Letter pubs.acs.org/OrgLett
Synthesis of Tetraaza[8]circulenes from Tetrathia[8]circulenes through an SNAr-Based Process Yuya Nagata, Shohei Kato, Yoshihiro Miyake,* and Hiroshi Shinokubo Department of Molecular and Macromolecular Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan S Supporting Information *
ABSTRACT: The synthesis of highly planar tetraaza[8]circulenes from tetrathia[8]circulenes through oxidation followed by a nucleophilic aromatic substitution reaction with arylamines is presented. Photophysical and electrochemical properties of tetraaza[8]circulenes were investigated and compared to those of tetrathia[8]circulenes. The tetraaza[8]circulene exhibited bright fluorescence both in solution and solid states. [8]Circulenes are unique macrocyclic arenes in which eightmembered rings are surrounded by eight-fused benzene units.1 Among them, hetero[8]circulenes are a class of [8]circulenes containing heteroaromatic rings and are expected to be applicable in organic electronic materials such as light-emitting diodes and field-effect transistors. A variety of hetero[8]circulenes2 annulated by furans,3 thiophenes,4−7 selenophenes,4c and siloles8 have been successfully synthesized. Aza[8]circulenes have also attracted considerable attention as functional motifs for organic materials based on the unique optical and electronic properties of pyrroles. The synthesis of azatrioxa-,9 diazadioxa-,9 diazadithia-,10 and diazadiselena[8]circulenes10 have been reported to date (Figure 1). However, until now, the construction of the tetraaza[8]circulene core has been limited to tetrabenzotetraaza[8]circulene reported by Osuka and co-workers.11 Recently, we reported the synthesis of tetrathia[8]circulenes 2 in which a variety of substituents can be introduced at the peripheral positions.7 We envisaged transformation of tetrathia[8]circulenes 2 into tetraaza[8]circulenes 1, which should provide a series of tetraaza[8]circulenes with various substituents (Scheme 1a). The transformation of dibenzothiophenes to carbazoles via a nucleophilic aromatic substitution (SNAr) reaction of dibenzothiophene dioxides has been reported by Yorimitsu and co-workers (Scheme 1b).12 Here, we report the synthesis of tetraaza[8]circulenes 1 from tetrathia[8]circulenes 2. The structure and properties of 1 are also disclosed. Tetraaza[8]circulenes 1 were synthesized from 2 in two steps as shown in Scheme 2. Oxidation of 2a and 2b with m© 2017 American Chemical Society
Figure 1. Examples of aza[8]circulenes.
chloroperoxybenzoic acid (mCPBA) afforded the corresponding tetrathia[8]circulene octaoxides 3a and 3b. We then attempted the SNAr reaction of 3 with amines. The reaction of 3a with p-toluidine (R′ = p-tol) in the presence of an excess amount of KN(SiMe3)2 (KHMDS, 0.5 M toluene solution) in Received: April 9, 2017 Published: May 10, 2017 2718
DOI: 10.1021/acs.orglett.7b01074 Org. Lett. 2017, 19, 2718−2721
Letter
Organic Letters Scheme 1. (a) Transformation of Dibenzothiophenes into Carbazoles; (b) Synthesis of 1 from 2 via SNAr Reaction
Figure 2. (a) Top and (b) side views of the molecular structure of 1a (atomic displacement parameters set at 50% probability; all hydrogen atoms and t-Bu groups in (a) and 4-t-BuC6H4 groups in (b) are omitted for clarity).
Scheme 2. Synthesis of 1 from 2 via SNAr Reaction
1a are 349.53° (N1), 354.90 (N3), and 353.80 (N4), indicating that these nitrogen atoms adopt shallow trigonal pyramidal geometry. The value (357.93°) of N2 is similar to that in 3,6diphenyl-9-(p-tolyl)-9H-carbazole (359.83°).12 The C−N bonds in pyrrole rings of 1a (1.397(3)−1.4071(17) Å) are slightly longer than those in tetrabenzotetraaza[8]circulene (1.37−1.38 Å).11 On the basis of these results, we concluded that that the structure around the nitrogen atoms in 1a is slightly distorted because of the steric repulsion among the aryl rings on the benzene and pyrrole rings. Electronic absorption spectra of 1a in CH2Cl2 and 2a in CHCl3 were measured (Figure 3a).14 As compared to 2a, the
1,4-dioxane/toluene at 85 °C for 24 h afforded tetraaza[8]circulene 1a in 5% yield along with the partial substitution products 4a (20%) and 5a (27%). Further substitution reaction of 4a and 5a under similar conditions furnished 1a in 24% and 12% yields, respectively. In sharp contrast, the reaction of 3b with p-tert-butylaniline (R′ = p-t-BuC6H4) under similar conditions furnished 1b in 25% yield without formation of the partial substitution products. We could not identify other byproducts. These results suggest that the electron withdrawing nature of p-CF3C6H4 on 3b promotes the SNAr reaction with amines effectively. Recrystallization of 1a from chloroform/acetonitrile afforded a single crystal suitable for X-ray diffraction analysis, which unambiguously confirmed the molecular structure of 1a (Figure 2).13 The mean plane deviation of the tetraaza[8]circulene core in 1a is 0.061 Å, and the sum of the inner angles of the central eight-membered ring is 1079.81°, which are close to that of a regular octagon (1080°). These results suggest that the tetraaza[8]circulene core is almost planar. The sums of the valence angles around the nitrogen atoms (N1, N3, and N4) in
Figure 3. (a) UV−vis absorption spectra of 1a in CH2Cl2 (red line) and 2a in CHCl3 (black line) and fluorescence spectra of 1a in CH2Cl2 (red dash line) and 1a in solid (blue dash line) excited at 400 nm. (b) Fluorescence photograph of 1a in CH2Cl2. (c) Fluorescence photograph of 1a in solid.
