On the bromination of the dihydroazulene/vinylheptafulvene photo-/thermoswitch

Introduction

1,8a-Dihydroazulene-1,1-dicarbonitrile (DHA, 1) is a yellow photochromic compound, which undergoes a light-induced 10-electron retro-electrocyclization to a red-colored vinylheptafulvene (VHF) (Scheme 1) [1-3]. The VHF compound is formed as the s-cis conformer, which, however, is in equilibrium with the more stable s-trans conformer. The s-cis VHF undergoes a thermally induced cyclization to regenerate the original DHA. The significant structural difference between the DHA and VHF forms, as reflected in their different colors and hence electronic properties, renders the system interesting as a light-controlled molecular switch in, for example, molecular electronics. Indeed, light-induced conductance switching was recently observed for a DHA derivative situated in a single-molecule junction [4]. For the further exploration of the DHA/VHF switch in this field, ongoing synthetic efforts are required for the incorporation of functional groups onto the system, especially in a regioselective manner.

We have recently developed an efficient protocol for functionalizing the DHA/VHF 1/2 (R = Ph) at position 7 (for numbering, see Figure 1) by a bromination–elimination protocol of 1, as shown in Scheme 2 [3,5-8]. Elimination of HBr from the intermediate 3 provided the bromo-functionalized DHA 4 that was employed for further cross-coupling reactions. The advantage of this method is that the “parent DHA” 1 can be prepared on a large scale in a few steps [9] and it is hence a convenient building block for further functionalization. Moreover, we have found that the treatment of DHA by aluminum chloride followed by water provided another means of inducing ring-opening of DHA to form VHF (Scheme 1) [10]. This method is particularly convenient for making VHF on a preparative scale, which is more tedious when employing a light source. Along this line, we became interested in investigating the possibility of brominating the VHF 2. It was previously shown by Kuroda and Asao [11] that the related VHF 5 underwent bromination by N-bromosuccinimide (NBS) to form the product 6 (Scheme 3), but any thermal conversion of this VHF to a DHA was not described. Here we describe NBS bromination of VHF 2 and isolation of the corresponding 3-bromo-DHA. In addition, the outcome of further bromination of this compound as well as of the 7-bromo-DHA 4 is presented.

Results and Discussion

Synthesis

DHA 1 was first opened to VHF 2 by aluminum chloride followed by quenching with water according to a previously described procedure (Scheme 4) [10]. The resulting VHF 2 was then treated with NBS and benzoyl peroxide in benzene and the mixture was subjected to irradiation from a 500 W halogen lamp source, presumably generating the intermediate, but short-lived (see below), species 7. No structural evidence for 7 was obtained, but after standard work-up, the ring-closed 3-bromo-DHA product 8 was isolated in an overall yield of 46%, suggesting that 7 is indeed formed as an intermediate. The structure of 8 was elucidated by a ¹H,¹H COSY NMR spectrum (Figure 2) and ultimately confirmed by X-ray crystallographic analysis (Figure 3a). Interestingly, when following the bromination by ¹H NMR spectroscopy, signals from the VHF 2 seem absent within 5 min, while instead signals from the DHA 8 quickly emerge (Figure 4). There are, however, some signals around 6.5 and 7.3 ppm (Figure 4c and Figure 4d) from unidentified intermediates, which decrease in intensity after irradiation for longer time. No characteristic signals from the suggested bromo-VHF intermediate 7 were observed, thus it must undergo rapid ring closure to the DHA 8 under the reaction conditions although the mixture is subjected to light.

Interestingly, when the known tolyl derivative 9 [12] (Figure 5) was subjected to the ring opening followed by NBS bromination, the 3-bromo-DHA 10 was formed in almost quantitative yield (estimated yield of 95%) instead of the benzylic bromide product. This compound was, however, difficult to purify without significant loss of material. When subjecting VHF 2 to bromination by Br₂ (Scheme 4), under the same conditions used to regioselectively brominate the corresponding DHA 1, many products (unidentified) were formed according to NMR spectroscopy. Thus, ionic bromination is not a useful method for the functionalization of the VHF. Gratifyingly, we managed to grow crystals suitable for X-ray diffraction studies of the VHF 2 (generated by the AlCl₃-induced ring opening of DHA 1 followed by a liquid–liquid extraction) by layering cyclohexane upon a solution of the VHF in benzene and allowing this mixture to crystallize at 5 °C (lower temperatures and nonpolar solvents reduce the rate of the undesired thermal back reaction [9]). The crystal structure of VHF 2 is shown in Figure 3b.

