Computational Assessment of Verdazyl Derivatives for Electrochemical Generation of Carbon-Centered Radicals

To expand the scope for carbon-centered radical generation by electrochemical activation of adducts based on stable free radicals, a test set of six simple electron-rich Kuhn verdazyl derivatives in conjunction with nine different alkyl leaving groups has been computationally assessed. Like triazinyls, adducts of simple verdazyl derivatives functionalized with electron-donating substituents favor mesolytic cleavage to carbon-centered radicals under mild electrochemical potentials (-0.7 to -0.2 V vs Fc⁺/Fc). Electrochemical oxidation was found to reduce the bond dissociation Gibbs free energy (298 K in acetonitrile) by 70 kJ mol^-1 on average, when comparing the homolytic cleavage pathway of the unoxidized adduct to the preferred mesolytic pathway of the oxidized adduct (i.e., to form either a verdazyl radical and a carbocation or a verdazyl cation and a carbon-centered radical). Considering the full thermochemical cycle, we illustrate that all the relevant free energy changes can be reduced to differences between the oxidation potentials of adducts and radicals, defining a series of criteria that govern the rational design of suitable candidates for oxidative carbon-centered radical cleavage. As a result of a tradeoff between promoting the oxidation of the adduct and enhancing the net reduction in BDFE upon oxidation, the best verdazyl derivatives for carbon-centered radical generation are those substituted with tBu substituents.