Single Electron

Research Article Cite This: ACS Catal. 2019, 9, 6188−6193

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Rh/Ag-Mediated Peri-Selective Heteroarylation/Single Electron Transfer Annulation Cascade of 1‑(Methylthio)naphthalenes and Analogues: Road Less Traveled to Benzo[de]thioacenes Shiping Yang, Rui Cheng, Min Zhang, Zhengyang Bin, and Jingsong You* Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, Sichuan University, 29 Wangjiang Road, Chengdu 610064, P. R. China Downloaded via IDAHO STATE UNIV on July 17, 2019 at 10:16:24 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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ABSTRACT: Reported herein is a one-shot synthesis of benzo[de]thioacenes via a Rh-catalyzed peri-selective heteroarylation/Ag-mediated SET intramolecular cyclization sequence of 1-(methylthio)naphthalenes and heteroatom-embedded analogues in HFIP. Moreover, the different alcohols enable an exquisite switch in the reaction process. t-BuOH instead of HFIP delivers biaryl sulfides rather than cyclized products. By separation of rhodacycle species, control experiments, EPR experiments, and especially capture and isolation of thiophene radical adduct with BHT, the mechanistic pathway has been illustrated clearly. The formation of a sulfur-bridged sixmembered ring via SET cyclization of the cation-radical to the methylthio group presented herein is intrinsically different from the conventional acidmediated cyclization of sulfoxides. The annulative π-extension developed herein exemplifies the high compatibility of oxidative C−H activation and radical chemistry. KEYWORDS: benzo[de]thioacenes, π-extension, peri-selectivity, oxidative C−H/C−H cross-coupling, SET cyclization

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INTRODUCTION Sulfur-containing polycyclic aromatic hydrocarbons (PAHs) have triggered a great of interest in organic functional materials, such as organic field-effect transistors (OFETs), organic lightemitting diodes (OLEDs), and organic photovoltaics (OPVs), owing to their good stability and unique optoelectronic properties.1 Among them, PAHs containing the sulfur-bridged six-membered ring such as benzo[de]thiochromene and πextended analogues are well documented as excellent Weitz-type donors (Scheme 1). 2 The extended bis(benzothia)quinodimethanes exhibit a diradical property.3 In addition to PAHs containing the sulfur-bridged six-membered ring, thienoacenes consisting of fused thiophene rings (sulfur-bridged five-membered ring) have been intensively studied as promising organic semiconductors in the field of OFETs due to good stability and high carrier mobility.4 For example, diacene-fused thieno[3,2-b]thiophenes (DAcTTs) are one of the most pronounced organic semiconductors owing to their low-lying HOMO energy levels (excellent stability) and large intermolecular orbital overlap (high mobility). Recently, Liu, Li, and coworkers demonstrated that a [3,4]-extended isomer (4-S-PTA), the pyrene skeleton fused by the sulfur-bridged six-membered ring, displyed more remarkable optoelectronic properties (i.e., pronounced spectral red-shift and high quantum yield) than [1,2]- and [2,1]-extended isomers (sulfur-bridged five-membered-ring-fused thienoacenes).5 Herein, we present a simple and efficient strategy to build a versatile library of benzo[de]© 2019 American Chemical Society

Scheme 1. Examples of PAHs Containing Thiochromene and Thiophene

thioacenes consisting of benzo[de]thiochromene and thiophene that is certain to find application in the exploration of novel organic functional materials (Scheme 1). Received: April 7, 2019 Revised: May 30, 2019 Published: May 31, 2019 6188

