Electronic and Torsional Effects on Hydrogen Atom Transfer from Aliphatic C-H Bonds: A Kinetic Evaluation via Reaction with the Cumyloxyl Radical

A kinetic study on the hydrogen atom transfer (HAT) reactions from the aliphatic C-H bonds of a series of 1-Z-pentyl, 1-Z-propyl, and Z-cyclohexyl derivatives and of a series of N-alkylamides and N-alkylphthalimides to the electrophilic cumyloxyl radical (CumO(center dot)) has been carried out. With 1-pentyl and 1-propyl derivatives, alpha-CH2 activation toward CumO(center dot) is observed for Z = Ph, OH, NH2, and NHAc, as evidenced by an increase in k(H) as compared to the unsubstituted alkane substrate. A decrease in k(H) has been instead measured for Z = OAc, NPhth, CO2Me, Cl, Br, and CN, indicative of alpha-CH2 deactivation with HAT that predominantly occurs from the most remote methylenic site. With cyclohexyl derivatives, alpha-CH activation is only observed for Z = OH and NH2, indicative of torsional effects as an important contributor in governing the functionalization selectivity of monosubstituted cyclohexanes. In the reactions of N-alkylamides and N-alkylphthalimides with CumO(center dot), the reactivity and selectivity patterns parallel those observed in the oxidation of the same substrates with H2O2 catalyzed by manganese complexes, supporting the hypothesis that both reactions proceed through a common HAT mechanism. The implications of these findings and the potential of electronic, stereoelectronic, and torsional effects as tools to implement selectivity in C-H oxidation reactions are briefly discussed.