Synthesis of Polycyclic Aromatic Hydrocarbons (PAHs) via a Transient

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Letter Cite This: Org. Lett. 2018, 20, 7620−7623

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Synthesis of Polycyclic Aromatic Hydrocarbons (PAHs) via a Transient Directing Group Ming Tang,† Qinqin Yu,† Ziqi Wang, Chen Zhang, Bing Sun, Ying Yi,* and Fang-Lin Zhang* School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, Wuhan 430070, People’s Republic of China

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ABSTRACT: In this work, an efficient synthetic route was developed to construct PAHs with diverse shape, width, and edge topology. The precursors of PAHs were obtained by using a direct arylation of arenes via a transient ligand-directed C−H functionalization strategy and the cycloaromatization was readily achieved by using a Brønsted acid catalyst. This novel route provides an opportunity to build up PAHs in a highly efficient manner. olycyclic aromatic hydrocarbons (PAHs) with five or more fused aromatic rings have captivated scientists for years, because of their unique chemical and physical properties.1 Their high electron density, narrow highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO−LUMO) gaps, low redox potentials, π-conjugated systems, strong π−π stacking abilities, rigid skeletons, aromaticity, and excellent chemical stability make PAHs excellent candidates for diverse applications.2 Many functional materials based on organic compounds containing PAH skeletons have been prepared.3 The degree of π-extension, the topology of the ring fusion, and the periphery are crucial not only for the electronic properties at a molecular level but also for the packing of the PAHs.2 PAHs have already been effectively used in many optoelectronic devices, such as organic field effect transistors (OFETs),4 organic photovoltaic cells (OPVs),5 organic light-emitting diodes (OLEDs),6 and organic chemosensors.7 A wide variety of derivatives has been designed and synthesized in order to adjust the properties of PAHs and to improve device performance.8 The traditional approaches to prepare PAHs include Diels− Alder, cyclotrimerization, cross-coupling, and cyclodehydrogenation reactions.9 In 2012, Wang et al. reported the construction of dibenzo[d,d′]phenanthrenes using a rhodium(II)-catalyzed intramolecular cyclization of bis(Ntosylhydrazone)s (Scheme 1).10 In 2000, Harvey also unveiled the construction of dibenzo[d,d′]phenanthrenes by conversion to diolefins by Wittig reaction followed by photocyclization.11 In 2016, Kubozono and co-workers reported the PAHs of a repeating W-shaped pattern by a repetition of Wittig reactions followed by photocyclization.12 In 2014, Greaney prepared trinaphthylene using 2-bromoaryl boronic ester by metalcatalyzed benzyne cyclotrimerization13 and in 2017, Dastan also reported the synthesis of trinaphthylene using benzobarrelene−bromostannylalkene by cyclotrimerization and Diels− Alder reaction.14 However, the intrinsic difficulties in preparing substrates and tedious processes to prepare the precursors of

P

© 2018 American Chemical Society

Scheme 1. Synthesis of PAHs

PAHs are major drawbacks for their further development and applications. Recently, C−H functionalization using a transient directing group has received significant attention as a reliable strategy.15 The Yu group and the Hu group achieved the palladiumcatalyzed benzylic C(sp3)−H arylation of benzaldehydes using a transient directing group.16 Amino acid reagents that reversibly react with benzaldehydes in situ via imine formation to serve as a transient directing group for activation of inert (sp3)−H bonds, which includes three processes: (i) the formation of a transient directing group; (ii) the C−H activation process; and (iii) the removal of the directing group. This elegant work represents a major advance in C−H functionalization chemistry. In addition, our previous works17 mainly focus on the synthesis of heterocycles through cascade reactions. Herein, we report a transient directing group enabled palladium-catalyzed ortho-C(sp3)−H arylation of benzaldehydes with aryl diiodides and subsequent synthesis of diverse PAHs (Scheme 1) with different shape, width and edge topology in the presence of a catalytic amount of trifluoromethanesulfonic acid. These edge regions correspond Received: October 21, 2018 Published: November 20, 2018 7620

