Regiocontrol Using Fluoro Substituent on 3,6-Disubstituted Arynes

Abstract

The effect of fluoro substituent on the regioselectivity of several reactions of 3,6-disubstituted arynes was studied. These arynes contained another inductively electron-withdrawing substituent other than fluorine. A reasonable degree of regiocontrol was achieved in the (3 + 2) cycloaddition reaction of 3,6-disubstituted aryne containing both fluorine and bromine atoms with benzyl azide. Furthermore, the insertion reaction of aryne into Sn–F σ-bonds and the three-component coupling reaction involving the insertion of aryne into C=O π-bonds also led to the high degree of regiocontrol.

Introduction

The use of substituted arynes has attracted considerable attention for the preparation of complex and structurally diverse multi-substituted arenes.^1–5) However, the use of unsymmetrically substituted arynes in these reactions is limited because of low regioselectivity. In general, the regiocontrol of such reactions can be achieved by introducing a directing group at the 3-position of arynes^6–18) (Fig. 1). For examples, 3-methoxyaryne A and 3-haloarynes B1–B3 react with nucleophiles at the C1 position.^6–10) The distortion models proposed by Garg and colleagues and Houk and colleagues can be used to explain these regioselectivities.^8,19) The optimized structures of 3-methoxyaryne A and 3-fluoroaryne B1 show significant distortion; thus, nucleophiles regioselectively attack the more linear carbon at the C1 position of A and B1. The attack of nucleophiles at the C1 position is also sterically favorable. Furthermore, the effect of halogen atoms on the regioselectivity decreases in the following order: 3-fluoroaryne B1 > 3-chloroaryne B2 > 3-bromoaryne B3.⁸⁾ This trend is consistent with the degree of aryne distortion (B1: X = F > B2: X = Cl > B3: X = Br).

Fig. 1. Regioselectivity of Arynes and Calculation Study by Houk and Garg

In contrast to 3-substituted arynes, controlling the regioselectivity of the reactions of 3,6-disubstituted arynes is difficult. Therefore, our research group is interested in the effect of fluoro substituent on 3,6-disubstituted arynes C and D, which contain another inductively electron-withdrawing substituent. Garg and Houk’s computational studies show that aryne C has a high degree of distortion, whereas D shows little distortion.¹⁹⁾ Therefore, we expected that the reaction of aryne C may proceed regioselectively, although the attack of nucleophiles at the more linear carbon of C is sterically unfavorable. In this study, we investigated the effect of fluoro substituent on the regioselectivity of several reactions of arynes C and D. To our best knowledge, a precursor for generating aryne C and the reactions of aryne C have not yet been reported.

Results and Discussion

Preparation of Aryne Precursor 1

We recently developed a simple and practical method for the preparation of ortho-(trimethylsilyl)aryl triflate precursors.²⁰⁾ As per this method, precursor 1 for generating aryne C can be prepared through O-trimethylsilylation, the migration of the trimethylsilyl group, and triflation (Chart 1). First, 2-bromo-5-fluorophenol 3 was reacted with hexamethyldisilazane (HMDS), to produce the O-trimethylsilylated intermediate. Subsequently, the in situ-generated O-trimethylsilylated intermediate was treated with Mg(TMP)₂·2LiCl to produce C-silylated phenol 4 in 70% yield via selective deprotonation (metalation). This reaction was followed by the migration of the trimethylsilyl group. Tokuyama and colleagues used Mg(TMP)₂·2LiCl for aryne generation.^21–23) In our method, Mg(TMP)₂·2LiCl was used in the selective deprotonation step without noticeable aryne generation. The precursor 1 was obtained in 88% yield by treating C-silylated phenol 4 with Tf₂O in the presence of (i-Pr)₂NEt. In our previous study,²⁰⁾ we reported the preparation of precursor 2 for generating aryne D.

Chart 1. Preparation of Aryne Precursor 1

Cycloaddition Reactions

First, we investigated the [4 + 2] cycloaddition reactions of 3,6-disubstituted arynes using 2-bromofuran as a diene (Chart 2). To estimate the reactivity of aryne C, the reaction of precursor 1 was compared with those of precursor 5 for 3-methoxyaryne A, precursor 8 for 3-chloroaryne B2, and precursor 2 for 3-fluoro-6-methoxyaryne D. Giles et al. reported that the reaction of A, which was generated from precursor 5, with 2-bromofuran produced a mixture of regioisomers 6a and 7a at a ratio of 63 : 37.²⁴⁾ We also reported that the reaction of B2, generated from precursor 8, affords regioisomers 6b and 7b in 46 and 30% yields, respectively (6b : 7b = approximately 6 : 4).²⁰⁾ In these reactions, sterically unfavorable syn-adducts 6a and 6b were obtained as the major products. Similarly, when ortho-(trimethylsilyl)aryl triflate 1 was used as a precursor for aryne C, a mixture of regioisomers 6c and 7c was obtained at a ratio of 7 : 3 because of the regiocontrol of the fluoro substituent.²⁵⁾ Although these [4 + 2] cycloaddition reactions show low regioselectivities, the formation of major isomers 6a–6c is consistent with the aryne distortion models reported by Garg and Houk.¹⁹⁾ In contrast, the reaction of 3-fluoro-6-methoxyaryne D, which was generated from precursor 2, proceeded with relatively lower regioselectivity, producing regioisomers 6d and 7d in 37 and 46% yields, respectively. Surprisingly, the use of precursor 2 resulted in the flipped selectivity from the distortion model, although the selectivity was low.

