A Synthetic Strategy for the Construction of Functionalized

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

pubs.acs.org/OrgLett

A Synthetic Strategy for the Construction of Functionalized Triphenylene Frameworks via Palladium Catalyzed Intramolecular Annulation/Decyanogenative C−H Bond Alkenylation Sachin S. Ichake,† Bharath Kumar Villuri,† Sabbasani Rajasekhara Reddy,†,‡ Veerababurao Kavala,† and Ching-Fa Yao*,† †

Department of Chemistry, National Taiwan Normal University, 88, Sec. 4, Ting-Chow Road, Taipei-116, Taiwan R.O.C. Department of Chemistry, School of Advanced Sciences, Vellore Institute of Technology (VIT), Vellore 632014, India

‡

Org. Lett. Downloaded from pubs.acs.org by LA TROBE UNIV on 03/18/19. For personal use only.

S Supporting Information *

ABSTRACT: The palladium catalyzed synthesis of 14-phenylbenzo[f ]tetraphene-9-carbonitrile derivatives as core polycyclic aromatic hydrocarbons (PAHs) was achieved via an intramolecular annulation and decyanogenative C−H bond alkenylation strategy. A readily synthesized Knoevenagel condensation product of [1,1′-biphenyl]-2,2′-dicarbaldehyde with benzyl cyanide converted successfully into 14-phenylbenzo[f ]tetraphene-9-carbonitrile derivatives in excellent yields up to 94%. The transformation involves an intramolecular cascade C−C bond formation along with a C−H bond cleavage sequence.

P

triphenylene in slightly lower yields via cycloaddition reaction of arenediazonium carboxylate and tetraphenylcyclopentadienone.9a Later, Parez and co-workers reported on the synthesis of substituted benzo[b]triphenylenes in higher yields by switching to o-(trimethylsilyl)aryl triflate as an aryne precursor (Scheme 1, eq 4).9b Cheng and Perez groups reported the efficient route for the synthesis of PAHs via palladium catalyzed [2 + 2 + 2] cyclotrimerization of oxabicyclic alkenes and benzynes to generate the corresponding norbornene anellated 9,10-dihydrophenantherene derivatives, which can be converted to corresponding triphenylene analogues. On the other hand, aryne precursors undergo cycloaddition reactions with dimethylacetylenedicarboxylate (DMAD) to yield the anticipated extended triphenylenes (Scheme 1, eq 5).10 Recently, Ichikawa co-workers described an interesting C−F bond activation strategy for selective synthesis of benzo[f ]tetraphenes and benzo[g]chrysenes, in which regioselectivity depends on aluminum catalysts used for the reaction (Scheme 1, eq 6).11 Although, there were a plethora of synthetic strategies that have been developed for the construction of PAHs, most of the reported methods suffered from multistep synthesis, involvement of highly sensitive starting materials such as arynes, less selectivity,

olycyclic aromatic hydrocarbons (PAHs) are widely present in nature and played a significant role in material chemistry.1 PAHs are of interest, due to their major applications in organic light emitting diodes (OLEDs),2 transistors, and photovoltaic cells.3 Among various PAHs, benzo[b]triphenylene analogues have been extensively studied, as it shows a wide range of applications in material sciences and engineering.4 The cyano substituted PAHs such as benzo[b]triphenylene-9,14-dicarbonitrile derivatives are utilized as an effective photosensitizer for generation of singlet oxygen (Scheme 1, eq 1). Moreover, benzo[b]triphenylene moieties have shown some excellent photochemical properties at gas−solid and liquid−solid interfaces in different fields. Additionally, they are stable under irradiation, exhibit high absorption in the visible region, and also could be easily embedded onto inert supports like silica and polystyrene.4c Substituted hexaalkoxybenzo[b]triphenylenes display columnar liquid crystal phases. Further, these types of materials are constituents of semiconductor devices (Scheme 1, eq 2).5 Earlier, several attempts were made for the synthesis of benzo[b]triphenylenes using aryne-based6 or non-aryne-based7 methodologies, among them cycloaddition reactions of arynes being the most commonly used strategy. Fitzgerald et al. reported on the synthesis of benzo[b]triphenylenes through the formation of an o-tolyl anionic intermediate generated from ring opening of benzocyclobutenoxides to halobenzene derived arynes (Scheme 1, eq 3).8 Pascal and co-workers reported on the synthesis of highly substituted benzo[b]© XXXX American Chemical Society

Received: February 11, 2019

A

DOI: 10.1021/acs.orglett.9b00532 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

14-phenylbenzo[f ]tetraphene-9-carbonitrile derivatives using a palladium catalyzed intramolecular annulation and decyanogenative C−H bond alkenylation strategy (Scheme 2).

