A concise synthesis of quinolinium, and biquinolinium salts and biquinolines from benzylic azides and alkenes promoted by copper(II) species

Abstract

A novel copper-promoted multiple aza-[4 + 2] cycloaddition reaction of N-methyleneanilines in situ generated from benzylic azide and alkenes afforded quinolinium salts, biquinolinium salts, biquinolines or substituted quinolines depending on the substitution on the phenyl ring of benzylic azide. The reaction of para substituted benzylic azides and 2 equivalents of alkenes afforded the corresponding substituted quinolinium salts, while benzylic azides without a para substituent provided biquinolinium salts. The copper-promoted cycloaddition reaction also allows biquinoline products to be obtained from ortho-substituted benzylic azides. These reactions work well with both terminal and internal alkenes. Unsymmetrical internal alkene reactions proceed with high regioselectivity. The reaction is likely started by Lewis acidic Cu^II-assisted rearrangement of benzylic azide to N-methyleneaniline, followed by a [4 + 2] cycloaddition with alkene. Detailed mechanistic studies suggest that the biquinoline and biquinolinium salts are likely formed via radical processes.

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Introduction

The Lewis acid (LA) mediated aza-[4 + 2] cycloaddition reaction of 2-azadiene and electron rich alkenes is an important tool to construct nitrogen heterocycles.¹ In particular, the Povarov reaction (PR)² a [4 + 2] cycloaddition of N-arylimines and alkenes is a known effective method for the synthesis of tetrahydroquinolines (THQs), which can be easily transformed to quinolines by dehydrogenation.³ Generally, the Povarov reaction is performed using either a protic or Lewis acid as the catalyst and an N-arylimine as azadiene, and electron-rich alkenes, such as cyclic and acyclic enol ethers, enamides, enamines, and conjugated dienes as a dienophile.⁴ Recently, arylmethyl azide was also shown to be a suitable substrate to undergo [4 + 2] annulation with alkene to afford THQs.⁵ In addition to neutral nitrogen heterocycles, the synthesis of cationic N-heterocycles also received great attention owing to the various biological activities and materials applications.⁶ In particular, applications of quinolinium salts in biological studies demands new synthetic methods for the synthesis of a wide range of substituted quinolinium salts.⁷ A well-known method to these compounds involves the reaction of quinolines with alkyl halides.⁸ However, the method is limited by the availability of substrates. Our continuing interest in the synthesis of N-heterocyclic salts has encouraged us to tackle this problem and to develop a straightforward route to the diversely substituted quinolinium salts.⁹

In this manuscript, we report a convenient method to access highly substituted quinolinium salts from benzyl azides and alkenes via a Cu^II-promoted double aza-[4 + 2] cycloaddition reactions (Scheme 1).

Scheme 1 Cu^II-Promoted multiple aza-[4 + 2] cycloaddition reactions.

Results and discussion

The reaction of 4-methylbenzyl azide (1a) and styrene (2a) in the presence of Cu(II) salt gave dihydro-1H-pyridoquinolinium salt (3aa), which possesses an unusual fused tricyclic structure (eqn (1)) consisting of one p-toluidine, two styrene and two methylene units. In order to optimize the reaction, we performed a series of studies by varying the oxidant, solvent, the ratio of the substrates and the reaction temperature and time. After an extensive screening, we found that treatment of 1a (0.64 mmol), 2a (0.32 mmol), CuSO₄ (1.12 mmol), NaBF₄ (0.20 mmol), and H₂O (2.22 mmol) in CH₃NO₂ (3 mL) at 100 °C for 24 h gave the product 3aa in 89% isolated yield and the reaction condition was denoted as condition A (see ESI, Table S1† for the detailed optimization studies). In view of the excess amount of copper(II) salt used in condition A, we try to reduce the amount of copper salt by using various inorganic and organic oxidants along with catalytic amount of CuSO₄ in the reaction of 1a and 2a to give 3aa (see Table S2†). The results of this screening reveal that by using (NH₄)₂S₂O₈ as the oxidant, CuSO₄ can be reduced to a catalytic amount of 0.08 mmol, a decrease of 14 times compared to that of condition A. As a result, treatment of 1a (0.64 mmol), 2a (0.32 mmol), CuSO₄ (0.08 mmol), (NH₄)₂S₂O₈ (0.4 mmol), NaBF₄ (0.2 mmol), and H₂O (2.22 mmol) in MeNO₂ (3 mL) at 100 °C for 24 h afford 3aa in 77% isolated yield. This optimized catalytic condition is referred as condition B. We also examined the effect of different metal salt on the product yield under reaction condition B; among the Fe^III-, Fe^II-, and In^III-salts tested, none of them is as active as CuSO₄ (Table S3†).

