An Air-Stable Organoboron Compound, Dithienooxadiborepine

These results proved dithienooxadiborepine 1 a potent π-conjugating building ..... TD-DFT calculations revealed that the lowest electronic transition...
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Article Cite This: J. Org. Chem. 2018, 83, 9096−9102

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An Air-Stable Organoboron Compound, Dithienooxadiborepine: Preparation and Functionalization Qifan Yan,* Mengxuan Yin, Cheng Chen, and Yuankun Zhang Key Laboratory for Advanced Materials, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China

J. Org. Chem. 2018.83:9096-9102. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 08/17/18. For personal use only.

S Supporting Information *

ABSTRACT: π-Conjugated organoboron molecules, which are easy to prepare, stable against moisture, and easy to functionalize, are scarce. Here, we report a one-pot synthesis of an air-stable organoboron compound, dithienooxadiborepine 1 in 17% yield on a 600 mg scale without separation or handling an air-sensitive intermediate. Dithienooxadiborepine 1 showed excellent stability under ambient conditions, allowing conventional column chromatography purification. Functionalization of 1 was realized via direct bromination using NBS and further Stille coupling reactions, giving access to longer π-conjugated molecules 5A and 5B. Single-crystal structures of compounds 4, 5A, and 5B not only unambiguously verified the chemical identity of dithienooxadiborepine 1 but also revealed that both the seven-member oxadiborepine ring and the 5−7−5 fused dithienooxadiborepine ring system are planar. UV−vis absorption and fluorescence emission measurements of 5A and 5B showed bathochromic shifted absorption and emission relative to 1, evidencing good π-conjugation. Cyclic voltammograms of 5A and 5B displayed two reduction peaks corresponding to two electron-accepting events at two boron atoms. These results proved dithienooxadiborepine 1 a potent π-conjugating building block for electron-accepting materials.

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π-system fused borepins, a seven-membered aromatic boracycle (Chart 1). Tovar and co-workers15 creatively synthesized two isomeric dithienoborepins and related derivatives with excellent air-stability. Recently, a series of B−N−B embedded phenalenyls was reported,16 along with other BN heterocycles.17 Although the study of boracycles has significantly advanced in the past few years, boron-containing π-conjugated molecules, which are both stable and easy to functionalize, are still very rare. Herein, we report the preparation and functionalization of an air- and moisture-stable dithienooxadiborepine 1 (4,6-bis(2,4,6-tris(trifluoromethyl)phenyl)dithieno[3,2-c:2′,3′-e][1,2,7]oxadiborepine) and its facile functionalization (Chart 1, Scheme 1). Dithienooxadiborepine 1 can be synthesized from 3,3′-dibromo-2,2′-bithiophene in one-pot fashion with 23% yield on a 100 mg scale or 17% yield on a 600 mg scale. Taking advantage of its air- and moisturestability, facile functionalization has been achieved through direct bromination and subsequent Stille cross-coupling reactions. X-ray crystallography reveals co-planar geometry of the 5−7−5 fused ring system, while photophysical and electrochemical results suggest good π-conjugation and unaffected electron affinity of its derivatives.

INTRODUCTION Fused organic π-conjugated molecules are an attractive class of materials that have found widespread applications in optoelectronics.1 Incorporating main-group elements, such as B, Ga, Si, Ge, Sn, and P, into fused conjugated molecules has proven to be a powerful tool to further modulate the properties of these functional materials.2−7 Specifically, embedding one or multiple electron-deficient boron atoms8 became very appealing due to a less-developed electron-accepting materials library and unique features of boron atoms. With a vacant p orbital on the boron atoms, such boron-containing πconjugated molecules not only display intrinsic electron affinity but also poor stability toward moisture or air. The instability of boron-containing compounds and synthetic intermediates made preparation of boron-containing πconjugated materials challenging and limited its development. Over the past decade, fused boracycles and their derivatives have received increasing attention because of their unique photophysical and electronic properties.2e Previously, Yamaguchi and co-workers reported derivatives of borafluorene in 20029 and several intriguing thiophene-fused ladder boroles,10 but air-sensitivity deriving from their high antiaromaticity limited their further functionalization (Chart 1). A series of borafluorenes were synthesized via B-chloroborafluorene intermediate by Rupar et al.,11 revealing improved stability of B-2,4,6-tris(trifluoromethyl)phenylborafluorene. Other borafluorene derivatives were also reported by several other groups using different synthetic methods.12 On the other hand, Tovar13 and Piers14 have independently reported the different © 2018 American Chemical Society

