α-Amino Radicals via Photocatalytic Single-Electron Reduction of Imine Derivatives

Formate and CO2 Enable Reductive Carboxylation of Imines: Synthesis of Unnatural α-Amino Acids

Herein, a photocatalytic umpolung strategy for reductive carboxylation of imines for the synthesis of α-amino acids was disclosed. Carbon dioxide radical anion (CO2•-) generated from formate is the key single electron reductant in the reactions. An unprecedentedly broad substrate scope of imines with excellent reaction yields was obtained with carbon dioxide (CO2) and formate salt as carbon sources.

Electrochemical Synthesis of C(sp3)-Rich Amines by Aminative Carbofunctionalization of Carbonyl Compounds

Alkylamines form the backbone of countless nitrogen-containing small molecules possessing desirable biological properties. Despite advances in amine synthesis through transition metal catalysis and photoredox chemistry, multicomponent reactions that leverage inexpensive materials to transform abundant chemical feedstocks into three-dimensional α-substituted alkylamines bearing complex substitution patterns remain scarce. Here, we report the design of a catalyst-free electroreductive manifold that merges amines, carbonyl compounds and carbon-based radical acceptors under ambient conditions without rigorous exclusion of air and moisture. Key to this aminative carbofunctionalization process is the chemoselective generation of nucleophilic α-amino radical intermediates that readily couple with electrophilic partners, providing straightforward access to architecturally intricate alkylamines and drug-like scaffolds which are inaccessible by conventional means.

Eosin, blue LEDs and DIPEA are employed in a simple synthesis of (poly)cyclic O,O- and N,O-acetals

A simple procedure for the synthesis of (poly)cyclic O,O- and N,O-acetals from various enol ethers, N-acyl enamines or Boc-protected enamines has been developed. The key step is a photocatalytic Stork-Ueno-type cylization using the very simple metal-free conditions of catalytic eosin, diisopropylamine in the green solvent ethanol with blue LED irradition.

Synthesis of Unsymmetrical 1,6-Dicarbonyl Compounds through Photocatalytic One-Pot Tandem Three-Component Reaction

Unsymmetrical 1,6-dicarbonyl compounds are useful building blocks in organic synthesis. However, the synthetic methods are limited. Herein we report a three-component one-pot reaction for the synthesis of unsymmetrical 1,6-diketones. This reaction utilizes sulfides as radical precursors and enol ethers as building blocks. It takes advantage of photocatalysis and Lewis acid catalysis. Its synthetic utility was demonstrated in the modification of biorelevant compounds.

Multicomponent Synthesis of α-Branched Amines via a Zinc-Mediated Carbonyl Alkylative Amination Reaction

Methods for the synthesis of α-branched alkylamines are important due to their ubiquity in biologically active molecules. Despite the development of many methods for amine preparation, C(sp3)-rich nitrogen-containing compounds continue to pose challenges for synthesis. While carbonyl reductive amination (CRA) between ketones and alkylamines is the cornerstone method for α-branched alkylamine synthesis, it is sometimes limited by the sterically demanding condensation step between dialkyl ketones and amines and the more restricted availability of ketones compared to aldehydes. We recently reported a "higher-order" variant of this transformation, carbonyl alkylative amination (CAA), which utilized a halogen atom transfer (XAT)-mediated radical mechanism, enabling the streamlined synthesis of complex α-branched alkylamines. Despite the efficacy of this visible-light-driven approach, it displayed scalability issues, and competitive reductive amination was a problem for certain substrate classes, limiting applicability. Here, we report a change in the reaction regime that expands the CAA platform through the realization of an extremely broad zinc-mediated CAA reaction. This new strategy enabled elimination of competitive CRA, simplified purification, and improved reaction scope. Furthermore, this new reaction harnessed carboxylic acid derivatives as alkyl donors and facilitated the synthesis of α-trialkyl tertiary amines, which cannot be accessed via CRA. This Zn-mediated CAA reaction can be carried out at a variety of scales, from a 10 μmol setup in microtiter plates enabling high-throughput experimentation, to the gram-scale synthesis of medicinally-relevant compounds. We believe that this transformation enables robust, efficient, and economical access to α-branched alkylamines and provides a viable alternative to the current benchmark methods.

