Stable Core-Modified Doubly N-Fused Expanded ... - ACS Publications

Note Cite This: J. Org. Chem. 2018, 83, 1584−1590

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Stable Core-Modified Doubly N‑Fused Expanded Dibenziporphyrinoids Sunit Kumar,† Malakalapalli Rajeswara Rao,‡ and Mangalampalli Ravikanth*,† †

Indian Institute of Technology Bombay, Powai, Mumbai 400076, India Indian Institute of Technology Dharwad, Dharwad 580011, Karnataka, India

‡

S Supporting Information *

ABSTRACT: We describe one-pot synthesis of stable doubly N-fused expanded dibenziporphyrinoids using readily available precursors under acid-catalyzed conditions. The doubly Nfused expanded dibenziporphyrinoids have been synthesized by adopting an inversion followed by fusion strategy. The studies showed that the dibenziporphyrinoids undergo mono fusion initially, but due to the high stability of doubly fused dibenziporphyrinoids, the monofused macrocycles undergo further fusion to form doubly fused dibenziporphyrinoids. The mono fusion and double fusion in these dibenziporphyrinoids were established by X-ray crystallography. bond between pyrrole N and that of an α- or β-carbon of a neighboring inverted pentacycle.11 However, such a fusion resulting in tripentacyclic rings is not common, and very few examples are available in the literature.11 Furuta and co-workers reported the first examples of N-fused porphyrin12 1, N-fused sapphyrin, 13 and other related macrocycles containing tripentacyclic rings. These fused macrocycles were synthesized from the appropriate N-confused porphyrinoid systems but not from the inverted porphyrinoids.12,13 Furthermore, Furuta and co-workers14 have also reported the synthesis of N-fused pentaphyrin 2, which was obtained during condensation reactions under mild acid-catalyzed conditions. The formation of N-fused pentaphyrin 2 was attributed to pyrrole inversion followed by fusion during the condensation reaction. These Nfused porphyrinoids were used as ligands to prepare metal complexes, and their properties were studied.15 Latos-Grażyński and co-workers16 isolated unique N-fused benzipentaphyrin 3 by adopting a similar pyrrole inversion followed by fusion strategy. Subsequently, Furuta and coworkers also succeeded in synthesizing doubly fused porphyrin17 4 and pentaphyrin18 from corresponding N-confused Nfused porphyrin and N-confused N-fused pentaphyrin, respectively (Figure 1). Thus, a perusal of the literature revealed that singly fused porphyrinoinds were formed via an inversion strategy followed by fusion, but that no examples on doubly fused porphyrinoids under this simple strategy have been explored. Herein, we report the first examples of doubly N-fused core-modified expanded dibenziporphyrins 5a−5d that were synthesized in a one-pot reaction via an inversion followed by fusion strategy. These novel macrocycles contain two N-

C

arbaporphyrinoids1 are macrocycles containing one or more carbon atoms along with other donor heteroatoms, such as N, S, O, Se, etc., inside their cores and have been explored as a favorite porphyrin-based ligand to synthesize organometallic complexes.2 Carbaporphyrinoids exist as fully aromatic species as well as nonaromatic and anti-aromatic compounds. The presence of carbocyclic rings and heterocycles in carbaporphyrinoid macrocyclic skeletons alters the electronic properties significantly, making them quite different from porphyrinoids. The pioneering works carried out by the research groups of Lash,1,2 Latos-Grażyński,3 Furuta,4 and others5 have established that carbaporphyrinoids are the most promising macrocycles with a multitude of applications that range from catalysis to material science and medicine. Benziporphyrinoids6 containing one or two benzene rings, along with other five-membered heterocycles, are the most widely studied carbaporphyrinoids and are known to exhibit intriguing spectroscopic, structural, and coordinating properties. Interestingly, expanded porphyrinoids7 containing more than four five-membered heterocycles have been extensively investigated over the years, but relatively few examples are available on expanded benziporphyrinoids even though the first example was reported in 1991.8 Another feature that has not yet been studied to a greater extent in expanded benziporphyrinoid9 is the chemistry of inversion of heterocyclic units, which is a well-established feature in expanded porphyrinoids.10 Many expanded porphyrinoids known in the literature exhibit inversion of heterocycle rings, such as pyrrole, thiophene, furan, selenophene, and more.10 The inversion of heterocycle rings in porphyrinoids may sometimes result in the formation of another very interesting macrocycle called N-fused porphyrinoid, which contains a unique fused tripentacyclic (TP) ring in the macrocyclic core due to the formation of a © 2018 American Chemical Society

