2021, Vol.94, No.5

Novel C,O-chelated bora-heterocycles were synthesized. Reaction of potassium acyltrifluoroborates with tetrahydroquinolin-8-ol furnished C,N-swapped boranils in a single step. All five obtained borates showed photoluminescence in solid state. One of them showed yellow-to-orange reversible mechanochromic luminescence. Further derivatization afforded a B-chiral borate with an O,C,O-tridentate ligand that showed significant thermochromism.

Luminescent tetra-coordinated borates play an important role in organic chemistry and materials science because of their excellent optical properties and high photochemical stability.1 They have applications in fluorescence imaging and electroluminescence devices.2 Significant progress has been made in the development of novel borate luminophores such as boron-dipyrromethene (BODIPY) dyes, boron β-diketonate complexes, and boranils.1 Most of these O- or N-chelated borates are prepared by the simple complexation between bidentate ligands and boron electrophiles such as BF3·Et2O (Scheme 1a).1,3 However, this approach is not suitable for the synthesis of C,O-chelated borates, because C–B bond formation in these compounds is difficult compared to N–B and O–B bond formation in O- or N-chelated borates. Consequently, C,O-chelated borate luminophores are limited to boron-containing π-extended cis-stilbenes and a few other compounds, despite their unique optical properties.4

To find a novel alternative, we focused on potassium acyltrifluoroborates (KATs) as a starting material. KATs have recently received significant attention because of their relatively high chemical stability, improved availability,5 and characteristic structure and reactivity. KATs exhibit intriguing reactivity such as KAT ligation, which results in amide bond formation by rapid condensation with hydroxylamine,6 and condensation with amines, which affords trifluoroborate iminium.7 Furthermore, the synthesis of bora-heterocycles from KATs has provided access to novel skeletons that are difficult to construct by conventional methods.8 In our previous study,8a C,N-chelated boron-containing heterocycles were synthesized by the condensation and cyclization between bi-electrophilic KATs and bi-nucleophilic hydrazinopyridines (Scheme 1b). This strategy has potential applications in the synthesis of other classes of borate luminophores by replacing hydrazinopyridine with other amines bearing a coordinating functional group. Herein, we report an approach for the synthesis of novel C,O-chelated borate luminophores, also known as C,N-swapped boranils, using KATs as the starting material (Scheme 1c).

The reaction of KATs and tetrahydroquinoline-8-ol successfully afforded five different C,N-swapped boranils in a single-step reaction. The obtained molecules showed luminescence of various colors. Single crystal X-ray structure analysis and luminescence measurements were conducted. Photoluminescence was observed for all the compounds, and one of the C,O-chelated borate luminophore showed mechanochromic luminescence.9 We also successfully synthesized a borate bearing a stereogenic boron center using an O,C,O-tridentate ligand.

We first attempted to synthesize novel C,O-chelated borates from KATs based on a previously reported procedure.8a 2-Aminophenol (2a) was employed as an amine bearing a coordinating functional group (Scheme 2a). A two-step reaction involving the condensation of 4-fluoro-substituted phenyl KAT (1a) and 2a, followed by cyclization, afforded the desired compound 4. To the best of our knowledge, this is the first synthesis of a C,N-swapped boranil, though the yield in this two-step reaction was low (6%). This low yield can be attributed to the low nucleophilicity of 2a and the potential formation of an undesired Z isomer of 3 in the iminium formation step. Thus, we next investigated 1,2,3,4-tetrahydroquinolin-8-ol (2b) instead of 2a to improve the reactivity with 1a. Surprisingly, the reaction of 1a with 2b furnished the corresponding bora-heterocycle 5a in a single step. Optimization of the reaction conditions revealed that the reaction with 1.5 equiv of hydrochloric acid in acetonitrile at 40 °C gave 5a in 78% yield. Compound 5a is a pale-yellow, air- and moisture-stable solid that can be isolated by column chromatography on silica gel. The spontaneous cyclization occurs due to the restricted rotation of the phenol moiety, which forces the hydroxy group to approach the boron atom in the E-iminium intermediate during the equilibrium between 1a + 2b and E- and Z-iminium intermediates.