Chart 2. [4 + 2] Cycloaddition Reactions of 3,6-Disubstituted Arynes with 2-Bromofuran

Next, the (3 + 2) cycloaddition reactions of 3,6-disubstituted arynes were examined using benzyl azide as a 1,3-dipole (Table 1). To study the effect of the fluoro substituent on regioselectivity, the reactions of aryne C generated from precursor 1 were compared with those of precursors 8 and 2. Good regioselectivities were observed in the (3 + 2) cycloadditions of precursors 8 and 1. As reported in our previous study,²⁰⁾ the Huisgen-type reaction of 3-chloroaryne precursor 8 proceeded regioselectively to give 9a in 91% yield; no other isomers were detected in this reaction (entry 1). When the reaction of precursor 1 was carried out at room temperature, a mixture of regioisomers 9b and 10b was obtained in 35% combined yield at a ratio of 9 : 1 (entry 2). When precursor 1 was treated with benzyl azide at 50 °C, the chemical yields of products 9b and 10b increased slightly (entry 3). As precursor 1 was consumed and not recovered in these reactions (entries 2 and 3), we carried out the cycloaddition reaction using precursor 1 and benzyl azide in a 2 : 1 ratio (entry 4). As expected, the chemical yield of the reaction improved, and a mixture of regioisomers 9b and 10b was obtained in 68% combined yield based on the amount of benzyl azide. When precursor 1 (3 equivalent (equiv.)) and benzyl azide (1 equiv.) were used, the chemical yield increased to 74% combined yield (entry 4). The major isomer 9b was isolated in the form of pure crystals by the recrystallization of this isomeric mixture. In contrast, no regioselectivity was observed in the reaction of 3-fluoro-6-methoxyaryne precursor 2 (entry 6). The regioisomers 9c and 10c were obtained in 33 and 33% yields, respectively, because the directing effect of the fluorine atom on aryne D was dramatically decreased by the 6-methoxy group, which is an inductively electron-withdrawing group. These results are consistent with the aryne distortion models.

Table 1. (3 + 2) Cycloaddition Reactions of 3,6-Disubstituted Arynes with BnN₃

Entry	Precursor	T	Product	Combined yield (%)a)
1b)	8 (1 equiv.)	r.t.	9a	91e)
2b)	1 (1 equiv.)	r.t.	9b : 10b = 9 : 1	35
3b)	1 (1 equiv.)	50 °C	9b : 10b = 9 : 1	41
4c)	1 (2 equiv.)	50 °C	9b : 10b = 9 : 1	68
5d)	1 (3 equiv.)	50 °C	9b : 10b = 9 : 1	74
6b)	2 (1 equiv.)	r.t.	9c : 10c = 1 : 1	66f)

a) Isolated yield based on the amount of BnN₃. b) Precursor (1 equiv.), BnN₃ (1 equiv.) and CsF (3 equiv.) were used. c) Precursor (2 equiv.), BnN₃ (1 equiv.) and CsF (6 equiv.) were used. d) Precursor (3 equiv.), BnN₃ (1 equiv.) and CsF (9 equiv.) were used. e) This result was reported in our previous paper.²⁰⁾ f) 9c and 10c could be separated and were obtained in 33 and 33% yields, respectively.

Insertion of Arynes into σ-Bonds

Next, we investigated the insertion of arynes into σ-bonds. First, we studied the insertion of 3,6-disubstituted arynes into the C–N σ-bond of 1,3-dimethyl-2-imidazolidinone (DMI)^26–28) (Chart 3). In the presence of CsF, the reactions of aryne precursors 8, 1, and 2 were carried out in DMI. As reported in our previous study,²⁰⁾ 3-chloroaryne B2, which was generated from precursor 8, regioselectively reacted with DMI to give the tetrahydrobenzodiazepine derivative 11a in 93% yield. In contrast, modest regioselectivity was observed in the insertion reaction of aryne precursor 1. The attack of bulky DMI at the linear carbon of aryne C was sterically suppressed to produce the mixture of regioisomers 11b and 12b at a lower ratio of 5 : 3. Hence, the aryne distortion models cannot be applied to reactions involving a bulky nucleophile. The reaction of 3-fluoro-6-methoxyaryne precursor 2 also afforded the regioisomers 11c and 12c in 56 and 24% yields, respectively. As a general trend, the chemical yields in the reactions of 1 were relatively low, because aryne C having a bromine atom would be unstable compared to other arynes B2 and D.

Chart 3. Insertion Reactions of 3,6-Disubstituted Arynes Using 1,3-Dimethyl-2-imidazolidinone (DMI)

We then studied the insertion of arynes into the Sn–F σ-bond of n-Bu₃SnF²⁹⁾ (Chart 4). In the presence of KF and 18-crown-6, the reaction of precursor 8 with n-Bu₃SnF was carried out in 1,2-dimethoxyethane (DME) at 0 °C. In our previous study,²⁰⁾ the insertion of 3-chloroaryne B2 into the σ-bond of n-Bu₃SnF afforded the desired regioisomer 13a in 92% yield. Next, the reaction of precursor 1 with n-Bu₃SnF was carried out in DME at 50 °C in the presence of CsF. Although the chemical yield was low, a 9 : 1 mixture of regioisomers 13b and 14b was obtained. In the case of precursor 2, the regioisomers 13c and 14c were obtained in 20 and 6% yields, respectively. In contrast to the insertion of precursor 8 into the C–N σ-bond of bulky DMI, the aryne distortion models work well for explaining the regioselectivity of the insertion of arynes into the Sn–F σ-bond of n-Bu₃SnF.

Chart 4. Insertion Reactions Using n-Bu₃SnF

Insertion of Aryne into π-Bonds

Finally, the insertion of aryne C into the C=O π-bond of N,N-dimethylformamide (DMF) was examined in a three-component coupling reaction^30–32) (Chart 5). As reported in our previous study,²⁰⁾ the use of aryne precursor 8 led to the selective formation of 16a. Next, the precursor 1 was treated with diethyl malonate 15 in DMF at room temperature in the presence of CsF. Subsequently, the desired regioisomer 16b was obtained as a single regioisomer in 87% yield via the formation of a [2 + 2]-type intermediate E and the trapping of intermediate F by the anion of 15. Notably, DMF is less hindered and can regioselectively attack the linear carbon of aryne C, leading to the high degree of regiocontrol expected from the aryne distortion model. In marked contrast, three-component coupling reaction using 3-fluoro-6-methoxyaryne precursor 2 gave the regioisomers 16c and 17c.

