催化学报  2015, Vol. 36 Issue (1): 48-56   PDF (6769 KB)    
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戚自松
李兴伟
Rh(III)-catalyzed C-H activation of benzamides: Coupling with quinones
Zisong Qi, Xingwei Li     
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
Received: 2014-09-25. Accepted: 2014-11-10. Published: 2015-01-20.
* Corresponding author. Tel: +86-411-84379089; E-mail: xwli@dicp.ac.cn
Foundation Item:This work was supported by the Dalian Institute of Chemical Physics, Chinese Academy of Sciences and the National Natural Science Foundation of China (21272231).
Abstract: Rh(III)-catalyzed C-H activation of N-(alkyl)benzamides in the oxidative coupling with various quinones. In addition, under redox-neutral conditions, 2-hydroxy-6H-benzo[c]chromen-6-ones were also obtained via a cascade of cross-coupling followed by lactonization.
© 2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
Key words: Rhodium(III)     C-H activation     Benzamides     Arylquinones    
三价铑催化苯甲酰胺与醌的C-H活化偶联
戚自松, 李兴伟     
中国科学院大连化学物理研究所, 辽宁大连116023
收稿日期: 2014-09-25. 接受日期: 2014-11-10. 出版日期: 2015-01-20.
基金来源：中国科学院大连化学物理研究所; 国家自然科学基金(21272231).
*通讯联系人.李兴伟,电话: (0411)84379089; 电子信箱: xwli@dicp.ac.cn
摘要：三价铑在氧化条件下催化N-烷基苯甲酰胺与醌反应生成芳基醌. 在氧化还原中性条件下, 经过偶联和内酯化反应可以得到2-羟基-6H-苯并[c]吡喃-6-酮.
关键词： 三价铑     碳氢活化     苯甲酰胺     芳基醌    

1. Introduction

Transition-metal-catalyzed direct functionalization of C-H bonds has become an increasingly important strategy for the construction of complex organic molecules [1, 2, 3]. Palladium, ruthenium, iridium, and copper catalysts are particularly well known to serve this purpose [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]. Although much progress has been made, the development of new synthetic methods is still necessary, especially via a C-H activation pathway. RhIIICp* represent one of the most frequently used catalysts that enabled a vast number of transformations through coupling of C-H bonds with alkenes along with other molecules [20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57]. However, the olefins are mostly limited to acrylates and styrenes. There are only a few reports on the applications of quinones as an olefin partner under oxidative conditions [58]. The rarity is probably due to the difficulty of β-hydride elimination following syn migratory insertion of a Rh-C bond into the C=C bond of a quinone. In addition, the steric effect of this special disubstituted olefin may also be accountable [59, 60, 61, 62, 63].

On the other hand, quinones represent an important class of biologically active molecules that are widely distributed in natural products [64, 65, 66, 67]. Both natural and synthetic medicinal agents have been developed that are based on the quinone structure due to their chemotherapeutic value such as antitumor, antifungal, and antibacterial [68, 69, 70, 71, 72]. The quinone moiety is also involved in many bioenergetic processes and plays a vital role in electron-transport processes [73]. Aryl-substituted quinones are very useful in photosynthesis and the dye industry owing to their unique visual and electronic properties [74, 75]. Thus, developing powerful synthetic methods of arylquinones is very necessary to fulfill the practical requirements. Arylation of quinones has been reported by some groups [76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90]. However, these approaches often suffer from poor atom-economy, substrate limitations, stoichiometric amounts of metals or high temperature. Although much progress has been made, the development of new synthetic methods is still necessary, especially via a C-H activation pathway. Herein we report Rh(III)-catalyzed C-H activation of N-(alkyl)benzamides in the coupling with quinones, leading to synthesis of arylated quinones.

2. Experimental
2.1. General

All reactions were carried out using Schlenk techniques or in a nitrogen-filled glovebox. NMR spectra were recorded on a Bruker 400 MHz NMR spectrometer in the solvents indicated. The chemical shift is given in dimensionless δ values and is frequency referenced relative to TMS in 1H and 13C NMR spectroscopy. HRMS data were obtained on an Agilent 6540 Q-Tof. Column chromatography was performed on silica gel (300-400 mesh) using ethyl acetate (EA)/petroleum ether (PE). All chemicals were obtained from commercial sources and were used as received unless otherwise noted.

2.2. Preparation of compounds
2.2.1. General procedure for the synthesis of compounds 3

N-(alkyl)benzamides (0.2 mmol), quinones (0.3 mmol), [RhCp*Cl2]2 (4 mol%), AgSbF6 (16 mol%), Ag2CO3 (2.0 equiv) and dioxane (3 mL) were charged into the sealed tube. The reaction mixture was stirred at 100 °C for 12 h. After cooled to room temperature, the solvent was removed under reduced pressure and the residue was purified by silica gel chromatography using PE/EA to afford compounds 3.

