On the Feasibility of Nickel-Catalyzed Trifluoromethylation of Aryl

Article pubs.acs.org/Organometallics

On the Feasibility of Nickel-Catalyzed Trifluoromethylation of Aryl Halides Jesús Jover,† Fedor M. Miloserdov,† Jordi Benet-Buchholz,† Vladimir V. Grushin,*,† and Feliu Maseras*,†,‡ †

Institute of Chemical Research of Catalonia (ICIQ), Avgda. Països Catalans, 16, 43007 Tarragona, Catalonia, Spain Departament de Química, Universitat Autònoma de Barcelona, 08197 Bellaterra, Catalonia, Spain

‡

S Supporting Information *

ABSTRACT: A computational screening of 42 bidentate phosphines (PP) has yielded promising candidates for Ph−CF3 reductive elimination from Ni(II) complexes of the type [(PP)Ni(Ph)(CF3)]. The computed barriers and synthetic accessibility considerations have identified two PP ligands, dippf and dcypf (ΔG⧧ = 22.6 and 23.2 kcal/mol, respectively), for experimental studies with 1-Np (1naphthyl) in place of Ph. Ligand exchange of [(Ph3P)2Ni(1-Np)Cl] with dippf or dcypf has cleanly produced [(dippf)Ni(1-Np)Cl] and [(dcypf)Ni(1-Np)Cl], the first examples of trans square-planar 1,1′-ferrocenediyl backbone-based diphosphine metal complexes devoid of M···Fe dative interactions. Treatment of these chlorides with CF3SiMe3/F−, AgCF3/MeCN or [(Ph3P)3Cu(CF3)] does not furnish isolable or 19F NMR-detectable [(PP)Ni(1-Np)(CF3)] (PP = dippf, dcypf). Other transformations have been observed instead, e.g., ligand exchange with the Ag and Cu complexes, the latter leading to [(dcypf)Cu(CF3)], a rare example of well-defined CF3Cu(I) species. With CF3SiMe3/F−, indirect evidence has been obtained for intermediacy of [(PP)Ni(1-Np)(CF3)] (PP = dippf, dcypf) and instantaneous decomposition via pathways other than C−CF3 reductive elimination. The first Ni(II) complexes with fluoride trans to a non-electron-deficient aryl, [(Cy3P)2Ni(1-Np)F] and [(i-PrXantphos)Ni(1-Np)F], have been prepared and fully characterized. Surprisingly, [(Cy3P)2Ni(1-Np)F] can be produced from [(Cy3P)2Ni(1-Np)Cl] and CsF rather than AgF that is conventionally used for the synthesis of late transition metal fluorides via X/F exchange. While [(Cy3P)2Ni(1-Np)F] is unreactive toward CF3SiMe3, [(i-PrXantphos)Ni(1-Np)F] is readily trifluoromethylated to produce robust [(i-PrXantphos)Ni(1-Np)(CF3)], a rare example of complexes of the type [(PP)Ni(Ar)(CF3)] with PP other than dippe.

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INTRODUCTION Aromatic and heteroaromatic compounds bearing a trifluoromethyl (CF3) group on the ring are important building blocks and intermediates for the synthesis of agrochemicals, pharmaceuticals, and specialty materials.1−3 Metal-catalyzed/ mediated coupling of aryl halides with CF3 nucleophiles (eq 1)

Scheme 1. Catalytic Cycle for Pd-Catalyzed Trifluoromethylation of Aryl Halides

is currently viewed3,4 as an alternative to the environmentally unfriendly and limited in scope Swarts-reaction-based old technology to produce trifluoromethylated aromatics. Since the pioneering work of McLoughlin and Thrower5 in the 1960s, considerable progress has been made in the area of Cupromoted/catalyzed trifluoromethylation of haloarenes.2,3,6−10 Apart from Cu, two more metals, Pd and Ni, are widely used in a diversity of coupling reactions of aryl halides.11 A decade ago, one of us12−14 explored the possibility of Pd-catalyzed trifluoromethylation of haloarenes to show that Ar−CF3 reductive elimination from Pd(II) is the most problematic elementary step of the proposed catalytic cycle (Scheme 1). For example, [(PP)Pd(Ph)(CH3)] (PP = dppe, dppp) undergo reductive elimination of toluene at 15−40 °C,16 whereas their © XXXX American Chemical Society

CF3 congeners produce PhCF3 in low to modest yield only after many hours at 145 °C.13 This contrast is in full accord with the previously recognized15 exceptional strength and inertness of the late transition metal−perfluoroalkyl bonds. In 2006, however, it was reported14a that [(Xantphos)Pd(Ph)(CF3)] undergoes reductive elimination of PhCF3 in a highly selective manner under mild conditions (50−80 °C). Since the discovery of that first example of clean and facile Ar−CF3 reductive elimination from Pd, only three more ligands have been found to promote this transformation, BrettPhos,17 Received: August 27, 2014

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dx.doi.org/10.1021/om5008743 | Organometallics XXXX, XXX, XXX−XXX

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RuPhos,17,18 and (CF3)2PCH2CH2P(CF3)2.19 Of these four ligands, only BrettPhos and RuPhos have been found17 suitable for Pd-catalyzed trifluoromethylation of haloarenes (aryl chlorides). Even less progress has been made toward Ar−CF3 bond formation at Ni. Perfluoroalkyl complexes of nickel are rare in general.20,21 There has been only one type of CF3Ni aryls reported in the literature. Vicic and co-workers21 have prepared [(dippe)Ni(Ar)(CF3)] (dippe =1,2-bis(diisopropylphosphino)ethane; Ar = Ph, 3-Tol, 2-naphthyl, 4-anisyl) and demonstrated21a that these complexes produce Ar2 and [(dippe)Ni(CF3)2] but not ArCF3 upon heating. The formation of the corresponding trifluoromethylarenes in 11−22% yield was observed only when the decomposition was performed in the presence of PhZnBr, ZnBr2, or water.21a In the current work, we have attempted to identify ligands that would promote Ar−CF3 reductive elimination from Ni(II). Toward this goal, we have employed a rather nontraditional methodology by first performing a broad screening of ligands by computational means, followed by experimental work. This combined study has produced a number of valuable insights pertaining to organometallic fluorine chemistry in general and Ni-catalyzed trifluoromethylation of aryl halides in particular.

in this database containing information for a total of 275 ligands. Analysis of the resulting LKB-PPscreen database with principal component analysis (PCA) captures the effects of changing backbones and substituents on ligand properties and allows the grouping of ligands according to their chemical similarity. For the current study, we selected 35 of those ligands arising from the combination of seven different backbones (Scheme 3) Scheme 3. Backbones Selected for the Current Study

