Selective Radical-Radical Cross-Couplings: Design of a Formal β-Mannich Reaction

Photoredox catalyzed reductive trifluoromethylation of imines via a radical umpolung strategy

Visible light-induced radical umpolung chemistry is utilized to synthesize trifluoromethylated unnatural α-amino acid and amine derivatives. This approach utilizes photoredox catalysis to perform a single-electron-transfer reduction of imines generating a N-centred radical that eventually migrates to the C-centre followed by a radical-radical cross-coupling to deliver reductive trifluoromethylation products.

Ligand-Enabled Nickel(II)-Catalyzed β-C(sp3)-H Thiolation of Ketones

We present the first example of nickel(II)-catalyzed β-C(sp3)-H thiolation of ketones, employing 2-hydrazinopyridine as an efficient directing group. This approach enables the thiolation of a diverse array of ketones at the β-position. The straightforward installation and subsequent removal of the directing group significantly enhance the synthetic versatility and practicality of this transformation.

Modular Synthesis of Heterobenzylic Amines via Carbonyl Azinylative Amination

Transformations enabling the synthesis of α-alkyl, α'-2-azinyl amines by addition of 2-heteroaryl-based nucleophiles to in situ-generated and non-activated alkyl-substituted iminium ions are extremely rare. Approaches involving classical 2-azinyl organometallics, such as the corresponding Grignard reagents, often fail to produce the desired products. Here, we report an operationally straightforward solution to this problem through the development of a multicomponent coupling process wherein a soft 2-azinyl indium nucleophile, generated in situ from the corresponding 2-iodo heteroarene and indium powder, adds to an iminium ion that is also formed directly in the reaction. This modular carbonyl azinylative amination (CAzA) displays a broad scope and only a metal reductant is needed to generate a reactive 2-azinyl nucleophile. Beyond the addition to iminium ions, the 2-azinyl addition to polyfluoromethyl ketones forms the corresponding tertiary alcohols. Together, the products of these reactions possess a high degree of functionality, are typically challenging to synthesize by other methods, and contain motifs recognized as privileged in the context of pharmaceuticals and agrochemicals.

Emerging Opportunities of Colloidal Quantum Dots for Photocatalytic Organic Transformations

Colloidal quantum dots (QDs) have emerged as a versatile photocatalyst for a wide range of photocatalytic transformations owing to its high absorption coefficient, large surface-to-volume ratio, high stability, and efficient charge and energy transfer dynamics. The past decades have witnessed a rapid development of QDs for artificial photocatalysis. In this review, the unique characteristics of QDs are focused on, including quantum size effect, compositional and structural diversity, tunable surface chemistry, and photophysics, that can be utilized for photocatalytic transformations. The recent advancements in photocatalytic organic transformations enabled by QDs photocatalysts are summarized. The unique opportunities of QDs are highlighted to tackle organic reactions that are previously unattainable with small molecule photocatalysts. Lastly, an outlook is provided for future directions in this field.

Silyl Radicals as Single-Electron Reductants: α-Aminoalkyl Radical Formation via a Photocatalytic Oxidatively Initiated Radical Chain Process

The α-amino-radical constitutes a versatile reactive intermediate that has been used to great effect in the synthesis of complex amine-containing products. Here, we report the development of a multicomponent photocatalytic platform enabling access to all-alkyl α-amino-radicals, exploiting the oxidative formation of silyl-radicals from commercially available tris(trimethylsilyl)silane. A key design element of the new process involves the role of silyl-radicals in generating α-amino-radicals from iminium ions as part of an oxidatively initiated photocatalytic radical chain process. This distinct activation mode is showcased by engaging the ensuing radicals in cross-radical coupling with persistent arene radical anions, enabling the arylation of in situ-generated all-alkyl iminium ions to furnish alkyl-substituted benzylamines.