2.2.2. General procedure for the synthesis of compounds 4

N-(tert-butyl)benzamides (0.2 mmol), quinones (0.3 mmol), [RhCp*Cl2]2 (4 mol%), AgSbF6 (30 mol%), AgOAc (0.2 equiv) and acetone (4 mL) were charged into the sealed tube. The reaction mixture was stirred at 100 °C for 12 h. After cooled to room temperature, the solvent was removed under reduced pressure and the residue was purified by silica gel chromatography using etroleum ether/ethyl acetate to afford compounds 4.

2.3. Spectral data for products

N-(tert-butyl)-2',5'-dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide (3a). 1H NMR (400 MHz, CDCl3) δ 7.53 (dd, J = 7.4, 1.5 Hz, 1H), 7.48 (dd, J = 7.4, 1.6 Hz, 1H), 7.44 (td, J = 7.5, 1.5 Hz, 1H), 7.29 (dd, J = 7.4, 1.2 Hz, 1H), 6.80 (d, J = 1.1 Hz, 2H), 6.75 - 6.72 (m, 1H), 6.01 (s, 1H), 1.38 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.6, 185.8, 168.4, 149.5, 137.6, 137.03, 136.97, 133.2, 131.3, 130.9, 130.4, 129.8, 126.5, 52.0, 28.7. HRMS: [M + H]+ calculated for C7H18NO3: 284.1287, found 284.1290.

N-(tert-butyl)-5-methyl-2',5'-dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide (3b). 1H NMR (400 MHz, CDCl3) δ 7.42 (d, J = 7.8 Hz, 1H), 7.24 (d, J = 7.8 Hz, 1H), 7.09 (s, 1H), 6.80 (s, 2H), 6.72 (s, 1H), 5.96 (s, 1H), 2.39 (s, 3H), 1.37 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.6, 185.9, 168.4, 149.8, 141.3, 137.1, 137.0, 134.9, 133.3, 131.2, 131.0, 130.3, 126.5, 51.9, 28.8, 21.4. HRMS: [M + H]+ calculated for C18H20NO3: 298.1443, found 298.1440.

N-(tert-butyl)-5-methoxy-2',5'-dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide (3c). 1H NMR (400 MHz, CDCl3) δ 7.46 (d, J = 8.5 Hz, 1H), 6.92 (dd, J = 8.5, 2.6 Hz, 1H), 6.80 (dd, J = 4.1, 2.2 Hz, 3H), 6.72 (d, J = 1.5 Hz, 1H), 5.93 (s, 1H), 3.84 (s, 3H), 1.37 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.6, 185.7, 168.1, 161.4, 149.8, 137.2, 136.9, 135.2, 131.0, 130.0, 128.1, 116.3, 114.4, 55.7, 51.9, 28.8. HRMS: [M + H]+ calculated for C18H20NO4: 314.1392, found 314.1391.

N-(tert-butyl)-5-fluoro-2',5'-dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide (3d). 1H NMR (400 MHz, CDCl3) δ 7.53 (dd, J = 8.5, 5.4 Hz, 1H), 7.13 (td, J = 8.3, 2.6 Hz, 1H), 7.01 (dd, J = 8.8, 2.6 Hz, 1H), 6.82 (d, J = 1.0 Hz, 2H), 6.73 (s, 1H), 5.94 (s, 1H), 1.37 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.3, 185.4, 167.5, 163.7 (d, J = 250.9 Hz), 148.3 (d, J = 1.5 Hz), 137.1, 137.0, 135.7 (d, J = 8.2 Hz), 134.0 (d, J = 3.4 Hz), 131.7, 128.6 (d, J = 8.7 Hz), 117.7 (d, J = 22.9 Hz), 116.3 (d, J = 21.1 Hz), 52.2, 28.7. HRMS: [M + H]+ calculated for C17H17FNO3: 302.1192, found 302.1181.

N-(tert-butyl)-5-chloro-2',5'-dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide (3e). 1H NMR (400 MHz, CDCl3) δ 7.47 (d, J = 8.2 Hz, 1H), 7.41 (dd, J = 8.2, 2.0 Hz, 1H), 7.28 (d, J = 2.1 Hz, 1H), 6.81 (s, 2H), 6.74 (s, 1H), 5.97 (s, 1H), 1.37 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.2, 185.4, 167.5, 148.1, 137.1, 137.0, 136.8, 136.1, 134.9, 131.7, 130.4, 129.7, 127.9, 52.2, 28.7. HRMS: [M + H]+ calculated for C17H17ClNO3: 318.0897, found 318.0898.

5-Bromo-N-(tert-butyl)-2',5'-dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide (3f). 1H NMR (400 MHz, CDCl3) δ 7.58 (dd, J = 8.2, 2.0 Hz, 1H), 7.44 (d, J = 1.9 Hz, 1H), 7.39 (d, J = 8.2 Hz, 1H), 6.81 (s, 2H), 6.74 (s, 1H), 5.95 (s, 1H), 1.37 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.2, 185.4, 167.6, 148.0, 137.1, 137.0, 136.5, 135.0, 133.3, 132.7, 131.8, 128.0, 124.9, 52.3, 28.7. HRMS: [M + H]+ calculated for C17H17BrNO3: 362.0392, found 362.0391.

