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This article discusses the use of photoredox catalysts in organic synthesis, specifically focusing on the activation of inert C-H bonds through hydrogen atom transfer. The authors examine the synthesis of carboxylic acid derivatives using a homemade reactor and various organic photocatalysts. They analyze the effects of different substituents on the reactivity of aldehydes and discuss the factors that influence the reaction outcome. The study also explores the photocatalytic aerobic photooxidation of benzaldehyde to benzoic acid using different organic dyes as photocatalysts, finding that 2-chloroanthraquinone under home white-light lamps resulted in efficient conversion. The study provides insights into the photocatalytic activity of anthraquinones and offers guidelines for reproducibility and further exploration of environmentally friendly methodologies. [Extracted from the article]

This article discusses a method for incorporating gem-difluoroalkene moieties into organic molecules, which have important applications in drug discovery and compound production. The authors propose a photoredox defluorosilylation reaction using silacarboxylic acids as silyl radical precursors. They demonstrate the feasibility of this method using model substrates and optimize the reaction conditions. The results show that this approach allows for the rapid synthesis of structurally diverse gem-difluoroalkenes under mild conditions. Additionally, the authors propose a radical pathway for the reaction based on their experimental results. Overall, this method provides a convenient strategy for accessing structurally diverse gem-difluoroalkenes. [Extracted from the article]

2 Triplet Ketone Catalysis via Hydrogen Atom Transfer 2.1 Triplet Ketones with Nickel Catalysis Arguably, the employment of light-excited ketones as HAT agents is the most abundant application among all types of triplet ketone catalysis. 3 Triplet Ketone Catalysis via Single-Electron Transfer As mentioned in the introduction section, the oxidative abilities of triplet ketones grant their applicability as oxidants in SET events with substrates or other reaction intermediates. Keywords: dual catalysis; triplet ketone; transition-metal catalysis; photocatalysis; photosensitizer; C-H functionalization EN dual catalysis triplet ketone transition-metal catalysis photocatalysis photosensitizer C-H functionalization 1289 1308 20 07/11/23 20230725 NES 230725 Graph 1 Introduction Over the last few decades, aromatic ketones have gained increased attention as inexpensive organic photocatalysts with highly tunable photophysical properties. In 2020, Li's group disclosed the synthesis of ketones via decarboxylative coupling of -keto acids with aryl halides by employing CFL-induced triplet ketone/Ni catalysis (Scheme 20). [Extracted from the article]

It should be mentioned that so far electrocarboxylation has been mainly explored for the carboxylation of olefins and imines, and only a few reports have been published on direct electrocatalytic C(sp SP 3 sp )-H bond carboxylation reactions. In previous studies, transition-metal or transition-metal-free methodologies proceeded through C(sp SP 3 sp )-H bond carboxylation reactions via enolate chemistry to trap a CO SB 2 sb molecule in order to give carboxylation products. Recent Advances on the Carboxylations of C(sp3)-H Bonds Using CO2 as the Carbon Source Keywords: carbon dioxide; transition-metal; transition-metal-free; thermochemical strategy; visible-light-induced photocatalysis; electrocatalysis; reductive C-H carboxylations; carboxylic acids EN carbon dioxide transition-metal transition-metal-free thermochemical strategy visible-light-induced photocatalysis electrocatalysis reductive C-H carboxylations carboxylic acids 1327 1342 16 07/11/23 20230725 NES 230725 Graph 1 Introduction The carboxylic acid unit is omnipresent in bioactive drug molecules, pharmaceuticals, natural products, and agrochemicals. [Extracted from the article]

Direct functionalization of alkynes under oxidative conditions is challenging, as alkynes are usually recalcitrant towards typical oxidants. Herein, we communicate a strategy for the divergent functionalization of alkynes with photoexcited acridinium organic dyes, presumably via the formation of vinyl cation radicals as key intermediates. Based on the nature of the nucleophiles, different types of difunctionalized products were obtained in moderate to good yields. Addition of lithium Lewis acids resulted in a surprising reversal of diastereocontrol. [ABSTRACT FROM AUTHOR]

A metal-free approach for C(sp3)–H activation followed by an intramolecular Giese reaction to construct a wide range of cyclic ether scaffolds of various ring sizes under environmentally benign and straightforward conditions is reported. An easily prepared pyrylium salt is employed as an organophotocatalyst for this visible-light-driven, highly atom-economical (PMI = 64.34 g/g for a 0.2 mmol scale), cost-effective, and chemoselective transformation. The reported method has a broad functional-group tolerance, resulting in good-quality products. Furthermore, downstream functionalizations of a product and a gram-scale synthesis (PMI = 17.41 g/g for a 10 mmol scale) are demonstrated, highlighting our method's advantages. [ABSTRACT FROM AUTHOR]

