Bis(Cyclooctadiene)Nickel(0)-Catalyzed Exhaustive C(sp2) and C(sp3)-X Hydrodehalogenation of Pyridines and Arenes Employing Ammonia Borane and Potassium Tert-Butoxide

The hydrodefluorination (HDF) of CF3 groups and the hydrodehalogenation (HDH) of C(sp2)-X (X = F, Cl, Br) bonds in pyridines was conducted employing [Ni(COD)2] (COD = 1,5-cyclooctadiene) as catalyst and NH3BH3 as the H source in the presence of KOtBu at room temperature. The system was found efficient for the exhaustive hydrodefluorination of CF3 groups and of C(sp2)-X bonds at the 2, 3 or 4-positions of pyridines and for the selective HDH of C(sp2)-X bonds of trifluoromethylpyridines while retaining the CF3 group. Additionally, the system also showed a good efficiency for the C(sp2)-X (X = F, Cl, Br) HDH of halogenated pyridines without CF3 groups and arenes with low to moderate halogenation degree (1-4 halogens). Mechanistic studies allowed to identify three simultaneous HDF cycles operative in the HDF of 2-trifluoromethylpyridine with the first cycle being the rate-determining and the reaction of the [Ni(COD)2] precatalyst with the H source as the entry pathway to the cycle.