N-(tert-butyl)-2'',5''-dioxo-2'',5''-dihydro-[1,1':3',1''-terphenyl]-4'-carboxamide (3g). 1H NMR (400 MHz, CDCl3) δ 7.65 (dd, J = 7.9, 1.6 Hz, 1H), 7.61-7.54 (m, 3H), 7.49-7.43 (m, 3H), 7.39 (t, J = 7.2 Hz, 1H), 6.82 (s, 2H), 6.80 (d, J = 1.3 Hz, 1H), 6.04 (s, 1H), 1.40 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.6, 185.9, 168.2, 149.6, 144.0, 139.5, 137.1, 137.1, 136.3, 133.8, 131.4, 129.3, 129.2, 128.4, 128.3, 127.3, 127.1, 52.1, 28.8. HRMS: [M + H]+ calculated for C23H22NO3: 360.1600, found 360.1600.

N-(tert-butyl)-2',5'-dioxo-5-(trifluoromethyl)-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide (3h). 1H NMR (400 MHz, CDCl3) δ 7.72 (dd, J = 8.0, 1.0 Hz, 1H), 7.64 (d, J = 8.0 Hz, 1H), 7.55 (s, 1H), 6.84 (d, J = 2.2 Hz, 2H), 6.80 (d, J = 2.1 Hz, 1H), 5.97 (s, 1H), 1.39 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.1, 185.4, 167.3, 147.9, 140.9, 137.3, 136.9, 134.0, 132.9 (q, J = 33.2 Hz), 132.2, 127.4 (q, J = 3.6 Hz), 127.1, 126.7 (q, J = 3.7 Hz), 123.4 (q, J = 267.7 Hz), 52.5, 28.7. HRMS: [M + H]+ calculated for C18H17F3NO3: 352.1161, found 352.1157.

Methyl 6-(tert-butylcarbamoyl)-2',5'-dioxo-2',5'-dihydro-[1,1'- biphenyl]-3-carboxylate (3i). 1H NMR (400 MHz, CDCl3) δ 8.10 (dd, J = 8.0, 1.4 Hz, 1H), 7.96 (d, J = 1.2 Hz, 1H), 7.59 (d, J = 8.0 Hz, 1H), 6.81 (d, J = 6.4 Hz, 3H), 6.04 (s, 1H), 3.94 (s, 3H), 1.39 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.3, 185.6, 167.7, 165.8, 148.4, 141.5, 137.2, 136.9, 133.4, 132.3, 132.0, 131.5, 131.0, 126.7, 52.7, 52.4, 28.7. HRMS: [M + H]+ calculated for C19H20NO5: 342.1341, found 342.1340.

N-(tert-butyl)-4-methyl-2',5'-dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide (3j). 1H NMR (400 MHz, CDCl3) δ 7.33 (s, 1H), 7.29 (d, J = 8.6 Hz, 1H), 7.17 (d, J = 7.7 Hz, 1H), 6.78 (s, 2H), 6.71 (s, 1H), 6.00 (s, 1H), 2.41 (s, 3H), 1.38 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.6, 186.0, 168.5, 149.4, 140.2, 137.6, 137.0, 136.9, 131.4, 131.0, 130.4, 130.2, 127.3, 51.9, 28.7, 21.3. HRMS: [M + H]+ calculated for C18H20NO3: 298.1443, found 298.1446.

N-(tert-butyl)-4-methoxy-2',5'-dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide (3k). 1H NMR (400 MHz, CDCl3) δ 7.23 (d, J = 8.4 Hz, 1H), 7.05 (d, J = 2.5 Hz, 1H), 7.00 (dd, J = 8.4, 2.6 Hz, 1H), 6.79 (d, J = 1.1 Hz, 2H), 6.72 (d, J = 1.1 Hz, 1H), 5.91 (s, 1H), 3.87 (s, 3H), 1.37 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.7, 186.2, 168.2, 160.9, 149.0, 139.2, 137.1, 137.0, 131.9, 130.9, 125.3, 115.1, 113.6, 55.9, 52.1, 28.8. HRMS: [M + H]+ calculated for C18H20NO4: 314.1392, found 314.1394.

N-(tert-butyl)-6-methoxy-2',5'-dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide (3k’). 1H NMR (400 MHz, CDCl3) δ 7.38 (t, J = 8.0 Hz, 1H), 7.11 (d, J = 7.7 Hz, 1H), 7.02 (d, J = 8.4 Hz, 1H), 6.84 (d, J = 10.1 Hz, 1H), 6.80 (dd, J = 10.1, 2.4 Hz, 1H), 6.74 (d, J = 2.4 Hz, 1H), 5.70 (s, 1H), 3.77 (s, 3H), 1.32 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.5, 186.3, 168.1, 157.4, 144.8, 139.2, 137.1, 136.8, 134.4, 130.6, 121.0, 118.9, 113.0, 56.2, 52.0, 28.7. HRMS: [M + H]+ calculated for C18H20NO4: 314.1392, found 314.1395.

