Elementary Reactions at Organocopper(III): A Gas-Phase and Theoretical Study

The role of copper­(III) in copper-catalyzed
coupling reactions
is a topic of much debate in the literature, in large part due to
the difficulty in isolating the typically reactive species. Advanced
mass spectrometry experiments allow the isolation and interrogation
of reactive species in the absence of any solvent, counterions, or
competing species. Macrocyclic aryl-X-copper­(III) complexes were isolated
in the gas phase and subjected to collision-induced dissociation experiments
to examine their unimolecular reactivity both qualitatively and quantitatively.
When X = Cl or Br, the complexes fragment solely by deprotonation
of nitrogen and concomitant loss of HX. The experimentally determined
energies of activation are 33.4 ± 0.9 kcal mol–1 (X = Cl) and 35.8 ± 0.9 kcal mol–1 (X = Br).
This process is analogous to nucleophile activation at a copper­(III)
center, and it is observed preferentially over C-X reductive elimination
in the gas phase due to the strong ligating ability of the employed
macrocyclic ligand. The measured activation energies for the observed
nucleophile activation were used to test the performance of a range
of popular DFT functionals for predicting reactivity at CuIII. Most dispersion-corrected functionals reproduced the experimental
results with reasonable errors (D3bj-corrected TPSSh performed best),
whereas the uncorrected values tended to significantly underestimate
the activation energies. When X = I, a second fragmentation pathway
becomes competitive, which involves loss of I• and
reduction of copper from CuIII to CuII. The
relative energetics of one-electron versus two-electron processes
at CuIII are discussed.