DOI: 10.1021/acscatal.9b01426 ACS Catal. 2019, 9, 6188−6193

Research Article

ACS Catalysis Scheme 2. Synthetic Routes to Benzo[de]thioacenes

the best choice, giving 3a in 90% yield (Table S1, entries 11− 16). Substrate Scope. With the optimal system in hand, the scope of naphthalenes and analogues was investigated. As shown in Scheme 3, the coupling reaction illustrates a broad substrate scope. A variety of 1-(methylthio)naphthalenes smoothly reacted with benzothiophene in t-BuOH, affording the coupled products 3a−f in moderate to excellent yields. However, using tBuOH as the solvent, the cross-coupled product 3g was obtained in only 20% yield. The replacement of t-BuOH with HFIP could significantly improve the yield to 42%. Moreover, the corresponding cyclized product was not formed probably because the strongly electron-deficient property of the substrate inhibited the further cyclic process. Naphthalenes with functional groups, such as methyl, bromo, cyanide, ester, alkenyl, and imide, could be tolerated in this reaction. Gratifyingly, acenaphthenes, fluoranthenes, phenanthrenes, pyrenes, and benzanthrones, which are important motifs frequently encountered in organic functional materials,10 smoothly underwent the coupling reaction (3h−l). Interestingly, both 3-(methylthio)benzothiophene and 4-(methylthio)benzothiophene could undergo the heteroarylation with peri-selectivity (3m and 3n). The isoquinoline derivative could even react with 2a at the electron-deficient pyridine ring to provide 3o in 68% yield, which further enlarges a library of benzo[de]thioacenes. Next, we investigated the scope of heteroarenes. A broad range of benzothiophenes and thiophenes could participate in the cross-coupling reaction in excellent yields (4a−j). The thiophenes with either an electron-donating (e.g., Me and OMe) or electron-withdrawing group (e.g., chloro, bromo, formyl, acetyl, and ester) smoothly experienced this transformation. Other electron-rich heteroarenes such as 2,2′-bithiophene, thieno[3,2-b]thiophene, benzofuran, furan, and pyrrole could also undergo the peri-selective heteroarylation (4k−o). The cross-coupled products 3n and 4o were obtained in low yields probably because the relevant substrates (3-(methylthio)benzo[b]thiophene and 1-tosyl-1H-pyrrole) had a relatively low reactivity. The regioisomers and other relevant byproducts were not detected in these reactions. We attempted some indole substrates, such as 1-methylindole, 1-benzylindole, 1-phenylindole, and 1-acetylindole, but did not detect the corresponding biheteroaryl sulfides. It is worthy of note that the heteroarylation

Traditionally, the synthesis of benzo[de]thioacenes would comprise the following steps. First, biaryl sulfide is prepared via transition metal-catalyzed cross-coupling of methylthioaryl halide with aromatic boronic acid (alternatively, aromatic halide with (2-(methylthio)aryl)boronic acid). The resulting biaryl sulfide is then oxidized to afford the sulfoxide by the reaction with H2O2 or m-CPBA (m-chloroperbenzoic acid), followed by sequential treatment with trifluoromethanesulfonic acid and demethylation with a water−pyridine mixture (Scheme 2a).2d,5,6 Undoubtedly, it would be the most streamlined route to benzo[de]thioacenes based on transition metal-catalyzed oxidative Ar−H/Ar−H cross-coupling reactions outlined in Scheme 2b.7 However, transition metal-catalyzed oxidative C− H/C−H peri-selective heteroarylation of naphthalenes remains challenging and unprecedented. While the directing group is installed at the C1 position of naphthalenes, the heteroarylation reaction is documented to preferentially take place at the C2 position rather than the peri-position due to steric effect.8 Thus, key development of this work is to establish an efficient strategy to implement the peri-selectivity of heteroarylation reaction. In this work, we disclose an efficient rhodium catalytic system that could meet the demand of peri-selective heteroarylation of naphthalenes and analogues with the assistance of thioether directing group.

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RESULTS AND DISCUSSION Optimization of the Reaction Conditions. We initiated our investigation by examining the cross-coupling between 1(methylthio)naphthalene (1a) and benzothiophene (2a) as a model reaction (Table S1). To our delight, the cross-coupled product 3a was obtained in 46% yield by employing the commonly used [RhCp*Cl2]2/AgSbF6 catalytic system (Table S1, entry 1).9 No 3a product was observed in the absence of [RhCp*Cl2]2 (Table S1, entry 2). The absence of AgSbF6 and pivalic acid (PivOH) would lead to significantly diminished yields (Table S1, entries 3 and 4). Other solvents such as THF, tAmOH, and 1,4-dioxane were less effective than t-BuOH (Table S1, entries 5−8). Among silver salts investigated, Ag2O was found to be the most efficient, and Ag2CO3 and AgOAc showed inferior efficiency (Table S1, entries 6, 9, and 10). Further screening of catalysts, [Cp*Rh(MeCN)3][SbF6]2 proved to be 6189


