Synthesis of 5,6-Dihydrophenanthridines via Palladium-Catalyzed Intramolecular Dehydrogenative Coupling of Two Aryl C−H Bonds

5,6-Dihydrophenanthridines are common aza heterocycle frameworks of natural products and pharmaceuticals. Herein, we reported the first palladium-catalyzed intramolecular C−H/C−H dehydrogenative coupling reaction of two simple arenes to generate 5,6-dihydrophenanthridines. The approach features a broad substrate scope and good tolerance of functional groups, offering an efficient alternative synthesis route for important 5,6-dihydrophenanthridine compounds.


Introduction
5,6-Dihydrophenanthridines are common aza heterocycle frameworks of natural products and pharmaceuticals [1][2][3][4][5][6][7][8][9][10][11][12], exhibiting various biological activities including antibiotic, anti-inflammatory, and anticancer activity (Figure 1) [13][14][15][16][17][18][19]. More recently, studies have shown that the current COVID-19 pandemic is caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and 5,6-dihydrophenanthridine derivatives can interact tightly with SARS-CoV-2 nucleocapsid protein and inhibit the replication of SARS-CoV-2 in vitro [20,21]. Owing to the synthetic challenges in their unique polycyclic skeleton structures, as well as their potential druggability, 5,6-dihydrophenanthridines have aroused considerable interest in synthetic chemists [22,23]. In view of the structural feature of 5,6-dihydrophenanthridines, the formation of their aryl−aryl bond is undoubtedly the key step. To date, there are three main strategies to forge the aryl−aryl bond of 5,6-dihydrophenanthridines (Scheme 1): (1) transition-metal-catalyzed cross-couplings of organometallic aryls with aryl halides (path A) [24][25][26][27][28][29][30]; (2) annulation via benzyne intermediates (path B) [31][32][33]; and (3) the direct arylation of nonactivated aryl C−H bonds with aryl halides (path C) [34][35][36][37][38][39][40][41][42][43][44][45]. Compared to paths A and B, the biggest advantage of path C is that the more expensive and difficult-to-prepare organometallic coupling partner is replaced in this transformation. Nevertheless, the simplest and ideal approach to access 5,6-dihydrophenanthridines is the dehydrogenative coupling of two nonactivated aryl C−H bonds (path D), particularly when aryl halides are not readily available. However, so far there has been no report on using the ideal strategy in the construction of 5,6-dihydrophenanthridine. The major hindrance to the ideal strategy lies in three challenges: (1) the low reactivity of the aryl C−H bond [46]; (2) the regioselectivity issue, especially when there are several reactive sites; and (3) the strong coordinative nitrogen atom of the substrate and product that can easily poison the metal catalyst [23]. In order to accomplish the ideal approach, we hypothesized that an appropriate directing group could be introduced into the substrate to control the regioselectivity whilst at the same time enhancing the reactivity of the aryl C−H bond. Meanwhile, the nitrogen atom of the substrate should be protected by a proper protecting group. In addition, an efficient catalytic the ideal approach, we hypothesized that an appropriate directing group could be introduced into the substrate to control the regioselectivity whilst at the same time enhancing the reactivity of the aryl C−H bond. Meanwhile, the nitrogen atom of the substrate should be protected by a proper protecting group. In addition, an efficient catalytic system should undoubtedly be sought. Herein, we document the successful execution of this hypothesis to realize the first palladium-catalyzed intramolecular dehydrogenative coupling of two aryl C−H bonds to construct 5,6-dihydrophenanthridines.

