Angewandte Eine Zeitschrift der Gesellschaft Deutscher Chemiker Chemie www.angewandte.de Akzeptierter Artikel Titel: Synthesis of Pyrrole-Fused Corannulenes via Unprecedented 1,3-Dipolar Cycloaddition of Azomethine Ylides to Corannulene Autoren: Yuki Tokimaru, Shingo Ito, and Kyoko Nozaki Dieser Beitrag wurde nach Begutachtung und Überarbeitung sofort als "akzeptierter Artikel" (Accepted Article; AA) publiziert und kann unter Angabe der unten stehenden Digitalobjekt-Identifizierungsnummer (DOI) zitiert werden. Die deutsche Übersetzung wird gemeinsam mit der endgültigen englischen Fassung erscheinen. Die endgültige englische Fassung (Version of Record) wird ehestmöglich nach dem Redigieren und einem Korrekturgang als Early-View-Beitrag erscheinen und kann sich naturgemäß von der AA-Fassung unterscheiden. Leser sollten daher die endgültige Fassung, sobald sie veröffentlicht ist, verwenden. Für die AA-Fassung trägt der Autor die alleinige Verantwortung. Zitierweise: Angew. Chem. Int. Ed. 10.1002/anie.201707087 Angew. Chem. 10.1002/ange.201707087 Link zur VoR: http://dx.doi.org/10.1002/anie.201707087 http://dx.doi.org/10.1002/ange.201707087 10.1002/ange.201707087 Angewandte Chemie COMMUNICATION Synthesis of Pyrrole-Fused Corannulenes via Unprecedented 1,3-Dipolar Cycloaddition of Azomethine Ylides to Corannulene Abstract: In the long history of corannulene chemistry, the 1,3dipolar cycloaddition to corannulene is unprecedented. Herein, we report the 1,3-dipolar cycloaddition of a polycyclic aromatic azomethine ylide to corannulene, which occurs exclusively at the rim bond of corannulene from the convex side in an exo fashion. The cycloadducts could be successfully converted by successive oxidative dehydrogenation to pyrrole-fused corannulenes, which exhibited pronounced solvatofluorochromism. Since its first synthesis in 1966, corannulene (1),[1,2] a C5vsymmetric polycyclic aromatic hydrocarbon (PAH) consisting of a central pentagon surrounded by five fused hexagonal benzene rings, has attracted much attention due to its unique structural and electronic properties, e.g. its bowl-shaped structure, its dipole moment, which is oriented perpendicular relative to the πsurface, and its high electron-accepting ability. The fascinating properties of 1 have led to much research on the applications of 1 in e.g. organic electronics, molecular recognition,[7,8] and supramolecular polymerization, whereby attention has predominantly been focused on the functionalization of 1. Most methods available for such functionalization rely on substitution reactions at the rim bond of 1 (Figure 1a), including electrophilic,[10–13] and radical as well as metal-catalyzed C–H substitutions, functionalization.[15,16] The rim bond of 1 is also amenable to addition reactions with various reagents such as alkyllithium and alkylradical species (Figure 1b). Conversely, addition reactions of carbocations occur at the spoke bond of 1 and afford isolable cationic species (Figure 1c). Cyclization/cycloaddition reactions represent another important functionalization mode for 1. The spoke double bond can be used for cycloaddition reactions such as the cyclopropanation with dichlorocarbene species (Figure 1d). Recently, palladium-catalyzed annulative π-extensions of 1 have been reported as a method to engage the rim double bond of 1 in a one-step cyclization (Figure 1e). However, to the best of our knowledge, cycloadditions involving the rim bond of pristine 1 have not yet been demonstrated experimentally (Figure 1f), even though theoretical calculations have predicted that the rim double bond of 1 may be reactive toward [4π+2π]-type cyclizations. [a] Mr. Y. Tokimaru, Dr. S. Ito, and Prof. Dr. K. Nozaki Department of Chemistry and Biotechnology Graduate School of Engineering The University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan E-mail: firstname.lastname@example.org, email@example.com Supporting information for this article is given via a link at the end of the document. 1,3-Dipolar cycloadditions represent a powerful synthetic tool to construct heterocyclic five-membered rings containing oxygen or nitrogen atoms. Azomethine ylides, which are characterized by a C=N+–C− moiety, are reactive and versatile 1,3-dipoles that allow the construction of pyrrolidine or 2,5-dihydropyrrole structures when treated with alkenes or alkynes. 