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 improve 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 properties 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 counterparts, whereas the addition of individual MeCN molecules diminishes the propensity toward transmetalation. The quantum 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.