Computational Evaluation of the Oxidative Cleavage of Triazine Derivatives for Electrosynthesis

Abstract

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 group. Results were obtained for homolytic cleavage of the neutral compound, and two possible mesolytic cleavage pathways for the oxidized form (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 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.

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

“…Triazinyls considered in our previous work (ref ). Substituents “X” and “Y’ are consistent with those in the verdazyl series of Figure .…”

“…We very recently illustrated, in silico, that R-groups attached to electron rich benzo-1,2,4-triazinyl radicals form adducts that are far better candidates for • cleavage, owing to their very low oxidation potentials (-0.7 -0.2 V for radicals, and -0.3 -0.4 V for adducts). [7][8][9] We found that for all derivatives considered, it should be possible to drive the cleavage of more electron rich…”

“…Our previous study of triazinyl systems from ref was undertaken using the CPCM solvation model with wB97X-D electronic densities. For fair comparison with the verdazyl-derived adducts, solvent corrections for triazinyls have been recalculated using COSMO-RS, and the updated numbers are reported in the Supporting Information.…”

“…We very recently illustrated in silico that R groups attached to electron-rich benzo-1,2,4-triazinyl radicals form adducts that are far better candidates for R • cleavage owing to their very low oxidation potentials (−0.7 to 0.2 V for radicals and −0.3 to 0.4 V for adducts). − We found that for all derivatives considered, it should be possible to drive the cleavage of more electron-rich radicals thanks to the stability of the triazinyl cation. We are still in the process of exploring the experimental capacity of these adducts; however, based on the promising outcomes of the computational study, we posit that other heterocyclic radicals might also possess desirable properties to further expand the scope of radical generation.…”

Polymorphism in a π stacked Blatter radical: structures and magnetic properties of 3-(phenyl)-1-(pyrid-2-yl)-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl

“…Blatter radicals have also been used in chemical synthesis. For example, they can act as initiators in controlled polymerizations, [35][36][37][38] as organic paramagnetic ligands in metal coordination complexes, [39][40][41] and as building blocks in high-spin diradicals and biradicaloids. 32,42,43 Blatter-type radicals can form stable thin films (without degradation) while retaining their paramagnetic character.…”