Metal-Free, Redox-Neutral, Site-Selective Access to Heteroarylamine via Direct Radical-Radical Cross-Coupling Powered by Visible Light Photocatalysis

Organophotocatalytic pyridination of N-arylglycines with 4-cyanopyridines by decarboxylative and decyanative radical-radical coupling

A photocatalytic decarboxylative aminoalkylation of 4-cyanopyridines with N-arylglycines is achieved, providing 4-(aminomethyl)pyridine derivatives in moderate to good yields. This organic photocatalytic reaction undergoes a radical-radical cross-coupling process under redox-neutral conditions, featuring simple operation, readily available N-arylglycines and a broad substrate scope. Mechanistic investigations indicated that a proton-coupled electron-transfer process was involved to enable the single electron transfer between the reduced photocatalyst and 4-cyanopyridine in the presence of N-arylglycines.

Photoelectrochemical Ni-catalyzed cross-coupling of aryl bromides with amine at ultra-low potential

Photoelectrochemical (PEC) cell is an ideal platform for organic transformation because of its green benefits and minimal energy consumption. As an emerging methodology, the reaction types of photoelectrocatalytic organic synthesis (PECOS) are limited to simple oxidation and C-H activation at current stage. Metal catalysis for the construction of C(sp2)-N bonds has not been touched yet in PECOS. We introduce here a PEC method that successfully engages Ni catalysis for the mild production of aniline derivatives. Experimental and computational investigations elucidate that the addition of photoanode-generated amine radical to Ni catalyst avoids the sluggish nucleophilic attack, enabling the reaction to proceed at an ultra-low potential (-0.4 V vs. Ag/AgNO3) and preventing the overoxidation of products in conventional electrochemical synthesis. This synergistic catalysis strategy exhibits good functional group tolerance and wide substrate scope on both aryl halides and amines, by which some important natural products and pharmaceutical chemicals have been successfully modified.

Gram-scale photosynthesis of polyfunctionalized dihydro-2-oxypyrroles using 3DPAFIPN as a halogenated dicyanobenzene-based photosensitizer via a consecutive visible-light-induced electron transfer process

Background

Typically, organic dyes show lower excited state lifetimes, a key hindrance in the development of efficient photoredox processes. Due to their distinctive qualities and efficiency, a particular class of organic chromophores has drawn considerable interest from the scientific community. Thermally activated delayed fluorescence (TADF), is only seen in molecules with a minimal energy gap (usually less than 0.2 eV) between their lowest two excited states, i.e., singlet excited state (S1) and triplet excited state (T1), is a distinctive property of the molecules under study. Isophthalonitriles are a promising family of chromophores for use as organic photocatalysts because of the ease with which their redox potentials may be adjusted and the prolonged singlet excited states resulting from TADF.

Methods

A sustainable process for the photosynthesis of polyfunctionalized dihydro-2-oxypyrroles has been developed using the Michael-Mannich cyclocondensation of amines, dialkyl acetylenedicarboxylates, and formaldehyde. The development of a green radical synthesis strategy for this family of chemicals is discussed in detail in the current work. This work used a novel halogenated dicyanobenzene-based photosensitizer was used as a photocatalyst. It was dissolved in ethanol, exposed to air at ambient temperature, and triggered by a blue light-emitting diode as a renewable energy source. This project's main goal is to use a novel conveniently accessible, reasonably priced donor-acceptor (D-A) based on halogenated cyanoarene.

Findings

When exposed to visible light, the 3DPAFIPN [2,4,6-tris(diphenylamino)-5-fluoroisophthalonitrile] photocatalyst, which is a thermally activated delayed fluorescence (TADF), can induce single-electron transfer (SET), providing a simple and green method that is highly effective, energy-efficient, and environmentally friendly. Also, we calculated the turnover number (TON) and turnover frequency (TOF) for polyfunctionalized dihydro-2-oxypyrroles. Gram-scale cyclization has also been shown to be a practical technique for use in industrial applications.