With an effective preparation procedure for C,N-swapped boranils in hand, we next synthesized their analogs 5b5e (Table 1). Compound 5b with a p-tolyl group was obtained in a good yield of 68%, while 5c with a more sterically hindered mesityl group was obtained in a moderate yield of 47%. This reaction is also compatible with KATs containing a heterocycle (5d: 48% yield) or an electron-donating amino group (5e: 64% yield). For the synthesis of 5e, the counter cation in acyl boron 1e was changed from potassium to n-Bu4N to facilitate its isolation by increasing the solubility of 1.8a All compounds of 5 were stable without decomposition at room temperature in air at least for 3 months.

Table
Table 1. Substrate scope of C,N-swapped boranils 5b5e.
Table 1. Substrate scope of C,N-swapped boranils 5b5e.

[a] Acyltrifluoroborate with tetrabutylammonium (n-Bu4N) cation was used instead of potassium acyltrifluoroborate.

Subsequently, we investigated the optical properties of 5a5e in the solid state. All the bora-heterocycles showed solid-state blue to yellow luminescence upon UV irradiation (Figure 1a). Compounds 5a5e are moderately emissive with emission quantum yields (φem) of 3.0–15.0% (Table S1). Emission lifetime measurements confirmed the fluorescence characteristics of 5a5e. Emission bands of 5a5c were similar, with emission maxima around 470 nm (Figure 1b). Emission maxima of 5d and 5e were further red-shifted to 511 and 550 nm, respectively. The absorption spectrum of 5a lay in the wavelength range lower than 400 nm (Figure 1c). Similar to the trend of the emission bands, the absorption bands of 5b and 5c were similar to that of 5a, while red-shifted maxima at 475 and 540 nm were observed for 5d and 5e, respectively. To investigate the fluorescent mechanism, density functional theory (DFT) and TD-DFT calculations of 5a at the B3LYP/6-31+G(d) level of theory were performed (Figure S3). The calculations revealed that the absorption maximum around 400 nm originated from HOMO–LUMO transition, while peaks below 400 nm could be attributed to the transition from molecular orbitals below HOMO to LUMO. There was no or faint luminescence in solution.10

We successfully grew single crystals of 5a5c and 5e and found that they exhibited similar packing structures (Table S2). Compound 5a crystallizes in the monoclinic space group P21/c. The six-membered bora-heterocycle is distorted, and the KAT-derived phenyl ring and bora-heterocycle are highly twisted with a dihedral angle of 57.66° (Figure 2a). These twisted structures of 5a5e enable efficient luminescence in the solid states by preventing tight π-stacking, which is a major cause of aggregation-induced quenching.1a In solution, the aromatic ring can rotate freely, causing non-radiative deactivation of the excited state of the molecule. The molecules in the crystals of 5a5c and 5e form multiple C–H⋯F–C and C–H⋯F–B hydrogen bonds and CH–π interactions between the neighboring molecules to form 2D molecular sheets in the ac plane (green and yellow dotted line in Figure 2b). Between the molecular sheets, molecules of 5a also form C–H⋯F–B hydrogen bonds and CH–π interactions (blue and pink dotted lines in Figure 2c). The crystal structures of 5b, 5c, and 5e also indicate similar conformations with dihedral angles of 38.48°–75.12° (Figures S5–S7).

Interestingly, 5e shows mechanochromic luminescence.11 Application of mechanical stimuli to single crystals of yellow-emitting 5e changed the emission color of the resulting powder, 5eground, to orange (Figure 3a), confirming the mechanochromic luminescence. However, 5a5c did not show mechanochromic luminescence. To gain insight into the mechanism underlying the mechanochromic luminescence of 5e, we conducted powder X-ray diffraction analyses. No changes in the peak patterns of 5ecrystal and 5eground were observed, indicating the absence of any crystal to crystal phase transition during grinding (Figure 3b).12 Therefore, this mechanochromism should be derived from the partial amorphization of 5ecrystal upon grinding to afford 5eground. The emission spectrum demonstrated a red-shift of the maximum from 550 to 600 nm upon mechanical stimulation. Annealing of 5eground decreased the emission intensity up to 90 °C (Figure 3c), which could be attributed to accelerated non-radiative deactivation; the emission color was recovered at 20 °C (5eannealed, 20 °C) after annealing at 90 °C (5eground, 90 °C). This recovery of the emission properties suggests that the crystallinity is restored from the metastable amorphous phase upon heating.13