Chart 5. Three-Component Coupling Reactions via Insertion of DMF

Conclusion

As predicted from the aryne distortion model, the reactions of 3,6-disubstituted arynes containing both fluorine and bromine atoms afforded regioisomers with reasonable regioselectivities. The steric repulsion between the 3,6-disubstituted arynes and the nucleophiles also affected the regioselectivity. In particular, the less sterically hindered nucleophiles regioselectively attacked the linear carbon of the arynes, resulting in a high degree of regiocontrol.

Experimental

General

IR spectra were measured on a JASCO FT/IR-4100. ¹H-NMR spectra were measured on a JEOL ECX-400 PSK (400 MHz) or Varian NMRS 600 (600 MHz) with CDCl₃ as an internal standard (7.26 ppm). ¹³C-NMR spectra were measured on a JEOL ECX-400 PSK (101 MHz) or Varian NMRS 600 (151 MHz) with CDCl₃ as an internal standard (77.0 ppm). ¹⁹F-NMR spectra were measured on a JEOL ECX-400 PSK (376 MHz) with C₆F₆ as an internal standard (–162.2 ppm). High-resolution (HR) mass spectra were recorded on a time-of-flight (TOF) mass spectrometer by use of microTOF-Q LC/electrospray ionization (ESI)-TOF (Bruker Daltonics) and LCMS™-9030 LCMS-Q-TOF (Shimadzu, Kyoto, Japan) mass spectrometers. For silica gel column chromatography, SiliCycle Inc., Canada, SiliaFlash F60 was used. Preparative TLC separations were carried out on precoated silica gel plates (E. Merck 60F₂₅₄).

Procedure for Preparing Mg(TMP)₂·2LiCl

To a suspension of magnesium metal (turning, 535 mg, 22.0 mmol) in freshly distilled tetrahydrofuran (THF) (55 mL) was added 1,2-dichloroethane (1.72 mL, 22.0 mmol) under argon atmosphere at room temperature and stirred at the same temperature for 1 h. In another two neck flask, n-BuLi (1.6 M in hexane, 27.5 mL, 44.0 mmol) was added dropwise to a solution of 2,2,6,6-tetramethylpiperidine (TMPH, 7.43 mL, 44.0 mmol) in freshly distilled THF (22 mL) under argon atmosphere at –80 °C. After being warmed to 0 °C, this mixture was stirred at the same temperature for 30 min. To Li(TMP) solution in two neck flask was added dropwise the MgCl₂ solution at 0 °C and the reaction mixture was stirred at the same temperature for 1 h.

6-Bromo-3-fluoro-2-(trimethylsilyl)phenol (4)

To a solution of phenol derivative 3 (3.82 g, 20.0 mmol) in freshly distilled THF (20 mL) was added 1,1,1,3,3,3-hexamethyldisilazane (HMDS, 6.26 mL, 30.0 mmol) under argon atmosphere at room temperature. After being stirred at 50 °C the same temperature for 5 h, the reaction mixture was concentrated at reduced pressure. The obtained O-trimethylsilylated intermediates were used for next trimethylsilyl (TMS)-migration reaction without purification. To a solution of O-trimethylsilylated intermediates (20.0 mmol) in freshly distilled THF (20 mL) was added dropwise Mg(TMP)₂·2LiCl solution under argon atmosphere at –80 °C. After being stirred at the same temperature for 1 h, the reaction temperature was raised to 0 °C and the reaction mixture was stirred at the same temperature for 3 h. The reaction mixture was diluted with iced 0.5 M sodium (+)-tartrate solution and then extracted with hexanes. The organic phase was dried over Na₂SO₄ and concentrated at reduced pressure. Purification of the residue by flash silica gel column chromatography (AcOEt : hexanes = 0 : 1–1 : 1) afforded the product 4 (3.70 g, 70%) as colorless oil. ¹H-NMR (CDCl₃) δ: 7.39 (1H, dd, J = 8.7, 5.5 Hz), 6.50 (1H, t, J = 8.7 Hz), 5.79 (1H, br d, J = 0.9 Hz), 0.36 (9H, d, J = 1.8 Hz); ¹³C-NMR (CDCl₃) δ: 167.0 (d, J = 244 Hz), 156.7 (d, J = 17 Hz), 133.3 (d, J = 11 Hz), 113.3 (d, J = 36 Hz), 109.1 (d, J = 30 Hz), 104.9 (d, J = 3 Hz), 0.3 (d, J = 3 Hz); ¹⁹F-NMR (CDCl₃) δ: −99.0 (1F, s); IR (KBr) cm⁻¹: 3510 (br), 2956, 1597, 1450, 1416, 1251; HR-MS (ESI) m/z: 260.9738 (Calcd for C₉H₁₁⁷⁹BrFOSi (M–H⁻): 260.9752), 262.9720 (Calcd for C₉H₁₁⁸¹BrFOSi (M–H⁻): 262.9732).

