Introduction

The extended polycyclic aromatic hydrocarbons (PAHs) have received significant interdisciplinary attention due to their semiconducting applications in diverse organic electronics as well as intriguing structural interests of well-defined graphene segments1,2,3,4,5,6,7,8. A variety of synthetic methodologies for constructing various π-extended PAHs have been developed towards achieving high efficiency and novelty5,6,7,8,9,10,11,12,13,14,15. Among them, since the seminal advance has been made by Müllen and Spiess et al. towards synthesizing hexa-peri-hexabenzocoronene (p-HBC) through a highly efficient oxidative intramolecular cyclodehydrogenation (Scholl reaction) of hexaphenylbenzene16,17, the selective oxidative aromatic coupling of unfunctionalized arenes with a net loss of hydrogens has become the most popular, straightforward and atom-economical synthetic strategy of various PAHs involving nanographene and graphene5,6,7,8,9. However, this methodology still has some problems such as unpredictable regioselectivities due to the different activity of aromatic C–H bonds, electronic property of arenes and stereo factors. In this context, the discovery of novel and expedient aromatization methods towards structurally diverse PAHs with selectivity control and high efficiency is highly desirable.

Recently, we have developed a novel FeCl3-mediated oxidative dehydrogenative spirocyclization of 1,2-di(9H-fluoren-9-ylidene)-1,2-diphenylethane for the synthesis of a new class of the dispirolinked π-system DSFIIF (Fig. 1a)18. In this study, the conjugated diene moiety in 1,2-di(9H-fluoren-9-ylidene)-1,2-diphenylethane was predicted to be more electron-rich than the fluorenyl and phenyl moieties by the frontier molecular orbital calculation, which suggests that it would easily undergo twofold single-electron oxidation preferentially to form a dication species, followed by the intramolecular Friedel-Crafts reaction to afford the corresponding dispirocycle DSFIIF. In light of this observation, we designed a new olefin substrate 9-(biphenyl-2-ylmethylene)-9H-fluorene (BPMF), in which the alkene moiety was calculated to possess relatively higher electron density compared to other moieties (Fig. 1b and Supplementary Fig. 1). Inspired by this observation, we anticipated that if a radical cation forms selectively at the alkene moiety of BPMF by employing appropriate single-electron oxidation systems, the subsequent spirocyclization and 1,2-aryl migration tandem process may take place to give the desired twisted molecule dibenzo[g,p]chrysene (DBC), which is an intriguing holding block of discotic molecules, fluorescent and charge carrier-transporting materials. Diverse synthetic methods of the DBC derivatives have been reported, such as oxidation of electron-rich bis(biaryl)acetylenes, oxidation of electron-rich tetraarylethylenes, intramolecular aromatic C–H/C–Br coupling, double Suzuki-Miyaura coupling, and Pd-catalysed coupling of C–H bonds of small PAHs with dimethyldibenzosilole13,14,15,19,20,21,22,23,24,25,26,27,28. On the basis of our reaction design, we herein describe a novel dehydrogenative PAH synthetic methodology based on the oxidative spirocyclization and 1,2-aryl migration tandem process. A variety of extended PAHs involving functionalized DBCs, hexabenzotetracene, tetrabenzophenanthrotetraphene, tetrabenzophenanthropicene and helicene derivatives have been readily synthesized in good to high yields without prefunctionalization of aromatic rings.

Figure 1: Single-electron oxidation of alkenes for synthesis of π-conjugated polycyclic hydrocarbons.
figure 1

(a) Our previous method for synthesis of dispirofluorene-indenoindenefluorene (DSFIIF) via FeCl3-promoted oxidative spirocyclization of 1,2-di(9H-fluoren-9-ylidene)-1,2-diphenylethane (DFDPE). (b) This study for the single-electron oxidation induced spirocyclization and 1,2-aryl migration tandem synthesis of PAHs.

