Synthesis of Molecular Nanohoops Bearing a Tetrahydro[6

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Synthesis of Molecular Nanohoops Bearing a Tetrahydro[6]cycloparaphenylene Fused to a Hydrogenated or a Bithiophene-Inserted Cycloparaphenylene Behzad Farajidizaji, Haresh Thakellapalli, Novruz G Akhmedov, and Kung K. Wang J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b02694 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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The Journal of Organic Chemistry

Synthesis of Molecular Nanohoops Bearing a Tetrahydro[6]cycloparaphenylene Fused to a Hydrogenated or a Bithiophene-Inserted Cycloparaphenylene Behzad Farajidizaji, Haresh Thakellapalli, Novruz G. Akhmedov, and Kung K. Wang* C. Eugene Bennett Department of Chemistry, West Virginia University, Morgantown, West Virginia, 26506-6045, United States Supporting Information Placeholder

ABSTRACT: A synthetic pathway to a molecule bearing two molecular nanohoops, including a tetrahydro[6]cycloparaphenylene (4H[6]CPP) fused through two 1,4-dimethoxybenzene units to a 4H[10]CPP, has been developed. Similarly, a molecule containing a 4H[6]CPP fused through two 1,4-dimethoxybenzene units to a molecular nanohoop bearing a [6]CPP inserted with two 2,2′-bithiophene-5,5′-diyl groups has been synthesized. The Diels‒Alder reactions of two (E,E)-1,4-diaryl-1,3-butadienes with 1,4-benzoquinone and the Ni-mediated homocoupling reactions are the key steps for the construction of macrocyclic ring structures. Oxidative aromatization with DDQ converted a hydrogenated system to a fully aromatized nanohoop with 10 aromatic units including a [6]CPP inserted with two 2,2′-bithiophene-5,5′-diyl groups. The UV‒vis and fluorescence spectra of the fused two-hoop systems were investigated.

INTRODUCTION We recently reported synthetic pathways to cycloparaphenylenes (CPPs) and related carbon nanohoops1 with paraconnected benzene units as the shortest repeating macrocyclic segments of armchair carbon nanotubes. 2 Specifically, the Diels‒Alder reaction between (E,E)-1,4-bis(4-bromophenyl)1,3-butadiene (1) and 1,4-benzoquinone (2) in the presence of BF3.OEt2 produced 3, which upon being treated with silver oxide gave 1,4-benzoquinone 4 in 80% isolated yield over two steps (Scheme 1).1a The presence of a 1,4-benzoquinone moiety in 4 allowed a second Diels‒Alder reaction with 1 to produce, after methylation, 5 with all four 4-bromophenyl groups cis to one another as the major isomer in 68% isolated yield. The isomer with the two sets of 4-bromophenyl groups trans to each other is the minor product (9%). The cis relationship among the four 4-bromophenyl groups in 5 allowed the use of Ni(cod)2mediated (cod being 1,5-cyclooctadiene) homocoupling reactions to form the two corresponding macrocyclic dimers of 5 and three of the four possible macrocyclic trimers. These macrocycles contain partially hydrogenated CPPs with bent and fused structures bearing armchair carbon nanotube-like connections. Synthetic pathways involving the use of a 1,3-butadiene

other than 1 for the Diels‒Alder reaction with 4 leading specifically to one of the two macrocyclic dimers and one of the four macrocyclic trimers were also reported. We now have extended this strategy to the synthesis of molecular nanohoops bearing two fused macrocyclic rings with different ring sizes by using two different 1,3-butadienes for the Diels‒Alder reactions with 2. Macrocycle 6 bearing a tetrahydro[6]CPP (4H[6]CPP) fused to a 4H[10]CPP and macrocycle 7 bearing a 4H[6]CPP fused to a molecular nanohoop containing a [6]CPP inserted with two 2,2′-bithiophene-5,5′-diyl groups have thus been constructed (Figure 1).

RESULTS AND DISCUSSION The synthetic pathway leading to 6 involved the preparation of 1,3-butadiene 9 by the Suzuki‒Miyaura coupling reactions between 1 and 83 (Scheme 2). The Diels‒Alder reaction between 4 and 9 in the presence of AlCl3 then produced, after methylation, 10 with all four aryl substituents cis to one another in 71% isolated yield. The isomer with the trans relationship between the two sets of the aryl groups was not isolated, and the 1 H NMR spectrum of the crude reaction products indicated that the trans isomer was formed at most in a small amount. Treatment of 10 at 1 mM concentration in THF with Ni(cod)2 in the presence of 2,2′-bipyridyl (bpy)4 promoted the homocoupling

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reactions to produce cyclic dimers 11 in 16% isolated yield and 12 in 17% isolated yield. The lack of diastereoselectivity during the first Ni(cod)2-mediated homocoupling reaction was responsibility for producing essentially a 1:1 mixture of 11 and 12. The cis relationship among the four 4-[4-(trimethylsilyl)phenyl]phenyl groups in 11 was confirmed by treatment of 11 with ICl at ‒78 °C to form the corresponding tetraiodide 13,1a,5 which upon being exposed to Ni(cod)2 in the presence of bpy produced macrocycle 6 bearing two fused molecular nanohoops. Transformation of 12 to the corresponding tetraiodide 14 followed by treatment with Ni(cod)2 in the presence of bpy failed to produce 6, indicating that 12 had the two sets of 4-[4-(trimethylsilyl)phenyl]phenyl groups trans to each other. Scheme 1. Diels‒Alder Reactions of 1,3-Butadiene 1 with 1,4-Benzoquinone (2) to Form 5

Figure 1. Molecular structures of 6 and 7.