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ACS Catalysis

reaction exclusively occurred at the peri-position and no C2coupling reactions were observed. With a variety of heteroarylated products in hand, we attempted to construct the benzo[de]thioacenes.11 For example, 3a was oxidized by m-CPBA to give the key precursor sulfoxide 3p in 83% yield,9 followed by treatment with trifluoromethanesulfonic acid and subsequent demethylation in water− pyridine mixture to afford 5a in 86% yield (Scheme 3).9 Investigation of One-Shot Synthesis of Benzo[de]thioacenes between Naphthalenes and Heteroarenes. After accomplishing the stepwise synthesis of the benzo[de]thioacenes, we paid attention to one-shot annulative reactions of 1-(methylthio)naphthalenes with benzothiophene. In recent years, transition metal-catalyzed C−H functionalization of thiophenes has become an area of intense research. A set of mechanistic pathways have been proposed, such as electrophilic aromatic substitution (SEAr), concerted metalation−deprotonation (CMD), and Heck-type process.7c,12 We lately reported that the thiophene ring could undergo a SET process in the oxidative C−H/C−H heteroarylation.8b The thiophene ring undergoes a SET pathway to form the cation−radical intermediate as electrophilic species. Considering that the methylthio group is more electron-rich, we envisaged that a heteroarylation/SET cyclization cascade would be favored energetically. The one-shot annulative π-extension could be performed after subtle change of the standard reaction conditions. The replacement of t-BuOH with HFIP as the solvent could deliver the benzo[de]thioacene 5a in 63% yield (Table S1, entries 21− 24). As summarized in Scheme 4, the naphthalenes with functional groups, such as methyl, bromo, formyl, and ester, smoothly participated in this annulation (5b−e). Extension of the π-system of benzo[de]thioacenes, phenanthrene, and fluoranthene was tested and provided the desired products 5f and 5g. Satisfyingly, the quinoline derivatives gave the target products 5h and 5i, indicating that the current strategy can feasibly assemble a structurally diverse library of benzo[de]thioacenes. Furthermore, the benzothiophenes with a variety of functional groups (e.g., methoxy, phenyl, p-methoxyphenyl, and 1-naphthalenyl) smoothly underwent the transformation (5j− m). Benzofuran reacted with 1-(methylthio)naphthalene (1a) to provide 5n but in a low yield. We also tried heteroarenes such as simple thiophenes, substituted furans, and N-protected pyrroles, but the corresponding fused benzo[de]thiochromenes were not detected under the optimized conditions. In addition, in the one-shot reactions, the byproducts associated were not observed except for noncyclized biheteroaryl sulfides and remaining starting materials, leading to the one-shot synthesis of benzo[de]thioacenes in moderate yields. In this context, we tried extending reaction time and/or raising reaction temperature but could not significantly improve these reactions. Mechanistic Study. To gain mechanistic insight into the oxidative cross-coupling, 1-(methylthio)naphthalene 1a was used to isolate the rhodacycle intermediate. Fortunately, 1a reacted stoichiometrically with [RhCp*Cl2]2 in the presence of NaOAc to afford the neutral rhodium(III) complex 7 (Scheme 5a).9 The cross-coupling reaction of 1a with 2a by using neutral 7 as the catalyst furnished the desired product 3a in only 8% yield (Scheme 5b). However, the yield of 3a increased to 62% when AgSbF6 was added into the reaction mixture (Scheme 5c). These results suggested the possible intermediacy of a cationic five-membered rhodacycle complex in the catalytic cycle.

Scheme 3. Substrate Scopes and the Representative Example for Acid-Mediated Cyclization of Sulfoxide (standard reaction conditions: 1 (0.2 mmol), 2 (0.6 mmol), [Cp*Rh(MeCN)3][SbF6]2 (0.01 mmol), Ag2O (0.4 mmol), PivOH (0.2 mmol), and t-BuOH (1.0 mL) at 100 °C under N2 atmosphere for 12 h)

a

A splitting of 3a was observed due to the free rotation of the benzothiophen-2-yl. bAg2O (0.6 mmol) and HFIP (1.0 mL) as the solvent for 24 h. t-BuOH = tert-butanol. HFIP = 1,1,1,3,3,3hexafluoro-2-propanol. 6190


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ACS Catalysis Scheme 4. One-Shot Synthesis of Benzo[de]thioacenes (standard reaction conditions: 1 (0.2 mmol), 2 (0.6 mmol), [Cp*RhCl2]Cl2 (0.01 mmol), AgSbF6 (0.04 mmol), Ag2O (0.6 mmol), PivOH (0.2 mmol), and HFIP (1.0 mL) at 100 °C under N2 atmosphere for 24 h)

Scheme 5. Synthesis of Rhodium(III) Complex 7, Catalytic Reactions by 7, and ORTEP Diagram of 7

Scheme 6. Mechanistic Studies of Annulation

a

PMP = p-methoxyphenyl. bNAP = 1-naphthalenyl.

