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Rh/Ag-Mediated Peri-Selective Heteroarylation/Single Electron Transfer (SET) Annulation Cascade of 1-(Methylthio)naphthalenes and Analogues: Road Less Travelled to Benzo[de]thioacenes Shiping Yang, Rui Cheng, Min Zhang, Zhengyang Bin, and Jingsong You ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b01426 • Publication Date (Web): 31 May 2019 Downloaded from http://pubs.acs.org on June 2, 2019
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ACS Catalysis
Rh/Ag-Mediated Peri-Selective Heteroarylation/Single Electron Transfer (SET) Annulation Cascade of 1-(Methylthio)naphthalenes and Analogues: Road Less Travelled 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 Supporting Information Placeholder ABSTRACT: Reported herein is an 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 to exquisitely switch the reaction process. t-BuOH instead of HFIP delivers the biaryl sulfides rather than the 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 sulfur-bridging 6-membered ring via a SET cyclization of the cation-radical to the methylthio group presented herein is intrinsically different from the conventional acid-mediated 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
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 light-emitting diodes (OLEDs) and organic photovoltaics (OPVs), owing to their good stability and unique optoelectronic properties.1 Among them, PAHs containing the sulfur-bridging 6-membered ring like benzo[de]thiochromene and π-extended analogues are well documented as excellent Weitz-type donors (Scheme 1).2 The extended bis(benzothia)quinodimethanes exhibit diradical 3 property. In addition to PAHs containing the sulfur-bridging 6-membered ring, thienoacenes consisting of fused thiophene rings (sulfur-bridging 5-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 co-workers demonstrated that a [3,4]-extended isomer (4-S-PTA), the pyrene skeleton fused by the sulfur-bridging 6-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-bridging 5-membered-ring-fused thienoacenes).5 Herein, we wish to present a simple and efficient strategy to build a versatile library of benzo[de]thioacenes consisting of benzo[de]thiochromene and thiophene that is certain to find application in the exploration of novel organic functional materials (Scheme 1). S
S
S S S benzo[de]thiochromene 1,6-dithiapyrene (Weitz-type donor) (DTP) 1 2
red OLEDs emitter (BTA) Mes
S S
S
3 4
S
S
S
Mes pro-aromatic as diradicaloid (BBB-Pen)
organic semiconductor (4-S-PTA)
S S
n
S
N
benzo[de]thioacenes (this work)
n organic semiconductors (DAcTTs: n = 0, BTB; n = 1, DNTT; n = 2, DATT)
Scheme 1. Examples of PAHs containing thiochromene and thiophene Traditionally, the synthesis of benzo[de]thioacenes would comprise the following steps. First, biaryl sulfide is
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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 wish to 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. (a) Possible route based on conventional C-X/C-M coupling S
S
multi-steps Br
S
(i) m-CPBA
ArB(OH)2
(ii) 1) TfOH 2) pyridine/H2O
S
S
difficult preparation (b) This work based on oxidative Ar-H/Ar-H coupling H
competitive [Rh]/[O]
+ S
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–3f in moderate to excellent yields. However, using t-BuOH 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 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-3l). 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.
H S
S H
efficiency (Table S1, entries 6, 9 and 10). Further screening of catalysts, [Cp*Rh(MeCN)3][SbF6]2 proved to be the best choice, giving 3a in 90% yield (Table S1, entries 11-16).
S H
S
(i) m-CPBA S (ii) 1) TfOH 2) pyridine/H2O
Rh/Ag-mediated one-shot annulation cascade
Scheme 2. Synthetic routes to benzo[de]thioacenes
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, t-AmOH 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
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ACS Catalysis
H
S
Ar
Ar'
[Cp*Rh(MeCN)3][SbF6]2 (5 mol%) Ag2O (2.0 equiv)
X
H
2 (X = O, S, NTs)
1
X
PivOH (1.0 equiv) t-BuOH, 100 C,12 h
yields (4a-4j). The thiophenes with either 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-4o). The cross-coupled products 3n and 4o were obtained in low yields probably because of the relevant substrates (3-(methylthio)benzo[b]thiophene and 1-tosyl-1H-pyrrole) had a relatively low reactivity. The regioisomers and other relevant by-products 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 reaction exclusively occurred at the peri-position and no C2-coupling reactions were observed.