Scheme 2. Synthesis of Macrocycle 6

The DFT-optimized structure of 6 shows that the 4H[6]CPP and the 4H[10]CPP units have oval-shaped structures with more severe bending of the two quaterphenyl groups in the 4H[10]CPP unit (Figure 2). The dihedral angles between the benzene rings of the biphenyl groups in the 4H[6]CPP unit are 40° on average and between those of the two quaterphenyl groups range from 33° to 47°. The dihedral angle between the benzene rings of the biphenyl groups in the 4H[6]CPP unit appears to vary with the structural features of the molecules, ranging from approximately 5° in a crystal structure of a fused twohoop molecule containing two 4H[6]CPP units1a to 49° in a crystal structure of a one-hoop 4H[6]CPP system.1b Attempts to grow a single crystal of 6 from several solvent systems for direct comparison with those of the earlier cases were unsuccessful. The rates of rotation of the two biphenyl groups in the 4H[6]CPP unit and the two quaterphenyl groups in the 4H[10]CPP unit are relatively slow on the 1H NMR time scale with coalescence of the 1H NMR signals of the biphenyl groups occurring at approximately 30 °C (Figure 3) and of the quaterphenyl groups at approximately ‒30 °C (Figure S1). At −5 °C, four doublets from the aromatic hydrogens on the 4H[6]CPP unit can be clearly discerned. The two more upfield-shifted doublets are assigned to H17 and H18, respectively, which point in the general direction toward the 4H[10]CPP unit into the magnetically shielded regions of the 4H[10]CPP unit and the 1,4-


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The Journal of Organic Chemistry dimethoxybenzene groups. The rotational barrier of the biphenyl groups was determined by variable temperature NMR studies with ΔH‡ = 18.7 kcal/mol, ΔS‡ = 9.9 cal/mol.K, and ΔG‡ = 16.0 kcal/mol at 25 °C (Figures S3 and S4).

6, aerial view

7 18

6 17

A molecular nanohoop containing a 4H[6]CPP inserted with two 2,2′-bithiophene-5,5′-diyl units was reported to undergo oxidative aromatization with DDQ at 75 °C for 2 h to form the corresponding fully aromatized macrocyclic system bearing ten aromatic units.7 The presence of two 2,2′-bithiophene-5,5′-diyl units with bent orientations of four 2,5-disubstituted thiophenes was most likely responsible for reducing the strain energy in the fully aromatized macrocyclic system, allowing a milder condition for oxidative aromatization. The possibility of a milder condition for oxidative aromatization prompted us to develop a synthetic pathway for 7. The synthetic sequence outlined in Scheme 3 illustrates the preparation of 18 as a key intermediate leading toward 7. Diiodide 15, obtained from 1 by bromine– iodine exchange,8 was treated with 2 for the Diels‒Alder reaction leading to 1,4-hydroquinone 16. Oxidation of 16 with Ag2O then gave 1,4-benzoquinone 17. A second Diels‒Alder reaction with 1 followed by methylation then afforded 18 in 55% isolated yield. Scheme 3. Diels‒Alder Reactions of 1,3-Butadienes 15 and 1 with 1,4-Benzoquinone (2) to Form 18

2 18' 17' 6, perspective view

1

Figure 2. DFT-optimized structure of 6 with carbons (black and gray), hydrogens (white), and oxygens (red). 18/18' 7

1

6

17/17'

2

temp/oC +62o +50o +40o +30o +25o +20o +15o +5o

17'

18'

18

0o 17

-5o 6.5

6.0

5.5

5.0

Figure 3. Temperature-dependent 600 MHz 1H NMR spectra of the 4H[6]CPP unit in 6 in CDCl3.

It was previously reported that attempts to promote oxidative aromatization of a one-hoop system containing a 4H[6]CPP unit with DDQ at temperatures ranging from 25 to 150 °C were unsuccessful.1b On the other hand, oxidative aromatization of a one-hoop system containing a 4H[10]CPP unit with DDQ at 150 °C for 2 h was successful in producing a fully aromatized [10]CPP derivative in 71% isolated yield.6 However, heating 6 with DDQ at 120 °C resulted in decomposition.

The different reactivities of the iodophenyl and bromophenyl groups in 18 toward boronic ester 199 were exploited for selective Suzuki‒Miyaura coupling reactions to give 20 (Scheme 4). The Ni-mediated homocoupling reactions of 20 gave a 1:1 mixture of 21 and 22, which were not separated, in 28% combined yield. Treatment of the mixture of 21 and 22 with N-iodosuccinimide (NIS)7,10 then gave 23 and 24, which were separated by silica gel column chromatography, with each in 43% isolated yield (combined yield = 86%). The Ni-mediated homocoupling reactions of 23 gave macrocycle 25 in 52% isolated yield, whereas 24 failed to produce 25, supporting the assignment of all cis relationship among the four aryl substituents in 23. It was gratifying to observe that upon being subjected to oxidative aromatization with DDQ at 90 °C for 6 h, 25 was transformed to 7 in 83% isolated yield. The dihedral angles between the benzene rings of the two biphenyl groups in the DFT-optimized structure of 7 are 15° and 24° (Figure 4). In addition, the sulfur atoms of the four thiophene groups point in the same general direction toward the 4H[6]CPP unit. The dihedral angles between the thiophene rings of the two bithiophenes are 34° and 39°. This is similar to the DFT-optimized structure of a one-hoop macrocycle bearing a [6]CPP inserted with two 2,2′-bithiophene-5,5′-diyl units with all four sulfur atoms pointing in the same general direction and a dihedral angle of 24°.7 Restricted rotations of the biphenyl


The Journal of Organic Chemistry groups of the 4H[6]CPP units in 7 and 25 were also observed by 1H NMR spectroscopy. Scheme 4. Synthesis of Macrocycle 7

7, aerial view

7, perspective view

Figure 4. DFT-optimized structure of 7 with carbons (black and gray), hydrogens (white), oxygens (red), and sulfurs (yellow).

