Letter Cite This: Org. Lett. 2018, 20, 6632−6635
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From Colorless to Near-Infrared S‑Heteroarene Isomers: Unexpected Cycloaromatization of Cyclopenta[b]thiopyran Catalyzed by PtCl2 Yifan Lu, Yanjun Qiao, Haodong Xue, and Gang Zhou* Lab of Advanced Materials, State Key Laboratory of Molecular Engineering of Polymers, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai 200438, P.R. China
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ABSTRACT: S-heteroarenes containing cyclopenta[b]thiopyran moieties were synthesized via intramolecular ringexpanding cycloisomerization from 1-acetyl-2-thienyl-substituted precursors catalyzed by PtCl2. In comparison to the other two S-heteroarene isomers experiencing common 6endo and 5-exo cyclization processes, the aromatic cyclopenta[b]thiopyran derivative demonstrates intriguing photophysical and electrochemical properties, such as near-infrared absorption, low oxidation potential, and excellent electrochemical stability. Therefore, this work provides an effective pathway to incorporate cyclopenta[b]thiopyran as building blocks for organic semiconductors with unique properties.
S
benzo[b]thiophene or methylenecyclopenta[b]thiophene derivatives. Consequently, different S-heteroarenes can be easily synthesized from an identical precursor. Cyclopenta[b]thiopyran, an isomer of benzo[b]thiophene, has been known as pseudoazulene due to their similar π-electron structures.10 However, the properties of cyclopenta[b]thiopyran derivatives have rarely been investigated probably due to their unfavorable synthetic routes.10,11 Herein, 1,5-di(1-decynyl)-2,6dithienylnaphthalene was designed and utilized to produce different rigid coplanar building blocks for S-heteroarenes. Unexpectedly, under the catalyzation of PtCl2, the cycloaromatization reaction went through neither a common 6-endo nor 5-exo cyclization process and gave a cyclopenta[b]thiopyran-fused polycyclic aromatic product 7,14-dioctylnaphtho[2,1-f:6,5-f ′]bis(cyclopenta[b]thiopyran) (6c, Scheme 2). The chemical structures of the S-heteroarene isomers were identified by 1H NMR, mass spectroscopies, and single-crystal X-ray analysis. A possible mechanism of this ring-expanding cyclization is proposed in this paper. Most interestingly, the asprepared cyclopenta[b]thiopyran derivative presents distinct optoelectronic properties, such as near-infrared (NIR) absorption, low oxidation potential, and excellent electrochemical stability. Therefore, our findings provide not only a new pathway for construction of thiopyran derivatives but also a novel kind of promising building block with unique properties for further optoelectronic applications. The synthetic approach to S-heteroarene isomers 6a, 6b, and 6c started from commercially available naphthalene-1,5-diol (1, Scheme 2). Initially, it was brominated to 2,6-dibromonaph-
-containing fused heteroarenes that feature rigid planar backbones and extended π-conjugation have received tremendous attention due to their unique electronic and optical characteristics, such as intense luminescence, tunable band gap, high charge-carrier mobility, and intriguing self-assembly behavior.1 Therefore, the construction of fused S-heteroarenes has been a hot topic in recent years. Until now, a number of synthetic methodologies have been developed for the preparation of such S-heteroarenes.2−9 The representative method is to lock two twistable aromatic rings by introducing a sulfur atom.2 Alternatively, rigid S-heteroarenes can be synthesized by direct fusion on S-containing aromatic systems.3 In particular, 1-acetyl-2-thienyl-substituted aromatic rings are utilized as precursors for the construction of S-containing polycyclic scaffolds. With the assistance of acid,4 base,5 or metalbased catalyst,6−8 straightforward 6-endo or 5-exo cyclization may occur (Scheme 1),9 which results in the formation of Scheme 1. Different Cyclization Pathways for 1-Acetyl-2thienyl-Substituted Aromatics
Received: August 8, 2018 Published: October 22, 2018 © 2018 American Chemical Society
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DOI: 10.1021/acs.orglett.8b02546 Org. Lett. 2018, 20, 6632−6635
Letter
Organic Letters
corresponding to the protons Hc′ on the vinyl groups. For isomer 6c, the appearance of the triplet peak at δ = 7.15 ppm is characteristic of the protons Hb″ on the thiopyran rings and indicates the successful ring enlargement. Moreover, singlecrystal X-ray diffraction analysis (Figure 1) reveals that the π-
Scheme 2. Synthetic Route for S-Heteroarenes 6a−6c
Figure 1. ORTEP diagrams with ellipsoid contour probability level of 50% and crystal packings (only carbon and sulfur atoms are shown for clarity) of isomeric S-heteroarenes 6a, 6b, and 6c.