Utilizing this simple synthetic method, we attempted to access a more complex borate luminophore with a tridentate ligand.14 Using ortho-benzyl-protected hydroxybenzoyl trifluoroborate 1f as the starting material, we obtained C,N-swapped boranil 5f as a synthetic intermediate. Treatment of 5f with BBr3 led to the deprotection of the benzyl group, followed by the spontaneous coordination of the resultant hydroxy group to the boron atom (Scheme 3). The resulting bora-heterocycle 6 with O,C,O-tridentate ligand was obtained as a yellow crystal, albeit in a low yield (16% over two steps). Although the single-crystal X-ray diffraction analysis of 6 indicated poor quality of the crystals, it was confirmed that 6 contained a chiral center at the boron atom and crystallized as racemic crystals in the polar space group Pca21 (Table S2). In addition, 6 was found to have a more planar conformation compared to 5, which can be attributed to the rigid tridentate ligand.

A solid sample of 6 exhibited thermochromic luminescence;14c the emission color changed from blue to yellowish green in the temperature range from −100 to 150 °C. Under UV light, the solid sample of 6 showed green emission at room temperature (Figure 4a). Upon excitation at 365 nm, a broad emission spectrum with a maximum at 495 nm (green spectrum in Figure 4b) was observed. Upon cooling 6 from room temperature, the luminescence changed from green to blue. The emission maxima of 6 was gradually blue-shifted upon cooling and reached 470 nm at −50 °C (light blue spectrum in Figure 4b). No further spectral changes were observed upon cooling 6 below −50 °C (blue spectrum in Figure 4b). However, when 6 was heated from room temperature, the emission color changed from green to yellowish green. The emission spectrum was also red-shifted, with the emission maximum increasing to 525 nm at 150 °C, along with a decrease in the luminescence intensity (orange spectrum in Figure 4b). The reversible nature of the thermochromic luminescence of 6 was also suggested with visual observation. We suppose that the red-shifted emission of 6 with the decrease in intensity upon heating is induced by enhancing partial structural relaxation from the excited states at high temperature even in solid state. This is partially supported by the weak luminescence in solution.15 In contrast to crystal 6, crystals 5a5e did not show thermochromic luminescence. The planar molecular scaffold of 6 is different from that of 5, which has twisted conformations. This planar structure can be a key structural factor for the thermochromic luminescence. However, further derivatization and detailed studies such as structural analyses are required for an in-depth understanding of this phenomenon; this will be undertaken in our future work.

In summary, we have synthesized C,N-swapped boranils 5a5e through one-pot iminium salt formation/cyclization using KATs and 1,2,3,4-tetrahydroquinolin-8-ol as starting materials. Compounds 5a5e exhibited significant luminescence in the solid state. In addition, 5e exhibited red-shifted mechanochromic luminescence attributable to a crystalline-to-amorphous transition. Furthermore, a novel class of borate luminophore 6 with an O,C,O-tridentate ligand was synthesized, and it showed remarkable thermochromism. The present study indicates that the optical properties our C,N-swapped boranils are similar to those of boranils: ILCT emission mechanisms, tunable absorption/emission wavelength by introducing electron donating and/or withdrawing groups, and luminescent thermo- and mechanochromic properties. Combining our previous and the present studies, we demonstrated the generality of our approach for the synthesis of useful and structurally unique borates from KATs and amines bearing a coordinating functional group. Our study would further stimulate the development of novel luminescent borates based on this approach.

