Research Article pubs.acs.org/acscatalysis
Mild and Efficient Ni-Catalyzed Biaryl Synthesis with Polyfluoroaryl Magnesium Species: Verification of the Arrest State, Uncovering the Hidden Competitive Second Transmetalation and LigandAccelerated Highly Selective Monoarylation Junya Wang, Ge Meng, Kun Xie, Liting Li, Huaming Sun, and Zhiyan Huang* Innovative Research Center of Medicine, Key Laboratory of Macromolecular Science of Shaanxi Province, Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University (SNNU), Xi’an 710062, China S Supporting Information *
ABSTRACT: Employing a nickel catalyst and electron-deficient polyfluoroaryl magnesium species, a highly selective monoarylation of polyfluoroarenes containing multiple identical coupling sites has been achieved for the first time, which represents a long-standing problem due to the competitive reactivity between the desired products and the starting polyfluoroarenes. Because of the negative fluorine effect, a surprisingly stable cis [Ni(ArF4)2(DPEPhos)] species 4 (ArF4 = 2,3,5,6tetrafluorophenyl) confirmed by X-ray crystallography is isolated, which acts as catalyst arrest state as proven by a thermal decomposition test. Further retro-transmetalation experiments uncover a hidden secondary transmetalation between ArF4-Ni-Ph and excess ArF4-MgCl that competes with the desired but reluctant reductive elimination at the high-valent nickel center. Accordingly, through the cooperation of newly developed DMM-DPEPhos, and a dioxane-mediated Schlenk equilibrium with Grignard reagent, the formation of the corresponding arrest state is remarkably inhibited. An excellent coupling efficiency and an excellent monoarylation selectivity are therefore generally accomplished with a widespread electrophile scope and good functional group tolerance under mild conditions. Importantly, our novel method shows great power in the gram-scale synthesis of thienyl-2,3,5,6-tetrafluorophenyl units that represent key components in materials science. KEYWORDS: nickel catalyst, monoarylation, Kumada−Tamao−Corriu coupling, polyfluoroarene, transmetalation
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magnesium halides (ArFn-MgX, where n = 2−5) or analogues with comparable functional group tolerant properties of Knochel’s are widely used to prepare interesting polyfluorinated aryl boron,5 silane,6 and phosphine7 derivatives via nucleophilic substitution or addition 8 reactions. Compared to the corresponding fluorinated arylboronic acids due to rapid protodeboronation,9 these important types of Grignard reagents are hence supposed to serve as more appropriate coupling partners in the synthesis of polyfluorinated biaryl but
INTRODUCTION Homogeneous nickel catalysis continues to become more important in modern synthetic chemistry, not only as a lowcost replacement but also for its own diverse novel catalytic reactivity.1 Among these developments, biaryl synthesis has attracted much more attention and been considerably successful with the electrophile scopes expanded widely from aryl (pseudo) halides to aryl ether.1b,c The Ni-catalyzed Kumada−Tamao−Corriu (KTC) coupling2 reactions also benefit from the development and exhibit increasingly powerful and practical applications in biaryl synthesis.3 The aryl nucleophile scope of KTC coupling is now applicable from simple Grignard reagents to synthetically versatile ones, such as Knochel-type functional Grignard reagents.4 Polyfluoroaryl © XXXX American Chemical Society
Received: August 8, 2017 Revised: September 6, 2017
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products according to the concerted metalation/deprotonation (CMD) mechanism.24 The inherent limitations pose considerable challenges to achieving selective monoarylation of common and cheap tri- and tetrafluorobenzenes, as well as their derivatives possessing more than one equal coupling site. Actually, their monoarylated adducts are more important synthetic intermediates that can readily convert into a wide array of valuable fluorinated derivatives through direct functionalization. Thus, a mechanistically different catalytic pathway to simultaneously produce polyfluorinated biaryl with high monoselectivity from these kinds of readily accessible polyfluoroarenes could enormously empower the direct arylation strategy.22 We herein report a novel selective monoarylation protocol for efficient synthesis of polyfluorinated biaryl, combining a nickel catalyst and extremely electron-deficient Grignard reagents ArFn-MgCl generated in situ from corresponding fluoroarenes (Figure 2, eq III). The exploration of this new synthesis route is started from the isolation of the catalysis arrest state, an unexpected stable cis [(ArF4)2Ni(DPEPhos)] complex (ArF4 = 2,3,5,6-tetrafluorophenyl) that originated from the second transmetalation that went undiscovered for a long time in KTC coupling between intermediate Ar-Ni-ArF4 and excess ArF4-MgCl. The competition between the undesired second transmetalation and the desired reductive elimination prompts us to search for a more powerful phosphine ligand to outcompete the former process. In the presence of newly developed DMM-DPEPhos and necessary additives, an efficient and general nickel catalysis is built up to prepare various important polyfluorinated biaryls in both excellent yield and monoselectivity (mono s) with a serial of aryl, heteroaryl, and vinyl triflates, as well as thiophene iodides under mild conditions.