Thermodynamic H-Abstraction Abilities of Nitrogen Centered Radical Cations as Potential Hydrogen Atom Transfer Catalysts in Y-H Bond Functionalization

Y-H bond functionalization has always been the focus of research interest in the area of organic synthesis. Direct hydrogen atom transfer (HAT) from the Y-H bond is one of the most efficient and practical methods to activate the Y-H bond. Recently, nitrogen centered radical cations were broadly utilized as H-abstraction catalysts to activate Y-H bonds via the HAT process. As a type of HAT catalyst, the H-affinity of nitrogen centered radical cations is a significant thermodynamic parameter to quantitatively evaluate the thermodynamic H-abstraction potentials of nitrogen centered radical cations. In this work, the pK a values of 120 protonated N-containing compounds in acetonitrile (AN) are predicted, and the H-affinities of 120 nitrogen centered radical cations in AN are derived from the reduction potentials of nitrogen centered radical cations and pK a of protonated N-containing compounds using Hess' law. This work focuses on the H-abstraction abilities of 120 nitrogen centered radical cations in AN to enrich the molecule library of novel HAT catalysts or H-abstractors and provides valuable thermodynamic guidelines for the application of nitrogen centered radical cations in Y-H bond functionalization.

Metal Free Functionalization of Saturated Heterocycles with Vinylarenes and Pyridine Enabled by Photocatalytic Hydrogen Atom Transfer

Saturated heterocycles are important class of structural scaffolds in small-molecule drugs, natural products, and synthetic intermediates. Here, we disclosed a metal free, mild, and scalable functionalization of saturated heterocycles using vinylarenes as a linchpin approach. Key to success of this transformation is the employing of simple and cheap benzophenone as a hydrogen atom transfer (HAT) catalyst. This operationally robust process was used for the making of diverse functionalized saturated heterocycles. Furthermore, aldehydes, alkane, and alcohol have been functionalized under the optimized conditions. The potential pharmaceutical utility of the procedure has also been demonstrated by late-stage functionalization of bioactive natural compounds and pharmaceutical molecules. Initial mechanism studies and control experiments were performed to elucidate the mechanism of the reactions.

Para-selective nitrobenzene amination lead by C(sp2)-H/N-H oxidative cross-coupling through aminyl radical

Arylamines, serving as crucial building blocks in natural products and finding applications in multifunctional materials, are synthesized on a large scale via an electrophilic nitration/reduction sequence. However, the current methods for aromatic C-H amination have not yet attained the same level of versatility as electrophilic nitration. Here we show an extensively investigated transition metal-free and regioselective strategy for the amination of nitrobenzenes, enabling the synthesis of 4-nitro-N-arylamines through C(sp2)-H/N-H cross-coupling between electron-deficient nitroarenes and amines. Mechanistic studies have elucidated that the crucial aspects of these reactions encompass the generation of nitrogen radicals and recombination of nitrobenzene complex radicals. The C(sp2)-N bond formation is demonstrated to be highly effective for primary and secondary arylamines as well as aliphatic amines under mild conditions, exhibiting exceptional tolerance towards diverse functional groups in both nitroarenes and amines (>100 examples with yields up to 96%). Notably, this C(sp2)-H/N-H cross-coupling exhibits exclusive para-selectivity.

Visible-Light-Triggered Synthesis of N-α-Ketoacylated Sulfoximines by Denitrogenative and Oxidative Functionalization of Vinyl Azides

We have introduced a sulfoximidation reaction initiated by visible light between α-phenyl vinyl azides and NH-sulfoximines. The cost-effective and readily accessible hypervalent iodine reagent (PIDA) easily promoted the oxidative sulfoximidation process to afford N-α-ketoacylated sulfoximines in good to high yields, involving the formation of two new C-O bonds and one C-N bond. Additionally, the protocol offers noteworthy advantages, including its metal-free and photocatalyst-free reaction and its broad substrate compatibility.

Photocatalytic redox-neutral α-C(sp3)-H pyridination of glycine derivatives and N-arylamines with cyanopyridines

A photo-induced α-C(sp3)-H decyanative pyridination of N-arylglycine derivatives with cyanopyridines was developed. This reaction was performed under organic photocatalytic and redox-neutral conditions via a radical-radical cross-coupling process. Besides, the protocol was also suitable for the C(sp3)-H pyridination of N-aryl tetrahydroisoquinolines as well as benzylamines.

Access to CF2COR-Containing Quinazolinones via Visible-Light-Induced Domino Difluoroalkylation/Cyclization of N-Cyanamide Alkenes

A green and highly efficient visible-light-induced radical cascade difluoroalkylation/cyclization reaction of N-cyanamide alkenes has been developed. A variety of CF2COR-containing quinazolinones have been obtained in high yields with cheap non-metallic 4CzIPN as the photocatalyst. This photocatalytic reaction provides rapid, facile, and practical access to valuable polycyclic quinazolinone, and it is amenable to the gram scale.