DOI: 10.1021/acs.orglett.8b03359 Org. Lett. 2018, 20, 7620−7623

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Organic Letters

found that the desired product (3ab) were afforded in 50% yield at 2.5 equiv of AgTFA. In order to further improve the yield of the desired product (3ab), we raised the temperature to 100 °C (Table 1, entries 10 and 11) and obtained the highest isolated yield of 70%. The cycloaromatization of 3ab was readily achieved by using 5 mol % of TfOH19 in CHCl3, which was sufficient to complete the reaction in 5 h at room temperature, leading to a 87% yield of 4ab. The optimized protocol proved to be applicable to a wide range of aromatic and heteroaromatic aldehydes in combination with diiodobenzenes (Scheme 2). The arylation of a wide range of ortho-methylbenzaldehyde derivatives proceeded with diiodobenzenes in good yields. Both electron-donating (3ab) and electron-withdrawing (3ac) substituents were welltolerated (72% and 55% yield, respectively). The use of meta-diiodobenzenes led to a lower yield (3bb, 63% and 3db, 52%). Ortho-substituted ortho-methylbenzaldehyde could react well with para-diiodobenzenes (3ai, 52%). The heteroaromatic aldehydes also gave good yield (3aj, 66% and 3bj, 57%). Larger conjugated systems (3bl and 3bm) were also compatible and generated the expected products with high efficiency, but the use of para-diiodobenzenes gave low efficiency (3al and 3am). More importantly, the cycloaromatization product (3cb) could be accessed directly, which were not for the others. Next, the reaction of cycloaromatization by using TfOH was explored with good to high yields. Regardless of whether the substrates contain electron-donating or electron-withdrawing moieties, all of them proved to be suitable for cycloaromatization except heteroaromatic substrates (4aj and 4bj). The products 4ah and 4bh were also obtained in very good yield with 20% TfOH. Heteroaromatic products were obtained by using In(OTf)320 in CH2ClCH2Cl at 100 °C. Small graphene nanoribbon precursor (4bn and 4bp) was also synthesized in high efficiency. The swallow-tail N-substituted21 PAH (4bo) with good solubility was synthesized with high efficiency. However, most of the PAHs cannot dissolve in common solvents (4ac−4af, 4al, 4am, 4bc−4bf, and 4bm). We next investigated whether the same strategy could be applied to diiodonaphthalene substrates (Scheme 3), aiming to further expand the shape, width, and edge topology of PAHs. We obtained the monocycloaromatization products (5eb and 5fb) with moderate yields when 2,7-diiodonaphthalene was used. The reactivity of α-position of diiodonaphthalenes was better than that of diiodobenzenes. When 2,4,5-trimethylbenzaldehyde was used as substrate, the monocycloaromatization and no-cycloaromatization products were both obtained (5eq′ and 5eq) and the monocycloaromatization dominated the product. 2,6-Diiodonaphthalene (5fb) and 1,5-diiodonaphthalene (5gq) provided moderate yield. Larger conjugated systems were also investigated (5em). To further expand the substrate diversity, diiodonaphthalenes were examined to construct PAHs bearing a naphthalene segment. Although the initial investigation using 3-methyl-2naphthaldehyde provided 24% yield of the desired product (5em′) under the optimal conditions for diiodobenzenes (Table 1, entry 11), we found that, by using 15 mol % Pd(OAc)2 and a 15:1 mixture of AcOH:H2O as the solvent at 110 °C, the yield of the products 5em and 5em′ could be substantially improved to 35% and 31%, respectively. We also

to the concave armchair edge (bay region), convex armchair edge (K-region) and zigzag edge (L-region) in graphene nomenclature (Figure 1). Their edge structures can be further modified and transformed into fully fused, small graphene nanoribbons.18

Figure 1. Structure of PAHs.

