American Chemical Society
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Expanded [23]-Helicene with Exceptional Chiroptical Properties via an Iterative Ring-Fusion Strategy

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journal contribution
posted on 2022-12-16, 17:10 authored by Gavin R. Kiel, Harrison M. Bergman, Adrian E. Samkian, Nathaniel J. Schuster, Rex C. Handford, August J. Rothenberger, Rafael Gomez-Bombarelli, Colin Nuckolls, T. Don Tilley
Expanded helicenes are an emerging class of helical nanocarbons composed of alternating linear and angularly fused rings, which give rise to an internal cavity and a large diameter. The latter is expected to impart exceptional chiroptical properties, but low enantiomerization free energy barriers (ΔGe) have largely precluded experimental interrogation of this prediction. Here, we report the syntheses of expanded helicenes containing 15, 19, and 23 rings on the inner helical circuit, using two iterations of an Ir-catalyzed, site-selective [2 + 2 + 2] reaction. This series of compounds displays a linear relationship between the number of rings and ΔGe. The expanded [23]-helicene, which is 7 rings longer than any known single carbohelicene and among the longest known all-carbon ladder oligomers, exhibits a ΔGe that is high enough (29.2 ± 0.1 kcal/mol at 100 °C in o-DCB) to halt enantiomerization at ambient temperature. This enabled the isolation of enantiopure samples displaying circular dichroism dissymmetry factors of ±0.056 at 428 nm, which are ≥1.7× larger than values for previously reported classical and expanded helicenes. Computational investigations suggest that this improved performance is the result of both the increased diameter and length of the [23]-helicene, providing guiding design principles for high dissymmetry molecular materials.