Received: November 10, 2017 Published: January 3, 2018 1584

DOI: 10.1021/acs.joc.7b02851 J. Org. Chem. 2018, 83, 1584−1590

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The Journal of Organic Chemistry

doubly N-fused expanded dibenziporphyrinoid 5a was synthesized by condensing 1 equiv of the appropriate diol 7a with 1 equiv of tripyrrane 8a in CH2Cl2 in the presence of a catalytic amount of TFA under an inert atmosphere for 2 h followed by oxidation with 2 equiv of DDQ in the open air for an additional 2 h (Scheme 1). TLC analysis distinctively showed one major polar spot and one less-polar minor spot. However, upon further addition of 2 equiv of DDQ followed by continuous stirring of the reaction mixture for an extra 2 h in the open air, we noticed that the major polar spot disappeared and the earlier minor spot had become the major spot. The crude compound was subjected to silica gel column chromatography and afforded the only macrocycle 5a as a purple solution using petroleum ether and CH2Cl2. The macrocycle 5a was freely soluble in common organic solvents and was stable in solution and in solid form. The single crystal of compound 5a was obtained readily from CH2Cl2/n-hexanes, and the crystal structure showed that the macrocycle obtained was the unprecedented doubly N-fused dibenziporphyrinoid 5a. To test the generality of the reaction, we carried out the [5+3] condensation with three other different diols (7b−7d) and three different tripyrranes (8b−8d) under the same mild acid-catalyzed conditions followed by oxidation with 4 equiv of DDQ, and we successfully obtained the doubly N-fused dibenziporphyrinoids 5b−5d in 10−15% yields. To know more about the other intermediate macrocycle which initially formed in these reactions but transformed to the doubly fused macrocycle, we made an attempt to isolate and characterize the intermediate macrocycle. Thus, condensation of diol 7a and tripyrrane 8a was carried out under mild TFA-catalyzed conditions for 2 h and was oxidized by only 2 equiv of DDQ. Column chromatographic purification yielded the intermediate macrocycle 9. Fortunately, we obtained the crystal structure of 9, and the structure revealed that the macrocycle was mono N-fused dihydrobenziporphyrin 9 (vide inf ra). Thus, the [5+3] condensation initially results in the formation of the intermediate mono N-fused dihydrobenziporphyrin 9 which transforms to doubly N-fused dibenziporphyrin upon further oxidation. The doubly N-fused dibenziporphyrinoids 5a−5d can therefore be obtained readily in a one-pot reaction under mild acid-catalyzed reaction conditions.

Figure 1. Structures of N-fused porphyrin, pentaphyrin, benzipentaphyrin, and dibenziporphyrinoids.

fused tripentacyclic rings due to the formation of a bond between nitrogens of two pyrrole rings and neighboring βcarbons of inverted thiophene rings. However, our initial aim was to synthesize core-modified expanded dibenziporphyrin 6, but serendipitously, we obtained the doubly fused expanded dibenziporphyrin 5a as confirmed by X-ray crystallography (vide inf ra). To establish our serendipitous discovery, we varied different diols and tripyrranes and synthesized three other doubly fused expanded dibenziporphyrins 5b−5d. The macrocycles 5a−5d were highly stable in the solid and solution states and were characterized by HR-MS, 1D and 2D NMR spectroscopy, and X-ray crystallography. The target macrocycles 5a−5d were prepared by following the [5+3] condensation method as shown in Scheme 1. The 10,10′-bis[(p-tolyl/anisyl/4-isopropylbenzene/4-tertbutylbenzene)hydroxymethyl]-1,3-bis(2-thienyl)benzenediols 7a−7d were synthesized in 45−63% yields by treating 1,3bis(2-thienyl)benzene with 2.5 equiv of n-BuLi followed by 2.5 equiv of the appropriate aryl aldehyde in THF at 0 °C and by subsequent purification by silica gel column chromatography. The 5,20-bis(p-tolyl/anisyl/4-isopropylbenzene/4-tert-butylbenzene)-27,28-bis(thia)-25,26-dihydrotripyrranes 8a−8d were prepared by treating the appropriate diol 7a−7d with 10 equiv of pyrrole under mild acid-catalyzed conditions followed by silica gel column chromatographic purification. The Scheme 1. Synthesis of Macrocycles 5a−5d

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indicating that the aromaticity of the former thiophenes is unperturbed. Also, C−C bond lengths of the TP rings exhibit lengths close to ∼1.40 Å, which is in line with the expected πdelocalization within the rings. The crystal structure of the intermediate mono N-fused dihydrobenziporphyrin 9 also possesses a highly contorted conformation with dihedral angles that closely match those of 5a. However, the central benzene rings maintain near planarity (∼3−5°) with their corresponding substituents (thiophene-thiophene or thiophene-TP ring), which could be attributed to the increased flexibility bestowed by two sp3 carbons. Overall, it is anticipated that the relatively electron-rich and structurally rigid TP ring (vs thiophene) in the intermediate compound 9 may facilitate the occurance of the fusion on the same side of the macrocycle. However, the high selectivity of formation of the cis-isomer over the transisomer is unclear at present, and more investigations are needed to understand the selective formation of the cis-isomer. The macrocycles 5a−5d were further characterized by detailed 1D and 2D NMR spectroscopy, and the representative 1D and 2D NMR spectra of 5a are shown in Figures 3 and S1. All the resonances of the macrocycles 5a−5d were identified based on position, integration, coupling constant, and crosspeak correlations in 1D and 2D NMR spectra, and the molecular structures of 5a−5d were deduced (see Supporting Information). We have attempted to characterize the intermediate macrocycle 9 by NMR spectroscopy. The 1H NMR recorded for compound 9 at room temperature as well as at 233 K showed a very complex spectrum with a large number of resonances due to the nonsymmetric nature of macrocycle 9. Since we could not carry out detailed 2D NMR studies at a low temperature because of our instrument limitations, the identification and assignment of all the resonances in the 1H NMR spectrum of macrocycle 9 was not done. The absorption and electrochemical properties were investigated for doubly N-fused dibenziporphyrinoids 5a−5d along with mono N-fused dihydrobenziporphyrinoid 9, and the data are presented in Table S4. The comparison of the absorption spectra of 5a and 9 is presented in Figure 4a, and the cyclic voltammograms are provided in Figure 4b. Both 5a and 9 showed typical nonaromatic absorption features with three broad absorption bands present in the 300−700 nm region. Unlike the reported macrocycles where meso-aryl groups lie orthogonal to the plane of the macrocycles in order to avoid steric interactions, the TP rings of 5a−5d facilitate the mesoaryls to remain in the plane, which led to the extended πdelocalization resulting in red-shifted absorption. The absorption bands of 5a were slightly bathochromically shifted compared to those of 9. These fused macrocycles 5a−5d exhibited two reversible oxidations at approximately 0.8 and

