C_sp2–H Amination Reactions Mediated by Metastable Pseudo-O_h Masked Aryl-Co^III-nitrene Species

Cobalt-catalyzed C-H amination via M-nitrenoid species is spiking the interest of the research community. Understanding this process at a molecular level is a challenging task, and here we report a well-defined macrocyclic system featuring a pseudo-Oh aryl-CoIII species that reacts with aliphatic azides to effect intramolecular Csp2-N bond formation. Strikingly, a putative aryl-Co═NR nitrenoid intermediate species is formed and is rapidly trapped by a carboxylate ligand to form a carboxylate masked-nitrene, which functions as a shortcut to stabilize and guide the reaction to productive intramolecular Csp2-N bond formation. On one hand, several intermediate species featuring the Csp2-N bond formed have been isolated and structurally characterized, and the essential role of the carboxylate ligand has been proven. Complementarily, a thorough density functional theory study of the Csp2-N bond formation mechanism explains at the molecular level the key role of the carboxylate-masked nitrene species, which is essential to tame the metastability of the putative aryl-CoIII═NR nitrene species to effectively yield the Csp2-N products. The solid molecular mechanistic scheme determined for the Csp2-N bond forming reaction is fully supported by both experimental and computation complementary studies.