Letter pubs.acs.org/OrgLett
Aromatic Metamorphosis of Dibenzofurans into Triphenylenes Starting with Nickel-Catalyzed Ring-Opening C−O Arylation Yuto Kurata, Shinya Otsuka, Norihito Fukui, Keisuke Nogi, Hideki Yorimitsu,* and Atsuhiro Osuka Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan S Supporting Information *
ABSTRACT: A new class of aromatic metamorphosis has been developed in which dibenzofurans were converted into triphenylenes. This transformation is composed of three successive operations: (1) nickel-catalyzed ring-opening C− O bond arylation with arylmagnesium bromides, (2) trifluoromethanesulfonylation (triflation) of the resulting hydroxy moiety with Tf2O, and (3) palladium-catalyzed or photoinduced ring closure. In the last ring-closing step, the photoinduced process has proven to be more productive than the palladium-catalyzed one. By employing π-extended dinaphthofuran as the substrate, dorsally benzo-fused [5]helicene was obtained in a satisfactory yield.
T
Scheme 1. Strategy for Transformation of Dibenzofurans into Triphenylenes
riphenylenes have gained increasing attention because of their unique physical and optical properties, which can be utilized as liquid crystals as well as materials in organic lightemitting diodes (OLEDs).1 Nevertheless, owing to the high symmetry and planarity of the triphenylene core, efficient and selective synthesis of triphenylenes remains difficult and can be further improved.2 Recently, we have become interested in developing novel methodologies, “aromatic metamorphosis”, in which an aromatic compound is converted into another cyclic compound through aromatic ring cleavage and reconstruction processes.3 Along this line, we achieved transformations of dibenzothiophenes into triphenylenes in four steps assisted by palladium catalysts.3a However, this four-step protocol has several drawbacks: (1) organosulfur substrates have to be activated as sulfonium salts by alkylation of the sulfur atom before both ring-breaking and ring-closing steps, (2) stoichiometric amounts of silver salts are necessary for the alkylation reactions, and (3) resulting sulfonium species are often unstable. To address these problems and expand the utility of aromatic metamorphosis, here we report conversions of dibenzofurans into triphenylenes in three steps powered by nickel-catalyzed ring-opening C−O bond arylation and palladium-catalyzed or photoinduced ring closure. Our strategy is depicted in Scheme 1. The first step is ringopening arylation of the C−O bond with an arylmetal reagent under nickel catalysis (step a). After the ring-opening arylation, the resulting aryloxy moiety would be readily converted into triflate with trifluoromethanesulfonic anhydride (Tf2O) (step b). Since the C−O bond of aryl triflate is highly reactive, we expected that the subsequent ring closure would proceed easily (step c). First, we needed to develop ring-opening arylation of dibenzofurans with arylmagnesium reagents. Although an ethereal C−O bond is much stronger than C−X (X = I, Br, Cl) bonds, a variety of catalytic C−O bond cleavages are rapidly emerging with the aid of electron-rich transition metal catalysts such as nickel complexes.4,5 Ring-opening arylation or © XXXX American Chemical Society
alkylation of the challenging endocyclic C−O bonds of heteroaromatic furan analogues was also accomplished under nickel5a,n,6 or rhodium catalysis.7 However, dibenzofurans have rarely undergone C−O bond cleavage. Although Chatani and Tobisu recently succeeded in nickel-catalyzed ring-opening alkylation of dibenzofuran with an alkylmagnesium reagent,5n there has been no example of arylative ring-opening of dibenzofurans. We first screened a series of nickel catalysts in the presence of p-tolylmagnesium bromide to execute ring-opening arylation of dibenzofuran (1a) as a model (Table 1). Phosphine ligands as well as a widely used carbene ligand IPr did not show catalytic activity (entries 1−3). Similar reaction conditions for the arylative C−O bond cleavage of benzofuran reported by Martin was not applicable to the conversion of dibenzofuran (entry 4).6c Fortunately, 1,3-dialkylimidazol-2-ylidene ligands, especially I(1-Ad) and ICy, afforded the arylated product 2a in Received: December 27, 2016
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DOI: 10.1021/acs.orglett.6b03861 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters Table 1. Optimization of Catalyst System
a
entry
Ni cat.
1 2 3 4a 5 6 7 8 9b
NiCl2(PCy3)2 Ni(cod)2 NiCl2(IPr) (PPh3) Ni(cod)2 Ni(cod)2 Ni(cod)2 Ni(cod)2 Ni(cod)2 Ni(cod)2
ligand SPhos SIPr·HCl ItBu·HBF4 I(1-Ad)·HBF4 ICy·HBF4 ICy·HCl I(2-Ad)·HCl
Scheme 2. Scope of Ring-Opening Arylation
NMR yield (%) 8 6 0 trace 3 36 77 80 91
1.0 equiv of LiCl was added. bPerformed for 2 h.
a
Triflation was performed with 2 equiv of Tf2NPh and 2 equiv of NEt3 at −78 °C to room temperature for 24 h. bNickel-catalyzed reaction was performed for 5 h. c3.0 mmol scale.