The single crystals of 5a were grown via slow evaporation of n-hexane in a CH2Cl2 solution at room temperature over a period of 3 days. The molecule crystallizes as a triclinic crystal system with P1̅ space group. The crystallographic data and crystal parameters are given in Table S1, while all the relevant bond distances are listed in Tables S2 and S3. It is evident from the X-ray structure that the macrocycle 5a is highly ruffled and resembles a Mobius conformation (Figure 2). However, the

Figure 2. Single-crystal X-ray structure of macrocycles (a) 5a and (b) 9 at 50% probability. For clarity, all of the hydrogen atoms have been omitted.

1,3-connection of dithienyl benzene provides no continuous πdelocalization leading to a nonaromatic system. Further, it renders unambiguous evidence toward the formation of two tripentacyclic (TP) rings of a cis fashion. The sulfur atoms of the fused thiophene rings point outward, while those of the free thiophene rings point inward. The flexibility of the large core and the rigid TP rings induced a large distortion at the meso positions between the TP rings and thiophenes with dihedral angles of 53.7° and 57.1°, which minimizes the π-delocalization pathway. Moreover, central benzenes exhibit asymmetric dihedral angles of 41.0° and 22.1° with respect to the TP rings, while the thiophenes on the nonfused side showcase 6.4° and 32.5° angles. The C−C bond lengths of the thiophenes in the TP rings match closely with those of the nonfused form,

Figure 3. 1H NMR spectrum of macrocycle 5a recorded in CDCl3. 1586


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Figure 4. (a) Comparison of the absorption spectra of compound 9 (black line) and compound 5a (red line) recorded in chloroform and (b) comparison of cyclic voltammograms of compound 9 (black line) and compound 5a (red line) and the differential pulse voltammogram (blue line) recorded in CH2Cl2 with 0.1 M TBAP as the supporting electrolyte at a scan rate of 50 mVs−1.

1.10 V and one irreversible reduction at approximately −1.20 V. The low oxidation potentials support the idea that the macrocycles are electron rich. In summary, we have synthesized the first stable doubly Nfused nonaromatic expanded dibenziporphyrins in one pot under mild reaction conditions. We have demonstrated that the double fusion can be induced in the expanded dibenziporphyrin by adopting a simple inversion strategy followed by fusion. The double fusion in expanded dibenziporphyrinoid occurs stepwise via the formation of an intermediate mono N-fused dihydrobenziporphyrin. Both mono N-fused dihydrobenziporphyrin and doubly N-fused dibenziporphyrin were structurally characterized, and the characterization showed the presence of tripentacyclic ring(s) leading to structural contortion in the macrocycles. The NMR and absorption studies supported the nonaromatic nature of the macrocycles, while electrochemical studies revealed the electron-rich behavior of these fused expanded dibenziporphyrinoids. Presently, studies are underway to understand the underdeveloped inversion followed by fusion strategy for the disclosure of the factors responsible for fusion in expanded carbaporphyrinoids.

