Computational Evaluation of the Oxidative Cleavage of Triazine Derivatives for Electrosynthesis

Computational chemistry was used to study the N–C bond dissociation Gibbs free energies (298 K, acetonitrile) for a test set of 54 triazine derivatives, comprising combinations of 6 triazinyl radicals with 9 different alkyl leaving groups. Results were obtained for homolytic cleavage of the neutral compound, and two possible mesolytic cleavage pathways for the oxidized form were found (cleavage to a triazinyl radical and a carbocation or cleavage to a triazinyl cation and a carbon-centered radical). Oxidation potentials of the adducts and triazine radicals were also assessed. The aim of the study was to assess triazines as an alternative to alkoxyamines as electrochemical sources of carbon-centered radicals or cations. Oxidation potentials of the triazine adducts (−0.1 to +0.2 V vs Fc/Fc+) are lower than those for corresponding alkoxyamines (0.7–1.2 V), making these compounds more functional group tolerant in electrosynthesis. In contrast to alkoxyamines, upon oxidation, they generally cleave to a carbon-centered radical rather than a carbocation and do so exergonically or at relatively low energies. Important exceptions are electron donating leaving groups for which mesolytic cleavage to a carbocation is preferred, though only in select cases is oxidative cleavage thermodynamically favored. The computational studies outlined herein support the possibility that an adduct based on the triazinyl radical is superior to an alkoxyamine for the electrochemical activation of carbon-centered radicals.M.L.C. gratefully acknowledges a Georgina Sweet ARC Laureate Fellowship (FL170100041) and generous allocations of supercomputing time on the National Facility of the Australian National Computational Infrastructure