S Ar
Ar'
3 or 4
Scope of naphthalenes and analogues
S
S
S
S
O
R 3a, R = H, 90% 3b, R = Me, 77% 3c, R = Br, 88% 3d, R = CN, 45% 3e, R = COOMe, 87%
O C 2H 5
N
3aa n
COO Bu
C 4H 9
3f, 86%
S S
S
S
3g, 42%b
S
S
S
S
S
S
3j, 72%
3i, 52%
3h, 51% S
S
S
S
S
S
N 3o, 68%b
S 3n, 18%
3m, 45%
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
S
S S
O 3l, 98%
3k, 75%
Scope of heteroarenes Cl R S
S
Br
S
4a, 95%
S
S
OHC 4c, R = Me, 86% 4d, R = OMe, 80% 4e, R = Cl, 91% S 4f, R = Br, 92% 4g, R = acetyl, 89% 4h, R = ethyl formate, 93% 4i, R = phenyl, 89%
S
4j, 91%
4b, 60% Br
S
S S
S
S
4k, 72%
S
O
S
O
4n, 50%
4m, 52%
4l, 71%
S
S
Ts N
S
4o, 20%
Conventional acid-mediated cyclization of sulfoxide
S
S
m-CPBA S DCM, rt
3a
S
O
i) TfOH, DCE, rt ii) pyridine/H2O 120 C
3p, 83%
S
S
5a, 86%
3p
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. aA 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 = 1,1,1,3,3,3-hexafluoro-2-propanol.
Next, we investigated the scope of heteroarenes. A broad range of benzothiophenes and thiophenes could participate in the cross-coupling reaction in excellent
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-5e). To extend π-system of
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benzo[de]thioacenes, phenanthrene and fluoranthene were 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-5m). Benzofuran could 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 non-cyclized biheteroaryl sulfides and remained 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.
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[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. aPMP = p-methoxyphenyl. bNAP = 1-naphthalenyl.
Mechanistic Study. To gain mechanistic insight into the oxidative cross-coupling, 1-(methylthio)naphthalene 1a was used to isolate 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. *Cp
S
Cl Rh S
[Cp*RhCl2]2/NaOAc
(a)
t-BuOH, 100 C, 24 h 1a
7, 11%
Ar3 H
S
Ar1 Ar2
Ar
1
S
[Cp*RhCl2]2 (5 mol%) AgSbF6 (20 mol%)
X
H
3
X
Ag2O (3.0 equiv) PivOH (1.0 equiv) HFIP,100 C, 24 h
2 (X = O, S)
7 (5 mol%) Ag2O (2.0 equiv)
S
S
Ar1
Ar2
1a
2a
3a, 8%
S
S S
S
S
S 1a
5a
5a, 63%
S
S
Br 5c, 71%
5b, 50%
S
S
COOMe 5e, 43%
5f, 65%
OMe
S S
S
S
S
5h, 38%
Ph
S
S
5k, 56%
S
5l, 48%a
S
5j, 55%
5i, 86% NAP
PMP
S
S
N
N 5g, 22%
S
S
O S
5m, 45%b
S
S
PivOH (1.0 equiv) t-BuOH, 100 C,12 h
(c)
Scheme 5. Synthesis of rhodium(III) complex 7, catalytic reactions by 7 and ORTEP diagram of 7.
S
5d, 92%
2a
7 (5 mol%) AgSbF6 (5 mol%) Ag2O (2.0 equiv)
3a, 62%
S
CHO
S (b)
5
S
S
S
PivOH (1.0 equiv) t-BuOH, 100 C,12 h
S
5n, 25%
Scheme 4. One-shot synthesis of benzo[de]thioacenes. Standard reaction conditions: 1 (0.2 mmol), 2 (0.6 mmol),
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ACS Catalysis a) Intramolecular cyclization of 3a
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
(i) standard conditions 26% (ii) Ag2O (1.2 equiv), PivOH, HFIP S
S
62%
S
(iii) Ag2O (1.2 equiv), HFIP 62% (iv) HFIP
3a
5a
N.D.
b) Verification of the intermediate of cyclization S
O
[Cp*RhCl2]2 (5 mol%) AgSbF6 (20 mol%)
S (i)
Ag2O (3.0 equiv) PivOH (1.0 equiv) HFIP,100 C, 24 h
1a
S
1p, N.D
standard conditions S
S
N.D.