have been scarcely documented in transition metal-catalyzed KTC coupling,10 and never for a nickel catalyst. The challenge mainly lies in the fact that these kinds of Grignard reagents can survive only at ambient temperature or lower temperatures, otherwise rapidly decomposing via benzyne or other pathways.11 Unfortunately, because of the negative fluorine effect,12 the notorious reductive elimination step at a high-valent metal center, as opposed to one of the coupling sites that is flanked by two fluorine atoms, is not likely to take place under such mild conditions. Another concern is that the zero-valent transition metal complex favors C−F bond activation in the presence of Grignard reagent.13 Furthermore, because elementary nickel is a relatively electropositive late transition metal compared with palladium, reductive elimination at the high-valent nickel center is correspondingly more difficult to perform.14 Polyfluorinated biaryl represents one of the key structural motifs in medicinal chemistry15 and materials science,16 including use in an analgesic and anti-inflammatory drug (Diflunisal),17 Moloney murine leukemia (PIM) kinase inhibitors,18 liquid crystal displays (LCDs),19 and electronic devices, such as organic light-emitting diodes (OLEDs) and field-effect transistors (FETs)16 (Figure 1). The rapid
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RESULTS AND DISCUSSION Design and Development of Highly Selective Monoarylation of Tetrafluorobenzene. Scheme 1 shows the possible directions in arylation of tetrafluorobenzene via KTC coupling. We reasoned that selective monoarylation of 1,2,4,5tetrafluorobenzene with two equal acidic C−H bonds might be achievable by employing in situ-generated ArF4-MgCl as a coupling partner, if the coupling could be realized with an efficient nickel catalyst under mild conditions. The idea is supported by our preliminary work on the properties of tetrafluoroaryl magnesium chloride (ArF4-MgCl) A.31 First, in situ-generated species A is stabilized by three coordinated solvents through X-ray crystallography and surprisingly has a long half-time as tracked by 19F nuclear magnetic resonance (NMR) at room temperature (see the Supporting Information), which leaves sufficient time for the nickel catalyst to fully convert aryl electrophiles. Second, the lower basicity of species A relative to that of iPrMgCl would retard secondary H−Mg exchange between A and monoarylated product B especially were dioxane used as a cosolvent (see the Supporting Information), consequently dramatically diminishing the extent of formation of C and bisarylated byproduct D (Scheme 1). Lastly, the good functional group tolerance of A would be beneficial with respect to practical synthesis. To identify an effective nickel catalyst, we initiated our investigation by studying the coupling between an easy electrophile PhOTf 1 and 1,2,4,5-tetrafluorobenzene 2 at room temperature. As expected, phosphine ligands dppe,
Figure 1. Compounds containing the polyfluorinated biaryl motif in biologically active, liquid crystal display, and electronic devices.