In order to probe the feasibility of the desired palladiumcatalyzed ortho-C(sp3)−H arylation of benzaldehydes, we chose the reaction of 1,4-diiodobenzene (1a) with 2,4dimethylbenzaldehyde (2b) as the model reaction for optimization (Table 1). Table 1. Optimization of the Palladium-Catalyzed orthoC(sp3)−H Arylation of Benzaldehydes with Aryl Diiodidesa

entry 1 2 3 4 5 6 7 8 9 10 11

AgTFA (equiv)

glycine (equiv)

T (°C)

yield (%)

(9:1)

3.0 3.0

1.0 1.0

90 90

44 37

(6:1) (9:1) (9:1) (9:1) (9:1) (9:1) (9:1) (9:1) (9:1)

3.0 3.0 3.0 3.0 2.5 2.0 2.5 2.5 2.5

1.0 0.6 0.8 1.2 1.0 1.0 0.8 1.0 0.8

90 90 90 90 90 90 90 100 100

40 49 58 50 47 36 50 62 70

solvent AcOH:H2O AcOH:H2O (12:1) AcOH:H2O AcOH:H2O AcOH:H2O AcOH:H2O AcOH:H2O AcOH:H2O AcOH:H2O AcOH:H2O AcOH:H2O

a

Isolated yield.

We were delighted to find that the desired product (3ab) could be formed in 44% isolated yield at 90 °C using 10 mol % Pd(OAc)2, 3.0 equiv of silver trifluoroacetate (AgTFA) and 1.0 equiv of glycine in acetic acid (AcOH) and H2O (9:1, v/v) (Table 1, entry 1). Systematic screening of the reaction conditions revealed that the solvent system is a critical reaction parameter. The addition of water would reduce the concentration of the imine intermediate and prevent its decomposition during the reaction. An appropriate proportion between AcOH and H2O is conducive to the balance between the imine formation step and the following ortho-C(sp3)−H activation (Table 1, entries 2 and 3). The reaction is also sensitive to the amount of glycine (Table 1, entries 4−6). The best result was obtained in the presence of 0.8 equiv of glycine. We also evaluated the effect of AgTFA (entries 7−9) and 7621

DOI: 10.1021/acs.orglett.8b03359 Org. Lett. 2018, 20, 7620−7623

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Organic Letters Scheme 2. Scope of the Substrates with Different Diiodobenzenesa

Scheme 3. Scope of the Benzaldehydes Reacted with Different Diiodonaphthalenesa

a

Isolated yield. bPd(OAc)2 (15 mol %), AcOH/H2O (15:1, v/v), 110 °C, 36 h.

We also investigated whether the same strategy could be applied to 1,3,5-triiodobenzene (Scheme 4) to access Scheme 4. Scope of the Reaction with 1,3,5Triiodobenzenea

a

a Isolated yield. bTfOH (20 mol %). CH2ClCH2Cl, 100 °C, 12 h.

c

Isolated yield.

trinaphthylenes. Under the optimal conditions, we obtained the desired products in good yield (41% and 49% for 7hb and 7 h, respectively). Then the corresponding trinaphthylene were successfully obtained via cycloaromatization using TfOH. In conclusion, the palladium-catalyzed ortho-C(sp3)−H arylation of benzaldehydes with aryl diiodides via a transient directing group to synthesize the precursors of PAHs was successfully developed. The high efficiency and practicality of the sequential intramolecular cationic cyclization and dehydration was demonstrated in the construction of PAHs with various shape, width and edge topology by using a catalytic amount of TfOH. This route provides an opportunity to rapidly build up PAHs. Furthermore, the substituents and the

In(OTf)3 (10 mol %),

investigate the reaction of cycloaromatization of these substrates by using TfOH, and they all could be well-tolerated. Triphenylene derivatives have attracted special attention since they were found to form discotic liquid crystals and have great potential as the basis of optical and electronic devices.22 7622

DOI: 10.1021/acs.orglett.8b03359 Org. Lett. 2018, 20, 7620−7623

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edge structures of the PAHs can be further modified and transformed into fully fused, small graphene nanoribbons.

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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.8b03359. Experimental details and NMR spectra for important compounds (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Fang-Lin Zhang: 0000-0002-7472-9713 Author Contributions †

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the financial support from the Fundamental Research Funds for the Central Universities (WUT: 2017IVA114, 2018IVB045).

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REFERENCES

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DOI: 10.1021/acs.orglett.8b03359 Org. Lett. 2018, 20, 7620−7623