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B(C₆F₅)₃-catalyzed TMSOTf-assisted intermolecular redox-neutral cyclization of N,N-dialkyl arylamines and allylic esters†

Qin-Long Wang‡ ^a, Wen-Jing Wang‡^a, Shuo Peng^b, Cuifen Lu^a, Junqi Nie^a, Sheng-Chao Huang*^b and Chao Ma*^a
aMinistry of Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, Hubei Key Laboratory for Precision Synthesis of Small Molecule Pharmaceuticals, College of Chemistry and Chemical Engineering, Hubei University, Wuhan, 430062, China. E-mail: machao@hubu.edu.cn
^bHubei Three Gorges Laboratory, Yichang, 443007, China. E-mail: huangshengchao027@qq.com

First published on 14th July 2025

Introduction

	Fig. 1 Tetrahydroquinoline drug molecules.

Various approaches have been established for constructing the THQ skeleton, such as hydrogenation of quinolines,⁶ intramolecular cyclizations,⁷ and the Povarov reaction.⁸ In recent years, hydride transfer involved cascade cyclization has also been developed into a powerful protocol for the synthesis of structurally diverse N-heterocycles including THQs,⁹ which relies on the use of starting materials containing ortho-substituted functionalities (e.g. aldehydes, imines or electron-deficient alkenes) as hydride acceptors. In this context, the oxidative Povarov reaction emerges as a significant strategy for the synthesis of tetrahydroquinoline derivatives from readily available starting materials.¹⁰ This dehydrogenative [4 + 2] annulation process involves N-alkylanilines and alkenes, enabling the formation of two new carbon–carbon bonds with high efficiency. However, this approach necessitates the use of stoichiometric oxidants to convert N-alkylanilines into iminium ions or α-aminoalkyl radical intermediates, which subsequently react with activated olefins bearing electron-donating or electron-withdrawing groups (Scheme 1a). Consequently, the development of a redox-neutral [4 + 2] annulation between N-alkylanilines and unactivated olefins for THQ synthesis, which eliminates the need for external oxidants, would be a highly desirable advancement.

	Scheme 1 Different catalytic strategies for THQ synthesis.

Over the past decade, the development of non-metallic catalysts has advanced rapidly. As a type of green non-metallic catalyst, B(C₆F₅)₃, which can catalyze a series of reactions, has drawn extensive attention and achieved remarkable progress.¹¹ B(C₆F₅)₃ activation of the α-C–H bond of N-alkylamines followed by a series of step- and atom-economical transformations is widely utilized.^12–15 The ability of B(C₆F₅)₃ to abstract α-H of amines results in the formation of iminium borohydride salts as key intermediates in these transformations. B(C₆F₅)₃-catalyzed intramolecular cyclization of adjacent olefin-substituted N,N-dialkyl arylamines for the synthesis of tetrahydroquinoline derivatives was developed by the Wang group and Paradies group, respectively (Scheme 1b).¹⁶ Nonetheless, this reaction requires the preparation of N,N-dialkyl arylamines containing a vinyl substituent at the ortho position of the aryl ring in advance. In 2022, our group developed an unprecedented B(C₆F₅)₃-catalyzed redox-neutral annulation reaction of N-alkylamines with electron-deficient alkynes to obtain tetrahydroquinoline derivatives (Scheme 1c).¹⁷ However, the efficient synthesis of tetrahydroquinoline derivatives through B(C₆F₅)₃-catalyzed intermolecular cyclization between N-alkylanilines and unactivated olefins under external oxidant-free conditions still remains elusive in the literature.

Herein, we report a B(C₆F₅)₃-catalyzed intermolecular redox-neutral cyclization reaction of N,N-dialkyl tertiary amines and allylic esters. This reaction proceeds via a Friedel–Crafts alkylation followed by a redox-neutral cyclization in one pot, enabling the synthesis of a series of tetrahydroquinoline derivatives with high efficiency (Scheme 1d).

