Room-Temperature Cu(II) Radical-Triggered Alkyne C–H Activation

A dimeric Cu(II) complex [Cu(II) 2 L 2 (μ 2 -Cl)Cl] ( 1 ) built from an asymmetric tridentate ligand (2-(((2-aminocyclohexyl)imino)methyl)-4,6-di- tert -butylphenol) and weakly coordinating anions has been synthesized and structurally characterized. In dichloromethane solution, 1 exists in a monomeric [Cu(II)LCl] ( 1′ ) (85%)–dimeric ( 1 ) (15%) equilibrium, and cyclic voltammetry (CV) and electron paramagnetic resonance (EPR) studies indicate structural stability and redox retention. Addition of phenylacetylene to the CH 2 Cl 2 solution populates 1′ and leads to the formation of a transient radical species. Theoretical studies support this notion and show that the radical initiates an alkyne C–H bond activation process via a four-membered ring (Cu(II)–O···H–C alkyne ) intermediate. This unusual C–H activation method is applicable for the efficient synthesis of propargylamines, without additives, within 16 h, at low loadings and in noncoordinating solvents including late-stage functionalization of important bioactive molecules. Single-crystal X-ray diffraction studies, postcatalysis, confirmed the framework’s stability and showed that the metal center preserves its oxidation state. The scope and limitations of this unconventional protocol are discussed.