Structures, Chiroptical Properties, and Unexpectedly Facile Helical Inversion of Highly Elongated Anthracene-Fused Expanded Helicenes

• Published in: Chemistry : a European journal 2024-12, p.e202404348
• Main Authors: Fukuda, Hiroki, Kobayashi, Moe, Tsurumaki, Eiji, Yamashina, Masahiro, Hasegawa, Masashi, Wakamatsu, Kan, Toyota, Shinji
• Format: Article
• Language: English
• ISSN: 0947-6539; 1521-3765; 1521-3765
• DOI: 10.1002/chem.202404348

Helical fused anthracenes were elongated by fusing additional aromatic units at both ends to yield novel expanded helicenes. Compounds [5]HA2N and [7]HA consisting of 19 and 21 benzene rings, respectively, were synthesized by fourfold cycloisomerization of the corresponding terminal alkyne precursors. The helical structures were confirmed by X-ray crystallographic analysis, where the aromatic frameworks stacked effectively with the helical turn numbers exceeding two. The enantiomers of the two compounds were resolved by chiral HPLC. Whereas [5]HA2N readily underwent enantiomerization at room temperature at the barrier to enantiomerization of 91 kJ mol , the barrier was enhanced to 99 kJ mol for the long analog [7]HA. The enantiomers of [7]HA exhibited strong responses in the circular dichroism (CD) and circularly polarized luminescence (CPL) spectra, as scaled by dissymmetry factors |g |=0.034 and |g |=0.012. Theoretical calculations by the r SCAN-3c method suggested stepwise mechanisms for the enantiomerization via helical inversion with acceptable barrier heights. The unexpectedly flexible nature of the aromatic frameworks of [5]HA2N and [7]HA was discussed on the basis of the proposed mechanism.