6-Bromo-3-fluoro-2-(trimethylsilyl)phenyl Trifluoromethanesulfonate (1)

To a solution of 4 (1.58 g, 6.0 mmol) and N,N-diisopropylethylamine (1.2 mL, 6.9 mmol) in anhydrous dichloromethane (15 mL) was added dropwise trifluoromethanesulfonic anhydride (1.11 mL, 6.6 mmol) under argon atmosphere at –40 °C. After being stirred at the same temperature for 5 h, the reaction mixture was diluted with iced water and then extracted with dichloromethane. The organic phase was dried over Na₂SO₄ and concentrated at reduced pressure. Purification of the residue by flash silica gel column chromatography (AcOEt : hexanes = 0 : 1–1 : 6) afforded the product 1 (2.08 g, 88%) as colorless oil. ¹H-NMR (CDCl₃) δ: 7.65 (1H, dd, J = 8.4, 6.0 Hz), 6.97 (1H, t, J = 8.4 Hz), 0.44 (9H, d, J = 1.8 Hz); ¹³C-NMR (CDCl₃) δ: 166.3 (d, J = 247 Hz), 147.7 (d, J = 14 Hz), 136.1 (d, J = 10 Hz), 125.4 (d, J = 36 Hz), 118.5 (q, J = 322 Hz), 116.8 (d, J = 29 Hz), 111.8 (d, J = 4 Hz), 0.3 (d, J = 4 Hz); ¹⁹F-NMR (CDCl₃) δ: −72.5 (3F, s), −94.5 (1F, m); IR (KBr) cm⁻¹: 2959, 1589, 1432, 1409, 1214; HR-MS (ESI) m/z: 392.9241 (Calcd for C₁₀H₁₀⁷⁹BrF₄O₃SSi (M–H⁻): 392.9245), 394.9218 (Calcd for C₁₀H₁₀⁸¹BrF₄O₃SSi (M–H⁻): 394.9224).

Diels–Alder-Type Reaction of Precursor 1 with 2-Bromofuran

To a suspension of CsF (183 mg, 1.2 mmol) in anhydrous acetonitrile (8.0 mL) were added 2-bromofuran (72.0 µL, 0.80 mmol) and precursor 1 (158 mg, 0.40 mmol) under argon atmosphere at room temperature. After being stirred at the same temperature for 1 h, the reaction mixture was filtrated with folded filter paper. The filtrate was concentrated at reduced pressure. The purification by preparative TLC (AcOEt:hexanes = 1 : 10, 2-fold development) afforded the mixture of 6c and 7c (51.8 mg, 40%, 7 : 3 ratio).²⁵⁾

Mixture of 1,5-Dibromo-8-fluoro-1,4-dihydro-1,4-epoxynaphthalene (6c) and 1,8-Dibromo-5-fluoro-1,4-dihydro-1,4-epoxynaphthalene (7c) in 7 : 3 Ratio

Colorless oil; ¹H-NMR (CDCl₃) δ: 7.16–7.04 (3H, m), 6.68 (7/10H, t, J = 8.4 Hz), 6.65 (3/10H, d, J = 8.7 Hz), 5.91 (3/10H, d, J = 1.8 Hz), 5.74 (7/10H, t, J = 1.8 Hz); ¹³C-NMR (CDCl₃) δ: 155.2 (7/10C, d, J = 253 Hz), 154.7 (3/10C, d, J = 249 Hz), 151.8 (7/10C, d, J = 3 Hz), 148.4 (3/10C, d, J = 4 Hz), 146.5 (7/10C), 146.0 (3/10C), 144.0 (3/10C), 143.6 (7/10C), 135.7 (7/10C, d, J = 17 Hz), 135.4 (3/10C, d, J = 20 Hz), 133.4 (3/10C, d, J = 6 Hz), 131.7 (7/10C, d, J = 7 Hz), 117.6 (7/10C, d, J = 24 Hz), 117.0 (3/10C, d, J = 23 Hz), 110.1 (3/10C, d, J = 3 Hz), 108.6 (7/10C, d, J = 2 Hz), 91.6 (3/10C, d, J = 2 Hz), 87.7 (7/10C, d, J = 3 Hz), 82.7 (7/10C), 78.8 (3/10C); ¹⁹F-NMR (CDCl₃) δ: −124.9 (3/10F, dd, J = 8, 4 Hz), −125.6 (7/10F, ddd, J = 8, 4, 2 Hz); IR (KBr) cm⁻¹: 3098, 2929, 1626, 1465, 1239; HR-MS (DUIS) m/z: 318.8775 (Calcd for C₁₀H₆⁷⁹Br₂FO (M + H⁺): 318.8764), 320.8758 (Calcd for C₁₀H₆⁷⁹Br⁸¹BrFO (M + H⁺): 320.8743), 322.8723 (Calcd for C₁₀H₆⁸¹Br₂FO (M + H⁺): 322.8735).

Diels–Alder-Type Reaction of Precursor 2 with 2-Bromofuran

To a suspension of CsF (183 mg, 1.2 mmol) in anhydrous acetonitrile (8.0 mL) were added 2-bromofuran (72.0 µL, 0.80 mmol) and precursor 2 (138 mg, 0.40 mmol) under argon atmosphere at room temperature. After being stirred at the same temperature for 1 h, the reaction mixture was filtrated with folded filter paper. The filtrate was concentrated at reduced pressure. The purification by preparative TLC (AcOEt:hexanes = 1 : 10, 2-fold development) afforded the isomeric products 6d (40.0 mg, 37%) and 7d (49.8 mg, 46%).²⁵⁾

1-Bromo-8-fluoro-5-methoxy-1,4-dihydro-1,4-epoxynaphthalene (6d)

Colorless oil; ¹H-NMR (CDCl₃) δ: 7.06 (1H, dd, J = 5.5, 1.8 Hz), 7.02 (1H, d, J = 5.5 Hz), 6.72 (1H, t, J = 8.7 Hz), 6.59 (1H, dd, J = 8.7, 3.0 Hz), 5.86 (1H, t, J = 1.8 Hz), 3.80 (3H, s); ¹³C-NMR (CDCl₃) δ: 150.9 (d, J = 246 Hz), 148.7 (d, J = 2 Hz), 146.0, 144.2, 137.2 (d, J = 2 Hz), 134.9 (d, J = 17 Hz), 116.2 (d, J = 24 Hz), 113.1 (d, J = 6 Hz), 87.9 (d, J = 3 Hz), 80.0, 56.2; ¹⁹F-NMR (CDCl₃) δ: −133.4 (1F, ddd, J = 8, 3, 2 Hz); IR (KBr) cm⁻¹: 2941, 2840, 1629, 1494, 1276, 1243; HR-MS (DUIS) m/z: 270.9770 (Calcd for C₁₁H₉⁷⁹BrFO₂ (M + H⁺): 270.9764), 272.9750 (Calcd for C₁₁H₉⁸¹BrFO₂ (M + H⁺): 272.9745).

