Visible-Light-Driven Bisfunctionalization of Unactivated Olefins via the Merger of Proton-Coupled Electron Transfer and Carbene Catalysis

Photocatalytic Synthesis of Isoquinolinediones via the Cascade Reaction of N-Alkyl-N-methylacrylamides with Diazo Compounds

Carbon radicals generated from acceptor diazo compounds prefer to add to electron-rich olefins and heteroarenes due to their containing electron-withdrawing groups. Herein, a cascade reaction of acceptor diazo compounds with electron-deficient olefins is developed. In the presence of visible light and Ru catalyst, diazo compounds generate carbon radicals via proton-coupled electron transfer process, followed by addition with various N-alkyl-N-methylacrylamides and subsequent cyclization to give a series of acyl-substituted isoquinolinediones.

Cu(II)/Photoredox-Catalyzed Aminoacylation of Vinyl Bromides

A novel C-C coupling reaction of vinyl bromides with carboxamides has been developed through Cu(OAc)2/photocatalysis. This process features regioselective carboxamido carbon-vinylation under mild reaction conditions. Mechanistic studies suggest that the formation of an acyl-Cu(I) intermediate plays a crucial role with regard to enabling this transformation.

Nickel-Catalyzed Branched Hydroalkylation of Alkenes with Diazo Compounds

A nickel-catalyzed, branched-selective hydroalkylation of alkenes using diazo compounds has been developed. This protocol enables the functionalization of both activated and unactivated alkenes, in both directed and nondirected manners. Mono-, di-, and trisubstituted alkenes can be effectively transformed. Highly diastereoselective hydroalkylations have also been demonstrated. The method provides a novel approach for introducing an α-carbonyl moiety to alkenes, which is currently inaccessible by existing methods. Preliminary mechanistic investigations suggest a carbene-type mechanism, which is unusual for nickel catalysis.

N-Heterocyclic carbene (NHC) organocatalysis: from fundamentals to frontiers

N-Heterocyclic carbenes (NHCs) have been used as organocatalysts for a multitude of C-C and C-heteroatom bond-forming reactions. They enable diverse modalities of activating a wide range of structurally distinct substrate classes and allow access to electronically distinct intermediates. The easy tunability of the NHC scaffold contributes to its versatility. Recent years have witnessed a surge of interest in various organocatalytic reactions of NHCs, leading to the forays of NHC catalysis into the relatively newer domains such as reactions involving radical intermediates, atroposelective synthesis, umpolung of electrophiles other than aldehydes, and the use of NHCs as non-covalent templates for enantioinduction. This tutorial review provides an overview of various important structural features and reactivity modes of NHCs and delves deep into some frontiers of NHC-organocatalysis.

Synergistic photocatalysis enables aerobic oxo-hydrazination of α-diazoacetates with azobenzenes

A photocatalytic oxo-hydrazination of α-diazoacetates with azobenzenes has been developed. With air as an oxygen source, the reaction proceeded smoothly and afforded previously unknown N,N'-diarylhydrazino-containing oxoacetates. Mechanistically, the reaction is enabled by cooperation of photoredox catalysis, energy transfer photocatalysis and direct photoexcitation.

Tunable Thiazolium Carbenes for Enantioselective Radical Three-Component Dicarbofunctionalizations

Asymmetric N-heterocyclic carbene (NHC) organocatalysis is a cornerstone of synthetic organic chemistry. The emerging concept of single-electron NHC catalysis broadened the scope of C-C bond-forming reactions, facilitating the synthesis of a variety of attractive racemic compounds. However, the development of effective and selective chiral NHC catalysts for asymmetric radical-mediated reactions has been challenging. In this report, we introduce a family of highly tunable chiral thiazolium carbenes with three distinct positions for broad electronic and steric modulation featuring bulky chiral flanking groups. We demonstrate the catalytic efficacy of these chiral carbenes in an enantioselective SET-type three-component acyl-difluoroalkylation of olefins using a broad range of aldehydes and difluoroalkyl bromides. This method provides straightforward access to a diverse set of β-difluoroalkylated α-chiral ketones (65 examples) with an up to 87% yield and excellent enantioselectivities of up to >99:1 er. The utility of this methodology is further outlined by enantio- and diastereoselective late-stage modifications of pharmaceutically relevant compounds and selective twofold orthogonal acyl-difluoroalkylations of linchpin reagents.

