Introduction

Heptalene has a fused structure of two cycloheptatrienes which is one of the non-aromatic bicyclic molecules with a 12π-electronic structure. Heptalene produces the stabilized aromatic dication and dianion with 10π- and 14π-electrons by two-electron oxidation and reduction, respectively. Fascinated by such unique properties, many researchers have studied the synthesis and characterization of various heptalene derivatives. Therefore, a number of synthetic methods for the heptalene derivatives have appeared in the literature. In 1961, the first synthesis of parent heptalene 1a was reported by Dauben Jr. and Bertelli as an instable compound1. Hansen et al. established the preparation of benzo-fused heptalene 2a, but 2a is still unstable and the synthetic protocol requires a multi-step pathway2. Recently, Fukazawa, Yamaguchi, and their colleagues have achieved the synthesis of kinetically stabilized heptalene 3 fused by four thiophenes by reductive cyclization of bisdehydro[12]annulene with sodium metal, as well as its dianion (Fig. 1)3.

Figure 1
figure 1

Heptalene derivatives 1a, 1b, 2a, 2b, and 3 appeared in the literature.

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In addition to the above synthetic procedures, preparation of the heptalene skeleton is established by using the reaction of azulene derivatives with dimethyl acetylenedicarboxylate (DMAD). In 1976, Hafner et al. reported the cycloaddition reaction of azulene itself with DMAD to afford heptalene derivative 1b with two-ester functions4. The reaction of benzo[a]azulene with DMAD gives benzene-fused heptalene derivative 2b, which is demonstrated by Yasunami et al.5. Hansen et al. have reported the synthesis and reactivity of several fused-heptalenes including benzene-fused derivatives6,7,8,9,10,11,12,13,14,15. However, there are few examples of the synthesis of heptalenes by the reaction of ring-fused azulene derivatives with DMAD, and the properties of the ring-fused heptalene derivatives have hardly been evaluated, although a number of azulene derivatives may become a promising precursor for the heptalene synthesis.

In recent years, we have investigated the novel synthetic methods for various azulene derivatives and explored their distinctive reactivity16,17,18,19,20,21,22,23,24,25. In these researches, we have reported the synthesis, optical and electrochemical properties of azulenothiophene derivatives23,26,27,28 that become good precursors for the thiophene-fused heptalenes. Although thiophene-fused heptalene derivative 3 has already been reported as described above, preparation of much simpler derivatives should be essential for better understanding the nature of these series.

In this paper, we describe a synthesis of thiophene-fused heptalene derivatives from the readily accessible starting materials, i.e., azulenothiophenes. The structural feature of the heptalene derivatives was elucidated by single-crystal X-ray structural analysis. The electronic properties of the thiophene-fused heptalenes obtained by this study were characterized by UV/Vis and fluorescence spectroscopy and theoretical calculations. The electrochemical features of these derivatives were also examined by voltammetry and spectroelectrochemical experiments.

Results and discussion

Azulenothiophenes 4a, 4b, and 5, precursors for the thiophene-fused heptalenes 6a, 6b, and 7, were prepared by the reaction of 2H-cyclohepta[b]furan-2-ones with the corresponding enamines29. The reaction of azulenothiophene 4a with DMAD in tetraline at 150 °C, subsequent chromatographic purification afforded the thiophene-fused heptalene 6a, but in low yield (15%), whereas the reaction under the higher temperature condition (200 °C) resulted in a significant increase of the yield of 6a (60%). Synthesis of 6b and 7 was also accomplished by a similar manner by the reaction of 4b and 5 in 54% and 23% yields, respectively (Fig. 2). Despite the parent heptalene 1a is thermally unstable, compounds 6a, 6b, and 7 did not exhibit the decomposition even after standing for more than a year under the ambient conditions.

Figure 2
figure 2

Synthesis of thiophene-fused heptalenes 6a, 6b, and 7 by the cycloaddition reaction of azulenothiophenes 4a, 4b, and 5 with DMAD; red-numbers show the numbering of ring-carbon.

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The presumed reaction mechanism for the formation of the thiophene-fused heptalenes is illustrated in Fig. 3. The reaction should be initiated by the nucleophilic addition of electron-rich azulene at the 8-position to DMAD to produce the transient zwitterion A, which immediately cyclizes by intramolecular nucleophilic addition to give cyclobutene intermediate B. Eventually, strained cyclobutene intermediate B is transformed by the retroelectrocyclization to give the thiophene-fused heptalenes.

Figure 3
figure 3

Presumed reaction mechanism for the formation of the thiophene-fused heptalenes.