1-Bromo-5-fluoro-8-methoxy-1,4-dihydro-1,4-epoxynaphthalene (7d)

Colorless oil; ¹H-NMR (CDCl₃) δ: 7.06 (1H, d, J = 5.5 Hz), 7.02 (1H, dd, J = 5.5, 2.1 Hz), 6.73 (1H, dd, J = 9.2, 7.4 Hz), 6.63 (1H, dd, J = 9.2, 3.7 Hz), 5.85 (1H, d, J = 1.8 Hz), 3.84 (3H, s); ¹³C-NMR (CDCl₃) δ: 150.1 (d, J = 2 Hz), 150.1 (d, J = 243 Hz), 146.6, 143.5, 136.2 (d, J = 3 Hz), 135.1 (d, J = 23 Hz), 116.0 (d, J = 23 Hz), 114.0 (d, J = 7 Hz), 88.8 (d, J = 2 Hz), 79.1, 56.8; ¹⁹F-NMR (CDCl₃) δ: −131.8 (1F, dd, J = 7,3 Hz); IR (KBr) cm⁻¹: 2941, 2839, 1628, 1494, 1276, 1243; HRMS (DUIS) m/z: 270.9771 (Calcd for C₁₁H₉⁷⁹BrFO₂ (M + H⁺): 270.9764), 272.9751 (Calcd for C₁₁H₉⁸¹BrFO₂ (M + H⁺): 272.9745).

(3 + 2) Cycloaddition of Precursor 1 with Benzylazide

To a suspension of CsF (273 mg, 1.8 mmol) in anhydrous acetonitrile (12.0 mL) were added benzylazide (25.0 µL, 0.20 mmol) and precursor 1 (237 mg, 0.60 mmol) under argon atmosphere at 50 °C. After being stirred at the same temperature for 1 h, the reaction mixture was diluted with water and then extracted with AcOEt. The organic phase was dried over Na₂SO₄ and concentrated at reduced pressure. Purification of the residue by flash silica gel column chromatography (AcOEt:hexanes = 0 : 1–1 : 6) afforded the mixture of 9b and 10b (45.1 mg, 74%, 9 : 1 ratio).²⁵⁾ Recrystallization of this mixture (AcOEt-hexanes) led to the pure major isomer 9b.

1-Benzyl-7-bromo-4-fluoro-1H-benzo[d][1,2,3]triazole (9b)

Colorless crystals; mp 77–78 °C (AcOEt-hexanes); ¹H-NMR (CDCl₃) δ: 7.45 (1H, dd, J = 8.4, 4.2 Hz), 7.35–7.27 (3H, m), 7.25–7.20 (2H, m), 6.94 (1H, dd, J = 9.6, 8.4 Hz), 6.19 (2H, s); ¹³C-NMR (CDCl₃) δ: 153.0 (d, J = 214 Hz), 137.5 (d, J = 20 Hz), 135.7, 133.5 (d, J = 5 Hz), 132.1 (d, J = 8 Hz), 128.8, 128.3, 127.1, 110.2 (d, J = 18 Hz), 96.8 (d, J = 4 Hz), 52.3; ¹⁹F-NMR (CDCl₃) δ: −125.2 (1F, dd, J = 9, 4 Hz); IR (KBr) cm⁻¹: 3070, 3035, 2958, 2927, 2856, 1955, 1849, 1722, 1617, 1585, 1505, 1241; HR-MS (DUIS) m/z: 306.0045 (Calcd for C₁₃H₁₀⁷⁹BrFN₃ (M + H⁺): 306.0037), 308.0022 (Calcd for C₁₃H₁₀⁸¹BrFN₃ (M + H⁺): 308.0017).

1-Benzyl-4-bromo-7-fluoro-1H-benzo[d][1,2,3]triazole (10b)

The following NMR data of minor isomer 10b were obtained from the mixture of 9b and 10b in 9 : 1 ratio. ¹H-NMR (CDCl₃) δ: 7.44 (1H, dd, J = 7.8, 3.6 Hz), 7.35–7.27 (3H, m), 7.25–7.20 (2H, m), 7.01 (1H, dd, J = 9.6, 7.8 Hz), 5.93 (2H, s); ¹³C-NMR (CDCl₃) δ: 147.2 (d, J = 251 Hz), 137.2 (d, J = 19 Hz), 134.8, 133.4 (d, J = 7 Hz), 133.2 (d, J = 4 Hz), 128.9, 128.5, 127.7, 113.2 (d, J = 18 Hz), 96.8 (d, J = 4 Hz), 53.9 (d, J = 2 Hz); ¹⁹F-NMR (CDCl₃) δ: −131.7 (1F, dd, J = 10, 4 Hz).

(3 + 2) Cycloaddition of Precursor 2 with Benzylazide

To a suspension of CsF (91.1 mg, 0.60 mmol) in anhydrous acetonitrile (4.0 mL) were added benzylazide (25.0 µL, 0.20 mmol) and precursor 2 (69.3 mg, 0.20 mmol) under argon atmosphere at room temperature. After being stirred at the same temperature for 1 h, the reaction mixture was diluted with water and then extracted with AcOEt. The organic phase was dried over Na₂SO₄ and concentrated at reduced pressure. Purification of the residue by preparative TLC (iPr₂O : CHCl₃ = 10 : 1) afforded the isomeric products 9c (17.1 mg, 33%) and 10c (17.2 mg, 33%).²⁵⁾

1-Benzyl-4-fluoro-7-methoxy-1H-benzo[d][1,2,3]triazole (9c)

Colorless crystals; mp 117–118 °C (CH₂Cl₂-hexanes); ¹H-NMR (CDCl₃) δ: 7.33–7.27 (5H, m), 6.88 (1H, dd, J = 9.6, 8.2 Hz), 6.62 (1H, dd, J = 8.2, 3.0 Hz), 5.99 (2H, s), 3.93 (3H, s); ¹³C-NMR (CDCl₃) δ: 147.4 (d, J = 251 Hz), 142.1 (d, J = 4 Hz), 137.8 (d, J = 20 Hz), 136.0, 128.7, 128.2, 127.8, 126.5 (d, J = 7 Hz), 108.2 (d, J = 19 Hz), 105.6 (d, J = 7 Hz), 56.0, 53.4; ¹⁹F-NMR (CDCl₃) δ: −135.4 (1F, dd, J = 9, 3 Hz); IR (KBr) cm⁻¹: 2924, 1529, 1240; HR-MS (ESI) m/z: 258.1040 (Calcd for C₁₄H₁₃FN₃O (M + H⁺): 258.1037).

