On the Feasibility of Nickel-Catalyzed Trifluoromethylation of Aryl Halides

Silyl-Osmium(IV)-Trihydride Complexes Stabilized by a Pincer Ether-Diphosphine: Formation and Reactions with Alkynes

Complex OsH₄{κ³-P,O,P-[xant(PⁱPr₂)₂]} (1) activates the Si-H bond of triethylsilane, triphenylsilane, and 1,1,1,3,5,5,5-heptamethyltrisiloxane to give the silyl-osmium(IV)-trihydride derivatives OsH₃(SiR₃){κ³-P,O,P-[xant(PⁱPr₂)₂]} [SiR₃ = SiEt₃ (2), SiPh₃ (3), SiMe(OSiMe₃)₂ (4)] and H₂. The activation takes place via an unsaturated tetrahydride intermediate, resulting from the dissociation of the oxygen atom of the pincer ligand 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene (xant(PⁱPr₂)₂). This intermediate, which has been trapped to form OsH₄{κ²-P,P-[xant(PⁱPr₂)₂]}(PⁱPr₃) (5), coordinates the Si-H bond of the silanes to subsequently undergo a homolytic cleavage. Kinetics of the reaction along with the observed primary isotope effect demonstrates that the Si-H rupture is the rate-determining step of the activation. Complex 2 reacts with 1,1-diphenyl-2-propyn-1-ol and 1-phenyl-1-propyne. The reaction with the former affords Os{C≡CC(OH)Ph₂}₂{=C=CHC(OH)Ph₂}{κ³-P,O,P-[xant(PⁱPr₂)₂]} (6), which catalyzes the conversion of the propargylic alcohol into (E)-2-(5,5-diphenylfuran-2(5H)-ylidene)-1,1-diphenylethan-1-ol, via (Z)-enynediol. In methanol, the hydroxyvinylidene ligand of 6 dehydrates to allenylidene, generating Os{C≡CC(OH)Ph₂}₂{=C=C=CPh₂}{κ³-P,O,P-[xant(PⁱPr₂)₂]} (7). The reaction of 2 with 1-phenyl-1-propyne leads to OsH{κ¹-C,η²-[C₆H₄CH₂CH=CH₂]}{κ³-P,O,P-[xant(PⁱPr₂)₂]} (8) and PhCH₂CH=CH(SiEt₃).

Synthesis, structure, and electrochemical properties of [LNi(Rf)(C4F8)]- and [LNi(Rf)3]- complexes

The new anionic nickelate complexes [(MeCN)Ni(C₄F₈)(CF₃)]^-, [(MeCN)Ni(C₄F₈)(C₂F₅)]^-, [(IMes)Ni(C₄F₈)(CF₃)]^-, [(IMes)Ni(CF₃)₃]^- (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), and [(F-NHC)Ni(R_f)₃]^- (F-NHC = 1,3-bis(2,4-F₂Ph), 2,4,6-F₃Ph- or 3,4,5-F₃Ph)imidazol-2-ylidene; (R_f = CF₃ or C₂F₅) were synthesized and structurally characterized. The electrochemical properties of all new compounds were revealed by cyclic voltammetry studies and compared to the known CF₃ analogue [(MeCN)Ni(CF₃)₃]^-. The IMes-coordinated complexes exhibited initial oxidation events that were well-separated from a second oxidation process in the cyclic voltammograms. The complexes containing F-substituted NHC ligands [(F-NHC)Ni(CF₃)₃]^- are structurally quite similar to the IMes derivative and reveal also two separated oxidation waves in their cyclic voltammograms. The absolute potentials as well as the separation between the two waves vary with the substitution pattern, suggesting that the NHC ligand environment (NHC = N-heterocyclic carbene) is an interesting platform for the development of new redox-triggered reactions that release trifluoromethyl and perfluoroalkyl radicals upon oxidation.

