Vertically π‐Expanded Imidazo[1,2‐ a ]pyridine: The Missing Link of the Puzzle

The dehydrogenative coupling of imidazo[1,2‐ a ]pyridine derivative has been achieved for the first time. In cases in which the most‐electron‐rich position of the electron‐excessive heterocycle was blocked by a naphthalen‐1‐yl substituent, neither oxidative aromatic coupling nor reaction under Scholl conditions enabled the fusion of the rings. The only method that converted the substrate into the corresponding imidazo[5,1,2‐ de ]naphtho[1,8‐ ab ]quinolizine was coupling in the presence of potassium in anhydrous toluene. Moreover, we discovered new, excellent conditions for this anion‐radical coupling reaction, which employed dry O 2 from the start in the reaction mixture. This method afforded vertically fused imidazo[1,2‐ a ]pyridine in 63 % yield. Interestingly, whereas the fluorescence quantum yield ( Φ fl ) of compound 3 , despite the freedom of rotation, was close to 50 %, the Φ fl value of flat naphthalene‐imidazo[1,2‐ a ]pyridine was only 5 %. Detailed analysis of this compound by using DFT calculations and a low‐temperature Shpol′skii matrix revealed phosphorescence emission, thus indicating that efficient intersystem‐crossing from the lowest‐excited S 1 level to the triplet manifold was the competing process with fluorescence.