1-Benzyl-7-fluoro-4-methoxy-1H-benzo[d][1,2,3]triazole (10c)

Colorless crystals; mp 80–81 °C (hexanes); ¹H-NMR (CDCl₃) δ: 7.35–7.29 (5H, m), 7.00 (1H, dd, J = 10.1, 8.2 Hz), 6.51 (1H, dd, J = 8.2, 2.8 Hz), 5.90 (2H, s), 4.06 (3H, s); ¹³C-NMR (CDCl₃) δ: 147.8 (d, J = 3 Hz), 141.9 (d, J = 241 Hz), 140.2 (d, J = 2 Hz), 135.3, 128.8, 128.4, 127.7, 124.1 (d, J = 16 Hz), 112.4 (d, J = 18 Hz), 102.6 (d, J = 6 Hz), 56.5, 53.4 (d, J = 2 Hz); ¹⁹F-NMR (CDCl₃) δ: −141.6 (1F, dd, J = 10, 3 Hz); IR (KBr) cm⁻¹: 2937, 1532, 1258; HR-MS (ESI) m/z: 258.1045 (Calcd for C₁₄H₁₃FN₃O (M + H⁺): 258.1037).

Reaction of Precursor 1 with DMI

To a suspension of CsF (91.1 mg, 0.60 mmol) and in distilled DMI (1.0 mL) was added precursor 1 (79.0 mg, 0.20 mmol) under argon atmosphere at room temperature. After being stirred at the same temperature for 3 h, the reaction mixture was diluted with AcOEt and then filtrated with folded filter paper. The filtrate was concentrated at reduced pressure. The purification of the residue by flash silica gel column chromatography (AcOEt:hexanes = 1 : 1–1 : 0) afforded the mixture of 11b and 12b (18.4 mg, 32%, 5 : 3 ratio). The isomeric products 11b and 12b could be separated by second purification using preparative TLC (AcOEt : hexanes = 1 : 10, 2-fold development) to give 11b and 12b.²⁵⁾

9-Bromo-6-fluoro-1,4-dimethyl-1,2,3,4-tetrahydro-5H-benzo[e][1,4]diazepin-5-one (11b)

Colorless oil; ¹H-NMR (CDCl₃) δ: 7.52 (1H, dd, J = 8.7, 5.7 Hz), 6.79 (1H, t, J = 8.7 Hz), 3.39 (2H, br s), 3.33 (2H, br t, J = 5.5 Hz), 3.19 (3H, s), 3.03 (3H, s); ¹³C-NMR (CDCl₃) δ: 164.7, 159.4 (d, J = 256 Hz), 145.3 (d, J = 5 Hz), 136.3 (d, J = 11 Hz), 124.3 (d, J = 13 Hz), 115.3 (d, J = 4 Hz), 114.0 (d, J = 23 Hz), 55.1, 48.7, 43.3, 33.8; ¹⁹F-NMR (CDCl₃) δ: −114.5 (1F, dd, J = 9, 6 Hz); IR (KBr) cm⁻¹: 2923, 1658, 1597, 1454; HR-MS (ESI) m/z: 309.0011 (Calcd for C₁₁H₁₂⁷⁹BrFN₂ONa (M + Na⁺): 309.0009), 310.9994 (Calcd for C₁₁H₁₂⁸¹BrFN₂ONa (M + Na⁺): 310.9990).

6-Bromo-9-fluoro-1,4-dimethyl-1,2,3,4-tetrahydro-5H-benzo[e][1,4]diazepin-5-one (12b)

mp Colorless crystals; 162–164 °C (CH₂Cl₂-hexanes); ¹H-NMR (CDCl₃) δ: 7.23 (1H, dd, J = 8.7, 4.3 Hz), 6.93 (1H, dd, J = 11.9, 8.7 Hz), 3.39 (2H, br s), 3.24 (2H, br s), 3.20 (3H, s), 2.96 (3H, d, J = 6.4 Hz); ¹³C-NMR (CDCl₃) δ: 167.1 (d, J = 2 Hz), 157.0 (d, J = 249 Hz), 134.7 (d, J = 3 Hz), 134.6 (d, J = 12 Hz), 128.7 (d, J = 8 Hz), 119.2 (d, J = 23 Hz), 115.0 (d, J = 2 Hz), 55.8, 48.2, 41.5 (d, J = 10 Hz), 33.7; ¹⁹F-NMR (CDCl₃) δ: −124.9 (1F, m); IR (KBr) cm⁻¹: 2921, 1658, 1595, 1456; HR-MS (ESI) m/z: 309.0003 (Calcd for C₁₁H₁₂⁷⁹BrFN₂ONa (M + Na⁺): 309.0009), 310.9986 (Calcd for C₁₁H₁₂⁸¹BrFN₂ONa (M + Na⁺): 310.9990).

