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

Abstract

The catalytic hydrodefluorination (HDF) of hexafluorobenzene, pentafluorobenzene, and pentafluoropyridine with alkylsilanes is catalyzed by the ruthenium N-heterocyclic carbene (NHC) complexes Ru(NHC)(PPh(3))(2)(CO)H(2) (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 C(6)F(5)H with Et(3)SiH 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 C(6)F(5)H and C(5)F(5)N, (ii) formation of deuterated fluoroarene products when reactions are performed in C(6)D(6) or C(6)D(5)CD(3), and (iii) a first-order dependence on [fluoroarene] and zero-order relationship with respect to [R(3)SiH]. Mechanisms are proposed for HDF of C(6)F(6) and C(6)F(5)H, the principal difference being that the latter occurs by initial C-H rather than C-F activation.