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Results

Investigation of single-electron oxidation conditions

Based on our previous oxidative spirocyclization18, we firstly examined the weak acidic oxidant FeCl3 using the readily available substrate 1a (Table 1, see also Supplementary Methods). Unfortunately, the use of FeCl3/FeO(OH) neutral systems resulted in no reaction, while the acidic FeCl3 in o-xylene at 80 °C only produced the spirocyclic product 3a (vide infra, Fig. 6) in 88% yield without formation of the desired product 2a. After investigation of various copper catalysts (20 mol%) combined with molecular oxygen (1 atm) in the presence of trifluoroacetic acid (TFA; 5 equiv), we were pleased to find that the desired tandem reaction did proceed with CuCl catalyst/O2 systems, affording the corresponding product 2a in 39% yield (Table 1, entry 1). The use of other oxidants such as tert-butyl hydroperoxide (TBHP) and di-tert-butyl peroxide (DTBP) in place of O2 have no beneficial effect in achieving a high yield of 2a (entries 2 and 3). To our delight, the yield of 2a was improved significantly when PhI(OAc)2 and PhCO3 tBu were used as oxidants in conjunction with the CuCl catalyst (entries 4 and 5); particularly, the latter oxidant produced 2a in nearly quantitative yield. The combination of a Cu(I) salt with PhCO3 tBu was reported to enable the formation of a PhCO2Cu(II) salt along with a tBuO radical species in situ to generate a allylic radical29, thus suggesting the involvement of a radical mechanism in the present tandem reaction. The use of PhCO3tBu, DTBP and PhI(OAc)2 as oxidants in the absence of the copper catalyst led to poor yields of 2a (entries 6–8), indicating the important role of the copper catalyst in the Cu(I)/oxidant systems. Further screening of other oxidants in the absence of the copper catalyst revealed that 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) and o-chloranil were also effective in achieving high yields of 2a (entries 9 and 10). It was noted that the use of TFA as an additive was necessary for rendering this transformation; the strong acid such as trifluoromethanesulfonic acid (TfOH) gave the spirocycle 3a in 95% yield, while the weak acid such as acetic acid resulted in no reaction. It has been demonstrated that the DDQ/strong acid system readily undergoes a single-electron oxidation of various electron donors with high oxidation potentials to afford the corresponding radical cation species23,30,31,32, implying the present transformation involves the radical cation formation process.

Table 1 Optimization of oxidation conditions.
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Figure 2: Substrate scope of oxidative aromatization.
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The isolated yields are shown. †CuCl (40 mol%), PhCO3tBu (3 equiv) and TFA (10 equiv) were used. ‡DDQ (3 equiv) and TFA (10 equiv) were used. ‡The reaction temperature is 40 °C.

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Figure 3: Synthesis of extended PAHs.
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Double annulations of (a) biphenyl-centered bis-methylenefluorenes, (b) p-terphenyl-centered bis-methylenefluorenes, (c) m-terphenyl-centered bis-methylenefluorenes, (d) 1,5-diphenylnaphthalene-centered bis-methylenefluorenes and (e) 1,1′:5′,1″-ternaphthalene-centered bis-methylenefluorenes.

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Figure 4: Construction of phenanthrene motif.
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The reaction with nonfluorenyl substrate 1r is able to produce the corresponding 9,10-diphenylphenanthrene 2r under the standard conditions.

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Figure 5: Ultraviolet–visible absorption and fluorescence spectra of extended PAHs.
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The ultraviolet–visible absorption (solid line) and fluorescence spectra (dash line) of 2k–m (a), and 2o–q (b) in diluted chloroform. The ultraviolet–visible absorption red-shifted as expending their π-conjugation lengths. The extended PAHs possess relatively large Stock shifts, indicating their weak structural rigidity.

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Figure 6: Reactions using the spirocyclic substrate 3a for the formation of 2a.
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Both single-electron oxidation conditions A and B afforded the corresponding product 2a, indicating the involvement of spirocyclic radical species B and C for subsequent 1,2-aryl migration.