Reaction of Precursor 2 with DMI

To a suspension of CsF (91.1 mg, 0.60 mmol) and in distilled DMI (1.0 mL) was added precursor 2 (69.2 mg, 0.20 mmol) under argon atmosphere at room temperature. After being stirred at the same temperature for 3 h, the reaction mixture was diluted with AcOEt and then filtrated with folded filter paper. The filtrate was concentrated at reduced pressure. Purification of the residue by flash silica gel column chromatography (AcOEt:hexanes = 0 : 1–1 : 0) afforded the isomeric products 11c (27.0 mg, 56%) and 12c (11.4 mg, 24%).²⁵⁾

6-Fluoro-9-methoxy-1,4-dimethyl-1,2,3,4-tetrahydro-5H-benzo[e][1,4]diazepin-5-one (11c)

Colorless crystals; mp 131–133 °C (CH₂Cl₂-hexanes); ¹H-NMR (CDCl₃) δ: 6.83 (1H, dd, J = 9.2, 5.0 Hz), 6.76 (1H, t, J = 9.2 Hz), 3.76 (3H, s), 3.38 (2H, br t, J = 5.5 Hz), 3.27 (2H, br t, J = 5.5 Hz), 3.17 (3H, s), 2.92 (3H, s); ¹³C-NMR (CDCl₃) δ: 165.8, 154.3 (d, J = 247 Hz), 151.2 (d, J = 3 Hz), 135.7 (d, J = 5 Hz), 121.6 (d, J = 14 Hz), 115.0 (d, J = 11 Hz), 110.8 (d, J = 23 Hz), 56.4, 56.1, 48.5, 42.0, 33.7; ¹⁹F-NMR (CDCl₃) δ: −124.0 (dd, J = 8, 5 Hz); IR (KBr) cm⁻¹: 2935, 1654, 1606, 1490; HR-MS (ESI) m/z: 261.0996 (Calcd for C₁₂H₁₅FN₂O₂Na (M + Na⁺): 261.1010).

9-Fluoro-6-methoxy-1,4-dimethyl-1,2,3,4-tetrahydro-5H-benzo[e][1,4]diazepin-5-one (12c)

Colorless oil; ¹H-NMR (CDCl₃) δ: 6.99 (1H, dd, J = 11.9, 9.2 Hz), 6.57 (1H, dd, J = 9.2, 3.2 Hz), 3.82 (3H, s), 3.39 (2H, br s), 3.22 (2H, br s), 3.17 (3H, s), 2.95 (3H, d, J = 6.4 Hz); ¹³C-NMR (CDCl₃) δ: 167.0, 153.4 (d, J = 8 Hz), 152.3 (d, J = 251 Hz), 133.9 (d, J = 12 Hz), 122.2, 118.1 (d, J = 23 Hz), 107.1 (d, J = 8 Hz), 56.5, 55.9, 48.3, 41.5 (d, J = 10 Hz), 33.5; ¹⁹F-NMR (CDCl₃) δ: −133.4 (1F, m); IR (KBr) cm⁻¹: 2932, 1653, 1605, 1491, 1255; HR-MS (ESI) m/z: 261.1011 (Calcd for C₁₂H₁₅FN₂O₂Na (M + Na⁺): 261.1010).

Reaction of Precursor 1 with n-Bu₃SnF

To a suspension of CsF (91.1 mg, 0.60 mmol) in anhydrous 1,2-dimethoxyethane (DME, 3.0 mL) were added tributyltin fluoride (61.8 mg, 0.20 mmol) and precursor 1 (119 mg, 0.30 mmol) under argon atmosphere at room temperature. After being stirred at 50 °C for 1 h, the reaction mixture was diluted with water and then extracted with AcOEt. The organic phase was dried over Na₂SO₄ and concentrated at reduced pressure. Purification of the residue by preparative TLC (AcOEt : hexanes = 1 : 10, 2-fold development) afforded the mixture of 13b and 14b (7.7 mg, 8%, 9 : 1 ratio).²⁵⁾

(3-Bromo-2,6-difluorophenyl)tributylstannane (13b)

The following NMR data of major isomer 13b were obtained from the mixture of 13b and 14b in 9 : 1 ratio. ¹H-NMR (CDCl₃) δ: 7.45 (1H, td, J = 8.2, 6.0 Hz), 6.73 (1H, dd, J = 8.7, 5.0 Hz), 1.56–1.48 (6H, m), 1.37–1.16 (12H, m), 0.88 (9H, t, J = 7.3 Hz); ¹³C-NMR (CDCl₃) δ: 166.4 (dd, J = 240, 18 Hz), 162.6 (dd, J = 239, 20 Hz), 134.4 (d, J = 9 Hz), 115.7 (t, J = 52 Hz), 112.1 (dd, J = 30, 4 Hz), 103.2 (dd, J = 28, 5 Hz), 28.8 (t, J = 11 Hz), 27.2 (t, J = 32 Hz), 13.6, 11.1; ¹⁹F-NMR (CDCl₃) δ: −85.98 (1F, d, J = 8 Hz), −93.85 (1F, t, J = 5 Hz).

(6-Bromo-2,3-difluorophenyl)tributylstannane (14b)

Two signals of minor isomer 14b were observed in ¹⁹F-NMR spectrum of the mixture of 13b and 14b in 9 : 1 ratio. ¹⁹F-NMR (CDCl₃) δ: −115.13 (1F, dd, J = 27, 9 Hz), −140.25 (1F, ddd, J = 27, 10, 4 Hz).

Mixture of 13b and 14b in 9 : 1 Ratio

Colorless oil; IR (KBr) cm⁻¹: 2924, 1589, 1441; HR-MS (DUIS) m/z: 424.9726 (Calcd for C₁₄H₂₀⁷⁹BrF₂¹²⁰Sn (M–Bu + H⁺): 424.9733), 426.9721 (Calcd for C₁₄H₂₀⁸¹BrF₂¹²⁰Sn (M–Bu + H⁺): 426.9712), 422.9723 (Calcd for C₁₄H₂₀⁷⁹BrF₂¹¹⁸Sn (M–Bu + H⁺): 422.9727).

