Catalytic asymmetric synthesis of 5-membered alicyclic α-quaternary β-amino acids via [3 + 2]-photocycloaddition of α-substituted acrylates

Unlocking Photocatalytic Activity of Acridinium Salts by Anion-Binding Co-Catalysis

The in situ generation of active photoredox organic catalysts upon anion-binding co-catalysis by making use of the ionic nature of common photosensitizers is reported. Hence, the merge of anion-binding and photocatalysis permitted the modulation of the photocatalytic activity of simple acridinium halide salts, building an effective anion-binding - photoredox ion pair complex able to promote a variety of visible light driven transformations, such as anti-Markovnikov addition to olefins, Diels-Alder and the desilylative C-C bond forming reactions. Anion-binding studies, together with steady-state and time-resolved spectroscopy analysis, supported the postulated ion pair formation between the thiourea hydrogen-bond donor organocatalyst and the acridinium salt, which proved essential for unlocking the photocatalytic activity of the photosensitizer.

Synthesis of 3-aminotetrahydro-1H-carbazols by visible-light photocatalyzed cycloaddition of cyclopropylanilines with 2-alkenylarylisocyanides

A visible-light-induced cycloaddition between 2-alkenylarylisocyanides and cyclopropylanilines is reported. This cascade radical reaction constructs two new C-C bonds and two rings to afford 3-aminotetrahydro-1H-carbazols with high atom and step economy. The mechanism is rationalized as involving sequential distonic radical cation formation/isocyanide insertion/5-exo-trig cyclization/intramolecular iminium ion addition/tautomerization.

Visible Light Promoted [3+2]-Cycloaddition for the Synthesis of Cyclopenta[b]chromenocarbonitrile Derivatives

In the manuscript, a novel method for the preparation of cyclopenta[b]chromenocarbonitrile derivatives via [3+2] cycloaddition reaction of substituted 3-cyanochromones and N-cyclopropyloamines initiated by visible light catalysis has been described. The reaction was performed in the presence of Eosin Y as a photocatalyst. The key parameters responsible for the success of the described strategy are visible light, a small amount of photoredox catalyst, an anhydrous solvent, and an inert atmosphere.

Diboron(4)-Catalyzed Remote [3+2] Cycloaddition of Cyclopropanes via Dearomative/Rearomative Radical Transmission through Pyridine

Ring structures such as pyridine, cyclopentane or their combinations are important motifs in bioactive molecules. In contrast to previous cycloaddition reactions that necessitated a directly bonded initiating functional group, this work demonstrated a novel through-(hetero)arene radical transmission concept for selective activation of a remote bond. An efficient, metal-free and atom-economical [3+2] cycloaddition between 4-pyridinyl cyclopropanes and alkenes or alkynes has been developed for modular synthesis of pyridine-substituted cyclopentanes, cyclopentenes and bicyclo[2.1.1]hexanes that are difficult to access using known methods. This complexity-building reaction was catalyzed by a very simple and inexpensive diboron(4) compound and took place via dearomative/rearomative processes. The substrate scope was broad and more than 100 new compounds were prepared in generally high yields. Mechanistic experiments and density function theory (DFT) investigation supported a radical relay catalytic cycle involving alkylidene dihydropyridine radical intermediates and boronyl radical transfer.

Visible-Light Organophotoredox-Mediated [3 + 2] Cycloaddition of Arylcyclopropylamine with Structurally Diverse Olefins for the Construction of Cyclopentylamines and Spiro[4.n] Skeletons

We developed a visible-light-mediated [3 + 2] cycloaddition of arylcyclopropylamine with structurally diverse olefins using QXPT-NPh as a highly efficient organic photoredox catalyst. We first achieved the use of various alkyl-substituted alkenes in intermolecular [3 + 2] cycloadditions with cyclopropylamine. We also developed a general and efficient strategy for the construction of structurally diverse cyclopentane-based spiro[4.n] skeletons with 1,3-difunctional groups, which broadly exist in natural products and synthetic molecules. Furthermore, we proposed a hydrogen-bond mode between the arylcyclopropylamine and the photocatalyst QXPT-NPh.

Ion-Pairing Catalysis in Stereoselective, Light-Induced Transformations

With the rapid development of photoredox catalysis, numerous concepts for asymmetric induction were successfully and broadly adapted from polar two-electron transformations to radical chemistry. While this applies to organocatalysis or transition metal chemistry, asymmetric ion-pairing catalysis remains a niche application within light-driven reactions today. This perspective gives an overview of recent examples, strategies, and their application in stereoselective transformations at the interface of ion-pairing and photo(redox) catalysis.

Activation of aminocyclopropanes via radical intermediates

Aminocyclopropanes are versatile building blocks for accessing high value-added nitrogen-containing products. To control ring-opening promoted by ring strain, the Lewis acid activation of donor-acceptor substituted systems is now well established. Over the last decade, alternative approaches have emerged proceeding via the formation of radical intermediates, alleviating the need for double activation of the cyclopropanes. This tutorial review summarizes key concepts and recent progress in ring-opening transformations of aminocyclopropanes via radical intermediates, divided into formal cycloadditions and 1,3-difunctionalizations.

