title: 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.
abstract: 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.
authors.affiliation: Dipartimento di Scienze e Tecnologie Chimiche, Università "Tor Vergata", Via della Ricerca Scientifica, 1 I-00133 Rome, Italy.
authors.forename: Michela
authors.initials: M
authors.lastname: Salamone
authors.affiliation: Dipartimento di Scienze e Tecnologie Chimiche, Università "Tor Vergata", Via della Ricerca Scientifica, 1 I-00133 Rome, Italy.
authors.forename: Vanesa B
authors.initials: VB
authors.lastname: Ortega
authors.affiliation: Dipartimento di Scienze e Tecnologie Chimiche, Università "Tor Vergata", Via della Ricerca Scientifica, 1 I-00133 Rome, Italy.
authors.forename: Massimo
authors.initials: M
authors.lastname: Bietti