N-(tert-butyl)-4-chloro-2',5'-dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide (3l). 1H NMR (400 MHz, CDCl3) δ 7.50 (d, J = 2.0 Hz, 1H), 7.47 (dd, J = 8.1, 2.1 Hz, 1H), 7.23 (d, J = 8.1 Hz, 1H), 6.81 (s, 2H), 6.74 (s, 1H), 5.96 (s, 1H), 1.38 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.3, 185.6, 167.2, 148.2, 139.2, 137.1, 137.0, 135.9, 131.7, 131.7, 131.5, 130.9, 127.0, 52.4, 28.7. HRMS: [M + H]+ calculated for C17H17ClNO3: 318.0897, found 318.0897.

N-(tert-butyl)-6-chloro-2',5'-dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide (3l’). 1H NMR (400 MHz, CDCl3) δ 7.53 (d, J = 7.8 Hz, 1H), 7.44-7.35 (m, 2H), 6.90-6.82 (m, 2H), 6.74 (d, J = 2.0 Hz, 1H), 5.68 (s, 1H), 1.31 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.0, 185.7, 167.1, 145.5, 139.7, 137.1, 136.8, 134.7, 134.3, 131.5, 130.9, 130.4, 125.1, 52.3, 28.7. HRMS: [M + H]+ calculated for C17H17ClNO3: 318.0897, found 318.0899.

N-(tert-butyl)-6-fluoro-2',5'-dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide (3m). 1H NMR (400 MHz, CDCl3) δ 7.42 (td, J = 8.0, 5.3 Hz, 1H), 7.33 (d, J = 7.2 Hz, 1H), 7.24 (t, J = 8.8 Hz, 1H), 6.87-6.79 (m, 3H), 5.89 (s, 1H), 1.36 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.1, 185.4, 167.2 (d, J = 2.7 Hz), 160.1 (d, J = 248.7 Hz), 141.8, 139.7 (d, J = 1.7 Hz), 137.1, 137.0, 134.4 (d, J = 3.0 Hz), 131.0 (d, J = 8.7 Hz), 122.4 (d, J = 3.4 Hz), 120.2 (d, J = 15.7 Hz), 118.2 (d, J = 22.9 Hz), 52.2, 28.7. HRMS: [M + H]+ calculated for C17H17FNO3: 302.1192, found 302.1196.

N-(tert-butyl)-3-methoxy-2',5'-dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide (3n). 1H NMR (400 MHz, CDCl3) δ 7.45-7.37 (m, 1H), 7.05 (d, J = 8.4 Hz, 1H), 6.96 (s, 1H), 6.89 (d, J = 7.6 Hz, 1H), 6.81-6.78 (m, 2H), 6.67 (d, J = 2.1 Hz, 1H), 3.92 (s, 3H), 1.36 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.8, 185.5, 165.0, 156.5, 150.9, 137.2, 136.9, 136.1, 131.4, 129.8, 125.2, 123.3, 113.4, 56.5, 51.7, 28.9. HRMS: [M + H]+ calculated for C18H20NO4: 314.1392, found 314.1398.

N-(tert-butyl)-3-fluoro-2',5'-dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide (3o). 1H NMR (400 MHz, CDCl3) δ 7.48-7.41 (m, 1H), 7.20 (ddd, J = 10.2, 8.4, 0.9 Hz, 1H), 7.09 (dd, J = 7.6, 0.7 Hz, 1H), 6.81 (d, J = 1.2 Hz, 2H), 6.73 (t, J = 1.1 Hz, 1H), 6.23 (s, 1H), 1.38 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.5, 185.4, 163.0 (d, J = 1.8 Hz), 159.3 (d, J = 245.7 Hz), 148.8 (d, J = 2.4 Hz), 137.1, 137.0, 136.2 (d, J = 3.5 Hz), 131.9 (d, J = 9.7 Hz), 131.4, 126.4 (d, J = 2.9 Hz), 124.7 (d, J = 15.8 Hz), 117.7 (d, J = 24.5 Hz), 52.4, 28.8. HRMS: [M + H]+ calculated for C17H17FNO3: 302.1192, found 302.1194.

N-(tert-butyl)-3-(3,6-dioxocyclohexa-1,4-dien-1-yl)-2-naphthamide (3p). 1H NMR (400 MHz, CDCl3) δ 7.97 (s, 1H), 7.87 (dd, J = 11.5, 4.7 Hz, 2H), 7.78 (s, 1H), 7.62-7.55 (m, 2H), 6.86 (s, 1H), 6.81 (s, 2H), 6.18 (s, 1H), 1.43 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.8, 186.4, 168.7, 149.7, 137.1, 137.0, 135.1, 133.9, 133.1, 131.2, 130.8, 130.4, 128.4, 128.3, 128.2, 128.1, 126.5, 52.1, 28.8. HRMS: [M + H]+ calculated for C21H20NO3: 334.1443, found 334.1442.