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CrystalClear-SM Expert 2.1 b24 software. The structures were solved by direct methods and refined by least-squares against F2 utilizing the software packages SHELXL-97,19a,b SIR-92,19c and WINGX.19d All non-hydrogen atoms were refined anisotropically. The CCDC (1564600 and 1564601 for 5a and 9, respectively) contains supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Density functional calculations (see Supporting Information) of the molecular structures (in the gas phase) and the molecular orbital energies were carried out at the B3LYP/6-31G(d) level as implemented in Gaussian 09.20 1 0, 1 0 ′- B i s [( p - to l yl / a n is y l/ 4- i s op r op yl b e n ze n e / 4 - te r tbutylbenzene)hydroxymethyl]-1,3-bis(2-thienyl)benzenediol Diols 7a−7d. Dry and distilled n-hexane (30 mL) was added to a 250 mL three-necked, round-bottom flask fitted with a rubber septum and gas inlet tube; the flask was flushed with nitrogen for 10 min. Tetramethylethylenediamine (TMEDA) (3.09 mL, 20.66 mmol) and n-butyllithium (12.91 mL of ca. 1.6 M solution in hexane) were added to the stirred solution, and the reaction temperature was maintained at 0 °C in an ice bath. 1,3-Bis(2-thienyl)benzene21 (2.00 g, 8.26 mmol) was added, and the solution was refluxed gently for 1 h. As the reaction progressed a white turbid solution formed, indicating the formation of the lithiated salt 1,3-bis(2-thienyl)benzene. The reaction mixture was allowed to attain room temperature. To this mixture was then added an ice-cold solution of the appropriate aryl aldehyde (20.66 mmol) in dry THF (40 mL) while the flask was kept in an ice bath, and the mixture was stirred for an additional 15 min at 0 °C. The reaction mixture was brought to room temperature and stirred for an additional 2 h, and the reaction was quenched by addition of an ice-cold NH4Cl solution (50 mL, ca. 1 M). The organic layer was diluted with ether, washed several times with water and brine, and dried over anhydrous Na2SO4. The solvent was removed in a rotary evaporator under reduced pressure to afford the crude compound. TLC analysis showed three spots corresponding to unchanged bithiabenzene, unreacted aldehyde, and the desired diol. The crude compound was loaded on silica and eluted with petroleum ether. The 1,3-bis(2-thienyl)benzene and aldehyde were removed with a petroleum ether/ethyl acetate (98:02) mixture, and the desired diol (7a−7d) was collected with a mixture of petroleum ether/ethyl acetate (∼70:30). The solvent was removed in a rotary evaporator to afford each diol as a white solid in a 45−63% yield. 7a: 63% yield, 2.40 g; 1H NMR (400 MHz, CDCl3, δ) 7.69−7.70 (m, 1H, benzene), 7.41 (d, J = 7.8 Hz, 2H, benzene), 7.35 (d, J = 7.8 Hz, 4H, Ar), 7.31 (t, J = 7.5 Hz, 1H, benzene), 7.18 (d, J = 8.2 Hz, 4H, Ar), 7.15 (d, J = 3.6 Hz, 2H, thiophene), 6.85 (d, J = 3.6 Hz, 2H, thiophene), 6.01 (s, 2H, −CH), 2.36 (s, 6H, tolyl-CH3); 13C NMR (125 MHz, CDCl3, δ) 148.2, 143.9, 140.2, 138.1, 135.1, 129.6, 129.5, 126.5, 125.9, 124.9, 123.2, 123.1, 72.7, 21.3; HRMS calcd for C30H25OS2 [M − OH]+ 465.1341, found 465.1344.

EXPERIMENTAL SECTION

General Experimental. The chemicals, such as BF3·OEt2, TFA, and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), were used as obtained from Aldrich. All other chemicals used for the synthesis were reagent grade unless otherwise specified. Column chromatography was performed on silica gel. The 1H NMR spectra were recorded in CDCl3 on Bruker 400 and 500 MHz instruments. The frequencies used for 13 C NMR were 100.06 and 125.77 MHz for the 400 and 500 MHz instruments, respectively. Tetramethylsilane [Si(CH3)4] was used as an internal standard for 1H and 13C NMR. Absorption spectra were obtained with an Agilent Technologies Cary 5000 UV−vis−NIR instrument. Cyclic voltammetric (CV) studies were carried out with a BAS electrochemical system utilizing a three electrode configuration consisting of glassy carbon (working electrode), platinum wire (auxiliary electrode), and saturated calomel (reference electrode) electrodes. The experiments were done in dry dichloromethane using 0.1 M tetrabutylammonium perchlorate as the supporting electrolyte. The HR mass spectra were recorded with a Q-TOF micro mass spectrometer. For UV−vis, the solution for each compound (1 × 10−5 M) was prepared by using spectroscopic grade CHCl3 solvent. For X-ray crystal structure analysis, single-crystal X-ray structure analysis was performed on a Rigaku Saturn 724 diffractometer that was equipped with a low temperature attachment. Data were collected at 100 K using graphite-monochromated Mo Kα radiation (λα = 0.71073 Å) by the ω-scan technique. The data were reduced by using 1587