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RESULTS AND DISCUSSION The reaction between boron trichloride and carbanion reagents (i.e., organolithium reagents) has been a common Received: May 10, 2018 Published: June 22, 2018 9096

DOI: 10.1021/acs.joc.8b01188 J. Org. Chem. 2018, 83, 9096−9102

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The Journal of Organic Chemistry

method for the preparation of organoboranes, such as borafluorenes.9,11 Thus, we attempted to synthesize dithienoborole through this synthetic route with corresponding 3,3′dilithio-2,2′-bithiophene generated from 3,3′-dibromo-2,2′bithiophene in situ. After treating it with borontrichloride and 2,4,6-tris(trifluoromethyl)phenyllithium sequentially, we were unable to separate any dithienoborole. To our surprise, dithienooxadiborepine 1 was obtained instead via column chromatography over silica gel as a major product (Scheme 1). The structure of 1 was characterized with NMR and mass spectroscopies and unambiguously verified with the single crystal structure of its brominated derivative 4 (vide infra). Under an optimized reaction condition (i.e., excess borontrichloride), dithienooxadiborepine 1 can be prepared from 3,3′dibromo-2,2′-bithiophene in 23% yield on a 100 mg scale. The yield of 1 decreased slightly to 17% on a larger scale of 600 mg. Ambient stability of 1 allows purification by conventional column chromatography technique. Notably, compound 1 is so stable that no sign of decomposition was observed in 1H NMR spectroscopy after a solution of 1 in CDCl3 had been stored for 2 weeks under ambient conditions (Figure 1). It is intriguing that under the same reaction conditions, 2,2′dibromobiphenyl gives borafluorene as the major product, while 3,3′-dibromo-2,2′-bithiophene affords dithienooxadiborepine 1. Thus, we probed the formation mechanism of 1 by quenching the reaction mixture with excess primary amines (i.e., 2-ethylhexylamine and aniline) prior to normal workup procedure. Two phenomena were observed regardless of which amine we used. First, compound 1 remained as the major product, indicative of its thermodynamic stability compared to nitrogen-containing analogues. Second, mass spectra of the

Chart 1. Structures of Several Boracycles (in Rectangles, DFT Calculated Aromaticity Indicated) and Their Annulated Derivatives

Scheme 1. Syntheses of Dithienooxadiborepine 1 and Its Derivatives

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DOI: 10.1021/acs.joc.8b01188 J. Org. Chem. 2018, 83, 9096−9102

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Figure 1. Comparison of 1H NMR spectra (partial, with assignments) of 1 in CDCl3 stored under ambient conditions after different periods of time (storage time indicated).

