HETEROCYCLES

Paper | Special issue | Vol. 86, No. 2, 2012, pp. 1103-1118
Received, 26th June, 2012, Accepted, 14th August, 2012, Published online, 24th August, 2012.
DOI: 10.3987/COM-12-S(N)57

■ RHODIUM-CATALYZED SYNTHESIS OF BENZOFURANS BY THE REACTION OF KETONES AND o-DIFLUOROBENZENES

Mieko Arisawa,* Soichiro Nakane, Manabu Kuwajima, and Masahiko Yamaguchi*

Department of Organic Chemistry, Graduate School of Pharmaceutical Sciences, Tohoku University, Aramaki, Aoba-ku, sendai 980-8578, Japan

Abstract

In the presence of catalytic amounts of RhH(PPh3)4 and 1,2-bis(diphenylphosphino)ethane (dppe), ketones reacted with o-difluorobenzenes giving benzofurans in high yields. The α-arylation of ketones and furan cyclization proceeded without in the absence of a base. Benzofuran cyclization was applied to substituted pentafluorobenzenes to give 6-substituted-4,5,7-trifluorobenzofuranes.

INTRODUCTION
Benzofurans are an important class of heterocyclic compounds exhibiting biological activities.1 Among their various synthetic methods, a one-pot approach using the reaction of ketones and o-dihalobenzenes is attractive. The transformation is generally conducted in the presence of a transition-metal catalyst and a stoichiometric amount of base using o-dibromobenzenes and o-bromoiodobenzenes.2 The use of o-difluorobenzene has not been reported, with the exception of the reaction of hexafluorobenzene with ketones in the presence of a stoichiometric amount of NaH without a metal catalyst.3
It was thus considered interesting to explore a transition-metal-catalyzed method of obtaining benzofurans from o-difluorobenzenes, particularly in the absence of a base. Such a method has potential for synthesizing novel benzofurans under mild conditions. Described in this study is the rhodium-catalyzed benzofuran cyclization of ketones and o-difluorobenzenes, which proceeds without using a base.4,5 Another advantage of this method is that polyfluorinated benzofurans can be obtained by reacting various readily available polyfluorinated benzenes. Conventional methods of synthesizing polyfluorobenzofurans employ stepwise cyclization, and a palladium-catalyzed reaction of dibromodifluorobenzene.6 Our method allows the synthesis of various polyfluorinated benzofurans from polyfluorobenzenes.
This furan cyclization involves a C-arylation reaction of ketone enolates with aryl fluorides. The palladium-catalyzed C-arylation of ketone enolates using aryl triflates, iodides, bromides, and chlorides in the presence of a stoichiometric amount of base has been reported.7 The present method entails ketone C-arylation using aryl fluoride in the absence of base, which was also briefly examined.

RESULTS AND DISCUSSION
When a mixture of 1,2-diphenyl-1-ethanone 1a and 3,4-difluorobenzonitrile 2a (2 equiv) in chlorobenzene was heated at reflux for 6 h under argon in the presence of RhH(PPh3)4 (5 mol%), 1,2-bis(diphenylphosphino)ethane (dppe, 10 mol%), and tributylsilane (2 equiv), 6-cyano-2,3-diphenylbenzofuran 3aa (90%) was obtained (Table 1, entry 1). X-Ray analysis of the product 3aa confirmed the structure (Figure 1).8 The rhodium complex and dppe were both essential for the reaction, and no reaction occurred in the absence of either substance. Tributylsilane was converted into tributylsilyl fluoride by trapping the fluoride atom of hydrogen fluoride. Using triphenylsilane in place of tributylsilane, 3aa was obtained in 77% yield (entry 2). The removal of hydrogen fluoride promoted the formation of benzofurans, and in the presence of RhH(PPh3)4 (20 mol%) and dppe (40 mol%) the reaction proceeded in 80% yield without the silane reagent in refluxing o-dichlorobenzene (180 °C) under nitrogen bubbling (entry 3). The yield decreased to 48% at 150 °C (entry 4). Without nitrogen bubbling, the yield decreased to 38% in refluxing o-dichlorobenzene (entry 5). The reaction temperature could be markedly reduced from 180°C for the o-dichlorobenzene reflux to 132 °C for the chlorobenzene reflux in the presence of the silane reagent, although the reagent itself is not essential for this reaction.

The formation of 6-cyanobenzofuran in this synthesis indicated that C-arylation occurred at the p-position of 2a followed by O-arylation at the m-position. Rhodium enolate was formed by the α−activation of the ketones, which was followed by C-arylation with difluorobenzenes giving α-aryl ketones. Then, the intramolecular O-arylation of another rhodium enolate gave the product. In this reaction, rhodium enolates were formed twice without using a base. The mechanism was supported by the following experiment. When a mixture of 1a and 2b was heated in the presence of RhH(PPh3)4 (20mol%) and dppe (40 mol%) in refluxing o-dichlorobenzene with nitrogen bubbling for 6 h, fluoroarylated ketone 7ab (27%) and benzofuran 3ab (15%) were obtained. The compound 7ab could be converted into 3ab in 76% yield under the same rhodium-catalyzed conditions (Scheme 1). The rhodium complex was essential for the second O-arylation step.

Various ketones reacted with o-difluorobenzenes giving benzofurans (Table 2). Benzyl phenyl ketones possessing either p-electron-donating or p-electron-withdrawing groups on their aroyl moiety reacted with 2a smoothly (entries 1-4 and 7). Difluorobenzenes with cyano-, benzoyl-, and nitro-groups at the 4-position also reacted in high yields (entries 4-6). No reaction, however, occurred with 1,2-difluoro-4-methylbenzene. 1-Phenyl-2-butanone and 2a gave the corresponding benzofuran 3fa in 81% yield (entry 8). Dibenzyl ketone gave 3ga (38%) and its arylated product 4ga (38%) (entry 9). The latter compound 4ga was formed by the diarylation of 1g at two benzyl positions followed by cyclization (Scheme 2). The reaction could be applied to 1,3-diketones (entries 10 and 11). An acetophenone derivative, 4-acetylbenzonitrile 1j, also reacted to give the product 4ja, which was formed by the diarylation of 1j at the α-methyl protons followed by cyclization (Scheme 3).

