BF₃·Et₂O-mediated intramolecular cyclization of unsaturated amides: convenient synthesis of dihydroquinolin-2-one-BF₂ complexes

Abstract

A facile and efficient synthesis of substituted dihydropyridone–BF₂ complexes is developed via intramolecular cyclization of α-acyl acrylamides and α-acyl cinnamamides mediated by BF₃·Et₂O.

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Introduction

Over the past decades, pyridin-2(1H)-ones and their analogues have attracted considerable attention in chemical and biological fields.^1,2 These structural motifs can serve as efficient catalysts in a variety of proton-dependent reactions³ and as valuable ligands in coordination chemistry.⁴ Furthermore, pyridin-2(1H)-ones are versatile intermediates in the synthesis of a wide range of aza-heterocycles, such as pyridines, piperidines, indolizidines, quinolines and quinolizidines.^5,6 In particular, pyridin-2(1H)-one is a key unit in numerous natural products and synthetic organic compounds such as elfamycin, cerpegin and camptothecin,⁷ along with diverse bio-, physio- and pharmacological activities. To date, a variety of synthetic approaches have been well established to access pyridin-2(1H)-ones and their analogues, which comprise the modification of the pre-constructed heterocyclic ring by pyridinium salt chemistry⁸ and N-alkylation,⁹ the construction of heterocyclic skeletons from appropriately substituted open-chain precursors via metal-catalyzed sp² C–H bond amination,¹⁰ ring closing metathesis,¹¹ and Diels–Alder reaction.¹²

On the other hand, organoboron compounds have emerged as one of the most important class of organic complexes for their excellent photophysical properties and potential use in molecular sensors,¹³ biomolecular probes¹⁴ and optoelectronic devices.¹⁵ Among those reported work, β-dicarbonyl compounds are most used ligands, and the boron difluoride β-diketonates have been extensively investigated and their promising luminescence make them good candidates for optical imaging and sensing applications.^16,17

During the course of our studies on the synthesis of heterocycles based on β-oxo amide derivatives, we developed the synthesis of a variety of substituted pyridin-2(1H)-ones under Vilsmeier conditions.¹⁸ Most recently, we achieved the synthesis of indeno[2,1-c]quinolin-6(7H)-ones from α-acyl cinnamamides mediated by PPA (eqn (1), Scheme 1),¹⁹ divergent synthesis of quinolin-2(1H)-ones (eqn (2), Scheme 1)²⁰ and pyridin-2(3H)-ones (eqn (3), Scheme 1) from 2-acyl penta-2,4-dienamides.²¹ Encouraged by the previous work, we are interested to examine the reaction behaviors of unsaturated amides toward BF₃·Et₂O. By this research, we developed a facile and convenient synthesis of dihydropyridone–BF₂ complexes under very mild conditions. Herein, we will report our experimental results and present a proposed mechanism involved in the cyclization reactions.

Scheme 1 Reactions of α,β-unsaturated amides.

Results and discussion

The substrates, unsaturated amides, were prepared by Knoevenagel condensation of commercially available β-oxo amides with aryl aldehydes in the presence of piperidine and acetic acid in good yields according to reported procedures.^19–22 Then, we selected 2-benzylidene-3-oxo-N-phenylbutanamide 1a as a model compound to investigate its reaction behavior in the presence of BF₃·Et₂O (1.0 equiv.) in CH₂Cl₂ at room temperature. The reaction could proceed and furnished a product, which was characterized as 2,2-difluoro-4-methyl-5-phenyl-5,10-dihydro-2H-[1,3,2]dioxa-borinino[4,5-b]quinolin-1-ium-2-uide 2a (in 76% yield) on the basis of its spectral and analytical data. A series of experiments revealed that the optimal results were obtained when the reaction of 1a was performed with BF₃·Et₂O (2.0 equiv.) in CH₂Cl₂ at room temperature, in which the yield of 2a reached 88% (Table 1, entry 1).

Table 1 Synthesis of substituted dihydroquinolin-2-one-BF₂ complexes 2 from α-Acyl N-arylcinnamamides 1^a

Entry	1	R^1	R^2	R^3	R^4	2	Yield^b (%)
a Reagents and conditions: 1 (2.0 mmol), BF3·Et2O (4.0 mmol), CH2Cl2 (5.0 mL), rt, 2.0–3.0 h.b Isolated yield.
1	1a	H	H	Me	H	2a	88
2	1b	4-Me	H	Me	H	2b	81
3	1c	2-Me	H	Me	H	2c	82
4	1d	3-Me	H	Me	H	2d	79
5	1e	2,4-Me2	H	Me	H	2e	80
6	1f	4-Cl	H	Me	H	2f	83
7	1g	4-MeO	H	Me	H	2g	96
8	1h	2-MeO	H	Me	H	2h	94
9	1i	H	H	Ph	H	2i	75
10	1j	H	4-Me	Me	H	2j	87
11	1k	H	2-Me	Me	H	2k	86
12	1l	H	2-MeO	Me	H	2l	85
13	1m	H	4-Cl	Me	H	2m	83
14	1n	H	H	Me	Et	2n	81

