Catalytic Hydrodefluorination of Aromatic Fluorocarbons by Ruthenium N-Heterocyclic Carbene Complexes

The catalytic hydrodefluorination (HDF) of hexafluorobenzene, pentafluorobenzene, and pentafluoropyridine with alkylsilanes is catalyzed by the ruthenium N-heterocyclic carbene (NHC) complexes Ru(NHC)(PPh₃)₂(CO)H₂ (NHC = SIMes (1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) 13, SIPr (1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) 14, IPr (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) 15, IMes (1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) 16). Catalytic activity follows the order 15 > 13 > 16 > 14, with 15 able to catalyze the HDF of C₆F₅H with Et₃SiH with a turnover number of up to 200 and a turnover frequency of up to 0.86 h⁻¹. The catalytic reactions reveal (i) a novel selectivity for substitution at the 2-position in C₆F₅H and C₅F₅N, (ii) formation of deuterated fluoroarene products when reactions are performed in C₆D₆ or C₆D₅CD₃, and (iii) a first-order dependence on [fluoroarene] and zero-order relationship with respect to [R₃SiH]. Mechanisms are proposed for HDF of C₆F₆ and C₆F₅H, the principal difference being that the latter occurs by initial C−H rather than C−F activation.