Synthesis and Reactivity of s‑Aryltetrazines Palladacycles Toward a Direct o‑Aryl C–H Bond Halogenation Achieved by Electrocatalysis

Electrocatalysis is considered as a promising approach toward cleaner, precisely controlled, and low waste-producing transition metal-catalyzed C–H bond activation/functionalization. However, some useful redox-active substrates could be challenging to use in electrocatalysis, depending on their redox potential and intrinsic reactivity. Accordingly, the topical s-aryltetrazines have not been employed, up to now, for their selective transition metal-promoted ortho-C–H functionalization using an electrocatalytic process. Herein, we addressed this challenging issue stepwise by synthesizing first a series of stable and well-defined dinuclear and mononuclear palladacycles of the s-diphenyltetrazine. These compounds are considered pertinent intermediates of the direct C–H functionalization of s-aryltetrazines in a palladium-catalyzed approach. The palladium complexes were characterized by multinuclear NMR spectroscopy in solution, and single crystal X-ray diffraction analysis in the solid state. Their electrochemical behavior was established by cyclic voltammetry. We successfully achieved the reductive elimination of o-C–H halogenated s-diphenyltetrazine under electrochemical conditions from its corresponding mononuclear o-metalated s-aryltetrazine palladium halide, establishing the electrochemically controlled palladium-promoted o-C–H halogenation of an s-aryltetrazine. From this elementary step achieved under electrocatalytic control, we devised simple conditions for effective electrochemical palladium-catalyzed C–H ortho-chlorination and iodination of s-aryltetrazines, using cheap lithium halide salt as a nucleophile source. While these conditions still suffer from multiple halogenation reactions, and especially the symmetrical bis-halogenation, they open the way, as proof-of-concept, to unprecedented electrochemically controlled direct functionalization of the redox active s-aryltetrazines.