Visible Light‐Initiated Catalyst‐Free One‐Pot, Multicomponent Construction of 5‐Substituted Indole Chromeno[2,3‐b]pyridines

Abstract

A visible light‐initiated pseudo four‐component reaction of salicylaldehydes, indole and malononitrile in aqueous ethyl lactate is described. This pot, atomic and step economic (PASE) methodology provides a practical approach for the preparation of various 5‐substituted indole chromeno[2,3‐b]pyridines from readily available starting materials at ambient temperature without any catalyst. High yields and excellent functional groups tolerance are observed.magnified image

Catalyst-Free Organic Transformations under Visible-Light

“…In 2019, the same research group applied visible-light to synthesize a number of 5-substituted indole chromeno[2,3b]pyridines 39.4 without a photocatalyst at room temperature (Scheme 39). 143 The same as the previous work, the best reaction conditions were obtained in EL/H 2 O under green light irradiation. As outlined in Scheme 39, under optimized conditions, a variety of salicylaldehydes 39.1 and indoles 39.3 participated in a one-pot reaction, affording the target adducts in good to excellent yields.…”

“…4 without a photocatalyst at room temperature (Scheme ). The same as the previous work, the best reaction conditions were obtained in EL/H 2 O under green light irradiation. As outlined in Scheme , under optimized conditions, a variety of salicylaldehydes 39 .…”

Copper-decorated covalent organic framework as a heterogeneous photocatalyst for phosphorylation of terminal alkynes

“…In the last few decades, significant advances have been made in visible-light-driven organic transformations under very mild conditions. 19 However, in most cases, photosensitive molecules such as noble metal complexes (such as iridium, ruthenium, rhodium, etc .) and organic dyes have been used as homogeneous photocatalysts for a variety of organic reactions.…”

Catalytic Approaches to Multicomponent Reactions: A Critical Review and Perspectives on the Roles of Catalysis

“…Isocyanide-based MCRs are sometimes performed in the absence of catalysts [ 62 ], but their catalytic versions proved to have many advantages. Some catalyst-free MCRs have been induced by alternative conditions, such as light-promoted reactions [ 63 ], microwave irradiation [ 64 ] and a few others [ 65 , 66 , 67 , 68 ], but catalysis effects rather than these conditions are of interest in the current article. Some MCRs have been labeled “catalyst-free”, but they have also been elegantly questioned [ 69 ], whereas other so-called “catalyst-free” MCRs could not be reproduced at all.…”