O₂-promoted photocyclization of arylimines: A mild strategy of generating phenanthridines

A visible light-induced cyclization of arylimines using trifluoromethanesulfonic acid (TfOH) as the catalyst is reported. The transformation, conducted under air and mild conditions utilizes O₂ as the oxidant, eliminating the need for leaving groups or excess acid. Optimization studies reveal the pivotal role of TfOH and visible light in the reaction. The methodology allows a broad substrate scope, accommodating various electron-donating and electron-withdrawing groups. The reaction conditions enable gram-scale synthesis and show inhibitory effects on cancer cell lines and low toxicity towards normal cells. Mechanistic insights indicate a visible light-induced 6π-electrocyclization. This method provides a practical and efficient approach for the synthesis of phenanthridine derivatives.