Halogenated dicyanobenzene-based photosensitizer (3DPAFIPN) as a thermally activated delayed fluorescence (TADF) used in gram-scale photosynthesis 3,4-dihydropyrimidin-2-(1H)-one/thione derivatives via a consecutive visible-light-induced electron-transfer pathway

Background: Organic dyes often have shorter lifetimes in the excited state, which is a major obstacle to the development of effective photoredox methods. The scientific community has shown a great deal of interest in a certain class of organic chromophores because of their unique characteristics and effectiveness. One characteristic of the molecules under research is thermally activated delayed fluorescence (TADF), which is only observed in molecules with a tiny energy gap (often less than 0.2 eV) between their lowest two excited states, i.e., singlet excited state (S1) and triplet excited state (T1). The extended singlet excited states arising from TADF and the simplicity with which their redox potentials may be altered make the isophthalonitrile family of chromophores an attractive option for organic photocatalyst applications. Methods: The Biginelli reaction between β-ketoesters, arylaldehydes, and urea/thiourea has been used to build a sustainable technique for the production of 3,4-dihydropyrimidin-2-(1H)-one/thione derivatives. In the present study, the development of a green radical synthesis approach for this class of compounds is addressed in depth. As a photocatalyst, a new halogenated dicyanobenzene-based photosensitizer was employed in this study. As a renewable energy source activated by a blue LED, it was dissolved in ethanol, at room temperature in air atmosphere. The primary objective of this research is to employ a novel donor-acceptor (D-A) based on halogenated cyanoarene that is affordable, easily available, and innovative. Findings: The 3DPAFIPN [2,4,6-tris(diphenylamino)-5-fluoroisophthalonitrile] photocatalyst, a thermally activated delayed fluorescence (TADF), induces single-electron transfer (SET) in response to visible light, offering a straightforward, eco-friendly, and highly efficient process. Additionally, we determined the 3,4-dihydropyrimidin-2-(1H)-one/thione derivatives turnover frequency (TOF) and turnover number (TON). It has also been demonstrated that gram-scale cyclization is a workable method for industrial purposes.

Photoinduced, Metal-Free Hydroacylation of Aromatic Alkynes for Synthesis of α,β-Unsaturated Ketones via C(sp3)-H Functionalization

Despite the notable advancements made over the past decade in achieving carbon-carbon bonds by transition-metal-catalyzed cross-coupling processes, metal-free cross-coupling reactions for hydroacylation of aromatic alkynes via C(sp3)-H functionalization are still rare and highly desired. Here we report a metal-free reliable approach for the synthesis of α,β-unsaturated ketones (chalcones) via C(sp3)-H functionalization using MeCN:H2O as green solvent, Eosin Y as organic photocatalyst, and ambient air as oxidant. More significantly, this strategy can effectively transform a variety of methyl arenes and aromatic alkynes into the desired product. With high atom efficiency, use of green solvents, metal-free nature, environmental friendliness, and visible light as a renewable energy source, this method is compatible with biologically active molecules.

Metal-Free, Hindered, Regioselective Access to Multifunctional Groups Diarylamines via SN Ar Substitution of P-Nitroso Aromatic Methyl Ether by Arylamines

Multifunctional groups diarylamines, an innovative product, efficiently produced from arylamines and p-nitrosoanisole derivatives by intermolecular SN Ar under weak acid conditions. This SN Ar proceeds under mild reaction conditions, and more significantly, the substrates involved do not necessarily require strong electron-withdrawing groups. Moreover, this SN Ar is characterized by resistance to space crowding, tolerance to halogen and nitroso functional groups, and high regioselectivity. Mechanistic observations suggest that the SN Ar is the result of the transfer of the positive charge center of the protonated nitroso group to the p-methoxy group.

Accelerated photochemical reactions at oil-water interface exploiting melting point depression

Water can accelerate a variety of organic reactions far beyond the rates observed in classical organic solvents. However, using pure water as a solvent introduces solubility constraints that have limited the applicability of efficient photochemistry in particular. We report here the formation of aggregates between pairs of arenes, heteroarenes, enamines, or esters with different electron affinities in an aqueous medium, leading to an oil-water phase boundary through substrate melting point depression. The active hydrogen atoms in the reactants engage in hydrogen bonds with water, thereby accelerating photochemical reactions. This methodology realizes appealingly simple conditions for aqueous coupling reactions of complex solid molecules, including complex drug molecules that are poorly soluble in water.