reaction mixture found an m/z peak corresponding to structure 3, suggesting an intermediate 2 (Scheme 1). Thus, the more stable compound 1 was yielded from intermediate 2 via hydrolysis by moisture in air during the workup procedure.18 The difference between formation of 2 from 3,3′-dilithio-2,2′bithiophene and 9-chloro-9-borafluorene19 from 2,2′-dilithiobiphenyl may be due to the different torsion angles between two adjacent aromatic rings. More specifically, sulfur−lithium interactions in 3,3′-dilithio-2,2′-bithiophene (Scheme 1) make the torsion angle of the two thiophene units close to 180° and the two carbanions spatially separated. In comparison, the torsion angle of the two phenyl units and the distance between two carbanions were more flexible in 2,2′-dilithiobiphenyl due to the absence of such weak coordination. Nevertheless, airstable compound 1 can be easily prepared in a one-pot fashion from cheap substrates, without separation of sensitive intermediate on a 600 mg scale. These features make dithienooxadiborepine 1 a convenient materials candidate in research. Bromination of 1 occurred smoothly upon treatment with NBS, affording dibromide 4 in 78% yield (Scheme 1). As a comparison, halogen-substituted borafluorene20 or dithienoborole derivatives require lengthy synthetic efforts, because direct bromination resulted in decomposition of substrates. Single crystals of dibromide 4 were obtained by slow evaporation of solution in ethyl acetate and were analyzed via X-ray crystallography, revealing key features of dithienooxadiborepine moiety (Figure 2a and 2b, Table 1). First, the oxadiborepine seven-membered ring in our compounds is planar, as suggested by the sum of the inner angles being 899.5°. Second, boron atoms adopted sp2 hybridization, as the three valence bonds merged from the boron centers are nearly co-planar. This suggested that a vacant, potential electronaccepting p orbital on the boron centers exists. Third, the C−B bond length (1.53 Å) in thiophene−oxadiborepine is evidently shorter than those in dithienoborole compounds (1.57 Å), indicating good boron-thiophene p-π conjugation. Last, the 5− 7−5 fused aromatic ring system is almost flat with a dihedral angle between the two thiophenes at 3.86°, making dithienooxadiborepine a good π-conjugating building block. High yield and good selectivity in the bromination of dithienooxadiborepine 1 paved way for its facile integration into a longer π-conjugated system. Extended π-conjugated compounds were expected to be produced through various transition-metal-catalyzed cross-coupling reactions. However,

Figure 2. Single crystal structures of 4 (a and b), 5A (c and d, 2,4,6tris(trifluoromethyl)phenyl groups omitted for clarity), and 5B (e and f, 2,4,6-tris(trifluoromethyl)phenyl groups omitted for clarity) with views perpendicular (a, c, and e) and parallel (b, d, and f) to the polycyclic planes. (Displacement ellipsoid set at 50% probability level; torsion angles between dithienooxadiborepine and periphery substituents indicated in c and e).

no desired coupling products were found in either Suzuki coupling or Pd-catalyzed direct C−H arylation reaction, and mass spectrometry indicated fragmented skeletons of dithienooxadiborepine (see SI). C−B bonds in the substrates were broken in the presence of a Pd catalyst together with a base. Considering the sensitivity of the C−B bond, we attempted milder reaction conditions, such as Stille coupling reaction, in which a base is not needed. Compounds 5A and 5B were successfully obtained through Stille cross coupling between dibromide 4 and corresponding tin reagents (Scheme 1). Several other catalysts and solvents, including Pd(PPh3)2Cl2, Pd2(dba)3/P(furyl)3, and Pd2(dba)3/(Cy)3P, in THF and chlorobenzene solution, were also applied to the reaction to improve yields but with no success. All dithienooxadiborepine derivatives in solid state exhibited excellent tolerance against moisture and oxygen and can be stored under air for several months without detectable decomposition. Access of compounds 5A and 5B showed that direct functionalization of 1 is feasible. 9098

DOI: 10.1021/acs.joc.8b01188 J. Org. Chem. 2018, 83, 9096−9102

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The Journal of Organic Chemistry Table 1. Selected Bond Lengths (Å) and Angles (o) for Compounds 4, 5A, and 5Ba compd 4 5A 5B

B−C bond length in 7-membered ring

sum of inner angles of the 7-membered ring

1.527 ± 0.006 (1.543), 1.531 ± 0.006 (1.543) 1.533 ± 0.004 (1.540), 1.537 ± 0.003 (1.540) 1.542 ± 0.003 (1.542), 1.536 ± 0.003 (1.542)

torsion angles between dithienooxadiborepine and peripheral substituents

899.47 (899.35) 899.81 (899.85)

