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An Air-Stable Organoboron Compound, Dithienooxadiborepine: Preparation and Functionalization Qifan Yan, Mengxuan Yin, Cheng Chen, and Yuankun Zhang J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b01188 • Publication Date (Web): 22 Jun 2018 Downloaded from http://pubs.acs.org on June 22, 2018

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The Journal of Organic Chemistry

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

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 600 mg scale without separation or handling 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 chemical identity of dithienooxadiborepine 1, but also revealing 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 1

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at two boron atoms. These results proved dithienooxadiborepine 1 a potent πconjugating building block for electron-accepting materials.

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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 less developed electron-accepting materials library and unique features of boron atoms. With a vacant p orbital on boron atoms, such boron containing π-conjugated molecules display intrinsic electron affinity, but poor stability towards moisture or air as well. The instability of boron containing compounds and synthetic intermediates made preparation of boron containing π-conjugated materials challenging and limited its development.

3



Chart 1. Structures of several boracycles (in rectangles, DFT calculated aromaticity indicated) and their annulated derivatives

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 coworkers reported derivatives of borafluorene in 2002,9 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

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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 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 πconjugated molecules, which are both stable and easy to functionalize, are still very rare. Herein we report the preparation and functionalization of an air- and moisturestable dithienooxadiborepine 1 (4,6-bis(2,4,6-tris(trifluoromethyl)phenyl)dithieno[3,2c: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 100 mg scale or 17% yield on 600 mg scale. Taking advantage of its air- and moisture-stability, facile functionalization has been achieved through direct bromination and subsequent Stille cross-coupling reactions. X-ray crystallography reveals coplanar 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.

Results and Discussion 5



The reaction between boron trichloride and carbanion reagents (i.e. organolithium reagents) has been a common 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 2,4,6tris(trifluoromethyl)phenyllithium sequentially, we were unable to separate any dithienoborole. And 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 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 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 two 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’-dibromo2,2’-bithiophene affords dithienooxadiborepine 1. Thus, we probed the formation mechanism of 1 by quenching the reaction mixture with excess primary amines (i.e., 2ethylhexylamine and aniline) prior to normal workup procedure. Two phenomena were observed regardless which amine we used. First, compound 1 remained as the major 6


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product, indicative of its thermodynamic stability compared to nitrogen containing analogues. Second, mass spectra of reaction mixture found a m/z peak corresponding to structure 3, suggesting an intermediate 2 (Scheme 1). Thus more stable compound 1 was yielded from intermediate 2 via hydrolysis by moisture in air during 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, respectively 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 180o 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 absence of such weak coordination. Nevertheless, air-stable compound 1 can be easily prepared in a one-pot fashion from cheap substrates, without separation of sensitive intermediate on 600 mg scale. These features make dithienooxadiborepine 1 a convenient materials candidate in research.

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Scheme 1. Syntheses of dithienooxadiborepine 1 and its derivatives

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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).

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 requires lengthy synthetic efforts, because direct bromination resulted in decomposition of substrates. Single crystals of dibromide 4 was obtained by slowly evaporate a solution in ethyl acetate and 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 suggested by the sum of inner angles of 899.5o. Second, boron atoms adopted sp2 hybridization as three valence bonds merged from the boron centers are nearly coplanar. This suggested that a vacant p-orbital on boron centers exists for potential electronaccepting. Third, C-B bond length (1.53 Å) in thiophene-oxadiborepine is evidently shorter than those in dithienoborole compounds (1.57 Å) indicating good boron9



thiophene p-π conjugation. Last, 5-7-5 fused aromatic ring system is almost flat with dihedral angle between two thiophenes of 3.86o making dithienooxadiborepine a good π-conjugating building block.

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, 2,4,6tris(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).

