Under nitrogen protection, 18.6 g (0.31 mol) of ethylenediamine was dissolved with 55.0 g (0.26 mol) of 9,10-phenanthrenequinone in 1000 mL of toluene to form a red suspension. A manifold was installed and the reaction mixture was heated to reflux for 24 hours. Upon completion of the reaction, 50.0 g of manganese dioxide was added to the resulting brown suspension, and heating and refluxing was continued until thin layer chromatography (TLC) detection showed complete disappearance of the intermediate. The reaction mixture was filtered while hot and the solid residue was washed with 200 mL of toluene. The filtrates were combined and concentrated under reduced pressure, and the resulting solid was dried under vacuum to give 58.0 g of dibenzo[f,h]quinoxaline as a pale yellow solid in 97% yield.
[1] Patent: CN101970448, 2016, B. Location in patent: Paragraph 0381; 0382
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