Next, we probe the scope of the copper promoted double aza-[4 + 2] cycloaddition reaction to form quinolinium salts. Various substituted benzyl azides (1a–i) were treated with styrene 2a under both conditions A and B and their results are shown in Scheme 2. Thus, the reaction of p-iPr, p-OMe, p-Ph, p-Cl, p-F substituted benzyl azides (1a–f) with 2a afforded the expected quinolinium salts 3ba–fa in good yields. 3,4-Disubstituted benzyl azide 1g was also effectively transformed into the respective quinolinium salt 3ga in 65% yield. The reaction of ortho substituted benzyl azide 1h gave a single [4 + 2] cycloaddition product, quinoline 3ha′, presumably due to the presence of the ortho methyl group in 1h that prevents a second [4 + 2] cycloaddition to give a quinolinium salt. Benzyl azide (1i) containing a methyl substituent at the benzylic position failed to undergo the expected [4 + 2] cycloaddition product (product 3ia). Similarly, the benzylic azides containing strong electron withdrawing groups such as p-CF₃, p-NO₂, p-CN were ineffective under the reaction conditions.

Scheme 2 Scope of benzyl azides in quinolinium salts formation. ^aCondition A: azide 1 (0.64 mmol), styrene 2a (0.32 mmol), CuSO₄ (1.12 mmol), NaBF₄ (0.20 mmol), and H₂O (2.22 mmol) in MeNO₂ (3 mL) at 100 °C for 24 h. ^bCondition B: azide 1 (0.64 mmol), styrene 2a (0.32 mmol), CuSO₄ (0.08 mmol), NaBF₄ (0.2 mmol), (NH₄)₂S₂O₈ (0.4 mmol), and H₂O (2.22 mmol) in MeNO₂ (3 mL) at 100 °C for 24 h. ^cIsolated yields are calculated based on 3, 0.16 mmol, as the upper limit (100%).

The copper-catalyzed synthesis of quinolinium salts from benzyl azides were effective with different alkenes (Scheme 3). Thus the reaction of 4-Me-, and 4-Br substituted styrenes afforded products 3ab and 3ac in good yields. In addition to vinyl arenes, simple alkyl alkene 2d also effective in the double aza–cycloaddition reaction to give 3ad in 61% yield. Treatment of β-methylstyrene (2e) with 1a gave the expected quinolinium salt 3ae in 69% yield. Interestingly, the reaction of allylbenzene (2f) with 1a gave quinolinium salt 3af′ in 74% yield with elimination of a benzyl group at the quinoline ring. The structure of 3af′ was confirmed by X-ray structure analysis.¹⁰ Similarly, α-methylstyrene (2g) reacted with 1a to give 3ag′ with the elimination of a methyl group.

Scheme 3 Scope of alkenes in the quinolinium salts formation. ^aCondition B: azide 1a (0.64 mmol), alkene 2 (0.32 mmol), CuSO₄ (0.08 mmol), NaBF₄ (0.2 mmol), (NH₄)₂S₂O₈ (0.4 mmol), and H₂O (2.22 mmol) in MeNO₂ (3 mL) at 100 °C for 24 h. ^bCondition A: azide 1a (0.64 mmol), alkene 2 (0.32 mmol), CuSO₄ (1.12 mmol), NaBF₄ (0.2 mmol), and H₂O (2.22 mmol) in MeNO₂ (3 mL) at 100 °C for 24 h. ^cIsolated yields are given based on product 3 (0.16 mmol).