Normalized Intensity

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250 The UV‒vis spectrum of 7 exhibited absorption maxima (λabs) at 259 and 384 nm (Figure 5). For the hydrogenated precursor 25, λabs occurred at 257, 290, and 374 nm. The longest wavelength of the λabs was red-shifted from 25 to 7 as the bithiophene-containing macrocyclic unit became fully aromatized. In comparison, the absorption maximum of 5,5′-diphenyl-2,2′-bithiophene occurred at 374 nm.11 The one-hoop macrocycle with a structure similar to the bithiophene-containing hoop-like structure in 7 showed the absorption maximum at 376 nm.7 The fluorescence maximum (λem) of 7 was observed at 551 nm, which was significantly red-shifted from those of 25 at 453 and 474 nm and that of the corresponding one-hoop system at 511 nm.7 Other thiophene-containing CPPs had also been synthesized to allow the investigation of their photophysical properties.12

350

450 550 Wavelength/nm

650

750

Figure 5. UV‒vis (solid lines) and fluorescence (dashed lines) spectra of 7 (blue) and 25 (red).

CONCLUSIONS In summary, synthetic pathways to 6 and 7 bearing two fused molecular nanohoops have been developed. The use of the Diels‒Alder reactions to place all four aryl substituents in 10 and 18 cis to one another is crucial to the success of the synthetic pathways. The homocoupling reactions of 13 and 23 are intramolecular processes, which are relatively efficient in producing the macrocyclic nanohoops. Restricted rotations of the benzene rings in 6 and 7 were observed by temperature-dependent NMR studies. DFT-optimized structure of 7 indicates that the sulfur atoms of the four thiophene groups all point in the


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The Journal of Organic Chemistry same general direction toward the 4H[6]CPP unit. The UV‒vis and fluorescence spectra of 7 are red-shifted from those of the hydrogenated precursor 25 and the corresponding one-hoop macrocycle.

EXPERIMENTAL SECTION General Experimental Methods. All reactions were conducted in oven-dried (120 °C) glassware. Chemicals, including 1,4-benzoquinone (2), silver(I) oxide (Ag2O), boron trifluoride diethyl etherate (BF3.OEt2), tetrakis(triphenylphosphine)palladium [Pd(PPh3)4], bis(1,5-cyclooctadiene)nickel [Ni(cod)2], 2,2′-bipyridyl (bpy), N-iodosuccinimide (NIS), anhydrous aluminium chloride (AlCl3), iodine monochloride (ICl), 4,4,5,5tetramethyl-2-(2-thienyl)-1,3,2-dioxaborolane (19),9 N,Nʹ-dimethylethylenediamine, and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), were purchased from chemical suppliers and were used as received. (E,E)-1,4-Bis(4-bromophenyl)-1,3-butadiene (1),13 dibromide 4,1a and 4,4,5,5-tetramethyl-2-[4-(trimethylsilyl)phenyl]-1,3,2-dioxaborolane (8)3 were prepared according to reported procedures. UV‒vis absorption spectra were recorded on a spectrophotometer with a 1 nm resolution, and the baseline was corrected with a solvent filled square quartz cell. Fluorescence spectra were recorded on a spectrofluorophotometer with a 2 nm resolution. Infrared (IR) spectra of solid samples were recorded on a Fourier transform infrared system equipped with a diamond crystal attenuated total reflectance sampling interface. HRMS spectra were obtained on an FT-ICR or an Orbitrap mass analyzer coupled with electrospray ionization (ESI). Preparation of 9. To a two-neck 250 mL flask containing (E,E)-1,4-bis(4-bromophenyl)-1,3-butadiene (1, 1.82 g, 5.00 mmol), 4,4,5,5-tetramethyl-2-[4-(trimethylsilyl)phenyl]-1,3,2dioxaborolane (8, 4.14 g, 15.0 mmol), and sodium carbonate (2.65 g, 25.0 mmol) and fitted with a condenser and a rubber septum were added toluene (150 mL), ethanol (30 mL), and water (20 mL). The solution was flushed with nitrogen for 10 min, and then Pd(PPh3)4 (0.578 g, 0.500 mmol) was added. The solution was again flushed with nitrogen for 20 min and then heated at reflux for 12 h before it was cooled to rt. The solvent was removed in vacuo, and the residue was filtered and then washed with water (2 × 100 mL) and ethanol (3 × 100 mL) to produce 9 (2.24 g, 4.45 mmol, 89% yield) as a pale yellow solid: mp 290–292 °C; IR 1253, 991, 838, 826, 800 cm‒1; 1H NMR (CDCl3, 400 MHz) δ 7.61 (8 H, s), 7.60 (4 H, d, J = 8.6 Hz), 7.53 (4 H, J = 8.6 Hz), 7.03 (2 H, m), 6.73 (2 H, m), 0.31 (18 H, s); 13C NMR (CDCl3, 100 MHz) δ 141.0, 140.2, 139.4, 136.5, 133.8, 132.4, 129.4, 127.3, 126.8, 126.2, –1.1; HRMS (ESI/FTICR) m/z [M]+ calcd for C34H38Si2 502.2507; found 502.2530. Preparation of 10. To a mixture of dibromide 4 (0.653 g, 1.39 mmol), 1,3-butadiene 9 (0.790 g, 1.57 mmol), and AlCl3 (0.025 g, 0.187 mmol) in a 100 mL flask under a nitrogen atmosphere was added via cannula anhydrous dichloromethane (50 mL). The reaction mixture was stirred at rt for 24 h before it was passed through a short silica gel column (5 cm high, 3 cm in diameter), and the column was further eluted with dichloromethane (2 × 100 mL). The combined eluates were concentrated to afford a yellow solid. The crude solid was used without further purification. To the yellow solid and potassium carbonate (0.553 g, 4.00 mmol) in dry acetone (50 mL) was added dimethyl sulfate (0.40 mL, 4.2 mmol) via a syringe, and the reaction mixture was heated at reflux for 12 h before it was cooled to rt. The reaction mixture was then passed through a short silica