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Table 1. Selected bond lengths (Å) and angles (o) for compounds 4, 5A, and 5B (data from DFT calculations showed in parentheses) compd

B−C bond length in 7-memebered ring

sum of inner angles of the 7 membered ring

4

1.527±0.006 (1.543)

899.47 (899.35)

torsion angles between dithienooxadiborepin e and peripheral substituents

1.531±0.006 (1.543) 5A

1.533±0.004 (1.540)

899.81 (899.85)

1.537±0.003 (1.540) 5B

1.542±0.003 (1.542)

9.50 (26.74) 27.36 (26.82)

898.73 (899.26)

1.536±0.003 (1.542)

3.81 (15.78) 9.68 (17.15)

High yield and good selectivity in the bromination of dithienooxadiborepine 1 paved way for its facile integration into longer π-conjugated system. Extended π-conjugated compounds were expected to be produced through various transition metal catalyzed cross-coupling reactions. However, no desired coupling products were found in neither Suzuki coupling nor Pd-catalyzed direct C-H arylation reaction, and mass spectrometry indicated fragmented skeletons of dithienooxadiborepine (SI). C-B bonds in the substrates were broken in the presence of a Pd-catalyst together with a base. Considering the sensitivity of 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 11



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 or oxygen, and can be stored under air for several months without detectable decomposition. Access of compounds 5A and 5B showed direct functionalization of 1 is feasible. Single crystals of compounds 5A and 5B were obtained by slowly evaporating a solution in petroleum ether and a binary solution in dichloromethane/heptane, respectively. 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 comparing 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 9.68o, indicating excellent planarity and πconjugation within the tetrathiophene backbone. The torsion angles in phenyl analogue 5A are 9.50 and 27.36o, slightly larger than the former. These single crystal structures implied that dithienooxadiborepine moiety is a building block that conjugates effectively with other π-moieties.

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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.

UV-vis absorption and fluorescence emission spectra of compounds 5A and 5B were recorded with comparison to those of 1 to investigate π-conjugation effect on dithienooxadiborepine (Figure 3 and Table 2).

Compounds 5A exhibited a single

absorption band peaked at 419 nm, bathochromic shifted obviously compared with 359 nm of that of dithienooxadiborepine 1. Compound 5B with thiophene substituents displayed even more redshifted absorption peak at 443 nm. A more pronounced bathochromic shift in absorption of 5B can be attributed to two reasons. First, thiophene units in 5B have stronger electron-donating ability than phenyl units in 5A. (Higher HOMO energy level was found in DFT calculation for 5B. see SI). Second, peripheral thiophene substituents in 5B with the smaller torsion angles resulted in better πconjugation. 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 13



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.

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

Electrochemical measurements were carried out on compounds 5A and 5B to study their electron-accepting ability (Figure 4, S5, and Table 2). Dithienoborole with one boron center can accept one electron reversibly.10 With two boron centers on dithienooxadiborepine moiety, two electron accepting behavior was 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 was estimated -2.90 and -3.04 eV, respectively, from the first reduction peak measured in differential pulse voltammetry (DPV). These values 14


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are comparable to that of dithienoborole10 and other similar molecules,21 showing dithienooxadiborepine moiety is a potent building blocks for electron-accepting molecules. The stability of compounds 5A and 5B did not sacrifice their electron affinity.

Table 2. Summary of photophysical and electrochemical data

a

Compd

λa abs/nm

λa,b em/nm

Фc

Ed1/2/V

LUMOe /eV

1

359

416

0.75

-2.17

-2.63 (-2.30)f

5A

419

480

0.63

-1.90, -2.38

-2.90 (-2.39)

5B

443

505

0.22

-1.76, -2.24

-3.04 (-2.51)

b

measured in toluene.

Excited at the longest absorption maximum wavelengths.

Fluorescence quantum yield determined absolutely with an integrating sphere. Determined by DPV.

e

ELUMO = −(4.8 + Ered).

f

c

d

DFT calculated LUMO values in

parentheses.