Visible-Light-Promoted Metal- and Photocatalyst-Free Reactions between Arylglyoxylic Acids and Tetraalkylthiuram Disulfides: Synthesis of α-Ketoamides

A convenient and new synthetic approach has been developed for the oxidative cross-coupling of the C-N bond through the reaction between arylglyoxylic acids and tetraalkylthiuram disulfides. The reaction proceeds under ambient air at room temperature in the presence of visible light. This reaction offers a metal-, base-, photocatalyst-, and solvent-free synthesis of various α-ketoamides with moderate to excellent yields via the radical pathway. In addition, this protocol demonstrates the potential application of a gram-scale synthesis.

Alkaline-Metal-Promoted Divergent Synthesis of 1-Aminoisoquinolines and Isoquinolines

Alkaline-metal-promoted divergent syntheses of 1-aminoisoquinolines and isoquinolines have been reported involving 2-methylaryl aldehydes, nitriles, and LiN(SiMe3)2 as reactants. In addition, the three-component reaction of 2-methylaryl nitriles, aldehydes, and LiN(SiMe3)2 has been developed to furnish 1-aminoisoquinolines. This protocol features readily available starting materials, excellent chemoselectivity, broad substrate scope, and satisfactory yields.

Activation Model and Origins of Selectivity for Chiral Phosphoric Acid Catalyzed Diradical Reactions

To develop new radical synthesis strategies, a profound understanding of the electronic transfer mechanism is critical. An activation model called relayed proton-coupled electron transfer (relayed-PCET) was developed and investigated for chiral phosphoric acid-catalyzed diradical reactions by density functional theory (DFT). The driving force of electron transfer from the nucleophile to the electrophile is the proton transfer that occurs via the chiral phosphoric acid (CPA) catalyst to the electrophile. Moreover, the origins of the selectivity can be explained by distortion of the catalyst, favorable hydrogen bonding, and strong interactions of the substrates with substituents of the CPAs.

Photocatalytic Anti-Markovnikov Hydroamination of Alkenes with Primary Heteroaryl Amines

We report a light-driven method for the intermolecular anti-Markovnikov hydroamination of alkenes with primary heteroaryl amines. In this protocol, electron transfer between an amine substrate and an excited-state iridium photocatalyst affords an aminium radical cation (ARC) intermediate that undergoes C-N bond formation with a nucleophilic alkene. Integral to reaction success is the electronic character of the amine, wherein increasingly electron-deficient heteroaryl amines generate increasingly reactive ARCs. Counteranion-dependent reactivity is observed, and iridium triflate photocatalysts are employed in place of conventional iridium hexafluorophosphate complexes. This method exhibits broad functional group tolerance across 55 examples of N-alkylated products derived from pharmaceutically relevant heteroaryl amines.

Ru(II)-Catalyzed N-Methylation of Amines Using Methanol as the C1 Source

Four ruthenium complexes were used as catalysts for the N-methylation of amines using methanol as the C1 source under weak base conditions. The (DPEPhos)RuCl2PPh3(1a) catalyst showed the best catalytic performance (0.5 mol %, 12 h). The deuterium labeling and control experiments suggested the reaction via the Ru-H mechanism. This study provides a new ruthenium catalyst system for N-methylation with methanol under weak base conditions.

3DPAFIPN as a halogenated dicyanobenzene-based photosensitizer catalyzed gram-scale photosynthesis of pyrano[2,3-d]pyrimidine scaffolds

