Visible-Light-Driven Organic Photochemical Reactions in the Absence of External Photocatalysts

Abstract

Visible-light-driven organic photochemical reactions have attracted substantial attention from the synthetic community. Typically, catalytic quantities of photosensitizers, such as transition metal complexes, organic dyes, or inorganic semiconductors, are necessary to absorb visible light and trigger subsequent organic transformations. Recently, in contrast to these photocatalytic processes, a variety of photocatalyst-free organic photochemical transformations have been exploited for the efficient formation of carbon–carbon and carbon–heteroatom bonds. In addition to not requiring additional photocatalysts, they employ low-energy visible light irradiation, have mild reaction conditions, and enable broad substrate diversity and functional group tolerance. This review will focus on a summary of representative work in this field in terms of different photoexcitation modes.1 Introduction2 Visible Light Photoexcitation of a Single Substrate3 Visible Light Photoexcitation of Reaction Intermediates4 Visible Light Photoexcitation of EDA Complexes between Substrates5 Visible Light Photoexcitation of EDA Complexes between Substrates and Reaction Intermediates6 Visible Light Photoexcitation of Products7 Conclusion and Outlook

Synthetic reactions driven by electron-donor–acceptor (EDA) complexes

“…Until the past few years, EDA-complex photochemistry has attracted more and more chemists and provided new opportunities for synthetic chemistry [ 8 ]. Moreover, diverse photocatalyst-free photochemical reactions have been employed to construct carbon–carbon and carbon–heteroatom bonds [ 9 ]. Among these methods, the product formations by aid of EDA complexes are the simplest and most rapid way.…”

Enantioselective Radical Functionalization of Imines and Iminium Intermediates via Visible-Light Photoredox Catalysis

“…6b On the other hand, the ability of some iminium ion intermediates to directly participate in the photoexcitation of substrates upon light excitation can realize previously inaccessible photocatalytic reactions without an additional photocatalyst. 24 Meanwhile, the chiral amine catalyst ensures effective stereochemical control of these reactions. As a result, this iminium ion activation mode allows the asymmetric -functionalization of unsaturated carbonyl compounds, leading to chiral -functionalized carbonyl compounds.…”

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

“…were employed in photocatalytic oxidative or reductive transformations [8a,b] . On the other hand, photocatalyst‐free processes have also been exploited in which the reaction substrates can absorb light energy or generate an electron donor‐acceptor (EDA) complex [8c] to promote the chemical transformations in the absence of external photocatalysts [8d] . Although there are several related reviews were reported in the field of indoles synthesis including photocatalyzed reactions [9] .…”