With the objective to generate DHA building blocks with more than one bromo functionality for further reactions, we subjected the 3-bromo-DHA 8 to the ionic bromination–elimination protocol, which, via the intermediate 11, provided the 3,7-dibromo-substituted DHA 12 (Scheme 5). In an alternative strategy (Scheme 6), we started out with the 7-bromo-DHA 4. Treatment with bromine at −78 °C generated in this case, however, a very labile intermediate, tentatively assigned to the structure 13, which underwent ready conversion, without the addition of base, to the azulene 14 together with a complex mixture of other nonisolated products. This product is not surprising, inasmuch as we have previously found that a solution of the related dibromide 3 over time underwent conversion to a mixture of 1-bromo-3-cyano-2-phenylazulene and 1-cyano-2-phenylazulene [5]. The conversion of 13 into azulenes was, however, so fast that we could not perform the controlled elimination of HBr by LiHMDS to generate the corresponding 7-bromo-DHA as we could from 3. The structure of the azulene 14 was confirmed by X-ray crystallographic analysis (Figure 3c). Functionalized azulenes are themselves interesting in materials chemistry for their optical and redox properties [13].

The two bromo positions of 14 showed very different reactivity. Thus, subjecting 14 to a Sonogashira coupling with triisopropylsilylacetylene by using the Pd(PPh₃)₂Cl₂/CuI catalyst system only gave the monocoupled product 15 (Scheme 7), confirmed by X-ray crystal structure analysis (Figure 3d). Moreover, a Suzuki cross-coupling reaction with boronic acid 16 gave the product 17, albeit in rather low yield (Scheme 7). The substitution was confirmed by NOESY 1D experiments (see Supporting Information File 1).

Finally, we observed that upon storage at room temperature, the dibromide 3 as well as the known tetrabromide 18 [5] slowly turned into a mixture of azulenes, which we isolated and identified as 19–22 shown in Scheme 8. While, 19 (X-ray crystal structure is given in [6]), 20 [5], and 21 [5,14,15] are already known, the dibromoazulene 22 is new.

UV–vis absorption and switching studies

The UV–vis absorption spectrum of the 3-bromo-DHA 8 in cyclohexane is shown in Figure 6. The compound exhibits an absorption maximum at 337 nm, which is blue-shifted by 17 nm relative to that of DHA 1 [9]. By irradiation at 337 nm, the DHA was gradually converted to the VHF 7 (Figure 6) exhibiting a characteristic absorption maximum at 453 nm (blue-shifted by 7 nm relative to that of VHF 2 [9]). Upon heating of the VHF at 50 °C, a gradual conversion to the DHA was observed. Although both the light-induced DHA-to-VHF conversion and the thermally induced VHF-to-DHA conversion were found to occur with isosbestic points in the absorption spectra, we found that the conversion was not fully reversible. First, the VHF could not be fully converted to DHA (Figure 6, broken curve), and, second, when a second cycle was performed, the final VHF absorption was at a lower intensity than after the first conversion. Nevertheless, the decay of the VHF absorption could be fitted to an exponential decay (see Supporting Information File 1).

From this fit, we obtained rough estimates of the rate constant and half-life of k = 5.1 × 10⁻⁴ s⁻¹ and t_1/2 = 27 min at 50 °C. For comparison, the values for conversion of VHF 2 to DHA 1 in cyclohexane are 8.3 × 10⁻⁵ s⁻¹ and t_1/2 = 139 min at 50 °C (found from Arrhenius plot, [9]). Thus, the influence of the bromo substituent in 7 is to enhance significantly the rate of the thermal ring closure, which is explained by its inductive electron withdrawal, hence stabilizing a zwitterionic VHF transition state, as previously suggested [9]. In accordance hereto, we have previously found that moving the bromo substituent to the seven-membered ring has the opposite effect; thus, the VHF derived from 7-bromo-DHA 4 underwent ring closure considerably slower than VHF 2 [7]. We are, however, not able to explain the remarkably fast formation of 8 from 7 under the bromination conditions described above, but it may be the result of the slight Lewis acidity under the reaction conditions.