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Synthesis of various PAHs with versatile functional groups

With the optimized conditions obtained from entries 5 and 9 in Table 1 as conditions A and B, the electronic effect of substituents on the fluorene and biphenyl moieties in the starting BPMFs was investigated to understand the influence on the construction of the DBC scaffold (Fig. 2). The reaction of 1b with two electron-rich t-butyl substituents on the fluorene moiety under condition A afforded a higher yield of the corresponding DBC 2b than that under condition B. Likewise, the condition A showed higher activity than the condition B for the reactions of 1c and 1d with two electron-poor substituents such as Br and I as R1. The electron-rich substrate 1b showed much higher reactivity as compared to the electron-poor substrates 1c and 1d under both conditions A and B. Similar electronic effects were also observed from the substrates with substituents on the biphenyl moiety. For example, the reactions of 1e and 1f with two electron-rich substituents such as Me and MeO as R2 produced the corresponding DBCs 2e and 2f in high yields under both conditions A and B, while the reactions of 1g and 1h having two electron-withdrawing substituents such as F and Cl as R2 afforded the corresponding products 2g and 2h in lower yields with longer reaction times. The BPMF 1i and its perdeuterated analogue 1i- d5 bearing electron-donating t-butyl and methoxy groups at fluorene and biphenyl moieties showed a high reactivity, which underwent the tandem annulation at 40 °C under both conditions A and B to afford the corresponding DBC derivatives 2i and 2i- d4 in good yields and no D–H exchanged products were observed for the reaction of 1i- d5. In the reactions of the benzothiophene-substituted substrate 1j, the DDQ-mediated condition B showed higher activity than the Cu-catalysed condition A, giving the corresponding DBC derivative 2j in moderate yields. Interestingly, the tandem reaction of 1k composed of binaphthyl instead of biphenyl also proceeded efficiently, giving rise to the helicene product 2k in high yields33.

The growing interests in the synthesis and potential application of extended PAHs as graphene molecules in optoelectronics led us to examine the construction of highly extended PAHs using the present tandem reaction1,2,3,4. In contrast to the activity for the synthesis of the DBC scaffold, the CuCl-catalysed condition A was less active than the DDQ-mediated condition B for the construction of the following extended PAHs. Under condition A, the double annulation reaction of 1l containing two biphenyl-centered methylenefluorene moieties produced the corresponding hexabenzo[a,c,fg,j,l,op]tetracene 2l in a low yield of 10% (Fig. 3a). It was noted that the monoannulated product 2l′ was dominantly formed in 60% yield (see Supplementary Information for the structure of 2l′). To our delight, the yield of 2l increased to 55% under the DDQ-mediated condition B with the formation of 2l′ in 16% yield. The structure of 2l was recently reported to possess an interesting helically twisted conformation14. We also examined the double annulation of the p-terphenyl-centered methylenefluorene substrates 1m and 1n (Fig. 3b). The reaction of 1m under condition B proceeded well to afford the corresponding tetrabenzo[a,c,f,m]phenanthro[9,10-k]tetraphene 2m (ref. 34) in 88% yield, while only a 20% yield of 2m was obtained under condition A. The combination of the CuCl catalyst with DDQ (condition C) further increased the yield of 2m to 95%. Although the condition B was not sufficiently effective for the double annulation of the t-butyl-substituted substrate 1n, fortunately, the condition C provided the corresponding product 2n (ref. 13) in a high yield of 86%. The present method was also successfully applied to the synthesis of several novel π-extended PAHs. When the m-terphenyl-centered methylenefluorene 1o was subjected to both conditions A and B, the corresponding new PAH of tetrabenzo[a,c,f,k]phenanthro[9,10-m]tetraphene 2o was obtained in 32% and 86%, respectively (Fig. 3c). Moreover, the 1,5-diphenylnaphthalene-centered methylenefluorene 1p also underwent the double tandem annulation efficiently under condition B to afford the novel π-extended PAH 2p in 72% yield, while the condition A resulted in decomposition of 1p (Fig. 3d). Remarkably, a novel π-extended S-type helicene product 2q could be constructed in 40% yield upon the double annulation of the 1,1′:5′,1″-ternaphthalene-centered methylenefluorene 1q under condition B (Fig. 3e). The different activity of conditions A to C awaits further clarification with respect to the yield and reactivity, but we assumed that the electron distribution of benzene components in various substrates should highly affect the single-electron oxidation of the alkene moiety.