Utilizing the Knoevenagel-Michael tandem cyclocondensation reaction of barbituric acid/1,3-dimethylbarbituric acid, malononitrile, and aryl aldehydes, a sustainable methodology for the photosynthesis of pyrano[2,3-d]pyrimidine scaffolds has been devised. The present study expounds on the development of a green radical synthetic approach toward this class of compounds. In this study, a novel halogenated dicyanobenzene-based photosensitizer was utilized in an aqueous solution, exposed to air at room temperature, and activated by a blue LED as a renewable energy source for the purpose of generating energy. The primary aim of this endeavor is to employ a recently developed, easily obtainable, and affordably priced halogenated cyanoarene-based donor-acceptor (D-A). The 3DPAFIPN [2,4,6-tris(diphenylamino)-5-fluoroisophthalonitrile]} photocatalyst, as a thermally activated delayed fluorescence (TADF), is capable of inducing single electron transfer (SET) upon irradiation with visible light, thereby offering a facile and efficient approach with a high degree of effectiveness, energy efficiency, and eco-friendliness. The aforementioned phenomenon facilitates the exploration of the temporal changes that have occurred in the interactions between the surroundings and chemical constituents. The present study aimed to investigate the turnover number (TON) and turnover frequency (TOF) for pyrano[2,3-d]pyrimidine scaffolds. Additionally, it has been demonstrated that gram-scale cyclization is a viable method for utilization in industrial applications.

Redox-neutral carbon-heteroatom bond formation under photoredox catalysis

Recently, visible-light-mediated photoredox catalysis has been emerging as one of the fastest growing fields in organic chemistry because of its low cost, easy availability and environmental benignness. In the past five years, a new yet challenging trend, visible-light-induced redox-neutral carbon-heteroatom bond formation reaction involving presumed radical intermediates, has been flourishing rapidly. Although mostly transition metal-based photoredox catalysts were reported, a few organophotoredox catalysts have also shown efficacy towards carbon-heteroatom bond formation reactions. This review intends to summarize the recent research progress in redox-neutral carbon-heteroatom bond formations based on active intermediate(s) involved under photoredox catalysis.

P(III)-Promoted Reductive Coupling of Aromatic and Aliphatic Nitro Compounds with Grignard Reagents

A phosphine-promoted reductive coupling of nitro compounds with Grignard reagents is described. Polyfunctional and pharmaceutically relevant diarylamines were generated by this reaction in moderate to high yields. Aliphatic nitro compounds that are highly challenging substrates undergo a combination of α-arylation and reductive coupling to afford the α-arylated arylamines efficiently. A series of valuable biaryl compounds with polyfluorinated and heteroaryl rings are co-generated in 56-94% yields.

Regioselective C(sp2)-H imidation of arenes by redox neutral visible-light photocatalysis

We report herein a redox neutral visible light-induced regioselective C(sp²)-H imidation of electron-rich arenes and heteroarenes using conceptually designed redox-active 1 as a source of the N-centered imidyl radical. Structurally diverse aromatic imides were obtained in moderate to good yields. This methodology has been successfully employed for the late stage imidation of complex molecules and has also been applied towards the formal total synthesis of the marine natural products carpatamides A, B and D. It has further been shown that the generated imides can easily be converted to the corresponding anilines in situ directly.

Gem-Difluoroallylation of Aryl Sulfonium Salts

The unprecedented photochemical late-stage defluorinative gem-difluoroallylation of aryl sulfonium salts, which are formed site-selectively by direct C(sp²)─H functionalization, is herein disclosed. This method is distinguished by its mild reaction conditions, wide scope, and excellent site-selectivity. As showcase examples, a Flurbiprofen and Pyriproxyfen derivatives could be late stage C(sp²)─H gem-difluoroallylated with high yields. Experimental and computational investigations were conducted.

Redox-neutral and metal-free synthesis of 3-(arylmethyl)chroman-4-ones via visible-light-driven alkene acylarylation

A metal- and aldehyde-free visible-light-driven photoredox-neutral alkene acylarylation with readily available cyanoarenes is described. A variety of 3-(arylmethyl)chroman-4-ones (i.e., homoisoflavonoids) and analogs are efficiently synthesized with good functional group tolerance. This mild protocol relies on a phosphoranyl radical-mediated acyl radical-initiated cyclization and selective radical-radical coupling sequence, and is also further highlighted by subsequent derivatization to chromone and 2H-chromene as well as its application in the three-component alkene acylarylation.

C(sp3)-H/C(sp3)-H Dehydrogenative Radical Coupling of Glycine Derivatives

Here we report a general C(sp³)-H/C(sp³)-H dehydrogenative coupling strategy for the preparation of various natural or unnatural amino acids from readily available glycine derivatives and hydrocarbons through a combination of SET and HAT process.