Repercussion of a 1,3-Hydrogen Shift in a Hydride-Osmium-Allenylidene Complex

An unusual 1,3-hydrogen shift from the metal center to the C_β atom of the C₃-chain of the allenylidene ligand in a hydride-osmium(II)-allenylidene complex is the beginning of several interesting transformations in the cumulene. The hydride-osmium(II)-allenylidene complex was prepared in two steps, starting from the tetrahydride dimer [(Os(H···H){κ³-P,O,P-[xant(PⁱPr₂)₂]})₂(μ-Cl)₂][BF₄]₂ (1). Complex 1 reacts with 1,1-diphenyl-2-propyn-1-ol to give the hydride-osmium(II)-alkenylcarbyne [OsHCl(≡CCH=CPh₂){κ³-P,O,P-[xant(PⁱPr₂)₂]}]BF₄ (2), which yields OsHCl(=C=C=CPh₂){κ³-P,O,P-[xant(PⁱPr₂)₂]} (3) by selective abstraction of the C_β–H hydrogen atom of the alkenylcarbyne ligand with K^tBuO. Complex 3 is metastable. According to results of DFT calculations, the migration of the hydride ligand to the C_β atom of the cumulene has an activation energy too high to occur in a concerted manner. However, the migration can be catalyzed by water, alcohols, and aldehydes. The resulting alkenylcarbyne-osmium(0) intermediate is unstable and evolves into a 7:3 mixture of the hydride-osmium(II)-indenylidene OsHCl(=C_IndPh){κ³-P,O,P-[xant(PⁱPr₂)₂]} (4) and the osmanaphthalene OsCl(C₉H₆Ph){κ³-P,O,P-[xant(PⁱPr₂)₂]} (5). Protonation of 4 with HBF₄ leads to the elongated dihydrogen complex [OsCl(η²-H₂)(=C_IndPh){κ³-P,O,P-[xant(PⁱPr₂)₂]}]BF₄ (6), while the protonation of 5 regenerates 2. In contrast to 4, complex 6 evolves to a half-sandwich indenyl derivative, [Os(η⁵-IndPh)H{κ³-P,O,P-[xant(PⁱPr₂)₂]}][BF₄]Cl (7). Phenylacetylene also provokes the 1,3-hydrogen shift in 3. However, it does not participate in the migration. In contrast to water, alcohols, and aldehydes, it stabilizes the resulting alkenylcarbyne to afford [Os(≡CCH=CPh₂)(η²-HC≡CPh){κ³-P,O,P-[xant(PⁱPr₂)₂]}]Cl (8).

Reactions of POP-pincer rhodium(I)-aryl complexes with small molecules: coordination flexibility of the ether diphosphine

Reactions of the aryl complexes Rh(aryl){κ3-P,O,P-[xant(PiPr2)2]} (1; aryl = 3,5-Me2C6H3 (a), C6H5 (b), 3,5-Cl2C6H3 (c), 3-FC6H4 (d); xant(PiPr2)2 = 9,9-dimethyl-4,5-bis-(diisopropylphosphino)xanthene) with O2, CO, and MeO2CC≡CCO2Me have been performed. Under 1 atm of O2, the pentane solutions of complexes 1 afford the dinuclear peroxide derivatives [Rh(aryl){κ2-P,P-xant(PiPr2)2}]2(μ-O2)2 (2a–2d) as yellow solids. In solution, these species are unstable. In dichloromethane, at room temperature, they are transformed into the dioxygen adducts Rh(aryl)(η2-O2){κ3-P,O,P-[xant(PiPr2)2]} (3a–3d), as a result of the rupture of the double peroxide bridge and the reduction of the metal center. Complex 3b decomposes in benzene, at 50 °C, to give diphosphine oxide, phenol, and biphenyl. Complexes 1 react with CO to give the square-planar mono carbonyl derivatives Rh(aryl)(CO){κ2-P,P-[xant(PiPr2)2]} (4a–4d), which under carbon monoxide atmosphere evolve to benzoyl species Rh{C(O)aryl}(CO){κ2-P,P-[xant(PiPr2)2]} (5a–5d), resulting from the migratory insertion of CO into the Rh-aryl bond and the coordination of a second CO molecule. The transformation is reversible; under vacuum, complexes 5 regenerate the precursors 4. The addition of the activated alkyne to complexes 1b and 1d initially leads to the π-alkyne intermediates Rh(aryl){η2-C(CO2Me)≡C(CO2Me)}{κ3-P,O,P-[xant(PiPr2)2]} (6b, 6d), which evolve to the alkenyl derivatives Rh{(E)-C(CO2Me)=C(CO2Me)aryl}{κ3-P,O,P-[xant(PiPr2)2]} (7b, 7d). The diphosphine adapts its coordination mode to the stability requirements of the different complexes, coordinating cis-κ2-P,P in complexes 2, fac-κ3-P,O,P in compounds 3, trans-κ2-P,P in the mono carbonyl derivatives 4 and 5, and mer-κ3-P,O,P in products 6 and 7.

