Biaryl Formation via Base-Promoted Direct Coupling Reactions of Arenes with Aryl Halides.

In the absence of ligand, Cs2CO3-promoted cross-coupling reaction of arenes with cyano-/nitro-substituted aryl halides in DMSO affording biaryls is reported. The cyano/nitro group in biaryls is useful and convenient for further transformation. The formation of dibenzofurans resulting from the reactions between arenes and 1-bromo-2-iodobenzene is also reported. On the basis of control experiments and theoretical studies, a radical mechanism is proposed for the formation of biaryls.

Introduction

Arene carbon–hydrogen bond activation and its cross-coupling reactions have opened new avenues in synthetic chemistry with atom- and step-economy.[1] Among many applications, the carbon–carbon bond formation via cross-coupling reactions between arene and aryl halides is particularly interesting as it provides an efficient access to functionalized biaryls, which have been well developed with the use of transition-metal complexes as catalysts.[2] In the past 2 decades, under transition-metal-free conditions, efficient procedures for biaryl syntheses from cross-coupling reactions of arene and aryl halides promoted by bases have also been developed.[3] As shown in Scheme , with the use of different ligands, the alkali salts of KOBu, NaOBu, and NaH have been found to be efficient promoters or catalysts for this transformation.[4]

Scheme 1

Base-Promoted Biaryl Syntheses via Coupling Reactions of Arenes with Aryl Halides

Recently, without a ligand, Cs2CO3 in dimethyl sulfoxide (DMSO) has been found to be an efficient and versatile promoter in different transformations reported by our group[5e,5f] and others.[5a−5d] In continuation of our earlier studies on arene C–H arylation,[6] and C–H activation/annulations,[7] we would like to describe herein the biaryl synthesis via the cross-coupling of arenes with cyano-/nitro-substituted aryl halides in DMSO promoted by Cs2CO3 under ligand-free conditions (Scheme ). The synthesis of biaryls bearing cyano and nitro groups is rarely reported with low to fair yields by transition-metal-free cross-coupling reactions of arenes with aryl halides.[4a,4f,4g,4i]

Results and Discussion

We selected the reaction between naphthalene (1a, 3.0 equiv) and 2-iodobenzonitrile (2a, 1.0 equiv) in DMSO at 100 °C in a sealed tube to optimize the reaction conditions. As shown in Table , in the presence of 2.5 equiv of KOBu, KOH, and K2CO3, the reactions under nitrogen for 24 h gave low yields or trace amounts of the desired coupling product 2-(naphthalen-1-yl)benzonitrile (3aa) (entries 1–3). When Cs2CO3 was chosen as a base, the yield of 3aa could be increased to 65% (entry 4). However, if tetrahydrofuran (THF), 1,4-dioxane, dimethylformamide (DMF), or toluene was used as the solvent to replace DMSO, 3aa formed in trace amounts only (entries 5–8). The decrease of either 1a (3.0 equiv to 2.0 equiv) or Cs2CO3 (2.5 equiv to 0.5 equiv) resulted in a considerable decrease in yields (entries 9–12). Therefore, the use of 3.0 equiv of Cs2CO3 and 1a is the best reaction condition to obtain 3aa in 72% yield (entry 13). In addition, when organic bases such as DABCO (1,4-diazabicyclo[2.2.2]octane) and DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) were used, or without any base, no reaction occurred at all (entries 14–16).

