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

A time-resolved kinetic study on the effect of metal ions (Mn+) 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 Mn+ 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 (kH) was observed indicating that interaction between Mn+ 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 kH with increasing Lewis acidity of Mn+ was observed (kH(MeCN) > kH(Li+) > kH(Mg2+)). This behavior was explained in terms of the stronger Lewis acid−base interaction of Mn+ 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 kH was measured after Mg(ClO4)2 addition up to a 1.5:1 amine/Mg(ClO4)2 ratio. At higher amine concentrations, very similar kH 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 Mn+ can dramatically influence kH, 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.