Three-Coordinate Copper(II) Alkynyl Complex in C–C Bond Formation: The Sesquicentennial of the Glaser Coupling
Abolghasem Bakhoda, Otome E. Okoromoba, Christine Greene, Mahdi Raghibi Boroujeni, Jeffery A. Bertke, Timothy H. Warren
Abstract
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.