The reaction was then applied to polyfluorobenzenes, which gave novel polyfluorinated benzofurans not available by conventional methods. When (diphenylamino)pentafluorobenzene 2e (2 equiv) was reacted with aryl benzyl ketone 1d in the presence of RhH(PPh3)4 (5 mol%), dppe (10 mol%), and tributylsilane (2 equiv) in refluxing chlorobenzene for 6 h, 6-diphenylamino-4,5,7-trifluorobenzofuran 3de was obtained in 83% yield (Table 3, entry 2). An X-ray analysis of 3de confirmed the structure (Figure 2).8 Various monosubstituted pentafluorobenzenes reacted smoothly, and the structures of 6-substituted benzofurans were in all cases determined by 19F-NMR, δ -151--154 for 4-fluorine (entries 1-8). X-Ray analysis was conducted for 3dd and 3di (entries 1 and 6, Figure 2).8

When hexafluorobenzene was subjected to the reaction, benzofuran 3dj (65%) was formed along with benzo[2,1-b;3,4-b]difuran 4dj (5%) (entry 7). The compound 4dj was formed by two cyclizations (Scheme 4). X-Ray analysis of the product 4dj confirmed the structure,8 in which two oxygen atoms occupy the o-position (Figure 3). It was also observed that the reaction of 3dj and 1d gave 4dj in 46% yield under the same conditions (Scheme 4). The results indicated that 1d reacted with 3dj at the 6-position.
The reaction of substituted pentafluorobenzenes gave 6-substituted-4,5,7-trifluorobenzofurans with either electron-donating groups such as diphenylamino, butylthio, p-tolyltio, and methyl groups or electron-withdrawing groups such as fluoro and benzoyl groups in high yields (Table 3, entries 1-8). The arylation generally proceeded at the p-position of the substituent in pentafluorobenzenes.

This synthesis includes the α-arylation of rhodium enolate. The rhodium-catalyzed α-arylation reaction of benzyl ketones was briefly examined. When a mixture of 1d (2 equiv) and 3-chloro-4-fluorobenzonitrile 7a in chlorobenzene was heated at reflux for 6 h in the presence of RhH(PPh3)4 (5 mol%), dppe (10 mol%), and tributylsilane (1 equiv), the arylated ketone 8da was obtained in 60% yield (Table 4, entry 1). Several other substituted benzyl phenyl ketones reacted smoothly. No reaction occurred in the absence of the rhodium complex or dppe. The rhodium-catalyzed enolate arylation proceeded using a fluorobenzene 7 without using a base.

In summary, the benzofuran cyclization of ketones and o-difluorobenzenes proceeded in the presence of a catalytic amount of rhodium complex without using a base. Polyfluorinated benzofurans were also synthesized using monosubstituted pentafluorobenzenes. It should be emphasized that rhodium catalysis could be used for the activation of ketones and o-difluorobenzenes.

EXPERIMENTAL
General: 1H-, 13C-, 19F-NMR spectra were recorded on a Varian Mercury (400 MHz). Tetramethylsilane and trifluoroacetic acid were used as standard. IR spectra were measured on a JASCO FT/IR-410 spectrophotometer. Melting points were determined with a Yanaco micro melting point apparatus without correction. High- and low-resolution mass spectra were measured on a JEOL JMS-DX-303, a JEOL JMS-700, or a JMS-T100GC. Merck silica gel 60 (63-200 µm) was employed for flash column chromatography.

6-Cyano-2,3-diphenylbenzofuran (3aa): In a two-necked flask equipped with a reflux condencer were placed 1,2-diphenylethanone 1a (0.25 mmol, 49 mg), 3,4-difluorobenzonitrile 2a (0.5 mmol, 69.5 mg), RhH(PPh3)4 (5 mol%, 14.4 mg), 1,2-bis(diphenylphosphino)ethane (10 mol%, 10.0 mg), and tributylsilane (0.5 mmol, 129 µL) in chlorobenzene (1 mL) under an argon atmosphere, and solution was stirred under reflux for 6 h. Then, the solvent was removed under reduced pressure, and the residue was purified by flash column chromatography on silica gel giving 3aa (66.3 mg, 90%) as colorless crystals. Mp 143-144 ºC (Hexane : EtOAc = 2 : 1). 1H-NMR (400 MHz, CDCl3) δ 7.33-7.37 (3H, m), 7.43-7.52 (5H, m), 7.56 (1H, dd, J = 8.0, 0.8 Hz), 7.66-7.68 (2H, m), 7.86 (1H, dd, J = 1.2, 0.8 Hz). 13C-NMR (100 MHz, CDCl3) δ 107.2, 115.2, 117.5, 119.5, 120.8, 126.6, 127.2, 128.2, 128.6, 129.2, 129.4, 129.6, 131.4, 134.4, 152.6, 154.0. IR (KBr) 3055, 2220, 1479, 1425, 1269, 1063, 764, 692 cm-1. MS (EI) m/z 295 (M+, 100%). HRMS Calcd for C21H13ON: 295.0997. Found: 295.0995. One carbon peak of aromatic region was piled up in 13C-NMR. CCDC 879726 contains the supplementary crystallographic data for compound 3aa.

