N‐Heterocycle‐Fused Pentalenes by a Gold‐Catalyzed Annulation of Diethynyl‐Quinoxalines and ‐Phenazines

The gold‐catalyzed annulation of diethynyl N‐heterocycles for the synthesis of quinoxaline‐/phenazine‐based pentalenes and the study of their optoelectronic properties are described. The inhibition of the gold catalyst by the nitrogen centers in the substrate and the product could be overcome by increasing the reaction temperature to 130 °C, which usually leads to catalyst decomposition in gold catalysis. At 130 °C, 6,7‐di(arylethynyl)quinoxalines in chlorobenzene give the corresponding pentalenes. The annulation of 2,3‐di(arylethynyl)quinoxalines requires an even higher temperature under microwave irradiation. The quinoxaline‐based pentalenes showed lower LUMO levels compared to the corresponding naphthalene‐based pentalenes. It′s gold again! The synthesis of quinoxaline‐/phenazine‐based dibenzo[a,e]pentalenes from the corresponding diynes by gold catalysts was developed. The reactivity of 6,7‐di(arylethynyl)quinoxalines and 2,3‐di(arylethynyl)quinoxalines is significantly different; the annulation of 6,7‐di(arylethynyl)quinoxalines proceeded at high temperature to give the corresponding pentalenes in good to high yields, but the reaction of 2,3‐di(arylethynyl)quinoxalines required high temperature under microwave irradiation (see scheme).