Optimization of Reaction Conditions
We commenced this research by investigating the phenol-protecting group, which had the potential directing feature for the ortho-functionalization of arenes. A series of phenol-protecting groups were tested with 10 mol% Pd(OAc)2 as the catalyst and acetyl as the protecting group of the nitrogen atom in dimethyl sulfoxide (DMSO) under an air atmosphere (Table 1). The reaction did not occur without the phenol-protecting group (entry 1). The protecting groups CONMe2 and O- (2-pyridyl)carbonyl are excellent directing groups in many C-H functionalization reactions of arenes, but they almost did not work in this reaction (entries 2 and 3). Gratifyingly, when the O- (2-pyridyl)sulfonyl group was used as the protecting group, the desired aryl C−H/C−H coupling product 2a was  the ideal approach, we hypothesized that an appropriate directing group could be introduced into the substrate to control the regioselectivity whilst at the same time enhancing the reactivity of the aryl C−H bond. Meanwhile, the nitrogen atom of the substrate should be protected by a proper protecting group. In addition, an efficient catalytic system should undoubtedly be sought. Herein, we document the successful execution of this hypothesis to realize the first palladium-catalyzed intramolecular dehydrogenative coupling of two aryl C−H bonds to construct 5,6-dihydrophenanthridines. Scheme 1. The main strategies to forge aryl−aryl bond of 5,6-dihydrophenanthridines.

Optimization of Reaction Conditions
We commenced this research by investigating the phenol-protecting group, which had the potential directing feature for the ortho-functionalization of arenes. A series of phenol-protecting groups were tested with 10 mol% Pd(OAc)2 as the catalyst and acetyl as the protecting group of the nitrogen atom in dimethyl sulfoxide (DMSO) under an air atmosphere (Table 1). The reaction did not occur without the phenol-protecting group (entry 1). The protecting groups CONMe2 and O- (2-pyridyl)carbonyl are excellent directing groups in many C-H functionalization reactions of arenes, but they almost did not work in this reaction (entries 2 and 3). Gratifyingly, when the O-(2-pyridyl)sulfonyl group was used as the protecting group, the desired aryl C−H/C−H coupling product 2a was Scheme 1. The main strategies to forge aryl−aryl bond of 5,6-dihydrophenanthridines.

