Reactions of Bifunctional Perfluoroarylsilanes with Activated C–F Bonds in Perfluorinated Arenes

Reactions of bifunctional perfluoroarylsilanes, p- and m-C6F4(SiMe3)2 as well as o-BrC6F4SiMe3, with substituted perfluoroarenes having electron-withdrawing groups were investigated using NMR and density functional theory calculation techniques. The C–F bond in perfluoroarenes was activated by the para-position of an electron-withdrawing group, such as CF3, C6F5, CN, and NO2. The reaction of C6F4(SiMe3)2 mainly occurred at the para-position of the perfluoroarenes and also occurred at the ortho-position of C6F5CN and C6F5NO2. Two equivalent reactions of perfluoroarenes with bifunctional p- and m-C6F4(SiMe3)2 provided disubstituted perfluoroarenes, along with a small amount of protonated monosubstituted perfluoroarenes. The reaction of o-BrC6F4SiMe3 with the CF3- and CN-substituted pentafluorobenzenes provided unexpected coupling products between C–Br and C–F bonds, in addition to the coupling products between C–SiMe3 and C–F bonds.


■ INTRODUCTION
Fluorinated aromatic compounds are expected to have unique physical properties because of the π-electron system connected with fluorine atoms, which bring high electronegativity, low polarization, and high bond energy to the C−F bond. Especially, bifunctional perfluoroaryl units can be the basic skeleton for developing materials that have peculiar properties. Past research on fluorinated materials has explored a variety of applications. Partially fluorinated poly(arylene ether)s have been developed for application as fuel cell membranes, and their polymerisability and degradation during sulfonation were examined. 1 A wide variety of bifunctional phenyl ethers having tetrafluorophenylene or octafluorobiphenylene moieties has been synthesized for optical or membrane applications. 2 Tetrafluorophenylene-bridged bisphospholes were synthesized to analyze their photophysical and electrochemical properties. 3 Similar perfluoroaromatic molecules have been explored for application to peptides and proteins; model cysteine compounds were prepared by nucleophilic aromatic substitution (S N Ar) with hexafluorobenzene and decafluorobiphenyl to study stapled peptides. 4 Perfluoroaromatic linkers have also been introduced to the backbones of model peptides. 5 The π-electron system associated with fluorine atoms has interesting properties involved in molecular recognition and electron transfer. Derivatives of decafluorobiphenyl and octafluoronaphthalene could act as acceptors for an anion due to anion−π interaction. 6 Polyfluorinated oxacalixarenes, which are potential macrocyclic molecules to make "guest− host" complexes, have been synthesized by reaction of perfluoro-m-xylene with tetrafluororesorcinol. 7 Aiming for use as organic conductors in printable electronics, perfluoroterphenyl derivatives were synthesized and studied for potential self-assembly using X-ray crystallography. 8 Other potential fluorinated organic materials for electronic applications include perfluorinated oligo(p-phenylene)s 9 and perfluorinated phenylene dendrimers, 10 which were synthesized and examined for an electron-transport layer and an n-type semiconductor. Therefore, the development of synthetic methods utilizing bifunctional perfluoroaryl units is in demand by a number of industrial fields.
We have been investigating the trimethylsilyl-based transfer reagent based on a pentafluorophenyl unit, C 6 F 5 SiMe 3 . The CO bond of hexafluoroacetone reacts with C 6 F 5 SiMe 3 to give perfluorinated aromatic ether by further reaction with C 6 F 5 CH 2 Br. 11 Both the CN bonds of perfluorinated cyclic imines 12 and CC bonds of highly branched perfluoroolefins 13 reacted with C 6 F 5 SiMe 3 to give the corresponding pentafluorophenyl cyclic imines and olefins via an addition− elimination (Ad N -E) mechanism. Interestingly, the paraposition of the introduced C 6 F 5 group in the resulting cyclic imines and olefins could react further with C 6 F 5 SiMe 3 molecules to provide perfluorobiphenyl and/or perfluoroterphenyl products. In cases of perfluorinated aromatic compounds containing electron-withdrawing substituents, this multiple pentafluorophenylation occurred at both the paraand ortho-positions of the electron-withdrawing substituents, resulting in not only para-phenylenes but also m-phenylenes. 14 A bifunctional trimethylsilyl-based tetrafluorophenylene transfer reagent, p-C 6 F 4 (SiMe 3 ) 2 , has been prepared from p-C 6 F 4 Br 2 with moderate yields. 15 Both trimethylsilyl sites reacted with aromatic aldehydes to provide diols having the tetrafluorophenylene skeleton. 16 However, reactions of p-C 6 F 4 (SiMe 3 ) 2 with fluorinated compounds, which may exploit the π-electron system associated with fluorine atoms, have not been reported. In the present study, we suggest synthetic methods for introducing a perfluorophenylene moiety into perfluoroarenes and investigate reactions of several perfluoroarenes containing electron-withdrawing substituents with p-C 6 F 4 (SiMe 3 ) 2 . In addition, preparation of geometrical isomers of C 6 F 4 (SiMe 3 ) 2 , which have not yet been reported, is examined to expand the range of potential methods to introduce the tetrafluorophenylene moiety.

