Dynamically Chiral Expanded Helicenes

Expanded helicenes are a class of chiral nanographenes with a screw-shaped skeleton and a larger diameter than their classical ortho-fused counterparts. The flexibility of this internal cavity results in helicenes that are often difficult to isolate as single enantiomers. Expanded helicenes are therefore an intriguing target for exploration of dynamic chirality, a phenomenon resulting from introduction of chiral auxiliaries to control structure in a racemic system. This contribution describes installation of chiral amine substituents into the cavity of a diformyl expanded [11]-helicene by mild, efficient, and reversible imine condensations. The resulting helicenes display strong molar circular dichroism up to |Δε| = 300 M^-1 cm^-1 and absorption dissymmetry factors of |g_abs| = 0.010. The magnitude of circular dichroism can be varied by using different chiral amine substituents, which is correlated to the diastereomeric ratios of the helicenes. This is the first instance of dynamic chirality being used to observe a chiroptic response in expanded helicenes and is a significant synthetic improvement over methods which require conventional enantiomer resolution.