Effect of Metal Ions on the Reactions of the Cumyloxyl Radical with Hydrogen Atom Donors. Fine Control on Hydrogen Abstraction Reactivity Determined by Lewis Acid–Base Interactions

Abstract

A time-resolved kinetic study on the effect of metal ions (M^<i>n</i>+) on hydrogen abstraction reactions from C–H donor substrates by the cumyloxyl radical (CumO^•) was carried out in acetonitrile. Metal salt addition was observed to increase the CumO^• β-scission rate constant in the order Li⁺ > Mg²⁺ > Na⁺. These effects were explained in terms of the stabilization of the β-scission transition state determined by Lewis acid–base interactions between M^<i>n</i>+ and the radical. When hydrogen abstraction from 1,4-cyclohexadiene was studied in the presence of LiClO₄ and Mg­(ClO₄)₂, a slight increase in rate constant (<i>k</i>_H) was observed indicating that interaction between M^<i>n</i>+ and CumO^• can also influence, although to a limited extent, the hydrogen abstraction reactivity of alkoxyl radicals. With Lewis basic C–H donors such as THF and tertiary amines, a decrease in <i>k</i>_H with increasing Lewis acidity of M^<i>n</i>+ was observed (<i>k</i>_H(MeCN) > <i>k</i>_H(Li⁺) > <i>k</i>_H(Mg²⁺)). This behavior was explained in terms of the stronger Lewis acid–base interaction of M^<i>n</i>+ with the substrate as compared to the radical. This interaction reduces the degree of overlap between the α-C–H σ* orbital and a heteroatom lone-pair, increasing the C–H BDE and destabilizing the carbon centered radical formed after abstraction. With tertiary amines, a >2-order of magnitude decrease in <i>k</i>_H was measured after Mg­(ClO₄)₂ addition up to a 1.5:1 amine/Mg­(ClO₄)₂ ratio. At higher amine concentrations, very similar <i>k</i>_H values were measured with and without Mg­(ClO₄)₂. These results clearly show that with strong Lewis basic substrates variations in the nature and concentration of M^<i>n</i>+ can dramatically influence <i>k</i>_H, allowing for a fine control of the substrate hydrogen atom donor ability, thus providing a convenient method for C–H deactivation. The implications and generality of these findings are discussed