lowest energy absorption band of 1a was slightly bathochromic shifted, and the molar extinction coefficient (ε) was slightly larger. This feature is in good agreement with the calculated absorption energies and oscillator strengths of the model compounds for 1a and 2a by the time-dependent density functional theory (TD-DFT) calculations at the B3LYP/631G(d) level of theory (Figures S5 and S6). A significant difference between 1a and 2a was observed in the fluorescence spectra. Tetraaza[8]circulene 1a exhibited blue fluorescence in a CH2Cl2 solution (Φ = 0.27), which resembles 2719
DOI: 10.1021/acs.orglett.7b01074 Org. Lett. 2017, 19, 2718−2721
Letter
Organic Letters
2601)” (JSPS KAKENHI Grant No. 26102003) and “Precisely Designed Catalysts with Customized Scaffolding (No. 2702)”(JSPS KAKENHI Grant No. 16H01013) from MEXT, Japan.
the mirror image of the absorption band (Figure 3a,b). In contrast, 2a showed only weak emission (Φ = 2.0σ(I)), Rw = 0.2055 (all data), GOF = 1.022. Crystallographic data for 1a·2CHCl3 have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-1532538. (14) The molar extinction coefficient of 1a could not be determined exactly in CHCl3 because 1a is slightly unstable in a CHCl3 solution.
Table 1. Summary of Electrochemical Dataa compd
Eox1 (V)
Eox2 (V)
Eox3 (V)
1a 2cb
0.15 0.72
0.59
0.94
a
Solvent: CH2Cl2. Supporting electrolyte: Bu4NClO4 (0.1 M).Working electrode: glassy carbon. Counter electrode: Pt wire. Reference electrode: Ag/AgClO4. Scan rate: 100 mV S−1. All potentials are referenced to the potential of ferrocene/ferrocenium cation couple. b Reference 7.
oxidation potentials, while tetrathia[8]circulene 2c showed one reversible potential. The first oxidation potential of 1a (0.15 V) is lower than that of 2c (0.72 V). This is attributed to the stronger electron-donating nature of nitrogen than that of sulfur. In summary, we have succeeded in synthesizing tetraaza[8]circulenes 1 from tetrathia[8]circulenes 2 through the nucleophilic aromatic substitution reaction of tetrathia[8]circulene octaoxides. The present route through the SNAr-based process allows direct transformation from sulfur to nitrogen on hetero[8]circulenes and modulation of photophysical and electrochemical properties on the basis of the distinct nature of the heteroatoms. This finding will open up a new aspect of the synthetic methodology for hetero[8]circulenes and diskshaped molecules including heteroaromatics. Further work is currently in progress in our group aimed at synthesizing novel PAHs incorporating heteroaromatic moieties and application of tetraaza[8]circulenes to p-type semiconductors.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b01074. Experimental procedures as well as spectroscopic and crystallographic data (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yoshihiro Miyake: 0000-0003-0247-531X Hiroshi Shinokubo: 0000-0002-5321-2205 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Scientific Research on Innovative Areas “pi-System Figuration (No. 2720
DOI: 10.1021/acs.orglett.7b01074 Org. Lett. 2017, 19, 2718−2721
Letter
Organic Letters Separately, we confirmed that the shape and the absorption bands of the spectrum of 1a in CHCl3 were similar to those of 1a in CH2Cl2 (Figure S3). (15) Octaoxide 3a exhibited no fluorescence in a CH2Cl2 solution. (16) A powder sample was prepared by the evaporation of a solution of 1a in CH2Cl2. After the powder of 1a was placed between two quartz plates, the fluorescence spectrum shown in Figure 3a was measured. (17) Unfortunately, we have not succeeded in the measurement of CV of 1b owing to its poor solubility in suitable solvents.
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DOI: 10.1021/acs.orglett.7b01074 Org. Lett. 2017, 19, 2718−2721