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Thiophene-fused heptalenes 6a, 6b, and 7 obtained by this study were characterized on the basis of their spectral data as summarized in the “Methods” section. High-resolution mass spectra (HRMS) of the new compounds ionized by MALDI-TOF showed the correct molecular ion peaks. The characteristic stretching absorption of the carbonyl group was observed at νmax = 1699 − 1701 cm−1 in their IR spectra, which supported the presence of the ester functions in these compounds.

Since the single crystals of 6a, 6b, and 7 were obtained by the slow evaporation of CH2Cl2/MeOH mixed solvent, the molecular structure of 6a, 6b, and 7 was clarified by single-crystal X-ray structure analysis (Fig. 4). The X-ray analysis of 6a, 6b, and 7 revealed their twisted structure between the 2 and 7 membered rings. The dihedral angles between the mean planes of these rings are observed as 49.78° (6a), 49.29° (6b), and 55.02° (7), whereas higher planarity between the thiophene and cycloheptatriene rings was observed with the dihedral angles of 19.25° (6a), 20.77° (6b), and 24.18° (7). Overall, 7 showed the lowest flatness compared to those of 6a and 6b; this may suggest a steric repulsion between the ring protons on the thiophene and heptalene rings in 7. Furthermore, both X-ray and 1H NMR spectral analyses denoted the bond alternation in the fused-ring structure as shown in Fig. 5 that means the non-aromatic character in the heptalene moiety of 6a, 6b, and 7.

Figure 4
figure 4

ORTEP drawings of thiophene-fused heptalenes (a) 6a (CCDC1965949), (b) 6b (CCDC1965950), and (c) 7 (CCDC1965951); ellipsoids are drawn at the 50% probability level.

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Figure 5
figure 5

Copyright CCDC 2001–2019 (https://www.ccdc.cam.ac.uk/mercury/).

Bond length of heptalene moieties of (a) 6a, (b) 6b, and (c) 7. The bond lengths in the single-crystal X-ray structure analysis were calculated using the following program; Mercury 4.1.0.

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UV/Vis and fluorescence spectra of 6a, 6b, 7, and 1b in CH2Cl2 are shown in Fig. 6. The longest wavelength absorption maxima and their coefficients (log ε) are summarized in Table 1. The absorption maxima of 6a, 6b, and 7 exhibited a bathochromic shift by 10 − 13 nm compared with that of 1b. In comparison with the longest wavelength absorption maximum of 3 (λmax = 399 nm), that of 6a, 6b, and 7 showed hypsochromic shift by 45 − 48 nm, although no significant differences among those of 6a, 6b, and 7 were observed. These results indicate that the fused-thiophene ring contributes to the expansion of the conjugated system, but this arrangement does not have a significant effect with respect to their UV/Vis spectra. Thiophene-fused heptalenes 6a, 6b, and 7 in CH2Cl2 showed weak luminescence at λflu = 436 − 439 nm. Since heptalene 1b also displayed the weak fluorescence in the similar region (λflu = 436 nm), this emission should originate from the heptalene skeleton.

Figure 6
figure 6

UV/Vis and fluorescence spectra of 1b (blue line), 6a (red line), 6b (light-green line), and 7 (purple line) in CH2Cl2.

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Table 1 Absorption (λmax) and fluorescence maxima (λflu) of heptalenes 6a, 6b, 7, and 1b in CH2Cl2.
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As mentioned in the Introduction section, heptalene derivatives generate the aromatic dication and dianion by two-electron oxidation and reduction, respectively. Heptalenes 6a, 6b, and 7 may produce the stabilized dication and dianion by the electrochemical redox reaction. Thus, the redox behavior of 6a, 6b, and 7 was examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) experiments (Table 2). The cyclic voltammograms of 6a, 6b, and 7 are summarized in the Supporting Information. Contrary to our expectations, heptalenes 6a, 6b, and 7 exhibited irreversible oxidation and reduction waves on the CV. Irreversibility on the CV waves of these compounds should be attributed to the instability of the cationic and anionic species generated by the electrochemical oxidation and reduction, respectively, due to their lower kinetic stability compared to that of thiophene-fused heptalene derivative 3 reported by Fukazawa et al.

Table 2 Redox potentials of heptalenes 6a, 6b, and 7.
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To observe these unstable cationic and anionic species spectroscopically, spectroelectrochemistry of 6a, 6b, and 7 was examined in benzonitrile solution containing Et4NClO4 (0.1 M) as a supporting electrolyte. We found that the thiophene-fused heptalenes 6a, 6b, and 7 exhibited spectral changes under both electrochemical oxidation and reduction conditions. These results suggest the formation of cationic and anionic species by the electrochemical redox reactions. However, the reverse redox reaction of the generated species did not recover the original absorption spectra of 6a, 6b, and 7, completely. This phenomenon indicates that the charged species generated electrochemically are quite unstable as suggested by the irreversibility on the redox waves as observed by the CV measurements. For instance, when the spectral changes of 6b were measured under the electrochemical oxidation conditions, the development of the peak at λmax = 354 nm was observed, along with the generation of the new absorption band at around 440 nm (Fig. 7, left). However, the reverse reduction of the oxidized species of 6b did not recover the original spectrum of neutral 6b, completely (Fig. 7, right).

