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

A dimeric Cu­(II) complex [Cu­(II)2L2(μ2-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 CH2Cl2 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–Calkyne) 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.