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

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 p K a and E ° values. The Eyring activation energies Δ G HAT ‡ do not correlate with the relatively constant Δ G° HAT , but do correlate close-to-linearly with Δ G° PT and Δ G° ET . The slopes of correlations are similar for the three radicals except that the t 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 G HAT ‡ vs Δ G ° PT/ET ), this effect is very small , only 5-10% of the typical Bell-Evans-Polanyi (BEP) effect of changing Δ G° 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 Δ G° 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.