Regioselective Synthesis of [3]Naphthylenes and Tuning of Their Antiaromaticity

Polycyclic conjugated
hydrocarbons containing four-membered cyclobutadienoids
(CBDs) are of great fundamental and technical interest due to the
antiaromaticity brought by CBD circuits. However, their synthesis
has been challenging, hampering the exploration and understanding
of such systems. We report efficient synthesis of a series of unprecedented
[3]­naphthylene regioisomers in high yields, where three naphthalenoids
are fused through two CBDs in linear, angular, and bent regioconnectivity.
Their synthesis was enabled by exclusively regioselective catalytic
arene–norbornene annulation (CANAL) between dibromonaphthalenes
and benzooxanorbornadienes, followed by aromatization. [3]­Naphthylene
regioisomers exhibited distinct optoelectronic properties. Nucleus-independent
chemical shift calculations, NMR spectroscopy, and X-ray crystallography
revealed the strong effect of the fusion pattern on the local antiaromaticity
and aromaticity in fused CBDs and naphthalenoids, respectively. Thus,
our synthetic strategy allows facile access to extended CBD-fused
π-systems with tunable local antiaromaticity and aromaticity.