Abstract
The mechanism of the hydrofluoroarylation of alkynes, RC≡CR, by nickel phosphine complexes, described by Nakao et al. ( Dalton Trans. 2010, 39, 10483), was studied by density functional theory (DFT) calculations. The oxidative addition of a C-H bond of partially fluorinated benzenes, C6FnH6-n (n = 0-5) to a Ni(0) phosphine complex is reversible, but the oxidative addition of a C-F bond yields a stable product via a high-energy barrier. A pathway via the Ni(II) hydride complex is eliminated on the basis of a calculated H/D kinetic isotope effect (KIE) that does not agree with the measured value. An alternate pathway was determined, using as reactant a Ni(phosphine)(alkyne) complex that is shown to be the major species in the reactive media under the catalytic conditions. This pathway is initiated by arene coordination to the Ni alkyne complex followed by proton transfer from the σ-C-H bond of the coordinated arene to the alkyne as the C-H activation step. Analysis of the charge distribution shows that the alkyne is strongly negatively charged when coordinated to the Ni(phosphine) species, which favors a C-H activation as a proton transfer, similar to that in CMD and AMLA but not previously seen between hydrocarbyl ligands for electron rich metals. The C-H activation step thus represents an example of a general class of mechanism that we term ligand-to-ligand hydrogen transfer (LLHT). The product of this reaction is a nickel(vinyl)(aryl) complex, which rearranges to place the aryl and vinyl groups cis to one another before undergoing reductive elimination of the arylalkene. An analysis of the calculated turnover frequencies shows that the rate-determining states that control the energy span are the alkyne complex + free arene and the transition state for the vinyl-aryl complex trans-to-cis rearrangement. The calculated KIE agrees with the observed lack of isotope effect. Analysis of the effects of fluorine substituents shows that the Ni-C(aryl) bond energies control the energy barriers for the arene C-H activation step and the energy spans. A correlation between bond dissociation energies for the Ni-C(aryl) bond and the arene C-H bond follows the behavior presented previously ( J. Am. Chem Soc. 2009, 131, 7817), in which the effects of ortho fluorine substituents are dominant. Consequently, fluorine substitution of the arene, especially at the ortho positions, strengthens the Ni-C bond and increases the TOF. The LLHT mechanism described here may also apply to nickel-catalyzed C-H activation reactions with other substrates.