Interestingly, treatment of benzyl azide (1j) with 2a under conditions A and B gave the biquinolinium salt product 4ja in 5% yield (eqn (2)) instead of 3ja. The structure of 4ja was unambiguously confirmed by X-ray structure analysis.¹⁰ To improve the yield of 4ja, we conducted a series of optimization studies and found that treatment of 1j (0.64 mmol), 2a (0.32 mmol), and Cu(BF₄)₂·6H₂O (0.80 mmol) in CH₃NO₂ at 100 °C for 24 h offered 4ja in 70% isolated yield (see Table S4†). With the optimized conditions in hand, we tested the reaction of different alkenes with 2a (Scheme 4). 4-Bromostyrene (2c) effectively underwent multiple cycloadditions to give 4jc in 69% yield. Luckily, hept-1-ene (2d) also gave biquinolinium cation in 50% yield. However, the reaction of other alkenes 2f–g with benzyl azide (1j) resulted in inseparable complex mixtures. Interestingly, the reaction of ortho substituted benzylic azide such as 2-methylbenzyl azide (1k) and styrene (2a) afforded 6,6′-biquinolinyl derivative 5ka in 80% yield (Scheme 5). The structure was confirmed by its ¹H and ¹³C NMR, HRMS, and single crystal X-ray analysis.¹⁰ Similarly, 2-bromobenzylic azide (1l) gave product 5la in 20% yield. Other vinylic substrates 2c and 2d reacted effectively with 1k to give products 5kc and 5kd in 74 and 53% yields, respectively. The reaction also proceeded smoothly with β-methyl styrene (2e) giving 5ke in 54% yield. On the other hand, the use of allylbenzene (2f) as olefin substrate for the reaction with 1k, afforded product 5kf′ instead of 5kf in 66% yield. The benzyl group in 2f was cleaved during this copper-promoted reaction, similar to the reaction of 1a with 2f to give product 3af′ (Scheme 3).

Scheme 4 Scope of alkenes in the synthesis of biquinolinium salts. ^aReaction conditions: benzylic azide 1 (0.64 mmol), alkene 2 (0.32 mmol), and Cu(BF₄)₂·6H₂O (0.80 mmol) in MeNO₂ (3 mL) at 100 °C for 24 h. ^bIsolated yields calculated based on product 4 (0.08 mmol).

Scheme 5 Synthesis of substituted biquinolines. ^aReaction conditions: benzylic azide 1 (0.64 mmol), alkene 2 (0.32 mmol), and Cu(BF₄)₂·6H₂O (0.80 mmol) in MeNO₂ (3 mL) at 100 °C for 24 h. ^bIsolated yields calculated based on product 5 (0.16 mmol).

To understand the mechanism of this multiple aza-[4 + 2] cycloaddition reaction, several controlled experiments were performed as shown in Scheme 6. The reaction of 1a and 2a under reaction condition A, but with an extra addition of 1 equivalent of radical scavenger 2,2,6,6-tetramethylpiperidinyloxy (TEMPO), is blocked nearly completely affording 3aa in less than 10% yield. Next, we used pre-formed 4-methyl-N-methyleneaniline (1A) with 2a under the reaction conditions to give 3aa in 25% yield. The result indicates that N-aryl imine is a possible intermediate in situ formed from benzylic azides under the reaction conditions. Furthermore, we conducted the reaction of D₁-1a with 2a and the product D₂-3aa showed 50% deuterium incorporation at the two adjacent carbons connected to the quinoline nitrogen atom. The result suggests that both N-carbons of 3aa are from the CH₂ group of 1a. In addition, two molecules of 1a are required for the formation of a molecule of 3aa. Next, we treated 4-phenyl-1,2,3,4-tetrahydroquinoline (1B) with formaldehyde and styrene (2a) in the presence of Cu^II-salt to give product 4ja in 87% yield. Similarly, treatment of 1B in the presence of Cu(BF₄)₂·6H₂O gave 6,6′-biquinoline derivative 5B in 45% yield along with 17% 4-phenylquinoline (1C). Finally, we treated 1C with styrene and formaldehyde under the reaction conditions and neither dimer product (Schemes 4 and 5) nor quinolinium product (Schemes 2 and 3) was observed.

Scheme 6 Mechanistic studies.

Based on the mechanistic studies and known literature a plausible mechanism for the multiple aza-[4 + 2] cycloaddition reaction using 4-methyl benzyl azide (1a) and allylbenzene (2f) is proposed as shown in Scheme 7. It is expected that 4-methyl-N-methyleneaniline (I) is formed in situ from benzylic azide.^5,11 A dual [4 + 2] cycloaddition of in situ formed imine intermediate with alkene 2f affords julolidine derivative VI.^4,12,13 Copper-mediated multiple single electron transfer (SET) oxidation process offers the final quinolinium cation. During the aromatization process of julolidine VI, a benzyl group instead of a hydrogen is eliminated. The cationic product 3ag is stabilized by the non-nucleophilic anion BF₄.^6,14 The presence of H₂O is important for the present reaction. Because the cascade reaction involves a second aza-[4 + 2] cycloaddtion of imine cation V with alkene, the formation of this imine cation needs one molecule of HCHO, which is generated from the reaction of N-methyleneaniline I with H₂O.