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COMPUTATIONAL STUDIES Knowledge-Guided Computational Search of Efficient Ligands. The experimental work by Vicic and co-workers21 has proven that complexes with an aryl and CF3 on Ni(II) are accessible, at least with dippe as the ancillary ligand. We have probed the experimentally unsuccessful Ar−CF3 reductive elimination from [(dippe)Ni(Ph)(CF3)]21a by DFT calculations (Scheme 2) to compute a prohibitively high free energy barrier of 36.3 kcal/mol.

with five P-substituents: methyl (Me), tert-butyl (t-Bu), phenyl (Ph), trifluoromethyl (CF3), and pentafluorophenyl (C6F5), aiming to cover the maximum ligand space. Note that the abbreviations presented in Scheme 3 refer to the backbones, not particular ligands. Selected similar ligands containing i-Pr on P were also included in the final set due to dippe being the supporting ligand for the only synthesized and fully characterized CF3Ni aryls.21 Although diphosphines bearing iPr on the P atoms were not originally in LKB-PPscreen, they could be easily imported from the data available in the LKB-PP database. A complete description of the ligand selection procedure and database expansion is detailed in the Supporting Information. The selected diphosphines are highlighted on the LKBPPscreen map presented in Figure 1, showing the final set spreading across most areas in the ligand space. The map was generated by PCA24 and contains useful information regarding ligand similarities (represented as proximity between the ligands on the map) and trends observed when systematic changes in the ligand structure are applied. The two first principal components contain a combination of electronic (M− L and σ-bonding descriptors) and steric effects (bite angles and the He8wedge and nHe8 parameters), which place the ligands on the southwest−northeast diagonal of the map. The He8wedge and nHe8 descriptors are steric measures that capture the interaction energy between a given ligand and a “wedge” of eight helium atoms placed in approximate ligand positions of an octahedral complex. The electron-rich ligands take up the top right side of the map (t-Bu, filled squares), while the electronpoor diphosphines are located in the southwest sector (CF3, triangles). Likewise, the smaller ligands are in the southeast area (dpm, blue markers), whereas the bulkier ones can be found in the northwest zone (Xantphos and dpf, black and red markers).

Scheme 2. Computed Reductive Elimination Barriers for (PP)Ni(Ph)CF3 Complexes

Modifications in bidentate phosphine ligands (PP) might yield lower barriers for reductive elimination from their complexes. It is widely known that both steric and electronic parameters determine key ligand properties including their ability to promote reductive elimination reactions.22 In general, electron-deficient diphosphines with longer backbones and bulkier substituents tend to lower reductive elimination barriers. To identify bidentate phosphines that can facilitate the desired Ar−CF3 bond formation at Ni(II), we employed one of the University of Bristol Ligand Knowledge Bases23 that provide a fast and rational way to select ligands, ensuring the right modeling for a given process. In the current work, we have used the LKB-PPscreen database,23d which has been constructed to account for systematic variations on the backbone and Psubstituents of cis-chelating diphosphine ligands. Altogether, 25 different backbones and 11 substituents covering most of the steric and electronic features of the ligands have been included B


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Figure 1. Final ligand selection projected on the LKB-PPscreen map.

The dippe ligand used for the synthesis of [(dippe)Ni(Ph)(CF3)]21a lies on the middle right side of the map (orange empty square), close to the small methyl-based electron-rich ligands such as 1,3-bis(dimethylphosphino)propane (dmpp) and 1,4-bis(dimethylphosphino)butane (dmpb). All of these ligands are expected to produce high Ph−CF3 reductive elimination barriers that should lower when moving to the left side of the map. The computed ΔG⧧ values for the Ph−CF3 reductive elimination from various complexes of the type [(PP)Ni(Ph)(CF3)] are presented in Table 1 and projected on the LKB-

Figure 2. Projected Ph−CF3 reductive elimination barriers (in kcal/ mol) on the LKB-PPscreen map with the lighter tones corresponding to lower barriers.

and, more importantly, would render the Ni(0) center incapable of activating the haloarene substrate via Ar−X oxidative addition.25 Another option is electron-rich bulky diphosphines that generally lead to intermediate barriers, e.g., 21.7 and 24.1 kcal/mol computed for the i-Pr- and t-Busubstituted dpb, dpx, and dpf species. Some of such complexes are expected to be reasonably accessible by synthesis. Although Xantphos-based ligands could provide one of the best platforms for Ar−CF3 reductive elimination from Pd(II),14 this is not the case with nickel because the aryl and the CF3 in such complexes are computed to be mutually trans, thereby impeding the reductive elimination. For the complexes with the rest of the ligands, both the cis and trans isomers were computed to ensure the lowest energy conformer was used to compute the reductive elimination. The cis complexes were found to be more stable in all cases. Although the barrier for the tBuXantphos complex was also studied, we were unable to reach convergence for the reductive elimination transition state. On the basis of the computational studies, two nonfluorinated ligands were identified as the most promising candidates: 1,4-bis(di-tert-butylphosphino)butane (dtbpb, ΔG⧧ = 21.7 kcal/mol) and 1,1′-bis(diisopropylphosphino)ferrocene (dippf, ΔG⧧ = 22.6 kcal/mol). These candidates were proposed for the experimental work reported in the following section. The synthetic studies, however, were not limited to these two specific ligands. Other similar yet more synthetically accessible diphosphines were also explored, even though they were originally absent from the computational database used.

Table 1. Reductive Elimination Barriers (in kcal/mol) backbone

a

R

dpm

dpe

Dpp

dpb

dpx

dpf

Xantphos

Me i-Pr t-Bu Ph CF3 C6F5

36.3 38.3 36.9 33.8 22.7 27.0

38.4 36.3 29.3 34.5 20.5 27.0

33.2 31.1 26.5 28.3 14.6 23.6

32.1 26.9 21.7 30.3 12.7 23.8

29.1 24.1 23.6 28.8 12.3 25.4

29.8 22.6 23.1 27.0 12.4 17.5

31.1 32.0 −a 28.9 19.0 17.4

The transition state for this species could not be correctly located.

PPscreen map in Figure 2. As can be seen from Figure 2, the barrier changes in the range >35 to 95% conversion after 1 h to give [(dcypf)Cu(CF3)] (ca. 50% yield) and a nickel fluoride (ca. 30% yield), which we formulate as [(Ph3P)2Ni(1Np)F] on the basis of the NMR data (19F: −336.9 ppm, t, JP−F = 45 Hz; 31P: 14.7 ppm, d, JP−F = 45 Hz). This reaction, however, did not involve CF3 transfer from Cu to Ni but rather resulted in phosphine ligand exchange leading to [(dcypf)Cu(CF3)]. The latter was also prepared independently from [(Ph3P)3Cu(CF3)] and dcypf (eq 3), isolated, and fully characterized. The 19F and 31P NMR spectra of [(dcypf)Cu(CF3)] displayed a triplet (−25.7 ppm, JP−F = 17 Hz) and a quartet (−7.7 ppm, JP−F = 17 Hz), respectively, in full accord with the solid-state structure obtained by single-crystal X-ray diffraction (Figure 5). No reaction occurred between [(dcypf)Ni(1-Np)Cl] and [(dcypf)Cu(CF3)] in C6D6 at room temperature. At 50 °C, only a slow reaction was observed, giving rise to CHF3, [(dcypf)Ni(1-Np)F] (see above), and minute quantities of unidentified species, likely perfluoroalkyl Ni and Cu derivatives (19F NMR). Trifluoromethylnaphthalene was not produced.

