Site-selective and metal-free C–H nitration of biologically relevant N-heterocycles

The site-selective and metal-free C–H nitration reaction of quinoxalinones and pyrazinones as biologically important N-heterocycles with t-butyl nitrite is described. A wide range of quinoxalinones were efficiently applied in this transformation, providing C7-nitrated quinoxalinones without undergoing C3-nitration. From the view of mechanistic point, the radical addition reaction exclusively occurred at the electron-rich aromatic region beyond electron-deficient N-heterocycle ring. This is a first report on the C7–H functionalization of quinoxalinones under metal-free conditions. In contrast, the nitration reaction readily takes place at the C3-position of pyrazinones. This transformation is characterized by the scale-up compatibility, mild reaction conditions, and excellent functional group tolerance. The applicability of the developed method is showcased by the selective reduction of NO2 functionality on the C7-nitrated quinoxalinone product, providing aniline derivatives. Combined mechanistic investigations aided the elucidation of a plausible reaction mechanism. Supplementary Information The online version contains supplementary material available at 10.1007/s12272-021-01351-5.


Introduction
Since the landmark discovery by Mitscherlich and Laurent in 1834 (Patel et al. 2021), nitro compounds are important and versatile building blocks in organic chemistry (Ono 2001), and their derivatives are widely utilized in various pharmaceuticals, agrochemicals, pigments, and dyes as well as a variety of fine chemicals such as solvents, perfumes, explosives, and polymers, as shown in Fig. 1 (Fan et al. 2004;McNamara et al. 2011;Nepali et al. 2019). For example, chloramphenicol (Rebstock et al. 1949) and metronidazole (Freeman et al. 1997) are well-known antibiotic and antiprotozoal drugs for the treatment of a number of infectious diseases. The biological properties of these molecules are closely related with (hetero)aryl motifs tethered with nitro functionality, but vary depending on the nature and position of substituents on (hetero)aryl rings.
Traditional methods for nitration reactions rely on an excess use of nitric acid or its mixture with sulfuric acid or dinitrogen pentoxide (Olah et al. 1990). However, from a synthetic point of view, these protocols represent the limited functional group tolerance and the generation of undesirable by-products as well as incomplete regioselectivity. To overcome these limitations, new nitrating agents such as nitrate (Manna et al. 2012;Zolfigol et al. 2012), nitrite salts (Fors et al. 2009;Li et al. 2013), and tert-butyl nitrite (TBN) have been intensively investigated (Wu and Neumann et al. 2011;Wu and Schranck et al. 2011;Shen et al. 2014).
With great advance on C-H functionalization reactions (Mishra et al. 2017Pandey et al. 2018;Sambiagio et al. 2018;Lee et al. 2019), direct C-H nitration of Abstract The site-selective and metal-free C-H nitration reaction of quinoxalinones and pyrazinones as biologically important N-heterocycles with t-butyl nitrite is described. A wide range of quinoxalinones were efficiently applied in this transformation, providing C7-nitrated quinoxalinones without undergoing C3-nitration. From the view of mechanistic point, the radical addition reaction exclusively occurred at the electron-rich aromatic region beyond electron-deficient N-heterocycle ring. This is a first report on the C7-H functionalization of quinoxalinones under metal-free conditions. In contrast, the nitration reaction readily takes place at the C3-position of pyrazinones. This transformation is characterized by the scale-up compatibility, mild reaction conditions, and excellent functional group tolerance. The applicability of the developed method is showcased by the selective reduction of NO 2 functionality on the C7-nitrated quinoxalinone product, providing aniline derivatives. Combined mechanistic investigations aided the elucidation of a plausible reaction mechanism. Site-selective and metal-free C-H nitration of biologically relevant N-heterocycles (hetero)arenes has been recently developed. The metal-free oxidative C-H nitration of phenols or amines has been explored, as shown in Fig. 2 (Koley et al. 2009;Kilpatrick et al. 2013;Li et al. 2014). However, none of these protocols represent a general strategy to allow for complete site-selectivity between the ortho-and para-positions. A great deal of effort on site-selectivity of nitration has been devoted to the transition-metal-catalyzed ortho-C-H nitration of N-heterocycles. For example, Liu reported the Pd(II)-catalyzed ortho-C-H nitration of nitrogen-containing heterocycles with silver nitrite in the presence of K 2 S 2 O 8 as an external oxidant (Liu et al. 2010). The ortho-C-H nitration of (hetero)arenes using nitrite salts was also realized with the Cu(II), Rh(III), Ru(0), and Ni(II) catalytic systems (Zhang et al. 2011;Xie et al. 2013;Katayev et al. 2014;Majhi et al. 2014;Fan and Ni 2016;Wan et al. 2017). In addition, the Pd(II)-catalyzed aerobic oxidative ortho-C-H nitration of arenes with tert-butyl nitrite and toluene as the radical precursors was demonstrated (Liang et al. 2015). The azaindole-assisted ortho-C-H nitration of arenes with tert-butyl nitrite affording various nitrated azaindole derivatives was disclosed .