conjugated backbones of all the three isomers are coplanar with dihedral angles between two neighboring aromatic rings less than 3°. Therefore, S-heteroarene isomers 6a, 6b, and 6c demonstrate offset face-to-face stacking structures with interplanar distance of 3.40, 3.39, and 3.48 Å, respectively. Isomers 6a and 6b present “fishbone” packing structures, with the angles between two crossing molecular planes being 83 and 88°, respectively. However, all the π-conjugated backbones of 6c molecules are parallel to each other. To further investigate the molecular structures, bond length analysis was conducted. The lengths of S−C bonds in isomers 6a and 6b range from 1.7103 to 1.7405 Å (Tables S3 and S4), which are typical bond lengths for S−C bonds in the thiophene ring, whereas the lengths of S−C bonds in the thiopyran ring of 6c are 1.7158(18) and 1.7376(17) Å, respectively (Table S5). Similar to those in the thiophene rings of isomers 6a and 6b, these values are between the lengths of well-acknowledged S−C single (1.778−1.882 Å)14 and double bonds (1.553−1.611 Å).15 This suggests that the lone pair electrons of the sulfur atoms are involved in the πconjugated backbone of isomer 6c. Furthermore, the lengths of the C−C bonds (Table S5) indicate that the resonance structure 6c is more predominant than the other 6c′ (Figure S5), giving two specific cyclopenta[b]thiopyran moieties in isomer 6c. Hence, the π-electron numbers of cyclopenta[b]thiopyran and those of the skeleton of isomer 6c are calculated to be 10 and 26, respectively, which perfectly match Hückel’s rule, (4n + 2)πelectrons. Therefore, it comes to a preliminary conclusion that Sheteroarene isomer 6c containing cyclopenta[b]thiopyran moieties presents aromatic characteristics. To gain insight into the aromaticity of the resulting Sheteroarene isomers, nucleus-independent chemical shift (NICS) calculations were performed.16 As shown in Figure S6, the NICS(1) values of all the rings in isomer 6a are lower than −9.2 ppm, verifying their highly aromatic property. However, the two fulvene rings in isomer 6b display NICS(1) values of −0.7 ppm, which suggest their nonaromaticity
thalene-1,5-diol (2) by bromine in acetic acid and then esterified to 2,6-dibromonaphthalene-1,5-diyl bis(trifluoromethanesulfonate) (3) by trifluoromethanesulfonic anhydride with pyridine as base. Subsequently, the key intermediate 1,5-di(1decynyl)-2,6-dithienylnaphthalene (5) was obtained via two selective Pd-catalyzed coupling reactions, that is, Sonogashira coupling12 with 1-decyne and Stille coupling13 with tributyl(2thienyl)stannane. With the assistance of basic 1,8-diazabicyclo[5.4.0]undec-7-ene, the decyne substituents at the orthopositions of the dithienyl groups went through a classic 6endo cyclization and produced isomer 7,14-dioctylchryseno[1,2-b:7,8-b′]dithiophene (6a). Alternatively, intermediate 5 underwent classic 5-exo cyclization and provided isomer 6,12dinonylidenenaphtha[1,2-f:5,6-f ′]bis(cyclopenta[b]thiophene) (6b) with Pd(OAc)2 as catalyst and 1,1′-bis(diisopropylphosphino)ferrocene as ligand. Interestingly and unexpectedly, when PtCl2 was used as catalyst, an unusual ringexpanding compound, 7,14-dioctylnaphtho[2,1-f:6,5-f ′]bis(cyclopenta[b]thiopyran) (6c), was obtained. The two thiophene units experienced ring enlargement and gave thiopyran rings in 58% yield. It should be noted that neither common 6-endo nor 5-exo product was formed in this case as monitored by thin-layer chromatography. Moreover, when the ring-expanding cycloaromatization was performed on a monofunctionalized precursor P1 (Table S1), a high yield of 83% was achieved. Furthermore, the reaction condition was further optimized and is shown in Table S1. It is found that neither polar solvents nor additive ligands contribute to the yields of cyclopenta[b]thiopyran derivatives. The structures of isomeric heteroarenes 6a, 6b, and 6c were verified by 1H NMR spectroscopies and single-crystal X-ray diffraction analysis. As shown in Figure S1, a clear singlet peak at δ = 8.48 ppm can be found in the 1H NMR spectrum of 6a, which is assigned to the proton signal of Hc. Similarly, the 1H NMR spectrum of 6b displays a triplet peak at δ = 7.20 ppm, 6633