2-(p-Benzoyl-o-fluorophenyl)-1,2-diphenyl-1-ethanone (7ab): In a two-necked flask equipped with a reflux condencer were placed 1a (0.25 mmol, 49 mg), 3,4-difluorobenzophenone 2b (0.5 mmol, 54.5 mg), RhH(PPh3)4 (20 mol%, 57.6 mg), and dppe (40 mol%, 39.8 mg) in o-dichlorobenzene (5 mL) under an argon atmosphere, and solution was stirred under reflux with nitrogen bubbling for 6 h. Then, the solvent was removed under reduced pressure, and the residue was purified by flash column chromatography on silica gel giving 6-benzoyl-2,3-diphenylbenzofuran 3ab (14.2 mg, 15%) and 7ab (26.9 mg, 27%) as colorless crystals. 3ab: Mp 134-135 ºC (Hexane : EtOAc = 2 : 1). 1H-NMR (400 MHz, CDCl3) δ 7.33-7.35 (3H, m), 7.43-7.53 (7H, m), 7.57 (1H, d, J = 8.0 Hz), 7.61 (1H, t, J = 8.0 Hz), 7.67-7.70 (2H, m), 7.77 (1H, dd, J = 8.0, 1.6 Hz), 7.84 (2H, d, J = 7.6 Hz), 8.03 (1H, d, J = 1.2 Hz). 13C-NMR (100 MHz, CDCl3) δ 113.6, 117.7, 119.6, 125.3, 127.2, 128.0, 128.3, 128.5, 129.1, 129.1, 129.7, 130.0, 130.0, 132.1, 132.1, 134.0, 134.2, 138.2, 153.2, 153.7, 196.3. IR (KBr) 3064, 1655, 1296, 715, 696 cm-1. MS (EI) m/z 374 (M+, 100%), 297 (M+-C6H5, 43%). HRMS Calcd for C27H18O2: 374.1307. Found: 374.1301. 7ab: Mp 123-124 ºC (Hexane : EtOAc = 2 : 1). 1H-NMR (400 MHz, CDCl3) δ 6.36 (1H, s), 7.18 (1H, t, J = 7.6 Hz), 7.3-7.6 (13H, m), 7.79 (2H, d, J = 7.2 Hz), 8.02 (2H, d, J = 7.2 Hz). 13C-NMR (100 MHz, CDCl3) δ 52.2 (d, J = 2.2 Hz), 116.5 (d, J = 23.5 Hz), 125.9 (d, J = 3.7 Hz), 127.8, 128.4, 128.7, 129.0, 129.2, 129.4, 130.0, 130.7 (d, J = 3.0 Hz), 131.7 (d, J = 15.2 Hz), 132.7, 133.4, 136.1, 136.3, 137.0, 138.3 (d, J = 6.8 Hz), 159.8 (d, J = 246.3 Hz), 194.9, 196.6. 19F-NMR (376 MHz, CDCl3) δ -118.4. IR (KBr) 3063, 1683, 1641, 1285, 1263, 1211, 703 cm-1. MS (EI) m/z 394 (M+, 11%), 105 (M+-COC6H5, 100%). HRMS Calcd for C27H19O2F: 394.1368. Found: 394.1386.

6-Cyano-2,3-bis(p-methoxyphenyl)benzofuran (3ba): Colorless crystals. Mp 157-158 ºC (Hexane : EtOAc = 2 : 1). 1H-NMR (400 MHz, CDCl3) δ 3.83 (3H, s), 3.89 (3H, s), 6.87 (2H, d, J = 8.4 Hz), 7.02 (2H, d, J = 8.8 Hz), 7.37 (2H, d, J = 8.8 Hz), 7.47 (1H, d, J = 8.0 Hz), 7.49 (1H, d, J = 8.8 Hz), 7.61 (2H, d, J = 9.6 Hz), 7.80 (1H, s). 13C-NMR (100 MHz, CDCl3) δ 55.3, 55.3, 106.5, 114.1, 114.7, 114.9, 115.7, 119.7, 120.4, 122.2, 123.7, 126.6, 128.7, 130.8, 134.9, 152.5, 154.1, 159.4, 160.4. IR (KBr) 3059, 2962, 2225, 1606, 1500, 1254, 1026 cm-1. MS (EI) m/z 355 (M+, 100%), 340 (M+-CH3, 16%). HRMS Calcd for C23H17O3N: 355.1208. Found: 355.1220.

6-Cyano-2-(p-methoxyphenyl)-3-phenylbenzofuran (3ca): Colorless crystals. Mp 131.5-132.5 ºC (Et2O). 1H-NMR (400 MHz, CDCl3) δ 3.82 (3H, s), 6.86 (2H, d, J = 8.8 Hz), 7.43-7.52 (7H, m), 7.60 (2H, d, J = 8.8 Hz), 7.81 (1H, s). 13C-NMR (100 MHz, CDCl3) δ 55.3, 106.6, 114.1, 115.0, 116.0, 119.6, 120.4, 122.0, 126.6, 128.1, 128.8, 129.2, 129.6, 131.7, 134.7, 152.5, 154.3, 160.5. IR (KBr) 3072, 2951, 2835, 2223, 1608, 1512, 1252, 1030 cm-1. MS (EI) m/z 325 (M+, 100%), 310 (M+-CH3, 17%). HRMS Calcd for C22H15O2N: 325.1103. Found: 325.1088.

2-(p-Chlorophenyl)-6-cyano-3-phenylbenzofuran (3da): Colorless crystals. Mp 163-164 ºC (Et2O). 1H-NMR (400 MHz, CDCl3) δ 7.32 (2H, d, J = 8.8 Hz), 7.43-7.56 (7H, m), 7.60 (2H, d, J = 8.8 Hz), 7.85 (1H, s). 13C-NMR (100 MHz, CDCl3) δ 107.5, 115.3, 118.0, 119.4, 120.9, 126.8, 127.9, 128.4, 128.5, 129.4, 129.5, 131.1, 134.3, 135.4, 152.6, 152.9. IR (KBr) 3054, 2223, 1480, 1427, 1092, 833, 826 cm-1. MS (EI) m/z 329 (M+, 100%). HRMS Calcd for C21H12ONCl: 329.0607. Found: 329.0589.

