Three-Coordinate Copper(II) Alkynyl Complex in C–C Bond Formation: The Sesquicentennial of the Glaser Coupling

Copper­(II) alkynyl species are proposed as key intermediates in numerous Cu-catalyzed C–C coupling reactions. Supported by a β-diketiminate ligand, the three-coordinate copper­(II) alkynyl [CuII]–CCAr (Ar = 2,6-Cl2C6H3) forms upon reaction of the alkyne H–CCAr with the copper­(II) tert-butoxide complex [CuII]–OtBu. In solution, this [CuII]–CCAr species cleanly transforms to the Glaser coupling product ArCC–CCAr and [CuI]­(solvent). Addition of nucleophiles R′CC–Li (R′ = aryl, silyl) and Ph–Li to [CuII]–CCAr affords the corresponding Csp–Csp and Csp–Csp2 coupled products RCC–CCAr and Ph–CCAr with concomitant generation of [CuI]­(solvent) and {[CuI]–CCAr}−, respectively. Supported by density functional theory (DFT) calculations, redox disproportionation forms [CuIII]­(CCAr)­(R) species that reductively eliminate R–CCAr products. [CuII]–CCAr also captures the trityl radical Ph3C· to give Ph3C–CCAr. Radical capture represents the key Csp–Csp3 bond-forming step in the copper-catalyzed C–H functionalization of benzylic substrates R–H with alkynes H–CCR′ (R′ = (hetero)­aryl, silyl) that provide Csp–Csp3 coupled products R–CCR via radical relay with tBuOOtBu as oxidant.