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.