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Under the identical conditions as for 2a, a range of reactions of α-acyl N-arylcinnamamides 1b–n were carried out and some of the results are summarized in Table 2. All the reactions of 1b–g bearing various electron-donating and electron-withdrawing substituents R¹ on the aryl amides proceeded smoothly to afford the corresponding dihydropyridone–BF₂ complexes 2b–g in high yields (Table 1, entries 2–9). In the case of 1d, 2,2-difluoro-4,8-dimethyl-5-phenyl-5,10-dihydro-2H-[1,3,2]dioxaborinino-[4,5-b]quinolin-1-ium-2-uide 2d was exclusively obtained in 79% yield, which suggests that 1d underwent the cyclization reaction in a regioselective manner (Table 1, entry 4). The efficiency of the cyclization proved to be suitable for 1j–m bearing various electron-donating and electron-withdrawing substituents R² on the benzene ring affording the corresponding substituted dihydropyridone–BF₂ complexes 2j–m in very good yields (Table 1, entries 10–13). In the same fashion, the validity of this dihydropyridone–BF₂ complex synthesis was further evaluated by performing 1n bearing secondary amide, in which dihydroquinolin-2-one-BF₂ complex 2n was obtained in high yield (Table 1, entry 14). The structure of 2g was further confirmed by the X-ray single crystal analysis (Fig. 1). The results shown above demonstrate the efficiency and synthetic interest of the cyclization reaction of variable α-acyl N-aryl cinnamamides 1.

Table 2 Synthesis of dihydropyridin-2(3H)-one-BF₂ complexes 5 from 2-acyl penta-2,4-dienamides^a

Entry	4	R^1	R^2	5	Yield^b (%)
a Reaction conditions: 1 (1.0 mmol), KOH (6.0 mmol), t-BuOH (10 mL), 80 °C. 1.0–2.0 h.b Isolated yields.
1	4a	4-Me	H	5a	70
2	4b	4-MeO	H	5b	72
3	4c	4-Cl	H	5c	68
4	4d	2-Cl	4-MeO	5d	71
5	4e	2-Me	4-MeO	5e	63
6	4f	4-Me	4-Me	5f	65

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Fig. 1 ORTEP drawing of 2g.

It should be mentioned that when dihydropyridone–BF₂ complex 2h was treated with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ, 1.0 equiv.) in CH₂Cl₂ at room temperature for 1.0 h, 3-acyl-6-methoxy-4-phenylquinolin-2(1H)-one 3h could be obtained in 80% yield (Scheme 2). Therefore, we provided a novel and convenient synthesis of dihydropyridone–BF₂ complexes 2 and an alternative synthesis of dihydroquinolin-2-ones 3 as well.

Scheme 2 Reaction of substituted dihydroquinolin-2-one-BF₂ complex 2h with DDQ.

Encouraged by the above results, we intended to explore the reaction of 2-acyl penta-2,4-dienamides under identical reaction conditions as for 2a. However, when 2-acyl-5-phenyl-N-(p-tolyl)penta-2,4-dienamide 4a was subjected to CH₂Cl₂ in the presence of BF₃·Et₂O at room temperature for 2.0 h, no reaction was observed. Then, the reaction of 4a was performed in (CH₂)₂Cl₂ under reflux for 1.0 h and furnished a product, which was characterized as 2,2-difluoro-4-methyl-7-phenyl-8-(p-tolyl)-7,8-dihydro-2H-[1,3,2]dioxaborinino[4,5-b]pyridin-1-ium-2-uide 5a (Table 2, entry 1).

Under the identical conditions as for 4a in Table 2 entry 1, a series of reactions of 2-acyl penta-2,4-dienamides 4b–f were carried out in the presence of BF₃·Et₂O, and some of the results are summarized in Table 2. All the reactions of 4b–f bearing different aryl amide groups for R¹ and aryl groups for R² could proceed smoothly to afford the corresponding dihydropyridin-2(3H)-one-BF₂ complexes 5b–f in good yields (Table 2, entries 2–7). The structure of 5d was elucidated by NMR (¹H, ¹³C) spectra and further confirmed by means of the X-ray single crystal analysis (Fig. 2).

Fig. 2 ORTEP drawing of 5d.

In contrast to the conventional acid-catalyzed Knorr quinolin-2(1H)-one synthesis,²³ α-acyl N-aryl cinnamamides 1 were found to undergo a distinct intramolecular cyclization in which the nucleophilic addition site was on the β-carbon of the α,β-unsaturated carbonyl compounds 1 instead of their α-acyl groups. On the basis of the results obtained above and the reported literatures, a plausible mechanism for the synthesis of dihydroquinolin-2-one-BF₂ complexes 2 is presented in Scheme 3. Mediated by BF₃·Et₂O, α-acyl N-arylcinnamamide 1 is activated by the formation of BF₂-complex intermediate A,^17d,e followed by an intramolecular Friedel–Crafts reaction to afford dihydropyridin-2(3H)-one-BF₂ complex 2.²⁴ It is most possible that the BF₂-complex moiety could not provide enough activation to promote further intramolecular cyclization for 2 under the investigated conditions. As for 2-acyl penta-2,4-dienamides 4, a BF₂-complex intermediate B is formed in the same way (Scheme 3). Here, it is worth noting that BF₂-complex intermediate B contains a 1-azatriene moiety, which under the investigated conditions may undergo a 6π-azaelectrocyclization reaction²¹ instead of the Friedel–Crafts reaction as α-acyl N-aryl cinnamamide 1 did. Just like the role of hydrogen bond did in our previous work, the BF₂-complex structure provides the driving force to keep the azadiene N=C–C=C of B in a cis conformation that may favor the subsequent 6π-azaelectrocyclization, and also stabilize the structure of product 5.

Scheme 3 Plausible mechanism for the reaction of unsaturated amides mediated by BF₃·Et₂O.

Conclusions

In summary, a facile and convenient synthesis of substituted dihydropyridone–BF₂ complexes 3 and 5 is developed via intramolecular cyclization of unsaturated amides, α-acyl N-ary lcinnamamides 1 and 2-acyl penta-2,4-dienamides 4, mediated by BF₃·Et₂O, respectively. The simple execution, readily available substrates, very mild conditions, good yields and wide range of synthetic potential of the products make this protocol much attractive. The extension of the scope of the methodology and its further applications are currently under investigation in our laboratory.