Despite the compelling progress on the C-H nitration reaction of various N-heterocycles, the site-selective nitration reaction of quinoxalinones and pyrazinones under milder reaction conditions is still unexplored. Driven by our ongoing interest in the C−H functionalization of N-heterocycles Ghosh et al. 2019Ghosh et al. , 2021An et al. 2020;Park et al. 2021), we herein describe the metal-free and site-selective C-H nitration reaction of quinoxalinones and pyrazinones with tert-butyl nitrite as a readily available nitrating agent. Notably, the gramscale reaction, selective reduction of a nitro group, and thiocarbonylation demonstrate the synthetic utility of the developed method.

General methods
Commercially available reagents were used without additional purification, unless otherwise stated. Quinoxalinones (1a-1m) and 5-aryl pyrazinones (4a-4j) were prepared according to the reported literature Guo et al. 2021). t-Butyl nitrite was purchased from Aldrich, Switzerland. All the reactions were performed in an oil bath by using hot plate magnetic stirrer (IKA universal, Guangzhou city, China). Sealed tubes were purchased from Fischer Scientific (13 × 100 mm, 1495925A; Mexico) and dried in oven for overnight and cooled at room temperature prior to use. Thin layer chromatography was carried out using plates coated with silica gel 60 F 254 (Merck KGaA, 64271 Darmstadt, Germany). For flash column chromatography, silica gel 60 Å (230-400 mesh, Merck, Germany) was used. Nuclear magnetic resonance spectra ( 1 H, 13 C, and 19 F NMR) were recorded on a Bruker Unity 400, 500, and 700 MHz spectrometers in CDCl 3 , CD 3 COCD 3 , and DMSO-d 6 solution and chemical shifts are reported as parts per million (ppm). Resonance patterns are reported with the notations s (singlet), br (broad), d (doublet), t (triplet), q (quartet), sext (sextet), dd (doublet of doublets), dt (doublet of triplets), dq (doublet of quartets), qd (quartet of doublets), td (triplet of doublets), tt (triplet of triplets), and m (multiplet). In addition, the notation br is used to indicate a broad signal. Coupling constants (J) are reported in hertz (Hz). IR spectra were recorded on a Varian 2000 Infrared spectrophotometer and are reported as cm −1 . High-resolution mass spectra (HRMS) were recorded on a JEOL JMS-600 spectrometer.  To an oven-dried sealed tube charged with 1-methylquinoxalin-2(1H)-one (1a) (32.0 mg, 0.2 mmol, 100 mol %) was added t-butyl nitrite (2a) (71.4 µL, 0.6 mmol, 300 mol %) and CH 3 CN (2 mL) under O 2 atmosphere at room temperature. After using O 2 balloon, the reaction mixture was allowed to stir at 60 °C for 20 h. The reaction mixture was cooled to room temperature, diluted with EtOAc (4 mL) and concentrated in vacuo. The residue was purified by flash column chromatography (n-hexanes/EtOAc = 10:1 to 2:1) to afford 3a (31.2 mg) in 76% yield.