Visible light-induced chemoselective 1,2-diheteroarylation of alkenes

Visible-light photocatalysis has evolved as a powerful technique to enable controllable radical reactions. Exploring unique photocatalytic mode for obtaining new chemoselectivity and product diversity is of great significance. Herein, we present a photo-induced chemoselective 1,2-diheteroarylation of unactivated alkenes utilizing halopyridines and quinolines. The ring-fused azaarenes serve as not only substrate, but also potential precursors for halogen-atom abstraction for pyridyl radical generation in this photocatalysis. As a complement to metal catalysis, this photo-induced radical process with mild and redox neutral conditions assembles two different heteroaryl groups into alkenes regioselectively and contribute to broad substrates scope. The obtained products containing aza-arene units permit various further diversifications, demonstrating the synthetic utility of this protocol. We anticipate that this protocol will trigger the further advancement of photo-induced alkyl/aryl halides activation.

Remote C-H Amination and Alkylation of Camphor at C8 through Hydrogen-Atom Abstraction

Camphor continues to serve as a versatile chiral building block for chemical synthesis. We have developed a novel method to functionalize the camphor skeleton at C8 using an intramolecular hydrogen atom abstraction. The key advance involves the use of a camphor-derived aminonitrile, which is converted to the corresponding nitrogen-centered radical under photoredox conditions to effect the 1,5-hydrogen atom transfer at C8. The resulting carbon-centered radical at C8 was utilized in a C-H amination to access topologically complex proline derivatives. Furthermore, the total synthesis of several sesquiterpenoids was accomplished by engaging the radical generated at C8 in alkylation reactions.

Unveiling Mechanistic Insights and Photocatalytic Advancements in Intramolecular Photo-(3 + 2)-Cycloaddition: A Comparative Assessment of Two Paradigmatic Single-Electron-Transfer Models

The synthesis of 1-aminonorbornane (1-aminoNB), a potential aniline bioisostere, through photochemistry or photoredox catalysis signifies a remarkable breakthrough with implications in organic chemistry, pharmaceutical chemistry, and sustainable chemistry. However, an understanding of the underlying mechanisms involved in these reactions remains limited and ambiguous. Herein, we employ high-precision CASPT2//CASSCF calculations to elucidate the intricate mechanisms regulating the intramolecular photo-(3 + 2)-cycloaddition reactions for the synthesis of 1-aminoNB in the presence or absence of the Ir-complex-based photocatalyst. Our investigations delve into radical cascades, stereoselectivity, particularly single-electron-transfer (SET) events, etc. Furthermore, we innovatively introduce and compare two SET models integrating Marcus electron-transfer theory and transition-state theory. These models combined with kinetic data contribute to recognizing the critical control factors in diverse photocatalysis, thereby guiding the design and manipulation of photoredox catalysis as well as the improvement and modification of photocatalysts.

Coligands Controlled Reactivities of Ruthenium(II) Precursors: Antiferromagnetically Coupled Ruthenium(III)-Phenoxyl versus Ruthenium(II)-Phenoxyl Forms

The study disclosed that the reactivities of [RuII (PPh3)3Cl2] and [RuII(PPh3)3(CO)(H)Cl] precursors toward a trimethoxyarylimino-phenol derivative are sensibly different. The former promotes methoxy demethylation reaction affording a [Phenolato-RuIII-Phenolato] unit, while the latter containing π-acidic CO and hydride as coligands leads to C-H activation reaction, generating a [Phenolato-RuII-Aryl] unit. Notably, the oxidized analogues of these two forms produce antiferromagnetically coupled [RuIII-phenoxyl] and paramagnetic [RuII-phenoxyl] forms, which exhibit diverse reactivities. Surprisingly, the magnetically coupled [RuIII-phenoxyl] form obtained from [Phenolato-RuIII-Phenolato] motif leads to coligand, PPh3 oxidation and undergoes dimerization, making a Ru-Ru bond (2.599(2) Å), while the [RuII-phenoxyl] form obtained from [Phenolato-RuII-Aryl] motif leads to C-C coupling and H abstraction reactions. The coupling reaction affords a 4,4'-dibenzosemiquinonate anion radical complex, but the H-abstraction of the phenoxyl form gives a [RuII-Phenol] complex. For comparison, [RuII(IQR 0)] and [RuII(ISQR·-)] complexes were also isolated, where IQR 0 and ISQR·- are p-R-o-iminobenzoquinone and p-R-o-iminobenzosemiquinonate anion radicals. However, they fail to promote any bond-formation reaction. The molecular and electronic structures of the ruthenium (II/III) complexes were confirmed by single-crystal X-ray crystallography, EPR spectroscopy, and DFT calculations.

