Computational study of the hydrodefluorination of fluoroarenes at [Ru(NHC)(PR₃)<sub style="vertical-align: sub;">2</sub>(CO)(H)<sub style="vertical-align: sub;">2</sub>]: predicted scope and regioselectivities

Density functional theory calculations have been employed to investigate the scope and selectivity of the hydrodefluorination (HDF) of fluoroarenes, C6F6-nHn (n = 0-5), at catalysts of the type [Ru(NHC)(PR3)(2)(CO)(H)(2)]. Based on our previous study (Angew. Chem., Int. Ed., 2011, 50, 2783) two mechanisms featuring the nucleophilic attack of a hydride ligand at a fluoroarene substrate were considered: (i) a concerted process with Ru-H/C-F exchange occurring in one step; and (ii) a stepwise pathway in which the rate-determining transition state involves formation of HF and a Ru-sigma-fluoroaryl complex. The nature of the metal coordination environment and, in particular, the NHC ligand was found to play an important role in both promoting the HDF reaction and determining the regioselectivity of this process. Thus for the reaction of C6F5H, the full experimental system (NHC = IMes, R = Ph) promotes HDF through (i) more facile initial PR3/fluoroarene substitution and (ii) the ability of the NHC N-aryl substituents to stabilise the key C-F bond breaking transition state through F center dot center dot center dot HC interactions. This latter effect is maximised along the lower energy stepwise pathway when an ortho-H substituent is present and this accounts for the ortho-selectivity seen in the reaction of C6F5H to give 1,2,3,4-C6F4H2. Computed C-F bond dissociation energies (BDEs) for C6F6-nHn substrates show a general increase with larger n and are most sensitive to the number of ortho-F substituents present. However, HDF is always computed to remain significantly exothermic when a silane such as Me3SiH is included as terminal reductant. Computed barriers to HDF also generally increase with greater n, and for the concerted pathway a good correlation between C-F BDE and barrier height is seen. The two mechanisms were found to have complementary regioselectivities. For the concerted pathway the reaction is directed to sites with two ortho-F substituents, as these have the weakest C-F bonds. In contrast, reaction along the stepwise pathway is directed to sites with only one ortho-F substituent, due to difficulties in accommodating ortho-F substituents in the C-F bond cleavage transition state. Calculations predict that 1,2,3,5-C6F4H2 and 1,2,3,4-C6F4H2 are viable candidates for HDF at [Ru(IMes)(PPh3)(2)(CO)(H)(2)] and that this would proceed selectively to give 1,2,4-C6F3H3 and 1,2,3-C6F3H3, respectively.</p

Room temperature regioselective catalytic hydrodefluorination of fluoroarenes with trans-[Ru(NHC)4H2] through a concerted nucleophilic Ru−H attack pathway

The authors acknowledge the EPSRC (grant EP/J009962/1, DTA) for financial supportThe efficient and highly selective room temperature hydrodefluorination (HDF) of fluoroarenes by the trans-[Ru(IMe4)4H2] catalyst, 3 , is reported. Mechanistic studies show 3 acts directly in catalysis without any ligand dissociation and DFT calculations indicate a concerted nucleophilic attack mechanism. The calculations fully account for the observed selectivities which corroborate earlier predictions regarding the selectivity of HDF.PostprintPeer reviewe

Unexpected Vulnerability of DPEphos to C-O Activation in the Presence of Nucleophilic Metal Hydrides

C−O bond activation of DPEphos occurs upon mild heating in the presence of [Ru(NHC)2(PPh3)2H2] (NHC=N-heterocyclic carbene) to form phosphinophenolate products. When NHC=IEt2Me2, C−O activation is accompanied by C−N activation of an NHC ligand to yield a coordinated N-phosphino-functionalised carbene. DFT calculations define a nucleophilic mechanism in which a hydride ligand attacks the aryl carbon of the DPEphos C−O bond. This is promoted by the strongly donating NHC ligands which render a trans dihydride intermediate featuring highly nucleophilic hydride ligands accessible. C−O bond activation also occurs upon heating cis-[Ru(DPEphos)2H2]. DFT calculations suggest this reaction is promoted by the steric encumbrance associated with two bulky DPEphos ligands. Our observations that facile degradation of the DPEphos ligand via C−O bond activation is possible under relatively mild reaction conditions has potential ramifications for the use of this ligand in high-temperature catalysis.</p