We also examined the triaryl-substituted ethene 1r bearing a diphenylmethylene unit instead of the fluorene moiety (Fig. 4). As expected, the reaction of 1r underwent the present tandem reaction efficiently, affording the corresponding 9,10-diphenylphenanthrene 2r in high yields under both conditions A and B. Other biphenyl-substituted ethenes replacing the 2,2-diphenyl unit in 1r with different substituents, such as Ph and Me, Ph and H, dimethyl and two hydrogens were examined to be totally inactive, demonstrating the importance of triaryl groups for the implementation of the present tandem transformation. The aryl groups are thought to be crucial for the radical cation stabilization and 1,2-aryl migration tandem process as shown in Fig. 7.

Figure 7: Proposed tandem reaction mechanism.
figure 7

Initially, the reaction takes place via a single-electron oxidation of the relatively electron-rich alkene moiety of 1a, followed by the intramolecular Friedel-Crafts reaction to form a spirocyclic radical species B, which is further oxidized to give a spirocyclic cation C. The cation C gives rise to the 1,2-aryl migration followed by aromatization to afford the desired dibenzochrysene 2a.

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Optical and electrochemical properties of 2k–q

The ultraviolet–visible absorption of extended PAHs of 2p (426 nm) and 2q (432 nm) in diluted chloroform solution show red-shifted onsets compared to that of 2k–o due to their large π-extension lengths (Table 2 and Fig. 5). The fluorescence spectra of the extended PAHs have emission maxima at the region of 436–475 nm and exhibit relatively large Stock shifts in the range of 17–65 nm, implying their weak structural rigidity. Interestingly, the S-type helicene 2q exhibits a much smaller Stock shift of 17 nm compared to the simple helicene 2k (65 nm), indicating the higher structural rigidity of the former. The compounds 2l, 2m and 2p in diluted chloroform have moderate fluorescence quantum yields of 0.27, 0.36 and 0.31, respectively, which are higher than the compound 2o (0.19). It was found that both helicene compounds 2k (0.06) and 2q (0.14) showed relatively low emission properties. The highest occupied molecular orbital (HOMO) energy level calculated from the oxidation potential by cyclic voltammetry (Supplementary Fig. 2) shows that the helicene 2k has the lowest HOMO of −5.65 eV and 2o has the highest HOMO of −5.25 eV compared to that of other four PAHs (−5.42 to −5.50 eV). The lowest unoccupied molecular orbital (LUMO) energy level estimated from the HOMO and the optical energy gap shows that 2k, 2m, 2p and 2q have lower LUMOs in the range of −2.51 to −2.63 eV compared to the PAHs 2l (−2.37 eV) and 2o (−2.18 eV). The large extended PAHs 2p and 2q show smaller HOMO–LUMO gaps than that of other PAHs, which is attributed to the decreased LUMO orbitals more than their HOMO orbitals.

Table 2 Optical and electrochemical properties of 2k–q.
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Discussion

To further understand the reaction pathways, the spirocyclic compound 3a was used as a starting substrate under both conditions A and B (Fig. 6), which was prepared as a byproduct during the optimization of reaction conditions. Although the yields of the desired product 2a were relatively low, the oxidation reactions did proceed under the standard conditions, suggesting the formation of spirocyclic radical B and cation C intermediates (Fig. 7) through the sequential single-electron oxidation of 3a. The formation of the cation species C may give rise to subsequent 1,2-aryl shift to produce the final product 2a. It should be mentioned that, however, the present tandem reaction of 1a to 2a may not involve the formation of 3a by the H atom abstraction. In addition, an intermolecular competing reactions between the protonated substrate 1i and the perdeuterated substrate 1i- d5 were studied in the same reaction vessel to understand the kinetic isotope effects (KIEs; Supplementary Fig. 3). When a 1:1 mixture of 1i and 1i- d5 was heated at 40 °C for 1 h under conditions A and B, respectively, the corresponding products 2i and 2i- d4 were obtained with similar KIE values of 1.21 and 1.20. The small isotope effect suggests that the present reaction involves a rapid aromatic deprotonation process and in other words, the C–C bond formation or cleavage steps can be presumed to be the rate-determining.