Experimental section

General

All reagents were purchased from commercial sources and used without treatment, unless otherwise indicated. The products were purified by column chromatography over silica gel. ¹H NMR and ¹³C NMR spectra were recorded at 25 °C at 300 MHz and 100 MHz, respectively, with TMS as internal standard. IR spectra (KBr) were recorded on FTIR-spectrometer in the range of 400–4000 cm⁻¹. All melting points were determined in open capillary tubes in a Thiele apparatus and are uncorrected.

Typical procedure for the synthesis of substituted unsaturated amides 1 (1a as an example)

To a 100 mL round-bottomed flask was added 3-oxo-N-phenylbutanamide (0.89 g, 5.0 mmol), 4-methylbenzaldehyde (0.60 g, 5.0 mmol), piperidine (0.5 mmol), acetic acid (0.5 mmol) and ethanol (30 mL). Then the mixture was stirred for 8.0 h at room temperature. The resulting mixture was slowly poured into saturated aqueous NaCl (100 mL), and extracted with dichloromethane (3 × 30 mL). The combined organic phase was washed with water (3 × 30 mL) and dried over anhydrous Na₂SO₄. The solvent was removed under reduced pressure, and the crude product was purified by flash chromatography (silica gel, petroleum ether : ethyl acetate10 : 1) to give 1a as a colorless solid (1.20 g, 86%).

Substrates 1a–k and 1n are known compounds (1a and 1j: J. Indian Chem. Soc., 1981, 58, 168, 1c and 1d: Comptes Rendus Hebdomadaires des Seances de I^′Academie des Sciences, 1949, 228, 576, 1b, 1e–k and 1n: Org. Lett., 2013, 15, 776.)

2-(4-Methoxybenzylidene)-3-oxo-N-phenylbutanamide (1l) [E/Z = 4 : 25]. Colorless solid: mp 91–96 °C; ¹H NMR (300 MHz, CDCl₃) (minor E-isomer): δ 2.45 (s, 3H), 3.86 (s, 3H), 6.94 (d, J = 6.0 Hz, 2H), 7.14–7.16 (m, 1H), 7.28–7.33 (m, 2H), 7.57–7.63 (m, 4H), 8.21 (s, 1H), 9.39 (s, 1H); (major Z-isomer): δ 2.45 (s, 3H), 3.81 (s, 3H), 6.86 (d, J = 6.0 Hz, 2H), 7.16 (t, J = 6.0 Hz, 1H), 7.35 (t, J = 6.0 Hz, 2H), 7.53–7.57 (m, 5H), 7.88 (s, 1H); ¹³C NMR (100 MHz, CDCl₃): δ 13.7, 20.5, 22.2, 26.4, 30.9, 54.8, 59.9, 114.0, 119.7, 124.0, 128.5, 131.2, 131.9, 137.3, 140.5, 145.1, 161.3, 165.8, 195.4; Anal. calcd for C₁₈H₁₇NO₃: C, 73.20; H, 5.80; N, 4.74. Found: C, 72.81; H, 5.85; N, 4.69.

2-(4-Chlorobenzylidene)-3-oxo-N-phenylbutanamide (1m) [E/Z = 2 : 5]. Colorless solid: mp 122–127 °C; ¹H NMR (300 MHz, CDCl₃) (minor E-isomer): δ 2.17 (s, 3H), 7.11–7.18 (m, 2H), 7.27–7.35 (m, 2H), 7.47–7.50 (m, 2H), 7.52 (s, 2H), 7.58 (d, J = 6.0 Hz, 2H), 9.28 (s, 1H); (major Z-isomer): δ 2.43 (s, 3H), 7.11–7.18 (m, 1H), 7.20 (d, J = 12.0 Hz, 1H), 7.27–7.35 (m, 4H), 7.40 (d, J = 9.0 Hz, 1H), 7.45 (s, 1H), 7.47–7.50 (m, 2H), 8.11 (d, J = 4.0 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃): δ 26.4, 31.1, 119.5, 119.9, 124.2, 128.6, 130.5, 135.8, 136.8, 143.1, 160.3, 165.1, 195.4, 206.3; Anal. calcd for C₁₇H₁₄ClNO₂: C, 68.12; H, 4.71; N, 4.67. Found: C, 68.44; H, 4.75; N, 4.62.

Typical procedure for the synthesis of dihydropyridone–BF₂ complexes 2 (2a as an example)

To a 50 mL round bottomed flask was added 1a (530.0 mg, 2.0 mmol), BF₃·Et₂O (5.0 mmol) and CH₂Cl₂ (10 mL). The mixture was stirred at room temperature for 1.0 h. After the substrate 1a was consumed completely as indicated by TLC, the mixture was poured into ice water, and then extracted with dichloromethane (3 × 20 mL), the combined organic phase was washed with water (3 × 20 mL), and dried over anhydrous Na₂SO₄. The solvent was removed under reduced pressure, and the crude product was purified by flash chromatography (silica gel, petroleum ether : ethyl acetate 5 : 1) to give 2a as colorless solid (551.1 mg, 88%).