Reaction of Precursor 2 with n-Bu₃SnF

To a suspension of CsF (72.9 mg, 0.48 mmol) in anhydrous 1,2-dimethoxyethane (DME, 2.4 mL) were added tributyltin fluoride (61.8 mg, 0.20 mmol) and precursor 2 (83.1 mg, 0.24 mmol) under argon atmosphere at room temperature. After being stirred at the same temperature for 2 h, the reaction mixture was diluted with water and then extracted with AcOEt. The organic phase was dried over Na₂SO₄ and concentrated at reduced pressure. Purification of the residue by preparative TLC (AcOEt:hexanes = 1 : 10, 2-fold development) afforded the isomeric products 13c (17.3 mg, 20%) and 14c (5.3 mg, 6%).²⁵⁾

Tributyl(2,6-difluoro-3-methoxyphenyl)stannane (13c)

Colorless oil; ¹H-NMR (CDCl₃) δ: 6.86 (1H, br td, J = 9.2, 5.5 Hz), 6.74 (1H, ddd, J = 8.7, 6.0, 1.4 Hz), 3.85 (3H, s), 1.60–1.45 (6H, m), 1.32 (6H, sex, J = 7.3 Hz), 1.25–1.08 (6H, m), 0.88 (9H, t, J = 7.3 Hz); ¹³C-NMR (CDCl₃) δ: 160.5 (dd, J = 232, 18 Hz), 155.7 (dd, J = 238, 20 Hz), 143.7 (dd, J = 16, 3 Hz), 115.3 (d, J = 48 Hz), 114.5 (dd, J = 10, 3 Hz), 109.6 (dd, J = 30, 4 Hz), 56.7, 28.9 (t, J = 10 Hz), 27.2 (t, J = 32 Hz), 13.6, 11.0; ¹⁹F-NMR (CDCl₃) δ: −103.6 (1F, t, J = 5 Hz), −113.3 (1F, d, J = 9 Hz); IR (KBr) cm⁻¹: 2956, 2923, 1579, 1467, 1436; HR-MS (DUIS) m/z: 377.0738 (Calcd for C₁₅H₂₃F₂O¹²⁰Sn (M–Bu⁺): 377.0734), 375.0731 (Calcd for C₁₅H₂₃F₂O¹¹⁸Sn (M–Bu⁺): 375.0728).

Tributyl(2,3-difluoro-6-methoxyphenyl)stannane (14c)

Colorless oil; ¹H-NMR (CDCl₃) δ: 7.02 (1H, dd, J = 18.8, 8.7 Hz), 6.47 (1H, ddd, J = 9.2, 2.5, 0.9 Hz), 3.73 (3H, s), 1.53–1.47 (6H, m), 1.31 (6H, sex, J = 7.3 Hz), 1.20–1.03 (6H, m), 0.88 (9H, t, J = 7.3 Hz); ¹³C-NMR (CDCl₃) δ: 159.6 (dd, J = 17, 2 Hz), 154.1 (dd, J = 236, 12 Hz), 145.2 (dd, J = 245, 19 Hz), 118.0 (d, J = 47 Hz), 117.0 (d, J = 19 Hz), 104.5 (dd, J = 10, 3 Hz), 55.7, 29.0 (t, J = 11 Hz), 27.2 (t, J = 31 Hz), 13.6, 11.1; ¹⁹F-NMR (CDCl₃) δ: −119.2 (1F, dd, J = 28, 9 Hz), −148.3 (1F, ddd, J = 28, 10, 3 Hz); IR (KBr) cm⁻¹: 2955, 1589, 1471, 1439; HR-MS (DUIS) m/z: 377.0740 (Calcd for C₁₅H₂₃F₂O¹²⁰Sn (M–Bu⁺): 377.0734), 375.0739 (Calcd for C₁₅H₂₃F₂O¹¹⁸Sn (M–Bu⁺): 375.0728).

Three-Component Coupling Reaction

To a suspension of CsF (91 mg, 0.60 mmol) and diethyl malonate (46 µL, 0.30 mmol) in anhydrous N,N-dimethylformamide (DMF, 2.0 mL) was added precursor 1 (79 mg, 0.20 mmol) under argon atmosphere at room temperature. After being stirred at the same temperature for 12 h, silica gel (0.50 g) was added to the reaction mixture, which was concentrated under reduced pressure. The purification of the residue by flash silica gel column chromatography (AcOEt : hexanes = 1 : 8–1 : 0 with 2% CH₂Cl₂) afforded the product 16b (55 mg, 87%).²⁵⁾

8-Bromo-5-fluoro-2-oxo-2H-1-benzopyran-3-carboxylic Acid, Ethyl Ester (16b)

Colorless crystals; mp 148–150 °C (acetone-hexanes); ¹H-NMR (CDCl₃) δ: 8.69 (1H, s), 7.83 (1H, dd, J = 8.7, 5.5 Hz), 6.99 (1H, t, J = 8.7 Hz), 4.43 (2H, q, J = 7.1 Hz), 1.41 (3H, t, J = 7.1 Hz); ¹³C-NMR (CDCl₃) δ: 162.2, 158.3 (d, J = 259 Hz), 154.7, 151.9 (d, J = 4 Hz), 140.8 (d, J = 3 Hz), 137.6 (d, J = 9 Hz), 119.1, 111.6 (d, J = 21 Hz), 109.4 (d, J = 20 Hz), 104.8 (d, J = 4 Hz), 62.3, 14.2; ¹⁹F-NMR (CDCl₃) δ: −117.4 (dd, J = 8, 6 Hz); IR (KBr) cm⁻¹: 3095, 3064, 1771, 1711, 1618, 1473, 1279, 1231; HR-MS (ESI) m/z: 336.9487 (Calcd for C₁₂H₈⁷⁹BrFO₄Na (M + Na⁺): 336.9482), 338.9470 (Calcd for C₁₂H₈⁸¹BrFO₄Na (M + Na⁺): 338.9463).

Conflict of Interest

The authors declare no conflict of interest.

Supplementary Materials

This article contains supplementary materials.

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