■ RESULTS AND DISCUSSION
By modification of the Ruppert method for C 6 F 5 SiMe 3 , 1,4bis(trimethylsilyl)tetrafluorobenzene [p-C 6 F 4 (SiMe 3 ) 2 ] has been previously prepared using ClSiMe 3 and P(NEt 2 ) 3 . 15 In this previous method, although an excess of ClSiMe 3 and P(NEt 2 ) 3 was used in refluxing hexane, the desired product was obtained at relatively low yields. In the present study, anhydrous CH 3 CN at a lower reaction temperature (−30°C) was used to prepare C 6 F 4 (SiMe 3 ) 2 from C 6 F 4 Br 2 . Using this lower reaction temperature, p-C 6 F 4 (SiMe 3 ) 2 could be obtained at higher yields than with the previous method. As shown in our previous study, 14 pentafluorophenylation using C 6 F 5 SiMe 3 could not progress for perfluoroarenes having an electronreleasing substituent. Therefore, we focused on four perfluoroarenes having electron-withdrawing substituents (X = CF 3 , C 6 F 5 ,CN,and NO 2 ) for the reaction of p-C 6 F 4 (SiMe 3 ) 2 (Scheme 1). The results are summarized in Table 1.
Besides pentafluorophenylation using C 6 F 5 SiMe 3 , nucleophilic attack occurred only at the para-position of C 6 F 5 CF 3 (1a). The reaction mixture of 1a with p-C 6 F 4 (SiMe 3 ) 2 became a white suspension, with p-C 6 F 4 (p-C 6 F 4 CF 3 ) 2 (4a) as a component of the suspended phase. The disubstituted product 4a was obtained by the reaction of both SiMe 3 groups of p-C 6 F 4 (SiMe 3 ) 2 with 1a via an addition−elimination (Ad N -E) mechanism. 12−14 After removing the white precipitate, the filtrate DMF solution contained a mixture of 4a and 4′-H(C 6 F 4 )(p-C 6 F 4 CF 3 ) (3a). Evaporation and successive Kugelrohr distillations provided both 3a and 4a as the isolated form. However, the protonated monosubstituted product 3a was obtained at very low yield because it was distilled out along with the DMF solvent. The protonated monosubstituted product 3a formed from the trimethylsilylated monosubstituted product, p-Me 3 Si(C 6 F 4 )C 6 F 4 CF 3 (2a), which was not stable enough to isolate. Thus, it was necessary to modify 4a by successive Ad N -E reactions with 1a and produce 3a by protonation.