Figure 4. ORTEP drawings of trans-[(dcypf)Ni(1-Np)Cl]·2THF (left) and trans-[(dippf)Ni(1-Np)Cl]·C6H6 (right) with the THF and C6H6 molecules and all H atoms omitted for clarity and thermal ellipsoids drawn at the 50% probability level.

free ligand at −9.3 ppm and appearance of a new resonance at 23.1 ppm, which was assigned to [(dcypf)Ni(COD)].34 The treatment of the latter (generated in situ) with 1-chloronaphthalene at 50 °C for 15 h produced [(dcypf)Ni(1-Np)Cl] in 98% yield (31P NMR). Reactions of [(dcypf)Ni(1-Np)Cl] and [(dippf)Ni(1Np)Cl] with CF3 Sources. Having prepared and characterized [(PP)Ni(1-Np)Cl] (PP = dcypf, dippf), we attempted their transformation to the desired trifluoromethyl complexes [(dippf)Ni(1-Np)(CF3)] and [(dcypf)Ni(1-Np)(CF3)]. The treatment of the chloro complexes with CF3SiMe3 in the presence of CsF in THF12−14 led to a complex reaction mixture not containing the targeted product in 19F NMR-detectable quantities. After 21 h of reaction of [(dcypf)Ni(1-Np)Cl] with CF3SiMe3 (3 equiv) and CsF (3 equiv) in THF at room temperature, ca. 70% conversion of the nickel complex was observed (31P NMR). This reaction gave rise to a species (ca. 25% yield) displaying a doublet (JP−F = 39 Hz) at −1.1 ppm and a broadened singlet at −365.5 ppm in the 31P and 19F NMR spectra, respectively. The 19F NMR singlet, however, resolved into a triplet with the same coupling constant of 39 Hz after stirring the mixture with freshly added CsF and CF3SiMe3 for 20 h at room temperature. We therefore assigned those signals to [(dcypf)Ni(1-Np)F]. In the 31P NMR spectrum, an unassigned broadened resonance at ca. 23.8 ppm (ca. 20%) and a singlet at −9.3 ppm (free dcypf; ca. 20%) were also observed. In addition to the aforementioned triplet at −365.5 ppm, the 19 F NMR spectrum displayed resonances at −416.4 and −419.8 ppm (apparently from other Ni−F species), −157.6 ppm (FSiMe3), −78.8 ppm (CHF3), and numerous unidentified low-intensity peaks in the −49 to −84 ppm region. Importantly, small quantities (ca. 2%) of C2F5H (−139.4 ppm, d, JF−H = 52 Hz, 2F; −86.3 ppm, s, 3F) were also detected. The reaction of [(dippf)Ni(1-Np)Cl] with CF3SiMe3 in the presence of CsF in THF occurred similarly. After 17 h, the formation of ca. 10−15% of [(dippf)Ni(1-Np)F] (31P NMR δ: 5.7 ppm, d; 19F NMR δ: −362.0 ppm, t, JP−F = 39 Hz) was observed at 75% conversion. The formation of Ni−F complexes and C2F5H suggested that NiCF3 species might have been produced as unstable intermediates that underwent α-Felimination to give difluorocarbene species. GC-MS analysis of the reaction mixture revealed the formation of binaphthyl and naphthalene. Fluoronaphthalene, trifluoromethylnaphthalene, and pentafluoroethylnaphthalene were not detected. E


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or even with CsF (50 °C) in THF (eq 4). This fluoride complex is exclusively trans in solution (31P NMR δ: 17.6, d, JP−F = 45 Hz; 19F NMR δ: −368.7, t, JP−F = 45 Hz) and in the solid state (Figure 6). As expected,12 a number of Ni−F···H−C

Figure 5. ORTEP drawing of [(dcypf)Cu(CF3)] with all H atoms omitted and thermal ellipsoids drawn at the 50% probability level.

Stirring [(dcypf)Ni(1-Np)Cl] with AgCF3 in MeCN for 10 min produced a yellow precipitate that was poorly soluble in CH2Cl2, MeCN, benzene, and DMF. The 31P NMR spectrum of the supernatant displayed a number of weak broad signals. Two resonances at −23.4 and −26.6 ppm that could be assigned to Ni−CF3 species were observed in the 19F NMR spectrum. As the peak at −23.4 ppm disappeared, the one at −26.6 ppm grew in intensity. Importantly, both Ni−CF3 resonances were singlets, suggesting that the dcypf ligand and Cl originally on Ni had been transferred to Ag to give [(dcypf)AgCl] (which precipitated out). The liquid phase would then contain CF3 nickel species that we tentatively formulate as [(MeCN)2Ni(1-Np)(CF3)] and [(MeCN)2Ni(CF3)2(1-Np)]−. Heating an aliquot of this solution under argon at 60 °C for 24 h did not produce 1-NpCF3. Unexpectedly, however, exposure of another aliquot of this solution to air produced 1-NpCOF (ca. 4%) and 1-NpCF3 (ca. 2%) after 3 h. In order to find out if the formation of these products was caused by oxygen or moisture, another portion of the solution was treated with deaerated water and another one with dry air. While neither 1-NpCF3 nor 1-NpCOF was produced upon addition of O2-free water,37 dry air prompted the formation of both. Apparently, oxidatively induced reductive elimination from Ni38 led to the formation of 1NpCF3. The other product, 1-NpCOF, was likely formed via αF-elimination that competed with the C−CF3 reductive elimination from phosphine-free Ni(III). After the facile hydrolysis of the resultant difluorocarbene on Ni to CO with adventitious water and subsequent migratory insertion, 1NpCO-F reductive elimination would produce the acid fluoride final product. This reaction pathway to ArCOF has been established for phosphine-stabilized CF3Pd aryls.12,35a,c CF3 and F Ni Complexes with Cy3P and i-PrXantphos Ancillary Ligands. As mentioned above, little is known about tertiary phosphine-stabilized nickel aryls bearing a CF3 ligand,21 and only one such complex, [(dippe)Ni(2-Np)(CF3)], has been structurally characterized.21a The poor stability and apparent propensity of [(dcypf)Ni(1-Np)(CF3)] and [(dippf)Ni(1-Np)(CF3)] to undergo α-F-elimination (see above) prompted us to attempt the synthesis of CF3Ni aryls with other phosphine ligands. First, we found that [(Cy3P)2Ni(1Np)Cl], preisolated or generated in situ from [(Ph3P)2Ni(1Np)Cl] and Cy3P,27 could be converted to [(Cy3P)2Ni(1Np)F] upon treatment with [Me4N]+F− (room temperature)