Scheme 7 Plausible reaction mechanism for the synthesis of quinolinium cations.

A possible pathway for the formation of 6,6′-biquinolinium cation is depicted in Scheme 8. It follows similar steps shown in Scheme 7 up to the formation of 1,2,3,4-tetrahydro quinoline derivative IV′. Then oxidative dimerization of IV′ to give dimer IX′ likely proceeds via a series of Cu^II induced SET processes, nucleophilic addition, deprotonation and protonation.¹⁵ Further condensation of IX′ with formaldehyde released from the Cu^II-catalyzed decomposition of benzyl azide give di-iminium ions X′. Again, aza–cycloaddition reaction with 2a offers XI′. Cu^II-oxidation via a SET process in the presence of stabilizing BF₄ anion provides the final product 4ja. While the ortho substituted benzylic azides gave 6,6′-biquinoline product 5 (Scheme 5) from intermediate IX′ through Cu^II-oxidation (Scheme 7).

Scheme 8 Possible reaction mechanism for the formation of biquinolinium cations.

The new quinolinium and biquinolinium salts exhibit strong photoluminescence properties. A preliminary studies were performed using 3aa and 4ja to understand the fluorescence properties in solution. As shown in Fig. 1, the quinolinium salts 3aa and 4ja showed blue emissions. In addition, the fluorescence intensity of these compounds is very high prompting us to determine their fluorescence quantum yields (Φ_f) using 9,10-diphenylanthracene as the reference.¹⁶ Surprisingly, Φ_f of >99% for 3aa and 80% for 4ja were observed in dichloromethane solution. It is worth to mention that this type of fluorescent molecules highly useful in nonlinear optical (NLO) applications, fluorescent stains for biological studies, particle size analysis and chemosensors.^7,17 Moreover, these compounds also can be potential candidates as dopants for efficient solution-process organic light emitting diodes (OLEDs).¹⁸

Fig. 1 The absorption and emission spectra of compounds 3aa and 4ja at a concentration of 1 × 10⁻⁵ m in CH₂Cl₂.

Conclusions

In summary, we have successfully demonstrated a copper promoted multiple aza-[4 + 2] cycloaddition reactions between benzylic azides and alkenes. Three different types of products including quinolinium cations, biquinolinium cations, and biquinolines are formed from para-substituted, unsubstituted, and ortho-substituted benzylic azides, respectively. A wide range of functional groups on the benzylic azides can be tolerated. Moreover, high regioselectivity can be achieved with unsymmetrical alkenes. The reaction mechanism involves a Lewis acidic copper promoted rearrangement of benzylic azide to N-arylimine, followed by aza-[4 + 2] cycloaddition with alkene to provide 1,2,3,4-tetrahydroquinoline product, which further undergoes condensation with formaldehyde followed by another [4 + 2] cycloaddition with alkene to give the julolidine derivative. The mechanistic studies suggest that the oxidation of 1,2,3,4-tetrahydroquinoline to aromatic quinoline moiety goes through a copper mediated SET process. However, further detailed mechanistic studies are needed to elucidate the exact reaction mechanism, which is ongoing in our laboratory.

Experimental section

General information

Unless otherwise stated, all reactions were performed under a nitrogen atmosphere on a dual-manifold Schlenk line and in oven-dried glassware. All reagents were purchased commercially and used without further purification. Reagent grade nitromethane was used as such from Alfa Aesar without further purification. NMR spectra (¹H and ¹³C) were measured on a Bruker Avance III 400 MHz spectrometer. High resolution (HR) mass data were measured with a JEOL JMS-700 spectrometer. Infrared spectra were recorded on a HORIBA FT-IR 720 using KBr plates. UV-visible absorption and fluorescence spectra were measured using Hitachi U-3300 spectrophotometer and Hitachi F-7000 spectrophotometer, respectively.

