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