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Catalytic hydrodefluorination of pentafluorobenzene by [Ru(NHC)(PPh3)2(CO)H2]: a nucleophilic attack by a metal-bound hydride ligand explains an unusual ortho-regioselectivity.

The selective synthesis of fluoroarene compounds is a subject of intense current interest, driven by the prominent role such species play in many pharmaceuticals, agrochemicals, and other industrially important products. One attractive route to selectively substituted fluoroarenes involves the activation and functionalization of aromatic C F bonds derived from readily available perfluoroarenes. The simplest example of such a process is the hydrodefluorination reaction (HDF), in which fluorine is substituted for hydrogen. Catalytic HDF of C6F6 and C6F5H has been reported by Milstein et al. [3] and Holland et al. using Rh and Fe catalysts. However, both these systems exhibit practical problems that limit the mechanistic understanding of the HDF cycle. For example the Rh system requires high pressures of H2 as well as a sacrificial amine to remove HF, while with Fe no C F activation is observed in the absence of a reductant. As a consequence, the development of more active Rh or Fe catalysts has not been forthcoming. We recently reported the HDF of C6F6 and C6F5H using the ruthenium N-heterocyclic carbene (NHC) dihydride complex 1 (NHC = IMes, SIMes, IPr, SIPr; see Scheme 1 a) in the presence of trialkylsilanes at 70 8C in THF. Isolation and characterization of 1 allowed detailed kinetic studies to be undertaken, and these supported a mechanism involving initial phosphine dissociation to form 2 followed by HDF of the substrate to give the Ru F species, 3. Isolation of this 16e complex allowed us to demonstrate its reaction with trialkylsilane in the presence of PPh3 to regenerate catalyst 1. The most unusual feature of this system was the high regioselectivity for the formation of 1,2,3,4-C6F4H2 upon HDF of C6F5H, in complete contrast to the Milstein and Holland systems where the 1,2,4,5-isomer dominated. To account for the unusual ortho-regioselectivity we postulated the involvement of a tetrafluorobenzyne intermediate (Scheme 1b). Such species have been reported previously and could be formed here from 2 by successive C H and ortho-C F activation of C6F5H. However, density functional theory (DFT) calculations (with NHC = IMes) have now shown that this species lies more than 200 kJmol 1 above the reactants, effectively ruling it out as a viable intermediate under the conditions used experimentally. Further calculations, however, have now allowed us to define a series of alternative pathways which are based on a novel nucleophilic attack mechanism whereby a hydride ligand reacts directly with C6F5H. [10] These processes produced significantly lower barriers and, moreover, the lowestenergy pathway was found to be consistent with the unusual ortho-regioselectivity observed experimentally. Our calculations have shown that, after initial phosphine loss from 1, nucleophilic attack of hydride at C6F5H can occur through two different pathways (Scheme 2). In the concerted pathway I, the hydride is transfered from the metal onto the arene ring and the displaced fluorine migrates directly onto the metal center. In the alternative stepwise pathway II, an harene adduct, 4, is formed prior to the hydride attack. In this case the different orientation of the arene precludes direct transfer of fluorine onto the metal. Instead an intermediate is formed, 5, from which HF can be lost to form a s-aryl species, 6. Protonolysis by HF with concomitant F transfer to metal then yields 1,2,3,4-C6F4H2 and the M F species 3. The lowest-energy reaction profile for the HDF of C6F5H by 1 to give 1,2,3,4-C6F4H2 is computed to proceed through pathway II, and full details are shown in Figure 1. Initial Scheme 1. a) Catalytic hydrodefluorination (HDF) of C6F5H to 1,2,3,4C6F4H2 by 1; b) postulated tetrafluorobenzyne intermediate.