General procedure for the synthesis of substituted benzylic azides^5b

Substituted benzyl bromide (1.0 equiv.) and sodium azide (1.5 equiv.) were dissolved in DMF (2.0 mL mmol⁻¹) and stirred at room temperature for overnight. At the end of the reaction, the mixture was diluted with water and extracted with diethyl ether. The combined organic solution was dried over MgSO₄ and concentrated in vacuo and the crude product was purified by a silica gel column (n-hexane/EtOAc, 90 : 10) to afford the substituted benzyl azide.

General procedure for the synthesis of quinolinium salts 3

Condition A. A sealed tube that contained CuSO₄ (178 mg, 1.12 mmol) and NaBF₄ (24 mg, 0.2 mmol) was evacuated and purged with nitrogen gas three times. MeNO₂ (2.0 mL) was then added to the tube, and the suspension was stirred for 2 min at ambient temperature. Then, para-substituted benzylic azide 1 (0.64 mmol), alkene 2 (0.32 mmol), H₂O (40 µL, 2.22 mmol), and additional MeNO₂ (1 mL) were added to the system via syringe sequentially. The reaction was stirred at 100 °C for 24 h. At the end of the reaction, the mixture was diluted with CH₂Cl₂ (10 mL), filtered through a Celite pad, and washed three times with CH₂Cl₂ (3 × 20 mL). The combined filtrate was concentrated in vacuo and the mixture was purified by a silica gel column using DCM/MeOH (95 : 5) as eluent to afford the desired pure product 3.

Condition B. A sealed tube that contained CuSO₄ (13 mg, 0.080 mmol), NaBF₄ (24 mg, 0.20 mmol) and (NH₄)₂S₂O₈ (92 mg, 0.40 mmol) was evacuated and purged with nitrogen gas three times. MeNO₂ (2.0 mL) was then added to the tube, and the suspension was stirred for 2 min at ambient temperature. Then, para-substituted benzylic azide 1 (0.64 mmol), alkene 2 (0.32 mmol), H₂O (40 µL, 2.22 mmol) and additional MeNO₂ (1 mL) were added to the system via syringe sequentially. The reaction was stirred at 100 °C for 24 h. At the end of the reaction, the mixture was diluted with CH₂Cl₂ (10 mL), filtered through a Celite pad, and washed three times with CH₂Cl₂ (3 × 20 mL). The combined filtrate was concentrated in vacuo and the mixture was purified by a silica gel column using DCM/MeOH (95 : 5) as eluent to afford the desired pure product 3.

General procedure for the synthesis of biquinolinium salts 4

A sealed tube containing Cu(BF₄)₂·6H₂O (276 mg, 0.80 mmol) was dissolved in MeNO₂ (2 mL) under nitrogen gas. Then, unsubstituted benzylic azide 1 (0.64 mmol), alkene 2 (0.32 mmol) and additional MeNO₂ (1 mL) were added to the system via syringe sequentially. The reaction was allowed to stir at 100 °C for 24 h. When the reaction was completed, the mixture was diluted with CH₂Cl₂ (10 mL) and filtered through a Celite pad and washed several times with CH₂Cl₂ (50 mL). The combined filtrate was concentrated in vacuo and the residue was purified by column chromatography on a silica gel column using DCM/MeOH (95 : 5) as eluent to afford the desired pure product 4.

General procedure for the synthesis of substituted biquinolines 5

A sealed tube containing Cu(BF₄)₂·6H₂O (276 mg, 0.80 mmol) was dissolved in MeNO₂ (2 mL) under nitrogen gas. Then, ortho-substituted benzyl azide 1 (0.64 mmol), alkene 2 (0.32 mmol) and additional MeNO₂ (1 mL) were added to the system via syringe sequentially. The reaction was allowed to stir at 100 °C for 24 h. When the reaction was completed, the mixture was diluted with CH₂Cl₂ (10 mL) and filtered through a Celite pad and washed several times with CH₂Cl₂ (50 mL). The combined filtrate was concentrated in vacuo and the residue was purified by column chromatography on a silica gel column using n-hexane/ethyl acetate (95 : 5) as eluent to afford the desired pure product 5.

Acknowledgements

We thank the Ministry of Science and Technology of the Republic of China (MOST-104-2633-M-007-001) for support of this research.

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Footnote

† Electronic supplementary information (ESI) available: General experimental procedures, characterization details and ¹H and ¹³C NMR spectra of new compounds. CCDC 1434531–1434533. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra11840b

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