Figure 6. ORTEP drawing of trans-[(Cy3P)2Ni(1-Np)F] showing intermolecular F···H−C contacts, with thermal ellipsoids drawn at the 50% probability level.

contacts shorter than the sum of the van der Waals radii of H and F (2.67 Å) were found in the crystal structure of [(Cy3P)2Ni(1-Np)F]. These contacts include two intermolecular interactions of the F ligand with the C−H bonds of the σnaphthyl in the 4 (2.38 Å) and 5 (2.43 Å) positions and four F···H−C contacts involving the cyclohexyl rings on the P centers (2.56, 2.59, 2.61, and 2.63 Å). Unfortunately, yet unsurprisingly,12 the thermal decomposition of [(Cy3P)2Ni(1-Np)F] did not result in C−F bond formation. The lack of reactivity of this Ni(II) fluoride toward CF3SiMe3, however, was totally unexpected, considering our experience in the successful preparation of Pd−CF3,12−14 Cu− CF3,7d and Rh−CF339 complexes from the corresponding fluorides. No reaction was observed between [(Cy3P)2Ni(1Np)F] and CF3SiMe3 (5 equiv) in THF at room temperature. At 60 °C, CHF3 and FSiMe3 were produced in a sluggish reaction as the only 19F NMR-observable species. Attempts to enhance the reactivity of the Ni−F bond toward CF3SiMe3 with such additives as KHF2 (30 mol %), [Me4N]+F− (30 mol %), or t-BuOH (2 equiv) were unsuccessful. In contrast, [(i-PrXantphos)Ni(1-Np)F] smoothly reacted with CF3SiMe3 to give [(i-PrXantphos)Ni(1-Np)(CF3)] quantitatively. First, the fluoride was prepared from the reaction of [(Ph3P)2Ni(1-Np)Cl] with i-PrXantphos and [Me4N]+F− in THF. This fluoro complex was isolated in 66% yield and found to be trans in the solid state and in solution (31P NMR δ: 6.7 ppm, d, JP−F = 41 Hz; 19F NMR δ: −372.5 ppm, br s). The two structurally characterized solvates of [(iPrXantphos)Ni(1-Np)(F)], one with a water molecule and one with a molecule of CH2Cl2 (Figure 7), display, in addition to conventionally observed12,40 F···HC contacts, stronger Hbonding interactions with the molecules of CH2Cl2 and H2O. Being a much stronger hydrogen bond donor, water forms a considerably shorter contact to the fluorine (F···HOH = 1.74 Å) than dichloromethane (F···HCHCl2 = 2.05 Å). As a result, the Ni−F bond in the H2O adduct (1.923(4) Å) is longer than in the dichloromethane solvate (1.868(2) Å), which naturally F


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Figure 7. ORTEP drawing of trans-[(i-PrXantphos)Ni(1-Np)F]·H2O and trans-[(i-PrXantphos)Ni(1-Np)F]·CH2Cl2 with thermal ellipsoids drawn at the 50% probability level.

Figure 8. ORTEP drawing of trans-[(i-PrXantphos)Ni(1-Np)(CF3)] with the solvent molecules and all H atoms omitted and thermal ellipsoids drawn at the 50% probability level.

translates into the shorter Ni−C bond distance trans to F··· HOH (1.848(6) Å) than to F···HCHCl2 (1.896(3) Å). The fluoride [(i-PrXantphos)Ni(1-Np)F] appeared to be remarkably thermally stable, displaying no signs of decomposition at 110 °C in xylenes. Only at 140 °C did sluggish decomposition begin to reach full conversion after 3 days. No C−F bond formation took place, and the only signal observed in the 19F NMR spectrum of the resultant solution was a doublet at −94.8 ppm with a large coupling constant of 1036 Hz, pointing to the characteristic12 P−F bond formation. Other products of the thermal decomposition of [(i-PrXantphos)Ni(1-Np)F] included naphthalene, binaphthyl, and 9,9-dimethylxanthene (GC-MS). Unlike [(Cy3P)2Ni(1-Np)F], which appeared unreactive toward Ruppert’s reagent (see above), [(i-PrXantphos)Ni(1Np)F] readily reacted with CF3SiMe3 to give [(i-PrXantphos)Ni(1-Np)(CF3)]. The latter was also more conveniently prepared in 53% isolated yield via a one-pot method from [(Ph3P)2Ni(1-Np)Cl], i-PrXantphos, CsF, and CF3SiMe3 in THF (Scheme 5). Like its Pd analogue,14b [(i-PrXantphos)Ni(1-Np)(CF3)] was found to be trans in the solid state (Figure 8) and in solution (31P NMR δ: 18.3 ppm, q, JP−F = 14 Hz; 19F NMR δ: −14.0 ppm, t, JP−F = 14 Hz). The decomposition of [(i-PrXantphos)Ni(1-Np)(CF3)] in xylenes at 120 °C was slow (16 h) and poorly selective, not giving rise to 1-NpCF3. All signals in the 19F NMR spectrum of the reaction mixture were broadened, probably because of the formation of paramagnetic Ni species. The presence of the previously observed doublet at −95 ppm with JP−F = ca. 1050 Hz (see above) and a Ni−F resonance at −390 ppm pointed to α-F-elimination and subsequent P−F bond formation. Additional support for this reaction pathway was provided by the detection (19F NMR and GC-MS) of small quantities (ca. 5%) of difluoromethylnaphthalene that must have been produced from a difluorocarbene intermediate. The lack of C−CF3 bond formation in this reaction is consistent with the high computed

barriers of 47.7 and 32.0 kcal/mol to Ar−CF3 reductive elimination from the Ni center of [(i-PrXantphos)Ni(Ar)(CF3)] for Ar = 1-Np and Ph, respectively. It is noteworthy that a closely related palladium complex, [(i-PrXantphos)Pd(Ph)(CF3)], does produce PhCF3 on heating at 100 °C, albeit slowly and with modest selectivity.14b

