Gallium Hydrides and O/N-Donors as Tunable Systems in C-F Bond Activation

The gallium hydrides (iBu)(2)GaH (1a), LiGaH4 (1b) and Me3NGaH3 (1c) hydrodefluorinate vinylic and aromatic C-F bonds when O and N donor molecules are present. 1b exhibits the highest reactivity. Quantitative conversion to the hydrodefluorination (HDF) products could be observed for hexafluoropropene and 1,1,3,3,3-pentafluoropropene, 94% conversion of pentafluoropyridine and 49% of octafluorotoluene. Whereas for the HDF with 1b high conversions are observed when catalytic amounts of O donor molecules are added, for 1a, the addition of N donor molecules lead to higher conversions. The E/Z selectivity of the HDF of 1,1,3,3,3-pentafluoropropene is donor-dependent. DFT studies show that HDF proceeds in this case via the gallium hydride dimer-donor species and a hydrometallation/elimination sequence. Selectivities are sensitive to the choice of donor, as the right donor can lead to an on/off switching during catalysis, that is, the hydrometallation step is accelerated by the presence of a donor, but the donor dissociates prior to elimination, allowing the inherently more selective donorless gallium systems to determine the selectivity.