Synthesis of Dibenzofurans from Cyclic Diaryliodonium Triflates and Water via Oxygen–Iodine Exchange Approach

Abstract

An efficient synthesis of a variety of dibenzofuran derivatives via Cu-catalyzed cyclization diaryliodonium salts in water is achieved. Various dibenzofuran derivatives could be obtained in good to excellent yields via this oxygen–iodine exchange approach. A concise synthesis of organic semiconducting material molecule has been achieved using this method.

Introduction

Dibenzofurans can be found in a wide range of natural products; they are used as pharmaceutical candidates¹ because of their unique properties, such as anticancer, antibacterial, antiallergy, antimalarial, and anti-inflammatory activities (Scheme 1).² In addition, fused dibenzofuran skeletons are considered to be potential photoelectronic materials in blue phosphorescent organic light-emitting diodes.³ As a result, many research groups devoted themselves to developing new approaches to construct dibenzofuran motifs,⁴ which can be divided into intermolecular and intramolecular categories: (A) intramolecular synthesis strategy: cyclization of diaryl ether derivatives,⁵ formation of C–O bonds using biaryls;⁶ (B) reaction of ortho-iodophenols with silylaryl triflates in the presence of CsF to afford the O-arylated products, which are subsequently cyclized using a Pd catalyst to dibenzofurans;⁷ reaction between 6-diazo-2-cyclohexenone and an ortho-haloiodobenzene through tandem Pd-catalyzed cross-coupling/aromatization followed by a Cu-catalyzed Ullmann coupling reaction.⁸ However, these strategies suffer from hash reaction conditions, tedious steps, limited substrate scope, and use of molar excess of catalyst.

Scheme 1. Selected Active 3-(2-Oxopropyl)-2-arylisoindolinone and the Synthetic Strategies.

Recently, cyclic diaryliodoniums have emerged as a new paradigm to synthesis of dibenzothiophenes,⁹ dibenzopyrroles,¹⁰ and dibenzoselenophens,¹¹ which avoids poor atom economy by generating 1 equiv of an iodoarene as waste. Considering our continuous interest in developing application of hypervalent iodine reagent in organic synthesis, we were focused mainly on construction of heterocyclic to exploit diverse diaryliodonium salt transformations.¹² Accordingly, intramolecular cyclizations of dibenziodolium triflates were established by our group for the synthesis of dibenzothiophenes¹³ and fluorenones.¹⁴

Results and Discussion

Inspired by the pioneering achievements, we postulated that the dibenzofurans can be prepared via double C–O bond formation. However, C–O bond formation has received much less attention, partly owing to the large energy gap between M–O HOMO and M–C LUMO frontier orbitals.¹⁵ From former investigation and proposal, diaryliodonium salts could be activated by Cu(I) species through oxidative addition.¹⁶ On the basis of the above work, we initiated our study by investigating the reaction of cyclic diaryliodonium 1a, a protocol based on CuI; K₂CO₃ in a mixture solvent (toluene/H₂O = 9:1) at 100 °C for 4 h provided the desired compound 2a in 13% yield (Table 1, entry 1). Encouraged by this promising result, catalysts were first investigated. Although various copper catalysts were tested, such as CuCl, CuBr, Cu(OAc)₂, Cu(OTf)₂, Cu₂O, and Cu(MeCN)₄PF₆, the yield could not be increased. Generally, the ligand backbone has a critical impact on the reactivity and selectivity of transition-metal catalytic reaction; therefore, ligand examination was carried out. Gratifyingly, the yield was remarkably improved when ligands were used; the reaction with L4 as ligand provided dibenzofuran 2a in 83% yield (Table 1, entry 11). Ligand screening indicated that generally monodentate ligand PPh₃ was not efficient and the bidentate nitrogen ligands were found well adapted to this reaction, such as L2, L3, L4, and L5. Next, the impact of base on the reaction was studied (entries 14–18); other inorganic bases, such as Cs₂CO₃, t-BuOK, NaOH, Na₂CO₃, led to lower yields; improved yield was not observed when organic base Et₃N was employed. A remarkable solvent effect was then observed during solvent investigation. To our delight, the product yield could be further improved to 92% in MeOH, whereas no reaction occurred in 1,4-dioxane and dimethylformamide (DMF). H₂O as the source of oxygen is essential for this reaction; we proceed the reaction in pure water in the presence of 5 mol % CuI, 10 mol % of L4, and 2 equiv of K₂CO₃ at 100 °C to yield the desired product 2a in only 28% yield at 4 h. However, the results were significantly improved when the reaction time prolonged to 12 and 24 h, as 2a was obtained in up to 96% yield.