2,2-Difluoro-4-methyl-5-phenyl-5,10-dihydro-2H-[1,3,2]dioxaborinino[4,5-b]quinolin-1-ium-2-uide (2a). Yellow solid: mp 239–240 °C; ¹H NMR (300 MHz, DMSO): δ 2.05 (s, 3H), 5.33 (s, 1H), 7.07 (d, J = 6.0 Hz, 1H), 7.11 (s, 1H), 7.17–7.22 (m, 2H), 7.25 (s, 1H), 7.28 (d, J = 2.4 Hz, 3H), 7.32 (d, J = 7.5 Hz, 1H), 12.08 (s, 1H); ¹³C NMR (100 MHz, DMSO): δ 20.4, 41.5, 96.6, 117.2, 125.5, 126.4, 126.7, 126.9, 127.8, 129.0, 129.5, 132.4, 146.3, 163.8, 179.4; IR (KBr, cm⁻¹): 3352, 1624, 1610, 1593, 1526, 1493, 1333, 1119, 762, 706; Anal. calcd for C₁₇H₁₄BF₂NO₂: C, 65.21; H, 4.51; N, 4.47. Found: C, 65.52; H, 4.48; N, 4.54.

2,2-Difluoro-4,7-dimethyl-5-phenyl-5,10-dihydro-2H-[1,3,2]dioxaborinino[4,5-b]quinolin-1-ium-2-uide (2b). Yellow solid: mp 210–212 °C; ¹H NMR (300 MHz, DMSO): δ 2.03 (s, 3H), 2.17 (s, 3H), 5.26 (s, 1H), 6.98 (d, J = 9.9 Hz, 1H), 7.03 (d, J = 9.9 Hz, 2H), 7.18 (t, J = 6.9 Hz, 1H), 7.25 (d, J = 6.9 Hz, 2H), 7.30 (t, J = 7.5 Hz, 2H), 12.01 (s, 1H); ¹³C NMR (100 MHz, DMSO): δ 20.4, 41.7, 96.7, 117.1, 126.3, 126.8 (1), 126.8 (2), 128.4, 129.0, 129.8, 130.0, 134.9, 146.4, 163.5, 178.9; Anal. calcd for C₁₈H₁₆BF₂NO₂: C, 66.09; H, 4.93; N, 4.28. Found: C, 65.87; H, 4.90; N, 4.22.

2,2-Difluoro-4,9-dimethyl-5-phenyl-5,10-dihydro-2H-[1,3,2]dioxaborinino[4,5-b]quinolin-1-ium-2-uide (2c). Colorless solid: mp 246–247 °C; ¹H NMR (300 MHz, DMSO): 2.09 (s, 3H), 2.34 (s, 3H), 5.31 (s, 1H), 6.99 (t, J = 7.5 Hz, 1H), 7.06 (d, J = 7.5 Hz, 1H), 7.12 (d, J = 7.5 Hz, 1H), 7.16–7.21 (m, 1H), 7.24–7.28 (m, 3H), 7.32 (d, J = 7.5 Hz, 1H), 11.22 (s, 1H); ¹³C NMR (100 MHz, DMSO): δ 17.8, 20.5, 41.8, 97.0, 125.4, 126.1, 126.8, 127.0, 127.4, 128.1, 128.9, 129.2, 129.8, 130.9, 146.5, 164.6, 179.9; Anal. calcd for C₁₈H₁₆BF₂NO₂: C, 66.09; H, 4.93; N, 4.28. Found: C, 66.35; H, 4.88; N, 4.31.

2,2-Difluoro-4,8-dimethyl-5-phenyl-5,10-dihydro-2H-[1,3,2]dioxaborinino[4,5-b]quinolin-1-ium-2-uide (2d). Yellow solid: mp 261–263 °C; ¹H NMR (300 MHz, DMSO): δ 2.04 (s, 3H), 2.33 (s, 3H), 5.27 (s, 1H), 5.76 (s, 1H), 6.90 (d, J = 7.8 Hz, 2H), 7.13 (d, J = 7.8 Hz, 1H), 7.19 (d, J = 7.5 Hz, 1H), 7.24 (d, J = 7.5 Hz, 2H), 7.30 (d, J = 7.5 Hz, 2H), 12.02 (s, 1H); ¹³C NMR (100 MHz, DMSO): δ 20.4, 20.5, 41.2, 54.8, 96.7, 117.4, 123.5, 126.2, 126.7, 129.0, 129.3, 132.1, 137.3, 146.5, 163.8, 179.2; Anal. calcd for C₁₈H₁₆BF₂NO₂: C, 66.09; H, 4.93; N, 4.28. Found: C, 66.42; H, 5.00; N, 4.22.

2,2-Difluoro-4,7,9-trimethyl-5-phenyl-5,10-dihydro-2H-[1,3,2]dioxaborinino[4,5-b]quinolin-1-ium-2-uide (2e). Yellow solid: mp 291–293 °C; ¹H NMR (300 MHz, DMSO): δ 2.08 (s, 3H), 2.14 (s, 3H), 2.30 (s, 3H), 5.24(s, 1H), 6.87 (s, 1H), 6.91 (s, 1H), 7.16–7.33 (m, 5H), 11.19 (s, 1H); ¹³C NMR (100 MHz, DMSO): δ 17.5, 20.2, 20.4, 41.8, 96.9, 125.8, 126.6, 126.7, 126.8, 127.6, 128.4, 129.0, 130.3, 134.4, 146.4, 164.2, 179.3; IR (KBr, cm⁻¹): 3337, 1626, 1601, 1526, 1485, 1327, 1146, 731, 706; Anal. calcd for C₁₉H₁₈BF₂NO₂: C, 66.89; H, 5.32; N, 4.11. Found: C, 66.52; H, 5.39; N, 4.17.

