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 beta-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) tertbutoxide complex [Cu-II] -(OBu)-Bu-i. In solution, this [Cu-II]-C CAr species cleanly transforms to the Glaser coupling product At-C C-C CAr and [Cu-I](solvent). Addition of nudeophiles 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 center dot 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 'BuOO'Bu as oxidant.