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 [Cu^II]-C≡CAr (Ar = 2,6-Cl₂C₆H₃) forms upon reaction of the alkyne H-C≡CAr with the copper(II) tert-butoxide complex [Cu^II]-O^tBu. In solution, this [Cu^II]-C≡CAr species cleanly transforms to the Glaser coupling product ArC≡C-C≡CAr and [Cu^I](solvent). Addition of nucleophiles R'C≡C-Li (R' = aryl, silyl) and Ph-Li to [Cu^II]-C≡CAr affords the corresponding C_sp-C_sp and C_sp-C_sp2 coupled products RC≡C-C≡CAr and Ph-C≡CAr with concomitant generation of [Cu^I](solvent) and {[Cu^I]-C≡CAr}^-, respectively. Supported by density functional theory (DFT) calculations, redox disproportionation forms [Cu^III](C≡CAr)(R) species that reductively eliminate R-C≡CAr products. [Cu^II]-C≡CAr also captures the trityl radical Ph₃C· to give Ph₃C-C≡CAr. Radical capture represents the key C_sp-C_sp3 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 C_sp-C_sp3 coupled products R-C≡CR via radical relay with ^tBuOO^tBu as oxidant.