Enhanced Reactivity in Hydrogen Atom Transfer from Tertiary Sites of Cyclohexanes and Decalins via Strain Release. Equatorial C-H Activation vs Axial C-H Deactivation

Absolute rate constants for hydrogen atom transfer (HAT) from cycloalkanes and decalins to the cumyloxyl radical (CumO•) were measured by laser flash photolysis. Very similar reactivities were observed for the C−H bonds of cyclopentane and cyclohexane, while the tertiary C−H bond of methylcyclopentane was found to be 6 times more reactive than the tertiary axial C−H bond of methylcyclohexane, pointing toward a certain extent of tertiary axial C−H bond deactivation. Comparison between the cis and trans isomers of 1,2-dimethylcyclohexane, 1,4-dimethylcyclohexane and decalin provides a quantitative evaluation of the role played by strain release in these reactions. kH values for HAT from tertiary equatorial C−H bonds were found to be at least 1 order of magnitude higher than those for HAT from the corresponding tertiary axial C−H bonds (kH(eq)/kH(ax) = 10−14). The higher reactivity of tertiary equatorial C−H bonds was explained in terms of 1,3-diaxial strain release in the HAT transition state. Increase in torsional strain in the HAT transition state accounts instead for tertiary axial C−H bond deactivation. The results are compared with those obtained for the corresponding C−H functionalization reactions by dioxiranes and nonheme metal-oxo species indicating that CumO• can represent a convenient model for the reactivity patterns of these oxidants.