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

Abstract

Non-alternant non-benzenoid hydrocarbons exhibit unique properties compared to their well-studied benzenoid counterparts. However, the synthesis of these structures with extended π-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 π-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.