DOI: 10.1021/acs.orglett.8b02546 Org. Lett. 2018, 20, 6632−6635
Letter
Organic Letters
methyl groups are substituted at the para-positions of the sulfur atoms in the thiopyran rings of T6. Therefore, it proves that the insertion of the carbon atom from the alkynyl group, which results in the ring expansion of thiophene ring, occurs at the sulfur−carbon bond, while other bonds remain unbroken. Although three isomers contain similar chemical structures, the absorption properties significantly differ from each other. As shown in Figure 3, isomer 6a with six fused aromatic rings only
characteristic. Interstingly, the NICS(1) values for the six rings in isomer 6c are calculated to be −6.2, −9.3, −10.7, −10.6, −9.0, and −6.1 ppm, respectively. It is notable that the cyclopentadiene rings in isomer 6c present high aromaticity as they are often regarded as antiaromatic.17 Moreover, the NICS(1) values for thiopyran rings in isomer 6c are slightly lower than those for the thiophene rings in isomers 6a and 6b, indicating mediate strong aromaticity of six-membered thiopyran rings.18 The scope of this Pt-catalyzed ring-expanding cyclization was further investigated by tuning the aryl backbones and the alkyl substituents. As shown in Table S6, a series of mono- and bisfunctionalized substrates with benzene, naphthalene, and fluorene as building blocks can be successfully converted to the cyclopenta[b]thiopyran derivatives. Moreover, when the substituent on the acetylene is functionalized by an electrondeficient trifluoromethyl group, this cyclization reaction is deactivated. A probable mechanism of the cycloaromatization has been proposed and is illustrated in Figure 2. The reaction
Figure 3. UV−vis−NIR absorption and PL spectra of S-heteroarene isomers 6a, 6b, and 6c in DCM solutions.
exhibits absorption in the ultraviolet region with the maximum absorption wavelength at 386 nm in dichloromethane (DCM) solution. Under the same conditions, a significant bathochromic shift of 90 nm can be observed for isomer 6b (λmax = 476 nm), which is attributed to the classic nonalternant hydrocarbon absorption.21 Interestingly, isomer 6c demonstrates unusual broad absorption spectra over the whole visible region extending into the NIR range up to 840 nm (λmax = 660 nm). As cyclopenta[b]thiopyran is a (5π + 7π)-electron system, it is isoelectronic to the famous azulene, which always displays an absorption band in the NIR region.22 Similar to azulene,23 the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are localized partially in different parts of space in cyclopenta[b]thiopyran (Figure S8). Therefore, the long wavelength absorption of 6c is mainly attributed to the electron transition from HOMO to LUMO, as also revealed by theoretical calculations (Table S12). The absorption edges of isomers 6a, 6b, and 6c locate at 397, 553, and 840 nm, respectively, which indicates that the band gap of the isomers dramatically decreases from 3.12 eV for 6a to 2.24 eV for 6b and further to 1.48 eV for 6c. Correspondingly, isomers 6a and 6b display photoluminescence (PL) maxima at 388 and 547 nm, respectively. However, the emission of 6c is too weak to be detected in the NIR region by the PL spectrometer. Therefore, it can be concluded that the photophysical properties and the energy band gap of S-heteroarene-based organic semiconductors can be easily tuned by varied cyclization reactions from identical starting materials. Figure 4 displays 50 cycles of the cyclic voltammograms for the isomers in DCM solutions (0.1 mM). A redox couple with oxidation onset potential at 0.65 and 0.35 V (vs Fc/Fc+, the same below) can be observed in the first cycle for isomers 6a and 6b, respectively, which is ascribed to the oxidation of the thiophene unit. However, during repetitive cycling, the currents gradually changed with a film formed on the surface of the working electrode. This indicates the electrochemical polymerization of 6a and 6b on the working electrode surface.24 Under the same
Figure 2. Proposed mechanism of PtCl2-catalyzed ring-expanding cycloaromatization.