6-Benzoyl-2-(p-chlorophenyl)-3-phenylbenzofuran (3db): Colorless crystals. Mp 159-160 ºC (Hexane : EtOAc = 2 : 1). 1H-NMR (400 MHz, CDCl3) δ 7.29 (2H, d, J = 8.8 Hz), 7.43-7.52 (7H, m), 7.55 (1H, d, J = 8.4 Hz), 7.58- 7.62 (3H, m), 7.76 (1H, dd, J = 1.2, 8.0 Hz), 7.83 (2H, d, J = 6.8 Hz), 8.01 (1H, d, J = 0.8 Hz). 13C-NMR (100 MHz, CDCl3) δ 113.5, 118.1, 119.7, 125.4, 128.2, 128.3, 128.3, 128.4, 128.8, 129.2, 129.6, 129.9, 131.7, 132.2, 134.0, 134.2, 134.9, 138.1, 152.5, 153.2, 196.1. IR (KBr) 3051, 1658, 1288, 1093 cm-1. MS (EI) m/z 408 (M+, 100%), 331 (M+-C6H5, 36%). HRMS Calcd for C27H17O2Cl: 408.0917. Found: 408.0901.

2-(p-Chlorophenyl)-6-nitro-3-phenylbenzofuran (3dc): Yellow crystals. Mp 169-170 ºC (Et2O). 1H-NMR (400 MHz, CDCl3) δ 7.33 (2H, d, J = 8.4 Hz), 7.45-7.56 (6H, m), 7.61 (2H, d, J = 8.8 Hz), 8.18 (1H, dd, J = 2.0, 8.4 Hz), 8.45 (1H, d, J = 1.6 Hz). 13C-NMR (100 MHz, CDCl3) δ 107.6, 118.1, 119.0, 119.9, 127.8, 128.5, 128.6, 129.0, 129.4, 129.5, 131.0, 135.7, 135.9, 145.2, 152.5, 154.6. IR (KBr) 3101, 1512, 1344, 1095, 825 cm-1. MS (EI) m/z 349 (M+, 100%), 303 (M+-NO2, 17%). HRMS Calcd for C20H12O3NCl: 349.0506. Found: 349.0494.

6-Cyano-2-(p-cyanophenyl)-3-phenylbenzofuran (3ea): Colorless crystals. Mp >220 ºC (EtOAc : Hexane = 2 : 1). 1H-NMR (400 MHz, CDCl3) δ 7.45 (2H, dd, J = 1.6, 8.0 Hz), 7.49-7.58 (5H, m), 7.62 (2H, d, J = 8.8 Hz), 7.77 (2H, d, J = 8.0 Hz), 7.88 (1H, s). 13C-NMR (100 MHz, CDCl3) δ 108.5, 112.5, 115.5, 118.3, 119.0, 120.4, 121.4, 126.9, 127.3, 128.9, 129.4, 129.5, 130.5, 132.3, 133.6, 134.0, 151.4, 152.8. IR (KBr) 3060, 2225, 1606, 845, 820, 702 cm-1. MS (EI) m/z 320 (M+, 100%). HRMS Calcd for C22H12ON2: 320.0950. Found: 320.0940.

6-Cyano-2-ethyl-3-phenylbenzofuran (3fa): Colorless crystals. Mp 112 ºC (Hexane : EtOAc = 2 : 1). 1H-NMR (400 MHz, CDCl3) δ 1.37 (3H, t, J = 8.0 Hz), 2.92 (2H, q, J = 8.0 Hz), 7.39-7.45 (3H, m), 7.48- 7.52 (3H, m), 7.61 (1H, d, J = 8.0 Hz), 7.77 (1H, s). 13C-NMR (100 MHz, CDCl3) δ 12.6, 20.4, 106.3, 115.0, 116.8, 119.7, 120.3, 126.4, 127.7, 129.0, 129.0, 131.3, 133.2, 152.8, 160.2. IR (KBr) 2979, 2924, 2225, 1429, 1182, 835, 706 cm-1. MS (EI) m/z 247 (M+, 100%), 232 (M+-CH3, 65%). HRMS Calcd for C17H13ON: 247.0997. Found: 247.0995.

2-Benzyl-6-cyano-3-phenylbenzofuran (3ga): Colorless crystals. Mp 147.5-148.5 ºC (Hexane : EtOAc = 2 : 1). 1H-NMR (400 MHz, CDCl3) δ 4.24 (2H, s), 7.26 (2H, dd, J = 7.6, 8.0 Hz), 7.33 (2H, dd, J = 6.4, 7.2 Hz), 7.42-7.54 (7H, m), 7.64 (1H, d, J = 8.0 Hz), 7.76 (1H, s). 13C-NMR (100 MHz, CDCl3) δ 32.9, 106.7, 115.3, 118.7, 119.5, 120.6, 126.6, 126.9, 128.0, 128.5, 128.8, 129.0, 129.1, 131.0, 133.0, 136.8, 153.0, 156.6. IR (KBr) 3032, 2924, 2220, 2495, 1431, 980, 700 cm-1. MS (EI) m/z 309 (M+, 100%), 232 (M+-C6H5, 54%). HRMS Calcd for C22H15ON: 309.1153. Found: 309.1166.

6-Cyano-2-(p-chloro-o-fluorophenyl)phenylmethyl-3-phenylbenzofuran (4ga): Colorless crystals. Mp 156-158 ºC (Hexane : EtOAc = 2 : 1). 1H-NMR (400 MHz, CDCl3) δ 6.04 (1H, s), 7.18 (2H, dd, J = 7.6, 1.6 Hz), 7.32-7.38 (6H, m), 7.42-7.51 (5H, m), 7.54 (1H, dd, J = 8.0, 1.2 Hz), 7.64 (1H, dd, J = 8.0, 0.8 Hz), 7.80 (1H, dd, J = 1.2, 0.8 Hz). 13C-NMR (100 MHz, CDCl3) δ 41.7 (d, J = 3.0 Hz), 107.5, 112.7 (d, J = 9.1 Hz), 115.7, 117.4 (d, J = 3.0 Hz), 119.2, 119.2 (d, J = 25.0 Hz), 119.9, 121.2, 126.9, 127.8, 128.3, 128.4, 128.4, 129.0, 129.1, 129.2, 130.2, 132.0 (d, J = 3.8 Hz), 132.7, 133.6 (d, J = 13.6 Hz), 138.4, 153.2, 154.8, 160.0 (d, J = 249.3 Hz). IR (KBr) 3072, 2229, 1568, 1495, 1415, 1255, 980, 702 cm-1. MS (EI) m/z 428 (M+, 74%), 308 (M+-C6H3CNF, 20%), 276 (M+-152, 100%). HRMS Calcd for C29H17ON2F: 428.1325. Found: 428.1316.