Scheme 1. Some of the Relevant Strategies for the Synthesis of PAHs

Scheme 2. Present work

This strategy involves the formation of two new C−C bonds and a C−H/C−C bond cleavage sequence. An initial experiment was conducted using Knoevenagel condensation product 1a as a model substrate using standard reagents (10 mol % palladium acetate, 0.2 equiv of PPh3, and 3 equiv of K2CO3 in 0.5 mL of DMSO solvent at 100 °C) produced the product (2a) in 50% isolated yield (Table 1, entry 1) along with unreacted starting material 1a. The structure of the isolated compound 2a was determined by the spectral data and single crystal X-ray analysis (Figure 1). Inspired by our preliminary results, next, the ligand screening was carried out to optimize the reaction conditions (Table 1, entries 1−4), and it was found that the yield of the desired product was improved slightly to 52% when PPh3 was replaced with PBu3, while the bulky phosphine ligand tri-otolylphosphine resulted in only a trace amount of the product. Remarkably, in the absence of ligand, the yield of 2a was improved to 58% (Table 1, entry 4). However, the reaction failed to deliver the desired product in the absence of palladium acetate (Table 1, entry 5). The reaction was found to be ineffective, when Cs2CO3 was used as a base (Table 1, entry 6). We next tested the reaction with stoichiometric amounts of different oxidants such as Cu(OAc)2, AgOAc, and 1,4-benzoquinone. The reaction with Cu(OAc)2 gave an excellent yield (92%) of the product; however, the reactions were found to be less productive in the case of AgOAc and 1,4-benzoquinone (Table 1, entries 7−9). Further, we also verified the reaction under an oxygen balloon atmosphere, which resulted in a low yield of the product (Table 1, entries 10). No noticeable improvement in the product yield was observed, when we used different solvents such as DMF and DMAc (Table 1, entries 11 and 12). The reaction on 1.0 mmol scale also produced the desired product 2a in 87% yield (Table 1, entry 13). Hence, the optimal reaction conditions to carry out the reaction were found to be Table 1, entry 8. Following the previously mentioned reaction conditions, we tested the feasibility of the optimized reaction conditions by varying the substitution on the benzyl cyanide moiety (Scheme 3). As presented in Scheme 3, the reaction with a neutral and an electron-donating p-methyl group containing derivatives provided the corresponding products 2a and 2b in 92% and 89% isolated yields, when the reaction was conducted on 0.1 mmol scale. Next, the scope of the method was examined with respect to the substrate containing a m-methyl substituent. Interestingly, in this case we have isolated two regioisomers 2c and 2c′ (5.8:3.2) in overall excellent yield. Substrate equipped with a bulky tert-butyl group at the para position of the phenyl ring gave the desired product 2d in excellent yield. Further, the

formation of byproducts, limited substrate scope, low yields of the desired products, etc. Therefore, some alternate and convenient protocols are required to broaden the scope of the synthesis of various functionalized PAHs, which remains a challenging task in the fields of both chemistry and materials. Thus, synhesis of functionalized PAHs from the simple and easily accessible starting materials without incorporating prefunctionalized starting materials such as C−X (X = Br, Cl, I, OTf) would be highly desirable.12 We hypothesized that this might be achieved via an intramolecular annulation and decyanogenative C−H bond alkenylation strategy, by employing a palladium catalyzed cascade reaction protocol.13 This could contribute enormous synthetic utility to achieve the desired transformations in the field of synthetic organic chemistry. As part of our continuous efforts on exploring the use of substituted benzylcyanide surrogates for the construction of various value-added organic compounds in the presence of transition metal catalysts,14 herein, we report the synthesis of B