Optimization of Reaction Conditions
We commenced this research by investigating the phenol-protecting group, which had the potential directing feature for the ortho-functionalization of arenes. A series of phenol-protecting groups were tested with 10 mol% Pd(OAc) 2 as the catalyst and acetyl as the protecting group of the nitrogen atom in dimethyl sulfoxide (DMSO) under an air atmosphere (Table 1). The reaction did not occur without the phenol-protecting group (entry 1). The protecting groups CONMe 2 and O-(2-pyridyl)carbonyl are excellent directing groups in many C-H functionalization reactions of arenes, but they almost did not work in this reaction (entries 2 and 3). Gratifyingly, when the O-(2-pyridyl)sulfonyl group was used as the protecting group, the desired aryl C−H/C−H coupling product 2a was generated with a 9% yield (entry 4). The structure of 2a was confirmed by single-crystal X-ray diffraction. We considered that the reactivity of the O-(2-pyridyl)-sulfonyl group should lie in it being not only a great directing group but also a good activating group to facilitate the formation of a phenyl−Pd complex through the ortho C−H bond activation of phenol [47,48]. Encouraged by the preliminary result, we then carefully examined other parameters of the reaction. The influence of the solvent showed that CF 3 CH 2 OH was the best option, providing 2a with a 12% yield (entries 5-7). After screening the oxidant, copper(II) trifluoroacetate hydrate provided the highest yield (31%, entries [8][9][10][11][12]. Note that without any oxidant the reaction gave 2a with a 8% yield under an argon atmosphere (entry 13). Next, the reaction temperature was checked (entries [14][15][16][17]. Increasing the temperature benefitted the reaction, and the yield was increased to 45% at 100 • C. It turned out that the N-protecting group was crucial for this reaction. As speculated, the replacement of the N-protecting group with Ts, Boc, methyl, or phenyl all led to inferior results (entries [18][19][20][21]. Then, the palladium source was investigated, indicating that Pd(TFA) 2 was the best catalyst (entries [22][23][24][25]. The investigation of the amount of Pd(TFA) 2 and Cu(TFA) 2 ·H 2 O indicated that Pd(TFA) 2 (15 mol%) and Cu(TFA) 2 ·H 2 O (2.2 equiv.) were the best choices (entries [26][27][28][29][30][31]. It is noteworthy that the reaction proceeded similarly under an argon atmosphere (entry 31). Accordingly, the optimized reaction conditions were identified as the following: Pd(TFA) 2 (15 mol%) and Cu(TFA) 2 ·H 2 O (2.2 equiv.) in CF 3 CH 2 OH at 100 • C under an air atmosphere for 20 h (entry 29). generated with a 9% yield (entry 4). The structure of 2a was confirmed by single-crystal X-ray diffraction. We considered that the reactivity of the O-(2-pyridyl)-sulfonyl group should lie in it being not only a great directing group but also a good activating group to facilitate the formation of a phenyl−Pd complex through the ortho C−H bond activation of phenol [47,48]. Encouraged by the preliminary result, we then carefully examined other parameters of the reaction. The influence of the solvent showed that CF3CH2OH was the best option, providing 2a with a 12% yield (entries 5-7). After screening the oxidant, copper(II) trifluoroacetate hydrate provided the highest yield (31%, entries [8][9][10][11][12]. Note that without any oxidant the reaction gave 2a with a 8% yield under an argon atmosphere (entry 13). Next, the reaction temperature was checked (entries [14][15][16][17]. Increasing the temperature benefitted the reaction, and the yield was increased to 45% at 100 °C. It turned out that the N-protecting group was crucial for this reaction. As speculated, the replacement of the N-protecting group with Ts, Boc, methyl, or phenyl all led to inferior results (entries [18][19][20][21]. Then, the palladium source was investigated, indicating that Pd(TFA)2 was the best catalyst (entries [22][23][24][25]. The investigation of the amount of Pd(TFA)2 and Cu(TFA)2•H2O indicated that Pd(TFA)2 (15 mol%) and Cu(TFA)2•H2O (2.2 equiv.) were the best choices (entries [26][27][28][29][30][31]. It is noteworthy that the reaction proceeded similarly under an argon atmosphere (entry 31). Accordingly, the optimized reaction conditions were identified as the following: Pd(TFA)2 (15 mol%) and Cu(TFA)2•H2O (2.2 equiv.) in CF3CH2OH at 100 °C under an air atmosphere for 20 h (entry 29).

Substrate Scope
With the optimal conditions in hand, we first examined the effect of the substituents on the right aromatic ring for the aryl C−H/C−H coupling (Scheme 2). A variety of aryls with both electron-donating and electron-withdrawing groups could be engaged in this transformation, providing the desired 5,6-dihydrophenanthridines with good-toexcellent yields. A broad range of functional groups such as alkyls (2b−g), halides (F, Cl, Br, 2h−j), trifluoromethoxy (2k), nitro (2l), trifluoromethyl (2m), and methyl sulfonyl group (Ms, 3n) were compatible with this process. These provided synthetically interesting results because such substituents acted as versatile handles for further transformations. The position of the substituent on the aromatic rings had almost no effect on the reactivity (2o−q). Next, we investigated the substituents on the left phenol ring. Again, the aryl C−H/C−H dehydrogenative coupling reaction was insensitive to the electronic property of the substituent groups such as electron-donating methyl and methoxy, and electronwithdrawing ester groups; all reactions proceeded successfully, affording the desired products with 75-82% yields (2r−t).

Large-Scale Experiment and Synthetic Application
To test the practicality of this dehydrogenative coupling, a large-scale experiment was carried out. With the above standard reaction conditions, 1a (1.146 g, 3.0 mmol) provided 5,6-dihydrophenanthridine 2a (832 mg) with a 73% yield (Figure 2).