gel column (5 cm high, 3 cm in diameter), and the column was further eluted with dichloromethane (2 × 100 mL). The combined organic eluates were concentrated in vacuo. The solid residue was further purified by flash column chromatography (silica gel/dichloromethane:hexanes = 1:6 to 1:3) to produce 10 (0.98 g, 0.98 mmol, 71% yield) as a white solid: mp 140‒142 °C; IR 1487, 1249, 850, 810 cm‒1; 1H NMR (CDCl3, 400 MHz) δ 7.55 (4 H, d, J = 8.2 Hz), 7.50 (4 H, d, J = 7.8 Hz), 7.33 (4 H, d, J = 8.2 Hz), 7.21 (4 H, d, J = 8.6 Hz), 7.04 (4 H, d, J = 8.2 Hz), 6.86 (4 H, d, J = 8.6 Hz), 6.19 (2 H, d, J = 3.5 Hz), 6.11 (2 H, d, J = 3.1 Hz), 4.93 (2 H, d, J = 3.1 Hz), 4.85 (2 H, d, J = 2.8 Hz), 3.56 (6 H, s), 0.29 (18 H, s); 13C NMR (CDCl3, 100 MHz) δ 152.3, 142.9, 142.8, 141.3, 138.78, 138.75, 133.7, 131.8, 131.1, 130.8, 129.3, 128.3, 128.2, 127.9, 126.8, 126.3, 119.8, 60.5, 40.7, 40.4, ‒1.1; HRMS (ESI/FT-ICR) m/z [M]+ calcd for C58H56Br2O2Si2 998.2180, 1000.2160, 1002.2139; found 998.2139, 1000.2169, 1002.2178. Preparation of Cyclic Dimers 11 and 12. An oven-dried 1 L flask fitted with a condenser was placed in a glovebox. To the flask were then added dibromide 10 (0.560 g, 0.559 mmol), 2,2′-bipyridyl (0.210 g, 1.35 mmol), and bis(1,5-cyclooctadiene)nickel (0.370 g, 1.35 mmol). The flask was capped with a rubber septum and then removed from the glovebox before 560 mL of anhydrous THF was added via cannula under an argon atmosphere. Then the reaction mixture was heated at reflux for 24 h before it was cooled to rt. The reaction mixture was then passed through a short silica gel column (5 cm high, 3 cm in diameter), and the column was further eluted with THF (2 × 100 mL). The combined eluates were concentrated, and the residue was purified by flash column chromatography (silica gel, dichloromethane:hexanes = 1:10 to 1:4) to produce cyclic dimer 12 (0.081 g, 0.048 mmol, 17% yield) and then cyclic dimer 11 (0.075 g, 0.045 mmol, 16% yield) as white solids: 12: mp >250 °C; IR 1249, 1114, 840, 809 cm‒1; 1H NMR (CDCl3, 600 MHz) δ 7.57 (16 H, s), 7.54 (8 H, d, J = 8.2 Hz), 7.40 (8 H, d, J = 8.2 Hz), 6.84 (4 H, dd, J = 4.4, 2.6 Hz), 6.62 (8 H, d, J = 8.2 Hz), 6.54‒6.47 (8 H, br d), 6.14 (4 H, d, J = 2.3 Hz), 5.05 (4 H, d, J = 2.4 Hz), 5.02 (4 H, br), 3.59 (12 H, s), 0.30 (36 H, s); 13C NMR (CDCl3, 100 MHz) δ 151.8, 144.2, 141.4, 139.8, 138.9, 138.8, 138.4, 134.4, 133.7, 133.1, 130.7, 128.1, 127.9, 127.4, 127.1, 126.4, 126.0, 61.1, 41.5, 38.3, ‒1.1; HRMS (ESI/FTICR) m/z [M + Na]+ calcd for C116H112O4Si4Na 1703.7530; found 1703.7534; 11: mp >250 °C; IR 1247, 836, 807, 757 cm‒ 1 1 ; H NMR (CDCl3, 400 MHz) δ 7.51 (8 H, d, J = 8.2 Hz), 7.48 (8 H, d, J = 8.2 Hz), 7.39 (16 H, d, J = 8.2 Hz), 6.84 (4 H, dd, J = 4.5, 2.5 Hz), 6.65 (8 H, d, J = 8.6 Hz), 6.52 (8 H, d, J = 7.4 Hz), 6.11 (4 H, d, J = 2.8 Hz), 5.05‒5.00 (8 H, br), 3.57 (12 H, s), 0.13 (36 H, s); 13C NMR (CDCl3, 100 MHz) δ 151.9, 144.2, 141.1, 139.9, 138.8, 138.7, 138.6, 134.4, 133.8, 133.3, 130.9, 128.2, 127.9, 127.3, 127.0, 126.23, 126.16, 61.2, 41.6, 38.4, ‒ 1.1; HRMS (ESI/FT-ICR) m/z [M]+ calcd for C116H112O4Si4 1680.7632; found 1680.7616. Preparation of 6. A solution of 11 (0.018 g, 0.011 mmol) in 10 mL of anhydrous dichloromethane under a nitrogen atmosphere was cooled to −78 °C. To the solution at −78 °C was added dropwise 0.10 mL of a 1 M solution of ICl in dichloromethane. After 5 min at −78 °C, the reaction was quenched with a saturated aqueous sodium thiosulfate solution. The reaction mixture was extracted with dichloromethane (3 × 20 mL), dried over sodium sulfate, and concentrated to afford crude 13 as a white solid. Crude 13 in a 100 mL flask, fitted with a condenser, was placed in a glovebox. To the flask were added 2,2′-bipyridyl (0.018 g, 0.116 mmol) and bis(1,5-cyclooctadiene)nickel