expansion of the application of polyfluorobiaryl benefits from the efficient transition metal-catalyzed cross coupling reactions. These synthetic methods mainly include (1) Suzuki−Miyaura coupling with polyfluorinated aromatic boronic acid,20 (2) atom-economic21 direct arylation of polyfluoroarenes,22 and (3) decarboxylative arylation with polyfluorobenzoates.23 The low reactivity of polyfluorinated aromatic boronic acids in the transmetalation step makes them poorer coupling partners. Under harsh conditions, i.e., high temperatures, generally required to facilitate transmetalation and reductive elimination, the latter two routes show great efficiency in the synthesis of pentafluorophenyl-aryl or their analogues without additional reactive C−H bonds. In striking contrast, the preparation of other common polyfluorinated biaryl containing acidic C−H bonds oftentimes encountered a multiarylation quandary even under very weak basic conditions, such as K2CO3,24 K3PO4,25 Na2CO3,26 Ag2CO3,27 AgOPiv,28 (NMe4)OC(CF3)3,29 CuF30 (Figure 2, eq I), or potassium polyfluorinated benzoate23d (Figure 2, eq II). Theoretically, the contaminations of multiarylation products in these two pathways both stem from the competitive reactivity of the desired monoarylation 7422
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Figure 2. Reactions for accessing polyfluorinated biaryls.
dehalogenation byproducts. Unfortunately, the conversion was destined to be ∼50% regardless of the increasing catalyst loading, the amount of tetrafluorobenzene, and/or the reaction temperature (eq 1). Occasionally, a pure yellow byproduct was
Scheme 1. A Presumed Pathway for Ni-Catalyzed Direct Arylation of 1,2,4,5-Tetrafluorobenzene
collected and carefully characterized via 1H, 19F, and 31P NMR spectra. Its composition was postulated to be DPEPhos and tetrafluorophenyl in a 1:2 ratio. Signals at 917.0878 and 745.0979 from further HRMS analysis implied the yellow solid probably was a cis nickel complex, most like the [Ni(ArF4)2(DPEPhos)] species (see the Supporting Information).32 As illustrated by Scheme 2, we attempted to synthesize this unexpected cis nickel complex independently. Complex [Ni(ArF4)2DPEPhos] can be prepared on a sufficient scale in a straightforward manner. With NiCl2 as a precursor, a yellow powder was collected in 80% yield by simply stirring the mixture of DPEPhos and ArF4-MgCl. The exact structure was then determined to be a quite stable cis ArF4-Ni-ArF4 complex supported by DPEPhos via X-ray crystallography. The stability should benefit from the enhanced fluorine effect caused by four ortho fluorine atoms around the nickel center. The characteristic data of complex 4 were in strong agreement with those from the real catalytic reaction (eq 1). We next examined the
dppp, dppb, and PCy3, frequently used in KTC coupling, were all inefficient whether the nickel precursor is Ni(cod)2 or Ni(II) salts, such as Ni(acac)2, NiCl2, and NiBr2. Interestingly, only a trace amount of easy-forming Grignard homocoupling product octafluorobiphenyl was detected.13d,e After a systematic screening of phosphine ligands, a breakthrough was made when commercially available DPEPhos was used as a ligand, which gave promising results with a 53% yield and 17:1 monoarylation selectivity (mono s), along with small amounts of 7423
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ACS Catalysis Scheme 2. Independent Synthesis of the cis Bis-ArF4-Ni(II) Complex and Its ORTEP Structure
elimination from the cis Ar-NiII-Ar complex through the release of the Grignard homocoupling byproduct. In our case, only Ni(cod)2, an active nickel(0) catalyst, could served as the metal source. As a result, the formation of cis complex 4 should apply in a totally different manner. The “closed-shell” transmetalation between two molecules of [Ni(Ar)(Br)(bpy)]33 and comproportionation−disproportionation of [Ni(Ar)(X)(DPPF)]34 were reported to be responsible for the formation of the cis Ni(Ar)2 complex. However, the isolated cis [Ni(ArF4)(Br)(DPEPhos)] complex was very stable and failed to give complex 4 under catalytic reaction conditions.35 Recently, in a Pd-catalyzed Negishi coupling with orthosubstituted aryl iodide as a coupling partner, Liu, Lei, and coworkers36 revealed that an unusual second transmetalation between intermediate Ar1-Pd-Ar2 and Ar2-ZnCl gave Ar2-PdAr2 and Ar1-ZnCl, respectively. We envisioned that our cis ArF4Ni-ArF4 complex 4 could also probably derive from that kind of second transmetalation at the nickel center. Because of the negative fluorine effect, the cis ArF4-Ni-Ph complex has difficulty in quickly reductively eliminating product 3a under mild conditions. Alternatively, the deceleration of this step leaves sufficient time for excess ArF4-MgCl to capture the ArF4-Ni-Ar intermediate leading to the second transmetalation, which results in the formation of stable cis ArF4-Ni-ArF4 complex 4 and Ph-MgCl. The small amounts of dehalogenation products detected by gas chromatography were in line with the production of complex 4. The data reported in previous nonpolyfluorinated KTC coupling, which have not attracted a great deal of attention for a long time, supported our assumptions.3a,37 Consequently, the competitive production of inert complex 4 terminated the catalytic cycle and caused only partial conversion of phenyl triflate (Scheme 3).
