Rhodium-Catalyzed Intermolecular Arylative [2 + 2 + 1] Annulation–Oxidation to Produce Electron-Deficient Azulene-Embedded Polycyclic Aromatic Hydrocarbons

Azulene derivatives have attracted much attention for their application in organic electronic materials and devices because of their large dipole moment and small HOMO–LUMO energy gap. As these physical properties of azulene depend on its substitution and condensation patterns, developing methods to synthesize functionalized and π-extended azulenes is desirable. However, synthesizing π-extended azulenes requires harsh reaction conditions, making it hard to achieve both functionalization and π-extension. Here, we report the synthesis of electron-deficient azulene-embedded polycyclic aromatic hydrocarbons (PAHs) with two alkoxycarbonyl groups by the rhodium-catalyzed intermolecular arylative [2 + 2 + 1] annulation of teraryl diynes with dialkyl acetylenedicarboxylates followed by oxidation at room temperature. Interestingly, for the electron-rich diyne, prolonged oxidation time after the arylative [2 + 2 + 1] annulation yields a helicene-like bis(azulene-embedded PAH) in good yield. Thus, obtained electron-deficient fused azulenes have small HOMO–LUMO energy gaps (up to E_g^elec = 1.52 and E_g^theo = 2.06), resulting in long-wavelength absorption extending into the near-infrared region. Due to bulky electron-withdrawing groups and π-extension, the molecule becomes saddle-shaped and highly polarized, and strong π–π stacking interactions are observed in both the solid and solution states.