Table 1

Optimizing Reaction Conditions for Biaryl Formationa

entry	1a (equiv)	base (equiv)	solvent	yield (%)b
1	3.0	KOtBu (2.5)	DMSO	16
2	3.0	KOH (2.5)	DMSO	10
3	3.0	K2CO3 (2.5)	DMSO	trace
4	3.0	Cs2CO3 (2.5)	DMSO	65
5	3.0	Cs2CO3 (2.5)	THF	trace
6	3.0	Cs2CO3 (2.5)	1,4-dioxane	trace
7	3.0	Cs2CO3 (2.5)	DMF	trace
8	3.0	Cs2CO3 (2.5)	toluene	trace
9	2.0	Cs2CO3 (2.5)	DMSO	53
10	2.5	Cs2CO3 (2.5)	DMSO	60
11	3.0	Cs2CO3 (0.5)	DMSO	33
12	3.0	Cs2CO3 (1.0)	DMSO	51
13	3.0	Cs2CO3(3.0)	DMSO	72
14	3.0	DABCO (3.0)	DMSO	0
15	3.0	DBU (3.0)	DMSO	0
16	3.0		DMSO	0

Naphthalene (1a, 3.0 mmol) and 2-iodobenzonitrile (2a, 1.0 mmol) in 4.0 mL of solvent in a seated tube under N2 at 100 °C for 24.

Yields are isolated yields.

Table summarizes the substrate scope for biaryl syntheses with the reaction conditions indicated in entry 13 of Table . The cross-coupling reactions of 1a with 2-bromobenzonitrile, 1-iodo-, and 1-bromo-2-nitrobenzene gave the corresponding biaryls (3aa and 3ab) in 58–73% yields. The bromobenzenes show a somewhat lower reactivity compared with iodobenzenes. The use of 1-iodo-4-methyl-2-nitrobenzene resulted in 51% yield of 3ac, indicating that aryl halides bearing an electron-donating group is unfavorable for biaryl formation. 1a reacted with 1-bromo-4-nitrobenzene affording 3ad in 68% yields. In addition, the reactions 1a with meta-iodobenzonitrile and meta-iodonitrobenzene afforded the corresponding biaryls 3ae and 3af in 51 and 55% yields, respectively. When benzene (1b) was used, the corresponding biaryls (3ba–3be) were obtained in 41–75% yields, resulting from the cross-coupling reactions with cyano- or nitro-substituted aryl bromides and iodides. When p-xylene and mesitylene were employed, the cross-coupling reactions with aryl halides produces the expected biaryls (3cc–3dc) in moderate to good yields. In addition, the reactions of anthracene and phenanthrene were also examined, and the cross-coupling reactions occurred with high regioselectivity to give 9-substituted anthracenes and phenanthrenes.

Table 2

Substrate Scope of Biaryl Formation

Noted that under the standard conditions, aryl chlorides such as 2-chlorobenzonitrile and 1-chloro-4-nitrobenzene cannot undergo the cross-coupling reactions with 1a or 1b.

Although the cross-coupling reactions between arenes and bromo-/iodo-benzene or electron-rich aryl halides bearing methoxy groups do not occur smoothly under the standard conditions, the intramolecular coupling reaction of 1-iodo-2-phenoxybenzene can produce dibenzofuran (5af) (eq ). Therefore, as shown in Table , under the modified reaction conditions, dibenzofuran derivatives can be prepared in fair to good yields from a two-component reaction between 1-bromo-2-iodobenzene and phenols.

Table 3

Dibenzofuran Formationa

Reactions were carried out using 4 (1.0 mmol), 2f (3.0 mmol), and Cs2CO3 (2.5 mmol) in 4.0 mL of DMSO in a sealed tube at 130 °C for 24 h.

The radical mechanism for biaryl formation via cross-coupling of arenes with aryl halides under different conditions has been proposed in several literature studies.[4,8] In the present work, a radical mechanism is also proposed on the basis of the control experiments and the computational studies.

At first, a free radical trapping experiment was conducted in the reaction of 1b with 2b in the presence of the free radical scavengers 2,2,6,6-tetramethyl-1-piperidin-1-oxyl (TEMPO) under the standard conditions. Although the corresponding O-aryl-TEMPO could not be detected by gas chromatography (GC)–mass spectrometry (MS) analysis of the reaction mixture, 3bb was formed in trace amounts only. The results indicate that Cs2CO3/DMSO-promoted biaryl formation resulted from a free radical way via the coupling reactions of arenes with aryl halides.