1,3-Dipolar cycloadditions involving azomethine ylides are widely used for the functionalization of curved PAHs such as fullerenes and carbon nanotubes. However, successful 1,3-dipolar cycloadditions involving 1 remain elusive, although some unsuccessful attempts have been reported. (a) (f) X X (b) spoke This work (e) hub X (c) Y flank rim (d) 1 X X X Figure 1. Functionalization of 1: (a) substitution at the rim bonds, (b) addition to the rim bonds, (c) addition to the spoke bonds, (d) cycloaddition to the spoke bonds, (e) substitutive cyclization at the rim bonds, and (f) cycloaddition to the rim bonds. The probability to engage 1 in unprecedented 1,3-dipolar cycloadditions should be maximized when employing highly reactive azomethine ylides. Previously, our group and those of Feng and Müllen have independently developed polycyclic aromatic azomethine ylide 3, which is generated in situ from iminium salt 2, and can be used for 1,3-dipolar cycloadditions to alkenes and alkynes to afford the corresponding pyrrolidines and 2,5-dihydropyrroles (Scheme 1). The most noteworthy feature of 3 is its high reactivity towards dipolarophiles, i.e., 3 even converts sterically crowded 1,2-diarylethynes into the corresponding cycloadducts. Herein, we report unprecedented 1,3-dipolar cycloadditions of azomethine ylide 3 to the rim double bond of 1, which affords the corresponding mono- and diadducts 4 and 5, respectively. This reaction represents the first example of a 1,3-dipolar cycloaddition to 1. Furthermore, upon subsequent aromatizations, cycloadducts 4 and 5 could be converted into pyrrole-fused corannulenes 6 and 7 (Scheme 1).[28,29] This article is protected by copyright. All rights reserved. Accepted Manuscript Yuki Tokimaru,[a] Shingo Ito,*[a] and Kyoko Nozaki*[a] 10.1002/ange.201707087 Angewandte Chemie tBu H H N H 1 (1.0 equiv) + Cl N iPr2NEt (8 equiv) DMSO 120 ºC, 1 h H H H H N + H H H N H H via N tBu tBu 2 (4.0 equiv) 4 (46%) tBu 5 (29%) tBu tBu 3 N DDQ (2.1 equiv) DDQ (4.2 equiv) 4 5 CH2Cl2 rt, 10 min N tBu 6 (99%) CH2Cl2 rt, 14 h N tBu 7 (80%) Scheme 1. 1,3-Dipolar cycloaddition of azomethine ylide 3 to corannulene 1 and subsequent dehydrogenation to form pyrrole-fused corannulenes 6 and 7. The 1,3-dipolar cycloaddition of 1 with 3 generated in situ from iminium salt 2 was carried out under the conditions outlined in Scheme 1. As a result, mono- (4) and diadducts (5) were obtained in 46% and 29% isolated yield, respectively. Singlecrystal X-ray diffraction analyses of 4 and 5 (vide infra) revealed that 3 was added to the rim C=C bond of 1 from the convex side in an exo fashion to give 4. The addition of a second molecule of 3 proceeded in a similar manner to give cis diadduct 5. It should be noted that other regio- and stereoisomers, which could be formed by a cycloaddition to the rim C=C bond in an endo fashion, or by a cycloaddition to a spoke C=C bond, were not detected. In sharp contrast, other typical azomethine ylides such as CH2=N+MeCH2− and CH2=N+BnCH−CO2Me as well as other aromatic azomethine ylides did not provide any cycloadducts under similar reaction conditions (Scheme S1). These results are consistent with the report by Scott et al., who examined 1,3dipolar cycloadditions involving 1. Subsequently, we treated cycloadducts 4 and 5 with 2,3-dichloro-5,6-dicyano-pbenzoquinone (DDQ) to afford dehydrogenated pyrroles 6 and 7 in good yields (Scheme 1). Single-crystal X-ray diffraction analyses on 4, 5, and 6 were conducted using crystals grown by slow diffusion of hexane/pentane into a dichloromethane solution (4 and 5) or a carbon disulfide solution (6) of the target compounds, whereas no single crystal of 7 suitable for structural determination could be obtained. As shown in Figure 2, the polycyclic cores of 4 and 6 exhibit a Cs-symmetric structure due to the attachment of a fused pyrrolidine and a fused pyrrole ring to the rim bond of C5vsymmetric 1. The structure of the corannulene core was slightly distorted due to the ring fusion. For example, the C2–C3 single bond in 4 [1.608(9) Å], i.e., the rim bond of 1 that is fused with the pyrrolidine ring, is even longer than typical Csp3–Csp3 single bonds (~1.54 Å). This elongation is presumably due to the strain of the corannulene moiety that arises from the elongation of the C–C single bond. The pyrrole-fused rim bond of 6 [1.472(5) Å] is also significantly elongated compared to the other rim bonds (1.37–1.39 Å) and the rim bonds of 1 (1.38 Å), as well as the C2–C3 bond of pyrrole (1.43 Å). Considering the shorter length of the rim bond in a porphyrin-fused corannulene (1.40 Å),[12a] this elongation could be attributed to the larger contribution of a single bond to the bond alternation caused by fusion of a pyrrole ring. This notion is supported by the decreased aromaticity of the pyrrole-fused six-membered ring, which can be estimated by NICS calculations conducted at the B3LYP/6-31G(d) level of theory. The NICS(0) values of the pyrrole-fused