Visible Light-Promoted Reaction of <i>N</i> -Allyl-2-arylindoles with Phosphine Oxides Access to Mono/Diphosphonylated Indolo[2,1- <i>a</i> ] Isoquinolines

An efficient synthesis of phosphonylated indolo[2,1-<i>a</i>]isoquinolines has been developed via visible light-promoted reaction of <i>N</i>-allyl-2-arylindoles with diarylphosphine oxides at room temperature. The reaction afforded monophosphonylated indolo[2,1-<i>a</i>]isoquinolines when indoles had a substituent on the C-3 position. When C-3 unsubstituted <i>N</i>-allyl-2-arylindoles were employed, we obtained diphosphonylated indolo[2,1-<i>a</i>]isoquinolines exclusively. This reaction features advantages such as broad scope of substituents, mild and green conditions, as well as good yields. Moreover, this approach provides a green and the first light-promoted strategy for the synthesis of phosphonylated indolo[2,1-<i>a</i>]isoquinolines derivatives.