To further understand the cascade process, several control experiments were performed. First, intramolecular cyclization of the biaryl sulfide (3a) afforded the annulation product 5a in 26% yield under the standard reaction conditions (Scheme 6a, (i)). We assumed that a relatively low yield of 5a could be relative to the partial decomposition of 3a in the presence excessive amount of oxidant Ag2O. Unexpectedly, treatment of 3a with 1.2 equiv of Ag2O in either the presence or absence of PivOH delivered 5a in 62% yield (Scheme 6a, (ii and iii)). Treatment of 3a only with HFIP did not lead to 5a, and 3a was recovered completely (Scheme 6a, (iv)). These results demonstrate that a cascade process involving oxidative C−H/C−H cross-coupling and silver-mediated cyclization is indicated in this catalytic reaction. Notably, PivOH only served as an additive to improve the yield of heteroarylation (Table S1, entry 4). Subsequently, the sulfoxides 1p and 3p as the intermediates were excluded (Scheme 6b). Radical scavengers, such as 2,2,6,6-tetramethylpiperidine nitroxide (TEMPO) and 2,6-di-tert-butyl-4-methylphenol (BHT), significantly inhibited the reaction (Table S2). Fortunately, a radical adduct of 3a with BHT was captured and

isolated in 46% yield (Scheme 6c).9 Furthermore, electron paramagnetic resonance (EPR) experiments indicated that the intramolecular cyclization of 3a was EPR-active, giving a g value 6191


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ACS Catalysis of 2.0041 (Scheme 6d and 6e), which is close to the typical g factor of 2.0023 for organic radicals, while no obvious EPR signal was observed in the absence of 3a (Figure S2). These results demonstrate that a SET pathway from the thiophene ring to Ag2O to form the radical intermediate is operative and a homolytic process of the methylthio group of 3a to generate the radical is disfavored. Proposed Catalytic Mechanism. Based on our experimental results and previous reports,8b,13 a plausible mechanistic pathway is proposed in Scheme 7. First, the cationic five-

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CIF data for 5a (CIF) CIF data for 7 (CIF) CIF data for 8 (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jingsong You: 0000-0002-0493-2388

Scheme 7. Proposed Mechanism

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from National NSF of China (nos. 21772128 and 21432005) and the Comprehensive Training Platform of Specialized Laboratory, College of Chemistry, Sichuan University.