Recent Advances in Photoinduced Modification of Amino Acids, Peptides, and Proteins

The chemical modification of biopolymers like peptides and proteins is a key technology to access vaccines and pharmaceuticals. Similarly, the tunable derivatization of individual amino acids is important as they are key building blocks of biomolecules, bioactive natural products, synthetic polymers, and innovative materials. The high diversity of functional groups present in amino acid-based molecules represents a significant challenge for their selective derivatization Recently, visible light-mediated transformations have emerged as a powerful strategy for achieving chemoselective biomolecule modification. This technique offers numerous advantages over other methods, including a higher selectivity, mild reaction conditions and high functional-group tolerance. This review provides an overview of the most recent methods covering the photoinduced modification for single amino acids and site-selective functionalization in peptides and proteins under mild and even biocompatible conditions. Future challenges and perspectives are discussed beyond the diverse types of photocatalytic transformations that are currently available.

Factors hindering the degradation of pharmaceuticals from human urine in an iron-activated persulfate system

This study investigated the degradation of clofibric acid (CFA), bezafibrate (BZF), and sulfamethoxazole (SMX) in synthetic human urine using a novel mesoporous iron powder-activated persulfate system (mFe-PS system), and identified the factors limiting their degradation in synthetic human urine. A kinetic model was established to expose the radical production in various reaction conditions, and experiments were conducted to verify the modeling results. In the phosphate-containing mFe-PS system, the 120 min removal efficiency of CFA decreased from 95.1% to 76.6% as the phosphate concentration increased from 0.32 to 6.45 mmol/L, but recovered to 90.5% when phosphate concentration increased to 16.10 mmol/L. Meanwhile, the increased concentration of phosphate from 0.32 to 16.10 mmol/L reduced the BZF degradation efficacy from 91.5% to 79.0%, whereas SMX removal improved from 37.3% to 62.9%. The mFe-PS system containing (bi)carbonate, from 4.20 to 166.70 mmol/L, reduced CFA and BZF removal efficiencies from 100% to 76.8% and 80.4%, respectively, and SMX from 83.5% to 56.7% within a 120-min reaction time. In addition, alkaline conditions (pH ≥ 8.0) inhibited CFA and BZF degradations, while nonacidic pH (pH ≥ 7.0) remarkably inhibited SMX degradation. Results of the kinetic model indicated the formation of phosphate (H2PO4·/HPO4·-) and/or carbonate radicals (CO3·-) could limit pharmaceutical removal. The transformation products (TPs) of the pharmaceuticals revealed more incompletely oxidized TPs occurred in the phosphate- and (bi)carbonate-containing mFe-PS systems, and indicated that H2PO4·/HPO4·- mainly degraded pharmaceuticals via a benzene ring-opening reaction while CO3·- preferentially oxidized pharmaceuticals via a hydroxylation reaction.

Kinetics of H· Transfer from CpCr(CO)3H to Various Enamides: Application to Construction of Pyrrolidines

The rate constants kH (kD) have been determined at 27 °C for H· (D·) transfer from CpCr(CO)3H(D) to the C=C bonds of various enamides. This process leads to the formation of α-amino radicals. Vinyl enamides with N-alkyl and N-phenyl substituents have proven to be good H· acceptors, with rate constants close to those of styrene and methyl methacrylate. A methyl substituent on the incipient radical site decreases kH by a factor of 4; a methyl substituent on the carbon that will receive the H· decreases kH by a factor of 380. The measured kH values indicate that these α-amino radicals can be used for the cyclization of enamides to pyrrolidines. A vanadium hydride, HV(CO)4(dppe), has proven more effective at the cyclization of enamides than Cr or Co hydrides-presumably because the weakness of the V-H bond leads to faster H· transfer. The use of the vanadium hydride is operationally simple, employs mild reaction conditions, and has a broad substrate scope. Calculations have confirmed that H· transfer is the slowest step in these cyclization reactions.

Straightforward synthesis of functionalized γ-Lactams using impure CO2 stream as the carbon source

Direct utilization of CO2 into organic synthesis finds enormous applications to synthesize pharmaceuticals and fine chemicals. However, pure CO2 gas is essential to achieve these transformations, and the purification of CO2 is highly cost and energy intensive. Considering this, we describe a straightforward synthetic route for the synthesis of γ-lactams, a pivotal core structure of bioactive molecules, by using commercially available starting materials (alkenes and amines) and impure CO2 stream (exhaust gas is collected from the car) as the carbon source. This blueprint features a broad scope, excellent functional group compatibility and application to the late-stage transformation of existing pharmaceuticals and natural products to synthesize functionalized γ-lactams. We believe that our strategy will provide direct access to γ-lactams in a very sustainable way and will also enhance the Carbon Capture and Utilization (CCU) strategy.

