Exploring C-C bond formation reactions for expanding azulene derivatives linked at the 2-and/or 6-positions

Non-alternant non-benzenoid hydrocarbons exhibit unique properties compared to their well-studied benzenoid counterparts. However, the synthesis of these structures with extended pi-conjugation or their integration into established benzenoid frameworks presents ongoing challenges in organic chemistry. In this study, we investigated different strategies for the synthesis of pi-extended, azulene-containing polyaromatic hydrocarbons (PAHs), the key challenge being the formation of intramolecular C-C bonds involving different positions of the azulene. The Pt-catalyzed alkyne benzannulation gave the best results as ring closure reactions have been accomplished simultaneously at the 1 and 5 positions of the azulene unit, allowing the preparation of three azulene-annulated anthracene structural isomers whose optical and electrochemical properties have been studied. Although the three isomers possess very similar properties in the neutral state, the mono- and diprotonated forms differ significantly with absorption extending up to the near-infrared region. Different strategies for the synthesis of pi-extended, azulene-containing polyaromatic hydrocarbons (PAHs) were investigated and the most efficient was used to prepare azulene-fused anthracene derivatives.