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

Abstract

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.