Conclusion

In conclusion, we have developed a method for functionalizing the DHA/VHF photo-/thermoswitch with a bromo substituent at position 3 in the DHA core (product 8). The synthesis explores the ready conversion of DHA 1 to VHF 2 on a preparative scale by using aluminum chloride followed by NBS bromination. This bromination was found to occur selectively in the presence of a tolyl group (product 10). The method contrasts the earlier bromination (Br₂)–elimination protocol on DHA for incorporating selectively a bromo substituent at position 7 in the DHA core without proceeding via the VHF as an intermediate. Subjecting the 3-bromo-DHA 8 to this bromination–elimination protocol furnished the 3,7-dibromo-DHA 12. In contrast, subjecting the 7-bromo-DHA 4 to an additional bromination with Br₂ generated a very labile compound that was readily converted to the fully unsaturated 3,7-dibromo-substituted azulene 14 together with other unidentified products. The instability of intermediate bromide addition products was also reflected by solid-state conversions of di- and tetrabromides to a variety of azulenes. The influence on the thermal VHF-to-DHA conversion exerted by a bromo substituent at position 3 (DHA numbering) is opposite to that of one at position 7; the former substitution enhances the ring closure while the latter retards it. The new bromo-substituted compounds will be interesting for future scaffolding in the quest for advanced photo- and thermoswitches. We note, however, that initial attempts at using the 3-bromo-substituted DHA for Sonogashira or Suzuki couplings (as previously accomplished for the 7-bromo-substituted DHA) have so far been unsuccessful. It seems that the 3-bromo functionality is not very reactive, which is reflected as well by the regioselective Sonogashira and Suzuki reactions on the 3,7-dibromoazulene 14 reported in this work.

Experimental

General Methods

NMR spectra were measured on 300 or 500 MHz instruments. All chemical shift values in the ¹H and ¹³C NMR spectra are referenced to the solvent (δ_H = 7.26 ppm, δ_C = 77.16 ppm). Thin-layer chromatography (TLC) was carried out on commercially available precoated plates (silica 60) with fluorescence indicator; color change of the spot from yellow (DHA) to red (VHF) upon irradiation by UV light indicated the presence of a DHA. For column chromatographic purification of DHAs, the column was covered by aluminium foil to exclude light; the isolated fractions were also kept in the dark. All melting points are uncorrected. All spectroscopic measurements (including photolysis of DHA to VHF and kinetics studies on the thermal conversion of VHF to DHA) were performed in a cuvette of 1 cm path length. Photoswitching experiments were performed by using a 150 W xenon arc lamp equipped with a monochromator. Elemental analyses were performed at the Department of Chemistry, University of Copenhagen.

3-Bromo-2-phenyl-1,8a-dihydroazulene-1,1-dicarbonitrile (8): To a stirred solution of DHA 1 (300 mg, 1.17 mmol) in CH₂Cl₂ (40 mL) at rt was added AlCl₃ (800 mg, 6.01 mmol). Stirring was continued for further 20 min and then the mixture was quenched with water (60 mL). The organic layer was separated, dried over anhydrous Na₂SO₄, filtered, and concentrated under reduced pressure. The resulting red compound was then dissolved in benzene (40 mL) and NBS (700 mg, 3.93 mmol) was added. The solution was stirred under an Ar atmosphere for 10 min and then benzoyl peroxide (13 mg, 0.05 mmol) was added. The solution was irradiated with a 500 W halogen lamp kept at a distance of ca. 1 m from the reaction mixture. After 4 h, the mixture was filtered and washed with water. The organic layer was washed with saturated aqueous NaHCO₃ solution (3 × 100 mL), dried over anhydrous Na₂SO₄, and filtered. The solvent was removed under reduced pressure and the crude residue was purified by dry column vacuum chromatography (SiO₂, 60% Et₂O/heptane) affording 8 (180 mg, 46%) as a yellow solid. R_f 0.73 (30% heptane/EtOAc); mp 126 °C; ¹H NMR (500 MHz, CDCl₃) δ 7.63 (m, 2H), 7.44 (m, 3H), 6.61 (dd, J = 11.3, 6.4 Hz, 1H), 6.50 (d, J = 6.1 Hz, 1H), 6.49–6.46 (m, 1H), 6.28 (ddd, J = 10.1, 6.1, 2.1 Hz, 1H), 5.70 (dd, J = 10.1, 4.0 Hz, 1H), 3.70 (m, 1H) ppm; ¹³C NMR (125 MHz, CDCl₃) δ 136.5, 135.8, 131.8, 130.8, 130.4, 130.3, 129.0, 128.8, 128.10, 126.9, 122.1, 119.6, 114.3, 111.9, 48.8, 47.4 ppm; FABMS m/z: [M]⁺ 334; Analysis calcd for C₁₈H₁₁BrN₂: C, 64.50; H, 3.31; N, 8.36; found: C, 63.90; H, 3.42; N, 7.86.