Stereocontrolled Pericyclic and Radical Cycloaddition Reactions of Readily Accessible Chiral Alkenyl Diazaborolidines

In this paper is described an easily synthesized chiral diazaborolidine that is inexpensive, stable, and provides excellent stereoselection across a number of reaction classes. These versatile compounds possess utility in four different classes of cycloaddition reactions, offering good yield and stereoselectivity. X-ray structure analysis provides insight about the origin of stereocontrol.

Asymmetric [3 + 2] photocycloadditions of cyclopropylamines with electron-rich and electron-neutral olefins

Radical addition to olefins is a common and useful chemical transformation. In the context of offering enantioenriched three-dimensional molecules via such a highly reactive process, chiral hydrogen-bonding (H-bonding) catalysis has been widely used to provide enantiocontrol. The current strategies for operating H-bonding induction are confined to following that are prevalent in ionic-type manifolds. Here, we report a novel protocol towards electron-rich olefins based on converting these species from acting as H-bonding donors to acceptors. It facilitates the first development of asymmetric [3 + 2] photocycloadditions with cyclopropylamines. The method is also effective for electron-neutral olefins, in which the successful construction of all-carbon quaternary stereocentres from 1,1-diaryl ethylenes that feature two structurally similar aryl substituents demonstrates the versatility of this new chiral H-bonding catalytic strategy. Furthermore, the importance of the obtained six kinds of products in pharmaceuticals and asymmetric catalysis underscores the practicability of this work.

Radical addition to olefins is a common and useful chemical transformation.

Cooperative Stereoinduction in Asymmetric Photocatalysis

Stereoinduction in complex organic reactions often involves the influence of multiple stereocontrol elements. The interaction among these can often result in the observation of significant cooperative effects that afford different rates and selectivities between the matched and mismatched sets of stereodifferentiating chiral elements. The elucidation of matched/mismatched effects in ground-state chemical reactions was a critically important theme in the maturation of modern stereocontrolled synthesis. The development of robust methods for the control of photochemical reactions, however, is a relatively recent development, and similar cooperative stereocontrolling effects in excited-state enantioselective photoreactions have not previously been documented. Herein, we describe a tandem chiral photocatalyst/Brønsted acid strategy for highly enantioselective [2 + 2] photocycloadditions of vinylpyridines. Importantly, the matched and mismatched chiral catalyst pairs exhibit different reaction rates and enantioselectivities across a range of coupling partners. We observe no evidence of ground-state interactions between the catalysts and conclude that these effects arise from their cooperative behavior in a transient excited-state assembly. These results suggest that similar matched/mismatched effects might be important in other classes of enantioselective dual-catalytic photochemical reactions.

Asymmetric synthesis of cyclic β-amino carbonyl derivatives by a formal [3 + 2] photocycloaddition

Herein, a visible-light mediated strategy unlocking a family of cyclic β-amino carbonyl derivatives bearing three contiguous stereogenic centres is introduced. The overall reactivity relies on the performance of the substrate-catalyst complex to assist both the enantiocontrol and the photoredox tasks. This transformation led to an enantioselective [3 + 2] photocycloaddition between coordinated α,β-unsaturated acyl imidazoles and cyclopropylamine derivatives.

Chiral Photocatalyst Structures in Asymmetric Photochemical Synthesis

Asymmetric catalysis is a major theme of research in contemporary synthetic organic chemistry. The discovery of general strategies for highly enantioselective photochemical reactions, however, has been a relatively recent development, and the variety of photoreactions that can be conducted in a stereocontrolled manner is consequently somewhat limited. Asymmetric photocatalysis is complicated by the short lifetimes and high reactivities characteristic of photogenerated reactive intermediates; the design of catalyst architectures that can provide effective enantiodifferentiating environments for these intermediates while minimizing the participation of uncontrolled racemic background processes has proven to be a key challenge for progress in this field. This review provides a summary of the chiral catalyst structures that have been studied for solution-phase asymmetric photochemistry, including chiral organic sensitizers, inorganic chromophores, and soluble macromolecules. While some of these photocatalysts are derived from privileged catalyst structures that are effective for both ground-state and photochemical transformations, others are structural designs unique to photocatalysis and offer insight into the logic required for highly effective stereocontrolled photocatalysis.

Asymmetric Photocatalysis Enabled by Chiral Organocatalysts

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.

Construction of Chiral Cyclic Compounds Enabled by Enantioselective Photocatalysis

Chiral cyclic molecules are some of the most important compounds in nature, and are widely used in the fields of drugs, materials, synthesis, etc. Enantioselective photocatalysis has become a powerful tool for organic synthesis of chiral cyclic molecules. Herein, this review summarized the research progress in the synthesis of chiral cyclic compounds by photocatalytic cycloaddition reaction in the past 5 years, and expounded the reaction conditions, characters, and corresponding proposed mechanism, hoping to guide and promote the development of this field.

Gold-Catalyzed Hydroarylation Reactions: A Comprehensive Overview

The hydroarylation of alkynes, alkene, and, allene is a cost-effective and efficient way to incorporate unsaturated moieties into aromatic substrates. This review focuses on gold-catalyzed hydroarylation, which produces aromatic alkenes,...

Syntheses of new chiral chimeric photo-organocatalysts

A new family of chiral chimeric photo-organocatalysts derived from phosphoric acid were synthesized and their spectroscopic and electrochemical properties were investigated. Then, the ability of these photo-activable molecules to catalyse an asymmetric tandem electrophilic β-amination of enecarbamates was evaluated.