Tandem [2 + 2] Cycloaddition/Rearrangement toward Carbazoles by Visible-Light Photocatalysis

Reported herein is the first example of the synthesis of carbazoles via oxidative cyclization of 3-alkenylindoles with styrenes under visible light. The irradiation of a catalytic amount of [Ir(dtbbpy)(ppy)2][PF6] as the photocatalyst enables various 3-alkenylindoles and styrenes to undergo tandem [2 + 2] cycloaddition/rearrangement, thereby leading to carbazole derivatives in good to excellent yields under aerobic conditions. Mechanistic studies reveal that photoinduced energy transfer followed by electron transfer is responsible for the tandem reaction.