Large-Scale Experiment and Synthetic Application
To test the practicality of this dehydrogenative coupling, a large-scale exp was carried out. With the above standard reaction conditions, 1a (1.146 g, 3.0 mm vided 5,6-dihydrophenanthridine 2a (832 mg) with a 73% yield ( Figure 2).  To demonstrate the synthetic application of this methodology, we employed dehydrogenative coupling as the key step to synthesize an inhibitor of potassium channels K V 1.3 and IK-1 (7) (Scheme 3) [6]. First, the 2-pyridysulfonyl group was readily removed by zinc in NH 4 Cl (aq)/THF (1:1) at room temperature, affording 3 in a quantitative yield. Then, the hydroxy group was transformed into a benzenesulfonate group (4), which was further removed to give product 5 [49,50]. Next, the acetyl group in product 5 could be readily eliminated using H 2 SO 4 in MeOH, affording the product phenanthridine 6 with a 76% yield. Finally, the inhibitor dihydrophenanthridine 7 was achieved by the activation of the imine structure in phenanthridine with acyl chlorides to give an intermediary imminium ion followed by in situ nucleophilic attack with indole [6].
Molecules 2023, 28, x FOR PEER REVIEW To demonstrate the synthetic application of this methodology, we employe drogenative coupling as the key step to synthesize an inhibitor of potassium c KV1.3 and IK-1 (7) (Scheme 3) [6]. First, the 2-pyridysulfonyl group was readily r by zinc in NH4Cl (aq)/THF (1:1) at room temperature, affording 3 in a quantitativ Then, the hydroxy group was transformed into a benzenesulfonate group (4), wh further removed to give product 5 [49,50]. Next, the acetyl group in product 5 c readily eliminated using H2SO4 in MeOH, affording the product phenanthridine 76% yield. Finally, the inhibitor dihydrophenanthridine 7 was achieved by the ac of the imine structure in phenanthridine with acyl chlorides to give an intermed minium ion followed by in situ nucleophilic attack with indole [6].

Mechanistic Investigations
To gain insight into the mechanism of this intramolecular dehydrogenative c of the two aryl C−H bonds, a kinetic isotope effect (KIE) experiment was perform KIE value of the two parallel competition reactions of 1a and [D7]-1a was found to (Figure 3). This implied that an electrophilic aromatic palladation mechanism likely, and the cleavage of the C−H bond on the right aromatic ring should be inv the rate-determining step. In addition, a radical-trapping experiment was perfor the presence of 1.0 equiv. of 2,2,6,6-tetramethylpiperidine oxide (TEMPO), un above standard conditions, the reaction of 1a proceeded well to give 2a with alm same yield, illustrating a low possibility for a free radical pathway.

Mechanistic Investigations
To gain insight into the mechanism of this intramolecular dehydrogenative coupling of the two aryl C−H bonds, a kinetic isotope effect (KIE) experiment was performed. The KIE value of the two parallel competition reactions of 1a and [D 7 ]-1a was found to be 3.04 (Figure 3). This implied that an electrophilic aromatic palladation mechanism was unlikely, and the cleavage of the C−H bond on the right aromatic ring should be involved in the rate-determining step. In addition, a radical-trapping experiment was performed.
In the presence of 1.0 equiv. of 2,2,6,6-tetramethylpiperidine oxide (TEMPO), under the above standard conditions, the reaction of 1a proceeded well to give 2a with almost the same yield, illustrating a low possibility for a free radical pathway. KIE value of the two parallel competition reactions of 1a and [D7]-1a was found to be 3.04 (Figure 3). This implied that an electrophilic aromatic palladation mechanism was unlikely, and the cleavage of the C−H bond on the right aromatic ring should be involved in the rate-determining step. In addition, a radical-trapping experiment was performed. In the presence of 1.0 equiv. of 2,2,6,6-tetramethylpiperidine oxide (TEMPO), under the above standard conditions, the reaction of 1a proceeded well to give 2a with almost the same yield, illustrating a low possibility for a free radical pathway.

