Pd(TFA)2-catalyzed direct arylation of quinoxalinones with arenes

Sanjay Paul ab, Hari Datta Khanal a, Chayan Dhar Clinton a, Sung Hong Kim c and Yong Rok Lee *a
aSchool of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea. E-mail: yrlee@yu.ac.kr; Fax: 82-53-810-4631; Tel: 82-53-810-2529
bDepartment of Chemistry, Behala College, Parnashree, Behala, Kolkata-700060, India
cAnalysis Research Division, Daegu Center, Korea Basic Science Institute, Daegu 41566, Republic of Korea

Received 16th November 2018 , Accepted 11th December 2018

First published on 12th December 2018


The direct C-3 arylation of quinoxalin-2-ones with arenes is achieved by a Pd(TFA)2-catalyzed cross-dehydrogenative coupling (CDC) reaction. A wide array of functionalities, including highly sensitive allyl- and benzyl group substituted quinoxalin-2(H)-ones, are readily assembled with arenes containing electron-donating and electron-withdrawing groups in this CDC reaction. Highly regioselective products are produced in good yields with stronger electron-donating groups than toluene and densely substituted arenes as coupling partners. This protocol represents an efficient technique to access various 3-arylbenzo[g]quinoxalinones and 2-aryl-4-methylpyrido[3,4-b]pyrazin-3(4H)-ones under mild conditions.


Introduction

The transition-metal-catalyzed cross-dehydrogenative coupling (CDC) reaction is a promising synthetic tool to access valuable organic molecules.1 The direct functionalization of C–H bonds has been demonstrated as an important cross-coupling strategy for the formation of a range of C–C bonds.2 Owing to its atom- and step-economical nature, the CDC reaction is widely used in industry and academia for C–C bond formation over traditional cross-coupling and direct cross-coupling.3 In particular, the Pd-catalyzed olefination and arylation of 2-substituted 1,2,3-triazole N-oxides,3f β-carboline-N-oxides,3g and pyridine N-oxides3h have been previously described. The abundance of unsaturated rings in pharmacologically important molecules and natural products makes sp2 C–H bond activation more appealing to organic and medicinal chemists than sp3 C–H bond activation.4 Although many CDC reactions have been described, the development of novel CDC reactions between heterocycles and arenes is highly desirable in medicinal and organic synthesis.

Quinoxalinones are found in many biologically and pharmaceutically significant molecules.5 For example, ataquimast and bamaquimast have been used as antiasthmatic drugs in the treatment of chronic obstructive bronchopneumopathy,6 whereas caroverine is a spasmolytic drug used to treat tinnitus.7 In particular, 3-substituted quinoxalinone derivatives possess a wide range of potent biological properties, such as antibacterial,8 anti-inflammatory,9 antitumor,10 anticancer,11 antiproliferative,12 aldose reductase inhibiting,13 STK33 inhibiting,14 and VEGFR-2 kinase inhibiting activities.15 The wide biological importance and utility of 3-substituted quinoxalinone derivatives have encouraged synthetic organic and medicinal chemists to develop novel and efficient methodologies.

Owing to their importance, a range of protocols for the synthesis of 3-substituted quinoxalin-2-ones based on intra- and intermolecular cyclization reactions have been reported.16 Among these, typical approaches for 3-arylquinoxalin-2-ones include visible light-induced radical cyclization of ethyl 2-(N-arylcarbamoyl)-2-chloroiminoacetates,16a cleavage of immobilized oxazolones with benzene-1,2-diamines,16b Ugi 4CC/aza-Wittig sequence,16c and acylation of benzene-1,2-diamines followed by cyclization.16d More general methodologies for 3-arylquinoxalin-2-ones include Pd(II)-catalyzed oxidative 3-arylation of quinoxalin-2-ones (method a, Scheme 1)17 or 3-halo-quinoxalinones with arylboronic acids (method b, Scheme 1).18 As part of an ongoing study to develop efficient synthetic methods for 3-arylquinoxalin-2-ones, the authors previously reported the iodosobenzene-promoted direct oxidative 3-arylation of quinoxalin-2-ones with arylhydrazines (method c, Scheme 1).19


image file: c8qo01250d-s1.tif
Scheme 1 Reported 3-arylation of quinoxalin-2-ones or 3-chloroquinoxalin-2-ones with arylboronic acids or arylhydrazines.

Despite their merits, more environmentally benign and novel approaches are needed to improve the reaction conditions and avoid the use of pre-functionalized starting materials. In this context, the direct CDC reaction is useful for the construction of 3-arylquinoxalin-2-ones. Recently, the direct phosphonation of quinoxalin-2-ones has been demonstrated under transition-metal-free conditions (eqn (1), Scheme 2).20 On the other hand, the direct CDC reaction of qunioxalin-2-ones with arenes has not been reported. The authors previously described the transition-metal-catalyzed direct dehydrogenative cross-coupling of tetrahydrofuran (THF) with phenols21a and 1,4-naphthoquinones21b to form C–C bonds. In connection with previous studies on CDC reactions, this paper reports the novel Pd(TFA)2-catalyzed direct dehydrogenative cross-coupling of quinoxalin-2-ones with arenes for the construction of diverse 3-aryl qunioxalin-2-ones (eqn (2), Scheme 2).


image file: c8qo01250d-s2.tif
Scheme 2 Direct dehydrogenative cross-coupling strategies for C–P or C–C bond formation.

