Asymmetric Photocatalysis Enabled by Chiral Organocatalysts

Abstract

Visible‐light photocatalysis has advanced as a versatile tool in organic synthesis. However, attaining precise stereocontrol in photocatalytic reactions has been a longstanding challenge due to undesired photochemical background reactions and the involvement of highly reactive radicals or radical ion intermediates generated under photocatalytic conditions. To address this problem and expand the synthetic utility of photocatalytic reactions, a number of innovative strategies, including mono‐ and dual‐catalytic approaches, have recently emerged. Of these, exploiting chiral organocatalysis, such as enamine catalysis, iminium‐ion catalysis, Brønsted acid/base catalysis, and N‐heterocyclic carbene catalysis, to induce chirality transfer of photocatalytic reactions has been widely explored. This Review aims to provide a current, comprehensive overview of asymmetric photocatalytic reactions enabled by chiral organocatalysts published through June 2021. The substrate scope, advantages, limitations, and proposed reaction mechanisms of each reaction are discussed. This review should serve as a reference for the development of visible‐light‐induced asymmetric photocatalysis and promote the improvement of the chemical reactivity and stereoselectivity of these reactions.

Asymmetric organocatalysis involving double activation

“…Besides, the double activation mechanism has not been fully elucidated in some cases because of the multiple interaction modes, thus detailed theoretical calculations might be helpful for better understanding the relevant reactions and inspiring new reaction designs. On the other hand, though photocatalysis and electrocatalysis are powerful tools for the generation of reactive radical intermediates, their integration with organocatalysis is still in infancy [103,118]. Besides, the combination of organocatalysis and biocatalysis remains underdeveloped [119], especially considering that multienzymatic systems have been widely witnessed in nature.…”

“…et al, have made great contributions by introducing NHCs, amines, Brønsted acids and other organocatalysts into photocatalysis. It should be noticed that cooperative catalysis or relay catalysis was involved in most examples, which have been well summarized in related reviews[103][104][105]. On the other hand, those based on a double activation strategy were much less disclosed.…”

Chiral Cyclopropenimine‐catalyzed Asymmetric Michael Addition of Bulky Glycine Imine to α,β‐Unsaturated Isoxazoles

“…The asymmetric Michael additions can be realized by various chiral metal-or organo-catalysts to provide Michael adducts in high yields and enantioselectivities. [1][2][3] With the Renaissance of organocatalysis, [4][5][6][7] many chiral organocatalysts have been discovered to inspire different activation modes in organic synthesis. Among them, chiral Brønsted acids, chiral hydrogenbonding catalysts, and chiral bifunctional amines are very attractive for their structural diversity and high efficiency in catalytic asymmetric synthesis.…”

Enantioselective Hydroalkylation of Alkenylpyridines Enabled by Merging Photoactive Electron Donor–Acceptor Complexes with Chiral Bifunctional Organocatalysis

“…In recent years, visible-light photocatalysis has emerged as a fascinating approach for unlocking novel transformations in a sustainable manner, which greatly enriched the synthetic toolbox of modern chemists. , Thus, multiple classes of organic- and metal-based photocatalysts featuring different photophysical and redox properties have been developed, allowing the activation of various substrates under mild conditions . In contrast, electron donor–acceptor (EDA) complexes, which are generated by a ground-state molecular association between electron-rich substrates and electron-poor substrates, can absorb visible light directly and trigger an intracomplex single-electron-transfer (SET) event to afford transient radical intermediates but in the absence of any external photocatalyst .…”