Abstract
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 4 orders of magnitude, has been measured under identical experimental conditions. A corresponding set of consistent gas-phase C–H bond dissociation enthalpies (BDEs) spanning 27 kcal mol–1 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 λ = 58 kcal mol–1, indicating that with the unsaturated group λ 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.
Highlights
The relationship is often employed as a mechanistic tool in C−H bond oxidations promoted by radical and radical-like species, where the observation of a correlation is taken as evidence for a C−H bond cleavage step that occurs through hydrogen atom transfer (HAT).3−7 From the correlation obtained for the reaction of a given HAT reagent with a series of substrates or, alternatively, of a given substrate with a series of HAT reagents,3,8 it is possible to predict rate constants for the corresponding reactions of additional substrates,9 as well as derive substrate bond dissociation enthalpies (BDEs) or the BDE of the new bond formed by the HAT reagent following abstraction
Because of large discrepancies in some of the available C−H BDEs, and the absence of BDEs for some of these substrates, a corresponding set of consistent gasphase C−H BDEs and bond dissociation f ree energies (BDFEs) spanning a range of 27 kcal mol−1
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Summary
The Evans−Polanyi (EP) relationship ( known as the Bell− Evans−Polanyi relationship) correlates reaction rate constants (or other activation parameters) with bond dissociation enthalpies (BDEs). The relationship is often employed as a mechanistic tool in C−H bond oxidations promoted by radical and radical-like species, where the observation of a correlation is taken as evidence for a C−H bond cleavage step that occurs through hydrogen atom transfer (HAT).− From the correlation obtained for the reaction of a given HAT reagent with a series of substrates or, alternatively, of a given substrate with a series of HAT reagents, it is possible to predict rate constants for the corresponding reactions of additional substrates, as well as derive substrate BDEs or the BDE of the new bond formed by the HAT reagent following abstraction.10,11In 1982, Tedder discussed the factors governing reactivity and selectivity in atom transfer reactions. He highlighted the relative importance of the strengths of both the bond being broken and the bond being formed and of polar and steric effects in these processes. In 1982, Tedder discussed the factors governing reactivity and selectivity in atom transfer reactions.. In 1982, Tedder discussed the factors governing reactivity and selectivity in atom transfer reactions.12 He highlighted the relative importance of the strengths of both the bond being broken and the bond being formed and of polar and steric effects in these processes. One of us found that the empirical extrathermodynamic relationship defined by EP holds quite well for HAT reactions over a wide range of driving force, when comparing similar radicals and similar substrates, for example for HAT from C−H bonds to oxygen-centered radicals.3a This work highlighted the limitations imposed by the difficulty in compiling a consistent set of C−H bond strengths. The same limitation, together with the importance of extending the correlation over a sufficiently broad range of C−H bond strengths, was evidenced in a recent work by Jackson and coworkers in the case of HAT reactions promoted by high-valent metal oxo species.
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