α-Difluoroalkylation of Benzyl Amines with Trifluoromethylarenes

An α-difluoroalkylation of benzyl amines with trifluoromethylarenes is disclosed herein. This protocol is characterized by its operational simplicity, excellent chemoselectivity and broad scope-even with advanced synthetic intermediates-, thus offering a new entry point to medicinally-relevant α-difluoroalkylated amines from simple, yet readily accessible, precursors.

Enantioconvergent 6π Electrocyclization Enabled by Photoredox Racemization

This study presents a novel photoredox-enabled enantioconvergent catalytic strategy used to construct chiral 2H-1,3-benzoxazines via an unprecedented oxa-6π electrocyclization utilizing racemic α-substituted glycinates as substrates. The approach leverages a cobalt-based chiral Lewis acid catalyst, which promotes the transformation under thermal or photoredox conditions. While the thermal reaction selectively converts only the (S)-configured glycinates into enantioenriched 2H-1,3-benzoxazines (up to 96:4 e.r.), the addition of 0.5 mol % of a commercially available iridium photocatalyst under visible light irradiation transforms the reaction into an enantioconvergent process. Detailed mechanistic and time course studies of optically pure α-deuterated substrates revealed the presence of an enantiospecific kinetic isotope effect, which helped to clarify the role of both the photo- and chiral Lewis acid catalyst in the reaction sequence. In this dual catalytic system, the photocatalyst promotes a dynamic interconversion between the substrate enantiomers─a process not accessible via ground-state chemistry─while the chiral Lewis acid selectively transforms only the (S)-configured substrates. Further mechanistic evidence for the proposed mechanism is provided by linear free energy relationship analysis, which suggests that the stereodetermining step involves a 6π electrocyclization under both thermal and photoredox conditions.

Cr-Catalyzed Asymmetric Cross Aza-Pinacol Couplings for β-Amino Alcohol Synthesis

Chiral β-amino alcohols are crucial structural motifs found in pharmaceuticals, natural products, and chiral ligands in asymmetric catalysis. Despite previous advances, the development of catalytic approaches to access β-amino alcohols bearing vicinal stereocenters from readily available chemicals remains a prominent challenge. Herein, we describe the Cr-catalyzed asymmetric cross aza-pinacol coupling of aldehydes and N-sulfonyl imines. This protocol proceeds in a radical-polar crossover manner from the intermediacy of an α-amino radical instead of a ketyl radical. Key to the success is using a chiral chromium catalyst, which plays a triple role in the chemoselective single-electron reduction of the imine, fast radical interception to inhibit radical addition to imines, and chemo- and stereoselective addition to aldehydes instead of imines. This method provides a modular and efficient approach to accessing diverse β-amino alcohols bearing vicinal stereocenters.

α-Tertiary Primary Amine Synthesis via Photocatalytic C(sp3)-H Aminoalkylation

Facile access to sterically hindered α-tertiary primary amines via photocatalytic radical coupling of native C(sp3)-H substrates with N-unsubstituted ketimines is reported. LiBr was used as a hydrogen atom transfer reagent to cleave C(sp3)-H bonds to get alkyl radicals. The in situ-generated HBr can then serve as a Bronsted acid to activate N-unsubstituted ketimines readily for single-electron reduction to deliver α-amino radicals. As a consequence, radical-radical coupling affords primary amines with a congested α-tertiary substituent. This reaction is highlighted by simple and mild conditions, 100% atom-economy, and broad hydrocarbon substrate scope for benzyl ethers, cyclic ethers, benzyl alcohols, alkylarenes, and carbocycles.

Reductive photoredox transformations of carbonyl derivatives enabled by strongly reducing photosensitizers

Visible-light photoredox catalysis is well-established as a powerful and versatile organic synthesis strategy. However, some substrate classes, despite being attractive precursors, are recalcitrant to single-electron redox chemistry and thus not very amenable to photoredox approaches. Among these are carbonyl derivatives, e.g. ketones, aldehydes, and imines, which in most cases require Lewis or Brønsted acidic additives to activate via photoinduced electron transfer. In this work, we unveil a range of photoredox transformations on ketones and imines, enabled by strongly reducing photosensitizers and operating under simple, general conditions with a single sacrificial reductant and no additives. Specific reactions described here are umpolung C-C bond forming reactions between aromatic ketones or imines and electron-poor alkenes, imino-pinacol homocoupling reactions of challenging alkyl-aryl imine substrates, and γ-lactonization reactions of aromatic ketones with methyl acrylate. The reactions are all initiated by photoinduced electron transfer to form a ketyl or iminyl that is subsequently trapped.

