English

• Tetrahydroquinolines and their derivatives are important scaffolds which are widely found in natural products and synthetic drugs [1-4]. Various methods have been developed for the synthesis of tetrahydroquinoline skeletons thus far [5, 6]. Among them, the oxidative annulation of N, N-dimethylaniline with maleimide holds special interest, which provides a convenient and efficient protocol to access tetrahydroquinolines [7-12]. As is shown, this reported oxidative annulation strategy usually involves the metal catalysts together with molecular oxygen or organic oxidants. Despite the remarkable success, this approach, however, generally requires harsh reaction conditions in most cases. Photocatalysis is considered as an eco-friendly and powerful synthetic technique, which can promote functional-group transformations with the photon as a green source of energy and a traceless reagent [13-21]. It provides a competitive alternative to tetrahydroquinolines synthesis [22-33], in which a variety of photoactive molecular metal complexes and organic dyes, including Ru(bpy)₃Cl₂, Ir(ppy)₂(dtbbpy)PF₆, [Cu(dap)₂]Cl and Eosin Y, have been employed as the photocatalysts. However, these photocatalysts are usually homogeneous, which would lead to the difficult catalyst separation and recovery. Therefore, the design and synthesis of recyclable and efficient heterogeneous photocatalysts for the photocatalytic aerobic annulation reactions are of great importance.

  Covalent organic frameworks (COFs), as an emerging class of organic crystalline porous materials with highly ordered and extended structures, have recently received increasing attention [34]. Due to their tuneability, high chemical stability, and excellent optical properties, COFs are found to be efficient heterogeneous photocatalysts for a series of organic transformations [35-48]. However, the COF-based photocatalytic aerobic annulation to tetrahydroquinolines has been rarely reported. Very recently, a Pt-metallated COF was reported to serve as the photocatalyst for oxidative annulation, in which the loaded Pt enhanced the photocatalytic activity of COF [49]. To meet the requirements of resource-saving and low-cost aerobic annulation, we envisioned that the metal-free porphyrin [50-52] COF with unique photophysical and photochemical properties could be the efficient photocatalyst for such transformation.

  Herein, we report, for the first time, a metal-free porphyrin COF (H₂P-Bph-COF)-based protocol for the synthesis of tetrahydroquinolines via photocatalytic aerobic annulation. In the presence of H₂P-Bph-COF, various tetrahydroquinolines were obtained by the combination of N, N-dimethylanilines with maleimides under visible-light irradiation at room temperature. Furthermore, the obtained H₂P-Bph-COF heterogeneous photocatalyst was robust, and could be reused at least five times without loss in its catalytic activity. To the best of our knowledge, the photocatalytic aerobic annulation reaction employing the metal-free COF as the heterogeneous photocatalyst has not been reported thus far.

  As shown in Fig. 1a, the porphyrin COF of H₂P-Bph-COF was synthesized by the acid-catalyzed Schiff-base condensation between 5, 10, 15, 20-tetra(p-amino-phenyl)porphyrin (H₂TAPP) and 4, 4′-biphenyldialdehyde under solvothermal conditions (HOAc, o-dichlorobenzene/n-BuOH, 120 ℃, 3 days). H₂P-Bph-COF was obtained as the purple crystalline solids [53, 54]. The formation of H₂P-Bph-COF was confirmed by Fourier transform infrared (FT-IR). The peaks of 1695 cm⁻¹ for C=O and 3338 cm⁻¹ for N-H were strongly attenuated, while a new characteristic C=N stretching vibration band at 1626 cm⁻¹ was observed in the FT-IR spectrum of H₂P-Bph-COF, implying the formation of the imine bond (Fig. S1 in Supporting information). Powder X-ray diffraction (PXRD) analysis indicated the good crystallinity of the obtained H₂P-Bph-COF. As shown in Fig. 1c, the PXRD diagram of H₂P-Bph-COF exhibited two intense peaks at 3.09° and 6.04°, which were attributed to (110) and (220) planes, respectively. The simulation of its PXRD pattern with Materials Studio (version 2018) suggested that H₂P-Bph-COF possessed the 2D network with eclipsed AA-type stacking mode. The Pawley refinement indicated a negligible difference between the simulated and experimental PXRD patterns. H₂P-Bph-COF was assigned to the space group P4 with optimized parameters of a = b = 41.49 Å, c = 3.67 Å, α = β = γ = 90°, residuals R_wp = 4.73% and R_p = 3.12%.

  Figure 1

  

  Figure 1.  (a) Schematic representation of the synthesis of H₂P-Bph-COF (HOAc, o-dichlorobenzene/n-BuOH, 120 ℃, 3 days). (b) Top view of the eclipsed structure proposed for H₂P-Bph-COF (gray, C; blue, N; and white, H). (c) Indexed experimental (red), Pawley-refined (black), and simulated (green) PXRD patterns of H₂P-Bph-COF. The difference plot is presented in blue.