(0.031 g, 0.113 mmol). The flask was fitted a rubber septum and removed from the glovebox before 50 mL of THF was added via cannula under a nitrogen atmosphere. The reaction mixture was heated at reflux for 24 h before it was cooled to rt. The reaction mixture was then passed through a short silica gel column, and the column was eluted with dichloromethane (100 mL). The combined eluates were concentrated, and the residue was purified by flash column chromatography (silica gel, dichloromethane:hexanes = 1:5 to 1:2) to produce 6 (0.007 g, 0.005 mmol, 46% yield from 11) as a white solid: mp >210 °C; IR 1300, 1065, 791 cm‒1; 1H NMR (CDCl3, 400 MHz) δ 7.65 (8 H, d, J = 8.6 Hz), 7.54 (8 H, d, J = 8.6 Hz), 7.52 (8 H, d, J = 8.6 Hz), 7.42 (8 H, d, J = 8.2 Hz), 6.71 (4 H, dd, J = 4.7, 2.7 Hz), 6.32 (4 H, d, J = 2.7 Hz), 5.05 (4 H, d, J = 2.8 Hz), 4.94‒4.86 (4 H, m), 3.14 (12 H, s); 13C NMR (CDCl3, 150 MHz) δ 151.5, 143.3, 140.1, 139.8, 139.2, 139.0, 138.6, 134.1, 133.7, 131.6, 130.1, 127.97, 127.95, 127.8, 127.2 (br), 127.1, 126.3 (br), 60.8, 41.2, 38.9; HRMS (ESI/FT-ICR) m/z [M]+ calcd for C104H76O4 1388.5738; found 1388.5793. Preparation of Diiodide 15. To a solution of (E,E)-1,4-bis(4bromophenyl)-1,3-butadiene (1, 3.64 g, 10.0 mmol) in dioxane (100 mL) were added CuI (0.381 g, 2.00 mmol), sodium iodide (6.00 g, 40.0 mmol), and N,N′-dimethylethylenediamine (0.431 mL, 4.00 mmol). The reaction mixture was heated at reflux for 24 h before it was cooled to rt and then poured into 100 mL of a 5% aqueous ammonia solution. The precipitate was collected, washed with water (2 × 100 mL) and cold ethanol (50 mL), and air dried for 8 h to give 15 (3.94 g, 8.60 mmol, 86% yield) as a white solid: mp 257−259 °C; IR 1481, 991, 851, 793 cm ‒1; 1H NMR (CDCl3, 400 MHz) δ 7.65 (4 H, d, J = 8.2 Hz), 7.17 (4 H, d, J = 8.6 Hz), 6.92 (2 H, dd, J = 11.8, 2.8 Hz), 6.59 (2 H, dd, J = 11.8, 2.7 Hz); 1H NMR (DMSO-d6, 400 MHz) δ 7.70 (4 H, d, J = 8.2 Hz), 7.32 (4 H, d, J = 8.6 Hz), 7.12 (2 H, dd, J = 11.7, 2.7 Hz), 6.71 (2 H, dd, J = 11.8, 2.7 Hz); 13C NMR (DMSO-d6, 100 MHz) δ 137.5, 136.5, 132.1, 130.0, 128.4, 93.5; HRMS (ESI/Orbitrap) m/z [M]+ calcd for C16H12I2 457.9023; found 457.9029. Preparation of Diiodide 16. To a solution of (E,E)-1,4-bis(4iodophenyl)-1,3-butadiene (15, 4.600 g, 10.04 mmol) and 1,4benzoquinone (2, 1.300 g, 12.03 mmol) in 200 mL of anhydrous dichloroethane under an argon atmosphere was added by using a syringe boron trifluoride diethyl etherate (1.0 mL, 8.1 mmol). The reaction mixture was heated at reflux for 96 h and then washed with water (4 × 100 mL). The organic layer was separated and then passed through a short silica gel column (8 cm high, 5 cm in diameter). The column was further eluted with 200 mL of diethyl ether. The combined eluates were dried over sodium sulfate and concentrated to afford a brown solid. The brown solid was further purified by flash column chromatography (silica gel/ethyl acetate:hexanes = 1:4 to 1:2) to produce 16 (4.95 g, 8.74 mmol, 87% yield) as a yellow solid: mp 153– 155 °C; IR 3460–3120 (br), 1482, 1286, 1006 cm‒1; 1H NMR (CDCl3, 400 MHz) δ 7.62 (4 H, d, J = 8.2 Hz), 7.02 (4 H, d, J = 8.2 Hz), 6.63 (2 H, s), 5.89 (2 H, d, J = 2.4 Hz), 4.76 (2 H, d, J = 2.4 Hz), 4.20 (2 H, s); 13C NMR (CDCl3, 100 MHz) δ 147.4, 143.0, 137.9, 130.0, 127.2, 124.7, 115.1, 92.1, 41.0; HRMS (ESI/FT-ICR) m/z [M]+ calcd for C22H16I2O2 565.9234; found 565.9240. Preparation of Diiodide 17. To a mixture of diiodide 16 (2.60 g, 4.59 mmol) and silver oxide (1.28 g, 5.52 mmol) was added 100 mL of anhydrous diethyl ether under a nitrogen atmosphere. The reaction mixture was stirred at rt for 12 h and then