General Information
All reactions were carried out under an air atmosphere. Unless noted otherwise, commercially available chemicals were used without further purification. Flash chromatography was performed with silica gel (200-300 mesh). An oil bath served as the heat source. NMR spectra were acquired on either a Bruker 400 MHz ( 1 H at 400 MHz, 13 C at 100 MHz) or Jeol 400 MHz ( 1 H at 400 MHz, 13 C at 100 MHz) device, and NMR spectra were recorded in CDCl3 (TMS, δ = 0.00 ppm for 1 H and δ = 77.10 ppm for 13 C), DMSO-d6 (δ = 2.50 ppm for 1 H and δ = 39.52 ppm for 13 C), or CD3OD (δ = 3.31 ppm for 1 H and δ = 49.00 ppm for 13 C) using the solvent residue peaks as the internal references. Coupling constants were reported in hertz (Hz). Data for 1 H NMR spectra were reported as follows: chemical shift (ppm, referenced to protium, s = singlet, d = doublet, t = triplet, q = quartet, dd = doublet of doublets, td = triplet of doublets, ddd = doublet of doublet of doublets, m = multiplet, coupling constant (Hz), and integration). Infrared (IR) data were acquired on a Bruker Invenio-RFT-IR spectrometer. Absorbance frequencies were reported in reciprocal centimeters (cm −1 ). Mass spectra were acquired on a BrukerDaltonics S2 MicroTof-Q II mass spectrometer. X-ray crystal structure analyses were measured on a Bruker Smart APEXIICCD instrument using Mo-Kα radiation. The structures were solved and refined using the SHELXTL software package.

General Information
All reactions were carried out under an air atmosphere. Unless noted otherwise, commercially available chemicals were used without further purification. Flash chromatography was performed with silica gel (200-300 mesh). An oil bath served as the heat source. NMR spectra were acquired on either a Bruker 400 MHz ( 1 H at 400 MHz, 13 C at 100 MHz) or Jeol 400 MHz ( 1 H at 400 MHz, 13 C at 100 MHz) device, and NMR spectra were recorded in CDCl 3 (TMS, δ = 0.00 ppm for 1 H and δ = 77.10 ppm for 13 C), DMSO-d 6 (δ = 2.50 ppm for 1 H and δ = 39.52 ppm for 13 C), or CD 3 OD (δ = 3.31 ppm for 1 H and δ = 49.00 ppm for 13 C) using the solvent residue peaks as the internal references. Coupling constants were reported in hertz (Hz). Data for 1 H NMR spectra were reported as follows: chemical shift (ppm, referenced to protium, s = singlet, d = doublet, t = triplet, q = quartet, dd = doublet of doublets, td = triplet of doublets, ddd = doublet of doublet of doublets, m = multiplet, coupling constant (Hz), and integration). Infrared (IR) data were acquired on a Bruker Invenio-RFT-IR spectrometer. Absorbance frequencies were reported in reciprocal centimeters (cm −1 ). Mass spectra were acquired on a BrukerDaltonics S2 MicroTof-Q II mass spectrometer. X-ray crystal structure analyses were measured on a Bruker Smart APEXIICCD instrument using Mo-Kα radiation. The structures were solved and refined using the SHELXTL software package.

General Procedure A for Preparation of Substrates
To a solution of N-(3-hydroxyphenyl)acetamide (30.0 mmol, 4.51g) in CH 2 Cl 2 (50 mL), Et 3 N (40.0 mmol, 4.04 g) was added. The mixture was stirred at room temperature for 30 min, and then pyridine-2-sulfonyl chloride (30.0 mmol, 5.31 g) was added. The mixture was stirred overnight. The solvent was removed by distillation, and then EtOAc (50 mL) was added. The resulting solution was washed with water (3 × 10 mL) and brine (3 × 5 mL), dried over MgSO 4 , and concentrated. The residue was used without further purification in the next step.
To a solution of residue (1 mmol) in anhydrous tetrahydrofuran (10 mL), NaH (60 mg, 1.5 mmol, 60%) was added. The mixture was stirred at room temperature for 30 min, and then benzyl bromide (1.2 mmol) was added. The mixture was stirring at room temperature until completion of the reaction (monitored by TLC). Then, the reaction mixture was filtered and purified by column chromatography (eluent: PET (petroleum ether):EA (ethyl acetate) = 2:1).