In addition, we also performed the reaction of 1b with 2b using 1:1 of 1b and 1b-d6 and found that the kinetic isotopic effect value is 1.06 for the formation of 3bb, indicating that the rate-determining step for biaryl formation is the formation of 2b′s radical anion, not the activation or cleavage of C–H/C–D (eq ).

The purposes of computational studies are to investigate the role of Cs2CO3 in the cross-coupling reactions between arenes and haloarenes without the assistance of the ligand. The computational studies on the formation of 3bb from the reaction of benzene (1b) with 1-iodo-2-nitrobenzene (2b) as the model reaction have been done, the energy profile is shown in Figure .

Figure 1

Energy Profile for 2b′ Radical Formation.

In the radical mechanism, M06/def2-TZVP was used with the SMD solvation model in DMSO, and thermal correction.[9] The computational studies show that the vertical single electron transfer (SET) from cesium carbonate to the electron acceptor of 2b, without structure relaxation, requires 45.9 kcal/mol of electronic energy or 39.4 kcal/mol of free energy change. After the relaxation of the structures of the radical cation of cesium carbonate and the radical anion of 2-iodonitrobenzene, the electronic energy change is 39.4 kcal/mol or the free energy change is 20.9 kcal/mol. Namely, the way from [Cs2CO3 + 2b] to [S* + ν] to S* to form an active radical 2b (S*) should overcome a relatively high energy ([S* + ν]). However, the other way via the vibrational excited state ([S + ν′]) requires 16.6 kcal/mol electronic energy or 17.8 kcal/mol free energy only and to give S* with 20.9 kcal/mol free energy change, which can be achieved easily in the reaction temperature. Therefore, the SET radical pair can be readily realized via the vibrational excitation process to promote the cross-coupling reaction.

Therefore, a proposed mechanism for the formation of 3bb is depicted in Scheme . Cs2CO3 provides signal election to 2b, generating anion radical and then 2b radical. 2b radical interacts with benzene (1b) to form C–C bond and transfer radical to benzene ring. The interaction between the iodine radical and the benzene radical affords the final product 3bb, accompanied by the formation of HI.

Scheme 2

Radical Mechanism for 3bb Formation

Conclusions

In conclusion, in the absence of a ligand, Cs2CO3-promoted cross-coupling reactions between arenes and aryl halides in DMSO affording biaryls in good yields is developed. Although aryl halides are limited to electron-deficient substrates, the electron-withdrawing groups (−CN and −NO2) in biaryls are important and useful for further synthetic applications. Dibenzofuran derivatives can also be synthesized in fair to good yields from the reactions between 1-bromo-2-iodobenzene and phenols. In addition, from the results of the controlling experiments and the computational studies on the role of Cs2CO3, a radical mechanism is proposed for the formation of biaryls.

Experimental Section

General Methods

All commercial reagents are analytically pure and used without further purification. The purity of Cs2CO3 is 99.99%. Nuclear magnetic resonance (NMR) spectra were recorded using CDCl3 at 298 K. 1H NMR (400 and 600 MHz) chemical shifts (δ) were referenced to internal standard TMS (for[1] H, δ = 0.00 ppm). 13C NMR (100 and 125 MHz) chemical shifts were referenced to internal solvent CDCl3 (for 13C, δ = 77.16 ppm). Mass spectra were obtained on a low-resolution GC–MS spectrometer, and high-resolution mass spectra were recorded on a high-resolution magnetic sector mass spectrometer with an electrospray ionization (ESI) source.