3-Acetyl-6-cyano-2-methylbenzofuran (3ha): Colorless crystals. Mp 131-132 ºC (Et2O). 1H-NMR (400 MHz, CDCl3) δ 2.65 (3H, s), 2.84 (3H, s), 7.60 (1H, d, J = 8.0 Hz), 7.76 (1H, s), 8.11 (1H, d, J = 8.0 Hz). 13C-NMR (100 MHz, CDCl3) δ 15.7, 31.1, 107.6, 115.1, 117.8, 119.0, 122.6, 127.8, 130.5, 152.2, 165.8, 193.2. IR (KBr) 3095, 2222, 1664, 1394 cm-1. MS (EI) m/z 199 (M+, 62%), 184 (M+-CH3, 100%). HRMS Calcd for C12H9O2N: 199.0633. Found: 199.0616.

3-Benzoyl-6-cyano-2-phenylbenzofuran (3ia): Colorless crystals. Mp 149-151 ºC (Hexane : EtOAc = 2 : 1). 1H-NMR (400 MHz, CDCl3) δ 7.31-7.38 (5H, m), 7.50-7.55 (2H, m), 7.66-7.68 (3H, s), 7.81 (2H, dd, J = 8.4, 1.2 Hz), 7.90 (1H, s). 13C-NMR (100 MHz, CDCl3) δ 108.2, 115.5, 115.9, 119.0, 122.4, 127.5, 128.3, 128.6, 128.6, 128.6, 129.7, 130.7, 132.7, 133.6, 137.1, 152.5, 160.6, 191.3. IR (KBr) 3059, 2224, 1641, 1377, 908 cm-1. MS (EI) m/z 323 (M+, 100%), 246 (M+-C6H5, 29%). HRMS Calcd for C22H13O2N: 323.0946. Found: 323.0943.

6-Cyano-3-(p-cyano-o-fluorophenyl)-2-(p-cyanophenyl)benzofuran (4ja): Colorless crystals. Mp >245 ℃ (EtOAc). 1H-NMR (400 MHz, Benzene-d6) δ 6.47 (1H, dd, J = 7.6, 7.6 Hz), 6.60 (1H, dd, J = 1.6, 9.2 Hz), 6.68 (1H, d, J = 7.6 Hz) 6.70 (1H, dd, J = 1.6, 7.6 Hz), 6.84 (2H, d, J = 8.4 Hz), 6.96 (1H, dd, J = 1.6, 8.0 Hz), 7.01 (2H, d, J = 8.4 Hz), 7.13 (1H, s). 13C-NMR (100 MHz, CDCl3) δ 109.2, 112.0, 113.6, 114.8 (d, J = 9.1 Hz), 116.0, 116.9 (d, J = 2.2 Hz), 118.0, 118.7, 120.6 (d, J = 25.0 Hz), 121.2, 124.2 (d, J = 15.1 Hz), 127.3, 127.5, 129.0 (d, J = 3.8 Hz), 132.66, 132.69, 132.8 (d, J = 3.8 Hz), 132.9, 153.0, 153.4, 159.5 (d, J = 252.3 Hz). 19F-NMR (376 MHz, CDCl3) -161.7. IR (KBr) 3072, 2924, 2227, 1603, 1412, 1267, 976, 829 cm-1. MS (EI) m/z 363(M+, 100%). HRMS Calcd for C23H10ON3F: 363.0807. Found: 363.0814.

2-(p-Chlorophenyl)-6-dimethylamino-3-phenyl-4,5,7-trifluorobenzofuran (3dd): Colorless crystals. Mp 138-139 ºC (Hexane). 1H-NMR (400 MHz, CDCl3) δ 2.99 (6H, t, J = 1.6 Hz), 7.25 (2H, d, J = 8.8 Hz), 7.43 (5H, br), 7.47 (2H, d, J = 8.8 Hz). 13C-NMR (100 MHz, CDCl3) δ 43.9 (dd, J = 3.0, 3.8 Hz), 115.2 (d, J = 16.7 Hz), 116.7, 127.7 (d, J = 10.6 Hz), 128.0, 128.1, 128.3, 128.7, 128.8, 130.0, 131.1, 134.7, 138.2 (m), 139.8 (dd, J = 3.8, 242.5 Hz), 140.3 (ddd, J = 6.0, 12.9, 253.1 Hz), 143.5 (ddd, J = 4.6, 12.2, 240.2 Hz), 150.5. 19F-NMR (376 MHz, CDCl3) -153.1 (dd, J = 17.7, 18.4 Hz), -153.4 (d, J = 17.7 Hz), -154.7 (d, J = 18.4 Hz). IR (KBr) 2951, 2873, 2806, 1518, 1433, 991 cm-1. MS (EI) m/z 401 (M+, 100%), 386 (M+-CH3, 17%). HRMS Calcd for C22H15ONF3Cl: 401.0794. Found: 401.0772. CCDC 879722 contains the supplementary crystallographic data for compound 3dd.