General procedure and characterization data for the thiocarbonylation of 3a into 6b
To an oven-dried sealed tube charged with 1-methyl-7-nitroquinoxalin-2(1H)-one (3a) (41.0 mg, 0.2 mmol, 100 mol %) and Lawesson's reagent (242.7 mg, 0.6 mmol, 300 mol %) was added toluene (2 mL) under air at room temperature. The reaction mixture was allowed to stir in an oil bath for 12 h at 120 °C. The reaction mixture was cooled to room temperature, diluted with EtOAc (5 mL) and concentrated in vacuo. The aqueous residue was extracted with EtOAc (3 × 15 mL). The combined organic layer was washed with brine, dried over MgSO 4 , and concentrated in vacuo. The residue was purified by flash column chromatography (n-hexanes/acetone = 10:1 to 1:2) to afford 6b (25.8 mg) in 58% yield.

General procedures for the control experiment using a radical scavenger TEMPO
To an oven-dried sealed tube charged with 1-methylquinoxalin-2(1H)-one (1a) (32.0 mg, 0.2 mmol, 100 mol %) and TEMPO (312.5 mg, 2.0 mmol, 10.0 equiv.) were added t-butyl nitrite (2a) (71.4 µL, 0.6 mmol, 300 mol %) and CH 3 CN (2 mL) under O 2 atmosphere at room temperature. After using O 2 balloon, the reaction mixture was allowed to stir in an oil bath for 8 h at 60 °C. The reaction mixture was cooled to room temperature, diluted with EtOAc (4 mL) and concentrated in vacuo. On TLC the desired product 3a was not detected.
Solvent screening revealed that this coupling reaction displayed the increased reactivity in CH 3 CN solvent to give 3a in 40% yield (Table 1, entries 2 − 4). Addition of oxidants such as K 2 S 2 O 8 , Na 2 S 2 O 8 , and AgNO 2 were found to be unsatisfactory in this transformation (Table 1, entries 5 − 7). To our delight, this reaction smoothly proceeded with three equiv. of 2a to afford the desired product 3a in 76% yield along with C5-nitrated compound 3aa in 9% yield (Table 1, entry 8). The reaction temperature is quite pivotal for this transformation, as shown in entries 9 and 10. It should be noted that molecular oxygen was needed for the formation of both 3a and 3aa (Table 1, entry 11), revealing that a NO radical, derived from the decomposition of tert-butyl nitrite, could be readily oxidized into a reactive NO 2 radical by molecular oxygen (O 2 gas). Finally, when the reaction was performed with increased loading of 2a, the lower formation of our desired product was observed (Table 1, entries 12 and 13).
With the optimal reaction conditions in hand, the scope of quinoxalinones was examined as shown in Table 2. The linear and branched N-alkylated quinoxalinones 1b − 1e were found to be suitable substrates for this coupling reaction to afford C7-nitrated quinoxalinones 3b − 3e in high yields. It is noteworthy that a linear alkyl halide 1e was completely compatible under the current reaction conditions, and the tolerance of bromo moiety presents valuable opportunities for further versatile synthetic transformations. In addition, N-benzylated quinoxalinones 1f and 1g were also coupled with 2a to provide the corresponding products 3f (55%) and 3g (31%). To our pleasure, the current protocol could be applied to N-arylated quinoxalinones 1h − 1k, producing the desired products 3h − 3k without undergoing the C−H nitration on the N-aryl ring. It is mentioned that electrondeficient NO 2 (1i) and CO 2 Et (1k) groups on the N-aryl moiety were found to be comparatively less reactive in this transformation, presumably due to the destabilization of radical and carbocation intermediates. To observe the steric and electronic effects on the quinoxalinone framework, the reactions of 1l with 2a under the standard and modified reaction conditions were subjected to afford the C7-nitrated adduct 3l in 20% and 35% yields. It should be mentioned that the nitration reaction of C7-substituted quinoxalinone 1m preferentially occurred at the C5-position, affording the nitrated product 3m as a single regioisomer in 45% yield.
