Mechanism-Driven Development of N‑(Quinolin-8-yl)-benzamide Coupling Reactions via C–H or N–H Activation

The mechanistic details of the oxidative coupling of compounds with a number of different redox centers are investigated using N-(quinolin-8-yl)-benzamide (L) as a model substrate. The control of the chemical or electrooxidation parameters in the absence or presence of a cobalt catalyst makes it possible to obtain regioselectively different oxidative coupling products (ortho- vs para-C–H/C–H vs C–H/N–H vs N–H/N–H). The results indicate that the operative mechanism depends on the type of oxidized reaction center and the oxidant nature. Oxidation affects the para-C–H bond in quinoline or the N–H fragment or the ortho-C–H bond in the benzene substituent in the molecule. The intermediate Co(II)[L^–H]₂ complex and C–H-activated Co^III metallacycle with benzamide ligands, which are shown to oxidize at close potentials, have been isolated and characterized by various techniques, including X-ray diffraction and voltammetry. The strength of the oxidizing agent affects the formation of a particular product, though not acting as the determining factor. Two-electron oxidation of Co(II)[L^–H]₂ yields to C–N coupling, but one-electron oxidation of Co(III) leads to ortho-C–C coupling. All electrochemical reactions are performed under mild conditions at room temperature without adding special reagents (oxidants, halides, phosphines, etc.).