2-(p-Chlorophenyl)-6-diphenylamino-3-phenyl-4,5,7,-trifluorobenzofuran (3de): Colorless crystals. Mp 162-163.5 ºC (Hexane). 1H-NMR (400 MHz, CDCl3) δ 7.03 (2H, t, J = 7.2 Hz), 7.08 (4H, d, J = 8.0 Hz), 7.25-7.30 (6H, m), 7.44-7.50 (7H, m). 13C-NMR (100 MHz, CDCl3) δ 116.9, 119.0 (m), 120.6 (m), 121.2, 122.9, 127.6, 128.3, 128.6, 128.9, 128.9, 129.3, 130.0, 130.7, 135.2, 138.0 (m), 140.6 (ddd, J = 3.8, 15.9, 252.3 Hz), 144.6 (td, J = 3.8, 250.8 Hz), 144.6 (ddd, J = 2.2, 14.4, 247.0 Hz), 146.2, 151.8. 19F-NMR (376 MHz, CDCl3) δ -148.4 (d, J = 13.2 Hz), -151.4 (d, J = 20.3 Hz), -151.5 (d, J = 13.2, 20.3 Hz). IR (KBr) 3059, 3035, 1591, 1516, 1495, 1450, 1093, 997, 694 cm-1. MS (EI) m/z 525 (M+, 100%). HRMS Calcd for C32H19ONF3Cl: 525.1107. Found: 525.1098. CCDC 879725 contains the supplementary crystallographic data for compound 3aa.

6-Butylthio-2-(p-chlorophenyl)-3-phenyl-4,5,7-trifluorobenzofuran (3df): Colorless crystals. Mp 79-81 ºC (Hexane). 1H-NMR (400 MHz, CDCl3) δ 0.91 (3H, t, J = 7.2 Hz), 1.45 (2H, tt, J = 6.8, 6.8 Hz), 1.57 (2H, tt, J = 7.6, 7.6 Hz), 2.92 (2H, t, J = 7.2 Hz), 7.28 (2H, d, J = 8.8 Hz), 7.46 (5H, br), 7.51 (2H, d, J = 9.2 Hz). 13C-NMR (100 MHz, CDCl3) δ 13.6, 21.6, 31.8, 35.0, 108.7 (dd, J = 20.5, 22.0 Hz), 117.0, 121.3 (d, J =15.9 Hz), 127.5, 128.4, 128.6, 128.8, 128.9, 129.9, 130.6, 135.4, 137.5 (dd, J = 6.0, 12.8 Hz), 140.0 (ddd, J = 3.8, 17.4, 250.8 Hz), 143.9 (ddd, J = 3.1, 3.8, 248.6 Hz), 146.4 (ddd, J = 2.3, 12.9, 238.7 Hz), 152.0. 19F-NMR (376 MHz, CDCl3) δ -135.8 (d, J = 17.7 Hz), -140.0 (d, J = 21.8 Hz), -151.7 (dd, J = 17.7, 21.8 Hz). IR (KBr) 2958, 1504, 1481, 1263, 989, 825 cm-1. MS (EI) m/z 446 (M+, 100%). HRMS Calcd for C24H18OSF3Cl: 446.0719. Found: 446.0714.

2-(p-Chlorophenyl)-3-phenyl-6-(p-tolylthio)-4,5,7-trifluorobenzofuran (3dg): Colorless crystals. Mp 139.5-140 ºC (Hexane). 1H-NMR (400 MHz, CDCl3) δ 2.30 (3H, s), 7.08 (2H, d, J = 7.6 Hz), 7.26-7.30 (4H, m), 7.44 (5H, br), 7.49 (2H, d, J = 8.8 Hz). 13C-NMR (100 MHz, CDCl3) δ 21.1, 108.5 (dd, J = 19.7, 21.2 Hz), 117.1, 122.3 (d, J = 15.2 Hz), 127.4, 128.5, 128.6, 128.9, 130.0, 130.3, 130.5, 131.0, 135.5, 137.5, 137.6 (m), 140.1 (ddd, J = 4.5, 17.4, 251.5 Hz), 143.8 (ddd, J = 3.0, 3.8, 250.1 Hz), 146.2 (ddd, J = 1.5, 14.4, 240.2 Hz), 152.4. 19F-NMR (376 MHz, CDCl3) δ -135.1 (d, J = 18.8 Hz), -139.3 (d, J = 21.4 Hz), -151.0 (dd, J= 18.8, 21.4 Hz). IR (KBr) 3035, 2918, 1504, 1493, 1479, 1261, 993, 829, 706 cm-1. MS (EI) m/z 480 (M+,100%). HRMS Calcd for C27H16OSF3Cl: 480.05625. Found: 480.05344. Two carbon peak of aromatic region was piled up in 13C-NMR.

2-(p-Chlorophenyl)-6-methyl-3-phenyl-4,5,7-trifluorobenzofuran (3dh): Colorless crystals. Mp 163-164.5 ºC (Hexane). 1H-NMR (400 MHz, CDCl3) δ 2.37 (3H, t, J = 2.4 Hz), 7.27 (2H, d, J= 8.8 Hz), 7.44- 7.47 (5H, m), 7.50 (2H, d, J = 8.8 Hz). 13C-NMR (100 MHz, CDCl3) δ 7.8, 111.2 (dd, J = 20.8, 17.8 Hz), 116.8, 118.6 (d, J = 15.6 Hz), 127.9, 128.3, 128.4, 128.8, 130.0, 130.0, 131.0, 135.0, 137.6 (dd, J = 12.6, 7.4 Hz), 139.8 (ddd, J = 247.8, 16.3, 4.4 Hz), 141.4 (d, J = 20.3 Hz), 144.7 (ddd, J = 238.9, 12.7, 6.0 Hz), 150.8. 19F-NMR (376 MHz, CDCl3) δ -146.5 (d, J = 18.8 Hz), -149.4 (d, J = 20.3 Hz), -153.8 (dd, J = 18.8, 20.3 Hz). IR (KBr) 3062, 2925, 1514, 1441, 1277, 1065, 951, 829 cm-1. MS (EI) m/z 372 (M+, 100%). HRMS Calcd for C21H12OF3Cl: 372.0529. Found: 372.0522.