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The Journal of Organic Chemistry 7b: 45% yield, 1.80 g; 1H NMR (500 MHz, CDCl3, δ) 7.72−7.71 (m, 1H, benzene), 7.44 (d, J = 8.0 Hz, 2H, benzene), 7.39 (d, J = 9.0 Hz, 4H, Ar), 7.32 (t, J = 7.5 Hz, 1H, benzene), 7.16 (d, J = 4.0 Hz, 2H, thiophene), 6.91 (d, J = 9.00 Hz, 4H, Ar), 6.88 (d, J = 4.0 Hz, 2H, thiophene), 6.00 (s, 2H, −CH), 3.81 (s, 6H, anisyl-OCH3); 13C NMR (125 MHz, CDCl3, δ) 159.6, 148.3, 143.8, 135.3, 135.1, 129.5, 127.8, 125.7, 124.9, 123.1, 123.0, 114.1, 72.4, 55.4; HRMS calcd for C30H25O3S2 [M − OH]+ 497.1240, found 497.1230. 7c: 60% yield, 2.70 g; 1H NMR (400 MHz, CDCl3, δ) 7.72−7.71 (m, 1H, benzene), 7.43 (d, J = 6.4 Hz, 2H, benzene), 7.40 (d, J = 8.0 Hz, 4H, Ar), 7.31 (t, J = 7.6 Hz, 1H, benzene), 7.25 (d, J = 6.80 Hz, 4H, Ar), 7.15 (d, J = 3.6 Hz, 2H, thiophene), 6.85 (d, J = 3.6 Hz, 2H, thiophene), 6.00 (s, 2H, −CH), 2.92 (sep, J = 7.2 Hz, 2H, iso-CH), 1.26 (d, J = 6.8 Hz, 12H, iso-CH3); 13C NMR (100 MHz, CDCl3, δ) 149.0, 148.1, 143.8, 140.5, 135.1, 129.4, 126.8, 126.5, 125.8, 124.9, 123.1, 123.0, 72.7, 34.0, 24.1; HRMS calcd for C34H33OS2 [M − OH]+ 521.1967, found 521.1960. 7d: 55% yield, 2.60 g; 1H NMR (500 MHz, CDCl3, δ) 7.69−7.70 (m, 1H, benzene), 7.40 (d, J = 7.5 Hz, 2H, benzene), 7.41 (bs, 8H, Ar), 7.32 (t, J = 7.5 Hz, 1H, benzene), 7.16 (d, J = 3.5 Hz, 2H, thiophene), 6.87 (d, J = 3.5 Hz, 2H, thiophene), 6.00 (s, 2H, −CH), 1.32 (s, 18H, tbu-CH3); 13C NMR (125 MHz, CDCl3, δ) 151.1, 147.9, 143.7, 139.9, 135.0, 129.3, 126.0, 125.6, 125.5, 124.8, 123.0, 122.9, 72.5, 34.6, 31.3; HRMS calcd for C36H37OS2 [M − OH]+ 549.2280, found 549.2277. 5,20-Bis(p-tolyl)-27,28-bis(thia)-25,26-dihydrotripyrrane (8a). To the solution of 10,10′-bis[(p-tolyl)hydroxymethyl]-1,3-bis(2-thienyl)benzenediol 7a (1.00 g, 2.07 mmol) in pyrrole (1.40 mL, 20.07 mmol) in a 100 mL round-bottom flask was added BF3·OEt2 (0.025 mL, 0.207 mmol) to initiate the reaction. The reaction mixture was stirred under nitrogen for 30 min at room temperature. Then, the mixture was diluted with dichloromethane (300 mL), washed with 0.1 N aq NaOH (100 mL) and water (2 × 100 mL), and dried over anhydrous Na2SO4. The solvent was removed under reduced pressure, and the unreacted pyrrole was removed by vacuum distillation at room temperature. The compound was purified by silica gel column chromatography using petroleum ether/ethyl acetate (90:10). The evaporation of the solvent on a rotary evaporator under reduced pressure gave the desired compound 8a as an orange oil in a 75% yield (0.90 g): 1H NMR (500 MHz, CDCl3, δ) 7.95 (bs, 1H, NH), 7.69−7.65 (m, 1H, benzene), 7.40 (d, J = 7.5 Hz, 2H, benzene), 7.30 (t, J = 8.0 Hz, 1H, benzene), 7.20−7.14 (m, 10H, Ar, thiophene), 6.77 (d, J = 3.5 Hz, 2H, thiophene), 6.73−6.72 (m, 2H, pyrrole), 6.18 (q, J = 5.5 Hz, 2H, pyrrole), 6.00−5.98 (m, 2H, pyrrole), 5.64 (s, 2H, −CH), 2.34 (s, 6H, tolyl-CH3); 13C NMR (125 MHz, CDCl3, δ) 147.2, 143.1, 139.7, 137.0, 135.1, 133.2, 129.5, 129.4, 128.4, 126.8, 124.6, 123.0, 122.8, 117.4, 108.5, 107.7, 45.8, 21.2; HRMS calcd for C38H32NaN2S2 [M + K]+ 603.1899, found 603.1896. 5,20-Bis(anisyl)-27,28-bis(thia)-25,26-dihydrotripyrrane (8b). 5,20-Bis(anisyl)-27,28-bis(thia)-25,26-dihydrotripyrrane 8b was prepared by following the same procedure as given for compound 8a by using 10,10′-bis[(anisyl)hydroxymethyl]-1,3-bis(2-thienyl)benzenediol 7b (1.00 g, 1.94 mmol), pyrrole (1.30 mL, 19.45 mmol), and BF3·OEt2 (0.02 mL, 0.19 mmol). The crude compound was purified by silica column chromatography, and the desired compound 8b was obtained using petroleum ether/ethyl acetate (90:10) to give an orange oil in a 65% yield (0.77 g): 1H NMR (400 MHz, CDCl3, δ) 8.03 (bs, 1H, NH), 7.75−7.74 (m, 1H, benzene), 7.46 (d, J = 7.6 Hz, 2H, benzene), 7.32 (t, J = 7.4 Hz, 1H, benzene), 7.26 (d, J = 8.6 Hz, 2H, Ar), 7.20 (d, J = 3.6 Hz, 2H, thiophene), 6.90 (d, J = 8.6 Hz, 2H, Ar), 6.80 (d, J = 3.5 Hz, 2H, thiophene), 6.75−6.74 (m, 2H, pyrrole), 6.23 (q, J = 5.64 Hz, 2H, pyrrole), 6.04−6.03 (m, 2H, pyrrole), 5.63 (s, 2H, −CH), 3.83 (s, J = 7.2 Hz, 6H, anisyl-OCH3); 13C NMR (100 MHz, CDCl3, δ) 158.6, 147.4, 142.8, 135.0, 134.8, 133.2, 129.4, 129.3, 126.7, 124.5, 122.9, 122.5, 117.4, 114.0, 108.3, 107.5, 55.3, 45.2; HRMS calcd for C38H33N2O2S2 [M + H]+ 613.1978, found 613.1982. 5,20-Bis(4-isopropylbenzene)-27,28-bis(thia)-25,26-dihydrotripyrrane (8c). 5,20-Bis(4-isopropylbenzene)-27,28-bis(thia)-25,26-dihydrotripyrrane 8c was prepared by following the same procedure as given for compound 8a by using 10,10′-bis[(4-isopropylbenzene)-

