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.