passed through a short silica gel column. The column was further eluted with diethyl ether (2 × 50 mL), and the combined eluates were concentrated. The solid residue was further purified by flash column chromatography (silica gel/ethyl acetate:hexanes = 1:10 to 1:5) to afford 17 (2.31 g, 4.09 mmol, 89% yield) as a yellow solid: mp 172‒174 °C; IR 1656, 1482, 1293, 864 cm‒1; 1H NMR (CDCl3, 400 MHz) δ 7.63 (4 H, d, J = 8.2 Hz), 7.02 (4 H, d, J = 8.6 Hz), 6.69 (2 H, s), 5.89 (2 H, d, J = 2.8 Hz), 4.70 (2 H, d, J = 2.7 Hz); 13C NMR (CDCl3, 100 MHz) δ 186.0, 141.6, 140.9, 137.8, 136.5, 130.4, 126.5, 92.5, 40.1; HRMS (ESI/FT-ICR) m/z [M ‒ 2H]+ calcd for C22H12I2O2 561.8921; found 561.8963. Preparation of 18. To a mixture of (E,E)-1,4-bis(4-bromophenyl)-1,3-butadiene (1, 1.39 g, 3.82 mmol) and diiodide 17 (2.05 g, 3.63 mmol) in 100 mL of anhydrous dichloromethane under a nitrogen atmosphere was added by using a syringe boron trifluoride diethyl etherate (1.00 mL, 8.1 mmol). The reaction mixture was stirred at rt for 16 h. The reaction mixture was then passed through a short silica gel column, and the column was further eluted with diethyl ether (2 × 100 mL). The combined eluates were concentrated to afford a yellow solid, which was used without further purification. To the yellow solid and potassium carbonate (2.00 g, 14.5 mmol) in dry acetone (100 mL) was added dimethyl sulfate (1.00 mL, 10.6 mmol) by using a syringe, and the reaction mixture was heated at reflux for 12 h before it was cooled to rt. The reaction mixture was then passed through a short silica gel column, and the column was further eluted with dichloromethane (2 × 100 mL). The combined organic eluates were concentrated in vacuo. The solid residue was further purified by flash column chromatography (silica gel/ethyl acetate:hexanes = 1:6 to 1:4) to afford 18 (1.91 g, 2.00 mmol, 55% yield) as a white solid: mp 180‒182 °C; IR 1485, 1299, 1008, 809 cm‒1; 1H NMR (CDCl3, 400 MHz) δ 7.42 (4 H, d, J = 8.2 Hz), 7.22 (4 H, d, J = 8.6 Hz), 6.80 (4 H, d, J = 8.6 Hz), 6.68 (4 H, d, J = 8.6 Hz), 6.08 (2 H, d, J = 3.1 Hz), 6.06 (2 H, d, J = 3.1 Hz), 4.80 (2 H, d, J = 2.7 Hz), 4.78 (2 H, d, J = 3.2 Hz), 3.52 (6 H, s); 13C NMR (CDCl3, 100 MHz) δ 152.1, 143.3, 142.6, 137.7, 137.1, 131.2, 131.1, 129.5, 129.2, 128.1, 128.0, 119.9, 91.4, 60.4, 40.5, 40.4; HRMS (ESI/Orbitrap) m/z [M]+ calcd for C40H30Br2I2O2 953.8696, 955.8676, 957.8655; found 953.8700, 955.8676, 957.8665. Preparation of 20. To a 100 mL two-neck flask fitted with a condenser and a rubber septum were added 18 (0.60 g, 0.63 mmol), 4,4,5,5-tetramethyl-2-(2-thienyl)-1,3,2-dioxaborolane (19, 0.265 g, 1.26 mmol), cesium carbonate (0.82 g, 2.5 mmol), and Pd(PPh3)4 (0.073 g, 0.063 mmol). The flask was flushed with nitrogen, and anhydrous THF (50 mL) was introduced via cannula. The reaction mixture was heated at 50 °C for 24 h before it was cooled to rt. The reaction mixture was then passed through a short silica gel column, and the column was further eluted with dichloromethane (2 × 50 mL). The combined eluates were concentrated in vacuo. The solid residue was further purified by flash column chromatography (silica gel/ethyl acetate:hexanes = 1:6 to 1:4) to produce 20 (0.366 g, 0.421 mmol, 67% yield) as a white solid: mp 258‒260 °C; IR 1486, 1302, 1010, 813 cm‒1; 1H NMR (CDCl3, 400 MHz) δ 7.37 (4 H, d, J = 8.2 Hz), 7.24‒7.19 (8 H, m), 7.04 (2 H, dd, J = 4.7, 3.5 Hz), 7.00 (4 H, d, J = 8.2 Hz), 6.83 (4 H, d, J = 8.2 Hz), 6.13 (2 H, d, J = 3.2 Hz), 6.11 (2 H, d, J = 3.2 Hz), 4.89 (2 H, d, J = 3.2 Hz), 4.83 (2 H, d, J = 2.8 Hz), 3.54 (6 H, s); 13C NMR (CDCl3, 100 MHz) δ 152.2, 144.3, 143.2, 142.7, 132.2, 131.5, 131.1, 130.9, 129.3, 128.2, 128.0, 127.9, 125.7, 124.4, 122.8, 119.8, 60.5, 40.8, 40.4; HRMS (ESI/Orbitrap) m/z [M]+ calcd for