moderate to good yields (entries 6−8). Eventually, I(2-Ad)5d,8 was found to be optimal for efficient ring opening to afford hydroxyteraryl 2a in 91% NMR yield (entry 9). After aqueous workup of the arylation, treatment of the crude reaction mixture with Tf2O furnished the corresponding teraryl triflate 3a in 88% isolated yield over two steps (Scheme 2). We then conducted the ring-opening C−O bond arylation and subsequent triflation with an array of arylmagnesium bromides. Although electron-rich 4-(dimethylamino)phenylmagnesium bromide underwent the C−O bond arylation to give the product 3c in good yield, electrondeficient 4-fluoro- or 4-(trifluoromethyl)phenylmagnesium bromide did not afford the corresponding product, and 1a was recovered. Sterically congested 1-naphthyl- or 9-phenanthrylmagnesium bromide provided the coupling products 3e or 3f albeit in moderate yields as well as in a longer reaction time. As for the scope of dibenzofurans, 2,8-diphenyldibenzofuran (1g) was also applicable to this transformation. The C−O bond arylation also proceeded with π-extended dinaphthofurans to furnish 3h and 3i. With teraryl triflates 3 in hand, we attempted the last ringclosing transformation to form triphenylenes (Scheme 3). In our previous report, ring closure of teraryl sulfonium compounds was accomplished by means of a palladium/ SPhos catalyst.3a As we expected, under similar reaction conditions (procedure A), ring closure of 3a also proceeded to afford 2-methyltriphenylene (4a) in 89% yield. However, when other teraryl triflates were employed, the yields of products were moderate or significantly low, and 4i was not obtained at all. Furthermore, teraryl triflate 3d afforded a
mixture of two regioisomers 4d and 4d′ with moderate selectivity. As an alternative method to execute smooth ring closure, we then focused on photoinduced cyclization with a loss of TfOH. In the literature,9 under photoirradiation conditions, 2halogenated stilbenes or o-terphenyls undergo dehydrohalogenative cyclization to afford phenanthrenes or triphenylenes. Moreover, this method was applicable to the synthesis of polycyclic aromatic hydrocarbons such as dibenzotetracenes.9b,e We thus conducted photoinduced ring closure of teraryl triflates 3. A dilute benzene solution of 3a was irradiated by means of a 100-W high-pressure mercury lamp at room temperature for 30 min. To our delight, 4a was successfully obtained in 90% yield. In contrast to the palladium-catalyzed system, this method was applicable to a range of teraryl triflates 3 and provided the corresponding triphenylenes 4 in moderate to good yields. It is noteworthy that photoinduced ring closure of 3d took place with exclusive regioselectivity to furnish 4d as a single isomer in 88% yield. It is assumed that the ring closure proceeds through photoinduced 6π electrocyclization followed by elimination of TfOH. The stronger double-bond character of the C1−C2 bond of the naphthalene ring than that of the C2−C3 bond would lead to the perfect 4d/4d′ regioselectivity. The ring closure of 3i gave benzo[5]helicene 4i in 44% yield. A larger scale synthesis (1.0 mmol of 3i in 100 mL of benzene) improved the yield of 4i up to 59%. This three-step access to B
DOI: 10.1021/acs.orglett.6b03861 Org. Lett. XXXX, XXX, XXX−XXX
Organic Letters
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Scheme 3. Formation of Triphenylenes 4
Letter
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] ORCID
Shinya Otsuka: 0000-0001-5598-9707 Keisuke Nogi: 0000-0001-8478-1227 Hideki Yorimitsu: 0000-0002-0153-1888 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Numbers. JP23655037, JP24685007, JP25107002, JP16H01019, JP16H01149, JP16H04109, and JP16H06887 as well as by ACT-C, JST. H.Y. thanks the Japan Association for Chemical Innovation, Tokuyama Science Foundation, and The Naito Foundation for financial support. S.O. and N.F. acknowledge support through JSPS Predoctoral Fellowships.
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a 5.0 mol % of Pd2(dba)3 and 10 mol % of SPhos was used. b0.43 mmol scale. c0.34 mmol scale. Performed for 45 h. d0.24 mmol scale. e 0.20 mmol scale for 2 h. f0.20 mmol scale for 10 h. 1.0 equiv of I2 was added. g1.0 mmol scale. Performed for 2 h.
benzo[5]helicene demonstrates the striking synthetic utility of our system because a limited number of reports have been available to synthesize dorsally arene-fused [5]helicenes.10 In conclusion, we achieved a new class of aromatic metamorphosis in which dibenzofurans were transformed into triphenylenes in three steps through sequential nickel-catalyzed C−O bond arylation, triflation of the resulting hydroxy moiety, and palladium-catalyzed or photoinduced ring closure. In the last step, the photoinduced cyclization is much more efficient than the Pd-catalyzed reaction. Compared with the aromatic metamorphosis of dibenzothiophenes into triphenylenes, the present protocol is more step-economical, less time-consuming, and more efficient. Further investigations to extend the scope of substrates as well as to efficiently synthesize new and/or large π-extended aromatic hydrocarbons are now in progress.
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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.6b03861. Detailed experimental procedures as well as full spectroscopic data for all new compounds (PDF) C
DOI: 10.1021/acs.orglett.6b03861 Org. Lett. XXXX, XXX, XXX−XXX
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