hydroxymethyl]-1,3-bis(2-thienyl)benzenediol 7c (1.00 g, 1.85 mmol), pyrrole (1.30 mL, 18.58 mmol), and BF3·OEt2 (0.02 mL, 0.19 mmol). The crude compound was purified by silica gel column chromatography, and the desired compound 8c was obtained using petroleum ether/ethyl acetate (90:10) to give an orange oil in a 70% yield (0.82 g): 1H NMR (400 MHz, CDCl3, δ) 7.94 (bs, 1H, NH), 7.72−7.71 (m, 1H, benzene), 7.44 (d, J = 6.0 Hz, 2H, benzene), 7.31 (t, J = 7.2 Hz, 1H, benzene), 7.21−7.29 (m, 8H, Ar), 7.19 (d, J = 3.6 Hz, 2H, thiophene), 6.80 (d, J = 3.6 Hz, 2H, thiophene), 6.73−6.71 (m, 2H, pyrrole), 6.20 (q, J = 6.0 Hz, 2H, pyrrole), 6.04−5.64 (m, 2H, pyrrole), 5.64 (s, 2H, −CH), 2.93 (sep, J = 7.2 Hz, 2H, iso-CH), 1.28 (d, J = 7.2 Hz, 12H, iso-CH3); 13C NMR (100 MHz, CDCl3, δ) 147.8, 147.2, 143.0, 140.0, 135.1, 133.1, 129.4, 128.4, 126.8, 124.6, 123.0, 122.8, 117.4, 108.5, 107.7, 45.8, 33.8, 24.1; HRMS calcd for C42H40KN2S2 [M + K]+ 675.2264, found 675.2265. 5,20-Bis(4-tert-butylbenzene)-27,28-bis(thia)-25,26-dihydrotripyrrane (8d). 5,20-Bis(4-tert-butylbenzene)-27,28-bis(thia)-25,26-dihydrotripyrrane 8d was prepared by following the same procedure as given for compound 8a by using 10,10′-bis[(4-tert-butylbenzene)hydroxymethyl]-1,3-bis(2-thienyl)benzenediol 7d (1.00 g, 1.76 mmol), pyrrole (1.20 mL, 17.66 mmol), and BF3·OEt2 (0.02 mL, 0.17 mmol). The crude compound was purified by silica column chromatography, and the desired compound 8d was obtained using petroleum ether/ethyl acetate (90:10) to give an orange oil in a 75% yield (0.88 g): 1H NMR (400 MHz, CDCl3, δ) 7.93 (bs, 1H, NH), 7.68−7.69 (m, 1H, benzene), 7.41 (d, J = 7.2 Hz, 2H, benzene), 7.35 (d, J = 8.4 Hz, 4H, Ar), 7.29 (t, J = 8.0 Hz, 1H, benzene), 7.23 (d, J = 8.4 Hz, 4H, Ar), 7.16 (d, J = 3.6 Hz, 2H, thiophene), 6.79 (d, J = 3.6 Hz, 2H, thiophene), 6.72−6.70 (m, 2H, pyrrole), 6.17 (q, J = 5.8 Hz, 2H, pyrrole), 6.02−6.01 (m, 2H, pyrrole), 5.63 (s, 2H, −CH), 1.33 (s, 18H, tbu-CH3); 13C NMR (100 MHz, CDCl3, δ) 150.1, 147.2, 143.0, 139.6, 135.1, 133.2, 129.4, 128.1, 136.8, 125.7, 124.6, 123.0, 122.8, 117.4, 108.5, 107.7, 45.8, 34.6, 31.5; HRMS calcd for C44H45N2S2 [M + H]+ 665.3019, found 665.3000. Compound 5a. Samples of 10,10′-bis[(p-tolyl)hydroxymethyl]-1,3bis(2-thienyl)benzenediol 7a (100 mg, 0.20 mmol) and 1 equiv of the appropriate 5,20-bis(p-tolyl)-27,28-bis(thia)-25,26-dihydrotripyrrane 8a (120 mg, 0.20 mmol) in dichloromethane (100 mL) were condensed in the presence of TFA (8.00 μL, 0.10 mmol) for 1 h under an inert atmosphere at room temperature. The mixture was then oxidized with 4 equiv of DDQ (188 mg, 0.80 mmol) for 4 h in open air. The crude compound was purified by silica gel column chromatography using petroleum ether/dichloromethane (70:30) to afford 5a as a brown solid in a 15% yield (30 mg): 1H NMR (400 MHz, CDCl3, δ) 7.71 (bs, 2H, benzene), 7.59−7.55 (m, 6H, Ar, thiophene), 7.37−7.30 (m, 8H, Ar, benzene), 7.29−7.24 (m, 4H, Ar), 7.24−7.15 (m, 8H, Ar, benzene, thiophene), 6.95 (d, J = 5.8 Hz, 2H, pyrrole), 6.45 (d, J = 5.8 Hz, 2H, pyrrole), 6.01 (s, 2H, inner thiophene), 2.40 (s, 6H, tolyl-CH3), 2.39 (s, 6H, tolyl-CH3); 13C NMR (125 MHz, CDCl3, δ) 147.1, 142.9, 142.0, 140.7, 139.2, 138.4, 138.2, 138.0, 136.2, 136.0, 134.0, 131.4, 131.2, 131.0, 129.8, 129.7, 129.1, 126.8, 126.2, 125.1, 124.3, 123.7, 123.4, 119.8, 118.3, 112.8, 109.8, 29.9, 21.5, 21.4; UV−vis (in CHCl3, λmax/nm, log ε) 345 (4.91), 451 (4.51), 537 (4.47); HRMS calcd for C68H46N2S4 [M]+ 1018.2538, found 1018.2534. Compound 5b. The compound 5b was prepared by following the same procedure as given for compound 5a by using 10,10′-bis[(anisyl) hydroxymethyl]-1,3-bis(2-thienyl)benzenediol 7b (100 mg, 0.19 mmol), 5,20-bis(anisyl)-27,28-bis(thia)-25,26-dihydrotripyrrane 8b (119 mg, 0.19 mmol), TFA (7.00 μL, 0.09 mmol), and DDQ (176 mg, 0.77 mmol). The crude compound was purified by silica gel column chromatography using petroleum ether/dichloromethane (60:40) to afford 5b as a brown solid in a 12% yield (25 mg): 1H NMR (500 MHz, CDCl3, δ) 7.71 (bs, 2H, benzene), 7.61−7.59 (m, 6H, Ar, thiophene), 7.40−7.35 (m, 6H, Ar, benzene), 7.26−7.19 (m, 6H, benzene, thiophene), 7.00 (d, J = 8.7 Hz, 4H, Ar), 6.95−6.91 (m, 6H, Ar, pyrrole), 6.43 (d, J = 5.7 Hz, 2H, pyrrole), 6.02 (s, 2H, inner thiophene), 3.86 (bs, 12H, anisyl-OCH3); 13C NMR (125 MHz, CDCl3, δ) 159.9, 158.3, 147.12, 141.6, 140.4, 134.0, 133.4, 132.8, 131.2, 129.6, 127.4, 126.7, 126.6, 126.0, 125.1, 124.2, 123.7, 123.0, 1588