3-Bromo-2-(4-methylphenyl)-1,8a-dihydroazulene-1,1-dicarbonitrile (10): To a stirred solution of 2-(4-methylphenyl)-1,8a-dihydroazulene-1,1-dicarbonitrile (9) (250 mg, 0.97 mmol) in CH₂Cl₂ (30 mL) at rt was added AlCl₃ (760 mg, 5.74 mmol). Stirring was continued for a further 10 min and then the mixture was quenched with water (50 mL). The organic layer was separated, dried over Na₂SO₄, filtered, and concentrated under reduced pressure. The residue was then dissolved in benzene (50 mL), and NBS (1.60 g, 9.50 mmol) was added. The solution was stirred under an Ar atmosphere for 10 min and then benzoyl peroxide (3 mg, 0.01 mmol) was added. The solution was irradiated with a 500 W halogen lamp for 2 h. The mixture was filtered and washed with water. The organic layer was washed with saturated aqueous NaHCO₃ solution (3 × 150 mL), dried over anhydrous Na₂SO₄, and filtered. The solvent was removed under reduced pressure affording 10 with minor impurities (321 mg, estimated yield of 95%). An analytically pure sample (yellow solid) could be obtained by dry column vacuum chromatography (SiO₂, Et₂O/heptane 3:2). R_f 0.79 (30% ethyl acetate/heptane); mp 115 °C; ¹H NMR (500 MHz, CDCl₃) δ 7.62 (d, J = 8.0 Hz, 2H), 7.32 (d, J = 8.0 Hz, 2H), 6.67 (dd, J = 11.3, 6.4 Hz, 1H), 6.58–6.49 (m, 2H), 6.34 (ddd, J = 10.1, 6.1, 2.1 Hz, 1H), 5.76 (dd, J = 10.1, 4.0 Hz, 1H), 3.79–3.72 (m, 1H), 2.43 (s, 3H) ppm; ¹³C NMR (125 MHz, CDCl₃) δ 140.8, 136.6, 136.0, 131.6, 130.3, 129.7, 128.6, 128.1, 127.8, 126.2, 121.8, 119.6, 114.4, 112.0, 48.8, 47.3, 21.5 ppm; EIMS m/z: [M]⁺ 348; Analysis calcd for C₁₉H₁₃BrN₂: C, 65.35; H, 3.75; N, 8.02; found: C, 65.50; H, 3.54; N, 7.94.

3,7-Dibromo-2-phenyl-1,8a-dihydroazulene-1,1-dicarbonitrile (12): 3-Br-DHA 8 (85 mg, 0.25 mmol) was dissolved in CH₂Cl₂ (5 mL) at −78 °C under a N₂ atmosphere and the mixture was excluded from light. Then a solution of Br₂ in CH₂Cl₂ (0.78 M, 327 µL) was slowly added over 2 min. The mixture was stirred for 30 min at −78 °C and then concentrated in vacuo to yield the crude product 11 as a yellow-brown solid (126 mg, 0.25 mmol). This compound was used without purification for the next step. It was dissolved in dry THF (5 mL) and cooled to −78 °C. Then LiHMDS (0.3 mL, 0.3 mmol, 1 M in toluene) was added dropwise, and the solution was stirred for 2 h, while the temperature was slowly raised to rt. The reaction mixture was diluted with Et₂O (20 mL) and then washed with saturated aqueous NH₄Cl (2 × 50 mL). The organic phase was dried over anhydrous Na₂SO₄ and filtered. The solvent was removed under reduced pressure and the crude residue was purified by dry column vacuum chromatography (SiO₂, 50% Et₂O/heptane) affording 12 (68 mg, 65%) as a pale green solid. Mp 118 °C; ¹H NMR (300 MHz, CDCl₃) δ 7.78–7.64 (m, 2H), 7.61–7.46 (m, 3H), 6.63 (d, J = 3.6 Hz, 2H), 6.52 (dt, J = 3.6, 1.7 Hz, 1H), 6.08 (d, J = 4.5 Hz, 1H), 3.80 (dd, J = 4.5, 1.7 Hz, 1H) ppm; ¹³C NMR (125 MHz, CDCl₃) δ 138.3, 137.9, 133.8, 131.3, 130.7, 130.3, 129.1, 128.7, 126.4, 121.2, 120.4, 120.2, 113.7, 111.5, 49.0, 46.9 ppm; Analysis calcd for C₁₈H₁₀Br₂N₂: C, 52.21; H, 2.43; N, 6.76; found: C, 52.58; H, 2.41; N, 6.69.