N-(tert-butyl)-3-(3,6-dioxocyclohexa-1,4-dien-1-yl)thiophene-2-carboxamide (3q). 1H NMR (400 MHz, CDCl3) δ 7.35 (d, J = 5.0 Hz, 1H), 6.99 (d, J = 5.0 Hz, 1H), 6.86 (d, J = 10.1 Hz, 1H), 6.80 (dd, J = 10.1, 2.3 Hz, 1H), 6.75 (d, J = 2.3 Hz, 1H), 5.79 (s, 1H), 1.39 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.4, 185.0, 161.4, 143.6, 137.6, 137.2, 136.6, 134.9, 132.1, 129.7, 126.5, 52.4, 28.8. HRMS: [M + H]+ calculated for C15H16NO3S: 290.0851, found 290.0849.

N-(tert-butyl)-2-(1,4-dioxo-1,4-dihydronaphthalen-2-yl)benzamide (3r). 1H NMR (400 MHz, CDCl3) δ 8.14-8.08 (m, 1H), 8.06-8.00 (m, 1H), 7.77-7.69 (m, 2H), 7.58-7.52 (m, 1H), 7.52-7.43 (m, 2H), 7.37 (dd, J = 7.4, 1.3 Hz, 1H), 6.98 (s, 1H), 6.03 (s, 1H), 1.36 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 185.2, 184.0, 168.7, 151.4, 138.1, 133.8, 133.7, 133.0, 132.7, 130.8, 130.5, 129.8, 126.8, 126.5, 126.3, 52.0, 28.8. Two carbons are not visible due to overlapping peaks. HRMS: [M + H]+ calculated for C21H20NO3: 334.1443, found 334.1440.

N-(tert-butyl)-3',4'-dimethyl-2',5'-dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide (3s). 1H NMR (400 MHz, CDCl3) δ 7.53-7.40 (m, 3H), 7.31-7.27 (m, 1H), 6.71 (s, 1H), 5.98 (s, 1H), 2.06 (s, 3H), 2.04 (s, 3H), 1.39 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.5, 186.2, 168.7, 149.0, 141.5, 141.3, 137.8, 133.7, 131.1, 130.8, 130.4, 129.6, 126.5, 52.0, 28.8, 12.7, 12.4. HRMS: [M + H]+ calculated for C19H22NO3: 312.1600, found 312.1605.

N-(tert-butyl)-4'-methyl-2',5'-dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide and N-(tert-butyl)-3'-methyl-2',5'- dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide (3t). Selected signals: Major: 1H NMR (400 MHz, CDCl3) δ 7.54-7.41 (m, 3H), 7.30-7.26 (m, 1H), 6.01 (s, 1H), 2.06 (d, J = 1.5 Hz, 3H), 1.39 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.6, 186.5, 168.6, 149.6, 146.4, 137.6, 133.7, 131.3, 130.9, 130.4, 129.7, 126.5, 52.0, 28.7, 16.2. Minor: 1H NMR (400 MHz, CDCl3) δ 7.54-7.41 (m, 3H), 7.30-7.26 (m, 1H), 6.00 (s, 1H), 2.09 (d, J = 1.5 Hz, 3H), 1.39 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 188.1, 186.1, 168.6, 149.3, 146.4, 137.8, 133.1, 131.5, 130.8, 130.5, 126.5, 52.0, 28.7, 15.8. HRMS: [M + H]+ calculated for C18H20NO3: 298.1443, found 298.1447.

N-(tert-butyl)-4'-methoxy-2',5'-dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide and N-(tert-butyl)-3'-methoxy-2',5'- dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide (3u). Selected signals: Major: 1H NMR (400 MHz, CDCl3) δ 7.55-7.41 (m, 3H), 7.28 (t, J = 6.2 Hz, 1H), 6.70 (s, 1H), 6.05 (s, 1H), 5.97 (s, 1H), 3.84 (s, 3H), 1.40 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 185.8, 182.1, 168.4, 159.1, 150.0, 137.9, 133.1, 130.7, 130.4, 129.8, 129.4, 126.5, 107.8, 56.3, 51.9, 28.7. Minor: 1H NMR (400 MHz, CDCl3) δ 7.55-7.41 (m, 3H), 7.28 (t, J = 6.2 Hz, 1H), 6.65 (d, J = 2.3 Hz, 1H), 6.05 (s, 1H), 5.96 (d, J = 2.3 Hz, 1H), 3.82 (s, 3H), 1.38 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.3, 180.6, 171.2, 159.4, 147.5, 137.4, 131.9, 130.8, 130.4, 129.7, 126.5, 107.7, 56.4, 52.0, 28.7. HRMS: [M + H]+ calculated for C18H20NO4: 314.1392, found 314.1395.

N-(tert-butyl)-4'-chloro-2',5'-dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide and N-(tert-butyl)-3'-chloro-2',5'- dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide (3v). Selected 1H NMR signals: 1) 1H NMR (400 MHz, CDCl3) δ 7.55-7.45 (m, 3H), 7.32-7.27 (m, 1H), 6.05 (s, 1H), 1.39 (s, 9H). 2) 1H NMR (400 MHz, CDCl3) δ 7.55-7.45 (m, 3H), 7.32-7.27 (m, 1H), 6.05 (s, 1H), 1.39 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 185.10, 183.62, 179.75, 178.53, 168.40, 168.28, 150.29, 149.84, 144.64, 144.45, 137.68, 137.50, 134.20, 134.07, 133.15, 132.68, 131.08, 131.00, 130.54, 130.33, 130.20, 126.49, 126.47, 52.28, 52.15, 28.77, 28.76. HRMS: [M + H]+ calculated for C17H17ClNO3: 318.0897, found 318.0903.