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DISCUSSION Using the LKB-PPscreen database,23d we have studied by computational means 42 bidentate phosphines as ancillary ligands (PP) for Ph−CF3 reductive elimination from complexes of the type [(PP)Ni(Ph)(CF3)]. Remarkably low barriers of ca. 15−18 kcal/mol have been computed for this process for certain diphosphines PP bearing CF3 or C6F5 groups on the phosphorus atoms. Experimental work toward the synthesis of such electron-deficient phosphines and the corresponding nickel complexes was not pursued, however, because Ni(0) bearing such ligands is unlikely to undergo Ar−X oxidative addition, the first key step in Ni-catalyzed coupling reactions of aryl halides.25 Apart from the fluorinated phosphines, the most promising ligand, dtbpb (21.7 kcal/mol), was disregarded due to the propensity of long-backbone diphosphines to form oligomeric metal complexes. We finally settled on the next two candidates, [(dippf)Ni(Ph)(CF3)] and [(dcypf)Ni(Ph)(CF3)], with the computationally predicted Ph−CF3 reductive elimination barriers of 22.6 and 23.2 kcal/mol, respectively. The experimental work was done, however, with σ-1-naphthyl rather than σ-phenyl complexes because of the easy accessibility and enhanced stability of the starting material, Cassar’s catalyst [(Ph3P)2Ni(1-Np)Cl]. The use of the naphthyl derivatives was justified by the similar ΔG⧧ values computed for Ar = 1-Np and Ph, 23.2 vs 22.6 and 27.7 vs 23.2 kcal/mol for dippf and dcypf, respectively. The synthesis of [(PP)Ni(1-Np)(CF3)] (PP = dcypf, dippf), however, appeared to be a much greater challenge than originally expected. Although both [(dcypf)Ni(1-Np)Cl] and

Scheme 5. Synthesis of [(i-PrXantphos)Ni(1-Np)(CF3)]

G


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PrXantphos)Ni(1-Np)F] toward CF3SiMe3. Numerous reported7d,12,14,39,45 examples of efficient trifluoromethylation of a transition metal−fluorine bond with CF3SiMe3 suggested that both Ni fluorides should undergo this transformation as easily. Surprisingly, however, trans-[(Cy3P)2Ni(1-Np)F] appeared to be unreactive toward CF3SiMe3 even at elevated temperatures and even in the presence of such promoters as KHF2, [Me4N]+F−, and t-BuOH. While the mechanism of F/CF3 exchange between transition metal fluorides and CF3SiMe3 remains unknown, coordination of the fluoride ligand on the metal to the Si atom is likely the initiation step of the CF3 transfer. We believe that the reaction of CF3SiMe3 with trans[(Cy3P)2Ni(1-Np)F] bearing two bulky phosphines might be precluded by steric constraints. In contrast, less crowded trans[(i-PrXantphos)Ni(1-Np)F] readily reacted with CF3SiMe3 to give trans-[(i-PrXantphos)Ni(1-Np)(CF3)], which, in full accord with the theoretical predictions, is reluctant to undergo C−CF3 reductive elimination.

[(dippf)Ni(1-Np)Cl] were easily prepared from [(Ph3P)2Ni(1Np)Cl] and the corresponding diphosphine, numerous attempts to trifluoromethylate the Ni−Cl bond with CF3SiMe3/CsF, AgCF3/MeCN, or [(Ph3P)3Cu(CF3)] were unsuccessful. With the Ag and Cu reagents, facile phosphine ligand transfer to the coinage metal occurred. The resultant copper complex, [(dcypf)Cu(CF3)] (Figure 5), a rare example of adequately characterized well-defined CF3Cu(I) species,7c−e,41 was isolated but failed to trifluoromethylate [(dcypf)Ni(1-Np)Cl]. Nonselective reactions occurred upon treatment of [(dcypf)Ni(1-Np)Cl] or [(dippf)Ni(1-Np)Cl] with CF3SiMe3/CsF to give naphthalene, binaphthyl, and Ni−F complexes that likely emerged via α-F-elimination from unstable NiCF3 intermediates. The poorly stable fluorides [(PP)Ni(1-Np)F] (PP = dippf, dcypf) generated in situ from [Me4N]+F− and the corresponding Ni chloro complex readily reacted with CF3SiMe3. However, no formation of the desired complexes [(PP)Ni(1Np)(CF3)] was observed (19F NMR); naphthalene and binaphthyl but not trifluoromethylnaphthalene were again detected in the reaction solutions. This outcome parallels the recently reported20l formation of biphenyl but not benzotrifluoride in the reactions of [(Ph3P)2Ni(O2CCF3)(CF3)] with PhMgBr, PhLi, or Ph2Zn in an attempt to prepare [(Ph3P)2Ni(Ph)(CF3)]. The lack of C−CF3 formation from [(PP)Ni(1Np)(CF3)] (PP = dippe, dcype) is likely due to the existence of less energetically demanding pathways. In addition to the aforementioned α-F-elimination, these processes likely involve transmetalation leading to [(PP)Ni(CF3)2] (detected in some cases) and [(PP)Ni(1-Np)2] that reductively eliminates binaphthyl. Single electron transfer prompting radical processes to give naphthalene are also conceivable. A detailed study of such side transformations, however, was beyond the scope of the current project. To gain deeper insight into trifluoromethyl Ni(II) aryls, we have explored the possibility of preparing [(PP)Ni(1-Np)(CF3)], where PP = (Cy3P)2 or i-PrXantphos. Although these studies have not yielded a CF3−Ar bond forming system, interesting results have been obtained that certainly merit discussion. Both 1-naphthyl Ni(II) fluorides prepared and isolated in the current work, trans-[(Cy3P)2Ni(1-Np)F] and trans-[(i-PrXantphos)Ni(1-Np)F], represent the first examples of fully characterized fluoro nickel aryls lacking electronwithdrawing substituents on the aromatic ring. Numerous complexes of the type trans-[(R3P)2Ni(Ar)F] reported in the literature have been synthesized via Ar−F oxidative addition of polyfluorinated (hetero)arenes to R3P-stabilized Ni(0),42 including for R = Cy.43 The enhanced electron-deficiency/πacidity of the σ-(hetero)aryl trans to the fluoro ligand in the resultant d8 square-planar Ni(II) complex is expected12,35c,44 to contribute considerably to the stabilization of the Ni−F bond. Since this stabilizing factor is strongly diminished for Ni(II) complexes with F trans to a conventional σ-aryl such as Ph or Np, it was not clear at all if trans-[(Cy3P)2Ni(1-Np)F] and trans-[(i-PrXantphos)Ni(1-Np)F] could exist. As we have found in the current work, not only can they be generated and detected, but are also isolable in good yield for full characterization. Another remarkable finding is the formation of [(Cy3P)2Ni(1-Np)F] from the corresponding chloride and CsF in THF, as in the vast majority of cases AgF must be used in order to exchange a heavier halide on a late transition metal center for fluoride. Particularly striking is the difference in the reactivity of trans-[(Cy 3 P) 2 Ni(1-Np)F] and trans-[(i-