Visible Light-Induced Reactions of Diazo Compounds and Their Precursors

In recent years, visible light-induced reactions of diazo compounds have attracted increasing attention in organic synthesis, leading to improvement of existing reactions, as well as to the discovery of unprecedented transformations. Thus, photochemical or photocatalytic generation of both carbenes and radicals provide milder tools toward these key intermediates for many valuable transformations. However, the vast majority of the transformations represent new reactivity modes of diazo compounds, which are achieved by the photochemical decomposition of diazo compounds and photoredox catalysis. In particular, the use of a redox-active photocatalysts opens the avenue to a plethora of radical reactions. The application of these methods to diazo compounds led to discovery of transformations inaccessible by the classical reactivity associated with carbenes and metal carbenes. In most cases, diazo compounds act as radical sources but can also serve as radical acceptors. Importantly, the described processes operate under mild, practical conditions. This Review describes this subfield of diazo compound chemistry, particularly focusing on recent advancements.

Palladium-catalyzed three-component radical-polar crossover carboamination of 1,3-dienes or allenes with diazo esters and amines

Herein, we report a visible-light-mediated palladium-catalyzed three-component radical-polar crossover carboamination of 1,3-dienes or allenes with diazo esters and amines, affording unsaturated γ- and ε-amino acid derivatives with diverse structures. In this methodology, the diazo compound readily transforms into a hybrid α-ester alkylpalladium radical with the release of dinitrogen. The radical intermediate selectively adds to the double bond of a 1,3-diene or allene, followed by the allylpalladium radical-polar crossover path and selective allylic substitution with the amine substrate, thereby leading to a single unsaturated γ- or ε-amino acid derivative. This approach proceeds under mild and simple reaction conditions and shows high functional group tolerance, especially in the incorporation of various bioactive molecules. The studies on scale-up reactions and diverse derivatizations highlight the practical utility of this multicomponent reaction protocol.

Photocatalytic Synthesis of Indanone, Pyrone, and Pyridinone Derivatives with Diazo Compounds as Radical Precursors

We disclose herein a photocatalytic radical cascade cyclization of diazoalkanes for the divergent synthesis of important carbocycles and heterocycles. Under the optimal reaction conditions, various indanone, pyrone, and pyridinone derivatives can be obtained in moderate to good yields. Mechanistic experiments support the formation of carbon-centered radicals from diazoalkanes through the proton-coupled electron transfer process. Scale-up reaction using continuous flow technology and useful downstream application of the formed heterocycles further render the strategy attractive and valuable.

Enantioselective [3 + 3] Annulation-Deoxalation Strategy for Rapid Access to δ-Oxoesters via N-Heterocyclic Carbene Catalysis

A new and unprecedented stereoselective synthetic approach to δ-oxoesters derivatives from readily available starting materials has been developed. This method, catalyzed by N-heterocyclic carbene, involves an annulation-deoxalation reaction of alkynyl aldehydes with 2,4-diketoesters and proceeds via the chiral α,β-unsaturated acylazolium intermediates. The annulation includes the in situ formation of dihydropyranones, which undergo ring-opening methanolysis with Lewis acid activation, followed by deoxalation to afford chiral 1,5-ketoesters in moderate to good yields.

