Copper‐Stabilized Nitrene Radical in N─N Coupling: Facile Synthesis of Hydrazides and Pyrazole

Abstract

Abstract Despite extensive studies of electrophilic copper‐nitrenoid species in nitrene transfer chemistry, the formation and application of Cu‐nitrene radicals remain elusive and underdeveloped. Nitrene radicals, being highly reactive, actively participate in radical‐type amination reactions under mild conditions. Using a well‐defined Cu(II)‐catalyst [Cu II ( L 1 )Cl 2 ] ( 1 ) bearing 2,6‐bis(phenyldiazenyl)pyridine ( L 1 ) as a ligand, the otherwise copper‐stabilized electrophilic nitrenoid species has been transformed to a nitrene radical species via selective catalyst to nitrene single electron transfer. The copper‐stabilized nitrene radical species facilitates the selective formation of N─N bonds, resulting in N ‐acyl hydrazides. Catalyst 1 exhibits a broad substrate scope, smoothly functionalizing dioxazolone derivatives derived from long‐chain fatty acids, including palmitic, stearic, oleic, and erucic acids, as well as biologically relevant acids such as 1‐adamantane acetic acid, α ‐lipoic acid, and glutamic acid. Important N ‐heterocyclic amines, such as quinolines, carbazoles, and azepines, were also successfully functionalized. Catalyst 1 is also compatible with intramolecular N─N coupling, enabling the direct synthesis of pyrazoles in a single step, thereby emphasizing the method's synthetic versatility. The reaction is scalable, affording excellent yields in gram‐scale synthesis. Detailed mechanistic investigations, control experiments, and EPR analysis, including DFT calculations, confirm the existence of a radical‐mediated pathway.