9.50 (26.74), 27.36 (26.82)

898.73 (899.26)

3.81 (15.78), 9.68 (17.15)

a

Data from DFT calculations are shown in parentheses.

pronounced bathochromic shift in the absorption of 5B can be attributed to two reasons. First, thiophene units in 5B have an electron-donating ability that is stronger than that of the phenyl units in 5A. (A higher HOMO energy level was found in DFT calculation for 5B; see SI). Second, peripheral thiophene substituents in 5B with smaller torsion angles resulted in better π-conjugation. Fluorescence emission spectra of dithienooxadiborepine 1 showed a blue emission at 416 nm with a quantum yield of 0.75, while 5A and 5B exhibited bathochromic-shifted emissions with vibronic fine structures at 480 and 505 nm, respectively. The high quantum yields and vibronic structures in emission spectra of 5A and 5B implied the rigidity of dithienooxadiborepine core and conjugated backbone. The results from UV−vis absorption and fluorescence emission spectra clearly indicate dithienooxadiborepine is a rigid building block with good π-conjugation. Electrochemical measurements were carried out on compounds 5A and 5B to study their electron-accepting ability (Figures 4 and S5, Table 2). Dithienoborole with one boron

Single crystals of compounds 5A and 5B were obtained by slowly evaporating a solution in petroleum ether and a binary solution in dichloromethane/heptane. Both single crystals were analyzed via X-ray crystallography, with selected geometry parameters summarized in Table 1. The geometry of dithienooxadiborepine moiety in 5A and 5B remained unaffected after cross-coupling reaction compared to dibromide 4 (Figure 2). In compound 5B, two terminal thiophene units and the core dithienooxadiborepine moiety adopt a quasi-planar structure with very small torsion angles of only 3.81° and 9.68°, indicating excellent planarity and πconjugation within the tetrathiophene backbone. The torsion angles in phenyl analogue 5A are 9.50° and 27.36°, slightly larger than the former. These single crystal structures implied that dithienooxadiborepine moiety is a building block that conjugates effectively with other π-moieties. UV−vis absorption and fluorescence emission spectra of compounds 5A and 5B were recorded to compare to those of 1 to investigate π-conjugation effect on dithienooxadiborepine (Figure 3 and Table 2). Compound 5A exhibited a single

Figure 3. UV−visible absorption spectra (left, solid lines) and normalized fluorescence emission spectra (right, dashed lines) of 1 (black), 5A (blue), and 5B (red) in toluene.

Figure 4. Cyclic voltammograms of 5A and 5B in THF (1.0 mM) with Bu4NPF6 (0.1 M as the supporting electrolyte).

Table 2. Summary of Photophysical and Electrochemical Data compd

λa abs (nm)

λa,b em (nm)

Φc

1 5A

359 419

416 480

0.75 0.63

5B

443

505

0.22

Ed1/2 (V) −2.17 −1.90, −2.38 −1.76, −2.24

center can accept one electron reversibly.10 With two boron centers on the dithienooxadiborepine moiety, two electronaccepting behaviors were expected. Indeed, cyclic voltammetry (CV) experiments on compounds 5A and 5B showed two reduction peaks each, suggesting two boron centers accepted an electron sequentially. LUMO energy levels of 5A and 5B were estimated to be −2.90 and −3.04 eV, respectively, from the first reduction peak measured in differential pulse voltammetry (DPV). These values are comparable to those of dithienoborole10 and other similar molecules,21 showing that dithienooxadiborepine moiety is a potent building block for electron-accepting molecules. The stability of compounds 5A and 5B did not sacrifice their electron affinity. DFT calculations were carried out on compounds 1, 5A, and 5B at the theory level of B3LYP/6-31G(d,p) for geometry and frontier molecular orbital (FMO) energy level and B3LYP/6311G+(d,p) for nuclear independent chemical shift (NICS). Geometry of these molecules were fully optimized without