Synthetic Access to Ring-Expanded N-Heterocyclic Carbene (RE-NHC) Copper Complexes and their Performance in Click Chemistry

The facile syntheses of ring-expanded N-heterocyclic carbene (RE-NHC) copper(I) halide complexes are reported. The method makes use of a weak inorganic base in a green solvent. The reaction times can be greatly reduced by use of this weak-base route under microwave irradiation. The easy access to these complexes permits an evaluation of the catalytic activity and reaction profiling of [Cu(RE-NHC)X] complexes in the Huisgen 1,3-cycloaddition reaction

Heterobimetallic ruthenium–zinc complexes with bulky N-heterocyclic carbenes: syntheses, structures and reactivity

The ruthenium–zinc heterobimetallic complexes, [Ru(IPr)2(CO)ZnMe][BArF4] (7), [Ru(IBiox6)2(CO)(THF) ZnMe][BArF4] (12) and [Ru(IMes)’(PPh3)(CO)ZnMe] (15), have been prepared by reaction of ZnMe2 with the ruthenium N-heterocyclic carbene complexes [Ru(IPr)2(CO)H][BArF4] (1), [Ru(IBiox6)2(CO)(THF)H][BArF4] (11) and [Ru(IMes)(PPh3)(CO)HCl] respectively. 7 shows clean reactivity towards H2, yielding [Ru(IPr)2(CO) (¿2-H2)(H)2ZnMe][BArF4] (8), which undergoes loss of the coordinated dihydrogen ligand upon application of vacuum to form [Ru(IPr)2(CO)(H)2ZnMe][BArF4] (9). In contrast, addition of H2 to 12 gave only a mixture of products. The tetramethyl IBiox complex [Ru(IBioxMe4)2(CO)(THF)H][BArF4] (14) failed to give any isol- able Ru–Zn containing species upon reaction with ZnMe2. The cyclometallated NHC complex [Ru(IMes)’ (PPh3)(CO)ZnMe] (15) added H2 across the Ru–Zn bond both in solution and in the solid-state to afford [Ru(IMes)’(PPh3)(CO)(H)2ZnMe] (17), with retention of the cyclometallati

N-Heterocyclic Carbene Non-Innocence in the Catalytic Hydrophosphination of Alkynes

Studies on alkyne hydrophosphination employing nickel-NHC catalysts (NHC=N-heterocyclic carbene) revealed that the free N-alkyl substituted NHCs themselves were catalytically active. DFT calculations showed the mechanism involves the NHC acting as a Bronsted base to form an imidazolium phosphide species which then undergoes rate-limiting nucleophilic attack at the terminal alkyne carbon. This mechanism explains the preference seen experimentally for reactions with aryl substituted phosphines and alkynes, while the rearrangements of the alkenyl anion formed upon P-C bond formation account for the observation of both Z- and E-regioisomers of the products

Stoichiometric and catalytic C-F bond activation by the<i> trans</i>-dihydride complex [Ru(IEt₂Me₂)₂(PPh₃)₂H₂] (IEt₂Me₂ = 1,3-diethyl-4,5-dimethylimidazol-2-ylidene)

The room temperature reaction of C6F6 or C6F5H with [Ru(IEt2Me2)2(PPh3)2H2] (1; IEt2Me2 = 1,3-diethyl-4,5-dimethylimidazol-2-ylidene) generated a mixture of the trans-hydride fluoride complex [Ru(IEt2Me2)2(PPh3)2HF] (2) and the bis-carbene pentafluorophenyl species [Ru(IEt2Me2)2(PPh3)(C6F5)H] (3). The formation of 3 resulted from C–H activation of C6F5H (formed from C6F6via stoichiometric hydrodefluorination), a process which could be reversed by working under 4 atm H2. Upon heating 1 with C6F5H, the bis-phosphine derivative [Ru(IEt2Me2)(PPh3)2(C6F5)H] (4) was isolated. A more efficient route to 2 involved treatment of 1 with 0.33 eq. of TREAT-HF (Et3N·3HF); excess reagent gave instead the [H2F3]− salt (5) of the known cation [Ru(IEt2Me2)2(PPh3)2H]+. Under catalytic conditions, 1 proved to be an active precursor for hydrodefluorination, converting C6F6 to a mixture of tri, di and monofluorobenzenes (TON = 37) at 363 K with 10 mol% 1 and Et3SiH as the reductant