Visible light-photocatalysed carbazole synthesis via a formal (4+2) cycloaddition of indole-derived bromides and alkynes

Abstract

We successfully developed an unprecedented route to carbazole synthesis through a visible light-photocatalysed formal (4+2) cycloaddition of indole-derived bromides and alkynes. This novel protocol features extremely mild conditions, a broad substrate scope and high reaction efficiency.

Iodo-Cycloisomerization of Aryl(indol-3-yl)methane-Tethered Propargyl Alcohols to 3-Iodocarbazoles via Selective 1,2-Alkyl Migration

“…9 Few elegant approaches were reported for the synthesis of iodocarbazoles, 10 the benzannulation at C-2 and C-3 positions of indole from an alkyne-tethered indole substrate is one of the attractive and challenging methods due to the selective formation and step economy. 11 Few research groups have studied the reactivity of alkynes tethered to indoles at the C-3 position; 12 nevertheless, the synthesis of iodocarbazoles from these substrates is not well studied. Alcaide and co-workers have reported the synthesis of 3-iodocarbazoles from (3-iodoindol-2-yl)butynols via a gold-catalyzed intramolecular iodine transfer reaction (Figure 1, eq i).…”

Transition Metal‐Free Carbazole Synthesis from Arylureas and Cyclohexanones

“…The Itami and Lei groups found that a Pd‐Cu‐Ag trimetallic system could convert indoles to carbazoles using electron‐deficient alkenes as the two‐carbon units . The Xiao group developed an efficient approach for carbazole synthesis from indole derivatives and alkynes under visible light‐photocatalyzed conditions . Despite the high efficiency of transition metal catalysis, residual transition metal impurities could profoundly affect the biological properties or device performance of the final products.…”

Recent Developments in Photo‐Catalyzed/Promoted Synthesis of Indoles and Their Functionalization: Reactions and Mechanisms

“…Xiao and co‐workers reported an effective way to produce carbazoles by photocatalytic formal [4+2] cycloaddition of indole‐derived bromides and alkynes (Scheme 45). [72] This transformation features mild conditions, broad substrate scopes and high efficiency. In this case, the excited state photocatalyst [Ir (III) ] reduces bromide to radical intermediate 127 , which further undergoes radical addition reaction with alkyne to generate new radical intermediate 128 .…”

“…[71] Xiao and co-workers reported an effective way to produce carbazoles by photocatalytic formal [4 + 2] cycloaddition of indole-derived bromides and alkynes (Scheme 45). [72] This transformation features mild conditions, broad substrate scopes and high efficiency.…”