The regioselectivity of the intramolecular cyclization of bifunctional α-phenyl alkenes can be controlled simply by the choice of the organic chromophore as the photocatalyst. The central photoredox catalytic reaction in both cases is a nucleophilic addition of the hydroxy function to the olefin function of the substrates. N , N -(4-Diisobutylaminophenyl)phenothiazine catalyzes exo -trig cyclizations, whereas 1,7-dicyanoperylene-3,4,9,10-tetracarboxylic acid bisimides catalyze endo -trig additions to products with anti-Markovnikov regioselectivity. We preliminarily report the photoredox catalytic conversions of 11 representative substrates into 20 oxaheterocycles in order to demonstrate the similarity, but also the complementarity, of these two variants in this photoredox catalytic toolbox. [ABSTRACT FROM AUTHOR]

Organic photoredox catalysts with a long excited-state lifetime have emerged as promising alternatives to transition-metal-complex photocatalysts. This paper explains the effectiveness of using long-lifetime photoredox catalysts for organic transformations, focusing on the structures and photophysics that enable long excited-state lifetimes. The electrochemical potentials of the reported organic, long-lifetime photocatalysts are compiled and compared with those of the representative Ir(III)- and Ru(II)-based catalysts. This paper closes by providing recent demonstrations of the synthetic utility of the organic catalysts. 1 Introduction 2 Molecular Structure and Photophysics 3 Photoredox Catalysis Performance 4 Catalysis Mediated by Long-Lifetime Organic Photocatalysts 4.1 Photoredox Catalytic Generation of a Radical Species and its Addition to Alkenes 4.2 Photoredox Catalytic Generation of a Radical Species and its Addition to Arenes 4.3 Photoredox Catalytic Generation of a Radical Species and its Addition to Imines 4.4 Photoredox Catalytic Generation of a Radical Species and its Addition to Substrates Having C≡X Bonds (X=C, N) 4.5 Photoredox Catalytic Generation of a Radical Species and its Bond Formation with Transition Metals 4.6 Miscellaneous Reactions of Radical Species Generated by Photoredox Catalysis 5 Conclusions [ABSTRACT FROM AUTHOR]

Reduction-and-oxidation (redox) reactions are one of the most utilized approaches for the synthesis of value-added compounds. With the growing awareness of green chemistry, researchers have searched for new and sustainable pathways for performing redox reactions. From this, a new field has gained tremendous attention, namely photoredox catalysis. Here, molecules can be easily oxidized or reduced with the use of one of Nature's biggest resources: visible light. This tutorial paper gives the basics of photoredox catalysis along with limited examples to encourage further research in this blooming research area. 1 Introduction 2 Redox Chemistry 3 Photochemistry 3.1 Laws of Photochemistry 3.2 Principles 3.3 Examples 4 Photoredox Catalysis 4.1 General Principles 4.2 Classification of Redox Processes 4.3 Other Mechanistic Considerations 4.4 Stern–Volmer Plots 4.5 Photophysical Properties 4.6 Redox Potentials 5 Photocatalysts 5.1 Metal-Based Photocatalysts 5.2 Organic Dyes 5.3 Semiconductors 6 Dual Catalysis 7 Conclusions [ABSTRACT FROM AUTHOR]

N -Phenylphenothiazines are an important class of photoredox catalysts because they are synthetically well accessible, they allow the tuning of the optoelectronic properties by different substituents, and they have strong reduction properties for activation of alkenes. One of the major disadvantages of N -phenylphenothiazines, however, is the excitation at 365 nm in the UV-A light range. We synthesized three differently dialkylamino-substituted N -phenylbenzo[ b ]phenothiazines as alternative photoredox catalysts and applied them for the nucleo-philic addition of alkohols to α-methyl styrene. The additional benzene ring shift the absorbance bathochromically and allows performing the photocatalyses by excitation at 385 nm and 405 nm. This type of photoredox catalysis tolerates other functional groups, as representatively shown for alcohols as substrates with C–C and C–N triple bonds. [ABSTRACT FROM AUTHOR]

Styrene derivatives were hydroacylated with exclusive anti-Markovnikov selectivity by using neutral eosin Y as a direct hydrogen-atom-transfer (HAT) catalyst under visible-light irradiation. Aldehydes and styrenes with various substituents were tolerated (>20 examples), giving the corresponding products in moderate to high yields. The key acyl radical intermediate was generated from a direct HAT process induced by photoexcited eosin Y. Subsequent addition to styrenes and a reverse HAT process generated the ketone products. [ABSTRACT FROM AUTHOR]

