Facile synthesis of tetrahydrocarbazoles under photocatalyst-free conditions

Herein, we report a visible-light-induced, photocatalyst-free protocol for the preparation of tetrahydrocarbazoles <i>via</i> cascade decarboxylative cyclization of indolyl NHPI esters with alkenes. This approach features a broad substrate scope and eliminates the need for photocatalysts, distinguishing it from prior methods. Tetrahydrodibenzo[<i>b</i>,<i>d</i>]furan and tetrahydrodibenzo[<i>b</i>,<i>d</i>]thiophene could also be obtained under the standard conditions. Preliminary mechanistic studies indicate that an EDA complex formation from KI, PPh₃ and NHPI esters facilitates the reaction. The synthetic utility of this method was further demonstrated through post-transformation of the products.