Table 1. Optimization of the Reaction Conditions^a,b.

entry	catalyst	ligand	base	solvent	yield (%)
1	CuI		K2CO3	toluene	13
2	CuCl		K2CO3	toluene	1
3	CuBr		K2CO3	toluene	3
4	Cu(OAc)2·H2O		K2CO3	toluene	4
5	Cu(OTf)2		K2CO3	toluene	7
6	Cu2O		K2CO3	toluene	10
7	Cu(MeCN)4PF6		K2CO3	toluene	12
8	CuI	L1	K2CO3	toluene	5
9	CuI	L2	K2CO3	toluene	32
10	CuI	L3	K2CO3	toluene	50
11	CuI	L4	K2CO3	toluene	83
12	CuI	L5	K2CO3	toluene	7
13	CuI	L4	Cs2CO3	toluene	67
14	CuI	L4	t-BuOK	toluene	27
15	CuI	L4	NaOH	toluene	39
16	CuI	L4	Na2CO3	toluene	77
17	CuI	L4	Et3N	toluene	78
18	CuI	L4	K2CO3	DCE	8
19	CuI	L4	K2CO3	THF	24
20	CuI	L4	K2CO3	1,4-dioxane	0
21	CuI	L4	K2CO3	MeCN	37
22	CuI	L4	K2CO3	ether	75
23	CuI	L4	K2CO3	DMF	0
24	CuI	L4	K2CO3	MeOH	92
25	CuI	L4	K2CO3	DMSO	4
26c	CuI	L4	K2CO3	H2O	28
27d	CuI	L4	K2CO3	H2O	66
28e	CuI	L4	K2CO3	H2O	96

^aReaction conditions: 1 (1.0 equiv), catalyst (0.05 equiv), ligand (0.1 equiv), base (2 equiv) in {solvents: H₂O [9:1 (v/v), 2 mL]}, 100 °C, 4 h.

^bGas chromatography yields with n-decane as an internal standard.

^cReaction conditions: 1 (1.0 equiv), catalyst (0.05 equiv), ligand (0.1 equiv), base (2 equiv) in 2 mL of H₂O, 100 °C.

^d12 h.

^e24 h.

On the day of submission, we learned Huang and Wen’s group demonstrated a similar idea on the preparation of dibenzofurans.¹⁷ Having established the optimal conditions for the dibenzofuran synthesis, we intended to determine its scope and limitation. A series of reactions of cyclic diaryliodonium salts 1 and H₂O were carried out under the standard conditions. All reactions of cyclic diaryliodonium salts 1 bearing varied R¹ and R² groups proceeded smoothly to afford the corresponding dibenzofuran 2 in good to excellent yields (Table 2). The transformation was sensitive to the electronic nature of the diaryliodonium salt substituent; generally, reactions of substrates containing electron-donating substituents in 2 position led to higher yields than the reactions of those bearing electron-withdrawing groups (2b–d vs 2e, 2f). The yields were found to be a little lower for the substrates bearing halogen groups at the meta position on the cyclic diaryliodonium salts (2g–i); however, the C4-substituted halogen salts were more favorable than those at meta position (2j, 2k vs 2g, 2h). Subsequently, the C2-substituted groups were also investigated (2l), and the yield was influenced slightly unfavorably by steric effects. Notably, disubstituted diaryliodonium in different positions could be applied in this methodology and afforded the desired products in modest-to-good yields (2n–p). Likewise, disubstitution on different aromatic rings was also tested in the reaction and relatively normal yields were obtained (2q–s). In addition, when fused cyclic diaryliodonium salts were employed, desired polycyclic products were detected in modest-to-good yields.