Letter

Organic Letters Table 1. Optimization Studies for the Synthesis of 14-Phenylbenzo[f ]tetraphene-9-carbonitrile

entrya

catalyst (mol %)

base (equiv)

ligand/oxidant (equiv)

solvent (2 mL)

yield%b (2a)

1 2 3 4 5 6 7 8 9 10c 11 12 13d

Pd(OAc)2 (10) Pd(OAc)2 (10) Pd(OAc)2 (10) Pd(OAc)2 (10) − Pd(OAc)2 (10) Pd(OAc)2 (10) Pd(OAc)2 (10) Pd(OAc)2 (10) Pd(OAc)2 (10) Pd(OAc)2 (10) Pd(OAc)2 (10) Pd(OAc)2 (10)

K2CO3(3.0) K2CO3(3.0) K2CO3(3.0) K2CO3(3.0) K2CO3(3.0) Cs2CO3(3.0) K2CO3(3.0) K2CO3(3.0) K2CO3(3.0) K2CO3(3.0) K2CO3(3.0) K2CO3(3.0) K2CO3(3.0)

PPh3 (0.2) PBu3 (0.2) P(o-tol)3 (0.2) − − − AgOAc (1.0) Cu(OAc)2 (1.0) 1,4-Benzoquinone (1.0) − Cu(OAc)2 Cu(OAc)2 Cu(OAc)2

DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMF DMAc DMSO

50 52 trace 58 − 16 60 92 25 22 80 84 87

All of the reactions were carried out on a 0.1 mmol of 1a, 0.5 mL of solvent at 100 °C for 12−16 h. bNMR yields. cReaction was carried out under an oxygen balloon atmosphere. dReaction was carried out on 1.0 mmol scale. a

dimethyl) biphenyl ring gave the desired product 2o in 85% yield. Next, the scope of the method was verified with respect to the substrate containing dimethoxy groups (4,4′-dimethoxy) on the biphenyl group. In this case, the resulted dimethoxy substituted benzo[f ]tetraphene-9-carbonitrile 2p was obtained in 77% yield. Further, we also investigated the reactions with halogen substitutions such as F and Cl (4,4′-dihalo) on the biphenyl group. In these cases, the desired dihalo substituted triphenylenes were produced in yields of 84% for 2q and 88% for 2r, respectively. Furthermore, the scope of the reaction was examined with an unsymmetrically functionalized Knoevenagel condensation product such as 4-CF3 and 4-OMe substituents on the phenyl ring 1s. Strikingly, only a 4-OMebearing phenyl ring underwent C−H bond functionalization to obtain the product 2s in 80% yield (Scheme 4). The structure of 2s was also confirmed by single crystal X-ray analysis (see Supporting Information Figure S8). On the basis of current results and existing literature reports16 the plausible reaction mechanism was depicted as shown in Figure 2. We assume that the reaction may be initiated by a cyano-group directed C−H bond activation by a palladium(II) species via aromatic proton abstraction by an acetate ligand to form the complex I, followed by sequential insertion of a Pd−C species halfway to the two double bonds to form the complex II. Subsequently, formation of a sixmembered ring generated complex III. β-Hydride elimination and subsequent decyanative aromatization/oxidation afford the product 2a. Simultaneously, the completion of the catalytic cycle occurs via the oxidation of the Pd(0) species to Pd(II) with the assistance of Cu(OAc)2. As the triphenylene derivatives are known to be fluorescently active, we investigated UV absorption and fluorescence emission of some representative compounds 2b, 2f, 2e, 2m, 2n, 2p, 2q, and 2r in dichloromethane solvent. It was found that the absorption maxima of almost all the compounds except 2p was similar which varied from 378 to 420 nm and emission maxima also similarly varied from 438 to 450 nm. However, an interesting deviation was observed for the 2p compound where the absorption maximum was found to be 338 nm and emission maximum was 517 nm. The absorption−

Figure 1. ORTEP diagram of compound 2a.