LUMOe (eV) −2.63 (−2.30)f −2.90 (−2.39) −3.04 (−2.51)

a

Measured in toluene. bExcited at the longest absorption maximum wavelengths. cFluorescence quantum yield determined absolutely with an integrating sphere. dDetermined by DPV. eELUMO = −(4.8 + Ered). f DFT calculated LUMO values in parentheses.

absorption band peaked at 419 nm, obviously bathochromicshifted compared with the 359 nm value of dithienooxadiborepine 1. Compound 5B with thiophene substituents displayed an even more red-shifted absorption peak at 443 nm. A more 9099

DOI: 10.1021/acs.joc.8b01188 J. Org. Chem. 2018, 83, 9096−9102

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useful building block for electron-accepting π-conjugated materials.

symmetric restrictions in the gas phase,22 and the analysis showed consistent results with single crystal data (Table 1). FMO energy levels and distributions are summarized in Table 2 and in the Supplementary Information (Figures 5 and S6−

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EXPERIMENTAL SECTION

General Methods. Chemicals were used as received unless otherwise indicated. All oxygen- or moisture-sensitive reactions were performed under a nitrogen atmosphere using the standard Schlenk method. Et2O was distilled over sodium. Toluene was distilled over CaH2 prior to use. NMR spectra were recorded on Bruker-400 (400 MHz) or Bruker-500 (500 MHz) instrument using CDCl3 or C6D6 as the solvent unless otherwise noted. Chemical shifts in 1H, 13C, and 19 F NMR spectra were reported in parts per million (ppm) in reference to TMS (0 ppm) or residual CHCl3 (7.26 ppm) for 1H NMR, CDCl3 (77.0 ppm) for 13C NMR, BF3·Et2O (0 ppm, external) for 11B NMR, and CF3COOH (0 ppm, external) for 19F NMR spectra, respectively.23 Mass spectra were recorded on a Waters GCT Premier mass spectrometer with electron impact (70 eV). Elemental analyses were performed using a German Vario EL III elemental analyzer. 3,3′,5,5′-Tetrabromo-2,2′-bithiophene24 and 3,3′-dibromo-2,2′-bithiophene25 were prepared following reported procedures. Compound 1. To a solution of 3,3′-dibromo-2,2′-bithiophene (310 mg, 0.963 mmol) in toluene (3 mL) was added a solution of nBuLi in hexane (1.2 mL, 1.93 mmol, 1.6 M) at room temperature. After the mixture was stirred for 3 h at room temperature, BCl3 (2.1 mL, 2.1 mmol, 1.0 M in hexane) was added slowly to the reaction suspension at −78 °C. Then, the reaction mixture was gradually warmed to room temperature and stirred overnight. The reaction mixture was concentrated to ca. 3 mL in vacuo to remove excess BCl3 and extra solvents and then added slowly to a solution of tris(trifluoromethyl)phenyllithium21 (4.2 mmol) in toluene (6 mL) at −78 °C. The reaction mixture was gradually warmed to room temperature and stirred overnight. After precipitates were removed by filtration, solvents were removed in vacuo. Column chromatography over silica gel eluted with hexane afforded compound 1 as a white solid (158 mg, 23%). 1H NMR (400 MHz, CDCl3, ppm) δ: 8.11 (s, 4H), 7.24 (d, J = 5.2 Hz, 2H), 6.84 (d, J = 5.2 Hz, 2H). 13C NMR (101 MHz, CDCl3, ppm) δ: 151.5, 141.1 (br, C−B), 136.6, 134.9 (br, C−B), 134.5 (q, 2JC−F = 33 Hz), 132.2 (q, 2JC−F = 34 Hz), 126.0, 124.9, 123.5 (q, 1JC−F = 276 Hz), 122.6 (q, 1JC−F = 274 Hz). 11B NMR (128 MHz, CDCl3, ppm) δ: 42.0. 19F NMR (376 MHz, CDCl3, ppm) δ: −58.0, −63.2. MS (EI) calcd for C26H8B2F18OS2: 764.0 (m/ z). Found: 764.0 (m/z). Elem. Anal. calcd for C26H8B2F18OS2: C, 40.87; H, 1.06. Found: C, 40.90; H, 1.19. Compound 4. To a solution of 1 (450 mg, 0.59 mmol) in THF (4 mL) was added NBS (223 mg, 1.24 mmol) in one portion. The mixture was stirred at room temperature for 1.5 h before it was quenched with water (5 mL). The organics were extracted with ether (3 × 50 mL). The combined organic phase was washed with water (10 mL) and then dried over Na2SO4. After solvents were removed in vacuo, column chromatography over silica gel eluted with petroleum ether afforded compound 4 as a yellow solid (419 mg, 78%). 1H NMR (400 MHz, CDCl3, ppm) δ: 8.11 (s, 4H), 6.75 (s, 2H). 13C NMR (126 MHz, CDCl3, ppm) δ: 151.3, 140.0 (br, C−B), 138.4, 136.1 (br, C−B), 134.4 (q, 2JC−F = 33 Hz), 132.6 (q, 2JC−F = 35 Hz), 126.2, 123.4 (q, 1JC−F = 276 Hz), 122.5 (q, 1JC−F = 273 Hz), 113.0. 11 B NMR (128 MHz, THF-d8, ppm) δ: 41.4. 19F NMR (376 MHz, CDCl 3 , ppm) δ: −58.0, −63.3. HRMS (EI) calcd for C26H6B2Br2F18OS2: 921.8105 (m/z). Found: 921.8123 (m/z). Elem. Anal.: calcd for C26H6B2Br2F18OS2: C, 33.87; H, 0.66. Found: C, 34.29; H, 0.92. General Procedure for Stille Cross-Coupling Reaction. A Schlenk tube charged with compound 4 (100 mg, 0.109 mmol), corresponding tin reagent (0.271 mmol), and Pd(PPh3)2Cl2 (8.0 mg, 0.01 mmol) was evacuated and backfilled with nitrogen three times. Degassed anhydrous toluene (2 mL) was added, and the tube was sealed under a nitrogen atmosphere. The reaction mixture was heated at reflux for 20 h before it was cooled to room temperature. KF (aq. sat. 2 mL) was added, and the mixture was stirred at room