six-membered ring of 6 (−2.6 ppm) and 7 (−2.4 ppm) are significantly higher than those of the other sixmembered rings in the corannulene moiety (Figure S16). The perpendicular distance between the center of the central fivemembered ring of the corannulene core and the parallel plane containing the middle points of each rim bond were calculated for 4 (0.70, 0.88, and 0.84 Å) and 6 (0.70, 0.95, and 0.86 Å). The elongation of the rim bond that carries the pyrrolidine and pyrrole rings leads to a release of bowl strain and shallower bowl depths compared to 1 (0.87 Å). Moreover, the comparatively longer distance to the plane containing C24–C25(C33–C34) in 6 (0.95 Å) should be noted, and probably be attributed to the repulsion of hydrogen atoms attached to C6(C21) and C24(C34), i.e., the repulsion between the annulated dibenzoullazine cores and the corannulene moiety. Figure 2. Single-crystal X-ray diffraction structures of (a) 4 and (b) 6 with thermal ellipsoids set to 50 % probability. Solvent molecules, tert-butyl groups, and hydrogen atoms except for those on the pyrrolidine ring of 4 (side view) were omitted for clarity. In order to elucidate the reason for the observed high regioand stereoselectivity of this 1,3-dipolar cycloaddition, DFT calculations were performed at the B3LYP-D3/6-31G(d) level of theory. In order to simplify the calculations, the t-butyl group of 3 and the cycloadducts were replaced with hydrogen atoms, and only concerted mechanisms were considered, while stepwise or radical addition mechanisms were disregarded in conformity with the results of previous calculations. We calculated the relative This article is protected by copyright. All rights reserved. Accepted Manuscript COMMUNICATION 10.1002/ange.201707087 Angewandte Chemie COMMUNICATION Figure 3. Energy profiles for the 1,3-dipolar cycloaddition of azomethine ylide 3’, in which the t-butyl group of 3 was replaced with a hydrogen atom, to 1, calculated at the B3LYP-D3/6-31G(d) level of theory: (a) convex-rim-exoaddition, (b) convex-rim-endo-addition, (c) convex-spoke-addition, (d) concave-rim-exo-addition, and (e) concave-rim-endo-addition. The conformation and the bowl inversion behavior of 6’, in which the t-butyl group of 6 was replaced with a hydrogen atom, was also investigated by DFT calculations at the B3LYP-D3/631G(d) level of theory (Figure 4). The butterfly-shaped conformation A, which was observed experimentally in the single-crystal X-ray diffraction analysis of 6 (Figure 2b), was identified as the most stable conformer, including twisted conformation B (∆G = 5.1 kcal/mol) and another butterfly-shaped conformation (A*; ∆G = 9.5 kcal/mol). Among TSAA*, TSAB, and TSBB, i.e., the calculated TSs between each conformer, TSAA* (11.1 kcal/mol) is the highest pathway. The calculated barrier is comparable to the bowl inversion barrier of 1 (experimental value: 10–11 kcal/mol; theoretical value: 10.4 kcal/mol) and those of similar phenanthrene-annulated corannulenes (11.0 kcal/mol). Figure 4. Interconversion pathways of 6’ calculated at the B3LYP-D3/631G(d) level of theory. Pink highlights indicate transition states that involve a bowl inversion of the corannulene core, while blue highlights indicate transition states that involve a flipping of the ortho-fused helicene structure. Finally, the optical properties of pyrrole-fused corannulenes 6 and 7 were evaluated by UV-vis absorption and fluorescence spectroscopy (Figures 5 and Table S3). The UV-vis absorption spectra of solutions of 6 and 7 in cyclohexane, benzene, THF, CH2Cl2, and DMSO exhibited broad peaks, whose absorption edges (6: ~490 nm; 7: ~540 nm) and absorption spectra were virtually inert to the polarity of solvents. The observed absorption wavelengths are bathochromically shifted relative to those of 1 and similar to those of related molecules bearing an ullazine structure.[26,27] The HOMO and LUMO energies of 6 and 7 were estimated by combining the results of UV absorption analysis and the oxidation potentials of 6 and 7 measured by cyclic voltammetry (Figure S14 and Table S4). The emission spectra of 6 and 7 exhibited remarkable solvatofluorochromism. Characteristic vibronic emission bands with small Stokes shifts were observed in non-polar solvents (6: 470, 503, and 540 nm; 7: 487, 522, and 562 nm; both in cyclohexane), whereas broad emission bands with large Stokes shifts were observed in polar solvents. These phenomena are consistent with a radiative deactivation from locally excited states in non-polar solvents, and an emission from a low-lying charge-transfer state by intramolecular charge transfer (ICT) in polar solvents. Considering that the LUMOs of 6 and 7 are