DFT calculations were carried out on compounds 1, 5A, and 5B at theory level of B3LYP/6-31G(d,p) for geometry and frontier molecular orbital (FMO) energy level, and B3LYP/6-311G+(d,p) for nuclear independent chemical shift (NICS). Geometry of these molecules were fully optimized without symmetric restrictions in the gas phase,22 and showed consistent results with single crystal data (Table 1). FMO energy levels and distributions were summarized in Table 2 and SI (Figure 5, S6-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 15



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 decreases 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 dithienooxadiborepine moiety a potent π-conjugating building block with good p-π conjugation from boron substituents. NICS(1)zz values of the oxadiborepine 7membered ring in 1, 5A, and 5B were found -0.35, -0.17, and 0.65, respectively, indicative of nonaromatic nature of oxadiborepine in all three compounds.

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 was calculated -6.10, -5.58, and -5.42 eV, respectively.)

Conclusion 16


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In summary, we have reported a convenient synthetic method to obtain the air- and moisture-stable boron substituted bithiophene, dithienooxadiborepine 1 on 600 mg scale. The nonaromatic 7-membered oxadiborepine ring was stabilized by electrondonating 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 ability, which are not sacrificed by 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 useful building blocks for electron-accepting π-conjugated materials.

Experimental General Methods. Chemicals were used as received unless otherwise indicated. All oxygen or moisture sensitive reactions were performed under 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) using CDCl3 or C6D6 as the solvent unless otherwise noted. Chemical shifts in 1H, 13C and 19F NMR spectra were reported in parts per million (ppm) 17



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with referencing 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, 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 n-BuLi in hexane (1.2 mL, 1.93 mmol, 1.6 M) at room temperature. After stirring for 3 hours at room temperature, BCl3 (2.1 mL, 2.1 mmol, 1.0 M in hexane) was added slowly 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 under vacuo to remove excess BCl 3 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 removed by filtration, solvents were removed under 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, 18


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ppm) δ 42.0.

19

F NMR (376 MHz, CDCl3, ppm) δ -58.0, -63.2. MS (EI): calc’d for

C26H8B2F18OS2, 764.0 (m/z). Found, 764.0 (m/z). Elem. Anal.: calc’d. 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 hours before 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 removed under 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. 11B NMR (128 MHz, THF-d8, ppm) δ 41.4. 19F NMR (376 MHz, CDCl3, ppm) δ -58.0, -63.3. HRMS (EI): calc’d for C26H6B2Br2F18OS2, 921.8105 (m/z). Found, 921.8123 (m/z). Elem. Anal.: calc’d. 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 for three times. Degassed anhydrous toluene (2 mL) was added, and the tube was sealed under nitrogen atmosphere. The reaction mixture was heated at reflux for 20 hours, before cooled down to room temperature. KF (aq. sat. 2 mL) was 19



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added, and the mixture was stirred at room temperature for another 2 hours. The organic phase was washed with brine and dried over Na2SO4. After solvents removed under 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.

19

F NMR

(376 MHz, CDCl3, ppm) δ −57.9, −63.2. HRMS (EI): calc’d for C38H16B2F18OS2, 916.0541 (m/z). Found, 916.0535 (m/z). Elem. Anal.: calc’d. 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): calc’d for C34H12B2F18OS4, 928.0 (m/z). Found 928.0 (m/z). Elem. Anal.: calc’d for C34H12B2F18OS4: C, 43.99; H, 1.30. Found: C, 43.88; H, 1.57.

Supporting Information 20


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The Supporting Information is available free of charge on the ACS Publications website. Synthetic trials, X-Ray crystallography, photophysical measurements, electrochemical measurements, and DFT calculation. (PDF) Single crystal data of 4 (CCDC 1826019). (CIF) Single crystal data of 5A (CCDC 1826020). (CIF) Single crystal data of 5B (CCDC 1826021). (CIF) AUTHOR INFORMATION Corresponding Author [email protected] ACKNOWLEDGMENT The work is financially supported by the National Natural Science Foundation of China (No. 21602061) and the Fundamental Research Funds for the Central Universities (No. WJ1714009). We thank Mr. Jiajun Xie of Peking University for DFT calculations.