Table 2. Cyclic Diaryliodonium Salts Scope^a,b.

^aReaction condition: 1 (0.1 mmol), CuI (0.05 equiv), L4 (0.1 equiv), K₂CO₃ (2.0 equiv), H₂O (2.0 mL), 100 °C, 24 h.

^bIsolated yield.

Prompted by these results, we sought to design an efficient synthetic route to various ladder-type heteroacenes¹⁸ that contain furan rings in the fused-ring framework. Benzo[1,2-b:4,5-b′]bisbenzofuran,¹⁹ an organic semiconducting material molecule, exhibiting an outstanding electron ability with both high charge carrier mobility and high stability, could be obtained in 72% yield from the double diaryliodonium salts 1v through the oxygen–iodine exchange reaction. Interestingly, when cyclic diaryliodonium salts 1w that contain thiophene rings were treated under the standard conditions, corresponding sulfone product 2w could be afforded in 50% yield, which might be a promising candidate for organic field-effect transistor materials (Scheme 2).

Scheme 2. Syntheses of Ladder-Type Heteroacenes.

Next, we decided to elucidate the reaction mechanism of the formation of dibenzofuran, from the reaction we know wherein the key was the source of oxygen atoms. Accordingly, some control experiments were designed under the standard conditions as shown in Figure 1. Treatment of cyclic diaryliodonium 1a with 5 mol % CuI and 10 mol % L4 in dry MeOH at 100 °C failed to deliver possible dibenzofuran 2a, suggesting that oxygen atoms might not be derived from air. To further explore the source of oxygen atoms, the isotope labeled water H₂¹⁸O was employed in the reaction and ¹⁸O labeled product 2a′ was isolated in 81% yield. These results indicated that the oxygen atom was derived from H₂O.

Figure 1.

Control experiments.

On the basis of the control experiment results and relevant literature, we proposed the mechanism of this cyclization reaction. A reasonable mechanism suggested that an oxygen–iodine exchange process is involved, as shown in Figure 2. Initially, the oxidative addition of cyclic diaryliodonium salts 1 with CuI and ligand L afforded intermediate A, containing intramolecular iodobenzene. Subsequently, ligand exchange of A with H₂O formed hydroiodic acid and complex B. Next, reductive elimination of intermediate B took place to get 2-[2′-iodophenyl]phenol C and Cu(I) intermediate; ligand L was released out simultaneously. Then, oxidative addition of 2-[2′-iodophenyl]phenol C to Cu(I) would provide a new electrophilic Cu(III) intermediate, which can be attacked by hydroxyl under basic conditions to form intermediate D. Finally, reductive elimination of D gave the desired product 2 and regenerated the catalyst.

Figure 2.

Proposed reaction mechanism.

Conclusions

In summary, a Cu-catalyzed cyclization of cyclic diaryliodonium salts in water has been developed via double C–O bond formation. The developed oxygen–iodine exchange methodology provides straightforward access to dibenzofuran derivatives in up to 96% yield. Applying this strategy, a concise synthesis of organic semiconducting material molecule has been accomplished in moderate yields. Further studies to explore the diaryliodonium salts are currently underway in our laboratory.