Transition-Metal-Free, Site-Selective C-F Arylation of Polyfluoroarenes via Electrophotocatalysis

Direct C_Ar-F arylation is effective and sustainable for synthesis of polyfluorobiaryls with different degrees of fluorination, which are important motifs in medical and material chemistry. However, with no aid of transition metals, the engagement of C_Ar-F bond activation has proved difficult. Herein, an unprecedented transition-metal-free strategy is reported for site-selective C_Ar-F arylation of polyfluoroarenes with simple (het)arenes. By merging N,N-bis(2,6-diisopropylphenyl)perylene-3,4,9,10-bis(dicarboximide)-catalyzed electrophotocatalytic reduction and anodic nitroxyl radical oxidation in an electrophotocatalytic cell, various polyfluoroaromatics (2F-6F and 8F), especially inactive partially fluorinated aromatics, undergo sacrificial-reagents-free C-F bond arylation with high regioselectivity, and the yields are comparable to those for reported transition-metal catalysis. This atom- and step-economic protocol features a paired electrocatalysis with organic mediators in both cathodic and anodic processes. The broad substrate scope and good functional-group compatibility highlight the merits of this operationally simple strategy. Moreover, the easy gram-scale synthesis and late-stage functionalization collectively advocate for the practical value, which would promote the vigorous development of fluorine chemistry.

Photochemistry Journey to Multielectron and Multiproton Chemical Transformation

The odyssey of photochemistry is accompanied by the journey to manipulate "electrons" and "protons" in time, in space, and in energy. Over the past decades, single-electron (1e^-) photochemical transformations have brought marvelous achievements. However, as each photon absorption typically generates only one exciton pair, it is exponentially challenging to accomplish multielectron and proton photochemical transformations. The multistep differences in thermodynamics and kinetics urgently require us to optimize light harvesting, expedite consecutive electron transfer, manipulate the interaction of catalysts with substrates, and coordinate proton transfer kinetics to furnish selective bond formations. Tandem catalysis enables orchestrating different photochemical events and catalytic transformations from subpicoseconds to seconds, which facilitates multielectron redox chemistries and brings consecutive, value-added reactivities. Joint efforts in molecular and material design, mechanistic understanding, and theoretical modeling will bring multielectron and proton synthetic opportunities for fuels, fertilizers, and chemicals with enhanced versatility, efficiency, selectivity, and scalability, thus taking better advantage of photons (i.e., sunlight) for our sustainable society.

Direct C(sp)-H/Si-H Cross-Coupling via Copper Salts Photocatalysis

Reported herein is the first example of C(sp)-H/Si-H cross-coupling by photocatalysis. In terms of cheap and readily available starting materials, a series of alkynylsilanes are prepared in good to excellent yields upon visible-light irradiation of CuCl and alkynes with silane. The large scale reaction with flow chemistry and late-stage functionalization of natural products shows the potential of the transformation in practical organic synthesis of the alkynylsilanes intermediates.

Identification of the Structure of Triethanolamine Oxygenation Products in Carbon Nitride Photocatalysis

Triethanolamine (TEOA) is one of the most commonly used sacrificial agents in photocatalysis. Due to its more complex structure compared to, for example, ethanol, and its sacrificial role in photocatalysis, it gives a mixture of products. The structures of these molecules are not usually analyzed. Herein, we obtain and isolate the products of TEOA and N-tert-butyl diethanolamine oxygenation under photocatalytic conditions with ≈15 % yield, and followingly characterized them by NMR and mass spectroscopy. The reaction is mediated by potassium poly(heptazine imide) (K-PHI) in the presence of O2 and affords formyl esters of β-hydroxyethylene formamides from the corresponding ethanolamines.

C-H chlorination of (hetero)anilines via photo/organo co-catalysis

Chlorinated (hetero)anilines are a class of important structural motifs that are widely present in synthetic building blocks and pharmaceuticals. Despite recent advancements, direct aniline chlorination still suffers from ortho/para and mono/poly chlorination selectivity problems. Herein, we disclose a photo-redox and organo co-catalyzed chlorination method for anilines. This method has great substrate generality and excellent mono-chlorination selectivity. Another merit of this method is the late-stage modification of drug molecules, which would be useful in medicinal chemistry.

