Uncovering a copper(II) Alkynyl Complex in C−C Bond Forming Reactions

<p>Copper(II) alkynyl species are proposed as key intermediates in numerous Cu−catalysed 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) <i>tert</i>−butoxide complex [Cu^II]−O<i>^t</i>Bu. In solution, this [Cu^II]−C≡CAr species cleanly transforms the to the Glaser coupling product ArC≡C−C≡CAr and [Cu^I](solvent). Addition of nucleophiles R′C≡CLi (R′ = aryl, silyl) and Ph–Li to [Cu^II]−C≡CAr affords the corresponding C_sp−C_sp and C_sp−C_sp2coupled products RC≡C−C≡CAr and Ph–C≡CAr with concomitant generation of [Cu^I](solvent) and {[Cu^I]−C≡CAr}⁻. Supported by DFT calculations, redox disproportionation forms [Cu^III](C≡CAr)(R) species that reductively eliminate R−C≡CAr products. [Cu^II]−C<a>≡</a>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 catalysed 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 <i>^t</i>BuOO<i>^t</i>Bu as oxidant.</p>