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

Abstract

The use of clean and renewable light sources is increasingly common in organic synthesis due to its safety, practicality and economy. Recently, photoredox catalysis has shown great application values in organic transformations because of its advantages of environmentally friendly and abundant resources. Indoles and their derivatives (indolines, oxindoles and isatins) are the core skeletons of some important organic compounds and widely‐present in various natural products and pharmaceuticals with different biological activities. Therefore, the research on the synthesis and modification of indoles is particularly important for chemists and pharmacologists. This review summarizes the effects of photocatalysis on indole synthesis and modification in recent decades. These transformations are accomplished by using metal photocatalysts (i. e., Ir, Ru, Ni, Cu, Fe, Au, Rh, TiO2, etc.) or non‐metallic photocatalysts (i. e., Rose Bengal, Eosin Y, quinones, naphthols, N‐heterocyclic carbenes, carbazoles, pyrylium salts, etc.), or without the need of photocatalysts. The detailed mechanisms of these photo‐catalyzed/promoted organic reactions are also highlighted deeply. And we hope this review will be helpful to researchers interested in this promising field of photocatalyzed transformations.

Progress in Photoinduced Radical Reactions using Electron Donor‐Acceptor Complexes

“…Although the contributions of photocatalyzed synthesis of indolines and 2,3‐dihydrobenzofurans, these approaches were performed using Ir and Ru based complexes as photocatalysts [16,165] . In this context, the advantage of the EDA complex is considered to be an alternative for the synthesis of heterocyclic compounds [35–37] .…”

“…Although the contributions of photocatalyzed synthesis of indolines and 2,3-dihydrobenzofurans, these approaches were performed using Ir and Ru based complexes as photocatalysts. [16,165] In this context, the advantage of the EDA complex is considered to be an alternative for the synthesis of heterocyclic compounds. [35][36][37] Paixão et al developed a visible light-induced intramolecular radical cyclization strategy for the synthesis of highly substituted indolines and 2,3-dihydrobenzofurans under mild reaction conditions in good to excellent yields (Scheme 52).…”

“…Advances for the photocatalyzed synthesis of indoles attract continuing attention [16] . However, some achievements require the use of Ir and Ru photocatalysts, eliminating the metallic residues is an important challenge.…”

“…In this case, some colored Ru‐ or Ir‐complexes or organic dyes were used as photocatalysts [11–15] . These photochemical transformations were successfully accomplished via an oxidative quenching cycle or a reductive quenching cycle [16–20] . On the other hand, if the ground state substrates contain an electron donor (D) and an acceptor (A), which can form an electron donor‐acceptor (EDA) complex via the diffusion event [21,22] .…”

“…[11][12][13][14][15] These photochemical transformations were successfully accomplished via an oxidative quenching cycle or a reductive quenching cycle. [16][17][18][19][20] On the other hand, if the ground state substrates contain an electron donor (D) and an acceptor (A), which can form an electron donor-acceptor (EDA) complex via the diffusion event. [21,22] The EDA complex can harvest the energy of light.…”