stability of 4 at different temperature and solvents. The data in Table 1 suggested that this complex be hardly decomposed at Table 1. Thermal Stability of Complex 4 in Different Solvents and at Different Temperaturesa
Scheme 3. Proposed Pathway to Complex 4
entry
solvent
T (°C)
time
yield (%)
1 2 3b 4b 5 6
DCM THF THF/dioxane THF/dioxane benzene dioxane
room temperature room temperature room temperature 60 75 90
24 h 24 h 24 h 12 h 120 min 30 min
99:1 >99:1 4.5:1 >99:1 >99:1 >99:1 >99:1 >99:1
a
For the reaction, 0.1 mmol of PhOTf was used (0.1 M). bGas chromatography (GC) yield. cMonoarylation selectivity (mono s) determined by GC. dYields in parentheses refer to the total yield of two isomers.
coupling efficiency caused by dioxane (entries 1 and 4).40 Decreasing the reaction temperature or catalyst loading resulted in a slightly lower reactivity (entries 5 and 6). Phenyl halides show different reactivities. Iodobenzene gave a moderate yield (entry 8), a poor yield with bromobenzene (entry 7), and a trace product with chlorobenzene. Under optimal conditions
Scheme 4. Scope of the Electrophiles for Ni-Catalyzed Selective Monoarylationa
a
Reaction conditions: aromatic triflate (0.5 mmol), fluoroarene (2.4 equiv), iPrMgCl (2.0 equiv), isolated yield. bIn 15% Ni(cod)2. 7426
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Octafluoro-1,1′-biphenyl and hexafluoro-1,1′-biphenyl with two and four identical C−H bonds, respectively, both exclusively afforded monoadducts in high yields (13 and 14 in 90 and 87% yield, respectively). For frequently used polyfluorinated arenes containing a single reactive site, the novel nickel catalysis exhibited the same catalytic efficiency. As illustrated in Scheme 5b, polyfluorinated biaryls 15 with silane, ether, alkyl, trifluoromethyl, and cinnamyl substituents were obtained in high yields. The thienyl-2,3,5,6-tetrafluorophenyl unit represents one of the prototypical substructures in organic semiconducting materials.16 Encouraged by our efficient nickel catalysis, we hence attempted to employ our nickel catalyst to prepare these important molecules with commercially available electrophiles. To our delight, using iodothiophene and diiodobisthiophene as coupling partners, the transformation also exhibited good efficiency with excellent monoarylation under the same standard conditions. Most importantly, our novel nickel catalyst could not only avoid the use of toxic tin reagents41 but also expand the scale with easy purification (Scheme 6, 16a−c, ≤94% yield on a gram scale), which is essential in practical synthesis with the growing concerns of environmental contamination. The X-ray analysis disclosed that compound 18 has a crystal structure arrangement similar to that of 17 (Scheme 6).41a The efficiency of this mild and selective Ni-catalyzed monoarylation protocol was further demonstrated by synthesis of unsymmetrical polyfluorinated polyaryl via manipulation of fluoroarenes. In light of the great success in materials science of symmetric polyfluorinated phenylene-thiophene compounds,41c we attempted to prepare their unsymmetrical counterparts based on our sufficient gram-scale monoarylation process. The subsequent thiophenation or arylation of 16b conducted under the same conditions afforded desired polyaryls 19−25 in ∼90% yield. The structure of compound 21 was characterized as the desired unsymmetrical fluorinated polyaryl via X-ray crystallography analysis (Scheme 7). Proposed Mechanism. The proposed catalytic cycle is outlined in Scheme 8. Because of the facile formation of the
Scheme 5. Scope of the Fluoroarenes for Ni-Catalyzed Selective Monoarylation
a Reaction conditions: aromatic triflates (0.5 mmol), fluoroarenes (2.4 equiv), iPrMgCl (2.0 equiv), isolated yield.