The above reaction of p-C 6 F 4 (SiMe 3 ) 2 could apply to C 6 F 5 C 6 F 5 (1b), which is a perfluoroarene having an aromatic C 6 F 5 group as the electron-withdrawing substituent instead of the CF 3 group. Similar to 1a, both C−SiMe 3 bonds of p-C 6 F 4 (SiMe 3 ) 2 reacted with 1b to produce linear p-(C 6 F 4 )(p-C 6 F 4 C 6 F 5 ) 2 (4b) predominately. Since the disubstituted product 4b had lower solubility in DMF, 4b was easily collected as a precipitate from the reaction mixture. Mean- while, the DMF solution filtered from the reaction mixture contained the starting material 1b and 4″-H(C 6 F 4 )(p-C 6 F 4 C 6 F 5 ) (3b). Although the protonated monosubstituted product 3b was formed at very low yield according to 19 F NMR of the DMF solution, 3b could be isolated by Kugelrohr distillation. In the reaction of 1b with C 6 F 5 SiMe 3 , multiple pentafluorophenylation occurred at the terminal C−F bonds at the para-position, providing perfluorinated oligophenylene products. 14 Both 3b and 4b had a reactive C−F bond for the Ad N -E reaction; however, the reactions of p-C 6 F 4 (SiMe 3 ) 2 did not substantially occur for either 3b or 4b because the intermediate carbanion formed from p-C 6 F 4 (SiMe 3 ) 2 had lower nucleophilicity than that formed from C 6 F 5 SiMe 3 . As described in a previous report, 14 reaction of C 6 F 5 CN (1c) with C 6 F 5 SiMe 3 occurred at both para-and orthopositions of the CN group. Even though the first attack of C 6 F 5 SiMe 3 predominately occurred at the para-position, the multiple pentafluorophenylation of C 6 F 5 SiMe 3 occurred at both the para-position of the introduced C 6 F 5 ring and the ortho-position of the original C 6 F 5 CN ring. Therefore, the C 6 F 5 CN (1c) with excess C 6 F 5 SiMe 3 yielded the star-shaped 2,4,6-trisubstituted derivatives. In the present reaction of 1c with p-C 6 F 4 (SiMe 3 ) 2 , however, the attack on the ortho-position of the CN group rarely occurred. Thus, the ortho-isomers, 4′-H(C 6 F 4 )(o-C 6 F 4 CN) (6c) and p-C 6 F 4 (o-C 6 F 4 CN)(p-C 6 F 4 CN) (7c), were obtained as trace amounts in addition to the para-isomers, 4′-H(C 6 F 4 )(p-C 6 F 5 CN) (3c) and p-C 6 F 4 (p-C 6 F 5 CN) 2 (4c). At the same time, the yield of the protonated monosubstituted product 3c was much higher than those of 3a and 3b, while the yield of the disubstituted product 4c was comparable to those of 4a and 4b. Our previous report showed that the ortho-position of C 6 F 5 NO 2 (1d) was less reactive against C 6 F 5 SiMe 3 than that of 1c; 14 the reaction of p-C 6 F 4 (SiMe 3 ) 2 at the ortho-position of C 6 F 5 NO 2 (1d) also occurred. That is, the protonation of the silylated intermediate 2d provided 4′-H(C 6 F 4 )(p-C 6 F 5 NO 2 ) (3d) as a major product, while 5d yielded 4′-H(C 6 F 4 )(o-C 6 F 5 NO 2 ) (6d) as a minor product, similar to the case of 1c. Furthermore, the reaction of the intermediates 2d and 5d with another 1d molecule gave p-C 6 F 4 (p-C 6 F 5 NO 2 ) 2 (4d) and p-C 6 F 4 (o-C 6 F 5 NO 2 )(p-C 6 F 5 NO 2 ) (7d) as major and minor products, respectively.
Next, to examine substituent effects of perfluoroarenes on product distribution, the syntheses and reactions of the positional isomer of C 6 F 4 (SiMe 3 ) 2 were investigated. As shown above, we improved the reaction procedure for the synthesis of p-C 6 F 4 (SiMe 3 ) 2 over a previous work. The improved reaction condition could apply to the meta-isomers of C 6 F 4 (SiMe 3 ) 2 ; 1,3-bis(trimethylsilyl)tetrafluorobenzene [m-C 6 F 4 (SiMe 3 ) 2 ] was obtained from m-C 6 F 4 Br 2 at slightly lower yields than the para-isomer, p-C 6 F 4 (SiMe 3 ) 2 . Reactions of the same perfluoroarenes, having electron-withdrawing substituents X (X = CF 3 , C 6 F 5 , CN, and NO 2 ), with m-C 6 F 4 (SiMe 3 ) 2 were examined in this study (Scheme 2). The results are summarized in Table 2.