3,7-Dibromo-2-phenylazulene-1-carbonitrile (14): To a solution of 4 excluded from light (154 mg, 0.46 mmol) (prepared as previously described [12]) in CH₂Cl₂ (4 mL) at −78 °C under a N₂ atmosphere was slowly added a 0.78 M solution of Br₂ (0.59 mL, 0.46 mmol) in CH₂Cl₂. After being stirred for 10 min at −78 °C, the reaction mixture was concentrated in vacuo. Purification by flash column chromatography (SiO₂, 50% CH₂Cl₂/heptane) afforded 14 (71 mg, 40%) as a green solid. Mp 220–223 °C; ¹H NMR (500 MHz, CDCl₃) δ 8.89 (d, J = 2.1 Hz, 1H), 8.52 (d, J = 9.9 Hz, 1H), 8.17 (dd, J = 10.4, 1.8 Hz, 1H), 7.84 (m, 2H), 7.64–7.49 (m, 3H), 7.38 (t, J = 10.4 Hz, 1H) ppm; ¹³C NMR (125 MHz, CDCl₃) δ 152.5, 143.3, 141.7, 139.5, 139.4, 137.2, 132.7, 130.3, 130.0, 128.9, 127.4, 124.4, 116.3, 105.4, 96.1 ppm; GC–MS (m/z): [M]⁺ 386.9; HRMS–ESI (m/z): [M + H]⁺ calcd for C₁₇H₁₀Br₂N, 385.9175; found, 385.9196.

3-Bromo-2-phenyl-7-(triisopropylsilylethynyl)azulene-1-carbonitrile (15): To a mixture of the azulene 14 (18 mg, 0.047 mmol), Pd(PPh₃)₂Cl₂ (4 mg) and CuI (2 mg) in argon-degassed THF (4 mL) under an Ar atmosphere were added iPr₂NH (40 μL, 0.283 mmol) and triisopropylsilylacetylene (30 μL, 0.134 mmol). After being stirred for 24 h at rt, additional triisopropylsilylacetylene (50 μL, 0.223 mmol) was added, and the mixture was stirred again for 24 h at rt. Concentration of the reaction mixture in vacuo and purification of the residue by flash column chromatography (50% CH₂Cl₂/heptane) afforded 15 (17.5 mg, 77%) as a green solid. Mp 181–183 °C; ¹H NMR (500 MHz, CDCl₃) δ 8.72 (d, J = 1.5 Hz, 1H), 8.51 (dd, J = 9.9, 0.7 Hz, 1H), 8.03 (ddd, J = 10.4, 1.5, 1.0 Hz, 1H), 7.86–7.80 (m, 2H), 7.60–7.56 (m, 2H), 7.54–7.51 (m, 1H), 7.51 (t, J = 10.2 Hz, 1H), 1.22–1.16 (m, 21H) ppm; ¹³C NMR (125 MHz, CDCl₃) δ 151.9, 143.7, 142.1, 139.4, 139.2, 138.0, 132.9, 130.3, 129.8, 128.8, 127.4, 124.1, 116.5, 108.9, 106.0, 97.6, 95.1, 18.9, 11.5 ppm; HRMS–ESI (m/z): [M + H]⁺ calcd for C₂₈H₃₁BrNSi, 488.1404; found, 488.1398.