Typical Experimental Procedure for the Synthesis of 2-(Naphthalen-1-yl)benzonitrile (3aa)

To a 25 mL tube equipped with a magnetic stirrer were added naphthalene (1a) (384.1 mg, 3.0 mmol), 2-iodobenzonitrile (2a) (229.4 mg, 1.0 mmol), Cs2CO3 (978.3 mg, 3.0 mmol), and DMSO (4.0 mL) under nitrogen. The tube was sealed and stirred at 100 °C for 24 h in an oil bath. After the reaction mixture was cooled to room temperature, it was poured into a solvent mixture of water (50.0 mL) and ethyl acetate (20.0 mL), and the two phases were then separated. The aqueous layer was extracted with ethyl acetate (3 × 20.0 mL). The combined organic extracts were dried over anhydrous Na2SO4. After removal of the solvent under reduced pressure, the residue was purified by column chromatography on silica gel with petroleum ether/ethyl acetate (gradient mixture ratio from 100:0 to 95:05) as an eluent to afford 3aa as a white solid 72% (165.3 mg, 0.72 mmol).

Typical Experimental Procedure for the Synthesis of Dibenzofuran (5af)

To a 25 mL tube equipped with a magnetic stirrer were added phenol (4a) (94.1 mg, 1.0 mmol), 1-bromo-2-iodobenzene (2f) (849.1 mg, 3.0 mmol), Cs2CO3 (978.3 mg, 3.0 mmol), and DMSO (4.0 mL) under nitrogen. The tube was sealed and stirred at 130 °C for 24 h in an oil bath. After the reaction mixture was cooled to room temperature, it was poured into a solvent mixture of water (50.0 mL) and ethyl acetate (20.0 mL), and the two phases were then separated. The aqueous layer was extracted with ethyl acetate (3 × 20.0 mL). The combined organic extracts were dried over anhydrous Na2SO4. After removal of the solvent under reduced pressure, the residue was purified by column chromatography on silica gel with petroleum ether/ethyl acetate (gradient mixture ratio from 100:0 to 90:10) as an eluent to afford 5af in 65% (109.3 mg, 0.65 mmol).

2-(Naphthalen-1-yl)benzonitrile (3aa)[10]

White solid (X = I, 165.3 mg, 72%; X = Br, 132.9 mg, 58%); 1H NMR (400 MHz, CDCl3): δ 7.96 (t, J = 6.9 Hz, 2H), 7.85 (d, J = 7.7 Hz, 1H), 7.70 (t, J = 7.5 Hz,1H), 7.63–7.44 (m, 7H); 13C NMR (100 MHz, CDCl3): δ 144.4, 135.9, 133.7, 133.2, 132.3, 131.6, 131.5, 129.2, 128.6, 128.0, 127.6, 126.7, 126.2, 125.2, 118.1, 113.6; GC–MS m/z: 229 (M+).

1-(2-Nitrophenyl)naphthalene (3ab)[11]

Orange solid (X = I, 181.7 mg, 73%; X = Br, 152.0 mg, 61%); 1H NMR (400 MHz, CDCl3): δ 8.09 (d, J = 8.1 Hz, 1H), 7.94 (d, J = 7.9 Hz, 2H), 7.69 (t, J = 7.0 Hz, 1H), 7.62–7.41 (m, 6H), 7.38 (d, J = 6.9 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ 149.8, 135.5, 135.2, 133.5, 133.1, 132.6, 131.5, 128.7, 128.6, 128.5, 126.6, 126.1, 125.3, 124.9, 124.3; GC–MS m/z: 249 (M+).

1-(4-Methyl-2-nitrophenyl)naphthalene (3ac)

Orange waxy oil (134.2 mg, 51%); 1H NMR (400 MHz, CDCl3): δ 7.93–7.87 (m, 3H), 7.55–7.45 (m, 4H), 7.44–7.32 (m, 3H), 2.54 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 149.6, 139.2, 135.7, 133.5, 133.4, 132.9, 132.4, 131.7, 128.5, 126.6, 126.2, 126.1, 125.3, 125.0, 124.6, 21.1; HRMS (ESI) m/z: [M + H]+ calcd for C17H14NO2, 264.0946; found, 264.0949.

