This document is downloaded from DR‑NTU (https://dr.ntu.edu.sg) Nanyang Technological University, Singapore. Synthesis of corannulene‑based nanographenes Muzammil, Ezzah M.; Halilovic, Dzeneta; Stuparu, Mihaiela Corina 2019 Muzammil, E. M., Halilovic, D., & Stuparu, M. C. (2019). Synthesis of corannulene‑based nanographenes. Communications Chemistry, 2(1), 58‑. doi:10.1038/s42004‑019‑0160‑1 https://hdl.handle.net/10356/141853 https://doi.org/10.1038/s42004‑019‑0160‑1 © 2019 The Author(s). This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Downloaded on 25 Sep 2021 18:40:16 SGT REVIEW ARTICLE https://doi.org/10.1038/s42004-019-0160-1 OPEN Synthesis of corannulene-based nanographenes Ezzah M. Muzammil1,2, Dzeneta Halilovic1,2 & Mihaiela C. Stuparu 1 Corannulene (C20H10) is a polycyclic hydrocarbon in which five six-membered rings surround 1234567890():,; a central five-membered ring to construct a bowl-like aromatic structure. Here we examine the development of synthetic strategies that allow for the growth of the peripheral aromatic rings as a means to extend the aromatic area of the central corannulene nucleus and provide access to unique nanocarbon molecules. p2-hybridised structures of carbon have fascinated the research community for a very long 1 s time. In 1985, buckminsterfullerene, otherwise known as C60, was discovered (Fig. 1a) .In this ball-shaped molecule, the curvature in the structure stems from the presence of five- membered rings. In 1991, carbon nanotubes arrived on the scene2. Here, the structure is cylindrical and composed of only rolled-up six-membered rings. In 2004, a sheet-like single layer from graphite—graphene—was isolated3. All of these materials were shown to have extra- ordinary electronic and mechanical properties due to their unique curved or planar sp2-hybri- dised aromatic structures. Inspired by these discoveries, chemists have been developing strategies to access such aromatic hydrocarbons through rational (‘bottom-up’) synthetic approaches. Scott’s 12-step chemical synthesis of fullerene C60 from a rationally designed precursor is a testament to the ingenuity and resourcefulness of organic chemists4. In planar structures, nanographenes (well-defined cutouts of graphene with nano-scale dimensions) can now be prepared on a regular basis with a very diverse portfolio5. It is expected that combining the planar structure of graphene with the curvature of fullerenes may produce hybrid materials with interesting properties6–8. To induce non-planarity into nanographenes, a practical approach would be to introduce a five- membered ring such as in the case of fullerene, C60. A perfect building block that allows for such a structural arrangement to happen is corannulene (1)—a molecule in which five six-membered rings surround a central five-membered ring to give a bowl-like structure (Fig. 1b)9–20. Cor- annulene also offers many beneficial features as a molecular building block. It has high solubility in common organic solvents. It can be derivatized in a well-defined manner. Due to synthetic ease, the derivatives can be prepared on a multigram scale. These attributes are important as they allow for the scalable preparation of carefully designed corannulene-based building blocks and the subsequent synthesis, purification and structural analysis of the larger (fused) aromatic systems. Recently, therefore, there has been a surge in employment of corannulene as a core molecule in the synthesis of extended aromatic structures. Our aim in this review article is to discuss 1 Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, and School of Materials Science and Engineering, Nanyang Technological University, 21 Nanyang Link, 637371 Singapore, Singapore. 2These authors contributed equally: Ezzah M Muzammil, Dzenta Halilovic. Correspondence and requests for materials should be addressed to M.C.S. (email: [email protected]) COMMUNICATIONS CHEMISTRY | (2019) 2:58 | https://doi.org/10.1038/s42004-019-0160-1 | www.nature.com/commschem 1 REVIEW ARTICLE COMMUNICATIONS CHEMISTRY | https://doi.org/10.1038/s42004-019-0160-1 a C60 Carbon nanotube Graphene b c Cl Cl 1 Cl Cl 2 Cl i Cl Cl Cl Cl Cl ii Cl Cl Cl Cl Cl 4 3 Fig. 1 Curved and planar polycyclic aromatic hydrocarbons. a Chemical structures of fullerene, carbon nanotube and graphene. b Chemical structure of corannulene 1. c Synthesis of carbon nanotube end-cap. (i) C38H53ClNO2PPd·CH3OC4H9, 2,6-dichlorophenyl zinc chloride, THF, 