2-(p-Chlorophenyl)-3-phenyl-4,5,7-trifluorobenzofuran (3di): Colorless crystals. Mp 119-120 ºC (Methanol). 1H-NMR (400 MHz, CDCl3) δ 6.96 (1H, ddd, J = 6.0, 9.6, 10.4 Hz), 7.27 (2H, d, J = 8.8 Hz), 7.44 (5H, s), 7.49 (2H, d, J = 8.8 Hz). 13C-NMR (100 MHz, CDCl3) δ 101.2 (dd, J = 21.9, 25.0 Hz), 117.0, 121.6 (td, J = 3.1, 15.1), 127.6, 128.4, 128.5, 128.9, 129.9, 130.0, 130.8, 135.3, 137.6 (dd, J = 7.5, 12.1 Hz), 139.9 (ddd, J = 3.8, 15.1, 248.5 Hz), 142.6 (ddd, J = 3.0, 10.6, 247.0 Hz), 145.3 (ddd, J = 9.1, 12.9, 241.0 Hz). 151.9. 19F-NMR (376 MHz, CDCl3) δ -141.8 (dd, J = 10.9, 19.1 Hz), -145.8 (dd, J = 10.9, 20.3 Hz), -153.1 (ddd, J = 6.0, 19.1, 20.3 Hz). IR (KBr) 3076, 1651, 1522, 1483, 1421, 1257, 1101, 968, 825 cm-1. MS (EI) m/z 358 (M+, 100%). HRMS Calcd for C20H10OF3Cl: 358.0372. Found: 358.0375. CCDC 879724 contains the supplementary crystallographic data for compound 3di.

2-(p-Chlorophenyl)-3-phenyl-4,5,6,7-tetrafluorobenzofuran (3dj): Colorless crystals. Mp 148-149 ºC (Hexane). 1H-NMR (400 MHz, CDCl3) δ 7.27 (2H, d, J = 8.8 Hz), 7.42-7.48 (7H, m). 13C-NMR (100 MHz, CDCl3) δ 115.7 (d, J = 15.6Hz), 116.4, 127.3, 128.3, 128.7, 128.89, 128.94, 129.9, 130.4, 134.0 (ddd, J = 5.2, 11.9, 250.8Hz), 135.4, 137.0 (m), 137.5 (td, J = 14.9, 244.8 Hz), 138.9 (td, J = 13.8, 247.8 Hz), 140.0 (tdd, J = 3.0, 12.6, 251.6 Hz), 152.0. 19F-NMR (376 MHz, CDCl3) δ -151.1 (dd, J = 19.1, 20.3 Hz), -163.6 (dd, J = 19.2, 20.3Hz), -165.1 (ddd, J = 2.3, 20.3, 20.3 Hz), -166.8 (ddd, J = 2.3, 20.3, 20.3). IR (KBr) 3070, 1531, 1477, 1093, 1082, 1005, 833 cm-1. MS (EI) m/z 376 (M+, 100%). HRMS Calcd for C20H9OF4Cl: 376.0278. Found: 376.0258.

2,8-Bis(p-chlorophenyl)-4,5-difluoro-3,6-diphenylbenzo[2,1-b;3,4-b]difuran (4dj): Colorless crystals. Mp 149-150 ºC (Hexane). 1H-NMR (400 MHz, CDCl3) δ 7.29 (4H, d, J = 8.4 Hz), 7.45-7.52 (10H, m), 7.56 (4H, d, J = 8.8 Hz). 13C-NMR (100 MHz, CDCl3) δ 117.7 (dd, J = 7.5, 10.4 Hz), 117.9, 128.2, 128.2, 128.3, 128.4, 128.8, 130.1, 131.4, 134.7, 140.3 (dd, J = 16.4, 247.1 Hz), 150.4. 19F-NMR (376 MHz, CDCl3) δ -156.2. IR (KBr) 3070, 1531, 1477, 1093, 1082, 1005, 833 cm-1. MS (EI) m/z 566 (M+, 100%). HRMS Calcd for C34H18O2F2Cl2: 566.0652. Found: 566.0657. One carbon peak of aromatic region was piled up in 13C-NMR. CCDC 879723 contains the supplementary crystallographic data for compound 4dj.

6-Benzoyl-2-(p-chlorophenyl)-3-phenyl-4,5,6-trifluorobenzofuran (3dk): Colorless crystals. Mp 167-168 ºC (Hexane). 1H-NMR (400 MHz, CDCl3) δ 7.30 (2H, d, J = 8.8 Hz), 7.48 (5H, br), 7.52 (4H, m), 7.66 (1H, t, J = 7.2 Hz), 7.93 (2H, d, J = 7.2 Hz). 13C-NMR (100 MHz, CDCl3) δ 113.9 (dd, J = 20.1, 19.3 Hz), 117.1, 122.8 (d, J = 16.4 Hz), 127.2, 128.6, 128.8, 128.9, 129.0, 129.0, 129.8, 130.0, 130.3, 134.4, 135.7, 136.8, 137.4 (dd, J = 6.7, 12.6 Hz), 140.0 (ddd, J = 4.5, 15.6, 252.3 Hz), 140.1 (ddd, J = 4.5, 4.5, 251.6 Hz), 143.0 (ddd, J = 5.2, 14.1, 244.8 Hz), 153.3, 187.6. 19F-NMR (376 MHz, CDCl3) δ -142.7 (d, J = 21.4 Hz), -147.1 (dd, J = 19.2 Hz), -151.4 (dd, J = 19.2, 21.4 Hz). IR (KBr) 3059, 1676, 1508, 1294, 1003 cm-1. MS (EI) m/z 462 (M+, 100%), 385 (M+-C6H5, 15%). HRMS Calcd for C27H14O2F3Cl: 462.0634. Found: 462.0623.

