Hydrodefluorination reactions (HDF) of per- and polyfluorinated olefins and arenes by cheap aluminum alkyl hydrides in non-coordinating solvents can be catalyzed by O and N donors. TONs with respect to the organocatalysts of up to 87 have been observed. Depending on substrate and concentration, high selectivities can be achieved. For the prototypical hexafluoropropene, however, low selectivities are observed (E/Z≈2). DFT studies show that the preferred HDF mechanism for this substrate in the presence of donor solvents proceeds from the dimer Me Al (μ-H) ⋅THF by nucleophilic vinylic substitution (S V)-like transition states with low selectivity and without formation of an intermediate, not via hydrometallation or σ-bond metathesis. In the absence of donor solvents, hydrometallation is preferred but this is associated with inaccessibly high activation barriers at low temperatures. Donor solvents activate the aluminum hydride bond, lower the barrier for HDF significantly, and switch the product preference from Z to E. The exact nature of the donor has only a minimal influence on the selectivity at low concentrations, as the donor is located far away from the active center in the transition states. The mechanism changes at higher donor concentrations and proceeds from Me AlH⋅THF via S V and formation of a stable intermediate, from which elimination is unselective, which results in a loss of selectivity.

We present herein the utilization of NHC-stabilized alane adducts of the type (NHC)·AlH 3 [NHC = Me 2 Im (1), Me 2 Im Me (2), iPr 2 Im (3), iPr 2 Im Me (4), Dipp 2 Im (5)] and (NHC)· AliBu 2 H [NHC = iPr 2 Im (6), Dipp 2 Im (7)] as novel hydride transfer reagents in the hydrodefluorination (HDF) of different fluoroaromatics and hexafluoropropene. Depending on the alane adduct used, HDF of pentafluoropyridine to 2,3,5,6-tetrafluoropyridine in yields of 15-99 % was observed. The adducts 1, 2, and 5 achieved a quantitative conversion into 2,3,5,6-tetrafluoropyridine at room temperature immediately after mixing the reactants. Studies on the HDF of fluorobenzenes with the (NHC)·AlH 3 adducts 1, 3, and 5 and (Dipp 2 Im)·AliBu 2 H (7) showed the decisive influence of the reaction temperature on [a] 4032 Scheme 3. Hydrodefluorination of hexafluorobenzene, pentafluorobenzene, and 1,2,4,5-tetrafluorobenzene using (NHC)·AlH 3 [NHC = Me 2 Im (1), iPr 2 Im (3), Dipp 2 Im (5)] and (Dipp 2 Im)·AliBu 2 H (7)..Eur. J. Inorg. Chem. 2018, 4031-4043 www.eurjic.org

The gallium hydrides (iBu) GaH (1 a), LiGaH (1 b) and Me N⋅GaH (1 c) hydrodefluorinate vinylic and aromatic C-F bonds when O and N donor molecules are present. 1 b 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 1 b high conversions are observed when catalytic amounts of O donor molecules are added, for 1 a, 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.

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Cp3Ln (Ln = Ce, Nd, Sm, Er, Yb) are applied as precatalysts in the presence of LiAlH4 for the C–F bond activation of hexafluoropropene, 1,1,3,3,3‐pentafluoropropene, trifluoropropene, chlorotrifluoroethene, and octafluorotoluene. 100 % conversion and TONs up to 155 could be observed for the hydrodefluorination reaction (HDF). For chlorotrifluoroethene hydrodefluorination occurs with high chemoselectivity favoring the C–F bond activation versus C–Cl bond activation.