It should be noted that the former, a difficult substrate because the C−H bond was not located at the middle of two C−F bonds, succeeded in providing monoarylated adduct 7 in 93% yield.25 1,3,5-Trifluorobenzene with three equal weaker acidic C−H bonds also gave monoarylated product 10 with excellent yield and selectivity. With respect to polyfluorinated heteroarenes, such as pyridines, this nickel catalyst also gave a good result (12, 87% yield). Perhaps most importantly, this monoarylation is not limited to simple polyfluoroarenes.
Scheme 6. Ni-Catalyzed Efficient Synthesis of Thienyl-2,3,5,6-tetrafluorophenyl Units on a Gram Scale
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ACS Catalysis Scheme 7. Manipulation of Fluoroarenes via Preparation of Nonsymmetric Fluorinated Polyaryls
a
Reaction conditions: electrophiles (0.5 mmol), fluoroarene (2.4 equiv), iPrMgCl (2.0 equiv), isolated yield of the second step. bIn 15% Ni(cod)2.
reductively eliminates the desired monoarylation product (route a). Because of the lower nucleophilicity of (ArFn)2Mg and mild coupling conditions employed, the secondary transmetalation between complex II and excess I is dramatically inhibited, therefore remarkably diminishing the level of formation of arrest state III (route b). Less basic intermediate I is quite compatible with the desired monoarylation product, which is crucial for ruling out the production of undesired magnesiated species IV that leads to bisarylation. The cooperation of the efficient phosphine ligand and the solvent effect ultimately leads to a high coupling efficiency, as well as excellent monoarylation.
Scheme 8. Proposed Mechanism
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CONCLUSION In summary, near room temperature, a novel one-pot strategy for achieving selective monoarylation of polyfluoroarenes with multiple identical coupling sites is established via a combination of a nickel catalyst and extremely electron-deficient polyfluorinated magnesium species generated in situ from corresponding fluoroarenes. The isolation and identification of the catalyst arrest state, a cis [Ni(ArF4)2(DPEPhos)] complex 4, uncover the second transmetalation at the nickel(II) center that had been hidden for a long time, which competes with the desired reductive elimination step. In the presence of newly developed DMM-DPEPhos and necessary additives, the generation of the arrest state is remarkably inhibited. Therefore, excellent coupling efficiency and monoarylation selectivity (mono s > 99%) are generally observed with widespread electrophile scope
stable ArFn-Ni-ArFn complex when a nickel(II) salt was used as the precursor, the catalytic reaction hence needs to start from zero-valent nickel species derived from Ni(cod)2 and DMMDPEPhos. According to the literature,39 in the presence of dioxane, the Schlenk equilibrium of ArFn-MgCl generated in situ would spontaneously shift to the right side to produce diaryl magnesium species (ArFn)2Mg I. Compound I hence first undergoes smooth transmetalation to form intermediate II. As the powerful driving force of DMM-DPEPhos, complex II then 7428
