Hydrogen Atom Transfer from Alkanols and Alkanediols to the Cumyloxyl Radical: Kinetic Evaluation of the Contribution of α-C-H Activation and ß-C-H Deactivation.

A kinetic study on the reactions of the cumyloxyl radical (CumO•) with a series of alkanols and alkanediols has been carried out. Predominant hydrogen atom transfer (HAT) from the α-C-H bonds of these substrates, activated by the presence of the OH group, is observed. The comparable kH values measured for ethanol and 1-propanol and the increase in kH measured upon going from 1,2-diols to structurally related 1,3- and 1,4-diols is indicative of ß-C-H deactivation toward HAT to the electrophilic CumO•, determined by the electron-withdrawing character of the OH group. No analogous deactivation is observed for the corresponding diamines, in agreement with the weaker electron-withdrawing character of the NH2 group. The significantly lower kH values measured for reaction of CumO• with densely oxygenated methyl pyranosides as compared to cyclohexanol derivatives highlights the role of ß-C-H deactivation. The contribution of torsional effects on reactivity is evidenced by the ∼2-fold increase in kH observed upon going from the trans isomers of 4- tert-butylcyclohexanol and 1,2- and 1,4-cyclohexanediol to the corresponding cis isomers. These results provide an evaluation of the role of electronic and torsional effects on HAT reactions from alcohols and diols to CumO•, uncovering moreover ß-C-H deactivation as a relevant contributor in defining site selectivity.

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.