1-(4-Nitrophenyl)naphthalene (3ad)[12]

White waxy oil (169.4 mg, 68%); 1H NMR (400 MHz, CDCl3): δ 8.37 (d, J = 8.2 Hz, 2H), 7.97–7.90 (m, 2H), 7.79 (d, J = 8.4 Hz, 1H), 7.68 (d, J = 8.8 Hz, 2H), 7.60–7.40 (m, 4H); 13C NMR (100 MHz, CDCl3): δ 147.8, 147.3, 137.9, 133.9, 131.1, 129.1, 128.7, 127.2, 126.9, 126.4, 125.4, 125.2, 123.7; GC–MS m/z: 249 (M+).

3-(Naphthalen-1-yl)benzonitrile (3ae)[13]

White solid (116.3 mg, 51%); 1H NMR (400 MHz, CDCl3): δ 8.05–7.86 (m, 6H), 7.71–7.64 (m, 2H), 7.62–7.50 (m, 3H); 13C NMR (100 MHz, CDCl3): δ 142.5, 136.2, 133.6, 133.1, 131.8, 131.0, 130.8, 129.8, 129.1, 128.4, 127.8, 126.8, 126.7, 126.3, 124.9, 119.0, 113.2; GC–MS m/z: 229 (M+).

1-(3-Nitrophenyl)naphthalene (3af)[13]

Yellow solid (136.7 mg, 55%);1H NMR (400 MHz, CDCl3): δ 8.58 (s, 1H), 8.23 (d, J = 8.1 Hz, 1H), 8.09 (s, 1H), 8.04 (d, J = 7.7 Hz, 1H), 8.00–7.87 (m, 3H), 7.75 (d, J = 8.8 Hz, 1H), 7.64 (t, J = 8.0 Hz, 1H), 7.59–7.52 (m, 2H), 13C NMR (100 MHz, CDCl3): δ 148.9, 142.9, 136.0, 133.6, 133.3, 133.2, 129.9, 129.1, 128.4, 127.8, 126.9, 126.8, 126.5, 124.9, 122.2, 122.1; GC–MS m/z: 249 (M+).

(1,1′-Biphenyl)-2-carbonitrile (3ba)[3c]

Yellow waxy oil (X = I, 98.3 mg, 55%; X = Br, 78.5 mg, 44%); 1H NMR (400 MHz, CDCl3): δ 7.77 (d, J = 7.7 Hz, 1H), 7.65 (t, J = 7.6 Hz, 1H), 7.62–7.41 (m, 7H); 13C NMR (100 MHz, CDCl3): δ 145.6, 138.2, 133.8, 132.9, 130.2, 128.8, 127.6, 118.8, 111.4; GC–MS m/z: 179 (M+).

2-Nitro-1,1′-biphenyl (3bb)[3c]

Yellow solid (X = I, 125.2 mg, 63%; X = Br, 105.5 mg, 58%); 1H NMR (400 MHz, CDCl3): δ 7.86 (d, J = 8.1 Hz, 1H), 7.62 (t, J = 7.5 Hz, 1H), 7.51–7.39 (m, 5H), 7.36–7.30 (m, 2H); 13C NMR (100 MHz, CDCl3): δ 149.4, 137.5, 136.4, 132.4, 132.0, 128.8, 128.3, 128.2, 128.0, 124.2; GC–MS m/z: 199 (M+).

4-Methyl-2-nitro-1,1′-biphenyl (3bc)[14]

Pale orange oil (87.2 mg, 41%); 1H NMR (400 MHz, CDCl3): δ 7.67 (s, 1H), 7.44–7.36 (m, 4H), 7.34–7.28 (m, 3H), 2.47 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 149.2, 138.8, 137.5, 133.6, 133.1, 131.8, 128.7, 128.1, 128.0, 124.5, 21.0; GC–MS m/z: 213 (M+).

4-Nitro-1,1′-biphenyl (3bd)[3c]

Pale yellow solid (149.3 mg, 75%); 1H NMR (400 MHz, CDCl3): δ 8.30 (d, J = 8.7 Hz, 2H), 7.74 (d, J = 8.7 Hz, 2H), 7.63 (d, J = 7.2 Hz, 2H), 7.54–7.41 (m, 2H); 13C NMR (100 MHz, CDCl3): δ 147.7, 147.1, 138.8, 129.2, 129.0, 127.9, 127.5, 124.2; GC–MS m/z: 199 (M+).