In the reaction of m-C 6 F 4 (SiMe 3 ) 2 with C 6 F 5 CN (1c), even though the total yields of the monosubstituted product [5′-H(C 6 F 4 )(p-C 6 F 4 CN) (9c)] and the disubstituted product [m-C 6 F 4 (p-C 6 F 4 CN) 2 (10c)] were almost the same as the total yields of 3c and 4c [from p-C 6 F 4 (SiMe 3 ) 2 ], the ratio between Scheme 2 Table 2. Reaction of 1,3- the monosubstituted and disubstituted products was different for the para-and meta-reagents. Thus, 9c and 10c were obtained evenly (29 and 27% for 9c and 10c, respectively; Table 3, entry 7), while the disubstituted 4c was preferred to the monosubstituted 3c (39% for 4c vs 16% for 3c, Table 1, were also formed at 3% yield for each (Run 7, Table 2) in contrast with the trace formation of the ortho-isomers, 4′-H (C 6 The increases of the protonated monosubstituted product and the ortho-positional isomers were more prominent in the reaction of m- ] also increased. Therefore, the increase of ortho-positional isomers was caused by increasing stability of the silylated intermediate 11 because of resonance of the CN and NO 2 groups. In addition, the increase of the protonated monosubstituted product was caused by lowering the nucleophilicity of monosubstituted silylate 8, becoming less reactive with other molecules 1. Next, the reaction of o-C 6 F 4 Br 2 with ClSiMe 3 and P(NEt 2 ) 3 was examined for preparing o-C 6 F 4 (SiMe 3 ) 2 , using the same preparation procedure as that of p-C 6 F 4 (SiMe 3 ) 2 and m-C 6 F 4 (SiMe 3 ) 2 . Although the white crystalline material precipitated at −30°C 3 h after the start of the reaction, no desired o-C 6 F 4 (SiMe 3 ) 2 was found in the precipitate, only 1bromo-2-trimethylsilyl-3,4,5,6-tetrafluorobenzene [o-Br(C 6 F 4 )-SiMe 3 ]. Neither longer reaction time nor higher reaction temperature produced o-C 6 F 4 (SiMe 3 ) 2 , even though both longer time and higher temperature decreased the yield of o-Br(C 6 F 4 )SiMe 3 in the reaction. Furthermore, pure o-Br(C 6 F 4 )-SiMe 3 could not be collected from the precipitate because it melted at ambient temperature. Vacuum distillation was not able to isolate o-Br(C 6 F 4 )SiMe 3 because the boiling point of the byproduct OP(NEt 2 ) 3 is close to that of the desired o-Br(C 6 F 4 )SiMe 3 . Therefore, reactions of the bifunctional trimethylsilylated reagent with perfluoroarenes were performed using o-Br(C 6 F 4 )SiMe 3 , including 16 wt % of OP(NEt 2 ) 3 (Scheme 3). The results are summarized in Table 3.
Meanwhile, the reaction of o-Br (C 6 (15c)] at relatively low yields (6% for 14c and 5% for 15c, Table 3, entry 11). Similar to 1a, the product 14c was produced by the coupling reaction between the C−F bond at the para-position of CN of 1c and the C−Si bond of o-Br(C 6 F 4 )SiMe 3 , and successive coupling reactions occurred at the para-position of CN of another 1c molecule with the C−Br bond in 14c. The route to the protonated monosubstituted product 17 is only seen in the reactions of o-Br(C 6 F 4 )SiMe 3 with 1b, but not with 1a and 1c. The formation of 15 is also possible through the intermediate 16, and if so, the intermediate 16 bifurcated only through 16b but not through 16a and 16c. If this is not the case, the route to 16 is only operative for 1b but not for 1a and 1c. It is not yet clear which of these synthesis routes can explain the observed results. Table 3. Reaction of 4,5, 14c (6) 15c (5) a The number-letter labels (e.g., 14a) refer to structures shown in Scheme 3 above. b Determined by 19 F NMR of Kugelrohr distillates.

ACS Omega
Article However, it is important to note that the C−Br bond participated in the Ar−Ar coupling as well as the C−Si bond under the experimental conditions. The products 15a, 15c, and 17b were totally unexpected, so we carefully examined their 19 F NMR data by density functional theory (DFT) calculation to confirm their structures. The unexpected Ar−Ar coupling between C−Br and C−F bonds suggests further study, probably for developing a very new Ar−Ar coupling reaction without any metal catalyst. That work, however, is beyond the scope of this paper and will be the subject of a future study.