Results and discussion

We initiated our study by exploring the intermolecular redox-neutral cyclization reaction of N,N-dialkyl tertiary amines and allylic esters using B(C₆F₅)₃ as the catalyst. To our delight, the reaction between 4-methyl-N,N-dimethylaniline 1a and allylic ester 2a proceeded smoothly at 80 °C using 10 mol% B(C₆F₅)₃ as the catalyst and m-xylene as the solvent. The Friedel–Crafts alkylation product 3a was obtained in >99% yield, but no cyclization product 4a was formed (Table 1, entry 1). Increasing the reaction temperature did not prove to be useful for the redox-neutral cyclization process (Table 1, entry 2). Inspired by our previous report,¹⁷ one equivalent of TMSOTf was added into the reaction mixture after completing the Friedel–Crafts alkylation reaction, and the reaction was continued at 80 °C for another 24 hours in one pot. Encouragingly, the desired product 4a was obtained in 70% yield, which demonstrated that TMSOTf was essential for the intramolecular cyclization process (Table 1, entry 3). Subsequently, when the one-pot reaction was carried out at 140 °C after the Friedel–Crafts alkylation process, product 4a was formed in 97% yield and 7:1 dr (Table 1, entry 4). Next, a few solvents were screened for this one-pot reaction. When toluene and dichloromethane were used as solvents, the yield slightly decreased (Table 1, entries 5 and 6). While 1,2-dichloroethane and 1,4-dioxane gave moderate yields (Table 1, entries 7 and 8), no desired products were obtained by using acetonitrile or tetrahydrofuran as the solvents (Table 1, entries 9 and 10). No product was formed in the absence of B(C₆F₅)₃ (Table 1, entry 11). Notably, several common metal Lewis acids such as FeCl₃, Fe(OTf)₃, Zn(OTf)₂ and Cu(OTf)₂ were screened as catalysts, and they turned out to be ineffective for the one-pot reaction (Table 1, entries 12–15).

Entry	Lewis acid	Additive	Solvent	Temp. (°C)	Yieldb (%)
a Unless otherwise stated, the reaction was stirred under N2 in 1.5 mL of solvent, 1a (0.3 mmol), 2a (0.2 mmol), and B(C6F5)3 (0.02 mmol) at 80 °C for 24 h. TMSOTf (0.2 mmol) was then added and stirred at 140 °C for 24 h.b Isolated yield; 7:1 dr was determined by 1H NMR.c Product 3a was obtained in >99% yield. n.d. = not detected. TMSOTf = trimethylsilyl trifluoromethanesulfonate.
1	B(C6F5)3	None	m-Xylene	80	n.d.c
2	B(C6F5)3	None	m-Xylene	140	n.d.c
3	B(C6F5)3	TMSOTf	m-Xylene	80	70
4	B(C6F5)3	TMSOTf	m-Xylene	140	97
5	B(C6F5)3	TMSOTf	Toluene	140	95
6	B(C6F5)3	TMSOTf	DCM	140	93
7	B(C6F5)3	TMSOTf	DCE	140	76
8	B(C6F5)3	TMSOTf	Dioxane	140	52
9	B(C6F5)3	TMSOTf	MeCN	140	n.d.
10	B(C6F5)3	TMSOTf	THF	140	n.d.
11	None	TMSOTf	m-Xylene	140	n.d.
12	FeCl3	TMSOTf	m-Xylene	140	n.d.
13	Fe(OTf)3	TMSOTf	m-Xylene	140	n.d.
14	Zn(OTf)2	TMSOTf	m-Xylene	140	n.d.
15	Cu(OTf)2	TMSOTf	m-Xylene	140	n.d.

With the optimal reaction conditions in hand (Table 1, entry 4), we investigated the generality of the B(C₆F₅)₃-catalyzed redox-neutral cyclization of N,N-dialkylanilines 1 with allylic ester 2a (Scheme 2). Various para electron-neutral and electron-donating group substituted N,N-dimethylanilines were compatible in this reaction and afforded the corresponding products 4a–4d in 89–97% yields. 4-(Allyloxy)-N,N-dimethylaniline was also tested, giving product 4e in 34% yield. In contrast, no desired products were observed when para electron-withdrawing group substituted N,N-dimethylanilines were treated with this reaction system, possibly due to the low reactivity for Friedel–Crafts alkylation. Disubstituted N,N-dimethylanilines underwent Friedel–Crafts alkylation at the sterically less demanding position, giving products 4f–4k in 56–97% yields. Trisubstituted N,N-dimethylaniline also worked smoothly under the standard conditions, delivering the desired product 4l in 97% yield with 3:1 dr. When one of the methyl groups on the amino moiety was replaced by a benzyl group, the reaction proceeded chemoselectively at the methyl group to deliver 4m in 95% yield with 9:1 dr. Changing the N-alkyl group from methyl to ethyl was feasible, giving product 4n in 65% yield with 10:1 dr. A piperidine derived substrate was also tested, and product 4o was obtained in 90% yield with 15:1 dr. Notably, substrates bearing drug molecules such as oxaprozin or isoxepac were tolerated under the standard conditions, providing products 4p and 4q in 89% and 81% yields, respectively. The structures of 4a and 4o were unambiguously confirmed by X-ray crystallographic analysis.