Radical Group Transfer of Vinyl and Alkynyl Silanes Driven by Photoredox Catalysis

Radical group transfer is a powerful tool for the formation of C-C bonds. These processes typically involve radical addition to C-C π bonds, followed by fragmentation of the resulting cyclic intermediate. Despite the advantageous lability of organosilanes in this context, silicon-tethered radical acceptor groups have remained underexplored in radical group transfer reactions. We report a general photoredox-catalyzed protocol for the radical group transfer of vinyl and alkynyl silanes onto sp3 carbons, using activated and unactivated iodides as radical precursors. Our method displays high diastereoselectivity and excellent functional group tolerance, and enables direct formation of group transfer products by in situ ring opening. Mechanistic investigations revealed that the reaction proceeds via an unusual dual catalytic cycle, resulting in an overall redox-neutral process.

Visible-light induced cross-electrophile coupling of imines and anhydrides to synthesize α-amino ketones

α-Amino ketones are important motifs in synthetic and medicinal chemistry. Efficient methods to directly access these motifs from feasible precursors are, however, limited. Herein, a visible-light mediated reductive cross-electrophile coupling of readily available imines and anhydrides was developed. Under mild reaction conditions, the umpolung reactivity of diverse imines engaged with anhydrides gives a variety of α-amino ketones with good yields and a broad functional group compatibility. Primary mechanistic studies revealed that this transformation might proceed through a radical-radical cross coupling pathway dominantly.

Photoredox-catalyzed diastereoselective dearomative prenylation and reverse-prenylation of electron-deficient indole derivatives

Prenylated and reverse-prenylated indolines are privileged scaffolds in numerous naturally occurring indole alkaloids with a broad spectrum of important biological properties. Development of straightforward and stereoselective methods to enable the synthesis of structurally diverse prenylated and reverse-prenylated indoline derivatives is highly desirable and challenging. In this context, the most direct approaches to achieve this goal generally rely on transition-metal-catalyzed dearomative allylic alkylation of electron-rich indoles. However, the electron-deficient indoles are much less explored, probably due to their diminished nucleophilicity. Herein, a photoredox-catalyzed tandem Giese radical addition/Ireland-Claisen rearrangement is disclosed. Diastereoselective dearomative prenylation and reverse-prenylation of electron-deficient indoles proceed smoothly under mild conditions. An array of tertiary α-silylamines as radical precursors is readily incorporated in 2,3-disubstituted indolines with high functional compatibility and excellent diastereoselectivity (>20:1 d.r.). The corresponding transformations of the secondary α-silylamines provide the biologically important lactam-fused indolines in one-pot synthesis. Subsequently, a plausible photoredox pathway is proposed based on control experiments. The preliminary bioactivity study reveals a potential anticancer property of these structurally appealing indolines.

Organophotoredox 1,6-Addition of 3,4-Dihydroquinoxalin-2-ones to para-Quinone Methides Using Visible Light

An organophotoredox 1,6-radical addition of 3,4-dihidroquinoxalin-2-ones to para-quinone methides catalyzed by Fukuzumi's photocatalyst is described under the irradiation of a HP Single LED (455 nm). The corresponding 1,1-diaryl compounds bearing a dihydroquinoxalin-2-one moiety (20 examples) are obtained with good to excellent yields under mild reaction conditions. Several experiments have been carried out in order to propose a reaction mechanism.

Defluoroalkylation of Trifluoromethylarenes with Hydrazones: Rapid Access to Benzylic Difluoroarylethylamines

Here, we report an efficient and modular approach toward the formation of difluorinated arylethylamines from simple aldehyde-derived N,N-dialkylhydrazones and trifluoromethylarenes (CF₃-arenes). This method relies on selective C-F bond cleavage via reduction of the CF₃-arene. We show that a diverse set of CF₃-arenes and CF₃-heteroarenes react smoothly with a range of aryl and alkyl hydrazones. The β-difluorobenzylic hydrazine product can be selectively cleaved to form the corresponding benzylic difluoroarylethylamines.