The structures of the perfluorinated arenes synthesized in this study were determined by 1 H, 19 F, and 13 C{ 1 H, 19 F} NMR spectroscopies and gas chromatography−mass spectrometry (GC−MS). The NMR analyses gave valuable information for the identification of the perfluorinated arenes having several geometrical isomers. However, the signal of synthesized perfluorinated arenes was significantly shifted due to the conjugated π-electrons associated with the electron-withdrawing substituent; therefore, the assignments of NMR signals were also confirmed by DFT calculations. In our previous reports, when the DFT calculation was performed at the B3LYP level using the gauge-independent atomic orbital (GIAO) level with the 6-31++G(d,p) basis set, the chemical shift of poly-and perfluorinated compounds could be reproduced unerringly. 12,13,17,18 The usefulness of DFT calculations for spectral assignments of 13 C, 15 N, and 19 F NMR spectra has been reported. 19,20 The present investigation confirmed that the DFT calculation is also applicable for the 19 F NMR spectral assignment of such complex perfluoroarenes. The geometries of the concerned perfluoroarenes were first optimized using the DFT calculation at the B3LYP hydride functional with 6-31G(d,p). On the basis of the geometry obtained at the B3LYP/6-31(G) level, we further calculated NMR shieldings using the B3LYP-GIAO/6-31++G(d,p) level for the signal assignment. Correlation between the experimentally determined and calculated NMR chemical shifts is illustrated in Figure 1 (Tables  S1−S8). A comparison between the experimentally determined and calculated NMR chemical shifts is also given in the Supporting Information (Tables S9−S14).
The calculated 19 F NMR shieldings showed reasonably good agreement with the experimentally determined values, although they tended to take slightly smaller values [Δδ(F) −0.19 to −7.82]. Larger differences were observed for the CF 3 groups [Δδ(F) −9.12 to −10.00] (Tables S9−S11). Among the perfluoroaryl cyclic imines, the difference between calculated and experimental values in aliphatic fluorine was larger than that in aromatic fluorine. 13 According to the assignment of the 19 F NMR signal, the most significant effect of the electron-withdrawing substituent appeared in the fluorine adjacent to the substituent [CN (ca. −145 ppm) <CF 3 (ca. −139 ppm) <C 6 F 5 (ca. −136 ppm) <NO 2 (ca. −134 ppm)]. The 19 F signal of the aromatic ring originating from C 6 F 4 (SiMe 3 ) 2 was slightly shifted by the electronwithdrawing substituent in the disubstituted product, while its chemical shift was changed by hydrogen and bromine atoms in the monosubstituted product. The site of the perfluorinated arene (para or ortho) attacked by the electron-withdrawing substituent could be easily determined by the signal patterns and chemical shifts of aromatic fluorine atoms, which were well reproduced by the DFT calculation.
In comparison with 19 F NMR values, the calculated 13 C NMR shieldings showed better agreement with experimentally determined values [Δδ(C) −3.89 to 4.07], except for the signals of CF 3 , CN, and C-Br groups (Tables S12−S14). The differences between the calculated and measured values were smaller in aromatic carbons connected with fluorine (135−160 ppm) than those of aromatic carbons connected with another carbon ring (95−115 ppm). The CF 3 and C-Br groups had similar calculated , which took on larger values than the experimentally determined values, as follows: CF 3 [Δδ(C) 6.18−7.14]; C-Br [Δδ (C) 9.17−10.18]. Meanwhile, the CN group exhibited smaller calculated 13 C NMR-shielding values (98−100 ppm) although the experimentally measured values for the CN group were larger than the corresponding calculated values [Δδ(C) −6.47 to −8.19]. For the experimentally determined 13 C NMR shieldings, the electron-withdrawing substituent significantly affected the carbon directly connected with it in the following order: CN (ca. 97 ppm) <CF 3 (ca. 112 ppm) <NO 2 (ca. 131 ppm) <C 6 F 5 (ca. 144 ppm). The tendency of signal shifts due to the electron-withdrawing substituent gave beneficial

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Article information for assigning signals to complicated perfluorinated arenes that were present as several geometrical isomers.