Atom Transfer Radical Addition Reactions of Quinoxalin-2(1H)-ones with CBr4 and Styrenes Using Mes-Acr-MeClO4 Photocatalyst

The atom transfer radical addition (ATRA) reaction is defined as a method for introducing halogenated compounds into alkenes via a radical mechanism. In this study, we present an ATRA approach for achieving regioselective functionalization of quinoxalin-2(1H)-ones by activating C-Br bonds of CBr4 and subsequent trihaloalkyl-carbofunctionalization of styrenes employing the 9-mesityl-10-methylacridinium perchlorate (Fukuzumi) photocatalyst under 3W blue LED (450-470 nm) irradiation. This three-component radical cascade process demonstrates remarkable efficiency in the synthesis of 1-methyl-3-(3,3,3-tribromo-1-(4-chlorophenyl)propyl)quinoxalin-2(1H)-one derivatives.

Photoinduced cerium-catalyzed C-H acylation of unactivated alkanes

Ketones are ubiquitous motifs in the realm of pharmaceuticals and natural products. Traditional approaches to accessing these species involve the addition of metal reagents to carboxyl compounds under harsh conditions. Herein, we report a cerium-catalyzed acylation of unactivated C(sp3)-H bonds using bench-stable acyl azolium reagents under mild and operationally-friendly conditions. This reaction exhibits excellent generality, accommodating a wide range of feedstock chemicals such as cycloalkanes and acyclic compounds as well as facilitating the late-stage functionalization of pharmaceuticals. We demonstrate further applications of our strategy with a three-component radical relay reaction and an enantioselective N-heterocyclic carbene (NHC) and cerium dual-catalyzed reaction.

Recent advances in combining photo- and N-heterocyclic carbene catalysis

N-Heterocyclic carbenes (NHCs) are unique Lewis basic catalysts that mediate various organic transformations by means of polarity reversal. Although the scope of research on two-electron reactions mediated by NHC catalysts has been expanding, the types of these reactions are limited by the inability of NHCs to engage sp3-electrophiles. However, the revival of photocatalysis has accelerated the development of free-radical chemistry, and combining photoredox catalysis and NHC catalysis to achieve NHC-mediated radical reactions under mild conditions could overcome the above-mentioned limitation. This review summarizes recent advances in combining photoredox and NHC catalysis, focusing on elucidation and exploration of mechanisms, with the aim of identifying challenges and opportunities to develop more types of catalytic models.

On the Redox Properties of the Dimers of Thiazol-2-ylidenes That Are Relevant for Radical Catalysis

We report the isolation and study of dimers stemming from popular thiazol-2-ylidene organocatalysts. The model featuring 2,6-di(isopropyl)phenyl (Dipp) N-substituents was found to be a stronger reducing agent (E_ox = -0.8 V vs SCE) than bis(thiazol-2-ylidenes) previously studied in the literature. In addition, a remarkable potential gap between the first and second oxidation of the dimer also allows for the isolation of the corresponding air-persistent radical cation. The latter is an unexpected efficient promoter of the radical transformation of α-bromoamides into oxindoles.

Photocatalytic Three-Component Synthesis of 3-Heteroarylbicyclo[1.1.1]pentane-1-acetates

Herein, we report a visible-light-induced three-component reaction involving [1.1.1]propellane, diazoates, and various heterocycles for the synthesis of 3-heteroarylbicyclo[1.1.1]pentane-1-acetates. Throughout this reaction, the radicals generated from diazoate species react with [1.1.1]propellane in an addition reaction to form bicyclo[1.1.1]pentane (BCP) radicals that subsequently react with heterocycles, leading to the formation of 1,3-disubstituted BCP acetates. Notably, this methodology exhibits excellent functional group compatibility, high atom economy, and mild reaction conditions, thus facilitating suitable synthetic access to 1,3-disubstituted BCP acetates.

Visible-Light-Induced Radical gem-Iodoallylation of 2,2,2-Trifluorodiazoethane

A visible-light-induced radical gem-iodoallylation of CF₃CHN₂ was developed under mild conditions, delivering a variety of α-CF₃-substituted homoallylic iodide compounds in moderate to excellent yields. The transformation features broad substrate scope, good functional group compatibility, and operational simplicity. The described protocol provides a convenient and attractive tool to apply CF₃CHN₂ as CF₃-introduction reagent in radical synthetic chemistry.

