Dissecting\nTransmetalation Reactions at the Molecular\nLevel: Aryl Transfer in Copper-Tetraarylborate Complexes

Boron–copper\ntransmetalation reactions are practically important,\nbut their detailed mechanisms remain elusive. To improve our understanding\nof these transformations, we have analyzed the systems Cu<sup>+</sup>/(BAr<sup>X</sup><sub>4</sub>)<sup>−</sup> (Ar<sup>X</sup> = <i>p</i>-X–C<sub>6</sub>H<sub>4</sub> with X\n= OMe, Me, H, F, Cl, CF<sub>3</sub>) by a combination of NMR spectroscopy,\nESI-mass spectrometry, gas-phase experiments, and quantum chemical\ncalculations. By probing the gas-phase fragmentation of mass-selected\nadducts of the type [Cu(BAr<sup>X</sup><sub>4</sub>)(BPh<sub>4</sub>)]<sup>−</sup> and [(MeCN)<sub>2</sub>Cu<sub>2</sub>(BAr<sup>X</sup><sub>4</sub>)]<sup>+</sup>, we obtain intimate insight into\nthe microscopic reactivity of these model complexes. In all cases,\ntransmetalation reactions occur, the relative efficiency of which\ndepends on the electronic properties of the aryl groups, the charge\nof the complex, and the number of solvent molecules bound to the latter.\nSpecifically, electron-rich aryl groups show a higher tendency toward\nbeing transferred to copper than their electron-poor counterparts,\nwhereas the addition of individual MeCN molecules diminishes the propensity\ntoward transmetalation. The quantum chemical calculations are essential\nfor the interpretation of the experimental results by providing structural\nand thermochemical information. The trends derived from the present\ngas-phase models promise to help in the mechanistic analysis of boron–copper\ntransmetalation in solution.