Experimental Section

General Information

All of the reactions was carried out under an air atmosphere condition. Various reagents were purchased from Aldrich, Acros, or Alfa. For column chromatography, 200–300 mesh silica gel was used. ¹H, ¹³C and ¹⁹F NMR were recorded on Bruker 400 or 500 MHz spectrometer in CDCl₃. High resolution mass spectrometry (HRMS) was performed on an Agilent 6540 Q-TOF mass spectrometer (ESI). Melting points were determined on a SGW X-4B melting point apparatus. Cyclic diaryliodoniums are prepared according to the literature.⁸

General Procedure for Preparation of Compound 2

In the pressure tube, a solution of diaryliodonium salts 1 (0.1 mmol), K₂CO₃ (0.2 mmol), CuI (0.05 mmol), and ligand L4 (0.1 equiv) in H₂O (2.0 mL) was stirred at 100 °C for 24 h. After completion of the reaction (observed on thin-layer chromatography), the solvent was evaporated under reduced pressure to obtain the crude mixture. The residues were purified by silica-gel column chromatography (ethyl acetate/petroleum ether = 1/50:1/10) to afford the pure product 2. The obtained product was analyzed by ¹H NMR, ¹³C NMR, and ¹⁹F NMR.

Dibenzo[b,d]furan (2a)

White solid, 96% yield. ¹H NMR (400 MHz, CDCl₃) δ 7.98 (d, J = 7.7 Hz, 2H), 7.59 (d, J = 8.1 Hz, 2H), 7.48 (t, J = 7.6 Hz, 2H), 7.37 (t, J = 7.5 Hz, 2H). See: J. Am. Chem. Soc. 2009, 131, 4194–4195.

3-Methyldibenzo[b,d]furan (2b)

White solid, 92% yield. ¹H NMR (400 MHz, CDCl₃) δ 7.95–7.91 (m, 1H), 7.84 (d, J = 7.9 Hz, 1H), 7.57 (d, J = 8.2 Hz, 1H), 7.44 (dt, J = 7.4, 1.3 Hz, 1H), 7.34 (dt, J = 7.6, 0.9 Hz, 1H), 7.19 (d, J = 7.8 Hz, 1H), 2.55 (s, 3H). See: J. Am. Chem. Soc. 2009, 131, 4194–4195.

3-Methoxydibenzo[b,d]furan (2c)

White solid, 87% yield. ¹H NMR (400 MHz, CDCl₃) δ 7.88–7.86 (m, 1H), 7.83 (d, J = 8.5 Hz, 1H), 7.54 (d, J = 8.0 Hz, 1H), 7.37 (dt, J = 8.0, 1.3 Hz, 1H), 7.34 (dt, J = 7.5, 0.9 Hz, 1H), 7.11 (d, J = 2.2 Hz, 1H), 6.96 (dd, J = 8.5, 2.3 Hz, 1H), 3.92 (s, 3H). See: J. Am. Chem. Soc. 2011, 133, 9250–9253.

3-Ethyldibenzo[b,d]furan (2d)

White solid, 89% yield. Mp: 82–84 °C. ¹H NMR (400 MHz, CDCl₃) δ 7.93 (d, J = 7.6 Hz, 1H), 7.87 (d, J = 7.8 Hz, 1H), 7.57 (d, J = 8.2 Hz, 1H), 7.46–7.43 (m, 2H), 7.34 (t, J = 7.6 Hz, 1H), 7.21 (d, J = 8.0 Hz, 1H), 2.84 (q, J = 7.6 Hz, 2H), 1.35 (t, J = 7.6 Hz, 3H). ¹³C NMR (125 MHz, CDCl₃) δ 156.7, 156.3, 144.3, 126.5, 124.4, 122.9, 122.6, 121.8, 120.3, 120.2, 111.6, 110.7, 29.3, 15.9. HRMS (ESI) calcd for C₁₄H₁₃O ([M + H]⁺): 197.0961; found 197.0958.

Ethyl Dibenzo[b,d]furan-3-carboxylate (2e)

White solid, 68% yield. ¹H NMR (400 MHz, CDCl₃) δ 8.27 (s, 1H), 8.10–8.18 (m, 1H), 8.03–7.99 (m, 2H), 7.63 (d, J = 8.2 Hz, 1H), 7.55 (t, J = 7.5 Hz, 1H), 7.40 (t, J = 7.5 Hz, 1H), 4.46 (q, J = 7.1 Hz, 2H), 1.47 (t, J = 7.1 Hz, 3H). See: Org. Lett. 2011, 13, 5504–5507.