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The Journal of Organic Chemistry C48H36Br2O2S2 866.0518, 868.0498, 870.0477; found 866.0525, 868.0503, 870.0486. Preparation of Cyclic Dimers 21 and 22. To an oven-dried 1 L flask were added dibromide 20 (0.532 g, 0.612 mmol) and 2,2′-bipyridyl (0.234 g, 1.50 mmol). The flask was flushed with argon and placed in a glovebox, and then bis(1,5-cyclooctadiene)nickel (0.420 g, 1.53 mmol) was added. The flask was fitted with a condenser and a rubber septum and then removed from the glovebox before 600 mL of anhydrous THF was added via cannula under an argon atmosphere. Then the reaction mixture was heated at reflux for 18 h before it was cooled to rt. The reaction mixture was then passed through a short silica gel column (5 cm high, 3 cm in diameter), and the column was further eluted with THF (2 × 100 mL). The combined eluates were concentrated, and the residue was purified by flash column chromatography (silica gel, ethyl acetate:hexanes = 1:8 to 1:4) to produce a 1:1 mixture of cyclic dimers 21 and 22 (0.122 g, 0.086 mmol, 28% yield) as a white solid: IR 1498, 1301, 1066, 815 cm‒1; 1H NMR (CDCl3, 600 MHz) δ 7.569 (8 H, d, J = 8.2 Hz), 7.562 (8 H, d, J = 8.2 Hz), 7.346 (8 H, d, J = 8.4 Hz), 7.343 (8 H, d, J = 8.4 Hz), 7.27 (4 H, dd, J = 3.6, 1.2 Hz), 7.24 (4 H, dd, J = 3.0, 1.2 Hz), 7.23 (4 H, dd, J = 5.1, 1.2 Hz), 7.10 (4 H, dd, J = 5.4, 1.2 Hz), 7.05 (4 H, dd, J = 4.8, 3.6 Hz), 6.94 (4 H, dd, J = 5.3, 3.5 Hz), 6.84 (4 H, dd, J = 4.8, 2.4 Hz), 6.82 (4 H, dd, J = 4.5, 2.7 Hz), 6.65 (8 H, d, J = 8.8 Hz), 6.63 (8 H, d, J = 8.8 Hz), 6.54‒6.45 (16 H, br), 6.09 (4 H, d, J = 2.3 Hz), 6.07 (4 H, d, J = 2.9 Hz), 5.03‒4.99 (16 H, m), 3.57 (12 H, s), 3.53 (12 H, s); 13C NMR (CDCl3, 100 MHz) δ 151.8, 151.7, 144.48, 144.45, 144.42, 144.36, 139.8, 139.7, 138.6, 138.5, 134.49, 134.45, 133.2, 133.1, 132.4, 130.6, 130.5, 128.3, 128.0, 127.9, 127.7, 127.3, 126.17, 126.08, 126.03, 125.98, 124.5, 124.4, 122.8, 61.17, 61.08, 41.6, 41.5; HRMS (ESI/FT-ICR) m/z [M]+ calcd for C96H72O4S4 1416.4308; found 1416.4295. Preparation of Cyclic Dimers 23 and 24. To a 25 mL flask were added a 1:1 mixture of 21 and 22 (0.060 g, 0.042 mmol) and N-iodosuccinimide (0.076 g, 0.34 mmol). The flask was capped with a rubber septum and flushed with argon for 5 min before 10 mL of CHCl3 and 1 mL of acetic acid were added. The reaction mixture was stirred in the dark at rt for 12 h. A saturated aqueous sodium thiosulfate solution (3 mL) was added to the reaction mixture, and the organic layer was separated, dried over Na2SO4, and concentrated in vacuo. The residue was purified by flash column chromatography (silica gel/dichloromethane:hexanes = 1:2 to 1:1) to produce 24 (0.035 g, 0.018 mmol, 43% yield) and then 23 (0.035 g, 0.018 mmol, 43% yield) with a combined yield of 86% as white solids: 24: mp >185 °C; IR 1498, 1300, 1065, 791 cm‒1; 1H NMR (CDCl3, 600 MHz) δ 7.46 (8 H, d, J = 8.2 Hz), 7.32 (8 H, d, J = 8.8 Hz), 7.19 (4 H, d, J = 4.1 Hz), 6.93 (4 H, d, J = 4.1 Hz), 6.85 (4 H, dd, J = 4.7, 2.3 Hz), 6.61 (8 H, d, J = 8.2 Hz), 6.47 (8 H, br d, J = 6.4 Hz), 6.08 (4 H, d, J = 2.9 Hz), 5.02‒4.99 (4 H, m), 4.99 (4 H, d, J = 2.9 Hz), 3.58 (12 H, s); 13C NMR (CDCl3, 100 MHz) δ 151.7, 150.4, 144.9, 139.7, 138.5, 137.8, 134.6, 133.1, 131.6, 130.3, 128.3, 127.7, 127.3, 126.1, 125.8, 124.2, 71.8, 61.1, 41.5, 38.3; HRMS (ESI/FT-ICR) m/z [M]+ calcd for C96H68I4O4S4 1920.0174; found 1920.0112; 23: mp >180 °C; IR 1497, 1297, 1064, 788 cm‒1; 1H NMR (CDCl3, 600 MHz) δ 7.47 (8 H, d, J = 8.2 Hz), 7.33 (8 H, d, J = 8.2 Hz), 7.09 (4 H, d, J = 4.1 Hz), 6.91 (4 H, d, J = 3.5 Hz), 6.83 (4 H, dd, J = 4.4, 2.6 Hz), 6.65 (8 H, d, J = 8.2 Hz), 6.51 (8 H, br), 6.06 (4 H, d, J = 2.3 Hz), 5.04‒5.01 (4 H, m), 4.99 (4 H, d, J = 2.3 Hz), 3.54 (12 H, s); 13 C NMR (CDCl3, 100 MHz) δ 151.9, 150.3, 144.9, 139.8, 138.6, 137.9, 134.6, 133.3, 131.6, 130.5, 128.4, 127.8, 127.3,