3-Nitro-1,1′-biphenyl (3be)[13]

Yellow solid (121.2 mg, 61%): 1H NMR (400 MHz, CDCl3): δ 8.43 (s, 1H), 8.18 (d, J = 8.1 Hz, 1H), 7.90 (d, J = 7.7 Hz, 1H), 7.65–7.39 (m, 6H); 13C NMR (100 MHz, CDCl3): δ 148.7, 142.2, 138.6, 133.0, 129.7, 129.1, 128.5, 127.1, 122.0, 121.8; GC–MS m/z: 199 (M+).

2′,4,5′-Trimethyl-2-nitro-1,1′-biphenyl (3cc)

Pale yellow oil (101.3 mg, 42%); 1H NMR (400 MHz, CDCl3): δ 7.79 (s, 1H), 7.42 (d, J = 7.7 Hz, 1H), 7.20 (d, J = 7.7 Hz, 1H), 7.16–7.07 (m, 2H), 6.90 (s, 1H), 2.49 (s, 3H), 2.32 (s, 3H), 2.05 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 149.0, 138.6, 137.4, 135.2, 133.9, 133.4, 132.7, 132.1, 129.9, 129.1, 128.8, 124.4, 21.0, 19.5; HRMS (ESI) m/z: [M + H]+ calcd for C15H16NO2, 242.1103; found, 242.1106.

2′,4′,6′-Trimethyl-[1,1′-biphenyl]-2-carbonitrile (3da)[10]

White solid (103.7 mg, 47%); 1H NMR (400 MHz, CDCl3): δ 7.77 (d, J = 7.7 Hz, 1H), 7.65 (t, J = 7.4 Hz, 1H), 7.45 (t, J = 7.6 Hz, 1H), 7.29 (d, J = 7.7 Hz, 1H), 6.98 (s, 2H), 2.34 (s, 3H), 1.99 (s, 6H); 13C NMR (100 MHz, CDCl3): δ 146.6, 138.1, 135.7, 135.0,133.0, 132.9, 130.6, 128.5, 127.5, 118.0, 113.4, 21.2, 20.3; GC–MS m/z: 221 (M+).

2,4,6-Trimethyl-2′-nitro-1,1′-biphenyl (3db)[15]

Yellow oil (X = I, 168.6 mg, 70%; X = Br, 159.0 mg, 66%); 1H NMR (400 MHz, CDCl3): δ 8.01 (d, J = 8.1 Hz, 1H), 7.66 (t, J = 7.4 Hz, 1H), 7.52 (t, J = 7.4 Hz, 1H), 7.27–7.23 (m, 2H), 6.93 (s, 2H), 2.33 (s, 3H), 1.95 (s, 6H); 13C NMR (100 MHz, CDCl3): δ 149.4, 137.5, 135.9, 135.5, 134.2, 133.1, 132.1, 128.2, 124.2, 21.2, 20.5; GC–MS m/z: 241 (M+).

2,4,4′,6-Tetramethyl-2′-nitro-1,1′-biphenyl (3dc)

Waxy yellow oil (96.8 mg, 38%); 1H NMR (400 MHz, CDCl3): δ 7.81 (s, 1H), 7.45 (d, J = 7.7 Hz, 1H), 7.12 (d, J = 7.8 Hz, 1H), 6.92 (s, 2H), 2.49 (s, 3H), 2.32 (s, 3H), 1.94 (s, 6H); 13C NMR (100 MHz, CDCl3): δ 149.2, 138.6, 137.4, 135.7, 134.3, 133.9, 132.9, 128.2, 124.6, 21.2, 21.1, 20.5; HRMS (ESI) m/z: [M + H]+ calcd for C16H18NO2, 256.1259; found, 256.1263.