Synthesis of Thioesters from Aldehydes via N-Heterocyclic Carbene (NHC) Catalyzed Radical Relay

We have developed an efficient N-heterocyclic carbene (NHC)-catalyzed thioesterification of aldehydes using N-thiosuccinimides as the thiolation reagent. This organocatalyzed transition involves the generation of sulfur radicals by single electron transfer of the Breslow enolate (generated from aldehyde and NHC catalyst) with N-thiosuccinimides. This method offers facile access to various highly functionalized thioesters and exhibits good chemical yields and functional group tolerance.

Iminoacylation of Alkenes via Photoredox N-Heterocyclic Carbene Catalysis

The iminoacylation of alkenes via photoredox N-heterocyclic carbene catalysis is developed with the employment of alkene-tethered α-imino-oxy acids and acyl imidazoles. The corresponding substituted 3,4-dihydro-2H-pyrroles were afforded in moderate to good yields with good to high diastereoselectivities in most cases. The reaction involves the 5-exo-trig radical cyclization of an alkene-tethered iminyl radical and the following coupling with a ketyl radical from acyl imidazole under NHC catalysis.

Visible light-mediated NHC and photoredox co-catalyzed 1,2-sulfonylacylation of allenes via acyl and allyl radical cross-coupling

Visible light-mediated NHC and photoredox co-catalyzed radical 1,2-sulfonylacylation of allenes via cross-coupling between an allyl radical and an NHC-stabilized acyl radical.

Photocatalytic Regioselective Difunctionalization of Alkenes with Diazo Compounds and tert-Butyl Nitrite: Access to γ-Oximino Esters

A visible-light photocatalytic regioselective difunctionalization of alkenes with diazo compounds and tert-butyl nitrite has been developed. The protocol provides an efficient approach to γ-oximino esters under mild conditions. Significantly, this transformation not only shows the good compatibility of nucleophilic diazo compounds and electrophilic tert-butyl nitrite but also displays diazo compounds generating alkyl radicals that preferred addition to alkenes over nitroso radicals.

Photoredox-Catalyzed Carbonyl Alkylative Amination with Diazo Compounds: A Three-Component Reaction for the Construction of γ-Amino Acid Derivatives

A photoredox-catalyzed reaction of secondary amines, aldehydes, diazo compounds, and Hantzsch ester is reported, affording biologically active γ-amino acid derivatives in high yields. This one-pot process tolerates a broad range of functional groups and various drug molecules and biologically active compounds. Remarkably, a gram-scale reaction and diverse transformations of γ-amino acid derivatives were successfully performed, and the utility of the products is demonstrated in the synthesis of therapeutic agent pregabalin.

Research Progress on N-Heterocyclic Carbene Catalyzed Reactions for Synthesizing Ketones through Radical Mechanism

NHC-catalyzed radical cross-coupling reactions have been recently developed; they provide an efficient method to access ketones from aldehydes or carboxylic acid derivatives with sp3-hybridized carbon radical precursors. This reaction has indirectly solved the limitations in the scope of coupling partners in NHC umpolung catalyzed reactions of aldehydes. In this short review, we present some recent advances in NHC-catalyzed radical reactions, with a focus on the construction of the C–C(CO) bond.

1 Introduction

2 Oxidative Generation of NHC-Derived Ketyl Radical

2.1 NHPI Redox-Active Esters

2.2 Katritzky Pyridinium Salts

2.3 Alkyl Halides

2.4 Aryl Halides

2.5 Compounds Containing N–O Bond

2.6 Diazo Esters

2.7 Others

3 Reductive Generation of NHC-Derived Ketyl Radical

3.1 Hantzsch Esters

3.2 Sulfinates

3.3 Electron-Rich Arenes

3.4 Amines

3.5 Organoborane Reagents

4 Conclusion