Coordination-Induced Radical Generation: Selective Hydrogen Atom Abstraction via Controlled Ti-C σ-Bond Homolysis

A method for the generation of transient alkyl radicals via homolytic Ti-C bond cleavage was developed by employing a tailor-made organotitanium half-cage complex. In contrast to established metal-mediated radical initiation protocols via thermal or photochemical M-C σ-bond homolysis, radical formation is triggered solely by coordination of a solvent molecule (thf) to a titanium(IV) center. During the reaction, the nonstabilized alkyl radical is formed along with a persistent titanium(III) metalloradical, thus taming the former transient radical (persistent radical effect). Radical coupling and hydrogen atom abstraction (HAT) reactions have been explored not only experimentally but also computationally and by means of kinetic analysis. Exploiting these findings led to the development of selective HAT transformations, for example, with 9,10-dihydroanthracene. Deuterium labeling studies using selectively deuterated alkyls and 9,10-dihydroanthracene-d4 confirmed a radical pathway, which was underpinned by developing a radical-radical cross-coupling reaction for transferring the alkyl radical to a stable Sn-centered radical. To set the stage for an application in organic synthesis, a 5-endo-trig radical cyclization based on our methodology was established, and a dihydroxylated sesquiterpene was thus prepared in high diastereomeric excess.

Visible Light-Promoted β-Functionalization of Carbonyl Compounds in the Presence of Organic Dyes

Herein, we investigate the use of organic photocatalysts in the visible light-promoted β-functionalization of carbonyl compounds. In particular, we studied the addition of aliphatic aldehydes to α,β-unsaturated compounds (β-Michael addition), and the reaction of cyclic ketones with either ketones (β-aldol condensation) or imines (β-Mannich reaction). Among the dyes tested, donor-acceptor cyanoarenes gave the best results, promoting the transformations of interest in moderate to good yields. The reaction scope was investigated on substrates with different steric and electronic properties. Fluorescence quenching analysis (Stern-Volmer experiments) led us to propose for these reactions a reductive quenching mechanism involving a transient 5πe- activation mode.

The Rapid Synthesis of Colibactin Warhead Model Compounds Using New Metal-Free Photocatalytic Cyclopropanation Reactions Facilitates the Investigation of Biological Mechanisms

Herein, we report the synthesis of a series of colibactin warhead model compounds using two newly developed metal-free photocatalytic cyclopropanation reactions. These mild cyclopropanations expand the known applications of eosin within synthesis. A halogen atom transfer reaction mode has been harnessed so that dihalides can be used as the cyclopropanating agents. The colibactin warhead models were then used to provide new insight into two key mechanisms in colibactin chemistry. An explanation is provided for why the colibactin warhead sometimes undergoes a ring expansion-addition reaction to give fused cyclobutyl products while at other times nucleophiles add directly to the cyclopropyl unit (as when DNA adds to colibactin). Finally, we provide some evidence that Cu(II) chelated to colibactin may catalyze an important oxidation of the colibactin-DNA adduct. The Cu(I) generated as a result could then also play a role in inducing double strand breaks in DNA.