2-(Anthracen-9-yl)benzonitrile (3ea)[16]

White solid (206.3 mg, 74%); 1H NMR (400 MHz, CDCl3): δ 8.59 (s, 1H), 8.09 (d, J = 8.5 Hz, 2H), 7.94 (d, J = 7.7 Hz, 1H), 7.82 (t, J = 7.7 Hz, 1H), 7.66 (t, J = 7.5 Hz, 1H), 7.58–7.37 (m, 7H); 13C NMR (100 MHz, CDCl3): δ 143.2, 133.3, 132.8, 132.6, 132.0, 131.3, 130.2, 128.6, 128.48, 128.44, 126.4, 125.6, 125.4, 117.7, 115.2; GC–MS m/z: 279 (M+).

9-(2-Nitrophenyl)anthracene (3eb)[11]

Pale yellow solid (236.3 mg, 79%); 1H NMR (400 MHz, CDCl3): δ 8.54 (s, 1H), 8.27 (d, J = 7.8 Hz, 1H), 8.07 (d, J = 8.5 Hz, 2H), 7.80 (t, J = 7.4 Hz, 1H), 7.73 (t, J = 7.7 Hz, 1H), 7.53–7.33 (m, 7H); 13C NMR (100 MHz, CDCl3): δ 150.4, 134.1, 134.0, 133.1, 131.8, 131.3, 129.9, 129.2, 128.8, 127.6, 126.2, 125.5, 125.3, 124.7; GC–MS m/z: 299 (M+)

2-(Phenanthren-9-yl)benzonitrile (3fa)

White solid (203.5 mg, 73%); 1H NMR (400 MHz, CDCl3): δ 8.80 (d, J = 8.3 Hz, 1H), 8.75 (d, J = 8.1 Hz, 1H), 7.93 (d, J = 7.8 Hz, 1H), 7.87 (d, J = 7.8 Hz, 1H), 7.77–7.52 (m, 9H); 13C NMR (100 MHz, CDCl3): δ 144.6, 134.7, 133.3, 132.5, 131.7, 131.1, 130.7, 130.69, 130.66, 129.1, 128.7, 128.1, 127.5, 127.1, 127.0, 126.2, 123.2, 122.7, 118.1, 113.8; HRMS (ESI) m/z: [M + H]+ calcd for C21H14N, 280.1048; found, 280.1052.

9-(4-Nitrophenyl)phenanthrene (3fd)[17]

Pale yellow solid (212.3 mg, 71%); 1H NMR (400 MHz, CDCl3): δ 8.81 (d, J = 8.3 Hz, 1H), 8.75 (d, J = 8.3 Hz, 1H), 8.39 (d, J = 8.5 Hz, 2H), 7.93 (d, J = 7.7 Hz, 1H), 7.79 (d, J = 8.2 Hz, 1H), 7.77–7.63 (m, 6H), 7.58 (t, J = 7.6 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ 147.9, 136.5, 131.1, 130.8, 130.4, 130.2, 129.0, 128.2, 127.5, 127.3, 127.1, 127.0, 126.3, 125.1, 123.8, 123.3, 122.7; GC–MS m/z: 299 (M+)

Dibenzofuran (5af)[17]

White solid (109.1 mg, 65%); 1H NMR (400 MHz, CDCl3): δ 7.97 (d, J = 7.6 Hz, 2H), 7.59 (d, J = 8.2 Hz, 2H), 7.47 (t, J = 7.7 Hz, 2H), 7.36 (t, J = 7.4 Hz, 2H); 13C NMR (100 MHz, CDCl3): δ 156.2, 127.2, 124.3, 122.8, 120.7, 111.7; GC–MS m/z: 168 (M+).