begins with the coordination of PtCl2 with the alkyne I, activating the alkyne for the following transformation and forms a π-complex II. Then the electron-rich sulfur atom approaches the activated, electron-deficient alkyne to form a new sixmembered ring and generates a thienoium moiety (III). The πelectrons of the alkene attack the electron-deficient α-carbon on the thienoium moiety, giving a three-membered ring transition state IV.19,20 Owing to the instability of the sulfur−carbon bond in thienoium caused by electron deficiency and huge ring tension in the three-membered ring, a ring expansion can easily take place, forming a very thermodynamically stable V. Subsequently, a platinum σ-bond complex VI is generated by [1,2]-alkyl shift from V, followed by the PtCl2 dissociation to give cyclopenta[b]thiopyran derivative VII. To verify the proposed mechanism, the reaction path and the relative free energies at 383.15 K for all stationary points were calculated using density function theory with LANL2DZ basis set for Pt and 6-31G(d) for other atoms. As shown in Figure S7, all the transition states are confirmed to connect the neighboring intermediates. The model reaction turns out to be exothermic at 27.2 kcal mol−1, and the calculated relative free energies support the instability of transition state IV and the easiness of ring expansion (barrier 7.5 kcal mol−1). To further support the proposed mechanism, a control experiment was conducted by using 3-methyl-substituted analogue P6 as the precursor (Table S6). The 1H NMR spectrum of the product indicates that the 6634
DOI: 10.1021/acs.orglett.8b02546 Org. Lett. 2018, 20, 6632−6635
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Letter
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51722301, 21674023, and 21733003).
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Figure 4. Cyclic voltammograms of S-heteroarene isomers 6a, 6b, and 6c in DCM solutions. The red and gray lines represent the 1st and 2nd to 50th cycles of the CV curves, respectively.
conditions, a further negatively shifted oxidation onset potential at 0.04 V can be observed for isomer 6c which exhibits two reversible redox couples with half-wave potentials of 0.11 and 0.39 V, respectively. Such two-step one-electron oxidations correspond to the sequential removal of electrons from the thiopyran moieties to form the stable radical cation and dication. Moreover, unlike isomers 6a and 6b, the CV curves of 6c overlap very well during repetitive cycling, which implies that its oxidized states are extremely electrochemically stable. In summary, an unexpected ring-expanding cycloaromatization of cyclopenta[b]thiopyran from 1-acetyl-2-thienyl derivative catalyzed by PtCl2 was developed. From an identical precursor, three rigid and planar S-heteroarene isomers were successfully synthesized. Interestingly, the three resulting isomeric S-heteroarenes demonstrated completely different photophysical and electrochemical properties. The NIR absorption, relatively low oxidation potential, and the excellent electrochemical stability will make aromatic cyclopenta[b]thiopyran derivatives as promising building blocks for organic semiconductors with unique properties. Our ongoing work is focused on the expansion of the substrate scope and their further applications in optoelectronics.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b02546. Synthesis and characterizations of S-heteroarenes 6a−6c; optimization of the cyclization conditions and scope of the ring-expanding cycloaromatization (PDF) Accession Codes
CCDC 1579132, 1579137, and 1854147 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Tel/Fax: +86-21-5163-0350. ORCID
Gang Zhou: 0000-0002-1533-7795 Notes
The authors declare no competing financial interest. 6635
DOI: 10.1021/acs.orglett.8b02546 Org. Lett. 2018, 20, 6632−6635