Recent Advances in C-H Functionalisation through Indirect Hydrogen Atom Transfer

The functionalisation of C-H bonds has been an enormous achievement in synthetic methodology, enabling new retrosynthetic disconnections and affording simple synthetic equivalents for synthons. Hydrogen atom transfer (HAT) is a key method for forming alkyl radicals from C-H substrates. Classic reactions, including the Barton nitrite ester reaction and Hofmann-Löffler-Freytag reaction, among others, provided early examples of HAT. However, recent developments in photoredox catalysis and electrochemistry have made HAT a powerful synthetic tool capable of introducing a wide range of functional groups into C-H bonds. Moreover, greater mechanistic insights into HAT have stimulated the development of increasingly site-selective protocols. Site-selectivity can be achieved through the tuning of electron density at certain C-H bonds using additives, a judicious choice of HAT reagent, and a solvent system. Herein, we describe the latest methods for functionalizing C-H/Si-H/Ge-H bonds using indirect HAT between 2018-2023, as well as a critical discussion of new HAT reagents, mechanistic aspects, substrate scopes, and background contexts of the protocols.

Organocatalytic synthesis of β-enaminyl radicals as single-electron donors for phenyliodine(III) dicarboxylates: direct one-pot alkylation-aminoxidation of styrenes

A direct one-pot alkylation-aminoxidation of styrene derivatives was achieved using in situ-generated alkyl and N-oxyl radicals. The corresponding O-alkylated hydroxylamine derivatives were obtained in moderate to good yields. The reaction features the generation of the alkyl radicals from phenyliodine(III) dicarboxylates via an organocatalytic process, the use of phenyliodine(III) dicarboxylates as the source of the alkyl radicals and oxidants for the generation of N-oxyl radicals, and the first generation of the β-enaminyl radicals via a HAT process and their use as single-electron donors.

Iridium-Catalyzed Intramolecular β-C-H Alkenylation of Ketones with Alkynes via a Hydride-Transfer Approach

Direct functionalization of carbonyl β C-H bonds without using directing groups has not been a trivial task, and it is even more challenging to realize the corresponding atom-economical transformations with common alkenes or alkynes as the coupling partner. Here, we describe the development of an iridium-catalyzed intramolecular direct β-alkenylation of ketones with regular alkynes. The reaction is redox neutral, avoids strong acids or bases, and tolerates various functional groups. The combined experimental and computational mechanistic studies reveal a hydride-transfer pathway, involving ketone α,β-desaturation, iridium-hydride-mediated alkyne insertion, conjugate addition, and α-protonation.

Redox-active molecules as organocatalysts for selective oxidative transformations - an unperceived organocatalysis field

Organocatalysis is widely recognized as a key synthetic methodology in organic chemistry. It allows chemists to avoid the use of precious and (or) toxic metals by taking advantage of the catalytic activity of small and synthetically available molecules. Today, the term organocatalysis is mainly associated with redox-neutral asymmetric catalysis of C-C bond-forming processes, such as aldol reactions, Michael reactions, cycloaddition reactions, etc. Organophotoredox catalysis has emerged recently as another important catalysis type which has gained much attention and has been quite well-reviewed. At the same time, there are a significant number of other processes, especially oxidative, catalyzed by redox-active organic molecules in the ground state (without light excitation). Unfortunately, many of such processes are not associated in the literature with the organocatalysis field and thus many achievements are not fully consolidated and systematized. The present article is aimed at overviewing the current state-of-art and perspectives of oxidative organocatalysis by redox-active molecules with the emphasis on challenging chemo-, regio- and stereoselective CH-functionalization processes. The catalytic systems based on N-oxyl radicals, amines, thiols, oxaziridines, ketone/peroxide, quinones, and iodine(I/III) compounds are the most developed catalyst types which are covered here.

Umpolung Reactivity of Imine Ester: Visible-Light Mediated Transfer Hydrogenation of α-Aryl Imino Esters by Phenylsilane and Water

A visible-light mediated chemoselective transfer hydrogenation of α-aryl imino esters was demonstrated. The methodology allowed the efficient and practical preparation of α-amino acid esters. The mechanism of the reaction was probed by DFT calculations, and deuteration experiments indicated deuterium was introduced into amino acid esters efficiently (up to 99 % D ratio), enabling a feasible way to obtain deuterated amino acids using D₂ O as a cheap deuterium source.