Ranking Ligands by Their Ability to Ease (C6F5)2NiIIL → Ni0L + (C6F5)2 Coupling versus Hydrolysis: Outstanding Activity of PEWO Ligands

The Ni^II literature complex cis-[Ni(C₆F₅)₂(THF)₂] is a synthon of cis-Ni(C₆F₅)₂ that allows us to establish a protocol to measure and compare the ligand effect on the Ni^II → Ni⁰ reductive elimination step (coupling), often critical in catalytic processes. Several ligands of different types were submitted to this Ni-meter comparison: bipyridines, chelating diphosphines, monodentate phosphines, PR₂(biaryl) phosphines, and PEWO ligands (phosphines with one potentially chelate electron-withdrawing olefin). Extremely different C₆F₅-C₆F₅ coupling rates, ranging from totally inactive (producing stable complexes at room temperature) to those inducing almost instantaneous coupling at 25 °C, were found for the different ligands tested. The PR₂(biaryl) ligands, very efficient for coupling in Pd, are slow and inefficient in Ni, and the reason for this difference is examined. In contrast, PEWO type ligands are amazingly efficient and provide the lowest coupling barriers ever observed for Ni^II complexes; they yield up to 96% C₆F₅-C₆F₅ coupling in 5 min at 25 °C (the rest is C₆F₅H) and 100% coupling with no hydrolysis in 8 h at -22 to -53 °C.

A General Rhodium Catalyst for the Deuteration of Boranes and Hydrides of the Group 14 Elements

Pinacolborane, catecholborane, triethylsilane, triphenylsilane, dimethylphenylsilane, 1,1,1,3,5,5,5-heptamethyltrisiloxane, triethylgermane, triphenylgermane, and triphenylstannane deuterated at the heteroatom position have been catalytically prepared in 50-70% isolated yield, through H/D exchange between the D₂ molecule and the respective boranes and hydrides of the group 14 elements, in the presence of the rhodium(I)-monohydride catalyst precursor RhH{κ³-P,O,P-[xant(PⁱPr₂)₂]} (xant(PⁱPr₂)₂ = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene).

Nickel-Mediated Trifluoromethylation of Phenol Derivatives by Aryl C-O Bond Activation

The increasing pharmaceutical importance of trifluoromethylarenes has stimulated the development of more efficient trifluoromethylation reactions. Tremendous efforts have focused on copper- and palladium-mediated/catalyzed trifluoromethylation of aryl halides. In contrast, no general method exists for the conversion of widely available inert electrophiles, such as phenol derivatives, into the corresponding trifluoromethylated arenes. Reported herein is a practical nickel-mediated trifluoromethylation of phenol derivatives with readily available trimethyl(trifluoromethyl)silane (TMSCF₃ ). The strategy relies on PMe₃ -promoted oxidative addition and transmetalation, and CCl₃ CN-induced reductive elimination. The broad utility of this transformation has been demonstrated through the direct incorporation of trifluoromethyl into aromatic and heteroaromatic systems, including biorelevant compounds.

High-Valent NiIII and NiIV Species Relevant to C-C and C-Heteroatom Cross-Coupling Reactions: State of the Art

Ni catalysis constitutes an active research arena with notable applications in diverse fields. By analogy with its parent element palladium, Ni catalysts provide an appealing entry to build molecular complexity via cross-coupling reactions. While Pd catalysts typically involve a M⁰/M^II redox scenario, in the case of Ni congeners the mechanistic elucidation becomes more challenging due to their innate properties (like enhanced reactivity, propensity to undergo single electron transformations vs. 2e⁻ redox sequences or weaker M–Ligand interaction). In recent years, mechanistic studies have demonstrated the participation of high-valent Ni^III and Ni^IV species in a plethora of cross-coupling events, thus accessing novel synthetic schemes and unprecedented transformations. This comprehensive review collects the main contributions effected within this topic, and focuses on the key role of isolated and/or spectroscopically identified Ni^III and Ni^IV complexes. Amongst other transformations, the resulting Ni^III and Ni^IV compounds have efficiently accomplished: i) C–C and C–heteroatom bond formation; ii) C–H bond functionalization; and iii) N–N and C–N cyclizative couplings to forge heterocycles.

