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

A kinetic study on the reactions of the cumyloxyl radical (Cumo(center dot)) with a series of alkanols and alkanediols has been carried out. Predominant hydrogen atom transfer (HAT) from the alpha-C-H bonds of these substrates, activated by the presence of the OH group, is observed. The comparable k(H) 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 beta-C-H deactivation toward HAT to the electrophilic Cumo(center dot), 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(center dot) with densely oxygenated methyl pyranosides as compared to cyclohexanol derivatives highlights the role of beta-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(center dot), uncovering moreover beta-C-H deactivation as a relevant contributor in defining site selectivity.