The proposed reaction mechanism is outlined in Figure 7 in terms of the experimental information, such as the indispensable role of single-electron oxidants, electronic effect of substrates, control reaction with the spirocyclic substrate and KIE values. Initially, a single-electron oxidation takes place at the more electron-rich alkene moiety of 1a by the CuCl/PhCO3tBu or DDQ oxidation system to form a radical cation species A (refs 9, 35). Subsequently, the spirocyclization proceeds through the intramolecular Friedel-Craft reaction to afford a spirocyclic radical species B, which may undergo a second single-electron oxidation by the excess oxidants to form a spirocyclic cation C. Next, the direct 1,2-aryl migration36,37,38 of the cation C or more likely through an arenium cation intermediate D affords the cation E, which undergoes the rapid deprotonation to give the stable aromatic product 2a.

In summary, we have disclosed a novel and efficient tandem synthetic method for the construction of various extended PAHs by the single-electron oxidation of various (o-biphenylyl)methylene-substituted fluorenes. The selective single-electron oxidation of the alkene moiety by the Cu catalyst/PhCO3tBu or DDQ oxidation system in the presence of TFA enabled the subsequent spirocyclization and 1,2-aryl migration tandem process to take place. Following this method, various known and new extended PAHs including the functionalized DBCs, benzo[f]naphtho[1,2-s]picene, hexabenzo[a,c,fg,j,l,op]tetracene, tetrabenzo[a,c,f,m]phenanthro[9,10-k]tetraphene, tetrabenzo[a,c,f,k]phenanthro[9,10-m]tetraphene, tetrabenzo[a,c,f,o]phenanthro[9,10-m]picene and S-type helicene have been readily synthesized. Further extension of the present tandem synthetic strategy to a new class of π-extended polycycles with diverse ring systems is in progress.

Methods

Materials

For NMR spectra of compounds in this manuscript, see Supplementary Figs 4–52. For the calculated electron distribution of HOMO orbital of 1a, see Supplementary Fig. 1. For the cyclic voltammograms of compounds 2k–m and 2o–q, see supplementary Fig. 2. For the KIE experiments, see Supplementary Fig. 3. For the general information, experimental procedures and analytic data of compounds synthesized, see Supplementary Methods.

General procedure for synthesis of 2a

To a o-xylene (1.2 ml) solution of CuCl (4 mg, 20 mol %) and PhCO3tBu (58.3 mg, 0.3 mmol) (condition A) or DDQ (68.1 mg, 0.3 mmol, condition B) were added TFA (114 mg, 1 mmol) and BPMF (1a) (66 mg, 0.2 mmol) at room temperature. The mixture was heated at 80 °C for 1.5 h (condition A) or 12 h (condition B). After cooling to room temperature, the reaction mixture was monitored by TLC and GC–MS. The reaction mixture was washed with water and extracted with CH2Cl2 for 2 times. After concentration of the CH2Cl2 solution, the resulting residue was purified by flash silica gel chromatography using a mixture of CH2Cl2/hexane as eluent to give the corresponding product 2a in 94% yield (61.8 mg, condition A) or in 87% yield (57.2 mg, condition B) as a colourless solid.

Data availability

The authors declare that the data supporting the findings of this study are available within the paper and its Supplementary Information files, and also are available from the corresponding author upon reasonable request.

Additional information

How to cite this article: Zhang, X. et al. Synthesis of extended polycyclic aromatic hydrocarbons by oxidative tandem spirocyclization and 1,2-aryl migration. Nat. Commun. 8, 15073 doi: 10.1038/ncomms15073 (2017).

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