REFERENCES (1) (a) Mei, J.; Diao, Y.; Appleton, A. L.; Fang, L.; Bao, Z. Integrated Materials Design of Organic Semiconductors for Field-Effect Transistors. J. Am. Chem. Soc. 2013, 135, 6724-6746. (b) Wang, C.; Dong, H.; Hu, W.; Liu, Y.; Zhu, D. Semiconducting πConjugated Systems in Field-Effect Transistors: A Material Odyssey of Organic 21



Electronics. Chem. Rev. 2012, 112, 2208-2267. (c) Kushida, T.; Shirai, S.; Ando, N.; Okamoto, T.; Ishii, H.; Matsui, H.; Yamagishi, M.; Uemura, T.; Tsurumi, J.; Watanabe, S.; Takeya, J.; Yamaguchi, S. Boron-Stabilized Planar Neutral π-Radicals with WellBalanced Ambipolar Charge-Transport Properties. J. Am. Chem. Soc. 2017, 139, 14336–14339. (d) Hertz, V. M.; Bolte, M.; Lerner, H-W.; Wagner, M. Boron‐Containing Polycyclic Aromatic Hydrocarbons: Facile Synthesis of Stable, Redox‐Active Luminophores. Angew. Chem., Int. Ed. 2015, 54, 8800–8804. (2) (a) Jäkle, F. Advances in the Synthesis of Organoborane Polymers for Optical, Electronic, and Sensory Applications. Chem. Rev. 2010, 110, 3985–4022. (b) Ren, Y.; Jäkle, F. Merging Thiophene with Boron: New Building Blocks for Conjugated Materials. Dalton Trans. 2016, 45, 13996–14007. (c) Ji, L.; Griesbeck, S.; Marder, T. B. Recent Developments in and Perspectives on Three-coordinate Boron Materials: a Bright Future. Chem. Sci. 2017, 8, 846–863. (d) Suzuki, K.; Kubo, S.; Shizu, K.; Fukushima, T.; Wakamiya, A.; Murata, Y.; Adachi, C.; Kaji, H. Triarylboron‐Based Fluorescent Organic Light‐Emitting Diodes with External Quantum Efficiencies Exceeding 20 %. Angew. Chem., Int. Ed. 2015, 54, 15231–15235. (e) Escande, A.; Ingleson, M. J. Fused Polycyclic Aromatics Incorporating Boron in the Core: Fundamentals and Applications. Chem. Commun. 2015, 51, 6257-6274. (f) Dou, C.; Saito, S.; Matsuo, K.; Hisaki, I.; Yamaguchi, S. A Boron‐Containing PAH as a Substructure of Boron‐Doped Graphene. Angew. Chem., Int. Ed. 2012, 51, 12206– 12210. (3) (a) Matsumoto, T.; Tanaka, K.; Tanaka, K.; Chujo, Y. Synthesis and Characterization 22


Page 22 of 28

Page 23 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of Heterofluorenes Containing Four-coordinated Group 13 Elements: Theoretical and Experimental Analyses and Comparison of Structures, Optical Properties and Electronic States. Dalton Trans. 2015, 44, 8697–8707. (b) Matsumoto, T.; Tanaka, K.; Chujo, Y. Synthesis and Optical Properties of Stable Gallafluorene Derivatives: Investigation of Their Emission via Triplet States. J. Am. Chem. Soc. 2013, 135, 4211−4214. (4) (a) Heeney, M.; Bailey, C.; Genevicius, K.; Shkunov, M.; Sparrowe, D.; Tierney, S.; McCulloch, I. Stable Polythiophene Semiconductors Incorporating Thieno[2,3b]thiophene. J. Am. Chem. Soc. 2005, 127, 1078–1079. (b) McCulloch, I.; Bailey, C.; Giles, M.; Heeney, M.; Love, I.; Shkunov, M.; Sparrowe, D.; Tierney, S. Influence of Molecular Design on the Field-Effect Transistor Characteristics of Terthiophene Polymers. Chem. Mater. 2005, 17, 1381–1385. (5) Priegert, A. M.; Rawe, B. W.; Serin, S. C.; Gates, D. P. Polymers and the p-block Elements. Chem. Soc. Rev. 2016, 45, 922–953. (6) Smith, R. C.; Protasiewicz, J. D. Conjugated Polymers Featuring Heavier Main Group Element Multiple Bonds: A Diphosphene-PPV. J. Am. Chem. Soc. 2004, 126, 2268–2269. (7) Bontemps, S.; Devillard, M.; Ladeira, S. M.; Bouhadir, G.; Miqueu, K.; Bourissou, D. Phosphino-Boryl-Naphthalenes: Geometrically Enforced, Yet Lewis Acid Responsive P→B Interactions. Inorg. Chem. 2013, 52, 4714−4720. (8) (a) Wakamiya, A.; Mori, K.; Araki, T.; Yamaguchi, S. A B−B Bond-Containing Polycyclic π-Electron System: Dithieno-1,2-dihydro-1,2-diborin and Its Dianion. J. Am. 23