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(1) (a) Takimiya, K.; Shinamura, S.; Osaka, I.; Miyazaki, E. Thienoacene-Based Organic Semiconductors. Adv. Mater. 2011, 23, 4347−4370. (b) Wang, C.; Dong, H.; Hu, W.; Liu, Y.; Zhu, D. Semiconducting π-Conjugated Systems in Field-Effect Transistors: A Material Odyssey of Organic Electronics. Chem. Rev. 2012, 112, 2208− 2267. (2) (a) Nakasuji, K.; Kubota, H.; Kotani, T.; Murata, I.; Saito, G.; Enoki, T.; Imaeda, K.; Inokuchi, H.; Honda, M.; Katayama, C.; Tanaka, J. Novel Peri-Condensed Weitz-Type Donors: Synthesis, Physical Properties, and Crystal Structures of 3,10-Dithiaperylene (DTPR), 1,6Dithiapyrene (DTPY), and Some of Their CT Complexes. J. Am. Chem. Soc. 1986, 108, 3460−3466. (b) Nakasuji, K.; Sasaki, M.; Kotani, T.; Murata, I.; Enoki, T.; Imaeda, K.; Inokuchi, H.; Kawamoto, A.; Tanaka, J. Methylthio- and Ethanediyldithio-Substituted 1,6-Dithiapyrenes and Their Charge-Transfer Complexes: New Organic Molecular Metals. J. Am. Chem. Soc. 1987, 109, 6970−6975. (c) Guldi, D. M.; Spänig, F.; Kreher, D.; Perepichka, I. F.; van der Pol, C.; Bryce, M. R.; Ohkubo, K.; Fukuzumi, S. Contrasting Photodynamiccs between C60−DithiaPyrene and C60−Pyrene Dyads. Chem. - Eur. J. 2008, 14, 250−258. (d) Wu, D.; Pisula, W.; Haberecht, M. C.; Feng, X.; Müllen, K. Oxygen- and SulfurContaining Positively Charged Polycyclic Aromatic Hydrocarbons. Org. Lett. 2009, 11, 5686−5689. (e) Du, C.; Ye, S.; Liu, Y.; Guo, Y.; Wu, T.; Liu, H.; Zheng, J.; Cheng, C.; Zhu, M.; Yu, G. Fused-Seven-Ring Anthracene Derivative with Two Sulfur Bridges for High Performance Red Organic Light-Emitting Diodes. Chem. Commun. 2010, 46, 8573− 8575. (3) (a) Zeng, Z.; Shi, X.; Chi, C.; López Navarrete, J. T.; Casado, J.; Wu, J. Pro-Aromatic and Anti-Aromatic π−Conjugated Molecules: an Irresistible Wish to Be Diradicals. Chem. Soc. Rev. 2015, 44, 6578− 6596. (b) Dong, S.; Herng, T. S.; Gopalakrishna, T. Y.; Phan, H.; Lim, Z. L.; Hu, P.; Webster, R. D.; Ding, J.; Chi, C. Extended Bis(benzothia)quinodimethanes and Their Dications: From Singlet Diradicaloids to Isoelectronic Structures of Long Acenes. Angew. Chem., Int. Ed. 2016, 55, 9316−9320. (4) (a) Liu, C.; Minari, T.; Lu, X.; Kumatani, A.; Takimiya, K.; Tsukagoshi, K. Solution-Processable Organic Single Crystals with Bandlike Transport in Field-Effect Transistors. Adv. Mater. 2011, 23, 523−526. (b) Mei, J.; Diao, Y.; Appleton, A. L.; Fang, L.; Bao, Z. Integrated Materials Design of Organic Semiconductors for FieldEffect Transistors. J. Am. Chem. Soc. 2013, 135, 6724−6746. (c) Mori, T.; Nishimura, T.; Yamamoto, T.; Doi, I.; Miyazaki, E.; Osaka, I.; Takimiya, K. Consecutive Thiophene-Annulation Approach to πExtended Thienoacene-Based Organic Semiconductors with [1]Benzothieno[3,2-b][1]benzothiophene (BTBT) Substructure. J. Am. Chem. Soc. 2013, 135, 13900−13913.

membered rhodacycle complex I is formed through the coordination of 1-(methylthio)naphthalene (1a) with the Cp*Rh(III) species and subsequent peri-C−H bond activation of naphthalene. The resulting intermediate I reacts with benzothiophene (2a) to produce the intermediate II, which further undergoes a reductive elimination to release the desired product 3a. The released Cp*Rh(I) species is reoxidized to the Cp*Rh(III) species by Ag(I) to furnish the catalytic cycle. When HFIP was used as the reaction solvent, the coupled product 3a encounters a SET process from the thiophene ring to Ag2O to form the cation-radical intermediate III, followed by an electrophilic cyclization/demethylation to afford the intermediate IV. We propose that HFIP has a stabilizing effect on the cation−radical intermediates in the reaction.14,15 Finally, the desired product 5a is formed by oxidation of Ag2O and deprotonation of IV.

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CONCLUSIONS The one-shot synthesis of benzo[de]thioacenes developed herein remarkably demonstrates a road less traveled to πextension and exemplifies the high compatibility of oxidative C− H activation and radical chemistry. Moreover, we also describe an artful strategy to switch the reaction process for delivering the biheteroaryl sulfides or the fused benzo[de]thiochromenes through different alcohols. Undoubtedly, the rapid pathway to benzo[de]thioacenes could open a new door for relevant study in materials science.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.9b01426. Detailed information on experimental procedures and characterization data (PDF) CIF data for 3a (CIF) CIF data for 3p (CIF) 6192


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