scope of the method was verified with the substrate having strong electron-donating groups such as methoxy, dimethoxy, and methylenedioxy on the phenyl ring. In these cases, the desired functionalized triphenylenes were produced in high yields (2e−2g: 81−87%). We also verified the scope of the protocol with the substrate containing a p-chloro phenyl group. In this case, the anticipated triphenylene derivative 2h was produced in 81% yield. However, the m-chloro substitution on the phenyl group gave two desired regioisomers, 2i and 2i′ (5.2:3.2), in overall 84% yield. The structures of the compounds 2i and 2i′ were confirmed by single-crystal X-ray analysis and are presented in the Supporting Information (Figures S6 and S7). Unfortunately, no product was formed with the reaction of o-chloro substitution on phenyl group 2j. On the other hand, the substrate having o-fluoro functionality on the phenyl group gave the desired product 2k in 34% yield. However, the substrate having a fluorine group at the para position of the phenyl ring gave an excellent yield of the product 2l. Further, the scope of the reaction was extended to a strong electron-withdrawing group (4-CF3) on the phenyl ring. To our delight, the reaction provided the desired 4-CF3 substituted benzo[f ]tetraphene-9-carbonitrile derivative 2m in 50% yield. We next examined the substrate possessing a naphthyl group to provide a high yield of respective highly aromatic compound 2n in 85% yield. Under the optimal reaction conditions, we extended the feasibility of the substrate scope, by switching the substitution from the part of benzyl cyanide to the biphenyl side. As presented in Scheme 3, methyl group substitution on the (5,5′C


Letter

Organic Letters Scheme 3. Scope for the Synthesis of 14Phenylbenzo[f ]tetra-phene-9-carbonitrile Derivativesa,b

Scheme 4. Scope for the Unsymmetrically Functionalized Substratea,b

a

Reaction conditions: 0.1 mmol of substrate, 3 equiv K2CO3, 10 mol % Pd(OAc)2, 1 equiv of Cu(OAc)2, DMSO (0.5 mL), 100 °C for 12 h. bIsolated yields.

Figure 2. Plausible reaction mechanism.

tives 1. The newly developed approach is regioselective and proceeds in a cascade manner to produce several functionalized polycyclic aromatic compounds through the formation of two new C−C bonds and C−H bonds. At present, the developed protocol was tested only for the construction of nitrile substituted benzo[b]triphenylenes prepared from Knoevenagel condensation product 1 obtained from respective [1,1′-biphenyl]-2,2′-dicarbaldehyde and benzyl cyanide only. However, our ongoing research is focused on broadening the scope of the method developed.

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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.9b00532. X-ray crystallographic data for 1a (CCDC 1895395), 1c (CCDC 1895396), 1d (CCDC 1895399), 1r (CCDC 1895434), 2a (CCDC 1837256), 2i (CCDC 1837015), 2i′ (CCDC 1838022), and 2s (CCDC 1895462); UV and fluorescence data of 2b, 2f, 2g, 2m, 2n, 2p, 2q, and 2r; 1H and 13C NMR spectral data for representative compounds (PDF)

a Reaction conditions: 0.1 mmol of substrate, 3 equiv of K2CO3, 10 mol % Pd(OAc)2, 1 equiv of Cu(OAc)2, DMSO (0.5 mL), 100 °C for 12 h. bIsolated yields.

emission spectra were provided in the Supporting Information (Figures S9−S16). The detailed studies of the fluorescent properties of the compounds are underway in our laboratory. In summary, we report on the synthesis of diverse substituted 14-phenylbenzo[f ]tetraphene-9-carbonitrile 2 derivatives through the palladium catalysis, using (2Z,2′Z)-3,3′([1,1′-biphenyl]-2,2′-diyl)bis(2-phenylacrylonitrile) deriva-

Accession Codes

CCDC 1837015, 1837256, 1838022, 1895395−1895396, 1895399, 1895434, and 1895462 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 D


Letter

Organic Letters

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Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ching-Fa Yao: 0000-0002-8692-5156 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Ministry of Science and Technology ROC (MOST 107-2113-M-003-003) and National Taiwan Normal University (103-07-C) for providing financial assistance. We are also thanking the Instrumentation Centre at National Taiwan Normal University. We are grateful to Xray technician Mr. Ting-Shen Kuo, Mass spectroscopy technician Ms. Hsiu-Min Huan, NMR technician Ms. ChiuHui He and Dr. Ram Ambre (UV Fluorescence studies) for providing the analytical data presented in this paper.

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A Synthetic Strategy for the Construction of Functionalized

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