Chem. Soc. 2009, 131, 10850–10851. (b) Das, A.; Hübner, A.; Weber, M.; Bolte, M.; Lernera, H-W.; Wagner, M. 9-H-9-Borafluorene Dimethyl Sulfide Adduct: A Product of a Unique Ring-Contraction Reaction and a Useful Hydroboration Reagent. Chem. Commun. 2011, 47, 11339-11341. (9) Yamaguchi, S.; Shirasaka, T.; Akiyama, S.; Tamao, K. Dibenzoborole-Containing ð-Electron Systems: Remarkable Fluorescence Change Based on the “On/Off” Control of the pπ-π* Conjugation. J. Am. Chem. Soc. 2002, 124, 8816–8817. (10) Iida, A.; Yamaguchi, S. Thiophene-Fused Ladder Boroles with High Antiaromaticity. J. Am. Chem. Soc. 2011, 133, 6952–6955. (11) Smith, M. F.; Cassidy, S. J.; Adams, I. A.; Vasiliu, M.; Gerlach, D. L.; Dixon, D. A.; Rupar, P. A. Substituent Effects on the Properties of Borafluorenes. Organometallics, 2016, 35, 3182−3191. (12) (a) Wakamiya, A.; Mishima, K.; Ekawa, K.; Yamaguchi, S. Kinetically Stabilized Dibenzoborole as An Electron-accepting Building Unit. Chem. Commun. 2008, 579– 581. (b) Araneda, J. F.; Neue, B.; Piers, W. E.; Parvez, M. Photochemical Synthesis of a Ladder Diborole: A New Boron-Containing Conjugate Material. Angew. Chem., Int. Ed. 2012, 51, 8546–8550. (c) Kim, S.; Song, K.; Kang, S. O.; Ko, J. The role of Borole in a Fully Conjugated Electron-rich System. Chem. Commun. 2004, 68–69. (d) Chen, D. M.; Qin, Q.; Sun, Z. B.; Peng, Q.; Zhao, C. H. Synthesis and Properties of B,NBridged p-Terphenyls. Chem. Commun. 2014, 50, 782–784. (13) Caruso, A. Jr.; Tovar, J. D. Conjugated “B-Entacenes”: Polycyclic Aromatics Containing Two Borepin Rings. Org. Lett. 2011, 13, 3106–3109. 24