Results and discussion

The model reaction of 1-methylquinoxalin-2(1H)-one (1a) with benzene (2a) as a reactant and solvent was investigated under several catalysts and oxidants (Table 1). The initial attempt with the oxidant Ag2O (1.5 equiv.) in the presence of 10 mol% of AuCl3 or Ni(COD)2 as catalysts at 110 °C for 30 h did not provide the desired product (entries 1 and 2, Table 1). With Ag2O and PdCl2 (10 mol%), 3a was produced in 20% yield (entry 3).
Table 1 Optimization of the double C–H bond activation reactiona

image file: c8qo01250d-u1.tif

Entry Catalyst (mol%) Oxidant (1.5 equiv.) Time (h) Yield (%)
a Reaction conditions: 1a (1.0 mmol), catalyst (mol%) and oxidant (1.5 equiv.) in 3 mL of 2a at 110 °C under a N2 atmosphere.
1 AuCl3 (10) Ag2O 30
2 Ni(COD)2 (10) Ag2O 30
3 PdCl2 (10) Ag2O 30 20
4 Pd(OAc)2 (10) Ag2O 20 68
5 Pd(TFA) 2 (10) Ag 2 O 20 75
6 Pd(TFA)2 (5) Ag2O 20 56
7 Pd(TFA)2 (15) Ag2O 20 75
8 Pd(TFA)2 (10) AgNO3 20 15
9 Pd(TFA)2 (10) Ag2CO3 20 70
10 Pd(TFA)2 (10) Cu(OAc)2 20 60
11 Pd(TFA)2 (10) PhI(OAc)2 20 16


Encouraged by this result, other palladium catalysts were screened. With 10 mol% of Pd(OAc)2, the yield of 3a increased to 68% (entry 4). The best yield (75%) was achieved with Ag2O (1.5 equiv.) and Pd(TFA)2 (10 mol%) at 110 °C for 20 h (entry 5). The yield was not improved by decreasing Pd(TFA)2 to 5 mol% or increasing to 15 mol% (entries 6 and 7). With other oxidants, such as AgNO3, Ag2CO3, Cu(OAc)2, and PhI(OAc)2, 3a was isolated in 15, 70, 60, and 16% yields, respectively (entries 8–11). The structure of 3a was determined by spectral data analysis and by a comparison with the reported spectra.11

Under the optimized reaction conditions, additional reactions of various quinoxalinones 1a–1d with different arenes 2a–2l were next examined to afford a variety of 3-aryl quinoxalinone derivatives (Table 2). A reaction of 1a with toluene (2b) at 110 °C for 20 h provided 3b (78%) as an inseparable mixture of para and ortho products (5[thin space (1/6-em)]:[thin space (1/6-em)]1). The ratio was determined by 1H NMR spectroscopy of 3b. A combination of 1a with cumene (2c) or anisole (2d) bearing an electron-donating group on the benzene ring provided only para-position-coupled products 3c (75%) and 3d (80%), respectively, without the isolation of ortho-oriented products. With p-xylene (2f) and 1,3-dimethoxybenzene (2g), the desired products 3e and 3f were obtained in 67 and 83% yields, respectively. Treatment of 1a with chlorobenzene (2h), fluorobenzene (2i), trifluorotoluene (2j), or 1,2-dichlorobenzene (2k) bearing an electron-withdrawing group on the benzene ring afforded products 3g–3j in 50–58% yields, whereas that of nitrobenzene (2l) with a strong electron-withdrawing group did not afford the desired cross-coupling product 3k. The benzylic and allylic positions are quite susceptible to arylation22 and oxidation reactions23 in the presence of arylating agents and oxidants. In this regard, the generality of this reaction was explored further using different N-substituted quinoxalinones 1b–1d bearing 1-benzyl, 1-(4-chlorobenzyl) and 1-geranyl groups. The presence of N-benzyl and N-allyl pendants on the quinoxalinone skeleton did not hamper the desired product formation. For example, the reaction of 1-benzylquinoxalin-2-one (1b) with 2d or 2k provided the desired products 3l and 3m in 70% and 48% yields, respectively. On the other hand, the reaction of 1-(4-chlorobenzyl)quinoxalin-2-one (1c) with m-xylene (2e) afforded 3n in 60% yield. Similarly, treatment of 1-geranyl quinoxalin-2-one (1d) with 2a gave the product 3o in 62% yield. These results show that the regioselectivity in the synthesized products was enhanced predominantly by introducing stronger electron-donating groups of 2c–2g than toluene as coupling partners. In addition, the yields of the products isolated from electron-rich arenes were higher (in the range of 60–83%) than those from electron-poor arenes (0–58%).