4.1 General Materials.

Materials were obtained from commercial suppliers and purified by standard procedures unless otherwise noted. Solvents were also purchased from commercial suppliers and dried over molecular sieves (MS 3A for acetonitrile or MS 4A for other solvents). 1H NMR spectra were recorded on JEOL JNM-ECX400P (400 MHz), and JNM-ECS400 (400 MHz) spectrometers and spectra are referenced to tetramethylsilane (0.0 ppm) or the residual protonated solvent (acetone-d6: 2.05 ppm). 13C NMR spectra were recorded on JEOL JNM-ECX400P (100 MHz), and JNM-ECS400 (100 MHz) spectrometers and spectra are referenced to the solvent (CDCl3: 77.16 ppm; acetone-d6: 29.84 ppm). High-resolution mass spectra were recorded at the Global Facility Center, Hokkaido University. Excitation and emission spectra were recorded on a Hitachi F-7000 spectrometer. The emission quantum yields of the solid samples were recorded on a Hamamatsu Quantaurus-QY spectrometer with an integrating sphere. Emission lifetime measurements were recorded on a Hamamatsu Quantaurus-Tau spectrometer (6 ns pulse width). Photographs were obtained using Olympus BX51 or SZX7 microscopes with Olympus DP72, Nikon D5100 cameras. Powder diffraction data were recorded on a Rigaku SmartLab diffractometer with Cu-Kα radiation and D/teX Ultra detector covering 5–60° (2θ).

4.2 General Experimental Procedures.

Trifluoroborates 1b1e were prepared according to the reported procedure.5d,8a

Procedure for the Preparation of Cyclized Compound 4:

Potassium (4-fluorobenzoyl)trifluoroborate 1a (414.0 mg, 1.8 mmol), 2-aminophenol 2a (235.7 mg, 2.2 mmol) were placed in an oven-dried two-neck flask. The flask was connected to a vacuum/nitrogen manifold through a rubber tube. It was evacuated and then backfilled with nitrogen. This cycle was repeated three times. Acetonitrile (20.0 mL) and AcOH (154.4 µL, 2.7 mmol) were then added in the flask through the rubber septum using syringes, and the resultant solution was then stirred at room temperature for 3 h. After the reaction was completed, the reaction mixture was passed through a short silica gel column eluting with ethyl acetate and concentrated in vacuo. The crude product was purified by Et2O wash to afford the desired product 3 (86.7 mg, 0.3 mmol, 17%) as a yellow solid. Product 3 was used in the next reaction without further purification.

Compound 3 (70.8 mg, 0.25 mmol) was placed in an oven-dried screw neck reaction vial. After the vial was sealed with a screw cap containing a Teflon-coated rubber septum, it was connected to a vacuum/nitrogen manifold through a rubber tube. It was evacuated and then backfilled with nitrogen. This cycle was repeated three times. Acetonitrile (2.5 mL), MeOH (15.2 µL, 0.375 mmol), and BF3·Et2O (47.2 µL, 0.375 mmol) were then added to the vial through the rubber septum using syringes, and the resultant solution was then stirred at 40 °C for 12 h. After the reaction was completed, the reaction mixture was passed through a short silica gel column eluting with ethyl acetate/hexane (1:1) and concentrated in vacuo. The crude product was purified by Et2O wash to afford the desired product 4 (24.3 mg, 0.09 mmol, 37%) as a yellow solid. 1H NMR (399 MHz, acetone-d6, δ): 3.79 (s, 1H), 6.96 (t, J = 7.2 Hz, 1H), 7.06 (d, J = 8.4 Hz, 1H), 7.36 (t, J = 7.0 Hz, 1H), 7.44 (t, J = 8.6 Hz, 2H), 7.63 (d, J = 6.4, 1H), 8.36 (dd, J = 5.2, 8.8 Hz, 2H). 13C NMR (100 MHz, acetone-d6, δ): 69.2 (C), 117.2 (d, JC–F = 22.1 Hz, CH), 119.7 (CH), 120.3 (CH), 122.4 (CH), 130.3 (C), 131.6 (CH), 134.1 (d, JC–F = 9.6 Hz, CH), 151.2 (C), 210.0 (C). The carbon directly attached to the boron atom was not detected, likely due to quadrupolar relaxation. HRMS-ESI (m/z): [M − H] calcd for C13H8ONBF3, 262.06565; found, 262.06599.