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  The porosity of H₂P-Bph-COF was investigated by N₂ adsorption-desorption measurements at 77 K (Fig. S2 in Supporting information). The Brunauer-Emmett-Teller (BET) surface area of H₂P-Bph-COF was determined as 413 m²/g, and the total pore volume was calculated to be 0.32 cm³/g. The pore size distribution was mainly centered at ca. 2.0 nm based on nonlocal density functional theory (NLDFT) (Fig. S3 in Supporting information). The morphology of H₂P-Bph-COF was examined by scanning electron microscopy (SEM), which showed that the obtained H₂P-Bph-COF features micrometer-scale lamellar morphology (Fig. S4 in Supporting information). Thermogravimetric analysis (TGA) indicated that H₂P-Bph-COF remains stable up to ca. 240 ℃, suggesting its good thermal stability (Fig. S5 in Supporting information).

  We initially choose N, N-dimethylaniline (1a) and N-phenylmaleimide (2a) as the model substrates to carry out the photocatalytic oxidative annulation reaction. When the mixture, containing the substrates and a catalytic amount of H₂P-Bph-COF (8 mol%) as the photocatalyst in DMF, was irradiated by blue LEDs (λ = 450 nm) under an oxygen atmosphere for 12 h, the desired annulated product of 3a was obtained in 50% yield (Table 1, entry 1). Screening of the solvents showed that CHCl₃ was the best to give the product in 71% yield, while only a trace amount of annulated product was detected in MeOH or THF under the given conditions (Table 1, entries 2–6). On the other hand, the less catalyst loading (6 mol%) resulted in a lower 66% yield (Table 1, entry 7), while the more catalyst loading of 10 mol% did not lead to higher yield (Table 1, entry 8). In addition, the longer reaction time of 14 h and 16 h caused a slightly higher 76% yield for 3a (Table 1, entries 9 and 10). Further assessment revealed that the oxygen atmosphere is necessary (Table 1, entry 11). Control experiments indicated that both H₂P-Bph-COF and visible-light were essential for this aerobic annulation reaction (Table 1, entries 12 and 13). As such, the optimized conditions for synthesis of tetrahydroquinoline are as follows: 0.2 mmol 1a, 0.1 mmol 2a, and 8 mol% H₂P-Bph-COF in 3 mL CHCl₃, irradiated by blue LEDs under an oxygen atmosphere for 14 h at room temperature.

  Table 1

  

  Table 1.  Optimization of the H₂P-Bph-COF catalyzed photocatalytic aerobic annulation reaction conditions.^a

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  The leaching test demonstrated that H₂P-Bph-COF was a typical heterogeneous catalyst (Fig. S6 in Supporting information), and the yield of the annulation product 3a was still up to 73% after five catalytic runs (Fig. S7 in Supporting information). The measured PXRD pattern of H₂P-Bph-COF was intact after five catalytic runs, implying that its excellent stability and recyclability of H₂P-Bph-COF in photocatalysis (Fig. S8 in Supporting information).

  With the optimal reaction condition in hand, we explored the generality of this aerobic reaction. As shown in Scheme 1, a variety of N, N-dimethylanilines with electron-donating or electron-withdrawing groups could smoothly react with N-phenylmaleimide (2a) under the optimized reaction conditions, affording the desired annulated products in moderate-to-good yields (3b-3f). Furthermore, the scope of N-phenylmaleimide was also examined. When the N, N-dimethylaniline (1a) reacted with electronically diverse N-aryl maleimides bearing halogens (3g-3j) and alkoxy (3k-3l) groups at the meta-, and para- positions of aryl ring, the corresponding tetrahydroquinolines were generated in moderate-to-good yields. We have also investigated the scope of N-alkyl maleimides. N-Benzyl maleimide could react with N, N-dimethylaniline to afford the desired product in 32% yield (3m). Only a trace amount of annulated product (3n) was detected for the reaction of N-methyl maleimide with N, N-dimethylaniline (Scheme S3 in Supporting information).

  Scheme 1

  

  Scheme 1.  Scope of the H₂P-Bph-COF catalyzed photocatalytic aerobic annulation reaction. Reaction conditions: 1 (0.2 mmol), 2 (0.1 mmol), 8 mol% H₂P-Bph-COF, 3 mL CHCl₃, blue LEDs (λ = 450 nm), 14 h, room temperature, in oxygen. Isolated yields are shown.

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  To understand the reaction process of the system, cyclic voltammetry of H₂P-Bph-COF was performed. As is shown in Fig. S9 (Supporting information), an onset potential at −0.80 V vs. Fc/Fc⁺, attributable to the redox potential of E_H was observed. In addition, the UV–vis diffuse reflectance spectrum of H₂P-Bph-COF displayed a broad absorption up to ca. 660 nm, and the optical band gap (E_g) was calculated to be 1.75 eV according to the Tauc plots (Fig. S10 in Supporting information). Thus, the reduction potential of E_H was determined to be 0.95 V vs. Fc/Fc⁺ [55-58]. On the other hand, the oxidation potential of 1a (E_1a^•/1a) was known as 0.34 V vs. Fc/Fc⁺ [59, 60]. These results proved the thermodynamic feasibility of the photoinduced electron transfer from 1a to H₂P-Bph-COF*.