3-(Trifluoromethyl)dibenzo[b,d]furan (2f)

White solid, 59% yield. ¹H NMR (400 MHz, CDCl₃) δ 8.07 (d, J = 8.1 Hz, 1H), 8.03 (d, J = 7.8 Hz, 1H), 7.86 (s, 1H), 7.65–7.63 (m, 2H), 7.56 (t, J = 8.0 Hz, 1H), 7.43 (t, J = 7.4 Hz, 1H). See: J. Am. Chem. Soc. 2011, 133, 9250–9253.

3-Fluorodibenzo[b,d]furan (2g)

White solid, 70% yield. ¹H NMR (400 MHz, CDCl₃) δ 7.93–7.87 (m, 2H), 7.58 (d, J = 8.2 Hz, 1H), 7.45 (dt, J = 7.4, 1.3 Hz, 1H), 7.37 (dt, J = 7.6, 0.9 Hz, 1H), 7.31–7.28 (m, 1H), 7.11 (dt, J = 9.2, 2.3 Hz, 1H). See: J. Am. Chem. Soc. 2009, 131, 4194–4195.

3-Chlorodibenzo[b,d]furan (2h)

White solid, 69% yield. ¹H NMR (400 MHz, CDCl₃) δ 7.93 (d, J = 7.7 Hz, 1H), 7.87 (d, J = 8.2 Hz, 1H), 7.59–7.57 (m, 2H), 7.49 (t, J = 7.2 Hz, 1H), 7.39–7.33 (m, 2H). See: J. Am. Chem. Soc. 2011, 133, 9250–9253.

3-Bromodibenzo[b,d]furan (2i)

White solid, 75% yield. ¹H NMR (400 MHz, CDCl₃) δ 7.94 (d, J = 7.6 Hz, 1H), 7.83 (d, J = 8.2 Hz, 1H), 7.76 (d, J = 1.5 Hz, 1H), 7.59–7.57 (m, 1H), 7.50 (dt, J = 7.4, 1.1 Hz, 2H), 7.38 (t, J = 7.6 Hz, 1H). See: Org. Lett. 2011, 13, 5504–5507.

2-Fluorodibenzo[b,d]furan (2j)

White solid, 80% yield. ¹H NMR (400 MHz, CDCl₃) δ 7.93 (d, J = 7.4 Hz, 1H), 7.63 (dd, J = 8.1, 2.6 Hz, 1H), 7.59–7.57 (m, 1H), 7.53–7.50 (m, 2H), 7.37 (t, J = 7.8 Hz, 1H), 7.19 (dt, J = 9.0, 2.7 Hz, 1H). See: J. Am. Chem. Soc. 2009, 131, 4194–4195.

2-Chlorodibenzo[b,d]furan (2k)

White solid, 73% yield. ¹H NMR (400 MHz, CDCl₃) δ 8.00–7.89 (m, 2H), 7.59 (d, J = 8.3 Hz, 1H), 7.51 (dd, J = 7.9, 4.9 Hz, 2H), 7.46–7.34 (m, 2H). See: Org. Lett. 2011, 13, 5504–5507.

4-Methyldibenzo[b,d]furan (2l)

White solid, 85% yield. ¹H NMR (400 MHz, CDCl₃) δ 7.94 (d, J = 7.6 Hz, 1H), 7.77 (s, 1H), 7.57 (d, J = 8.2 Hz, 1H), 7.46 (t, J = 8.6 Hz, 2H), 7.34 (t, J = 7.5 Hz, 1H), 7.29–7.28 (m, 1H), 2.54 (s, 3H). See: Org. Lett. 2012, 14, 4838–4841.