As described above, bifunctional perfluoroarylsilanes, C 6 F 4 (SiMe 3 ) 2 , are useful reagents for manufacturing fluorine materials having not only bifunctional units but also various positional isomers on biphenyl and terphenyl skeletons. The electron-withdrawing substituents on these molecules can be easily converted to a reactive site for another bifunctional molecule having the terphenyl unit. The NMR analysis presented in this study can also apply to perfluorinated arenes having several aromatic rings, which may serve as fluorinated materials for industrial applications. In the future, we are planning to investigate synthesis and NMR analysis of fluorinated arenes having more complicated skeletons, from the point of view of the utilization of perfluorinated materials for a wide range of industrial fields.

■ EXPERIMENTAL SECTION
General Remarks. The 1 H and 13 C NMR spectra were measured on a Varian INOVA-300 spectrometer with CDCl 3 as the solvent operated at 299.95 and 75.42 MHz, respectively. The 19 F NMR spectra were measured using the same solvent and spectrometer operated at 282.24 MHz; positive δ values were downfield from the internal reference, CFCl 3 . The GC− MS data were obtained with a JEOL jms-kg/STK Ultra Quad GC/MS instrument, using electron-impact ionization at 70 eV. The TD-GC−MS data were obtained with a Shimadzu GCMS-QP2010 Ultra instrument, which used electron-impact ionization at 70 eV after the sample was sublimed from 100 to 600°C. All solvents were purchased as superdehydrated solvents commercially and were used without further purification.
Reaction of Octafluorotoluene (1a) with p-C 6 F 4 (SiMe 3 ) 2 . A solution of p-C 6 F 4 (SiMe 3 ) 2 (90 mg, 0.306 mol) and a catalytic amount (10 mg) of KHF 2 in 2 mL of anhydrous DMF was placed in a 30 mL Teflon vessel, and then a solution of octafluorotoluene 1a (146 mg, 0.618 mmol) with 1 mL of anhydrous DMF was added while gently stirring with a magnetic stirrer. After stirring at room temperature (RT) for 20 h, the reaction mixture turned to a light-yellow suspension. A white solid was collected from the suspension and was purified using Kugelrohr distillation to provide p-C 6 F 4 (p-C 6 F 4 CF 3 ) 2 (4a) as white needles. On the other hand, a lightyellow solid was obtained by evaporation of the filtrate and was distilled by the Kugelrohr apparatus to provide 4′-H(C 6 F 4 )(p-C 6 F 4 CF 3 ) (3a) as the lower-temperature fraction and p-C 6 F 4 (C 6 F 4 CF 3 ) 2 (4a) as the higher-temperature fraction. The yield of 3a isolated from the Kugelrohr distillate was 2%, while that of 4a was 34%, isolated from the Kugelrohr distillates of both the precipitate and filtrate.
Reaction of Decafluoro-1,1′-biphenyl (1b) with p-C 6 F 4 (SiMe 3 ) 2 . A solution of p-C 6 F 4 (SiMe 3 ) 2 (90 mg, 0.306 mmol) and 11 mg of KHF 2 in 2 mL of anhydrous DMF was placed in a 30 mL Teflon vessel, and then a solution of decafluorobiphenyl 1b (205 mg, 0.613 mmol) with 1 mL of anhydrous DMF was added while gently stirring. After stirring at RT for 60 h, the reaction mixture became a white suspension. A white solid was collected by filtration of the light-yellow suspension and was purified using Kugelrohr distillation to provide p-C 6 F 4 (p-C 6 F 4 C 6 F 5 ) 2 (4b) as a white solid. A light-yellow solid was obtained by evaporation of the filtrate and was distilled with the Kugelrohr apparatus to provide 4″-H(C 6 F 4 )(p-C 6 F 4 C 6 F 5 ) 2 (3b) as a white solid.