Procedure for the Preparation of Cyclized Compounds 5:
2,2-Difluoro-3-(4-fluorophenyl)-2,5,6,7-tetrahydro-[1,4,2]oxazaborinino[6,5,4-ij]quinolin-4-ium-2-uide (5a);

1,2,3,4-Tetrahydroquinolin-8-ol (2b) was purchased from a commercial supplier. 1a (34.5 mg, 0.15 mmol) and 2b (26.9 mg, 0.18 mmol) were placed in an oven-dried screw test tube. The test tube was connected to a vacuum/nitrogen manifold through a rubber tube. It was evacuated and then backfilled with nitrogen. This cycle was repeated three times. Acetonitrile (1.5 mL) and hydrogen chloride (ca. 4 mol/L in 1,4-dioxane) (56.3 µL, 0.23 mmol) were then added in the test tube through the Teflon packing using syringes, and the resultant solution was then stirred at 40 °C for 3 h. After the reaction was completed, the reaction mixture was passed through a short silica gel column eluting with ethyl acetate and concentrated in vacuo. The crude product was purified by Et2O wash to afford the desired product 5a (35.2 mg, 0.12 mmol, 78%) as a pale yellow solid. 1H NMR (396 MHz, acetone-d6, δ): 2.15–2.21 (m, 2H), 3.05 (t, J = 6.2 Hz, 2H), 4.40–4.43 (m, 2H), 6.82 (d, J = 7.7 Hz, 1H), 6.88 (d, J = 8.2 Hz, 1H), 7.29 (t, J = 7.9 Hz, 1H), 7.38 (t, J = 8.8 Hz, 2H), 7.72 (dd, J = 5.4, 8.6 Hz, 2H). 13C NMR (100 MHz, acetone-d6, δ): 22.6 (CH2), 27.6 (CH2), 54.3 (CH2), 111.1 (C), 116.5 (d, JC–F = 22.9 Hz, CH), 118.8 (CH), 120.5 (CH), 131.2 (CH), 131.5 (d, JC–F = 9.5 Hz, CH), 133.0 (C), 134.6 (C), 152.3 (C), 164.8 (d, JC–F = 249.9 Hz, C). The carbon directly attached to the boron atom was not detected, likely due to quadrupolar relaxation. HRMS-ESI (m/z): [M + Na]+ calcd for C16H13ONBF3Na, 326.09345; found, 326.09370.

2,2-Difluoro-3-(p-tolyl)-2,5,6,7-tetrahydro-[1,4,2]oxazaborinino[6,5,4-ij]quinoline-4-ium-2-uide (5b);

5b (101.8 mg, 0.34 mmol) was prepared in 68% yield from 1b (113.0 mg, 0.5 mmol) according to the procedure for the synthesis of 5a. 1H NMR (401 MHz, acetone-d6, δ): 2.12–2.18 (m, 2H), 2.43 (s, 3H), 3.04 (t, J = 6.2 Hz, 2H), 4.39–4.42 (m, 2H), 6.80 (d, J = 7.6 Hz, 1H), 6.88 (d, J = 8.0 Hz, 1H), 7.27 (t, J = 7.8 Hz, 1H), 7.40 (d, J = 8.4 Hz, 2H), 7.54 (d, J = 7.6 Hz, 2H). 13C NMR (99 MHz, acetone-d6, δ): 21.5 (CH3), 22.6 (CH2), 27.5 (CH2), 54.0 (CH2), 69.2 (C), 118.7 (CH), 120.4 (CH), 129.0 (CH), 129.9 (CH), 130.9 (CH), 132.3 (C), 132.7 (C), 142.1 (C), 152.0 (C). The carbon directly attached to the boron atom was not detected, likely due to quadrupolar relaxation. HRMS-ESI (m/z): [M + Na]+ calcd for C17H16ONBF2Na, 322.11852; found, 322.11856.