O
S
(ii)
S
Ag2O (1 equiv), HFIP N.D. 3p
5a
c) Radical trapping experiment S S S
S
BHT
Ag2O HFIP
5a N.D. t
3a
t
Bu
Bu
OH 8, 46% (isolated yield) e)
d) EPR experiment
S
S
Ag2O (1 equiv) HFIP, 1 h EPR-active
5a
3a
Scheme 6. Mechanistic studies of annulation 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 convinced 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 Besides, the electron paramagnetic resonance (EPR) experiments indicated that the intramolecular cyclization of 3a was EPR-active, giving a g value 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-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. H S
S +
H+ S
Ag0
S
1a
Cp*RhIIIX2 X = SbF6- or PivO-
AgI + 2XAg2O
H S
S
HX
*Cp Rh
5a
V
S
Cp*RhI
S
S
I
Ag0 Ag2O S
S
III
2a
*Cp Rh
SET IV
S S HX
3a II
Scheme 7. Proposed mechanism
CONCLUSIONS In conclusion, the one-shot synthesis of benzo[de]thioacenes developed herein remarkably demonstrates a road less travelled to π-extension 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
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benzo[de]thiochromenes through the different alcohols. Undoubtedly, the rapid pathway to benzo[de]thioacenes would open a new door for relevant study in materials science.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected] ORCID Jingsong You: 0000-0002-0493-2388
Notes
The authors declare no competing financial interests.
ASSOCIATED CONTENT Supporting Information. Detailed information on experimental procedures and characterization data, are available in the Supplementary Information. This material is available free of charge via the internet at http://pubs.acs.org.
ACKNOWLEDGMENT 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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ACS Catalysis crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre. (10) (a) Yu, M.-X.; Chang, L.-C.; Lin, C.-H.; Duan, J.-P. Wu, F.-I.; Chen, I.-C.; Cheng, C.-H. Luminescence Properties of Aminobenzanthrones and Their Application as Host Emitters in Organic Light-Emitting Devices. Adv. Funct. Mater, 2007, 17, 369–378. (b) Hoheisel, T. N.; Schrettl, S.; Szilluweit, R.; Frauenrath, H. Nanostructured Carbonaceous Materials from Molecular Precursors. Angew. Chem. Int. Ed. 2010, 49, 6496–6515. (c) Mochida, K.; Kawasumi, K.; Segawa, Y.; Itami, K. Direct Arylation of Polycyclic Aromatic Hydrocarbons through Palladium Catalysis. J. Am. Chem. Soc, 2011, 133, 10716–10719. (d) Pavliček, N.; Mistry, A.; Majzik, Z.; Moll, N.; Meyer, G.; Fox, D. J.; Gross, L. Synthesis and Characterization of Triangulene. Nat. Nano. 2017, 12, 308–312. (11) The thioether group is documented to be easily removed or converted into other synthetically useful functional groups. To further illuminate the synthetic utility of this protocol, the operation was performed to remove the directing group using 3a as a representative example. The methylthio group proved to be easily removed by selective reduction by Ni(COD)2 and dimethylethylsilane to deliver 2-(naphthalen-1-yl)benzo[b]thiophene (6) (For details, see SI, Part VIII). (12) For selected examples, see: (a) Join, B.; Yamamoto, T.; Itami, K. Iridium Catalysis for C−H Bond Arylation of Heteroarenes with Iodoarenes. Angew. Chem. Int. Ed. 2009, 48, 3644–3647. (b) Kirchberg, S.; Tani, S.; Ueda, K.; Yamaguchi, J.; Studer, A.; Itami, K.
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Table of Content
competitive
H S H
Ar2 Ar3 Ar
+
S
[Rh]/[Ag]
S
1
S
one-shot H -extension
Ar2 Ar3 Ar1
Mechanistic investigation S *Cp
Cl Rh
S S
t
rhodacycle species
t
Bu
Bu
OH
radical adduct with BHT
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