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.