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

Abstract

The applicability of the Evans–Polanyi (EP) relationship to HAT reactions from C(sp 3 )–H bonds to the cumyloxyl radical (CumO • ) has been investigated. A consistent set of rate constants, k H , 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 k H ′ 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 Δ G ⧧ HAT vs Δ G ° 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(sp 3 )–H donors.

C–H oxidation in fluorenyl benzoates does not proceed through a stepwise pathway: revisiting asynchronous proton-coupled electron transfer

“… 37–39 This concept has been developed in a Marcus framework 34,39,40 and cited recently to account for two series of H-atom transfer rate constants with unusual rate/driving force relationships. 41 In light of this prior literature, perhaps ‘non-perfect synchronization,’ or ‘imbalanced’ transfer could be more appropriate terms than ‘asynchronous’. We feel, however, that ‘asymmetric’ PCET is not the best term because readers may confuse that with enantioselective or the like.…”

Stereoretentive and regioselective selenium-catalyzed intermolecular propargylic C–H amination of alkynes

“…Notably this slope is greater than observed in two other studies of C–H activation of weak C–H bonds (0.36 and 0.23). 50,51 Our observed slope implies that any factors that stabilize the allenylselenium intermediate will substantially accelerate the reaction because the transition state develops substantial product-like character. Thus, the preference for activation of tertiary C–H bonds over secondary and primary can be explained by the fact that more substitution at the propargylic position results in formation of a more substituted, and thus more stable, allenylselenium intermediate.…”

A Desaturative Approach for Aromatic Aldehyde Synthesis via Synergistic Enamine, Photoredox and Cobalt Triple Catalysis

“…Next, we looked at ortho ‐substituted derivatives and by exploiting the simple functionalization chemistry of tetrahydrophtalic anhydride [16] we prepared 30 – 37 (also on gram‐scale) that are frequently encountered in the patented literature on drug development. Importantly, these derivatives demonstrated compatibility of several O‐, N‐ and S‐based groups as well as HAT‐labile positions [26] ( 31 ) and the B(pin) functionality ( 36 ).…”