Bimodal Evans−Polanyi Relationships in Hydrogen Atom Transfer from C(sp3)--H Bonds to the Cumyloxyl Radical. A Combined Time-Resolved Kinetic and Computational Study

The applicability of the Evans-Polanyi (EP) relationship to HAT reactions from C(sp3)-H bonds to the cumyloxyl radical (CumO) has been investigated. A consistent set of rate constants kH, for HAT from the C-H bonds of 56 substrates to CumO, spanning a range of more than four orders of magnitude, has been measured under identical experimental conditions. A corresponding set of consistent gas-phase C-H BDEs spanning 26 kcal/mol has been calculated using the (RO)CBS-QB3 method. The log kH vs C-H BDE plot shows two distinct EP relationships, one for substrates bearing benzylic and allylic C-H bonds (unsaturated group) and the other one, with a steeper slope, for saturated hydrocarbons, alcohols, ethers, diols, amines and carbamates (saturated group), in line with the bimodal behavior observed previously in theoretical studies of reactions promoted by other HAT reagents. The parallel use of BDFEs instead of BDEs allows the transformation of this correlation into a linear free energy relationship, analyzed within the framework of the Marcus theory. The ΔG‡ HAT vs ΔG°HAT plot shows again distinct behaviors for the two groups. A good fit to the Marcus equation is observed only for the saturated group, with lambda = 58 kcal mol-1, indicating that with the unsaturated group lambda must increase with increasing driving force. Taken together these results provide a qualitative connection between Bernasconi’s Principle of Nonperfect Synchronization and Marcus theory and suggest that the observed bimodal behavior is a general feature in the reactions of oxygen-based HAT reagents with C(sp3)-H donors.