Theoretical Exploration of Energy Transfer and Single Electron Transfer Mechanisms to Understand the Generation of Triplet Nitrene and the C(sp3)-H Amidation with Photocatalysts

Mechanistic explorations and kinetic evaluations were performed based on electronic structure calculations at the CASPT2//CASSCF level of theory, the Fermi's golden rule combined with the Dexter model, and the Marcus theory to unveil the key factors regulating the processes of photocatalytic C(sp³)-H amidation starting from the newly emerged nitrene precursor of hydroxamates. The highly reactive nitrene was found to be generated efficiently via a triplet-triplet energy transfer process and to be benefited from the advantages of hydroxamates with long-range charge-transfer (CT) excitation from the N-centered lone pair to the 3,5-bis(trifluoromethyl)benzoyl group. The properties of the metal-to-ligand charge-transfer (MLCT) state of photocatalysts, the functionalization of chemical moieties for substrates involved in the charge-transfer (CT) excitation, such as the electron-withdrawing trifluoromethyl group, and the energetic levels of singlet and triplet reaction pathways may regulate the reaction yield of C(sp³)-H amidation. Kinetic evaluations show that the triplet-triplet energy transfer is the main driving force of the reaction rather than the single electron transfer process. The effects of electronic coupling, molecular rigidity, and excitation energies on the energy transfer efficiency were further discussed. Finally, we investigated the inverted behavior of single-electron transfer, which is correlated unfavorably to the catalytic efficiency and amidation reaction. All theoretical explorations allow us to better understand the generation of nitrene with visible-light photocatalysts, to expand highly efficient substrate sources, and to broaden our scope of available photosensitizers for various cross-coupling reactions and the construction of N-heterocycles.

Site-Selective N-1 and C-3 Heteroarylation of Indole with Heteroarylnitriles by Organocatalysis under Visible Light

Site-selective N-1 and C-3 arylation of indole has been sought after because of the prevalent application of arylindoles and the intricate reactivities associated with the multiple sites of the N-unsubstituted indole. Represented herein is the first regioselective heteroarylation of indole via a radical-radical cross-coupling by visible-light irradiation. Steady and time-resolved spectroscopic and computational studies revealed that the hydrogen-bonding interaction of organic base and its conjugated acid, namely with indole and heteroarylnitrile, determined the reaction pathway, which underwent either proton-coupled electron-transfer or energy-transfer for the subsequent radical-radical cross-coupling, leading to the regioselective formation of C-3 and N-1 heteroarylation of indoles, respectively. The parallel methodologies for regioisomeric N-1 and C-3 heteroaryl indoles with good functional group compatibility could be applied to large-scale synthesis and late-stage derivatization of bioactive compounds under extremely mild reaction conditions.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

A Convergent Paired Electrolysis Strategy Enables Cross-Coupling of Methylarenes with Imines

In this report, we have developed a metal-free convergent paired electrolysis strategy for α-benzyl amine synthesis from readily available imines and methylarenes, taking advantage of both anodic oxidation and cathodic...

Visible-light-driven photocatalyst-free deoxygenative alkylation of imines with alcohols

Upon easy access and direct photoexcitation of xanthate anions, visible-light-driven deoxygenative alkylation of imines with a wide variety of alcohols has been achieved via a phosphine-assisted one-pot protocol, without any photocatalysts.

Photocatalytic Redox-Neutral Reaction of γ-Indolyl α-Keto Esters

The direct γ-C(sp3)-H activation of saturated α-keto esters has long been an elusive transformation. We found that photo-redox catalysis IrIII in combination with DABCO as dual hydrogen-bonding donor and organic...

Radical umpolung chemistry enabled by dual catalysis: concept and recent advances

We present a perspective on recent advances in radical umpolung chemistry; some selected examples in this area have been highlighted.