mainly located on the corannulene core (Figures S17 and S18), the ICT may occur from the ullazine moiety to the corannulene core in polar solvents. Notably, the highest quantum yield was observed in DMSO (6: 34%, 7: 33%; Table S3). These results stand in sharp contrast to the typical behavior of solvatofluorochromic molecules, whose fluorescence quantum yields decrease with increased polarity of the solvent. To the best of our knowledge, this is the first example of a solvatofluorochromic molecule that bears a corannulene moiety. This article is protected by copyright. All rights reserved. Accepted Manuscript G [kcal/mol] Gibbs energies for the starting materials, transition state (TS), and products for all the possible regioisomers arising from: (a) convex-rim-exo-addition, (b) convex-rim-endo-addition, (c) convex-spoke-addition, (d) concave-rim-exo-addition, and (e) concave-rim-endo-addition (Figure 3). Among all calculated TSs, the TS of the rim-exo addition (12.8 kcal/mol) is the lowest and affords the most stable product (−12.8 kcal/mol). In other words, the convex-rim-exo pathway (a) is thermodynamically and kinetically the most favourable pathway. The preference of the convex addition of 3 to 1 should be interpreted in terms of the larger coefficients of the lowest-unoccupied molecular orbitals (LUMOs) at the convex side relative to those at the concave side, considering that in typical 1,3-dipolar cycloadditions, the highest-occupied molecular orbital (HOMO) of the 1,3-dipole interacts with the LUMO of the dipolarophile. As the LUMO of 1 is mainly located at the rim bonds, the 1,3-dipolar cycloaddition of 3 should occur there, and this preference is consistent with the experimental results, i.e., that only cis adduct 5 was obtained. Nevertheless, the high selectivity toward the rim addition stands in sharp contrast to the spoke-selectivity of the addition of carbene species to 1, in which the LUMO of the electrophilic carbene species reacts with the HOMO of 1. 10.1002/ange.201707087 Angewandte Chemie    Figure 5. Absorption (dashed lines: 5.0 × 10−6 M) and emission spectra (solid lines: 5.0 × 10−7 M; λex = 350 nm) of (a) 6 and (b) 7 at room temperature; purple: cyclohexane; blue: benzene; yellow: THF; green: CH2Cl2; red:  DMSO.   In summary, we have developed a new method for synthesizing pyrrole-fused corannulenes 6 and 7 in a 2-step procedure involving an unprecedented 1,3-dipolar cycloaddition of azomethine ylide 3 to corannulene. The cycloaddition to corannulene occurs selectively at one of the rim C=C bonds, from its convex side in an exo fashion. The results of theoretical calculations strongly support the observed selectivity. Pyrrolefused corannulenes 6 and 7 exhibited bathochromically shifted absorption spectra relative to that of pristine corannulene, and a remarkable solvatofluorochromism. We believe that this novel πextension method, based on the 1,3-dipolar cycloaddition to corannulene, should become a useful tool for the development of highly desirable corannulene-based functional materials.     Acknowledgements This work was supported by JSPS KAKENHI No. 16H06030, Japan. A part of this work was conducted at the Research Hub for Advanced Nano Characterization, The University of Tokyo, supported by MEXT, Japan. The theoretical calculations were performed using computational resources provided by Research Center for Computational Science, National Institutes of Natural Sciences, Okazaki, Japan. Y.T. is grateful to the Japan Society for the Promotion of Science (JSPS) for a Research Fellowship for Young Scientists.      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For the details of optical properties of 6 and 7, see Table S3. This article is protected by copyright. All rights reserved. Accepted Manuscript COMMUNICATION 10.1002/ange.201707087 Angewandte Chemie COMMUNICATION Entry for the Table of Contents (Please choose one layout) Layout 1: COMMUNICATION Text for Table of Contents Author(s), Corresponding Author(s)* Title ((Insert TOC Graphic here)) Layout 2: COMMUNICATION Y. Tokimaru, S. Ito,* K. Nozaki* Page No. – Page No. Here we report the 1,3-dipolar cycloaddition of a polycyclic aromatic azomethine ylide to corannulene, which occurs exclusively at the rim bond of corannulene from the convex side in an exo fashion. The cycloadducts could be converted by oxidative dehydrogenation to pyrrole-fused corannulenes, which exhibited pronounced solvatofluorochromism. Synthesis of Pyrrole-Fused Corannulenes via Unprecedented 1,3Dipolar Cycloaddition of Azomethine Ylides to Corannulene This article is protected by copyright. All rights reserved. Accepted Manuscript Page No. – Page No.