N-(tert-butyl)-2',5'-dioxo-2',5'-dihydro-[1,1':4',1''-terphenyl]-2-carboxamide and N-(tert-butyl)-2',5'-dioxo-2',5'-dihydro- [1,1':3',1''-terphenyl]-2-carboxamide (3w). Selected signals: Major: 1H NMR (400 MHz, CDCl3) δ 6.85 (d, J = 2.6 Hz, 1H), 6.75 (d, J = 2.6 Hz, 1H), 6.11 (s, 1H), 1.32 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.6, 185.4, 168.1, 150.2, 146.8, 137.1, 133.9, 133.4, 132.8, 131.0, 130.9, 130.7, 129.8, 129.7, 129.4, 128.2, 126.6, 51.9, 28.7. Minor: 1H NMR (400 MHz, CDCl3) δ 6.89 (s, 1H), 6.82 (s, 1H), 6.09 (s, 1H), 1.39 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 186.9, 186.0, 168.5, 148.9, 146.2, 137.8, 133.1, 132.9, 132.7, 131.8, 130.5, 130.0, 129.5, 128.5, 126.52 51.9, 28.7. HRMS: [M + H]+ calculated for C23H22NO3: 360.1600, found 360.1600.

N-(tert-butyl)-2-(3,6-dioxocyclohexa-1,4-dien-1-yl)cyclopent-1-enecarboxamide (3x). 1H NMR (400 MHz, CDCl3) δ 6.82 (d, J = 10.2 Hz, 1H), 6.73 (dd, J = 10.2, 2.5 Hz, 1H), 6.55 (d, J = 2.5 Hz, 1H), 5.36 (s, 1H), 2.76-2.66 (m, 4H), 2.11-2.02 (m, 2H), 1.31 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.4, 185.0, 164.8, 146.4, 140.5, 139.9, 137.6, 136.5, 130.7, 51.6, 38.2, 34.9, 28.9, 22.6. HRMS: [M + H]+ calculated for C16H20NO3: 274.1443, found 274.1446.

N2,N2''-di-tert-butyl-2',5'-dioxo-2',5'-dihydro-[1,1':3',1''-terphenyl]-2,2''-dicarboxamide (3aa). 1H NMR (400 MHz, CDCl3) δ 7.56-7.52 (m, 2H), 7.49-7.40 (m, 6H), 6.80 (s, 2H), 6.04 (s, 2H), 1.21 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 187.2, 185.4, 168.4, 148.7, 137.8, 132.3, 132.2, 130.4, 130.2, 129.6, 127.3, 52.1, 28.7. HRMS: [M + H]+ calculated for C28H31N2O4: 459.2284, found 459.2288.

N-isopropyl-2',5'-dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide (3ba). 1H NMR (400 MHz, CDCl3) δ 7.57-7.43 (m, 3H), 7.30 (d, J = 7.2 Hz, 1H), 6.84-6.77 (m, 2H), 6.74 (d, J = 1.8 Hz, 1H), 6.02 (d, J = 7.0 Hz, 1H), 4.12-4.00 (m, 1H), 1.21 (d, J = 6.6 Hz, 6H). 13C NMR (100 MHz, CDCl3) δ 187.6, 185.9, 168.0, 149.4, 137.1, 136.9, 136.5, 133.4, 131.5, 131.1, 130.5, 129.9, 126.6, 42.3, 22.8. HRMS: [M + H]+ calculated for C16H16NO3: 270.1130, found 270.1134.

N-cyclohexyl-2',5'-dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide (3ca). 1H NMR (400 MHz, CDCl3) δ 7.58-7.45 (m, 3H), 7.30 (d, J = 7.5 Hz, 1H), 6.86-6.78 (m, 2H), 6.74 (d, J = 2.0 Hz, 1H), 5.99 (d, J = 7.5 Hz, 1H), 3.82-3.71 (m, 1H), 2.01-1.91 (m, 2H), 1.79-1.69 (m, 2H), 1.63 (d, J = 10.6 Hz, 2H), 1.45-1.31 (m, 2H), 1.27-1.16 (m, 2H). 13C NMR (100 MHz, CDCl3) δ 187.6, 185.9, 167.9, 149.5, 137.2, 136.9, 136.6, 133.4, 131.5, 131.1, 130.5, 129.9, 126.6, 49.1, 33.2, 25.7, 25.0. HRMS: [M + H]+ calculated for C19H20NO3: 310.1443, found 310.1445.