A Proton-Coupled Electron Transfer Strategy to the Redox-Neutral Photocatalytic CO2 Fixation

Herein, we report our study on the design and development of a novel photocarboxylation method. We have used an organic photoredox catalyst (PC, 4CzIPN) and differently substituted dihydropyridines (DHPs) in combination with an organic base (1,5,7-triazabicyclodec-5-ene, TBD) to access a proton-coupled electron transfer (PCET) based manifold. In depth mechanistic investigations merging experimental analysis (NMR, IR, cyclic voltammetry) and density-functional theory (DFT) calculations reveal the key activity of a H-bonding complex between the DHP and the base. The thermodynamic and kinetic benefits of the PCET mechanism allowed the implementation of a redox-neutral fixation process leading to synthetically relevant carboxylic acids (18 examples with isolated yields up to 75%) under very mild reaction conditions. Finally, diverse product manipulations were performed to demonstrate the synthetic versatility of the obtained products.

Photoredox Promoted Barbier-Type Reaction of Alkyl Iodides with N-Alkyl and N-Aryl Imines

The reaction of organozinc reagents with unactivated imines is accelerated when performed in the presence of a photocatalyst under blue light irradiation. Coordination between Lewis acidic zinc iodide and the imine is a key factor responsible for the reaction efficiency. The method can be carried out using alkyl iodides under Barbier conditions.

Switching the Photoreactions of Ir(III) Diamine Complexes between C-N Coupling and Dehydrogenation under Visible Light Irradiation

The selective photoreactions under mild conditions play an important role in synthetic chemistry. Herein, efficient and mild protocols for switching the photoreactions of Ir(III)-diamine complexes between the interligand C-N coupling and dehydrogenation are developed in the presence of O₂ in EtOH solution. The photoreactions of achiral diamine complexes rac-[Ir(L)₂(dm)](PF₆) (L is 2-phenylquinoline or 2-(2,4-difluorophenyl)quinoline, dm is 1,2-ethylenediamine, 1,2-diaminopropane, 2-methyl-1,2-diamino-propane, or N,N'-dimethyl-1,2-ethylenediamine) are competitive in the oxidative C-N coupling and dehydrogenation at room temperature, which can be switched into the interligand C-N coupling reaction at 60 °C, affording hexadentate complexes in good to excellent yields, or the dehydrogenative reaction in the presence of a catalytic amount of TEMPO as an additive, affording imine complexes. Mechanism studies reveal that ¹O₂ is the major reactive oxygen species, and metal aminyl is the key intermediate in the formation of the oxidative C-N coupling and imine products in the photoreaction processes. These will provide a new and practical protocol for the synthesis of multidentate and imine ligands in situ via the postcoordinated strategy under mild conditions.

Photocatalytic Alkyl Addition to Access Quaternary Alkynyl α-Amino Esters

A regioselective alkylation of β,γ-alkynyl-α-imino esters by visible-light photocatalysis has been developed. This method enables 1,2-addition of methyl, primary, secondary, and tertiary alkyl radicals to the conjugated imines under mild conditions to produce a variety of quaternary alkynyl α-amino acid and cyclic amino acid motifs. Alkyl radicals are generated from alkyl bis(catecholato)silicates with an organic photocatalyst. This process is effective under an air atmosphere, providing operational benefits compared to conventional alkylation using organometallic reagents.

Direct Reductive Arylation of Imines with Electron-Deficient (Hetero) Arenes via Electrosynthesis to Access Benzylic Amines

In this paper, a direct access to benzylic amines from imines and electron-deficient (hetero) arenes via electrochemical radical-radical cross-coupling is described. The reaction has the characteristics of the wide range of substrates, excellent functional group tolerance, metal-free and highly atom-economical. The insensitivity to air and moisture makes this synthetic strategy more efficient for the construction of various benzylic amine derivatives.

Nickel-Catalyzed Reductive Alkylation of Heteroaryl Imines

The preparation of heterobenzylic amines by a Ni-catalyzed reductive cross-coupling between heteroaryl imines and C(sp3 ) electrophiles is reported. This umpolung-type alkylation proceeds under mild conditions, avoids the pre-generation of organometallic reagents, and exhibits good functional group tolerance. Mechanistic studies are consistent with the imine substrate acting as a redox-active ligand upon coordination to a low-valent Ni center. The resulting bis(2-imino)heterocycle⋅Ni complexes can engage in alkylation reactions with a variety of C(sp3 ) electrophiles, giving heterobenzylic amine products in good yields.

Recent advances in visible light-induced C(sp3)–N bond formation

Synthetic chemists have long focused on selective C(sp ³)-N bond-forming approaches in response to the high value of this motif in natural products, pharmaceutical agents and functional materials. In recent years, visible light-induced protocols have become an important synthetic platform to promote this transformation under mild reaction conditions. These photo-driven methods rely on converting visible light into chemical energy to generate reactive but controllable radical species. This Review highlights recent advances in this area, mostly after 2014, with an emphasis placed on C(sp ³)-H bond activations, including amination of olefins and carbonyl compounds, and cross-coupling reactions.