4-Propyldibenzo[b,d]furan (2m)

Colorless oil, 84% yield. ¹H NMR (400 MHz, CDCl₃) δ 7.94 (d, J = 7.6 Hz, 1H), 7.86 (d, J = 7.8 Hz, 1H), 7.57 (d, J = 8.2 Hz, 1H), 7.46–7.41 (m, 2H), 7.34 (t, J = 7.5 Hz, 1H), 7.19 (d, J = 7.8 Hz, 1H), 2.78 (t, J = 7.4 Hz, 2H), 1.80–1.72 (m, 2H), 1.00 (t, J = 7.3 Hz, 3H). ¹³C NMR (125 MHz, CDCl₃) δ 156.6, 156.2, 142.7, 126.6, 124.4, 123.5, 122.6, 121.9, 120.4, 120.2, 111.6, 111.4, 38.5, 24.9, 13.9. HRMS (ESI) calcd for C₁₅H₁₅O ([M + H]⁺): 211.1117; found 211.1112.

2,4-Dimethyldibenzo[b,d]furan (2n)

White solid, 76% yield. ¹H NMR (400 MHz, CDCl₃) δ 7.94 (d, J = 7.6 Hz, 1H), 7.62–7.60 (m, 2H), 7.49–7.45 (m, 1H), 7.36 (t, J = 7.4 Hz, 1H), 7.12 (s, 1H), 2.61 (s, 3H), 2.52 (s, 3H). See: Org. Lett. 2013, 15, 2754–2757.

3,4-Dimethoxydibenzo[b,d]furan (2o)

White solid, 82% yield. ¹H NMR (400 MHz, CDCl₃) δ 7.85 (d, J = 7.5 Hz, 1H), 7.54 (d, J = 8.1 Hz, 1H), 7.38 (t, J = 6.3 Hz, 2H), 7.34–7.30 (m, 1H), 7.14 (s, 1H), 4.01 (s, 3H), 3.99 (s, 3H). See: J. Org. Chem. 2006, 71, 9104–9113.

Benzo[b][1,3]dioxolo[4,5-f]benzofuran (2p)

White solid, 69% yield. ¹H NMR (400 MHz, CDCl₃) δ 7.82 (d, J = 7.4 Hz, 1H), 7.52 (d, J = 8.1 Hz, 1H), 7.39–7.35 (m, 1H), 7.33–7.29 (m, 2H), 7.08 (s, 1H), 6.07 (s, 2H). See: Aust. J. Chem. 2013, 66, 1334–1341.

2-Fluoro-7-methyldibenzo[b,d]furan (2q)

White solid, 80% yield. Mp: 75–76 °C. ¹H NMR (400 MHz, CDCl₃) δ 7.79 (d, J = 7.9 Hz, 1H), 7.57 (dd, J = 8.2, 2.6 Hz, 1H), 7.48 (dd, J = 8.9, 4.1 Hz, 1H), 7.38 (s, 1H), 7.21–7.11 (m, 2H), 2.55 (s, 3H). ¹³C NMR (125 MHz, CDCl₃) δ 158.9 (d, J = 237.4 Hz), 157.6, 152.2, 138.5, 125.3 (d, J = 10.2 Hz), 124.1, 121.4, 120.4, 113.7 (d, J = 25.7 Hz), 112.2, 106.4 (d, J = 25.1 Hz), 22.0. ¹⁹F NMR (376 MHz, CDCl₃) δ −120.8. HRMS (ESI) calcd for C₁₃H₁₀FO ([M + H]⁺): 201.0710 found 201.0705.

2-Fluoro-7-methoxydibenzo[b,d]furan (2r)

White solid, 85% yield. ¹H NMR (400 MHz, CDCl₃) δ 7.98 (d, J = 8.6 Hz, 1H), 7.52 (dd, J = 8.3, 2.6 Hz, 1H), 7.46 (dd, J = 8.9, 4.1 Hz, 1H), 7.08 (dt, J = 11.4, 2.7 Hz, 2H), 6.95 (dd, J = 8.6, 2.2 Hz, 1H), 3.92 (s, 3H). See: J. Org. Chem. 2018, 83, 805–811.

