Visible-Light-Mediated Rose Bengal- or [Ru(bpy)₃]²⁺-Catalyzed Radical [4 + 2] Cycloaddition: An Efficient Route to Tetrahydrocarbazoles

A visible-light-induced Rose Bengal- or [Ru(bpy)₃]²⁺-catalyzed radical [4 + 2] cycloaddition of redox-active indole <i>N</i>-hydroxyphthalimide esters with electron-deficient alkenes has been developed. This base-free protocol provides a facile and powerful route for the synthesis of functionalized and biologically significant tetrahydrocarbazoles under mild conditions. On one hand, when an organic photocatalyst-Rose Bengal was employed under green light, the desired tetrahydrocarbazoles were obtained in up to 82% yield. On the other hand, the reaction yield increased to up to 93% in the presence of [Ru(bpy)₃Cl₂]·6H₂O under blue light. The success of the gram-scale and transformation experiments, as well as the photopromoted radical [5 + 2] cycloaddition further highlight the practicality and robustness of this protocol. Mechanistic studies also support the generation of a crucial alkyl radical intermediate.