Switchable, Reagent-Controlled Diastereodivergent Photocatalytic Carbocyclisation of Imine-Derived α-Amino Radicals

A reagent-controlled stereodivergent carbocyclisation of aryl aldimine-derived, photocatalytically generated, α-amino radicals possessing adjacent conjugated alkenes, affording either bicyclic or tetracyclic products, is described. Under net reductive conditions using commercial Hantzsch ester, the α-amino radical species underwent a single stereoselective cyclisation to give trans-configured amino-indane structures in good yield, whereas using a substituted Hantzsch ester as a milder reductant afforded cis-fused tetracyclic tetrahydroquinoline frameworks, resulting from two consecutive radical cyclisations. Judicious choice of the reaction conditions allowed libraries of both single and dual cyclisation products to be synthesised with high selectivity, notable predictability, and good-to-excellent yields. Computational analysis employing DFT revealed the reaction pathway and mechanistic rationale behind this finely balanced yet readily controlled photocatalytic system.

Modular Photocatalytic Synthesis of α-Trialkyl-α-Tertiary Amines

Molecules displaying an α-trialkyl-α-tertiary amine motif provide access to an important and versatile area of biologically relevant chemical space but are challenging to access through existing synthetic methods. Here, we report an operationally straightforward, multicomponent protocol for the synthesis of a range of functionally and structurally diverse α-trialkyl-α-tertiary amines, which makes use of three readily available components: dialkyl ketones, benzylamines, and alkenes. The strategy relies on the of use visible-light-mediated photocatalysis with readily available Ir(III) complexes to bring about single-electron reduction of an all-alkyl ketimine species to an α-amino radical intermediate; the α-amino radical undergoes Giese-type addition with a variety of alkenes to forge the α-trialkyl-α-tertiary amine center. The mechanism of this process is believed to proceed through an overall redox neutral pathway that involves photocatalytic redox-relay of the imine, generated from the starting amine-ketone condensation, through to an imine-derived product. This is possible because the presence of a benzylic amine component in the intermediate scaffold drives a 1,5-hydrogen atom transfer step after the Giese addition to form a stable benzylic α-amino radical, which is able to close the photocatalytic cycle. These studies detail the evolution of the reaction platform, an extensive investigation of the substrate scope, and preliminary investigation of some of the mechanistic features of this distinct photocatalytic process. We believe this transformation will provide convenient access to previously unexplored α-trialkyl-α-tertiary amine scaffolds that should be of considerable interest to practitioners of synthetic and medicinal chemistry in academic and industrial institutions.

Copper-Catalyzed Sulfonylation of Cyclobutanone Oxime Esters with Sulfonyl Hydrazides

A copper-catalyzed radical cross-coupling of cyclobutanone oxime esters with sulfonyl hydrazides has been developed. The copper-based catalytic system proved crucial for cleavage of the C−C bond of cyclobutanone oximes and for selective C–S bond-formation involving persistent sulfonyl-metal radical intermediates. This protocol is distinguished by the low-cost catalytic system, which does not require ligand, base, or toxic cyanide salt, and by the use of readily accessible starting materials, as well as broad substrate scope, providing an efficient approach to various diversely substituted cyano-containing sulfones.

Visible-light-promoted olefinic trifluoromethylation of enamides with CF3SO2Na

A visible-light-promoted olefinic C-H trifluoromethylation of enamides was developed by employing cheap and stable Langlois' reagent as the CF₃ source. A series of β-CF₃ enamides were obtained in moderate to good yields with high E-isomer selectivity under mild conditions. Preliminary mechanistic studies suggest that molecular oxygen acts as the terminal oxidant for this net oxidative process, and the E isomer selectivity could be well explained by a base-assisted deprotonation of the cation intermediate.

