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<sup>II</sup>]−C≡CAr (Ar = 2,6−Cl<sub>2</sub>C<sub>6</sub>H<sub>3</sub>) forms upon reaction of the alkyne H−C≡CAr with the copper(II) <i>tert</i>−butoxide complex [Cu<sup>II</sup>]−O<i><sup>t</sup></i>Bu. In solution, this [Cu<sup>II</sup>]−C≡CAr species cleanly transforms the to the Glaser coupling product ArC≡C−C≡CAr and [Cu<sup>I</sup>](solvent). Addition of nucleophiles R′C≡CLi (R′ = aryl, silyl) and Ph–Li to [Cu<sup>II</sup>]−C≡CAr affords the corresponding C<sub>sp</sub>−C<sub>sp</sub> and C<sub>sp</sub>−C<sub>sp2</sub>coupled products RC≡C−C≡CAr and Ph–C≡CAr with concomitant generation of [Cu<sup>I</sup>](solvent) and {[Cu<sup>I</sup>]−C≡CAr}<sup>−</sup>. Supported by DFT calculations, redox disproportionation forms [Cu<sup>III</sup>](C≡CAr)(R) species that reductively eliminate R−C≡CAr products. [Cu<sup>II</sup>]−C<a>≡</a>CAr also captures the trityl radical Ph<sub>3</sub>C• to give Ph<sub>3</sub>C−C≡CAr. Radical capture represents the key C<sub>sp</sub>−C<sub>sp3</sub> 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<sub>sp</sub>−C<sub>sp3</sub> coupled products R−C≡CR via radical relay with <i><sup>t</sup></i>BuOO<i><sup>t</sup></i>Bu as oxidant.</p>