Tuning hydrogen atom abstraction from the aliphatic C-H bonds of basic substrates by protonation. Control over selectivity by C-H deactivation

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 (CumO(center dot)) 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 alpha-C-H sigma* 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 gamma-C-H bonds and for the reversibility of this approach was also provided. With TFA no reaction was observed up to [amine] = [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] similar to 4, and a curved plot was observed with increasing [amine], as a result of the acid-base 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.