2-(o-Chloro-p-cyanophenyl)-1-(p-chlorophenyl)-2-phenyl-1-ethanone (8da): In a two-necked flask equipped with a reflux condencer were placed 1d (0.5 mmol, 115.0 mg), 7a (0.25 mmol, 38.8 mg), RhH(PPh3)4 (5 mol%, 14.4 mg), 1,2-bis(diphenylphosphino)ethane (10 mol%, 10.0 mg), and tributylsilane (0.25 mmol, 65 µL) in chlorobenzene (1 mL) under an argon atmosphere, and solution was stirred under reflux for 6 h. Then, the solvent was removed under reduced pressure, and the residue was purified by flash column chromatography on silica gel giving 8da (54.9 mg, 60%) as colorless crystals. Mp 130-131 ºC (Hexane : EtOAc = 2 : 1). 1H-NMR (400 MHz, CDCl3) δ 6.39 (1H, s), 7.08 (1H, d, J = 8.0 Hz), 7.27 (2H, d, J = 6.8 Hz), 7.33- 7,42 (5H, m), 7.47 (1H, dd, J = 2.0, 8.0 Hz), 7.69 (1H, d, J = 2.0 Hz), 7.93 (2H, d, J = 8.8 Hz). 13C-NMR (100 MHz, CDCl3) δ 56.7, 112.5, 117.3, 128.3, 129.1, 129.4, 129.6, 130.2, 130.4, 131.8, 132.5, 134.3, 134.6, 135.3, 140.0, 143.1, 195.1. IR (KBr) 3066, 3028, 2233, 1732, 1685, 1587, 1495, 1317, 1128, 1055, 742, 698 cm-1. MS (EI) m/z 365(M+, 1%), 226(M+-COC6H4Cl, 2%), 139 (COC6H4Cl, 100%). HRMS Calcd for C21H13ONCl2: 365.0374. Found: 365.0357.

2-(o-Chloro-p-cyanophenyl)-1,2-diphenyl-1-ethanone (8aa): Colorless crystals. Mp 141.5-142 ºC (Hexane : EtOAc = 2 : 1). 1H-NMR (400 MHz, CDCl3) δ 6.47 (1H, s), 7.11 (1H, d, J = 8.0 Hz), 7.28 (2H, d, J = 7.2 Hz), 7.33 (1H, t, J = 7.2 Hz), 7.33-7.44 (4H, m), 7.46 (1H, dd, J = 1.6, 8.0 Hz), 7.54 (1H, t, J = 7.6 Hz), 7.69 (1H, d, J = 1.6 Hz), 8.00 (2H, d, J = 7.6 Hz). 13C-NMR (100 MHz, CDCl3) δ 56.6, 112.3, 117.4, 128.1, 128.8, 128.9, 129.5, 130.4, 131.8, 132.4, 133.4, 134.6, 135.6, 135.9, 143.5, 196.3. IR (KBr) 3062, 3033, 2229, 1682, 1217, 704 cm-1. MS (EI) m/z 331 (M+, 0.4%), 105 (COPh, 100%). HRMS Calcd for C21H14ONCl: 331.0764. Found: 331.0764.

1,2-Bis(p-methoxyphenyl)-2-(o-chloro-p-cyano)-1-ethanone (8ba): Colorless oil. 1H-NMR (400 MHz, CDCl3) δ 3.78 (3H, s), 3.82 (3H, s), 6.36 (1H, s), 6.89 (2H, d, J = 8.8 Hz), 6.90 (2H, d, J = 8.8 Hz), 7.12 (1H, d, J = 8.0 Hz), 7.20 (2H, d, J = 8.8 Hz), 7.45 (1H, dd, J = 1.2 Hz, 8.0 Hz), 7.67 (1H, d, J = 2.4 Hz), 7.98 (2H, d, J = 8.4 Hz). 13C-NMR (100 MHz, CDCl3) δ 55.2, 55.4, 55.8, 112.0, 113.9, 114.7, 117.5, 127.8, 128.9, 130.3, 130.5, 131.2, 131.8, 132.3, 134.5, 144.1, 159.2, 163.6, 194.9. IR (neat) 2935, 2839, 2231, 1676, 1599, 1510, 1254, 1169, 1032 cm-1. MS (EI) m/z 391 (M+, 10%), 376 (M+-CH3, 5%) 135 (H3CC6H4CO, 100%). HRMS Calcd for C23H18O3NCl: 391.0975. Found: 391.0967.

2-(o-Chloro-p-nitrophenyl)-1-(p-chlorophenyl)-2-phenyl-1-ethanone (8db): Colorless oil. 1H-NMR (400 MHz, CDCl3) δ 6.42 (1H, s), 7.15 (1H, d, J = 8.4 Hz), 7.28 (2H, d, J = 6.8 Hz), 7.34-7.44 (5H, m), 7.94 (2H, d, J = 8.8 Hz), 8.03 (1H, dd, J = 2.4, 8.8 Hz), 8.29 (1H, d, J = 2.4 Hz). 13C-NMR (100 MHz, CDCl3) δ 56.7, 121.7, 124.4, 128.4, 129.2, 129.4, 129.7, 130.3, 131.8, 134.2, 134.6, 135.3, 140.1, 144.8, 147.3, 195.0. IR (neat) 3099, 2925, 1685, 1520, 1350, 768 cm-1. MS (EI) m/z 385 (M+, 2%), 139 (ClC6H4CO, 100%). HRMS Calcd for C20H13O3NCl2: 385.0272. Found: 385.0264.

ACKNOWLEDGEMENTS
This work was supported by a Grant-in-Aid for Scientific Research (No. 21229001) and the GCOE program from JSPS. M. A. expresses her appreciation to the financial supports from the Grant-in-Aid for Scientific Research (No. 22689001), Japan Science Technology Agency, and also to the Asahi Glass Foundation.

References

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8. Crystallographic data excluding structure factors have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication number CCDC 879722-879726 for compounds 3aa, 3dd, 3de, 3di, and 4dj. A copy of the data can be obtained free of charge from CCDC, 12 Union Road, Cambridge CB2 1EZ, UK or e-mail: deposit@ccdc.cam.ac.uk.