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A.; Helm, M.; Knochel, P. Angew. Chem., Int. Ed. 2008, 47, 6802− 6806. (d) Knochel, P.; Gavryushin, A.; Brade, K. In The Chemistry of Organomagnesium Compounds; John Wiley & Sons, Ltd.: New York, 2008; p 511−593. (e) Haag, B.; Mosrin, M.; Ila, H.; Malakhov, V.; Knochel, P. Angew. Chem., Int. Ed. 2011, 50, 9794−9824. (5) (a) Yang, D.-T.; Mellerup, S. K.; Peng, J.-B.; Wang, X.; Li, Q.-S.; Wang, S. J. Am. Chem. Soc. 2016, 138, 11513−11516. (b) Piers, W. E.; Marwitz, A. J. V.; Mercier, L. G. Inorg. Chem. 2011, 50, 12252−12262. (c) Stephan, D. W. Acc. Chem. Res. 2015, 48, 306−316. (6) Weicker, S. A.; Stephan, D. W. Chem. - Eur. J. 2015, 21, 13027− 13034. (7) Caputo, C. B.; Hounjet, L. J.; Dobrovetsky, R.; Stephan, D. W. Science 2013, 341, 1374−1377. (8) Reddy, J. S.; Anand, V. G. J. Am. Chem. Soc. 2008, 130, 3718− 3719. (9) (a) Kinzel, T.; Zhang, Y.; Buchwald, S. L. J. Am. Chem. Soc. 2010, 132, 14073−14075. (b) Yang, Y.; Oldenhuis, N. J.; Buchwald, S. L. Angew. Chem., Int. Ed. 2013, 52, 615−619. (10) Martin, R.; Buchwald, S. L. J. Am. Chem. Soc. 2007, 129, 3844− 3845. (11) Polyfluoroaryl magnesium halides are prone to forming benzyne at higher than ambient temperatures: (a) Brewer, J. P. N.; Eckhard, I. F.; Heaney, H.; Marples, B. A. J. Chem. Soc. C 1968, 664−676. (b) Nishimura, T.; Noishiki, A.; Hayashi, T. Chem. Commun. 2012, 48, 973−975. (12) (a) Zhang, W.; Ni, C.; Hu, J. In Fluorous Chemistry; Horváth, I. T., Ed.; Springer: Berlin, 2012; p 25−44. (b) Nishihara, Y.; Onodera, H.; Osakada, K. Chem. Commun. 2004, 192−193. (13) For transition metal-catalyzed C−F bond activation, see reviews: (a) Amii, H.; Uneyama, K. Chem. Rev. 2009, 109, 2119−2183. (b) Clot, E.; Eisenstein, O.; Jasim, N.; Macgregor, S. A.; McGrady, J. E.; Perutz, R. N. Acc. Chem. Res. 2011, 44, 333−348. (c) Ahrens, T.; Kohlmann, J.; Ahrens, M.; Braun, T. Chem. Rev. 2015, 115, 931−972. (d) Yoshikai, N.; Mashima, H.; Nakamura, E. J. Am. Chem. Soc. 2005, 127, 17978−17979. (e) Yoshikai, N.; Matsuda, H.; Nakamura, E. J. Am. Chem. Soc. 2009, 131, 9590−9599. (14) Lanni, E. L.; McNeil, A. J. J. Am. Chem. Soc. 2009, 131, 16573− 16579. (15) (a) Müller, K.; Faeh, C.; Diederich, F. Science 2007, 317, 1881− 1886. (b) Purser, S.; Moore, P. R.; Swallow, S.; Gouverneur, V. Chem. Soc. Rev. 2008, 37, 320−330. (c) Wang, J.; Sánchez-Roselló, M.; Aceña, J. L.; del Pozo, C.; Sorochinsky, A. E.; Fustero, S.; Soloshonok, V. A.; Liu, H. Chem. Rev. 2014, 114, 2432−2506. (d) Zhou, Y.; Wang, J.; Gu, Z.; Wang, S.; Zhu, W.; Aceña, J. L.; Soloshonok, V. A.; Izawa, K.; Liu, H. Chem. Rev. 2016, 116, 422−518. (e) Gillis, E. P.; Eastman, K. J.; Hill, M. D.; Donnelly, D. J.; Meanwell, N. A. J. Med. Chem. 2015, 58, 8315−8359. (16) Babudri, F.; Farinola, G. M.; Naso, F.; Ragni, R. Chem. Commun. 