Hydrogen Atom Transfer from Alkanols and Alkanediols to the Cumyloxyl Radical: Kinetic Evaluation of the Contribution of α-C-H Activation and β-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 k values measured for ethanol and 1-propanol and the increase in k measured upon going from 1,2-diols to structurally related 1,3- and 1,4-diols is indicative of β-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 NH group. The significantly lower k values measured for reaction of CumO with densely oxygenated methyl pyranosides as compared to cyclohexanol derivatives highlights the role of β-C-H deactivation. The contribution of torsional effects on reactivity is evidenced by the ∼2-fold increase in k 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 β-C-H deactivation as a relevant contributor in defining site selectivity.