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CONCLUSIONS All data accrued in the current study prompt us to look at the development of Ni-catalyzed nucleophilic trifluoromethylation of aryl halides as a big challenge. The DFT computational screening of a broad variety of bidentate phosphines as auxiliary ligands for Ar−CF3 reductive elimination from Ni(II) has identified a number of viable candidates. Two of them, dippf and dcypf, were selected for experimental work on the basis of the computed barriers and synthetic accessibility considerations. However, the seemingly straightforward synthesis of [(PP)Ni(Ar)(CF3)] (PP = dippf, dcypf; Ar = 1-Np) has failed after numerous attempts using three different nucleophilic CF3transferring reagents under a variety of conditions. In a way, the outcome of our studies echoes the famous Jagger−Richards line “You can’t always get what you want, but if you try sometime you just might find you get what you need”.46 We note that this is frequently the case with innovative science explorations in general and organometallic fluorine research in particular.12 Although Ar−CF3 reductive elimination from Ni(II) still remains unachieved, our work on the current project has yielded the following key learnings: 1. While in theory some CF3Ni(II) aryls might have low barriers to the desired Ar−CF3 reductive elimination (e.g., [(dippf)Ni(Ph)(CF 3 )] and [(dcypf)Ni(Ph)(CF3)]), other lower activation energy pathways exist for the same systems, as evidenced by experimental facts. 2. Nucleophilic trifluoromethylation of the Ni−X bond, the second elementary step in the proposed catalytic loop (shown for Pd in Scheme 1), may represent a much bigger problem than originally thought. 3. At least in some cases, Ni(II) fluoro complexes can be formed from the corresponding heavier halides upon treatment with CsF rather than with AgF, which is conventionally used for the synthesis of late transition metal fluorides via X/F exchange. 4. Ni(II) complexes with fluoride trans to a non-electrondeficient aryl can exist and be stable. 5. 1,1′-Ferrocenediyl backbone-based diphosphine ligands can coordinate to Ni(II) in a trans manner without Ni··· Fe dative interactions. 6. Complexes of the type [(PP)Ni(Ar)(CF3)] can be prepared for diphosphines PP other than dippe. H


Organometallics

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Article

Cy2P), 0.31 (m, 2H, Cy2P), 0.60 (m, 2H, Cy2P), 0.76 (m, 2H, Cy2P), 0.84−1.93 (m, 32H, Cy2P), 3.01 (br m, 2H, CH in Cy2P), 3.20 (br m, 2H, CH in Cy2P), 4.18 (m, 2H, Cp), 4.23 (m, 2H, Cp), 4.34 (m, 2H, Cp), 4.63 (m, 2H, Cp), 7.20 (m, 1H, C10H7Ni), 7.30 (m, 1H, C10H7Ni), 7.41 (d, J = 8.0, Hz 2H, C10H7Ni), 7.57 (m, 2H, C10H7Ni), 7.64 (d, J = 8.1 Hz, 2H, C10H7Ni), 11.10 (d, J = 8.1 Hz, 1H, C10H7Ni). 31 1 P{ H} NMR (C6D6, 162 MHz), δ: −5.5 (s, major isomer, 96%), −9.2 (s, minor isomer, ca. 4%).33 X-ray quality single crystals of trans[(dcypf)Ni(1-Np)Cl]·2THF were obtained by slow evaporation of its solution in THF (Figure 4). [(dippf)Ni(1-Np)Cl]. In a glovebox, a mixture of [(Ph3P)2Ni(1Np)Cl] (224 mg, 0.30 mmol), dippf (151 mg, 0.36 mmol), and benzene (6 mL) was agitated with a Teflon-coated magnetic stir-bar for 0.5 h to produce small needle-shaped crystals. Extra benzene (50 mL) was added to dissolve the crystals, and the reaction mixture was filtered through a pad of Celite. The flask was rinsed with benzene (3 × 10 mL), and the washings were filtered through the same pad. After evaporation of the combined filtrate and the washings to ca. 3 mL with a flow of argon and addition of n-hexane (5 mL), the precipitated crystals were separated, washed with n-hexane (3 × 3 mL), and dried under vacuum. The yield of [(dippf)Ni(1-Np)Cl] was 175 mg (91%). Anal. Calcd for C32H43ClFeNiP2: C, 60.1; H, 6.8. Found: C, 59.7; H, 6.6. 1H NMR (C6D6, 400 MHz), δ: 0.00 (q, J = 7.9 Hz, 6H, CH3 in iPr2P), 0.75 (q, J = 5.9 Hz, 6H, CH3 in i-Pr2P), 1.01 (q, J = 6.5 Hz, 6H, CH3 in i-Pr2P), 1.59 (q, J = 8.3 Hz, 6H, CH3 in i-Pr2P), 1.82 (m, 2H, CH in i-Pr2P), 3.43 (m, 2H, CH in i-Pr2P), 4.17 (m, 2H, Cp), 4.20 (m, 2H, Cp), 4.31 (m, 2H, Cp), 4.57 (m, 2H, Cp), 7.22 (t, J = 7.4 Hz, 1H, C10H7Ni), 7.33 (t, J = 7.4 Hz, 1H, C10H7Ni), 7.47 (m, 2H, C10H7Ni), 7.66 (m, 2H, C10H7Ni), 11.26 (d, J = 8.3 Hz, 1H, C10H7Ni). 31P{1H} NMR (C6D6, 162 MHz), δ: 1.3 (s). X-ray quality single crystals of trans-[(dippf)Ni(1-Np)Cl]·C6D6 precipitated out of the NMR sample solution in the tube after 3 days at room temperature (Figure 4). [(Cy3P)2Ni(1-Np)F]. (A) In a glovebox, a 50 mL screw-cap FEP tube was charged with a Teflon-coated magnetic stir-bar, [(PPh3)2Ni(1-Np)Cl] (373 mg, 0.50 mmol), PCy3 (280 mg, 1.00 mmol), and THF (20 mL). After stirring for 5 min, the starting complex had completely dissolved due to the formation of [(Cy3P)2Ni(1-Np)Cl]. After the addition of NMe4+F− (65 mg, 0.7 mmol) and agitation for 20 h complete conversion of [(Cy3P)2Ni(1-Np)Cl] (s, 14.4 ppm) to [(Cy3P)2Ni(1-Np)F] (d, 17.6 ppm, J = 45 Hz) was observed by 31P NMR. The fluoro complex was isolated in the glovebox. The reaction mixture was diluted with THF (35 mL) and filtered through a pad of Celite. The reaction tube was rinsed with THF (2 × 10 mL), and the washings were filtered through the same Celite pad. After evaporation of the combined filtrate and the washings with a flow of argon to ca. 5 mL, the mixture was kept at −32 °C for 1 day to complete the precipitation. The yellow solid was separated, washed with Et2O (3 × 5 mL), and dried under vacuum to give 298 mg of crude [(Cy3P)2Ni(1-Np)F]. For further purification, the crude product was dissolved in benzene (15 mL) at ca. 70 °C and filtered hot through a pad of Celite, which was then washed with hot benzene (2 × 3 mL). The combined filtrate and the washings were allowed to cool to room temperature and treated with ether (15 mL). The yellow microcrystalline solid was separated, washed with ether (2 × 5 mL), and dried under vacuum. The yield of the thus isolated [(Cy3P)2Ni(1Np)F] containing ca. 0.4 molecule of C6H6 per Ni (1H NMR) was 230 mg (58%). Anal. Calcd for C46H73FNiP2·0.4C6H6: C, 72.9; H, 9.5. Found: C, 72.8; H, 9.2. 1H NMR (THF-d8, 500 MHz), δ: 0.75−2.19 (m, 6PCy3, 66H), 6.86 (t, J = 7.5 Hz, 1H, C10H7Ni), 7.08 (d, J = 7.9 Hz, 1H, C10H7Ni), 7.23 (t, J = 7.4 Hz, 1H, C10H7Ni), 7.30 (s, C6H6), 7.37 (t, J = 7.5 Hz, 1H, C10H7Ni), 7.44 (d, J = 8.1 Hz, 1H, C10H7Ni), 7.68 (d, J = 6.8 Hz, 1H, C10H7Ni), 9.87 (d, J = 8.3 Hz, 1H, C10H7Ni). 31 1 P{ H} NMR (THF-d8, 203 MHz), δ: 17.6 (d, JP−F = 45 Hz). 19F NMR (THF-d8, 377 MHz), δ: −368.7 (t, JP−F = 45 Hz). X-ray quality single crystals of [(Cy3P)2Ni(1-Np)F] were grown by slow evaporation of its solution in THF inside the glovebox (Figure 6). (B) In a glovebox, a 20 mL vial was charged with a Teflon-coated magnetic stir-bar, [(Cy3P)2Ni(1-Np)Cl] (50 mg, 0.06 mmol), PCy3 (18 mg, 0.06 mmol), CsF (58 mg, 0.38 mmol), and THF (4 mL). The vial was sealed and brought out. After agitation for 24 h at 60 °C (oil