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117.8, 114.6, 113.9, 113.9, 112.3, 109.8, 55.5, 37.2, 29.8, 14.28; UV− vis (in CHCl3, λmax/nm, log ε) 342 (4.97), 458 (4.54), 543 (4.53); HRMS calcd for C68H46N2O4S4 [M]+ 1082.2335, found 1082.2330. Compound 5c. The compound 5c was prepared by following the same procedure as given for compound 5a by using 10,10′-bis[(4isopropylbenzene)hydroxymethyl]-1,3-bis(2-thienyl)benzenediol 7c (100 mg, 0.18 mmol), 5,20-bis(4-isopropylbenzene)-27,28-bis(thia)25,26-dihydrotripyrrane 8c (120 mg, 0.18 mmol), TFA (7.00 μL, 0.09 mmol), and DDQ (168 mg, 0.74 mmol). The crude compound was purified by silica gel column chromatography using petroleum ether/ dichloromethane (75:25) to afford 5c as a brown solid in a 10% yield (22 mg): 1H NMR (500 MHz, CDCl3, δ) 7.62 (d, J = 8.1 Hz, 4H, Ar), 7.38 (d, J = 8.0 Hz, 4H, Ar), 7.32−7.31 (m, 8H, Ar, benzene), 7.29− 7.22 (m, 8H, Ar, benzene, thiophene), 7.00 (d, J = 3.6 Hz, 2H, thiophene), 6.94 (d, J = 5.7 Hz, 2H, pyrrole), 6.44 (d, J = 5.7 Hz, 2H, pyrrole), 5.59 (s, 2H, inner thiophene), 2.98−2.90 (m, 4H, iso-CH), 1.30−1.28 (m, 24H, iso-CH3); 13C NMR (125 MHz, CDCl3, δ) 149.2, 147.1, 145.5, 142.5, 141.4, 141.2, 139.0, 138.3, 137.9, 136.4, 134.6, 131.6, 131.5, 129.4, 127.2, 126.5, 126.4, 126.2, 125.7, 125.4, 125.1, 125.0, 123.8, 118.6, 112.6, 112.4, 111.5, 34.2, 34.1, 34.0, 24.1, 24.1, 24.0, 22.5, 14.2; UV−vis (in CHCl3, λmax/nm, log ε) 338 (4.92), 452 (4.50), 535 (4.45); HRMS calcd for C76H62N2S4 [M]+ 1130.3790, found 1130.3795. Compound 5d. The compound 5d was prepared by following the same procedure as given for compound 5a by using 10,10′-bis[(4-tertbutylbenzene)hydroxymethyl]-1,3-bis(2-thienyl)benzenediol 7d (100 mg, 0.17 mmol), 5,20-bis(4-tert-butylbenzene)-27,28-bis(thia)-25,26dihydrotripyrrane 8d (117 mg, 0.17 mmol), TFA (6.50 μL, 0.08 mmol), and DDQ (160 mg, 0.70 mmol). The crude compound was purified by silica gel column chromatography using petroleum ether/ dichloromethane (80:20) to afford 5d as a brown solid in a 14% yield (30 mg): 1H NMR (400 MHz, CDCl3, δ) 7.62 (d, 4H, J = 8.4 Hz, Ar), 7.46 (d, J = 8.4 Hz, 8H, Ar), 7.45−7.35 (m, 8H, Ar), 7.35−7.30 (m, 4H, benzene), 7.26−7.20 (m, 4H, benzene, thiophene), 7.17 (bs, 2H, benzene), 7.01 (d, J = 3.7 Hz, thiophene), 6.96 (d, J = 5.1 Hz, 2H, pyrrole), 6.45 (d, J = 5.8 Hz, 2H, pyrrole), 5.60 (s, 2H, inner thiophene), 1.37 (s, 18H, tbu-CH3), 1.36 (s, 18H, tbu-CH3); 13C NMR (125 MHz, CDCl3, δ) 149.4, 145.5, 142.5, 141.5, 141.3, 139.1, 138.0, 137.9, 138.0, 137.9, 136.4, 134.6, 131.5, 131.5, 131.2, 129.4, 128.9, 126.7, 126.6, 126.1, 126.0, 125.7, 125.4, 125.3, 125.1, 125.0, 123.8, 123.2, 118.5, 112.3, 111.5, 55.5, 34.5, 34.7, 31.5, 29.8, 22.8, 14.2; UV−vis (in CHCl3, λmax/nm, log ε) 337 (4.93), 452 (4.50), 536 (4.45); HRMS calcd for C80H70N2S4 [M]+ 1186.4416, found 1186.4415. Compound 9. 