	Scheme 2 Scope of redox-neutral annulation between N,N-dialkylanilines and (E)-1,3-diphenylallyl 2,2,2-trifluoroacetate. The reaction was stirred under N2 in 1.5 mL of m-xylene, 1 (0.3 mmol), 2a (0.2 mmol), and B(C6F5)3 (0.02 mmol) at 80 °C for 24 h. TMSOTf (0.2 mmol) was then added and stirred at 140 °C for 24 h. The dr value was determined by 1H NMR.

Next, we investigated the compatibility of (E)-1,3-diarylallyl-2,2,2-trifluoroacetate 2 under the standard conditions (Scheme 3). Allylic esters 2 with electron-neutral and electron-donating groups at the ortho, meta and para positions of the aryl rings were well tolerated, and products 4r–4u were obtained in moderate to excellent yields ranging from 58% to 98% with high diastereoselectivity. In addition, 2-naphthyl substituted allylic ester was also tested, providing product 4v in 82% yield with 9:1 dr. However, no desired products were observed when allylic esters with electron-withdrawing substituents on the aryl rings were treated under the standard conditions. It should be noted that (E)-1,3-di(thiophen-2-yl)prop-2-en-1-ol 5 worked smoothly in the Friedel–Crafts alkylation. After further redox-neutral annulation reaction, the desired product 4w was obtained in 53% yield in two steps.

	Scheme 3 Scope of redox-neutral annulation between N,N,4-trimethylaniline and (E)-1,3-diarylallyl-2,2,2-trifluoroacetate. The reaction was stirred under N2 in 1.5 mL of m-xylene, 1a (0.3 mmol), 2 (0.2 mmol), and B(C6F5)3 (0.02 mmol) at 80 °C for 24 h. TMSOTf (0.2 mmol) was then added and stirred at 140 °C for 24 h. The dr value was determined by 1H NMR.

To demonstrate the utility and practicality of this method in organic synthesis, a 1 mmol scale reaction of allyl ester 2a and 4-methyl-N,N-dimethylaniline 1a was carried out under the standard reaction conditions, affording 318 mg (97%) of 4a, which is comparable with the 0.2 mmol scale reaction (Scheme 4a). To further showcase the synthetic applicability of the tetrahydroquinoline products, some valuable transformations were examined next (Scheme 4b). Tetrahydroquinoline 4a could be oxidized to amide 6 in 73% yield. Debenzylation of 4m produced 7 in 67% yield with a consistent dr value. The methyl group of 4d could be removed by BBr₃ to deliver compound 8 in 94% yield. Phenol 8 underwent sulfonylation with trifluoromethylsulfonic anhydride (Tf₂O), followed by a Pd/C catalyzed desulfonylation to remove the para-hydroxy group from the phenyl ring, affording tetrahydroquinoline 10 in good yield.

	Scheme 4 Scale-up reactions and synthetic transformation of tetrahydroquinolines 4.

Based on the experimental results as well as previous reports,^16,17 a plausible reaction mechanism was proposed, as shown in Scheme 5. The reaction is initiated by the coordination of B(C₆F₅)₃ to the carbonyl functionality of allylic ester 2a to form adduct I, which weakens the C–O bond, facilitating its cleavage to generate carbocation II and boron anion III. N,N-Dimethylaniline 1a then nucleophilically attacks the carbocation II to form intermediate IV. Subsequent deprotonation and rearomatization affords intermediate 3a and regenerates B(C₆F₅)₃. α-Hydride abstraction of 3a is promoted by B(C₆F₅)₃ to yield iminium borohydride salt V, which then undergoes cyclization to generate intermediate VI with high diastereoselectivity. TMSOTf may undergo a hydride exchange with the borohydride anion to regenerate B(C₆F₅)₃ and produce intermediate VII. The pentacoordinate anion [Me₃Si(OTf)H]⁻ is a better hydride donor than [HB(C₆F₅)₃]⁻,^16a,18 which facilitates hydride addition to the cyclic benzylic cation to afford product 4a.

	Scheme 5 Proposed catalytic cycle.

Conclusions

In summary, we have developed a B(C₆F₅)₃-catalyzed one-pot intermolecular redox-neutral cyclization reaction of N,N-dialkyl arylamines and allylic esters to construct the tetrahydroquinoline skeleton. This step-economical strategy involves Friedel–Crafts alkylation/hydride abstraction/cyclization/hydride reduction processes without using a transition metal or an external oxidant. A wide range of 1,2,3,4-tetrahydroquinoline derivatives were obtained in high yields (up to 98% yield) with high diastereoselectivity (up to >20:1 anti:syn).

Author contributions

Conflicts of interest

Data availability

Acknowledgements

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