EXPERIMENTAL SECTION

Anhydrous NiCl2 (98% purity) was purchased from Aldrich. 1Chloronaphthalene (Aldrich, technical grade) was found by 1H NMR to contain 3% of 2-chloronaphthalene. Zn dust (≥98% purity; 120° and no M−Fe bond are rare.31f−h Interestingly, the obtuse P−Ni−P angle in the structures of [(dcypf)Ni(1-Np)Cl] and [(dippf)Ni(1-Np)Cl] is accommodated by means of tilting of the Cp rings rather than their rotation. This is manifested by the Cp(centroid)−Fe−Cp(centroid) angles of 171° and 173° and the P−Cp(centroid)−Cp(centroid)−P torsion angles of 3.8° and 0.6° (nearly ideal eclipsed conformation) in the structures of [(dcypf)Ni(1-Np)Cl] and [(dippf)Ni(1-Np)Cl], respectively. (b) For the first example, see: Seyferth, D.; Hames, B. W.; Rucker, T. G.; Cowie, M.; Dickson, R. S. Organometallics 1983, 2, 472. (c) For a general review, see: Gramigna, K. M.; Oria, J. V.; Mandell, C. L.; Tiedemann, M. A.; Dougherty, W. G.; Piro, N. A.; Kassel, W. S.; Chan, B. C.; Diaconescu, P. L.; Nataro, C. Organometallics 2013, 32, 5966. (d) For examples of Ni ferrocene-based complexes with a Ni−Fe bond, see: Takemoto, S.; Kuwata, S.; Nishibayashi, Y.; Hidai, M. Inorg. Chem. 1998, 37, 6428. (e) For examples of Ni ruthenocene-based complexes with a Ni−Ru bond, see: Akabori, S.; Kumagai, T.; Shirahige, T.; Sato, S.; Kawazoe, K.; Tamura, C.; Sato, M. Organometallics 1987, 6, 2105. (f) Krafft, M. J.; Bubrin, M.; Paretzki, A.; Lissner, F.; Fiedler, J.; Záliš, S.; Kaim, W. Angew. Chem., Int. Ed. 2013, 52, 6781. (g) Reddy, N. D.; Fanwick, P. E.; Walton, R. A. Inorg. Chem. 2001, 40, 1732. (h) Kawano, H.; Nishimura, Y.; Onishi, M. Dalton Trans. 2003, 1808. (32) In principle, this could also be attributed to cis−trans isomerization that is still fast on the NMR time scale at −90 °C. This is unlikely, however, considering the computed difference in energy between the cis and trans isomers (5.0 kcal/mol at −90 °C). (33) For all isolated samples of [(dcypf)Ni(1-Np)Cl], the 31P NMR spectra displayed a low-intensity peak at −9.3 ppm (ca. 4%) in addition to the main singlet resonance at −5.4 ppm. In the 1H NMR spectra, weak extra resonances were also observed at 9.13 (d, J = 8.3 Hz) and 9.29 (d, J = 7.1 Hz) ppm (C6D6). These small signals could