7-Chloro-2,2-difluoro-4-methyl-5-phenyl-5,10-dihydro-2H-[1,3,2]dioxaborinino[4,5-b]quinolin-1-ium-2-uide (2f). Yellow solid: mp 218–219 °C; ¹H NMR (300 MHz, DMSO): δ 2.05 (s, 3H), 5.37 (s, 1H), 7.11 (d, J = 6.0 Hz, 1H), 7.20–7.24 (m, 1H), 7.28–7.36 (m, 6H), 12.20 (s, 1H); ¹³C NMR (100 MHz, DMSO): δ 20.5, 41.3, 96.1, 119.0, 126.8, 127.1, 127.8, 128.5, 129.0, 129.2, 131.5, 145.8, 163.8, 179.9; IR (KBr, cm⁻¹): 3348, 3333, 1622, 1609, 1593, 1520, 1489, 1140, 746, 710, 696; Anal. calcd for C₁₇H₁₃BClF₂NO₂: C, 58.75; H, 3.77; N, 4.03. Found: C, 59.10; H, 3.69; N, 4.06.

2,2-Difluoro-7-methoxy-4-methyl-5-phenyl-5,10-dihydro-2H-[1,3,2]dioxaborinino[4,5-b]quinolin-1-ium-2-uide (2g). Yellow solid: mp 226–228 °C; ¹H NMR (400 MHz, DMSO): δ 2.03 (s, 3H), 3.66 (s, 3H), 5.29 (s, 1H), 6.82–6.85 (m, 2H), 7.02–7.05 (m, 1H), 7.17–7.22 (m, 1H), 7.25–7.29 (m, 3H), 7.32 (d, J = 7.5 Hz, 1H), 11.99 (s, 1H); ¹³C NMR (100 MHz, DMSO): δ 21.2, 43.5, 55.5, 96.1, 113.2, 115.2, 118.0, 124.8, 126.9, 127.3, 129.2, 145.2, 157.5, 163.3, 181.0; IR (KBr, cm⁻¹): 3348, 1703, 1647, 1620, 1597, 1529, 1499, 1269, 1130, 733, 698; Anal. calcd for C₁₈H₁₆BF₂NO₃: C, 63.01; H, 4.70; N, 4.08. Found: C, 63.18; H, 4.69; N, 4.10.

Crystal data for 2g: C₁₈H₁₆BF₂NO₃, colorless crystal, M = 343.13, monoclinic, C2/c′, a = 26.916(3) Å, b = 8.0515(9) Å, c = 17.954(2) Å, α = 90.00°, β = 123.571(2)°, γ = 90.00°, V = 3241.9(6) ų, Z = 8, T = 293(2) K, F000 = 1512, R = 0.0474.

2,2-Difluoro-9-methoxy-4-methyl-5-phenyl-5,10-dihydro-2H-[1,3,2]dioxaborinino[4,5-b]quinolin-1-ium-2-uide (2h). Colorless solid: mp 239–240 °C; ¹H NMR (300 MHz, DMSO): δ 2.06 (s, 3H), 3.83 (s, 3H), 5.28 (s, 1H), 6.84 (d, J = 8.1 Hz, 1H), 6.90 (d, J = 8.1 Hz, 1H), 7.04 (t, J = 8.1 Hz, 1H), 7.15–7.23 (m, 1H), 7.25–7.31 (m, 4H), 11.50 (s, 1H); ¹³C NMR (100 MHz, DMSO): δ 20.5, 41.5, 56.1, 96.7, 110.2, 121.0, 121.6, 125.8, 126.7, 126.9, 127.4, 129.0, 146.2, 147.8, 164.0, 179.5; IR (KBr, cm⁻¹): 3319, 1608, 1593, 1543, 1495, 1271, 1103, 750; Anal. calcd for C₁₈H₁₆BF₂NO₃: C, 63.01; H, 4.70; N, 4.08. Found: C, 62.82; H, 4.74; N, 3.99.

2,2-Difluoro-4-methyl-5-(p-tolyl)-5,10-dihydro-2H-[1,3,2]dioxaborinino[4,5-b]quinolin-1-ium-2-uide (2i). Yellow solid: mp 192–194 °C; ¹H NMR (300 MHz, DMSO): δ 2.04 (s, 3H), 2.22 (s, 3H), 5.27 (s, 1H), 7.05–7.11 (m, 4H), 7.15 (d, J = 8.1 Hz, 2H), 7.19–7.25 (m, 2H), 12.04 (s, 1H); ¹³C NMR (100 MHz, DMSO): δ 20.5, 41.2, 54.8, 96.6, 117.1, 125.4, 126.6, 127.7, 129.5 (1), 129.5 (2), 132.3, 136.0, 143.5, 163.7, 179.2; IR (KBr, cm⁻¹): 3344, 1628, 1595, 1526, 1491, 1329, 810, 762; Anal. calcd for C₁₈H₁₆BF₂NO₂: C, 66.09; H, 4.93; N, 4.28. Found: C, 65.79; H, 5.01; N, 4.33.

2,2-Difluoro-4-methyl-5-(o-tolyl)-5,10-dihydro-2H-[1,3,2]dioxaborinino[4,5-b]quinolin-1-ium-2-uide (2j). Yellow solid: mp 251–253 °C; ¹H NMR (300 MHz, DMSO): δ 1.92 (s, 3H), 2.34 (s, 3H), 5.52 (s, 1H), 7.03–7.08 (m, 3H), 7.12–7.18 (m, 3H), 7.20–7.26 (m, 2H), 12.08 (s, 1H); ¹³C NMR (100 MHz, DMSO): δ 19.0, 20.8, 96.5, 117.0, 125.5, 125.8, 126.6, 126.9, 127.8, 129.3, 131.4, 132.3, 134.2, 144.4, 163.6, 179.0; IR (KBr, cm⁻¹): 3354, 1622, 1591, 1521, 1493, 1047, 764; Anal. calcd for C₁₈H₁₆BF₂NO₂: C, 66.09; H, 4.93; N, 4.28. Found: C, 66.41; H, 4.89; N, 4.32.