Table 2 Pd(TFA)2-catalyzed CDC reactions of N-substituted quinoxalinones 1a–1d with various arenes 2a–2l
image file: c8qo01250d-u2.tif


Considering the generality of this protocol using N-substituted quinoxalin-2-ones, various quinoxalin-2-ones bearing substituents on the benzene ring were next employed to afford a range of 3-arylquinoxalinone derivatives (Table 3). Treatment of 1e bearing electron-donating groups on the benzene ring with toluene (2b) provided products 4a (80%) as a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 para/ortho mixture, whereas that of 1e with anisole (2d) or 1,2-dichlorobenzene (2k) gave 4b and 4c in 50 and 48% yields, respectively. Similarly, the reaction of 1f with anisole (2d) provided the desired product 4d in 70% yield. Further combination of 1g or 1h bearing electron-withdrawing groups on the benzene ring with benzene (2a), anisole (2d) or m-xylene (2e) provided products 4e–4h in 64–75% yields. Similarly, the coupling of 1i or 1j bearing an electron-withdrawing F group with m-xylene (2e) and p-xylene (2f) provided products 4i and 4j in 58 and 54% yields, respectively. Moreover, the treatment of 6,7-dibromo-1-methylquinoxalin-2(1H)-one (1k) with anisole (2d) provided 4k in 53% yield. Similarly, the reaction between methyl 1-methyl-2-oxo-1,2-dihydroquinoxaline-6-carboxylate (1l) bearing an ester group on the benzene ring and anisole (2d) as the coupling partner afforded the product 4l in 68% yield.

Table 3 Pd(TFA)2-catalyzed CDC reactions of various quinoxalinones 1e–1l bearing substituents on the benzene ring and arenes 2a, 2b, 2d–2f and 2k
image file: c8qo01250d-u3.tif


Having confirmed the general applicability of this CDC protocol, the possibility of CDC reactions of benzo[g]quinoxalin-2(1H)-ones leading to 3-arylbenzo[g]quinoxalin-2(1H)-ones was next examined (Table 4). A combination of 1-methylbenzo[g]quinoxalin-2(1H)-one (1m) with benzene (2a), anisole (2d), m-xylene (2e), p-xylene (2f), or 1,3-dimethoxybenzene (2g) provided the desired products, 5a–5e, in 54–80% yields. Treatment of 1-benzylbenzo[g]quinoxalin-2(1H)-one (1n) with anisole (2d) or chlorobenzene (2h) afforded 5f and 5g in 75 and 52% yields, respectively.

Table 4 Pd(TFA)2-catalyzed CDC reactions of benzo[g]quinoxalinones 1m–1n with various arenes 2a, 2d, 2e–2h
image file: c8qo01250d-u4.tif


A further reaction of 4-methylpyrido[3,4-b]pyrazin-3(4H)-one (6), a nitrogen-containing quinoxalin-2-one, was next carried out as an application of this protocol (Scheme 3). For example, treatment of 6 with 2d readily afforded the desired product 7 in 52% yield.


image file: c8qo01250d-s3.tif
Scheme 3 Application reaction of 4-methylpyrido[3,4-b]pyrazin-3(4H)-one (6) with anisole (2d) for the formation of 7.

Based on the previously reported protocols24 and the present experimental findings, a plausible mechanistic pathway for the arylation reaction is depicted in Scheme 4. The CDC reaction is initiated by the heteroatom-guided palladation of 1a with Pd(OCOCF3)2 to form C-3 palladated species 9 and CF3CO2H via intermediate 8. The intermediate 9 undergoes subsequent coordination with benzene (2a) and C–H metalation affords the intermediate 10, which releases CF3CO2H to furnish another intermediate 11. Finally, reductive elimination of 11 provides the C-3-arylated product 3a and palladium(0) species. The Pd0 obtained by the reductive elimination subsequently oxidizes to Pd2+ with Ag2O, which resumes the catalytic cycle. During the oxidation of Pd0 to Pd2+, Ag2O gets reduced to Ag0,25 which was confirmed by XRD analysis (see the ESI).


image file: c8qo01250d-s4.tif
Scheme 4 Proposed mechanism for the formation of 3a.

Conclusions

A direct Pd(TFA)2-catalyzed cross-dehydrogenative coupling reaction was developed for the construction of diverse 3-arylquinoxalin-2-ones. The developed protocol was effective for the arylation of a wide range of substituted quinoxalin-2-one, benzo[g]quinoxalinone, and pyrido[3,4-b]pyrazinone derivatives. An important feature of this approach over other typical arylation methods is the atom-economical strategy, which does not require pre-functionalization.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (2018R1A2B2004432) and the Priority Research Centers Program (2014R1A6A1031189). This research was supported by the Nano Material Technology Development Program of the Korean National Research Foundation (NRF) funded by the Korean Ministry of Education, Science, and Technology (2012M3A7B4049675).

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Footnote

Electronic supplementary information (ESI) available: Experimental details, compound characterization, NMR spectra and XRD. See DOI: 10.1039/c8qo01250d

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