Figure 7
figure 7

Continuous change in the visible spectrum of 6b: constant-voltage electrochemical oxidation at + 1.00 V (left) and electrochemical reduction of the oxidized species at ± 0 V (right) in benzonitrile containing Et4NClO4 (0.1 M) at 30 s intervals.

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Conclusion

In this paper, we described the preparation of thiophene-fused heptalene derivatives 6a, 6b, and 7 by the cycloaddition reaction of the corresponding azulenothiophenes with DMAD. The single-crystal X-ray structural analysis of 6a, 6b, and 7 revealed their twisted structure, as well as their non-aromatic character denoted by their bond alternation. The measurements of UV/Vis spectra of 6a, 6b, and 7 revealed that the fused-thiophene ring directly contributed to the expansion of the conjugated system. In the spectroelectrochemistry measurements, 6a, 6b, and 7 exhibited the spectral changes under both electrochemical oxidation and reduction conditions, although the complete recovery of the original absorption spectra was not observed by the reverse redox reactions. These results indicate the generation of labile cationic and anionic species electrochemically from the thiophene-fused heptalene derivatives 6a, 6b, and 7.

Methods19,30

Melting points were determined with a Yanagimoto MPS3 micro melting apparatus and are uncorrected. Voltammetry measurements were carried out with a BAS 100B/W electrochemical workstation equipped with Pt working and auxiliary electrodes and a reference electrode formed from Ag/AgNO3 (0.01 M) in acetonitrile containing tetrabutylammonium perchlorate (0.1 M). High-resolution mass spectra were obtained with a Bruker Daltonics APEX III instrument (dithranol as a matrix substance and/or CF3CO2Ag as an auxiliary agent). IR and UV/Vis spectra were measured with JASCO FT/IR-4100 and Shimadzu UV-2550 spectrophotometers. 1H and 13C NMR spectra were recorded in CDCl3 with a JEOL ECA500 at 500 MHz and 125 MHz, respectively.

Synthesis of 6a

A solution of 4a (800 mg, 4.34 mmol) and DMAD (925 mg, 6.51 mmol) in tetralin (20 mL) was stirred at 200 °C for 45 min under an Ar atmosphere. After the reaction, the crude product was purified by alumina column chromatography with AcOEt as an eluent and reversed-phase chromatography with 80% MeOH to afford 6b (843 mg, 2.58 mmol, 60%) as reddish-orange crystals.

IR (AT-IR): νmax = 3,094 (w), 2,954 (w), 1717 (m), 1701 (s), 1,630 (w), 1603 (w), 1572 (w), 1,540 (w), 1507 (w), 1,437 (m), 1,405 (w), 1,387 (w), 1,349 (w), 1,290 (m), 1,262 (s), 1,246 (m), 1,206 (m), 1,157 (w), 1,120 (m), 1,103 (w), 1,060 (w), 1,034 (w), 1,007 (w), 984 (w), 948 (w), 933 (w), 903 (w), 889 (w), 865 (w), 840 (w), 805 (w), 792 (w), 775 (m), 752 (w), 731 (w), 721 (m), 693 (w), 666 (w), 653 (m) cm−1; UV/Vis (CH2Cl2): λmax = 289 (4.39), 351 (3.61) nm; 1H NMR (500 MHz, CDCl3): δH = 7.83 (s, 1H), 7.22 (d, J = 5.2 Hz, 1H), 6.94 (d, J = 5.2 Hz, 1H), 6.52 − 6.51 (m, 2H), 6.31 − 6.28 (m, 1H), 6.17 (t, J = 3.6 Hz, 1H), 5.92 (d, J = 10.9 Hz, 1H), 3.75 (s, 3H, CO2Me), 3.67 (s, 3H, CO2Me) ppm; 13C NMR (125 MHz, CDCl3): δC = 167.70, 167.44, 143.84, 142.49, 137.67, 137.15, 133.16, 132.20, 131.15, 130.71, 130.00, 129.62, 126.68, 126.47, 125.21, 124.65, 52.18, 51.68 ppm; HRMS (MALDI-TOF, positive): calcd for C18H14O4S+ [M]+ 326.0607, found: 326.0595, C18H14O4S + Ag+ [M + Ag]+ 432.9658, found: 432.9686.