Iridium-Promoted B-B Bond Activation: Preparation and X-ray Diffraction Analysis of a mer-Tris(boryl) Complex

The tris(boryl) complex Ir(Bcat)₃{κ³-P,O,P-[xant(PⁱPr₂)₂]} [Bcat = catecholboryl; xant(PⁱPr₂)₂ = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene] has been prepared and characterized by X-ray diffraction analysis. The boryl ligands are disposed in a mer arrangement. The Ir-B bonds situated mutually trans are ∼0.1 Å longer than that disposed cis to the other two. An energy decomposition analysis method coupled to natural orbitals for chemical valence has revealed that the level of π-back-donation from the metal to the p _z atomic orbital of the boron atom decreases ∼43% in the longer bonds with respect to the shorter one, while the level of σ-bonding interaction diminishes by only ∼8%.

Nickel-catalyzed aryl trifluoromethyl sulfide synthesis: a DFT study

A Ni(i)/Ni(iii) catalytic cycle allows aryl trifluoromethyl sulfides to be obtained at room temperature from [NMe₄][SCF₃] and aryl halides.

Modifications to the Aryl Group of dppf-Ligated Ni σ‑Aryl Precatalysts: Impact on Speciation and Catalytic Activity in Suzuki-Miyaura Coupling Reactions

There is currently significant interest in the development of efficient nickel precatalysts for cross-coupling. In this work, 14 nickel(II) precatalysts of the form (dppf)Ni(aryl)(X) (dppf = 1,1'-bis(diphenylphosphino)-ferrocene, X = Cl, Br) were synthesized. In particular, both the electronic and steric properties of the aryl group were modified to understand how this affects precatalyst activation. Using EPR spectroscopy, it was demonstrated that the amount of off-cycle nickel(I) species which are formed via comproportionation during precatalyst activation varies depending on the nature of the aryl group. For example, sterically bulky aryl groups reduce comproportionation. Additionally, the catalytic activity of the family of precatalysts was evaluated in five different Suzuki-Miyaura coupling reactions. The results from these catalytic studies provide information about how precatalyst structure affects catalytic efficiency, which may be useful for the rational design of improved nickel precatalysts for cross-coupling.

Nickel-Alkyl Complexes with a Reactive PNC-Pincer Ligand

Based on previous work related to the design and application of rigid tridentate phosphine-pyridine-phenyl coordination offered by a PNC-pincer ligand upon cyclometalation to nickel, the synthesis, spectroscopic and solid state characterization and redox-reactivity of two NiII(PNC) complexes featuring either a methyl (2CH3 ) or CF3 co-ligand (2CF3 ) are described. One-electron oxidation is proposed to furnish C-C reductive elimination, as deduced from a combined chemical, electrochemical, spectroscopic and computational study. One-electron reduction results in a ligand-centered radical anion, as supported by electrochemistry, UV spectroelectrochemistry, EPR spectroscopy, and DFT calculations. This further attenuates the breadth of chemical reactivity offered by such PNC-pincer ligands.

Featuring Xantphos

This review highlights the use of the bisphosphine ligand group in homogeneous catalysis.

Quantitative DFT modeling of product concentration in organometallic reactions: Cu-mediated pentafluoroethylation of benzoic acid chlorides as a case study

DFT calculations are widely used for computing properties, reaction mechanisms and energy profiles in organometallic reactions. A qualitative agreement between the experimental and the calculated results seems to usually be enough to validate a computational methodology but recent advances in computation indicate that a nearly quantitative agreement should be possible if an appropriate DFT study is carried out. Final percent product concentrations, often reported as yields, are by far the most commonly reported properties in experimental metal-mediated synthesis studies but reported DFT studies have not focused on predicting absolute product amounts. The recently reported stoichiometric pentafluoroethylation of benzoic acid chlorides (R-C₆H₄COCl) with [(phen)Cu(PPh₃)C₂F₅] (phen = 1,10-phenanthroline, PPh₃ = triphenylphosphine) has been used as a case study to check whether the experimental product concentrations can be reproduced by any of the most popular DFT approaches with high enough accuracy. To this end, the Gibbs energy profile for the pentafluoroethylation of benzoic acid chloride has been computed using 14 different DFT methods. These computed Gibbs energy profiles have been employed to build kinetic models predicting the final product concentration in solution. The best results are obtained with the D3-dispersion corrected B3LYP functional, which has been successfully used afterwards to model the reaction outcomes of other simple (R = o-Me, p-Me, p-Cl, p-F, etc.) benzoic acid chlorides. The product concentrations of more complex reaction networks in which more than one position of the substrate may be activated by the copper catalyst (R = o-Br and p-I) are also predicted appropriately.

