Dissecting Transmetalation Reactions at the Molecular Level: Aryl Transfer in Copper-Tetraarylborate Complexes

Boron-copper transmetalation reactions are practically important, but their detailed mechanisms remain elusive. To im-prove our understanding of these transformations, we have analyzed the systems Cu+/(BArX4)− (ArX = p-X−C6H4 with X = OMe, Me, H, F, Cl, CF3) by a combination of NMR spectroscopy, ESI-mass spectrometry, gas-phase experiments, and quantum chemical calculations. By probing the gas-phase fragmentation of mass-selected adducts of the type [Cu(BArX4)(BPh4)]− and [(MeCN)2Cu2(BArX4)]+, we obtain intimate insight into the microscopic reactivity of these model complexes. In all cases, transmetalation reactions occur, the relative efficiency of which depends on the electronic prop-erties of the aryl groups, the charge of the complex, and the number of solvent molecules bound to the latter. Specifically, electron-rich aryl groups show a higher tendency toward being transferred to copper than their electron-poor counter-parts, whereas the addition of individual MeCN molecules diminishes the propensity toward transmetalation. The quan-tum chemical calculations are essential for the interpretation of the experimental results by providing structural and thermochemical information. The trends derived from the present gas-phase models promise to help in the mechanistic analysis of boron-copper transmetalation in solution.