2,2-Difluoro-5-(4-methoxyphenyl)-4-methyl-5,10-dihydro-2H-[1,3,2]dioxaborinino[4,5-b]quinolin-1-ium-2-uide (2k). Yellow solid: mp 204–206 °C; ¹H NMR (300 MHz, DMSO): δ 2.04 (s, 3H), 3.68 (s, 3H), 5.26 (s, 1H), 6.86 (d, J = 9.0 Hz, 2H), 7.08 (t, J = 9.0 Hz, 2H), 7.17 (d, J = 9.0 Hz, 2H), 7.21–7.25 (m, 2H), 12.03 (s, 1H); ¹³C NMR (100 MHz, DMSO): δ 20.8, 41.0, 55.4, 97.1, 114.7, 117.5, 125.8, 127.2, 128.0, 128.2, 129.9, 132.6, 139.0, 158.4, 164.1, 179.7; Anal. calcd for C₁₈H₁₆BF₂NO₃: C, 63.01; H, 4.70; N, 4.08. Found: C, 63.39; H, 4.77; N, 4.16.

5-(4-Chlorophenyl)-2,2-difluoro-4-methyl-5,10-dihydro-2H-[1,3,2]dioxaborinino[4,5-b]quinolin-1-ium-2-uide (2l). Yellow solid: mp 218–220 °C; ¹H NMR (300 MHz, DMSO): δ 2.07 (s, 3H), 5.40 (s, 1H), 7.10 (t, J = 7.5 Hz, 2H), 7.24 (t, J = 7.5 Hz, 2H), 7.31 (d, J = 8.4 Hz, 2H), 7.39 (d, J = 8.4 Hz, 2H), 12.12 (s, 1H); ¹³C NMR (100 MHz, DMSO): δ 20.5, 40.9, 96.3, 117.3, 125.6, 125.9, 128.0, 129.0, 129.5, 131.7, 132.4, 145.2, 163.7, 179.6; IR (KBr, cm⁻¹): 3333, 1630, 1595, 1529, 1493, 1323, 1057, 760, 719; Anal. calcd for C₁₇H₁₃BClF₂NO₂: C, 58.75; H, 3.77; N, 4.03. Found: C, 58.44; H, 3.84; N, 4.12.

2,2-Difluoro-4,5-diphenyl-5,10-dihydro-2H-[1,3,2]dioxaborinino[4,5-b]quinolin-1-ium-2-uide (2m). Colorless solid: mp 282–284 °C; ¹H NMR (400 MHz, DMSO): δ 5.36 (s, 1H), 6.90 (d, J = 7.6 Hz, 2H), 7.09–7.19 (m, 5H), 7.26 (t, J = 7.6 Hz, 1H), 7.37 (d, J = 7.6 Hz, 1H), 7.45–7.50 (m, 4H), 7.56 (t, J = 6.8 Hz, 1H), 12.41 (s, 1H); ¹³C NMR (100 MHz, DMSO): δ 41.4, 97.2, 117.1, 125.7, 126.1, 126.3, 126.7, 127.8, 127.9, 128.6, 128.7, 129.4, 131.2, 132.4, 133.6, 145.6, 165.1, 174.8; IR (KBr, cm⁻¹): 3312, 1628, 1589, 1580, 1522, 1489, 1132, 762, 702; Anal. calcd for C₂₂H₁₆BF₂NO₂: C, 70.43; H, 4.30; N, 3.73. Found: C, 70.01; H, 4.19; N, 3.81.

10-Ethyl-2,2-difluoro-4-methyl-5-phenyl-5,10-dihydro-2H-[1,3,2]dioxaborinino[4,5-b]quinolin-1-ium-2-uide (2n). Colorless solid: mp 189–191 °C; ¹H NMR (400 MHz, DMSO): δ 1.31 (t, J = 6.9 Hz, 3H), 2.12 (s, 1H), 4.14–4.24 (m, 2H), 5.34 (s, 1H), 7.16–7.23 (m, 3H), 7.25–7.32 (m, 4H), 7.36 (d, J = 7.8 Hz, 1H), 7.39 (t, J = 7.8 Hz, 1H), 7.45 (d, J = 7.8 Hz, 1H); ¹³C NMR (100 MHz, DMSO): δ 13.0, 20.8, 39.1, 41.4, 97.5, 116.9, 126.3, 127.0, 127.4, 128.5, 129.5, 130.4, 133.8, 146.2, 163.6, 179.3; IR (KBr, cm⁻¹): 1603, 1580, 1518, 1487, 1337, 1138, 756, 700; Anal. calcd for C₁₉H₁₈BF₂NO₂: C, 66.89; H, 5.32; N, 4.11. Found: C, 67.23; H, 5.26; N, 4.17.

The procedure for the synthesis of substituted quinolin-2(1H)-one 3h

To a 50 mL round bottomed flask was added 2 h (686.3 mg, 2.0 mmol), DDQ (3.0 mmol) and CH₂Cl₂ (10 mL). The mixture was stirred at room temperature for 1.0 h. After the substrate 2g was consumed completely as indicated by TLC, the mixture was poured into ice water, and then extracted with dichloromethane (3 × 20 mL), the combined organic phase was washed with water (3 × 20 mL), and dried over anhydrous Na₂SO₄. The solvent was removed under reduced pressure, and the crude product was purified by flash chromatography (silica gel, petroleum ether : ethyl acetate 4 : 1) to give 3h as colorless solid (469.3 mg, 80%).

