Synthesis, Structure, and Chiroptical Properties of a Double [7]Heterohelicene.

The synthesis of 11a,25a-dibora-11,12,25,26-tetraoxatetranaphtho[1,2-a:2',1'-f:1″,2″-j:2‴,1‴-o]perylene, a double [7]heterohelicene containing OBO units, has been achieved via tandem demethylation-borylation, representing the highest double helicene reported thus far with all six-membered rings. Single-crystal X-ray analysis clearly demonstrated a significantly twisted structure with the terminal aromatic rings overlapping at both ends, giving the first example of a double helicene with intramolecular π-layers. Such structural features resulted in a high theoretical isomerization barrier of 45.1 kcal/mol, which is the highest value for all the double helicenes ever reported, rendering the achieved molecule with high chiral stability. The (P,P)- and (M,M)-isomers were separated by chiral HPLC and the chiroptical properties were investigated, revealing opposite circular dichroism responses.