Additionally, N-alkyl-5-aryl-substituted pyrazinones 4b-4e reacted with 2a to afford C3-nitrated pyrazinone adducts 5b-5e in moderate to good yields. In addition, N-benzylsubstituted pyrazinones 4f and 4g were also compatible under the current reaction conditions to give the corresponding products 5f (45%) and 5g (61%). The complete regioselectivity was observed in all cases. Finally, the C3-nitration reaction of N-aryl-substituted pyrazinones 4h-4j smoothly proceeded, resulting in the formation of the desired products 5h-5j. The functional group compatibility of nitrile and acetyl moieties (5g and 5i) allows further synthetic elaboration of the products.
To demonstrate the robustness and practicality of this process, the scale-up experiments and synthetic transformations were performed (Fig. 4). The nitration reaction of 1a was readily scaled up to 1 g (6.3 mmol) for the formation of 3a (0.69 g) in 68% yield. In addition, the gram-scale reaction of 4a (1 g, 5.4 mmol) with 2a was successfully achieved to afford 0.81 g of 5a in 65% yield. Meanwhile, the selective reduction of a nitro moiety of the product 3a was performed by the single electron reduction protocol using by Fe/NH 4 Cl to furnish the desired aniline adduct 6a in 65% yield. Moreover, treatment of 3a with Lawesson's reagent resulted in the formation of 7-nitroquinoxaline-2(1H)-thione 6b in 58% yield.

Discussion
To support the mechanistic pathway for this reaction, the nitration reaction of 1a and 2a was performed in the presence of a radical scavenger TEMPO (Fig. 5). No formation of C7-nitrated quinoxalinone 3a was observed. The nitration reaction was completely inhibited by 1,1-diphenylethylene (2b) as a radical polymerization mediator, and 1,1-diphenyl-2-nitroethylene (7a) was obtained in 56% yield. These results support that a radical pathway is involved in this process. Based on preliminary mechanistic investigation and reported literatures (Liang et al. 2015;Chun et al. 2018), a proposed reaction mechanism is outlined in Fig. 6. In the presence of molecular oxygen, a reactive NO 2 radical can be derived from the thermal decomposition of t BuONO, followed by subsequent aerobic oxidation of a NO radical. A NO 2 radical can undergo the radical addition into the C7-position on quinoxalinone 1a, affording intermediate A. The single-electron transfer (SET) process by the assistance of NO 2 or t BuO radical followed by aromatization provides the C7-nitrated product 3a. In case of the nitration of pyrazinone 4a, the radical addition can occur at the C3-position, delivering a nitrogen radical species C, which further undergoes the SET reaction and elimination reaction to produce 6a. The site-selectivity between the C7-and C3-positions of quinoxalinone 1a can be rationalized by the electronic density between the electron-rich aromatic ring Table 2 Scope of quinoxalinones a a Reaction conditions: 1a − 1m (0.2 mmol), 2a (0.6 mmol, 3 equiv.), CH 3 CN (2 mL) under O 2 atmosphere at 60 °C for 20 h in reaction tubes b Isolated yield by flash column chromatography c 2a (1.0 mmol, 5 equiv.) was used and electron-deficient N-heterocycle ring in the electrophilic radical addition step. Moreover, the C7-selectivity over C6,C8-positions on aromatic ring of 1a can be explained by the relative stability of a radical intermediate A. However, the site-selectivity between C7-and C5-position still remains unclear, and the detailed mechanistic investigations on the site-selectivity of this process are underway.
In summary, we described the synthesis of biologically relevant C7-nitrated quinoxalinones and C3-nitrated pyrazinones through metal-free C-H nitration with t-butyl nitrite. From the mechanistic point of view, the radical addition to quinoxalinones with t-butyl nitrite exclusively occurred at the electron-rich aromatic region beyond electron-deficient N-heterocycle ring. In contrast, the nitration reaction of pyrazinones readily takes place at the C3-position via the single electron transfer process of a nitrogen radical intermediate followed by elimination reaction. This protocol is characterized by the scale-up compatibility, mild reaction conditions, and excellent functional group tolerance. The selective reduction of a NO 2 group and thiocarbonylation on the synthesized products highlight the importance of the developed methodology.