  In order to determine the function of molecular oxygen, we performed the electron-spin resonance (ESR) measurement to study the reactive oxygen species (ROS) generated in the system. Thereinto, 2, 2, 6, 6-tetramethylpiperidine (TEMP) was employed as a probe to trap ¹O₂, and 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) was used to capture O₂^{• −}. As shown in Fig. 2, the characteristic signal of ¹O₂ was observed when the system containing H₂P-Bph-COF and TEMP was irradiated by blue LEDs, and the ¹O₂ signal was strongly attenuated after the addition of 1a. In contrast, no signal was detected in the system containing DMPO and H₂P-Bph-COF irradiated by blue LEDs. However, the characteristic signal of O₂^{• −} captured by DMPO was clearly observed after the addition of 1a, indicating the formation of O₂^{• −}, a crucial species for the oxidative annulation reaction, via a photoinduced electron transfer process.

  Figure 2

  

  Figure 2.  ESR measurements of (a) CHCl₃ solution of H₂P-Bph-COF and TEMP irradiated by blue LEDs; (b) CHCl₃ solution of H₂P-Bph-COF, 1a and TEMP irradiated by blue LEDs; (c) CHCl₃ solution of H₂P-Bph-COF and DMPO irradiated by blue LEDs; (d) CHCl₃ solution of H₂P-Bph-COF, 1a and DMPO irradiated by blue LEDs.

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  Based on above results and previous reports [22-33], we proposed the reaction mechanism for this transformation. As shown in Scheme 2, upon irradiation by visible-light, the H₂P-Bph-COF was activated to its excited state H₂P-Bph-COF*. The electron transfer from 1a to H₂P-Bph-COF* led to the formation of 1a and H₂P-Bph-COF^{• −}. The generated H₂P-Bph-COF^{• −} further reacted with O₂ to produce O₂^{• −}, accompanying the regeneration of H₂P-Bph-COF. Meanwhile, the generated 1a further lose one proton to afford α-aminoalkyl radical 1a, which followed to react with 2a to produce radical 4. Subsequently, the generated 4 underwent cyclization to form intermediate 5. Finally, the oxidation and deprotonation of intermediate 5 assisted by O₂^{• −} afforded the desired product 3a, with the formation of H₂O₂.

  Scheme 2

  

  Scheme 2.  Proposed reaction mechanism.

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  In summary, we report a metal-free porphyrin COF and its application in photocatalytic oxidative annulation reaction of N, N-dimethylanilines with maleimides. Upon visible-light irradiation, the metal-free H₂P-Bph-COF can be a highly efficient heterogeneous and reusable photocatalyst to promote the formation of various substituted tetrahydroquinolines in moderate to good yields under mild conditions. We believe that the photoactive COF-based synthetic protocol herein not only provides a green and facile way to access tetrahydroquinolines synthesis, but also significantly expands the application scope of COF-based catalysts.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  We are grateful for the financial support from the National Natural Science Foundation of China (Nos. 22101158, 22171169, 21971153 and 21772116), the Natural Science Foundation of Shandong Province (No. ZR2021MB088), the Taishan Scholars Climbing Program of Shandong Province, the Taishan Scholar Project of Shandong Province, and the Major Basic Research Projects of Shandong Natural Science Foundation (No. ZR2020ZD32).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.01.065.

  
  
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• 

  Figure 1  (a) Schematic representation of the synthesis of H₂P-Bph-COF (HOAc, o-dichlorobenzene/n-BuOH, 120 ℃, 3 days). (b) Top view of the eclipsed structure proposed for H₂P-Bph-COF (gray, C; blue, N; and white, H). (c) Indexed experimental (red), Pawley-refined (black), and simulated (green) PXRD patterns of H₂P-Bph-COF. The difference plot is presented in blue.

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  Scheme 1  Scope of the H₂P-Bph-COF catalyzed photocatalytic aerobic annulation reaction. Reaction conditions: 1 (0.2 mmol), 2 (0.1 mmol), 8 mol% H₂P-Bph-COF, 3 mL CHCl₃, blue LEDs (λ = 450 nm), 14 h, room temperature, in oxygen. Isolated yields are shown.

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  Figure 2  ESR measurements of (a) CHCl₃ solution of H₂P-Bph-COF and TEMP irradiated by blue LEDs; (b) CHCl₃ solution of H₂P-Bph-COF, 1a and TEMP irradiated by blue LEDs; (c) CHCl₃ solution of H₂P-Bph-COF and DMPO irradiated by blue LEDs; (d) CHCl₃ solution of H₂P-Bph-COF, 1a and DMPO irradiated by blue LEDs.

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  Scheme 2  Proposed reaction mechanism.

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  Table 1.  Optimization of the H₂P-Bph-COF catalyzed photocatalytic aerobic annulation reaction conditions.^a

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