Polar Effects in Hydrogen Atom Transfer Reactions from a Proton-Coupled Electron Transfer (PCET) Perspective:...

Rate constants for hydrogen atom transfer (HAT) reactions of substituted toluenes with tert-butyl, tert-butoxy, and tert-butylperoxyl radicals are reanalyzed here using the free energies of related proton transfer (PT) and electron transfer (ET) reactions, calculated from an extensive set of compiled or estimated pK_a and E° values. The Eyring activation energies ΔG_HAT^‡ do not correlate with the relatively constant ΔG°_HAT, but do correlate close-to-linearly with ΔG°_PT and ΔG°_ET. The slopes of correlations are similar for the three radicals except that the Bu^• barriers shift in the opposite direction from the oxyl radical barriers─a clear example of the qualitative "polar effect" in HAT reactions. When cast quantitatively in free energy terms (Δ, only 5-10% of the typical Bell-Evans-Polanyi (BEP) effect of changing ΔG°_HAT. This analysis also highlights connections between polar effects and the concepts of "asynchronous" or "imbalanced" HAT reactions in which the PT and ET components of ΔG°_HAT contribute differently to the barrier. Finally, these observations are discussed in light of the traditional explanations of polar effects and the potential for a rubric that could predict the extent to which contra-thermodynamic selectivity may be achieved in HAT reactions.