2,2-Difluoro-3-mesityl-2,5,6,7-tetrahydro-[1,4,2]oxazaborinino[6,5,4-ij]quinoline-4-ium-2-uide (5c);

5c (76.1 mg, 0.23 mmol) was prepared in 47% yield from 1c (127.3 mg, 0.5 mmol) according to the procedure for the synthesis of 5a. 1H NMR (396 MHz, acetone-d6, δ): 2.17 (s, 6H), 2.18–2.26 (m, 2H), 2.31 (s, 3H), 3.08 (t, J = 6.3 Hz, 2H), 4.05 (t, J = 5.7 Hz, 2H), 6.83 (d, J = 7.7 Hz, 1H), 6.93 (d, J = 8.2 Hz, 1H), 7.00 (s, 2H), 7.31 (t, J = 7.7 Hz, 1H). 13C NMR (99 MHz, acetone-d6, δ): 19.5 (CH3), 21.0 (CH3), 22.6 (CH2), 27.4 (CH2), 52.7 (CH2), 118.7 (CH), 120.4 (CH), 125.5 (C), 129.4 (CH), 131.5 (CH), 132.5 (C), 133.0 (C), 133.7 (CH), 139.6 (C), 152.6 (C). The carbon directly attached to the boron atom was not detected, likely due to quadrupolar relaxation. HRMS-ESI (m/z): [M + Na]+ calcd for C19H20ONBF2Na, 350.14982; found, 350.15000.

2,2-Difluoro-3-(thiophen-2-yl)-2,5,6,7-tetrahydro-[1,4,2]oxazaborinino[6,5,4-ij]quinoline-4-ium-2-uide (5d);

5d (70.2 mg, 0.24 mmol) was prepared in 48% yield from 1d (145.5 mg, 0.5 mmol) according to the procedure for the synthesis of 5a. 1H NMR (396 MHz, acetone-d6, δ): 2.23–2.29 (m, 2H), 3.05 (t, J = 6.1 Hz, 2H), 4.71–4.74 (m, 2H), 6.79 (dd, J = 1.1, 7.5 Hz, 1H), 6.86 (d, J = 8.2 Hz, 1H), 7.23 (t, J = 7.9 Hz, 1H), 7.43 (dd, J = 3.6, 5.1 Hz, 1H), 8.16 (d, J = 4.1 Hz, 1H), 8.28 (dd, J = 1.0, 4.9 Hz, 1H). 13C NMR (100 MHz, acetone-d6, δ): 22.9 (CH2), 27.5 (CH2), 54.4 (CH2), 118.8 (CH), 120.4 (CH), 125.7 (C), 129.1 (CH), 130.5 (CH), 132.8 (C), 135.4 (C), 138.1 (CH), 139.9 (CH), 151.7 (C). The carbon directly attached to the boron atom was not detected, likely due to quadrupolar relaxation. HRMS-ESI (m/z): [M + Na]+ calcd for C14H12ONBF2NaS, 314.05929; found, 314.05960.

3-[4-(Diphenylamino)phenyl]-2,2-difluoro-2,5,6,7-tetrahydro-[1,4,2]oxazaborinino[6,5,4-ij]quinolin-4-ium-2-uide (5e);

5e (28.7 mg, 0.06 mmol) was prepared in 64% yield from 1e (58.8 mg, 0.1 mmol) according to the procedure for the synthesis of 5a. 1H NMR (392 MHz, acetone-d6, δ): 2.13–2.19 (m, 2H), 3.04 (t, J = 6.4 Hz, 2H), 4.49–4.52 (m, 2H), 6.78 (d, J = 7.7 Hz, 1H), 6.87 (d, J = 7.2 Hz, 1H), 7.01–7.04 (m, 2H), 7.20–7.26 (m, 7H), 7.40–7.44 (m, 4H), 7.61 (d, J = 8.6 Hz, 2H). 13C NMR (99 MHz, acetone-d6, δ): 23.0 (CH2), 27.6 (CH2), 54.0 (CH2), 118.6 (CH), 119.5 (CH), 120.4 (CH), 125.9 (CH), 127.1 (CH), 130.4 (CH), 130.7 (CH), 132.3 (C), 132.4 (CH), 147.3 (C), 151.6 (C), 152.2 (C). The carbon directly attached to the boron atom and the carbon next to it were not detected, likely due to quadrupolar relaxation. HRMS-ESI (m/z): [M + Na]+ calcd for C28H23ON2BF2Na, 475.17637; found, 475.17645.

