(Invited) Stereoselective Synthesis of Molecular Nanographenes

Chirality is an important and fascinating concept which has not been properly addressed in carbon nanoscience 1. We have previously reported the first inherently chiral bilayer nanographene with a helicene linker, both as the racemate and the M isomer 2,3. Helical bilayer nanographenes (HBNGs) are chiral π-extended aromatic compounds consisting in two π-π stacked hexa-benzocoronenes (HBC) joined by a helicene, thus resembling van der Waals layered 2D materials. Recently, we have synthesized and compared 9HBNG, 10HBNG and 11HBNG helical bilayers endowed with 9, 10 and 11helicenes embedded in their structure, respectively. Interestingly, the helicene length defines the overlapping degree between the two HBCs (number of benzene rings involved in π-π interactions between the two layers), being of 26, 14 and 10 benzene rings, respectively, according to the X-ray analysis 4. More interesting, this overlapping also controls the properties. Compounds constituted only by carbon atoms, such as optically active molecular nanographenes have, so far, been obtained as racemates followed by time-consuming chiral chromatographic separations. In this presentation, an enantioselective strategy of a new bilayer nanographene which uses two pivotal stereocontrolled key synthetic steps to introduce and extend stereogenic elements into a polycyclic aromatic Csp 2 -based structure will be discussed. The resulting helical core of six ortho-fused rings embedded in the nanographene ensures stereochemical stability and good chiroptic activity 5. Acknowledgements : This work has been supported by the Ministerio de Educación, Cultura y Deporte of Spain (Grant PID2020-114653RB-100). References 1 J. M. Fernández-García, P. J. Evans, S. Filippone, M. A. Herranz, N. Martín, “Chiral Molecular Carbon Nanostructures”, Acc. Chem. Res . 2019 , 52 , 1565-1574. 2 P. J. Evans, et al., Angew. Chem. Int. Ed . 2018 , 57 , 6774 –6779. 3 Z. Zhou, J. M. Fernández-García, et al. Angew. Chem. Int. Ed. 2022 , 61, e20211574. 4 Patricia Izquierdo-García, et al. J. Am. Chem. Soc. 2023 , 145 , 11599-11610. 5 M. Buendía-Mateos, et al. Manuscript submitted, 2023 .