Corannulene-based Quintuple [6]/[7]helicenes: Well-preserved Bowl Core, Inhibited Bowl Inversion and Supramolecular Assembly with Fullerenes

Herein, corannulene-based quintuple [6]helicenes (Q[6]H-1 andQ[6]H-2) and [7]helicene (Q[7]H) were synthesized via penta-fold Heck and Mallory reaction. Notably, Q[7]H represents the highest reported helicene based on corannulene. X-ray crystallography reveals that Q[6]H-2 adopts a propeller-shaped conformation with a well-preserved corannulene core, while Q[6]H-1 and Q[7]H exhibit quasi-propeller-shaped conformations. Upon heating, conformer Q[6]H-1 undergoes conversion to the thermodynamically more stable conformer Q[6]H-2, whereas conformer Q[7]H remains unchanged due to larger steric congestion. Racemization of the enantiomer of Q[6]H-1 and conformational conversion were observed simultaneously at elevated temperature, with DFT studies indicating a racemization barrier of 28.64 kcal·mol-1. In contrast, the racemization barrier for Q[6]H-2 was calculated to be 45.46 kcal·mol-1, indicating exceptional chiral stability. Surprisingly, the bowl inversions of Q[6]H-1 and Q[6]H-2 conformers are somewhat inhibited by the helical blades, whereas this was not observed for other possible conformers of Q[6]H. These results first demonstrated that subtle conformational variations can lead to significant changes in chiral stability and bowl inversions of multihelicenes. Due to the well-preserved corannulene core, propeller-shaped conformation and electron complementarity, Q[6]H-2 can recognize fullerenes in both solution and solid state, which is a rare instance of co-crystallization assembly between multiple helicenes and fullerenes.