Figure 5. DFT calculated FMO distributions of 1, 5A, and 5B (isovalue was set as 0.02; HOMO energy levels of 1, 5A, and 5B were calculated as −6.10, −5.58, and −5.42 eV, respectively).

S8). HOMO−4 to HOMO of these three compounds (except for HOMO−3 of 1) exhibited significant contributions from the main π-conjugating backbones, namely, 2,2′-bithiophene (1) and diphenyl/thienyl-substituted 2,2′-bithiophene (5A/ 5B). LUMOs of these three compounds showed contributions from p orbitals of boron atoms, implying p-π conjugation, while higher LUMO+1 to LUMO+4 resided on MesF substituents. LUMO levels decrease in the sequence of 1, 5A, 5B, in agreement with electrochemical results. TD-DFT calculations revealed that the lowest electronic transitions (S0− S1) in UV−vis absorption of these three compounds were HOMO to LUMO transitions (100%), namely, π−π* transitions (Figure S9). These results together proved the dithienooxadiborepine moiety a potent π-conjugating building block with good p−π conjugation from boron substituents. NICS(1)zz values of the oxadiborepine 7-membered ring in 1, 5A, and 5B were found to be −0.35, −0.17, and 0.65, respectively, indicative of the nonaromatic nature of oxadiborepine in all three compounds.

