Ruthenium-hydride species of the type [Ru(NHC)(PPh3)2(CO)H2] (NHC = N-heterocyclic carbene = IMes, SIMes, IPr, SIPr) catalyze the hydrodefluorination (HDF) of C6F5H at 70�C with an unusual ortho-selectivity to give 1,2,3,4-C6F4H2. Density functional theory (DFT) calculations showed that C-F activation occurs via a novel nucleophilic attack mechanism whereby a hydride ligand reacts directly with the fluoroarene substrate. Two pathways have been computed: Pathway I, a one step process featuring concerted Ru-H/C-F exchange; and Pathway II which proceeds via initial formation of an ?2-arene adduct prior to the hydride attack. From this intermediate, formation of HF and a Ru-s-fluoroaryl species occurs, corresponding to the rate-limiting transition state. Protonolysis by HF with concomitant F transfer to metal then yields 1,2,3,4-C6F4H2 and Ru-F, as observed experimentally. Pathway II accounts for the unusual ortho-selectivity if this HDF process.This argument implies that other M-H systems that undergo C-F bond activation may proceed via nucleophilic attack by the hydride, and therefore differ from traditional oxidative addition or electron transfer mechanisms.Another ruthenium-hydride complex, cis-[Ru(dmpe)H2] (dmpe = Me2PCH2CH2PMe2) has shown unusual selectivity for aromatic C-F bond activation over C-H bond activation. For instance, cis-[Ru(dmpe)H2] reacts with C6F5H well below room temperature (-78�C) to generate the C-F activated tetrafluorophenyl hydride complex, trans-[Ru(dmpe)(C6F4H)H], and HF. The nature of this unusual C-F bond insertion will be discussed in terms of our nucleophilic attack mechanism. In addition, it should be noted that the use of NHC ligand gives a Ru-F species whereas the dmpe ligands affords a Ru-fluoroaryl complex.

C-F bond activation of fluoroaromatics at Ru-hydride complexes: Theory explains the unusual selectivity

248th National Meeting (2014)

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