posted on 2022-12-16, 17:10authored byGavin
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 (ΔG‡e) 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 ΔG‡e. 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 ΔG‡e 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.