2-Methyldibenzofuran (5bf)[18]

Waxy oil (65.5 mg, 36%); 1H NMR (400 MHz, CDCl3): δ 7.94 (d, J = 7.0 Hz, 1H), 7.76 (s, 1H), 7.57 (d, J = 8.2 Hz, 1H), 7.50–7.42 (m, 2H), 7.34 (t, J = 7.4 Hz, 1H), 7.30–7.23 (m, 1H), 2.53 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 156.5, 154.4, 132.4, 132.2, 128.3, 127.7, 127.0, 122.6, 120.7, 120.6, 111.7, 111.2, 21.4; GC–MS m/z: 182 (M+).

2-Nitrodibenzofuran (5cf)[19]

Pale yellow solid (153.2 mg, 72%); 1H NMR (400 MHz, CDCl3): δ 8.88 (d, J = 2.3 Hz, 1H), 8.44–8.38 (m, 1H), 8.04 (d, J = 7.7 Hz, 1H), 7.68–7.62 (m, 2H), 7.58 (t, J = 7.7 Hz, 1H), 7.46 (t, J = 7.5 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ 159.3, 157.6, 144.0, 129.1, 125.1, 124.0, 123.2, 123.1, 121.4, 117.3, 112.4, 112.1; GC–MS m/z: 213 (M+).

2-Chlorodibenzofuran (5df)[18]

White solid (90.8 mg, 45%); 1H NMR (400 MHz, CDCl3): δ 7.92–7.87 (m, 2H), 7.57 (d, J = 8.2 Hz, 1H), 7.52–7.46 (m, 2H), 7.43–7.33 (m, 2H); 13C NMR (100 MHz, CDCl3): δ 156.8, 154.5, 128.2, 127.9, 127.2, 125.7, 123.4, 123.1, 120.9, 120.5, 112.7, 111.9; GC–MS m/z: 202 (M+).

1,3-Dimethoxydibenzofuran (5ef)

White solid (118.4 mg, 52%); 1H NMR (400 MHz, CDCl3): δ 8.01 (d, J = 8.3 Hz, 1H), 7.49 (d, J = 7.4 Hz, 1H), 7.37–7.26 (m, 2H), 6.71 (s, 1H), 6.41 (s, 1H), 4.01 (s, 3H), 3.90 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 161.1, 158.3, 156.1, 155.6, 124.8, 123.9, 122.9, 121.8, 110.8, 107.3, 94.0, 88.5, 55.9, 55.8; HRMS (ESI) m/z: [M + H]+ calcd for C14H13O3, 229.0786; found, 229.0789.

Naphtho[1,2-b]benzofuran (5ff)[18]

White solid (137.4 mg, 63%); 1H NMR (400 MHz, CDCl3): δ 8.47 (d, J = 8.2 Hz, 1H), 8.06–7.95 (m, 3H), 7.80 (d, J = 8.5 Hz, 1H), 7.74 (d, J = 8.2 Hz, 1H), 7.67 (t, J = 7.5 Hz, 1H), 7.59 (t, J = 7.5 Hz, 1H), 7.50 (t, J = 7.7 Hz, 1H), 7.42 (t, J = 7.4 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ 156.0, 152.1, 133.1, 128.5, 126.5, 126.3, 126.1, 125.1, 123.4, 123.0, 121.4, 121.0, 120.4, 119.2, 118.6, 111.9; GC–MS m/z: 218 (M+).

Naphtho[2,1-b]benzofuran (5gf)[18]

White solid (152.7 mg, 70%); 1H NMR (400 MHz, CDCl3): δ 8.63 (d, J = 8.3 Hz, 1H), 8.40 (d, J = 8.4 Hz, 1H), 8.04 (d, J = 8.2 Hz, 1H), 7.93 (d, J = 8.9 Hz, 1H), 7.81–7.69 (m, 3H), 7.60–7.45 (m, 3H); 13C NMR (100 MHz, CDCl3): δ 155.9, 154.4, 130.5, 129.3, 129.1, 128.6, 127.2, 125.9, 125.0, 124.5, 123.5, 122.0, 117.4, 112.8, 112.0; GC–MS m/z: 218 (M+)..