be assigned to [(dcypf)Ni(2-Np)Cl]. The latter might originate from the use of conventionally available 1-chloronaphthalene of technical grade (≥85% from Aldrich) containing the 2-isomer for the synthesis of [(Ph3P)2Ni(1-Np)Cl], the starting material for the preparation of the dcypf complex. However, the purified [(Ph3P)2Ni(1-Np)Cl] used in the synthesis of [(dcypf)Ni(1-Np)Cl] displayed only one singlet in the 31P NMR spectrum, suggesting the absence of [(Ph3P)2Ni(2-Np) Cl]. It is therefore more plausible that the extra 31P NMR signal at −9.3 ppm observed for the isolated [(dcypf)Ni(1-Np)Cl] is from the nucleus of the uncoordinated P atoms of [(dcypf-κ1P)2Ni(1-Np)Cl] and/or oligomeric [(μ-dcypf)Ni(1-Np)Cl]n, whose other signals overlap with the intense singlet from [(dcypf)Ni(1-Np)Cl]. Indeed, on cooling the sample of [(dcypf)Ni(1-Np)Cl] to −90 °C (THF-d8), the minor 31P NMR peak at −9.3 ppm was replaced with five weak singlet resonances at 55.9, −5.5, −5.8, −8.5, and −9.0 ppm. A set of two doublets that would be expected from cis-[(dcypf)Ni(1-Np)Cl] was not observed within the temperature range used (25−−90 °C). The formation of [(PP-κ1P)2Ni(1-Np)Cl] and/or oligomeric [(μ-PP) Ni(1-Np)Cl]n is believed to be less likely for PP = dippf than for bulkier PP = dcypf. (34) (a) The formation of [(dppf)Ni(COD)] from [Ni(COD)2] and dppf has been reported.34b,c Reactions of [Ni(COD)2] with ferrocene-derived tertiary phosphine ligands have been used for the synthesis of acrylic acid and ethylene Ni complexes34d. (b) van Soolingen, J.; Brandsma, L. European Patent EP0613720, 1994. (c) Grushin, V.; Casalnuovo, A. L. U.S. Patent 8212075, 2012. (d) Langer, J.; Fischer, R.; Görls, H.; Walther, D. Eur. J. Inorg. Chem. 2007, 2257. (35) (a) Fraser, S. L.; Antipin, M. Yu.; Khroustalyov, V. N.; Grushin, V. V. J. Am. Chem. Soc. 1997, 119, 4769. (b) Pilon, M. C.; Grushin, V. V. Organometallics 1998, 17, 1774. (c) Grushin, V. V. Chem.Eur. J. 2002, 8, 1006. (d) Grushin, V. V.; Marshall, W. J. Angew. Chem., Int. Ed. 2002, 41, 4476. (e) Marshall, W. J.; Grushin, V. V. Organometallics 2003, 22, 555. (f) Grushin, V. V.; Marshall, W. J. Organometallics 2007, 26, 4997. (g) Grushin, V. V.; Marshall, W. J. J. Am. Chem. Soc. 2009, 131, 918. (36) (a) The use of AgCF 3 36b in the preparation of bis(trifluoromethyl)nickel complexes has been reported20j. (b) Tyrra, W. E. J. Fluorine Chem. 2001, 112, 149. (37) After the addition of deaerated water, a precipitate formed and the 19F NMR signals at −23.4 and −26.6 ppm were replaced with broad weaker resonances at −27.3 ppm (Ni−CF3) and −182 ppm (hydrated fluoride). No CHF3 was produced. (38) For selected examples of oxidatively induced reductive elimination from Ni, see: (a) Koo, K.; Hillhouse, G. L. Organometallics 1995, 14, 4421. (b) Koo, K.; Hillhouse, G. L. Organometallics 1996, 15, 2669. (c) Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc. 2001, 123, 4623. (d) Lin, B. L.; Clough, C. R.; Hillhouse, G. L. J. Am. Chem. Soc. 2002, 124, 2890. (e) Higgs, A. T.; Zinn, P. J.; Simmons, S. J.; Sanford, M. S. Organometallics 2009, 28, 6142. (f) Higgs, A. T.; Zinn, P. J.; Sanford, M. S. Organometallics 2010, 29, 5446. (39) (a) Goodman, J.; Grushin, V. V.; Larichev, R. B.; Macgregor, S. A.; Marshall, W. J.; Roe, D. C. J. Am. Chem. Soc. 2009, 131, 4236. (b) Goodman, J.; Grushin, V. V.; Larichev, R. B.; Macgregor, S. A.; Marshall, W. J.; Roe, D. C. J. Am. Chem. Soc. 2010, 132, 12013. (40) Grushin, V. V.; Marshall, W. J. Angew. Chem., Int. Ed. 2002, 41, 4476. (41) (a) Dubinina, G. G.; Furutachi, H.; Vicic, D. A. J. Am. Chem. Soc. 2008, 130, 8600. (b) Dubinina, G. G.; Ogikubo, J.; Vicic, D. A. Organometallics 2008, 27, 6233. (42) (a) Fahey, D. R.; Mahan, J. E. J. Am. Chem. Soc. 1977, 99, 2501. (b) Clot, E.; Eisenstein, O.; Jasim, N.; Macgregor, S. A.; McGrady, J. E.; Perutz, R. N. Acc. Chem. Res. 2011, 44, 333 and references therein. (43) Kanyiva, K. S.; Kashihara, N.; Nakao, Y.; Hiyama, T.; Ohashi, M.; Ogoshi, S. Dalton Trans. 2010, 39, 10483. (44) (a) Caulton, K. G. New J. Chem. 1994, 18, 25 and references therein. (b) Gil-Rubio, J.; Weberndörfer, B.; Werner, H. J. Chem. Soc., Dalton Trans. 1999, 1437. (c) Mezzetti, A.; Becker, C. Helv. Chim. Acta 2002, 85, 2686. L


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(45) (a) Huang, D.; Caulton, K. G. J. Am. Chem. Soc. 1997, 119, 3185. (b) Vicente, J.; Gil-Rubio, J.; Bautista, D. Inorg. Chem. 2001, 40, 2636. (c) Taw, F. L.; Scott, B. L.; Kiplinger, J. L. J. Am. Chem. Soc. 2003, 125, 14712. (d) Vicente, J.; Gil-Rubio, J.; Guerrero-Leal, J.; Bautista, D. Organometallics 2004, 23, 4871. (e) Taw, F. L.; Clark, A. E.; Mueller, A. H.; Janicke, M. T.; Cantat, T.; Scott, B. L.; Hay, P. J.; Hughes, R. P.; Kiplinger, J. L. Organometallics 2012, 31, 1484. (f) Bramananthan, N.; Carmona, M.; Lowe, J. P.; Mahon, M. F.; Poulten, R. C.; Whittlesey, M. K. Organometallics 2014, 33, 1986. (46) Quoted from the lyrics to the Jagger−Richards song “You Can’t Always Get What You Want” on the Rolling Stones LP album “Let it Bleed” originally released on the Decca (U.K.) and London (U.S.A.) labels in 1969. (47) Kolomeitsev, A. A.; Seifert, F. U.; Röschenthaler, G.-V. J. Fluorine Chem. 1995, 71, 47. (48) Asensio, G.; Cuenca, A. B.; Esteruelas, M. A.; Medio-Simón, M.; Oliván, M.; Valencia, M. Inorg. Chem. 2010, 49, 8665. (49) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J. F., Fox, D. J. Gaussian09, Revision B.01; Gaussian, Inc.: Wallingford, CT, 2009. (50) (a) Becke, A. D. J. Chem. Phys. 1997, 107, 8554. (b) Grimme, S. J. Comput. Chem. 2006, 27, 1787. (51) (a) Hariharan, P. C.; Pople, J. A. Theor. Chem. Acc. 1973, 28, 213. (b) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. V. R. J. Comput. Chem. 1983, 294. (c) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80, 3265. (52) (a) Dunning, T. H.; Hay, P. J. In Modern Theoretical Chemistry, Vol. 3; Schaefer, H. F., III, Ed.; Plenum Press: New York, 1976; p 1. (b) Bergner, A.; Dolg, M.; Küchle, W.; Stoll, H.; Preuss, H. Mol. Phys. 1993, 80, 1431. (53) (a) Tannor, D. J.; Marten, B.; Murphy, R.; Friesner, R. A.; Sitkoff, D.; Nicholls, A.; Honig, B.; Ringnalda, M.; Goddard, W. A. J. Am. Chem. Soc. 1994, 116, 11875. (b) Marten, B.; Kim, K.; Cortis, C.; Friesner, R. A.; Murphy, R. B.; Ringnalda, M. N.; Sitkoff, D.; Honig, B. J. Phys. Chem. 1996, 100, 11775. (54) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2009, 113, 6378.

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On the Feasibility of Nickel-Catalyzed Trifluoromethylation of Aryl

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