126.2, 125.8, 124.2, 72.3, 61.2, 41.6, 38.5; HRMS (ESI/Orbitrap) m/z [M]+ calcd for C96H68I4O4S4 1920.0174; found 1920.0190. Preparation of 25. To an oven-dried 100 mL flask inside a glovebox were added 23 (0.060 g, 0.031 mmol), 2,2′-bipyridyl (0.029 g, 0.18 mmol), and bis(1,5-cyclooctadiene)nickel (0.050 g, 0.18 mmol). After 25 mL of anhydrous THF was added, the flask was fitted with a condenser and a rubber septum and removed from the glovebox. Then the reaction mixture was heated at reflux for 24 h before it was cooled to rt. The reaction mixture was passed through a short silica gel column (3 cm high, 1 cm in diameter), and the column was further eluted with THF (2 × 50 mL). The combined eluates were concentrated in vacuo, and the residue was purified by flash column chromatography (silica gel, dichloromethane:hexanes = 1:1 to 2:1) to produce 25 (0.023 g, 0.016 mmol, 52% yield) as a yellow solid: mp >220 °C; IR 1499, 1458, 1300, 1065, 799 cm‒1; 1H NMR (CDCl3, 400 MHz) δ 7.66 (8 H, d, J = 8.6 Hz), 7.36 (8 H, d, J = 8.2 Hz), 7.16 (4 H, d, J = 3.5 Hz), 7.06 (4 H, d, J = 3.5 Hz), 6.79 (4 H, dd, J = 4.7, 2.7 Hz), 6.71 (8 H, d, J = 8.6 Hz), 6.14 (4 H, d, J = 2.4 Hz), 5.03‒4.98 (4 H, m), 4.94 (4 H, d, J = 2.4 Hz), 3.30 (12 H, s); 13C NMR (CDCl3, 100 MHz) δ 151.7, 145.2, 143.6, 140.1, 138.8, 136.8, 134.6, 133.5, 131.8, 130.9, 128.6, 127.4, 127.1, 126.3, 126.0, 122.4, 61.3, 41.4, 38.8; HRMS (ESI/FT-ICR) m/z [M]+ calcd for C96H68O4S4 1412.3995; found 1412.4028. Preparation of 7. To a 5 mL flask were added 25 (0.006 g, 0.0042 mmol) and DDQ (0.019 g, 0.084 mmol). The flask was flushed with argon, and then 1 mL of chlorobenzene was introduced by using a syringe. The reaction mixture was heated at 90 °C for 6 h before it was cooled to rt. Dichloromethane (50 mL) was added, and the solution was passed through a basic aluminum oxide column (2 cm high, 1 cm in diameter). The column was further eluted with dichloromethane (2 × 50 mL). The combined eluates were concentrated in vacuo, and the residue was washed with pentane to afford 7 (0.005 g, 0.0035 mmol, 83% yield) as a yellow solid: mp >164 °C; IR 1498, 1456, 1407, 1299, 812 cm‒1; 1H NMR (CDCl3, 600 MHz) δ 7.53 (8 H, d, J = 8.8 Hz), 7.44 (4 H, s), 7.42 (8 H, d, J = 8.2 Hz), 7.06 (4 H, d, J = 3.5 Hz), 6.98 (4 H, dd, J = 4.4, 2.6 Hz), 6.92 (4 H, d, J = 4.1 Hz), 6.90 (8 H, d, J = 8.8 Hz), 6.74 (8 H, br), 5.43 (4 H, br), 3.60 (12 H, s); 13C NMR (CDCl3, 100 MHz) δ 149.4, 145.6, 141.2, 140.4, 138.6, 137.3, 137.2, 133.07, 133.00, 132.0 130.6, 127.9, 127.4, 127.0, 126.36, 126.27, 122.7, 122.3, 60.3, 38.2; HRMS (ESI/FT-ICR) m/z [M]+ calcd for C96H64O4S4 1408.3682; found 1408.3579.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Computational coordinates and energetic details of DFT calculations; UV‒visible and fluorescence spectra; 1H and 13C NMR spectra of all new compounds (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.



ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under CHE – 1464026. The NMR spectrometer on which this work was carried out was supported by the Major Research Instrumentation (MRI) Program of the NSF (CHE – 1228336). DFT calculations were performed on the Super Computing System (Spruce Knob) at West Virginia University, which is funded in part by the NSF EPSCoR RII #1003907, WVEPSCoR via the Higher Education Policy Commission, and West Virginia University.

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Synthesis of Molecular Nanohoops Bearing a Tetrahydro[6

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