10,10′-Bis[(p-tolyl)hydroxymethyl]-1,3-bis(2thienyl)benzenediol 7a (100 mg, 0.20 mmol) and 5,20-bis(p-tolyl)27,28-bis(thia)-25,26-dihydrotripyrrane 8a (120 mg, 0.20 mmol) were dissolved in dichloromethane (100 mL), and TFA (8.00 μL, 0.10 mmol) was added to initiate the reaction. The reaction mixture was stirred in the dark at room temperature under nitrogen for 1 h. Two equivalents of DDQ (94 mg, 0.40 mmol) were added, and the mixture was stirred for an additional 2 h. The crude compound was purified by silica gel column chromatography using petroleum ether/dichloromethane (60:40) to afford 9 as a brown solid in an 18% yield (40 mg): 1 H NMR (500 MHz, CDCl3, δ) 7.89 (bs, 2H), 7.68 (bs, 1H), 7.89 (d, J = 8.0 Hz, 2H), 7.47−7.38 (m, 4H), 7.30−7.09 (m, 17H), 6.94 (d, J = 5.8 Hz, 1H), 6.83 (m, 1H), 6.50 (bs, 1H), 6.40 (d, J = 5.7 Hz, 1H), 6.10−5.98 (m, 4H), 5.66−5.51 (m, 4H), 2.41 (s, 3H, tolyl-CH3), 2.40 (s, 3H, tolyl-CH3), 2.35 (s, 3H, tolyl-CH3), 2.33 (s, 3H, tolyl-CH3); 13 C NMR (125 MHz, CDCl3, δ) 147.3, 138.9, 138.5, 138.3, 138.0, 136.9, 136.8, 136.2, 135.8, 135.0, 134.4, 134.0, 131.6, 131.5, 131.1, 129.8, 129.4, 129.1, 128.4, 127.4, 126.3, 126.2, 125.3, 124.5, 124.1, 123.5, 123.2, 122.9, 121.6, 121.1, 118.4, 112.8, 55.5, 45.9, 32.1, 31.6, 30.3, 29.9, 29.8, 29.5, 27.0, 22.8, 21.5, 21.4, 21.2; UV−vis (in CHCl3, λmax/nm, log ε) 308 (4.30), 460 (4.30), 552 (4.27); HRMS calcd for C68H51N2S4 [M + H]+ 1023.2930, found 1023.2933.

Note

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b02851. Characterization data for all reported compounds (HRMS, 1H and 13C NMR spectral, absorption, electrochemical, and computational data) (PDF) Crystallographic data for compound 9 (CIF) Crystallographic data for compound 5a (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 91-22-5723480. Tel: 91-22-5767176. ORCID

Mangalampalli Ravikanth: 0000-0003-0193-6081 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.R. thanks the SERB Department of Science and Technology and the Government of India (EMR/2015/002196). S.K. thanks the UGC for a fellowship.

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REFERENCES

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