2007, 1003−1022. (17) Adamski-Werner, S. L.; Palaninathan, S. K.; Sacchettini, J. C.; Kelly, J. W. J. Med. Chem. 2004, 47, 355−374. (18) (a) Burger, M. T.; Han, W.; Lan, J.; Nishiguchi, G.; Bellamacina, C.; Lindval, M.; Atallah, G.; Ding, Y.; Mathur, M.; McBride, C.; Beans, E. L.; Muller, K.; Tamez, V.; Zhang, Y.; Huh, K.; Feucht, P.; Zavorotinskaya, T.; Dai, Y.; Holash, J.; Castillo, J.; Langowski, J.; Wang, Y.; Chen, M. Y.; Garcia, P. D. ACS Med. Chem. Lett. 2013, 4, 1193−1197. (b) Burger, M. T.; Nishiguchi, G.; Han, W.; Lan, J.; Simmons, R.; Atallah, G.; Ding, Y.; Tamez, V.; Zhang, Y.; Mathur, M.; Muller, K.; Bellamacina, C.; Lindvall, M. K.; Zang, R.; Huh, K.; Feucht, P.; Zavorotinskaya, T.; Dai, Y.; Basham, S.; Chan, J.; Ginn, E.; Aycinena, A.; Holash, J.; Castillo, J.; Langowski, J. L.; Wang, Y.; Chen, M. Y.; Lambert, A.; Fritsch, C.; Kauffmann, A.; Pfister, E.; Vanasse, K. G.; Garcia, P. D. J. Med. Chem. 2015, 58, 8373−8386. (19) Kirsch, P.; Bremer, M. Angew. Chem., Int. Ed. 2000, 39, 4216− 4235. (20) Korenaga, T.; Kosaki, T.; Fukumura, R.; Ema, T.; Sakai, T. Org. Lett. 2005, 7, 4915−4917. (21) Trost, B. Science 1991, 254, 1471−1477.
and good functional group tolerance. This novel nickel catalyst also exhibits powerful efficiency in the gram-scale synthesis of thienyl-2,3,5,6-tetrafluorophenyl units, as well as in the preparation of symmetry and unsymmetry polyfluorinated polyaryls, which represent key components in materials science. On the basis of our studies, polyfluorinated magnesium species generated in situ from corresponding simple polyfluoroarenes have their own advantages as KTC coupling partners, including (1) their efficient transmetalation rate, (2) their good functional group tolerance even at room temperature, (3) interestingly the fact they are not likely to form homocoupling byproducts and engage in C−F bond activation, and (4) most importantly their potential coupling partners in the large-scale monoarylation of fluoroarenes. These unique properties make the current ArFn-MgX-involved Ni-catalyzed KTC coupling strategy complementary in the synthesis of various important polyfluorobiaryls. Employing polyfluorinated magnesium species to synthesize other interesting polyfluoroaromatics is currently under investigation.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b02618. Experimental procedure and characterization data (PDF) Copies of 1H, 13C, 19F, and 31P NMR spectra of all new compounds (PDF) X-ray data for compound ArF4-MgCl (CIF) X-ray data for compound 4 (CIF) X-ray data for compound 18 (CIF) X-ray data for compound 21 (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Zhiyan Huang: 0000-0001-8151-6188 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (21302115), the Natural Science Foundation of Shaanxi Province (2015JM2064), the Fundamental Research Funds for the Central Universities (GK201706006), and Funded Projects for the Academic Leaders and Academic Backbones, Shaanxi Normal University (16QNGG009), for financial support.
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REFERENCES
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DOI: 10.1021/acscatal.7b02618 ACS Catal. 2017, 7, 7421−7430
Research Article
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DOI: 10.1021/acscatal.7b02618 ACS Catal. 2017, 7, 7421−7430