General Procedure B for Preparation of Substrates
To a solution of 3-aminophenol (30.0 mmol, 4.51 g) in CH 2 Cl 2 (50 mL), NEt 3 (40.0 mmol, 4.04 g) was added. The mixture was stirred at room temperature for 30 min, and then pyridine-2-sulfonyl chloride (30.0 mmol, 5.31 g) was added. The mixture was stirred overnight. The solvent was removed by distillation, and then EtOAc (50 mL) was added. The resulting solution was washed with water (3 × 10 mL) and brine (3 × 5 mL), dried over MgSO 4 , and concentrated. The residue was used without further purification in the next step.
To a solution of residue (1 mmol) in CH 2 Cl 2 (10 mL), acetic anhydride (122 mg, 1.2 mmol) was added. The mixture was stirred overnight. The solvent was removed by distillation, and then CH 2 Cl 2 (15 mL) was added. The resulting solution was washed with water (3 × 5 mL) and brine (3 × 5 mL), dried over MgSO 4 , and concentrated. The residue was purified by column chromatography (eluent: PET:EA = 2:1) to give the product.
To a solution of above product (1 mmol) in anhydrous tetrahydrofuran (10 mL), NaH (60 mg, 1.5 mmol, 60%) was added. The mixture was stirred at room temperature for 30 min, and then benzyl bromide (1.2 mmol) was added. The mixture was stirring at room temperature until completion of the reaction (monitored by TLC). Then, the reaction mixture was filtered and purified by column chromatography (eluent: PET:EA = 2:1).   5-Acetyl-5,6-dihydrophenanthridin-1-yl benzenesulfonate (4): The compound 3 (0.4 mmol, 96 mg) was dissolved in DCM (10 mL) in a bottle filled under an argon atmosphere. The solution was cooled to 0 • C. Then, NEt 3 (0.078 mL, 0.56 mmol, 1.4 equiv) was added dropwise to the solution, which was followed by the addition of benzenesulfonyl chloride (84.5 mg, 0.48 mmol, 1.2 equiv). After 5 min, the ice bath was removed, and the reaction was monitored by TLC. Once the phenol was completely consumed, the reaction was stopped. The solvent was evaporated under a vacuum, and the residue was purified by flash column chromatography to give 4 as a colorless gel (144mg, 95%). 1  Phenanthridin (6) [57]: A stock solution was prepared by dropwise addition of concentrated sulfuric acid (1 mL) to reagent-grade methanol (5 mL) in a scintillation vial (20 mL in volume). The stock solution was stirred for 5 min, and then the solution was cooled to room temperature. A bottle equipped with a spin bar was charged with 1-(phenanthridin-5(6H)-yl)ethan-1-one 5 (0.26 mmol). A fraction of the stock solution (0.5 mL) was added slowly to the solid reagent, and the reaction mixture was stirred at 60 • C for 15 min. Then, the reaction mixture was cooled to room temperature, and distilled water (2 mL), EtOAc (5 mL), and saturated Na 2 CO 3 solution (2 mL) were added dropwise to the reaction mixture. The aqueous phase was extracted with EA (2 × 8 mL), and the combined organic fractions were dried over anhydrous Na 2 SO 4 or MgSO 4 . The combined organic fractions were filtered through celite, and the filtrate was concentrated. The residue was purified by flash column chromatography to give phenanthridine 6 as a white solid (35.7 mg, 76%). Immunosuppressant (7) [58]: Phenanthridine (0.2 mmol) was dissolved in dry tetrahydrofuran (2.0 mL) under an argon atmosphere. After cooling to 0 • C, the appropriate acetyl chloride (1.2 mmol) was added dropwise. The mixture was stirred for 2 h at room temperature and then cooled to 0 • C again. Triethylamine (0.3 mmol) was added followed by 1H-indole (1.5 mmol). After stirring for 3 h at room temperature, water was added, and the mixture was extracted several times with ethyl acetate. After washing the combined organic phases with brine and drying over Na 2 SO 4 , the solvent was removed in vacuo.