Photocatalytic Cyclization Cascades by Radical Relay toward Pyrrolo[1,2- a ]indoles: Synthesis, Mechanism, and Application.

A photocatalytic annulation cascade of unactivated N -alkene-linked indoles with Langlois' reagent by a radical relay is developed at room temperature under blue LED irradiation. The reaction afforded a series of tri/difluoromethylated pyrrolo[1,2- a ]indoles in moderate to good yields. The DFT study suggests that the reaction is ascribed to a rhodamine 6G-induced cyclization cascade involving vinyl addition-radical relay and hydrogen-atom-abstraction (HAA) processes, and interestingly, pyrrolo[1,2- a ]indoles are applied as fluorescent dyes into the fluorescence spectrum and live-cell imaging. This paper represents an initial example on photocatalytic cyclization cascades by radical relay and the HAA process.