7-Chloro-2-fluorodibenzo[b,d]furan (2s)

White solid, 55% yield. Mp: 77–79 °C. ¹H NMR (400 MHz, CDCl₃) δ 7.83 (d, J = 8.3 Hz, 1H), 7.60–7.57 (m, 2H), 7.52 (dd, J = 8.9, 4.0 Hz, 1H), 7.35 (dd, J = 8.3, 1.7 Hz, 1H), 7.20 (dt, J = 8.9, 1.9 Hz, 1H). ¹³C NMR (125 MHz, CDCl₃) δ 159.3 (d, J = 238.6 Hz), 152.6, 133.5, 124.5 (d, J = 10.3 Hz), 123.6, 122.8, 121.5, 114.9 (d, J = 25.8 Hz), 112.6, 106.8 (d, J = 25.1 Hz). ¹⁹F NMR (376 MHz, CDCl₃) δ −119.7. HRMS (ESI) calcd for C₁₂H₇ClFO ([M + H]⁺): 221.0164 found 221.0160.

Naphtho[2,3-b]benzofuran (2t)

White solid, 81% yield. ¹H NMR (400 MHz, CDCl₃) δ 8.48 (d, J = 8.1 Hz, 1H), 8.04–8.01 (m, 3H), 7.81 (d, J = 8.5 Hz, 1H), 7.74 (d, J = 8.2 Hz, 1H), 7.67 (t, J = 7.1 Hz, 1H), 7.59 (t, J = 7.1 Hz, 1H), 7.51 (t, J = 7.2 Hz, 1H), 7.43 (t, J = 7.3 Hz, 1H). See: Tetrahedron2005, 61, 1353–1362.

Phenanthro[9,10-b]benzofuran (2u)

White solid, 73% yield. ¹H NMR (400 MHz, CDCl₃) δ 8.83–8.76 (m, 2H), 8.68 (d, J = 7.7 Hz, 1H), 8.56–8.52 (m, 1H), 8.43–8.40 (m, 1H), 7.82–7.84 (m, 4H), 7.72–7.70 (m, 1H), 7.56–7.50 (m, 2H). See: J. Org. Chem. 2017, 82, 9133–9143.

Benzo[1,2-b:4,5-b′]bisbenzofuran (2v)

White solid, 67% yield. ¹H NMR (400 MHz, CDCl₃) δ 8.37 (d, J = 7.4 Hz, 1H), 8.04–8.01 (m, 2H), 7.74 (d, J = 8.1 Hz, 1H), 7.67 (d, J = 8.0 Hz, 1H), 7.62 (d, J = 8.4 Hz, 1H), 7.54–7.50 (m, 3H), 7.42 (t, J = 7.4 Hz, 1H). See: J. Am. Chem. Soc. 2011, 133, 9250–9253.

Benzo[b]benzo[4,5]thieno[3,2-g]benzofuran 12,12-Dioxide (2w)

White solid, 50% yield. Mp: 172–174 °C. ¹H NMR (400 MHz, CDCl₃) δ 8.47 (d, J = 7.5 Hz, 1H), 7.91 (d, J = 7.5 Hz, 1H), 7.86–7.82 (m, 2H), 7.81–7.79 (m, 1H), 7.69–7.62 (m, 3H), 7.57–7.49 (m, 2H). ¹³C NMR (125 MHz, CDCl₃) δ 157.4, 137.7, 134.1, 132.5, 130.2, 129.7, 129.6, 126.8, 124.5, 124.2, 122.4, 121.3, 120.6, 120.4, 119.4, 116.6, 111.9. HRMS (ESI) calcd for C₁₈H₁₁O₃S ([M + H]⁺): 307.0423 found 307.0418.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02345.

• Spectral data for all new compounds (PDF)

The authors declare no competing financial interest.