1) < bicyclo[2.2.2]octane (9.2) < adamantane (146); cis-1,2-dimethylcyclohexanewas observed to be 7-fold more reactive than its trans isomer, demonstrating remarkable discrimination for equatorial vs axial C-H attack (also noticed in the case of cis- and trans-decalin). The relative rate of oxidation of cumene vs ethylbenzene was found to be ca. 3.1 (after statistical correction), Le., in sharp excess over values usually recorded in classical radical H-atom abstraction from benzylic position. Rate constants determined for the reactions of cumene and of ethylbenzene show the title dioxirane (Ib) is more reactive than dimethyldioxirane (la) by factors of ca. 600 and over 700, respectively. The whole of the observations is better accommodated by an "oxenoid" mechanism, involvingconcerted 0-atom insertion by dioxirane into C-H bonds of hydrocarbons.