3-Bromo-7-(2’,5’-dimethoxyphenyl)-2-phenylazulene-1-carbonitrile (17): To a solution of the azulene 14 (50 mg, 0.129 mmol) and 2,5-dimethoxyphenylboronic acid (16) (47 mg, 0.259 mmol) under an Ar atmosphere in an argon-degassed toluene/water 9:1 mixture were added K₃PO₄ (110 mg, 0.517 mmol), Pd(OAc)₂ (6 mg, 0.0026 mmol) and RuPhos (2-dicyclohexylphosphino-2',6'-diisopropoxy-1,1'-biphenyl) (24 mg, 0.051 mmol). The mixture was vigorously stirred and heated at 100 °C for 4 d. Purification by flash column chromatography (SiO₂, CH₂Cl₂) afforded 17 (15 mg, 26%) as a blue solid. Mp 169–171 °C; ¹H NMR (500 MHz, CDCl₃) δ 8.81 (d, J = 1.7 Hz, 1H), 8.56 (dd, J = 10.0, 0.9 Hz, 1H), 8.00 (ddd, J = 10.2, 1.7, 0.9 Hz, 1H), 7.86–7.83 (m, 2H), 7.64 (t, J = 10.1 Hz, 1H), 7.60–7.55 (m, 2H), 7.53–7.47 (m, 1H), 7.06–6.88 (m, 3H), 3.85 (s, 3H), 3.80 (s, 3H) ppm; ¹³C NMR (125 MHz, CDCl₃) δ 154.1, 151.0, 150.6, 142.5, 142.4, 139.5, 139.4, 139.3, 137.2, 133.3, 132.9, 130.3, 129.5, 128.8, 127.7, 117.2, 117.1, 114.6, 113.0, 104.2, 96.8, 56.5, 56.1 ppm; HRMS–ESI (m/z): [M + H]⁺ calcd for C₂₅H₁₉BrNO₂, 444.0594; found, 444.0617.

Solid-state conversion of dibromide 3 into azulenes (19–21)

The dibromide 3 (416 mg, 1.00 mmol) was stored in the dark at rt in a closed vessel (50 mL) for 30 d. The resulting black solid was dissolved in CH₂Cl₂ and purification by dry column vacuum chromatography (SiO₂, 12.6 cm², 0–100% toluene/heptanes, 12.5% steps, then 0–35% CH₂Cl₂ in toluene, 7% steps, 40 mL fractions) gave 19 (119.4 mg, 57%) and an inseparable mixture (ca. 4:1) of 20 and 21 (55.8 mg, 24%) (known compounds: [5,14,15]).

2-Phenylazulene-1,3-dicarbonitrile (19): ¹H NMR (500 MHz, CDCl₃) δ 8.80 (d, J = 9.7 Hz, 2H), 8.08 (t, J = 9.7 Hz, 1H), 8.05 (d, J = 7.4 Hz, 2H), 7.87 (t, J = 9.7 Hz, 2H), 7.62 (t, J = 7.4 Hz, 2H), 7.58–7.55 (m, 1 H) ppm; ¹³C NMR (125 MHz, CDCl₃) δ 155.6, 145.4, 141.7, 138.1, 131.8, 131.7, 130.9, 129.7, 129.6, 116.1, 97.0 ppm; for X-ray data, see [6].

Solid-state conversion of tetrabromide 18 into azulenes 19, 20, and 22

The tetrabromide 18 (576 mg, 1.00 mmol) was stored in the dark at rt in a closed vessel (50 mL) for 30 d. The resulting black solid was dissolved in CHCl₃ and purification by flash column chromatography (SiO₂, toluene) gave 19 (78.9 mg, 38%) as a pink solid, 20 (50.8 mg, 17%) as a violet solid, and 22 (97.7 mg, 20%) as a green solid. TLC (toluene); R_f 0.06–0.09 (19), 0.25–0.30 (20), 0.43–0.47 (22).

3,6-Dibromo-2-phenylazulene-1-carbonitrile (22): ¹H NMR (300 MHz, CDCl₃) δ 8.32 (d, J = 8.2 Hz, 1H), 8.28 (d, J = 8.6 Hz, 1H), 7.94 (dd, J = 5.2 Hz, J = 1.9 Hz, 1H), 7.90 (dd, J = 4.6 Hz, J = 1.9 Hz, 1H), 7.81–7.85 (m, 2H), 7.49–7.61 (m, 3H) ppm; ¹³C NMR (75 MHz, CDCl₃) δ 151.3, 142.1, 138.6, 137.9, 136.2, 134.4, 132.8, 132.2, 131.7, 130.2, 129.8, 128.9, 116.2, 106.6, 98.5 ppm; Analysis calcd for C₁₇H₉Br₂N: C, 52.75; H, 2.34; N, 3.62; found: C, 52.97; H, 2.03; N, 3.55.