Visible-Light-Catalyzed Radical-Radical Cross-Coupling Reaction of Benzyl Trifluoroborates and Carbonyl Compounds to Sterically Hindered Alcohols

We report here an organic dye catalyzed direct radical-radical cross-coupling reaction based on the persistent free-radical effect (PRE), which is powered by visible light and does not require any external oxidants or reductants. In this reaction, benzyl trifluoroborates are oxidized by excited-state 4Cz-IPN to generate benzyl radicals, and the resulting boron trifluoride acts as a Lewis acid to reduce the reduction potential of carbonyl compounds. The dual roles of benzyl trifluoroborates enable aldehydes, ketones, diketones, and ketone esters to react with benzyl trifluoroborates to generate various sterically hindered alcohols.

A general electron donor-acceptor complex for photoactivation of arenes via thianthrenation

General photoactivation of electron donor-acceptor (EDA) complexes between arylsulfonium salts and 1,4-diazabicyclo[2.2.2]octane with visible light or natural sunlight was discovered. This practical and efficient mode enables the production of aryl radicals under mild conditions, providing an unrealized opportunity for two-step para-selective C-H functionalization of complex arenes. The novel mode for generating aryl radicals via an EDA complex was well supported by UV-vis absorbance measurements, nuclear magnetic resonance titration experiments, and density functional theory (DFT) calculations. The method was applied to the regio- and stereo-selective arylation of various N-heterocycles under mild conditions, yielding an assembly of challengingly linked heteroaryl-(hetero)aryl products. Remarkably, the meaningful couplings of bioactive molecules with structurally complex drugs or agricultural pharmaceuticals were achieved to display favorable in vitro antitumor activities, which will be of great value in academia or industry.

Visible-Light-Induced α,γ-C(sp3)-H Difunctionalization of Piperidines

Herein, we describe a novel protocol for visible-light-induced α,γ-C(sp³)-H difunctionalization of piperidines. This redox-neutral, atom-economical protocol, which exhibits a broad substrate scope and good functional group compatibility, constitutes a concise, practical method for constructing piperidine-containing bridged-ring molecules. Preliminary mechanistic studies indicated that highly regioselective activation of the inert γ-C(sp³)-H bond of piperidines was achieved through a 1,5-hydrogen atom transfer reaction of a nitrogen radical generated in situ.

Hydropyridylation of α,β-Unsaturated Esters through Electroreduction of 4-Cyanopyridine

A mild and highly efficient method for the hydropyridylation of α,β-unsaturated esters has been developed. This protocol provides the products smoothly with a wide substrate scope in an undivided cell under ambient conditions. Moreover, studies showed that the scope could be extended to other unsaturated compounds, including enones and aldehydes.

Hydrogen bond serving as a protecting group to enable the photocatalytic [2+2] cycloaddition of redox-active aliphatic-amine-containing indole derivatives

Redox-sensitive functionalities such as aliphatic amines with low oxidation potentials and easily oxidized by photocatalysts are generally not compatible with photocatalytic reactions. We describe a hydrogen-bond-assisted visible-light-mediated [2+2] cycloaddition of redox-sensitive aliphatic-amine-containing indole derivatives providing a range of cyclobutane-fused polycyclic indoline derivatives, especially bridged-cyclic indolines. Mechanistic studies indicated that the success of the reaction was based on on the formation of H-bonds between the N-atom and alcohol proton of TFE or HFIP, with this formation preventing or blocking the single-electron transfer from the aliphatic amine functionality to the excited photocatalyst.

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.

Strategies to Generate Nitrogen-centered Radicals That May Rely on Photoredox Catalysis: Development in Reaction Methodology and Applications in Organic Synthesis

For more than 70 years, nitrogen-centered radicals have been recognized as potent synthetic intermediates. This review is a survey designed for use by chemists engaged in target-oriented synthesis. This review summarizes the recent paradigm shift in access to and application of N-centered radicals enabled by visible-light photocatalysis. This shift broadens and streamlines approaches to many small molecules because visible-light photocatalysis conditions are mild. Explicit attention is paid to innovative advances in N-X bonds as radical precursors, where X = Cl, N, S, O, and H. For clarity, key mechanistic data is noted, where available. Synthetic applications and limitations are summarized to illuminate the tremendous utility of photocatalytically generated nitrogen-centered radicals.

Ruthenium-catalyzed reaction of diazoquinones with arylamines to synthesize diarylamines

The diarylamine scaffold is common in bioactive molecules. Herein, we report a Ru(ii)-catalyzed C–N cross-coupling reaction of diazoquinones with arylamines, which provides access to a range of functionalized diarylamines in 43–97% yields.