Synthesis and Chiroptical Characterization of a Negatively Curved Helicene with a Central Tropone Unit

We report the synthesis of a π-expanded [5]helicene incorporating a 7-membered tropone unit. The synthesis was based on a cycloaddition-cyclization strategy that exploited the versatile reactivity of alkynes. X-ray crystallographic analysis of the racemic compound revealed its highly twisted structure and provided insights into the structural impacts of the tropone moiety on the [5]helicene subunit. Enantiomeric resolution by chiral HPLC followed by an assessment of the configurational stability revealed a significantly reduced barrier for racemization compared to pristine [5]helicene. Density functional theory calculations on the racemization mechanism identified the crucial role of the flexible tropone unit in preorganizing the molecular framework towards the transition-state geometry, thereby lowering the activation barrier.