4″H-Perfluro(1,1′:4′,1″-terphenyl) (3b). Yield: 2% (determined by 19  Reaction of Pentafluorobenzonitrile (1c) with p-C 6 F 4 (SiMe 3 ) 2 . A solution of p-C 6 F 4 (SiMe 3 ) 2 (90 mg, 0.306 mmol) and 10 mg of KHF 2 in 2 mL of anhydrous DMF was placed in a 30 mL Teflon vessel, and a solution of pentafluorobenzonitrile 1c (118 mg, 0.611 mmol) in 1 mL of anhydrous DMF was added under gentle stirring. After stirring at RT for 20 h, the reaction mixture became a lightyellow solution. The reaction mixture was evaporated to give a light-yellow liquid with a white solid. Kugelrohr distillation provided mono-and bis-C 6 F 5 CN substituted products in pure forms. That is, the lower-temperature fraction provided 4′-H(C 6 F 4 )(p-C 6 F 4 CN) (3c) as a white solid, while the highertemperature fraction provided p-C 6 F 4 (p-C 6 F 4 CN) 2 (4c). Positional isomers 4′-H(C 6 F 4 )(o-C 6 F 4 CN) (6c) and p-C 6 F 4 (o-C 6 F 4 CN)(p-C 6 F 4 CN) (7c) were also obtained; however, 6c was mixed with 3c and 7c with 4c, and none of these constituents could be isolated. The structures of these positional isomers were determined by 19 F NMR and GC− MS of the mixture with the para isomers. The yield of 6c was below 1%, as determined by 19  Reaction of Pentafluoronitrobenze (1d) with p-C 6 F 4 (SiMe 3 ) 2 . A solution of p-C 6 F 4 (SiMe 3 ) 2 (90 mg, 0.306 mmol) and 10 mg of KHF 2 in 2 mL of anhydrous DMF was placed in a 30 mL Teflon vessel, and a solution of pentafluoronitrobenzene 1d (133 mg, 0.624 mmol) in 1 mL of anhydrous DMF was added while gently stirring. After reaction at RT for 20 h, the mixture turned to an orange solution. The mixture was evaporated to give an orange oil, which was distillated using the Kugelrohr apparatus. The first fraction provided a mixture of 4′-H(C 6 F 4 )(p-C 6 F 4 NO 2 ) (3d) and a small amount of its positional isomer, 4′-H(C 6 F 4 )(o-C 6 F 5 NO 2 ) (6d), while the second fraction yielded 3d in the pure form. The third fraction consisted of p-C 6 F 4 (C 6 F 4 NO 2 ) 2 (4d) with a small amount of its isomer, p-C 6 F 4 (o-C 6 F 5 NO 2 )(p-C 6 F 5 NO 2 ) (7d); however, 4d could not be isolated in the pure form.
Reaction of Pentafluorobenzonitrile (1c) with m-C 6 F 4 (SiMe 3 ) 2 . A solution of m-C 6 F 4 (SiMe 3 ) 2 (178 mg, 0.61 mmol) and 10 mg of KHF 2 in 2 mL of anhydrous DMF was placed in a 30 mL Teflon vessel. A solution of pentafluorobenzonitrile 1c (232 mg, 1.20 mmol) in 1 mL of anhydrous DMF was added while stirring. After 20 h of RT reaction, the mixture became an orange solution. The solution was evaporated to yield a brown oil, which was distilled with the Kugelrohr apparatus. The first fraction provided a mixture of 5′-H(C 6 F 4 )(p-C 6 F 4 CN) (9c) and a small amount of its positional isomer, 5′-H(C 6 F 4 )(o-C 6 F 4 CN) (12c). The second fraction consisted of m-C 6 F 4 (p-C 6 F 4 CN) 2 (10c) with a small amount of its isomer, m-C 6 F 4 (o-C 6 F 4 CN)(p-C 6 F 4 CN) (13c), while a third fraction was composed of 10c in the pure form.
5′H-Perfluoro-(4-nitro-1,1′-biphenyl) (9d). Yield: 25% (determined by 19  Computational Method. Density functional theory (DFT) calculations were performed using the Gaussian 09 program package. 21 All geometries were optimized at the B3LYP hybrid functional 22,23 with the 6-31G(d,p) basis set. Calculations of vibrational frequencies were performed at the same level of theory to confirm minimum. Isotropic NMRshielding tensors were calculated at the B3LYP level using the gauge-independent atomic orbital (GIAO) method 24−26 with the 6-31++G(d,p) basis set. The 19 F NMR shifts δ were calculated from the shielding (σ) as δ = σ ref − σ, where σ ref is the shielding of CFCl 3 (σ ref = 179.3792 ppm). The calculated 13 C NMR shifts were derived in the same fashion as the 19

Notes
The authors declare no competing financial interest.