Catalytic α-Deracemization of Ketones Enabled by Photoredox Deprotonation and Enantioselective Protonation

This study reports the catalytic deracemization of ketones bearing stereocenters in the α-position in a single reaction via deprotonation, followed by enantioselective protonation. The principle of microscopic reversibility, which has previously rendered this strategy elusive, is overcome by a photoredox deprotonation through single electron transfer and subsequent hydrogen atom transfer (HAT). Specifically, the irradiation of racemic pyridylketones in the presence of a single photocatalyst and a tertiary amine provides nonracemic carbonyl compounds with up to 97% enantiomeric excess. The photocatalyst harvests the visible light, induces the redox process, and is responsible for the asymmetric induction, while the amine serves as a single electron donor, HAT reagent, and proton source. This conceptually simple light-driven strategy of coupling a photoredox deprotonation with a stereocontrolled protonation, in conjunction with an enrichment process, serves as a blueprint for other deracemizations of ubiquitous carbonyl compounds.

Visible-Light Radical–Radical Coupling vs. Radical Addition: Disentangling a Mechanistic Knot

A highly enantioselective protocol has been recently described as allowing the synthesis of five-membered cyclic imines harnessing the selective generation of a β-Csp3-centered radical of acyl heterocyclic derivatives and its subsequent interaction with diverse NH-ketimines. The overall transformation represents a novel cascade process strategy crafted by individual well-known steps; however, the construction of the new C-C bond highlights a crucial knot from a mechanistically perspective. We believe that the full understanding of this enigmatic step may enrich the current literature and expand latent future ideas. Therefore, a detailed mechanistic study of the protocol has been conducted. Here, we provide theoretical insight into the mechanism using quantum chemistry calculations. Two possible pathways have been investigated: (a) imine reduction followed by radical–radical coupling and (b) radical addition followed by product reduction. In addition, investigations to unveil the origin behind the enantioselectivity of the 1-pyrroline derivatives have been conducted as well.

Unusual Behavior of Ketoximes: Reagentless Photochemical Pathway to Alkynyl Sulfides

The unique properties of ketoximes are used prominently for the synthesis of heterocycles. In contrast, their potential to absorb light and photoelectron transfer processes remains challenging. Widespread interest in controlling direct excitation of ketoxime tacticity unlocks unconventional reaction pathways, enabling photochemical intramolecular skeletal modification to constitute alkynyl sulfides that cannot be realized via traditional activation. Despite decades of advancements, the alkynyl sulfides, particularly those composed of polar functionalities and derived from renewable sources, remain unknown. These findings demonstrate the importance of decelerated ketoxime from β-oxodithioester for the identification of reaction conditions. The method uses mild reaction conditions to generate excited-state photoreductant for the functionalization of an array of alkynyl sulfides. Additionally, a fundamental understanding of elementary steps using electrochemical and spectroscopic techniques/experiments revealed a PCET pathway to this transformation, while the involved substrates and their properties with improved economical tools indicated the translational potential of this method.

Synthesis of Chiral Amines by C–C Bond Formation with Photoredox Catalysis

Chiral amines are key substructures of biologically active natural products and drug candidates. The advent of photoredox catalysis has changed the way synthetic chemists think about building these substructures, opening new pathways that were previously unavailable. New developments in this area are reviewed, with an emphasis on C–C bond constructions involving radical intermediates generated through photoredox processes.

1 Introduction

2 Radical–Radical Coupling of α-Amino Radicals

2.1 Radical–Radical Coupling Involving Amine Oxidation

2.2 Radical–Radical Coupling Involving Imine Reduction

2.3 Couplings Involving both Amine Oxidation and Imine Reduction

3 Addition Reactions of α-Amino Radicals

3.1 Conjugate Additions of α-Amino Radicals

3.2 Addition of α-Amino Radicals to Heteroaromatic Systems

3.3 Cross Coupling via Additions to Transition Metal Complexes

4 Radical Addition to C=N Bonds Using Photoredox Catalysis

4.1 Intramolecular Radical Addition to C=N Bonds

4.2 Intermolecular Radical Addition to C=N Bonds

5 Conclusion

Direct access to α-acyloxycarbonyl compounds and esters via oxidative esterification of aldehydes under visible light

An efficient synthesis of α-acyloxycarbonyl compounds and esters from aldehydes and α-bromocarbonyl compounds/benzyl bromide derivatives via photoredox catalysis has been developed.