Procedure for the Preparation of Cyclized Compound 6:

Potassium [2-(benzyloxy)phenyl]trifluoroborate 1f was synthesized according to the reported procedure.5d Note that there are small amounts of impurities mixed in because 1f is a potassium salt and it is difficult to perfectly isolate. 1H NMR (401 MHz, CDCl3, δ): 4.85 (s, 2H), 6.65–6.75 (m, 2H), 7.00–7.17 (m, 5H), 7.40 (s, 1H), 7.68 (d, J = 6.8 Hz, 1H). 13C NMR (99 MHz, CDCl3, δ): 71.1 (CH2), 114.6 (CH), 119.1 (C), 121.1 (CH), 127.4 (CH), 127.8 (CH), 128.6 (CH), 129.2 (CH), 132.0 (CH), 136.8 (C), 156.5 (C). The carbon directly attached to the boron atom and the carbon next to it were not detected, likely due to quadrupolar relaxation. HRMS-ESI (m/z): [M − K] calcd for C14H11O2BF3, 279.08097; found, 279.08113.

5f (118.0 mg, 0.30 mmol) was prepared in 60% yield from 1f (159.1 mg, 0.5 mmol) according to the procedure for the synthesis of 5a. The product 5f was used in the next reaction without further purification.

5f (118.0 mg, 0.30 mmol) was placed in an oven-dried screw neck reaction vial. After the vial was sealed with a screw cap containing a Teflon-coated rubber septum, it was connected to a vacuum/nitrogen manifold through a rubber tube. It was evacuated and then backfilled with nitrogen. This cycle was repeated three times. CH2Cl2 (3.0 mL) was then added in the vial through the rubber septum using syringes, and the solution was cooled to 0 °C. BBr3 in CH2Cl2 (1 M, 600 µL, 0.60 mmol) was added dropwise and the resultant solution was then stirred at 0 °C for 2 h. After reaction was completed, the reaction mixture was passed through a short silica gel column eluting with ethyl acetate/hexane (1:1) and concentrated in vacuo. The crude product was purified by Et2O wash to afford the desired product 6 (22.8 mg, 0.08 mmol, 27%) as a yellow solid. 1H NMR (392 MHz, acetone-d6, δ): 2.19–2.25 (m, 1H), 2.39–2.48 (m, 1H), 3.06 (t, J = 6.4 Hz, 2H), 4.40–4.46 (m, 1H), 4.72–4.79 (m, 1H), 6.85 (d, J = 6.3 Hz, 1H), 6.99–7.05 (m, 3H), 7.25 (t, J = 7.9 Hz, 1H), 7.66 (t, J = 8.6 Hz, 1H), 7.94 (dd, J = 0.9, 8.2 Hz, 1H). 13C NMR (99 MHz, acetone-d6, δ): 22.0 (CH2), 26.7 (CH2), 51.9 (CH2), 116.8 (CH), 119.4 (CH), 120.4 (CH), 120.7 (CH), 130.0 (CH), 132.7 (C), 140.2 (CH), 150.0 (C), 174.5 (C), 178.0 (C). The carbon directly attached to the boron atom was not detected, likely due to quadrupolar relaxation. HRMS-ESI (m/z): [M + Na]+ calcd for C16H13O2NBFNa, 304.09156; found, 304.09183.

This work was financially supported by JSPS KAKENHI grants JP17H06370, JP19H02784 and JP19H04555. This work was also supported by the Institute for Chemical Reaction Design and Discovery (ICReDD), established by the World Premier International Research Initiative (WPI) of MEXT, Japan.

Photophysical properties, details of DFT calculations, data for single crystal and powder X-ray structural analyses, and NMR spectra, including Figures S1–S7 and Tables S1–S2. This material is available on https://doi.org/10.1246/bcsj.20210024.

Hajime Ito

Hajime Ito was born in Osaka, Japan, in 1968. He completed his PhD in 1996 under the direction of the late Professor Yoshihiko Ito. He then worked as an Assistant Professor at Tsukuba University in corroboration with Professor Akira Hosomi, and moved to Institute for Molecular Science. He also joined Professor Kim D. Janda's research group at the Scripps Research Institute as a research associate in 2001. In 2002 he was appointed as an Associate Professor at Hokkaido University, working with Professor Masaya Sawamura. He was promoted to a full Professor at the same university in 2010.