N-(1-adamantanyl)-2',5'-dioxo-2',5'-dihydro-[1,1'-biphenyl]-2-carboxamide (3da). 1H NMR (400 MHz, CDCl3) δ 7.56-7.42 (m, 3H), 7.31-7.26 (m, 1H), 6.86-6.78 (m, 2H), 6.74 (d, J = 2.0 Hz, 1H), 5.84 (s, 1H), 2.09 (s, 3H), 2.03 (d, J = 2.6 Hz, 6H), 1.69 (s, 6H). 13C NMR (100 MHz, CDCl3) δ 187.6, 185.8, 168.2, 149.6, 137.8, 137.1, 137.1, 133.2, 131.3, 130.9, 130.5, 129.9, 126.5, 52.8, 41.6, 36.5, 29.6. HRMS: [M + H]+ calculated for C23H24NO3: 362.1756, found 362.1761.

2-Hydroxy-6H-benzo[c]chromen-6-one (4a).1H NMR (400 MHz, DMSO-d6) δ 9.73 (s, 1H), 8.33-8.21 (m, 2H), 7.98-7.91 (m, 1H), 7.72-7.66 (m, 1H), 7.62 (d, J = 2.7 Hz, 1H), 7.28 (d, J = 8.9 Hz, 1H), 7.02 (dd, J = 8.9, 2.8 Hz, 1H). 13C NMR (100 MHz, DMSO-d6) δ 160.9, 154.7, 144.4, 135.7, 134.7, 130.2, 129.7, 122.9, 121.1, 118.9, 118.7, 118.6, 108.7. HRMS: [M + H]+ calculated for C13H9O3: 213.0555, found 213.0552.

2-Hydroxy-8-methyl-6H-benzo[c]chromen-6-one (4b). 1H NMR (400 MHz, DMSO-d6) δ 9.69 (s, 1H), 8.15 (d, J = 8.1 Hz, 1H), 8.04 (s, 1H), 7.74 (d, J = 8.1 Hz, 1H), 7.56 (d, J = 2.3 Hz, 1H), 7.25 (d, J = 8.8 Hz, 1H), 6.97 (dd, J = 8.8, 2.5 Hz, 1H), 2.46 (s, 3H). 13C NMR (100 MHz, DMSO-d6) δ 160.9, 154.7, 144.1, 139.6, 136.7, 132.2, 129.9, 122.9, 120.9, 118.8, 118.46, 118.4, 108.4, 21.2. HRMS: [M + H]+ calculated for C14H11O3: 227.0708, found 227.0706.

9-Chloro-2-hydroxy-6H-benzo[c]chromen-6-one (4c). 1H NMR (400 MHz, DMSO-d6) δ 9.74 (s, 1H), 8.38 (d, J = 1.9 Hz, 1H), 8.22 (d, J = 8.5 Hz, 1H), 7.70 (dd, J = 8.5, 1.9 Hz, 1H), 7.65 (d, J = 2.7 Hz, 1H), 7.27 (d, J = 8.9 Hz, 1H), 7.04 (dd, J = 8.9, 2.7 Hz, 1H). 13C NMR (100 MHz, DMSO-d6) δ 160.2, 154.8, 144.8, 141.1, 136.6, 132.4, 129.8, 122.89, 119.9, 119.6, 118.6, 117.8, 109.2. HRMS: [M + H]+ calculated for C13H8ClO3: 247.0162, found 247.0161.

9-Bromo-2-hydroxy-6H-benzo[c]chromen-6-one (4d). 1H NMR (400 MHz, DMSO-d6) δ 9.73 (d, J = 3.8 Hz, 1H), 8.48 (d, J = 11.4 Hz, 1H), 8.12 (t, J = 8.0 Hz, 1H), 7.82 (t, J = 7.9 Hz, 1H), 7.69-7.59 (m, 1H), 7.26 (dd, J = 8.6, 6.9 Hz, 1H), 7.04 (dt, J = 8.8, 3.2 Hz, 1H). 13C NMR (100 MHz, DMSO-d6) δ 160.4, 154.8, 144.7, 136.6, 132.6, 132.2, 130.3, 125.8, 120.2, 119.6, 118.6, 117.6, 109.2. HRMS: [M + H]+ calculated for C13H8BrO3: 290.9657, found 290.9659.

3. Results and discussion

The reaction of N-(tert-butyl)benzamide 1a and 1,4- benzoquinone 2a was selected as a model for screening of the reaction parameters (Table 1). We initially performed this reaction in the absence of any external oxidant but with 4 mol% of [RhCp*Cl2]2 and 16 mol% of AgSbF6. Only traces of the desired product 3a were detected by GC-MS (Table 1, entry 1). When Cu(OAc)2 (1 equiv) was employed as an oxidant, 3a was isolated in 17% yield (entry 2). However, no product was obtained when AgSbF6 was omitted (entry 3). With Ag2CO3 or AgOAc being an oxidant, the yield of 3a was increased to 54% and 21%, respectively (entries 4 and 5). We were pleased to find that 3a was isolated in 85% yield when the amount of Ag2CO3 was increased to 2.0 equiv (entry 6). Further examination of the solvent revealed that DCE, THF, diglyme, PhCl, and acetone all gave inferior results (entries 7-11). Finally, the optimized conditions include 4 mol% of [RhCp*Cl2]2 as catalyst, 16 mol% of AgSbF6 as an additive, and 2.0 equiv of Ag2CO3 in 1,4-dioxane at 100 °C.