3-Acetyl-6-methoxy-4-phenylquinolin-2(1H)-one (3h). Yellow solid: mp 260–261 °C; ¹H NMR (400 MHz, DMSO): δ 2.21 (s, 3H), 3.57 (s, 3H), 6.64 (d, J = 2.4 Hz, 1H), 7.24–7.27 (dd, J₁ = 9.2 Hz, J₂ = 2.4 Hz, 1H), 7.31 (s, 1H), 7.33 (d, J = 2.4 Hz, 1H), 7.37 (d, J = 9.2 Hz, 1H), 7.48–7.53 (m, 3H), 12.14 (s, 1H); ¹³C NMR (100 MHz, CDCl₃): δ 31.4, 55.2, 109.0, 117.0, 119.5, 119.8, 128.5, 128.7, 133.0, 133.7, 134.2, 146.4, 154.2, 158.8, 201.7; IR (KBr, cm⁻¹): 3446, 1703, 1647, 1597, 1497, 1281, 733, 702; Anal. calcd for C₁₈H₁₅NO₃: C, 73.71; H, 5.15; N, 4.78. Found: C, 74.16; H, 5.20; N, 4.83.

Typical procedure for the synthesis of dihydropyridone–BF₂ complexes 5 (5a as an example)

To a 50 mL round bottomed flask was added 4a (610.7 mg, 2.0 mmol), BF₃·Et₂O (3.0 mmol) and DCE (10 mL). The mixture was stirred at 80 °C for 2.0 h. After the substrate 4a was consumed completely as indicated by TLC, the mixture was poured into ice water, and then extracted with dichloromethane (3 × 20 mL), the combined organic phase was washed with water (3 × 20 mL), and dried over anhydrous Na₂SO₄. The solvent was removed under reduced pressure, and the crude product was purified by flash chromatography (silica gel, petroleum ether : ethyl acetate 4 : 1) to give 5a as a colorless solid (494.4 mg, 70%).

2,2-Difluoro-4-methyl-7-phenyl-8-(p-tolyl)-7,8-dihydro-2H-[1,3,2]dioxaborinino[4,5-b]pyridin-1-ium-2-uide (5a). Yellow solid: mp 189–192 °C; ¹H NMR (300 MHz, DMSO): δ 2.21 (s, 3H), 2.27 (s, 3H), 4.55 (d, J = 7.5 Hz, 1H), 6.04–6.11 (dd, J₁ = 15.6 Hz, J₂ = 7.5 Hz, 1H), 6.34 (d, J = 15.6 Hz, 1H), 6.84 (d, J = 8.1 Hz, 1H), 6.99 (d, J = 8.1 Hz, 1H), 7.04 (s, 1H), 7.18–7.29 (m, 5H), 8.20 (s, 1H); ¹³C NMR (100 MHz, DMSO): δ 20.1, 20.6, 94.2, 117.0, 124.5, 126.5, 127.6, 128.0, 128.6, 129.8, 130.5, 131.9, 134.9, 136.2, 163.5, 178.6; IR (KBr, cm⁻¹): 3346, 1622, 1595, 1529, 1501, 1167, 816, 770, 746, 689; Anal. calcd for C₂₀H₁₈BF₂NO₂: C, 68.02; H, 5.14; N, 3.97. Found: C, 67.83; H, 5.21; N, 4.07.

2,2-Difluoro-8-(4-methoxyphenyl)-4-methyl-7-phenyl-7,8-dihydro-2H-[1,3,2]dioxaborinino[4,5-b]pyridin-1-ium-2-uide (5b). Yellow solid: mp 210–211 °C; ¹H NMR (300 MHz, CDCl₃): δ 2.25 (s, 3H), 3.79 (s, 3H), 4.61 (d, J = 7.5 Hz, 1H), 6.08–6.16 (dd, J₁ = 15.6 Hz, J₂ = 7.5 Hz, 1H), 6.38 (d, J = 15.6 Hz, 1H), 6.76–6.79 (dd, J₁ = 8.7 Hz, J₂ = 2.7 Hz, 1H), 6.82 (d, J = 2.7 Hz, 1H), 6.91 (d, J = 8.7 Hz, 1H), 7.22–7.30 (m, 5H), 8.00 (s, 1H); ¹³C NMR (100 MHz, DMSO): δ 19.9, 55.4, 93.8, 113.4, 114.6, 118.1, 126.0, 126.1, 126.4, 127.6, 128.1, 128.6, 131.8, 136.2, 157.0, 178.1; Anal. calcd for C₂₀H₁₈BF₂NO₃: C, 65.07; H, 4.91; N, 3.79. Found: C, 65.53; H, 4.85; N, 3.91.

8-(4-Chlorophenyl)-2,2-difluoro-4-methyl-7-phenyl-7,8-dihydro-2H-[1,3,2]dioxaborinino[4,5-b]pyridin-1-ium-2-uide (5c). Yellow solid: mp 219–221 °C; ¹H NMR (300 MHz, CDCl₃): δ 2.28 (s, 3H), 5.43 (d, J = 3.9 Hz, 1H), 5.49–5.54 (dd, J₁ = 10.2 Hz, J₂ = 3.9 Hz, 1H), 6.46 (d, J = 10.2 Hz, 1H), 6.84 (d, J = 8.7 Hz, 1H), 7.06–7.10 (m, 2H), 7.26 (s, 1H), 7.27–7.31 (m, 1H), 7.32 (d, J = 1.5 Hz, 1H), 7.34 (d, J = 1.5 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃): δ 19.6, 68.5, 95.2, 118.0, 118.5, 127.7, 128.5, 129.1, 129.2, 129.6, 134.8, 135.8, 137.8, 164.8, 174.0; Anal. calcd for C₁₉H₁₅BClF₂NO₂: C, 61.08; H, 4.05; N, 3.75. Found: C, 61.53; H, 4.16; N, 3.86.