Page 24 of 28

Page 25 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14) Mercier, L. G.; Piers, W. E.; Parvez, M. Benzo‐ and Naphthoborepins: Blue‐ Emitting Boron Analogues of Higher Acenes. Angew. Chem., Int. Ed. 2009, 48, 6108– 6111. (15) (a) Levine, D. R.; Siegler, M. A.; Tovar, J. D. Thiophene-Fused Borepins As Directly Functionalizable Boron-Containing π-Electron Systems. J. Am. Chem. Soc. 2014, 136, 7132−7139. (b) Messersmith, R. E.; Yadav, S.; Siegler, M. A.; Ottosson, H.; Tovar, J. D. Benzo[b]thiophene Fusion Enhances Local Borepin Aromaticity in Polycyclic Heteroaromatic Compounds. J. Org. Chem. 2017, 82, 13440−13448. (16) Wei, H.; Liu, Y.; Gopalakrishna, T. Y.; Hoa, P.; Huang, X.; Bao, L.; Guo, J.; Zhou, J.; Luo, S.; Wu, J.; Zeng, Z. B–N–B Bond Embedded Phenalenyl and Its Anions. J. Am. Chem. Soc. 2017, 139, 15760−15767. (17) (a) Wang, X. Y.; Zhuang, F. D.; Wang, R. B.; Wang, X. C.; Cao, X. Y.; Wang, J. Y.; Pei, J. A Straightforward Strategy toward Large BN-Embedded π-Systems: Synthesis, Structure, and Optoelectronic Properties of Extended BN Heterosuperbenzenes. J. Am. Chem. Soc. 2014, 136, 3764–3767. (b) Dou, C.; Ding, Z.; Zhang, Z.; Xie, Z.; Liu, J.; Wang, L. Developing Conjugated Polymers with High Electron Affinity by Replacing a C-C Unit with a B←N Unit. Angew. Chem., Int. Ed. 2015, 54, 3648 –3652. (c) Wakamiya, A.; Taniguchi, T.; Yamaguchi, S. Intramolecular B–N Coordination as a Scaffold for Electron‐Transporting Materials: Synthesis and Properties of Boryl‐ Substituted Thienylthiazoles. Angew. Chem., Int. Ed. 2006, 45, 3170–3173. (18) Abel, E. W.; Dandegaonker, S. H.; Gerrard, W.; Lappert, M. F. The Preparation and Characterising Constants of the Phenylboron Chlorides. J. Chem. Soc. 1956, 4697-4699. 25



(19) Biswas, S.; Oppel, I. M.; Bettinger, H. F. Synthesis and Structural Characterization of 9-Azido-9-Borafluorene: Monomer and Cyclotrimer of a Borole Azide. Inorg. Chem. 2010, 49, 4499-4506. (20) Adams, I. A.; Rupar, P. A. A Poly(9-Borafluorene) Homopolymer: An ElectronDeficient Polyfluorene with “Turn-On” Fluorescence Sensing of NH3 Vapor. Macromol. Rapid Commun. 2015, 36, 1336−1340. (21) Yin, X. D.; Chen, J. W.; Lalancette, R. A.; Marder, T. B.; Jäkle, F. Highly Electron‐ Deficient and Air‐Stable Conjugated Thienylboranes. Angew. Chem., Int. Ed. 2014, 53, 9761–9765. (22) The structures of 5A and 5B in single crystals were not used in DFT calculation due to disorders of trifluoromethyl groups in crystallography. (23) Adachi, Y.; Ohshita, J. Synthesis and Properties of Benzo[d]dithieno[b,f]borepins. Organometallics, 2018, 37, 869−881. (24) Beaujuge, P. M.; Tsao, H. N.; Hansen, M. R.; Amb, C. M.; Risko, C.; Subbiah, J.; Choudhury, K. R.; Mavrinskiy, A.; Pisula, W.; Brédas, J.-L.; So, F.; Müllen, K.; Reynolds, J. R. Synthetic Principles Directing Charge Transport in Low-Band-Gap Dithienosilole–Benzothiadiazole Copolymers. J. Am. Chem. Soc. 2012, 134, 89448957. (25) Lu, G.; Usta, H.; Risko, C.; Wang, L.; Facchetti, A.; Ratner, M. A.; Marks, T. J. Synthesis, Characterization, and Transistor Response of Semiconducting Silole Polymers with Substantial Hole Mobility and Air Stability. Experiment and Theory. J. Am. Chem. Soc. 2008, 130, 7670-7685. 26


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An Air-Stable Organoboron Compound ... - ACS Publications

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