Synthesis of 6b

A solution of 4b (450 mg, 1.99 mmol) and DMAD (424 mg, 2.99 mmol) in tetralin (20 mL) was stirred at 200 °C for 20 min under an Ar atmosphere. After the reaction, the crude product was purified by alumina column chromatography with AcOEt as an eluent and reversed-phase chromatography with 80% MeOH to afford 6b (398 mg, 1.08 mmol, 54%) as reddish-orange crystals. M.p. 148 − 149 °C; IR (AT-IR): νmax = 3,108 (w), 2,956 (w), 1699 (s), 1578 (w), 1,435 (m), 1,365 (w), 1,299 (m), 1,279 (s), 1,260 (s), 1,236 (m), 1,215 (w), 1,191 (m), 1,169 (m), 1,120 (m), 1,092 (w), 1,058 (m), 1,034 (w), 1,003 (w), 964 (w), 950 (w), 902 (w), 891 (w), 873 (w), 854 (w), 840 (w), 802 (m), 787 (s), 769 (m), 752 (m), 739 (s), 730 (s), 678 (m), 661 (w) cm−1; UV/Vis (CH2Cl2): λmax = 289 (4.40), 354 (3.62) nm; 1H NMR (500 MHz, CDCl3): δH = 7.81 (s, 1H), 7.20 (d, J = 5.4 Hz, 1H), 6.95 (d, J = 5.4 Hz, 1H), 6.45 − 6.52 (m, 2H), 6.09 (d, J = 5.7 Hz, 1H), 5.72 (s, 1H), 3.75 (s, 3H, CO2Me), 3.67 (s, 3H, CO2Me), 2.56 (sept, J = 6.9 Hz, 1H, i-Pr), 1.15 (d, J = 6.9 Hz, 6H, i-Pr) ppm; 13C NMR (125 MHz, CDCl3): δC = 167.96, 167.87, 149.64, 143.44, 142.19, 137.08, 136.89, 134.50, 132.25, 130.41, 129.64, 129.01, 128.06, 126.15, 124.54, 120.99, 52.17, 51.59, 35.74, 22.53 ppm; HRMS (MALDI-TOF, positive): calcd for C21H20O4S+ [M]+ calcd: 368.1077, found: 368.1098, C18H14O4S + Ag+ [M + Ag]+ calcd: 475.0128, found: 475.0142.

Synthesis of 7

A solution of 5 (500 mg, 2.72 mmol) and DMAD (578 mg, 4.07 mmol) in tetralin (20 mL) was stirred at 200 °C for 45 min under an Ar atmosphere. After the reaction, the crude product was purified by alumina column chromatography with AcOEt as an eluent and reversed-phase chromatography with 80% MeOH to afford 6b (208 mg, 0.638 mmol, 23%) as reddish-orange crystals. M.p. 159 − 161 °C; IR (AT-IR): νmax = 3,108 (w), 2,956 (w), 1699 (s), 1578 (w), 1,435 (m), 1,365 (w), 1,299 (m), 1,279 (s), 1,260 (s), 1,236 (m), 1,215 (w), 1,191 (m), 1,169 (m), 1,120 (m), 1,092 (w), 1,058 (m), 1,034 (w), 1,003 (w), 964 (w), 950 (w), 902 (w), 891 (w), 873 (w), 854 (w), 840 (w), 802 (m), 787 (s), 769 (m), 752 (m), 739 (s), 730 (s), 678 (m), 661 (w) cm−1; UV/Vis (CH2Cl2): λmax = 293 (4.48), 354 (3.65) nm; 1H NMR (500 MHz, CDCl3): δH = 7.85 (s, 1H), 7.57 (d, J = 5.2 Hz, 1H), 6.70 (d, J = 5.2 Hz, 1H), 6.56 (dd, J = 11.0, 6.3 Hz, 1H), 6.50 (dd, J = 11.0, 6.3 Hz, 1H), 6.30 (dd, J = 11.0, 6.3 Hz, 1H), 5.99 (d, J = 6.6 Hz, 1H), 5.93 (d, J = 11.2 Hz, 1H), 3.75 (s, 3H, CO2Me), 3.67 (s, 3H, CO2Me) ppm; 13C NMR (125 MHz, CDCl3): δC = 167.45, 167.41, 144.07, 143.91, 136.92, 134.99, 132.75, 132.42, 131.91, 131.61, 130.40, 129.58, 128.95, 128.82, 125.24, 124.38, 52.23, 51.66 ppm; HRMS (MALDI-TOF, positive): calcd for C18H14O4S+ [M]+ calcd: 326.0607, found: 326.0580, C18H14O4S + Ag+ [M + Ag]+ calcd: 432.9658, found: 432.9679.