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CONCLUSION In summary, we have reported a convenient synthetic method to obtain the air- and moisture-stable boron-substituted bithiophene, dithienooxadiborepine 1 on a 600 mg scale. The nonaromatic 7-membered oxadiborepine ring was stabilized by the electron-donating effect of the oxygen atom. Excellent stability also facilitates functionalization of 1. Direct bromination reaction on 1 with high selectivity and yield allows further derivatization through Stille coupling reactions. Single crystal X-ray crystallography analysis of its derivatives not only verified its chemical identity but also revealed the excellent planarity of these compounds. Photophysical and electrochemical study on dithienooxadiborepine derivatives discovered good π-conjugation and two electron-accepting abilities, which are not sacrificed by the electron-donating effect of the oxygen atom. Furthermore, such properties could be tuned through diverse substituent choice. In consideration of the excellent stability and convenient synthetic route to both substrate and derivatives, dithienooxadiborepine can serve as a 9100

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The Journal of Organic Chemistry

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temperature for another 2 h. The organic phase was washed with brine and dried over Na2SO4. After solvents were removed in vacuo, column chromatography over silica gel eluted with petroleum ether afforded the product. Compound 5A. A yellow solid (52 mg, 52% yield). 1H NMR (400 MHz, C6D6, ppm) δ: 7.85 (s, 4H), 7.29−7.32 (m, 4H), 7.24 (s, 2H), 6.95−6.96 (m, 6H). 13C NMR (101 MHz, CDCl3, ppm) δ: 150.4, 143.9, 141.1 (br, C−B), 136.3 (br, C−B), 134.5 (q, 2JC−F = 33 Hz), 132.7, 132.2 (q, 2JC−F = 35 Hz), 131.8, 129.1, 128.5, 126.3, 126.1, 123.5 (q, 1JC−F = 276 Hz), 122.6 (q, 1JC−F = 274 Hz). 11B NMR (128 MHz, CDCl3, ppm) δ: 42.5. 19F NMR (376 MHz, CDCl3, ppm) δ: −57.9, −63.2. HRMS (EI) calcd for C38H16B2F18OS2: 916.0541 (m/ z). Found: 916.0535 (m/z). Elem. Anal. calcd for C38H16B2F18OS2: C, 49.81; H, 1.76. Found: C, 49.36; H, 1.98. Compound 5B. A yellow solid (57 mg, 56% yield). 1H NMR (400 MHz, CDCl3, ppm) δ: 8.13 (s, 4H), 7.26−7.28 (m, 2H), 7.20−7.21 (m, 2H), 7.03 (dd, J = 5.2, 3.6 Hz, 2H), 6.83 (s, 2H). 13C NMR (101 MHz, CDCl3, ppm) δ: 149.4, 140.7 (br, C−B), 137.0, 136.4 (br, C− B), 135.3, 134.5 (q, 2JC−F = 33 Hz), 132.3 (q, 2JC−F = 35 Hz), 132.0, 128.1, 126.2, 125.9, 125.4, 123.5 (q, 1JC−F = 276 Hz), 122.6 (q, 1JC−F = 274 Hz). 11B NMR (128 MHz, CDCl3, ppm) δ: 42.7. 19F NMR (376 MHz, CDCl3, ppm) δ: −57.9, −63.2. MS (EI) calcd for C34H12B2F18OS4: 928.0 (m/z). Found: 928.0 (m/z). Elem. Anal. calcd for C34H12B2F18OS4: C, 43.99; H, 1.30. Found: C, 43.88; H, 1.57.

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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/acs.joc.8b01188. Single crystal data of 4 (CCDC 1826019) (CIF) Single crystal data of 5A (CCDC 1826020) (CIF) Single crystal data of 5B (CCDC 1826021) (CIF) Synthetic trials, X-ray crystallography, photophysical measurements, electrochemical measurements, and DFT calculation (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Qifan Yan: 0000-0002-5784-5560 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21602061) and the Fundamental Research Funds for the Central Universities (WJ1714009). We thank Mr. Jiajun Xie of Peking University for DFT calculations.

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REFERENCES

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