Table 1
Coupling of N-(tert-butyl)benzamide with quinone under various conditions.

Having determined the optimal conditions, we next surveyed various substrates to define the scope of the reaction (Table 2). N-(tert-butyl)benzamides bearing electron-donating groups at the para-position gave the corresponding products in moderate to good yields (3a-3c, 3g). With a halogen group at the 4-position of phenyl ring, the arylated quinones were isolated in slightly lower yield (3d-3f). However, using electron-withdrawing groups such as trifluoromethyl and methoxycarbonyl at the para-position decreased the yield of the products (3h and 3i). The meta methyl substituted substrate also gave product in good yield (3j) and C-H activation occurred selectively at the less hindered position. However, the meta methoxy (3k and 3k’) or chloro (3l and 3l’) substituted benzamide gave two regio-isomeric products that could be chromatographically separated. In contrast, C-H activation occurred selectively at the more hindered ortho site for the meta fluoro substituted benzamide (3m), on the basis of 1H and 13C NMR analysis. ortho-Methoxy substituted benzamide was also effective, furnishing the arylated quinone in 68% yield (3n). However, ortho-fluoro substrate exhibited lower reactivity (3o), implying that the electronic effect seems more significant than the steric effect at this position. Notably, the reactions for N-(tert-butyl)-2-naphthamide and N-(tert-butyl)thiophene-2- carboxamide proceeded smoothly to generate products in moderate yields (3p and 3q). The N-substituent is not limited to a tBu group. Thus benzamides bearing N-alkyl groups such as isopropyl, cyclohexyl, and adamantyl also underwent smooth coupling (3ba-3da). The scope of quinone substrate was next investigated. Subjection of 1,4-naphthoquinone and 2,3- dimethyl-1,4-benzoquinone to the coupling with 1a afforded 3r and 3s in 90% and 64% yields, respectively. In contrast, the coupling of a mono-substituted quinone with 1a provided two regioisomers in moderate total yields (3t-3w). Furthermore, extension to an olefinic substrate bearing this type of directing group proved successful, where product 3x was isolated in 52% yield. In addition, when an excess (3.0 equiv) of 1a was used, the diarylation product was isolated as the major one (3aa).

Table 2
Substrate scope.

It is noteworthy that a C-H insertion/lactonization product 4a was observed during the screening studies. It has been reported that the coupling of benzoic acid and benzoquinone afforded the same 2-hydroxy-6H-benzo[c]chromen-6-one under Ir(III) catalysis, however a rather high catalyst loading and a relatively high temperature were necessary to give synthetically useful yields [91]. Encouraged by these results, the reaction parameters were optimized for the synthesis of 4a. It was found that benzamides bearing methyl (4b), chloro (4c), and bromo (4d) groups at the 3- or 4- position underwent smoothly coupling under these redox-neutral conditions in acetone to give lactones in moderate to good yields (Scheme 1). However, we attempted but failed to achieve this type reaction for 1,4-naphthoquinone, where only quinone 3r was isolated. We noted that lactonization via C-H activation of other amides has been reported [92, 93, 94].

Scheme 1. Rh(III)-catalyzed synthesis of 2-hydroxy-6H-benzo[c]chromen-6-ones.

To probe the mechanism of this direct arylation of quinones, preliminary mechanistic studies were carried out. A notable primary kinetic isotope effect (KIE, kH/kD = 4.6) was observed, indicating that C-H bond cleavage of 1a is probably involved in the rate-limiting step. Furthermore, a competition reaction has been performed, where two benzamides 1b and 1e differing in electronic effects were allowed to compete in the coupling with 2a (1b:1e:2a = 1:1:1 ratio). 1H NMR analysis of the resulting mixture revealed that 3b and 3e were generated in 2.2:1 ratio, suggesting that coupling of an electron-rich arene is faster than that of a more electron-deficient one.

Based on these experiments and literature precedents [58, 95], a plausible mechanism is proposed in Scheme 2. O-Coordination of the amide to the Rh(III) center and subsequent ortho C-H bond activation afforded a five-membered rhodacyclic intermediate A. Insertion of quinone 2a to Rh-C bond of A gave a seven-membered rhodacycle B, which is followed by epimerization at one of the a-positions, allowing for b-hydrogen elimination to provide the desired product 3a together with the Rh(I) species (with elimination of HX). The Rh(I) was re-oxidized by Ag2CO3 to regenerate Rh(III) for the next catalytic cycle.

Scheme 2. Proposed mechanism of arylation of quinones.
4. Conclusions

We have developed Rh(III)-catalyzed C-H activation of N-(alkyl)benzamides in the coupling with quinones. A broad scope of substrates has been examined and good functional group compatibility has been realized. In addition 2-hydroxy-6H-benzo[c]chromen-6-ones were obtained via cascade reaction involving C-H activation and arylation-lactonization reaction. Competition experiment and KIE study were conducted, and a plausible mechanism has been proposed. This method may find useful applications in the synthesis of complex structures.

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