A kinetic study of the hydrogen atom abstraction reactions from propanal (PA) and 2,2-dimethylpropanal (DMPA) by the cumyloxyl radical (CumO•) has been carried out in different solvents (benzene, PhCl, MeCN, t-BuOH, MeOH, and TFE). The corresponding reactions of the benzyloxyl radical (BnO•) have been studied in MeCN. The reaction of CumO• with 1,4-cyclohexadiene (CHD) also has been investigated in TFE solution. With CHD a 3-fold increase in rate constant (k(H)) has been observed on going from benzene, PhCl, and MeCN to TFE. This represents the first observation of a sizable kinetic solvent effect for hydrogen atom abstraction reactions from hydrocarbons by alkoxyl radicals and indicates that strong HBD solvents influence the hydrogen abstraction reactivity of CumO•. With PA and DMPA a significant decrease in k(H) has been observed on going from benzene and PhCl to MeOH and TFE, indicative of hydrogen-bond interactions between the carbonyl lone pair and the solvent in the transition state. The similar k(H) values observed for the reactions of the aldehydes in MeOH and TFE point toward differential hydrogen bond interactions of the latter solvent with the substrate and the radical in the transition state. The small reactivity ratios observed for the reactions of CumO• and BnO• with PA and DMPA (k(H)(BnO•)/k(H)(CumO•) = 1.2 and 1.6, respectively) indicate that with these substrates alkoxyl radical sterics play a minor role.

A kinetic study on the effect of acetic (AcOH) and trifluoroacetic acid (TFA) on hydrogen abstractions from the C-H bonds of basic substrates by the cumyloxyl radical (CumOc) was carried out in acetonitrile. With tetrahydropyran no significant effect on k H was observed after acid addition. With the more basic tertiary amines acid addition led to greater than 4-orders of magnitude decreases in k H . Protonation at nitrogen decreases the degree of overlap between the a-C-H s* orbital and the lone-pair leading to an increase in the strength of the C-H bond and a destabilization of the radical formed after abstraction.Evidence that C-H deactivation extends up to the g-C-H bonds and for the reversibility of this approach was also provided. With TFA no reaction was observed up to [amine] ¼ [TFA], pointing towards stoichiometric protonation. At [amine] > [TFA], k H values that are very similar to those measured in the absence of added acid were obtained. With the weaker acid, AcOH, no reaction was observed up to[AcOH]/[amine] $4, and a curved plot was observed with increasing [amine], as a result of the acidbase equilibrium between AcOH and the amine. With 1,4-dimethylpiperazine, a quantitative evaluation of the C-H deactivation determined by sequential protonation of one or both nitrogen centers was obtained. These observations show that protonation provides an extremely efficient, precise and tunable method for the deactivation of the C-H bonds of basic substrates allowing moreover for careful control over the hydrogen abstraction selectivity. The implications of this approach are discussed.