100 °C, 5 h; 52%. (ii) Flash vacuum pyrolysis, 1100 °C, 0.25 Torr; 3%. c is partially reprinted with permission from ref. 23. Copyright 2012 American Chemical Society recent advances in this fascinating area of research. In light of the coupling with 2,6-dichlorophenylzinc chloride to present pre- two comprehensive review articles written by the pioneers of the cursor 3. The C–Cl bonds are cleaved during the pyrolysis of 3 to field, Scott9 and Siegel10, we limit our discussion to examples generate aryl radicals that join to form a web of five-membered published after 2006. The discussion is categorised on the basis of rings in polyarene 4. X-ray analysis of crystals of 4 confirm the synthetic method and a chronological order is maintained in each structure and measure a bowl depth of 5.16 Å. A CS2 molecule section. was read in the crystal structure in the ‘basket’ of the polyarene— a sulfur atom hovering above the centre of the structure and the Pyrolytic method carbon atom hovering below the plane of the rim carbons. This Barth and Lawton’s first synthesis of corannulene was a true feat of work demonstrates that the FVP method originally developed for organic synthesis. It comprised 17 synthetic steps and allowed the preparation of corannulene and fullerene C60 is still relevant access to this beautiful molecule in a <1% overall yield21.Following and can be a valuable synthetic tool in the preparation of carbon- this elegant work, the field remained dormant for the next quarter based nano-tubular architectures through rational synthesis of a century until Scott’s group demonstrated flash vacuum pyr- pathways. The reader is referred to a recent conference paper by olysis (FVP) as an alternative to Barth and Lawton’s solution-phase Scott for an insightful discussion on this approach to carbon method. Scott’s method allowed access to corannulene in a nanotubes and its prospects for the future24. remarkably practical fashion (3-step synthesis with an overall yield While FVP has been critical in rejuvenating the field, the high of 26%)22. This work breathed new life into the research area of temperatures limit the range of functionalities on the corannulene non-planar aromatics and rejuvenated the field of corannulene. scaffold. Solution-phase methods alleviate this situation by In FVP, high temperatures are employed to overcome the employing milder reaction conditions. In this regard, the reac- energy barrier of introducing the necessary strain onto the tions may be aided by metal catalysis. Alternatively, metal-free molecular structure. Conversion of the precursor to the desired conditions can be employed to achieve the same purpose. In the product depends on both the heating time and temperature. The following sections, we examine both pathways for the extension power of this synthetic tool can be appreciated in the final syn- of the corannulene nucleus. thetic step of a hemispherical polyarene (C50H10) that could serve as a carbon nanotube end-cap (Fig. 1c)23. The first step of the Reactions involving metal catalysis synthesis is a five-fold chlorination of corannulene with iodine Pd-catalysed coupling.Scott’s group in 2007 reported the synth- monochloride. The pentachloro product 2 follows a Negishi esis of extended corannulene structures; pentaindenocorannulene 2 COMMUNICATIONS CHEMISTRY | (2019) 2:58 | https://doi.org/10.1038/s42004-019-0160-1 | www.nature.com/commschem COMMUNICATIONS CHEMISTRY | https://doi.org/10.1038/s42004-019-0160-1 REVIEW ARTICLE a Cl Cl Cl Cl Cl B(OH) i ii Cl + 2 Cl Cl Cl Cl Cl 256 b Cl Cl Br Br Cl B(OH) i ii Br Br + 2 Cl Cl 789 c 10 11 12 13 14 d e OTf Ar Ar Cl Cl iii TMS Cl Cl 15 iii Cl Cl iv + Cl Cl 19 i 16 17 Cl Cl vi v Cl Cl 18 20 Fig. 2 Corannulene extension pathways involving palladium catalysis. a Synthesis of pentaindenocorannulene. (i) Pd2(dba)3, 1,3-bis(2,6-diisopropylphenyl) imidazolium chloride, Cs2CO3, dioxane, 80 °C, 48 h; 48%. (ii) Pd(PCy3)2Cl2, DBU, DMAc, 180 °C (microwave), 45 min; 35%. b Synthesis of tetraindenocorannulene. (i) Pd(PPh3)4,K2CO3, toluene/EtOH/H2O, 85 °C, 24 h; 91%. (ii) Pd(PCy3)2Cl2, DBU, DMAc, 170 °C (microwave), 40 min; 13%. c Mono, di, and triindenocorannulenes prepared by palladium-catalysed coupling reaction. d Wu’s synthesis of buckybowls 15–18. (i) Pd(OAc)2,C6H5I, AgOAc, p-xylene, 110 °C, 36 h; 60% (Ar = 2,6-C6H3Cl2). (ii) Pd(PCy3)2Cl2, DBU, DMF, 160 °C, 36 h; 31%. (iii) 2-butyne, Rh(PPh3)3Cl, p-xylene, 110 °C, 60 h; 99%.