8-(2-Chlorophenyl)-2,2-difluoro-7-(4-methoxyphenyl)-4-methyl-7,8-dihydro-2H-[1,3,2]dioxaborinino[4,5-b]pyridin-1-ium-2-uide (5d) (d/r = 5 : 3). Yellow solid: mp 151–155 °C; ¹H NMR (major isomer) (400 MHz, DMSO): δ 2.29 (s, 3H), 3.73 (s, 3H), 5.50 (d, J = 4.0 Hz, 1H), 5.62–5.66 (dd, J₁ = 12.0 Hz, J₂ = 4.0 Hz, 1H), 6.42 (d, J = 8.0 Hz, 1H), 6.91 (d, J = 12.0 Hz, 2H), 7.13 (d, J = 8.0 Hz, 2H), 7.21 (t, J = 8.0 Hz, 1H), 7.44 (t, J = 8.0 Hz, 1H), 7.72 (d, J = 8.0 Hz, 1H); ¹H NMR (minor isomer) (400 MHz, DMSO): δ 2.27 (s, 3H), 3.68 (s, 3H), 5.57–5.60 (dd, J₁ = 12.0 Hz, J₂ = 4.0 Hz, 1H), 5.93 (s, 1H), 6.71 (d, J = 12.0 Hz, 1H), 6.78 (d, J = 12.0 Hz, 2H), 6.99 (d, J = 8.0 Hz, 2H), 7.35–7.39 (m, 2H), 7.49–7.53 (m, 1H), 7.87 (d, J = 8.0 Hz, 1H); ¹³C NMR (100 MHz, DMSO): δ 19.7, 19.8, 55.4, 55.5, 66.0, 67.5, 95.2, 95.3, 113.8, 114.6, 118.0, 118.7, 119.2, 119.3, 128.1, 128.9, 130.4, 131.1, 134.4, 135.1, 159.9, 160.0, 163.8, 164.9, 173.3, 174.4; Anal. calcd for C₂₀H₁₇BClF₂NO₃: C, 59.52; H, 4.25; N, 3.47. Found: C, 59.91; H, 4.36; N, 3.40.

Crystal data for 5d: C₂₀H₁₇BClF₂NO₃, colorless crystal, M = 871.29, P-1, a = 7.643(5) Å, b = 12.269(5) Å, c = 12.840(5) Å, α = 101.954(5)°, β = 104.028(5)°, γ = 98.284(5)°, V = 1118.6(10) ų, Z = 1, T = 293(2) K, F000 = 450, R = 0.0123.

2,2-Difluoro-7-(4-methoxyphenyl)-4-methyl-8-(o-tolyl)-7,8-dihydro-2H-[1,3,2]dioxaborinino[4,5-b]pyridin-1-ium-2-uide (5e). Yellow solid: mp 182–183 °C; ¹H NMR (300 MHz, CDCl₃): δ 2.27 (s, 3H), 2.31 (s, 3H), 3.79 (s, 3H), 4.62 (d, J = 7.2 Hz, 1H), 5.94–6.01 (dd, J₁ = 15.6 Hz, J₂ = 7.2 Hz, 1H), 6.30 (d, J = 15.6 Hz, 1H), 6.81 (d, J = 8.7 Hz, 2H), 7.06–7.08 (m, 2H), 7.15–7.18 (m, 1H), 7.23 (s, 1H), 7.66 (s, 1H); ¹³C NMR (100 MHz, CDCl₃): δ 16.5, 20.7, 40.8, 55.3, 94.2, 114.0, 124.1, 124.4, 125.5, 127.4, 127.7, 128.6, 128.7, 129.7, 130.4, 159.4, 164.3, 181.7; IR (KBr, cm⁻¹): 1630, 1601, 1560, 1512, 1252, 1180, 827, 764; Anal. calcd for C₂₁H₂₀BF₂NO₃: C, 65.82; H, 5.26; N, 3.66. Found: C, 66.03; H, 5.31; N, 3.57.

2,2-Difluoro-4-methyl-7,8-di-p-tolyl-7,8-dihydro-2H-[1,3,2]dioxaborinino[4,5-b]pyridin-1-ium-2-uide (5f). Yellow solid: mp 219–221 °C; ¹H NMR (300 MHz, CDCl₃): δ 2.24 (s, 3H), 2.30 (s, 3H), 2.31 (s, 3H), 4.57 (d, J = 7.5 Hz, 1H), 6.02–6.10 (dd, J₁ = 15.6 Hz, J₂ = 7.5 Hz, 1H), 6.34 (d, J = 15.6 Hz, 1H), 6.85 (d, J = 8.1 Hz, 1H), 7.03 (d, J = 8.1 Hz, 1H), 7.09 (d, J = 8.1 Hz, 3H), 7.21 (d, J = 8.1 Hz, 2H), 8.02 (s, 1H); ¹³C NMR (100 MHz, CDCl₃): δ 20.6, 20.9, 21.1, 40.8, 94.4, 116.9, 124.0, 126.4, 128.9, 129.0, 129.2, 129.6, 129.7, 129.8, 133.3, 135.8, 137.7, 163.8, 180.4; IR (KBr, cm⁻¹): 3344, 1626, 1599, 1529, 1500, 1207, 1163, 814; Anal. calcd for C₂₁H₂₀BF₂NO₂: C, 68.69; H, 5.49; N, 3.81. Found: C, 68.36; H, 5.54; N, 3.84.

Acknowledgements

Financial support of this research by the National Natural Science Foundation of China (21172211) and Department of Science and Technology of Jilin Province (201105030 and 201205027) are greatly acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, spectral and analytical data, copies of ¹H NMR and ¹³C NMR spectra for new compounds 1–3 and 5, and CIF files for 2g and 5d. CCDC 922666 and 938933. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra06151a

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