Copper-Catalyzed Umpolung of N-2,2,2-Trifluoroethylisatin Ketimines for the Enantioselective 1,3-Dipolar Cycloaddition with Benzo[b]thiophene Sulfones

A copper-catalyzed umpolung of N-2,2,2-trifluoroethylisatin ketimines for the enantioselective 1,3-dipolar cycloaddition with benzo[b]thiophene sulfones was developed. Using a catalyst system consisting of an (S,S_p)-^tBu-Phosferrox ligand, Cu(OTf)₂, and Cs₂CO₃, a range of pentacyclic spirooxindoles containing pyrrolidine and benzo[b]sulfolane subunits were obtained in high efficiency with excellent regio-, diastereo-, and enantioselectivites under mild conditions. The practicality and versatility of the reaction were also demonstrated.

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis

Redox processes are at the heart of synthetic methods that rely on either electrochemistry or photoredox catalysis, but how do electrochemistry and photoredox catalysis compare? Both approaches provide access to high energy intermediates (e.g., radicals) that enable bond formations not constrained by the rules of ionic or 2 electron (e) mechanisms. Instead, they enable 1e mechanisms capable of bypassing electronic or steric limitations and protecting group requirements, thus enabling synthetic chemists to disconnect molecules in new and different ways. However, while providing access to similar intermediates, electrochemistry and photoredox catalysis differ in several physical chemistry principles. Understanding those differences can be key to designing new transformations and forging new bond disconnections. This review aims to highlight these differences and similarities between electrochemistry and photoredox catalysis by comparing their underlying physical chemistry principles and describing their impact on electrochemical and photochemical methods.

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.

Excited-state palladium-catalysed reductive alkylation of imines: scope and mechanism

Palladium catalysis induced by visible-light irradiation is a promising tool for promoting unusual chemical reactivity. Here, the hybrid alkyl radical/Pd(i) species generated is used to promote the reductive alkylation of imines.

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.

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.

Photoredox-Catalyzed Synthesis of α-Amino Acid Amides by Imine Carbamoylation

An operationally simple protocol for the photocatalytic carbamoylation of imines is reported. Easily available, bench-stable 4-amido Hantzsch ester derivatives serve as precursors to carbamoyl radicals that undergo rapid addition to N-aryl imines. The reaction proceeds under blue light irradiation in the presence of the photocatalyst 3DPAFIPN and Brønsted/Lewis acid additives. Mechanistic studies indicated a photoredox mechanism that involves carbamoyl radicals.

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.

Photoredox-catalyzed multicomponent Petasis reaction in batch and continuous flow with alkyl boronic acids

Multicomponent reactions (MCRs) are ideal platforms for the generation of a wide variety of organic scaffolds in a convergent and atom-economical manner. Many strategies for the generation of highly substituted and diverse structures have been developed and among these, the Petasis reaction represents a viable reaction manifold for the synthesis of substituted amines via coupling of an amine, an aldehyde and a boronic acid (BA). Despite its synthetic utility, the inherent drawbacks associated with the traditional two-electron Petasis reaction have stimulated continuous research towards more facile and tolerant methodologies. In this regard, we present the use of free alkyl BAs as effective radical precursors in this MCR through a single-electron transfer mechanism under mild reaction conditions. We have further demonstrated its applicability to photo-flow reactors, facilitating the reaction scale-up for the rapid assembly of complex molecular structures.

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Multicomponent photocatalyzed Petasis reaction

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Boronic acids as alkyl radical precursors under mild reaction conditions

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Photo-flow chemistry application for Petasis reaction

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.

Photoredox-Catalyzed C-H Functionalization Reactions

The fields of C-H functionalization and photoredox catalysis have garnered enormous interest and utility in the past several decades. Many different scientific disciplines have relied on C-H functionalization and photoredox strategies including natural product synthesis, drug discovery, radiolabeling, bioconjugation, materials, and fine chemical synthesis. In this Review, we highlight the use of photoredox catalysis in C-H functionalization reactions. We separate the review into inorganic/organometallic photoredox catalysts and organic-based photoredox catalytic systems. Further subdivision by reaction class-either sp² or sp³ C-H functionalization-lends perspective and tactical strategies for use of these methods in synthetic applications.