C-H Bond Trifluoromethylation of Arenes Enabled by a Robust, High-Valent Nickel(IV) Complex

The robust, high-valent Ni^IV complex [(Py)₂ Ni^IV F₂ (CF₃ )₂ ] (Py=pyridine) was synthesized and fully characterized by NMR spectroscopy, X-ray diffraction, and elemental analysis. It reacts with aromatic compounds at 25 °C to form the corresponding benzotrifluorides in nearly quantitative yield. The monomeric and dimeric Ni^III CF₃ complexes 2⋅Py and 2 were identified as key intermediates, and their structures were unambiguously determined by EPR spectroscopy and X-ray diffraction. Preliminary kinetic studies in combination with the isolation of reaction intermediates confirmed that the C-H bond-breaking/C-CF₃ bond-forming sequence can occur both at Ni^IV CF₃ and Ni^III CF₃ centers.

Decarboxylative Trifluoromethylating Reagent [Cu(O2 CCF3 )(phen)] and Difluorocarbene Precursor [Cu(phen)2 ][O2 CCF2 Cl]

This article describes the new economic decarboxylative trifluoromethylating reagent [Cu(phen)(O₂ CCF₃ )] (1; phen=1,10-phenanthroline) and the efficient difluorocarbene precursor [Cu(phen)₂ ][O₂ CCF₂ Cl] (2). Treatment of copper tert-butoxide with phen and subsequent addition of trifluoroacetic acid or chlorodifluoroacetic acid afforded air-stable complexes 1 and 2, respectively, which were characterized by X-ray crystallography. The copper(I) ion in 1 is coordinated by a bidentate phen ligand, a monodentate trifluoroacetate group, and a molecule of CH₃ CN in a distorted tetrahedral coordination geometry. The molecular structure of 2 adopts an ionic form that consists of a [Cu(phen)₂ ]⁺ cation and a chlorodifluoroacetate anion. Complex 1 reacted with a variety of aryl and heteroaryl halides to form trifluoromethyl (hetero)arenes in good yields. The corresponding Hammett plot exhibited a linear relationship and a reaction parameter (ρ)=+0.56±0.02, which indicated that the trifluoromethylation reaction proceeded via a nucleophilic reactive species. Complex 2 reacts with phenols to produce aryl difluoromethyl ethers in modest-to-excellent yields. Mechanistic investigations revealed that the difluoromethylation reaction proceeds by initial copper-mediated formation of difluorocarbene and subsequent concerted addition of difluorocarbene to the phenol to form a three-center transition state.

Oxidation of Ni(II) to Ni(IV) with Aryl Electrophiles Enables Ni-Mediated Aryl-CF3 Coupling

This communication describes the synthesis and reactivity of Ni(IV)(aryl)(CF3)2 complexes supported by trispyrazolylborate and 4,4'-di-tert-butylbipyridine ligands. We demonstrate that isolable Ni(IV) complexes can be accessed under mild conditions via the oxidation of Ni(II) precursors with S-(trifluoromethyl)dibenzothiophenium triflate as well as with diaryliodonium and aryl diazonium reagents. The Ni(IV) intermediates undergo high yielding aryl-CF3 bond-forming reductive elimination. These studies support the potential viability of Ni(IV) intermediates in nickel-catalyzed coupling reactions involving diaryliodonium and aryldiazonium electrophiles.

Nickel-Catalyzed Alkyl-Alkyl Cross-Couplings of Fluorinated Secondary Electrophiles: A General Approach to the Synthesis of Compounds having a Perfluoroalkyl Substituent

Fluorinated organic molecules are of interest in fields ranging from medicinal chemistry to polymer science. Described herein is a mild, convenient, and versatile method for the synthesis of compounds bearing a perfluoroalkyl group attached to a tertiary carbon atom by using an alkyl-alkyl cross-coupling. A nickel catalyst derived from NiCl2 ⋅glyme and a pybox ligand achieves the coupling of a wide range of fluorinated alkyl halides with alkylzinc reagents at room temperature. A broad array of functional groups is compatible with the reaction conditions, and highly selective couplings can be achieved on the basis of differing levels of fluorination. A mechanistic investigation has established that the presence of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) inhibits cross-coupling under these conditions and that a TEMPO-electrophile adduct can be isolated.