Formation of phenonium ions through anchimeric assistance (neighboring-group participation) of aryl rings has been known since 1949. Although these reactive intermediates have been studied extensively by physical organic chemists, their potential as control elements in synthesis is underutilized. Presented here are our laboratory's recently reported first examples of regiodivergent openings of nonsymmetrical phenonium ions with chloride nucleophiles. The selectivity of these processes is under reagent control. The reactions are operationally simple and permit the stereospecific synthesis of complex chiral building blocks from readily accessible epoxide starting materials. 1 Introduction 2 Select Examples of Phenonium Ion Methodology 3 Regiodivergent Opening of Nonsymmetrical Phenonium Ions 4 Summary and Outlook [ABSTRACT FROM AUTHOR]

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The C–H to C–C conversion in aliphatic oxygen heterocycles (dioxolanes, 1,3-dioxane, or cyclic carbonates) by photocatalytic hydrogen atom transfer and subsequent trapping of the resulting radical with phenyl vinyl sulfone was investigated. The performance of three different photocatalysts, namely tetrabutylammonium decatungstate and the aromatic ketones thioxanthone and 9-fluorenone, was compared. The UV-light-absorbing decatungstate anion is more efficient and permits the use of a smaller excess of hydrogen donor than the aromatic ketones, although the ketones could be excited by visible light. Further intramolecular selectivity studies revealed that aromatic ketones afforded a higher proportion of functionalization at the acetalic versus the ethereal positions than did the decatungstate anion. [ABSTRACT FROM AUTHOR]

Catalyst-free photochemistry provides numerous opportunities toward sustainable synthesis because catalyst separation can usually be avoided, which is consistent with green chemistry principles. Complementary to the well-reviewed photoredox chemistry, this review specifically summarizes the synthetic applications of photochemistry without external catalysts reported since 2000. The selected examples include both natural product synthesis and new methodology development. This review is arranged based on the type of chromophore. It is our hope that this review will inspire more synthetic chemists to embrace photochemistry into their research plans. 1 Introduction 2 Photochemistry of Olefins 2.1 [2+2] Cycloaddition of Enones and Olefins 2.2 Cycloaddition of Olefins without Carbonyl Groups 2.3 Z/E Isomerization 2.4 Cyclization 2.5 Others 3 Photochemistry of C=O 3.1 The Paternò--Büchi Reaction 3.2 The Yang Photoenolization 3.3 The Norrish Type I Reaction 3.4 The Norrish Type II Reaction 3.5 Others 4 Photochemistry of Nitrogen-Containing Functional Groups 5 Photochemistry of Halogen-Containing Compounds 6 Conclusion and Outlook [ABSTRACT FROM AUTHOR]

In this Account, the authors retrace their development of remote functionalisation reactions relying on photoredox catalysis. Inextricably connected to this is their discovery of small molecule organocatalysts that behave as formal reductive activators of diazonium salts, while requiring no promotion by visible light. This ultimately paved the way for the development of an unusual class of reactions in which the catalyst is also the nucleophile. [ABSTRACT FROM AUTHOR]

Herein is disclosed a novel visible-light photocatalytic double C-H functionalization of indoles. The reaction affords 2,3-difunctionalized indoles in up to 84% yield, but the reaction rate depends strongly on electronic substituent effects. Mechanistic DFT studies and control experiments suggest that the secondary functionalization occurs through an independent photocatalytic oxidation of bromide ions formed during the reaction to generate molecular bromine capable of electrophilic C-3 bromination. [ABSTRACT FROM AUTHOR]

A range of phenols bearing multiple useful functionalities at their meta positions were prepared from the corresponding quinols under the cooperative effects of visible light irradiation, Ru(bpy)3Cl2 photosensitizer, Hünig's base, LiBF4, and MeCN solvent. The process involves visible-light-enabled photocatalytic cleavage of C--O bond as the strategic event. [ABSTRACT FROM AUTHOR]

A new approach to the preparation of tetrahydrofurans and tetrahydropyrans through a photoredox catalytic process is described. The introduction of a phenylsulfanyl auxiliary group permits the substrates to be readily oxidized to form cationic intermediates for sequential intramolecular cyclization. The method features mild reaction conditions and operational simplicity. [ABSTRACT FROM AUTHOR]

Protocol simplification is an important aspect in the development of organic reactions such as heteroarene functionalizations. An operationally facile protocol for the direct C-H alkylation of heteroarenes with unactivated ethers has been developed. The Miniscitype radical addition process involves thermolysis of potassium persulfate, circumventing the need for transition-metal catalysts. [ABSTRACT FROM AUTHOR]

A visible-light-activated enantioselective radical perfluoroalkylation of 2-acyl imidazoles with perfluoroalkyl iodides (CF3I, C3F7I, C4F9I, C6F13I, C8F17I and C10F21I) and perfluorobenzyl iodide at the α-position of the carbonyl group is reported. Enantioselectivities with up to >99.5% ee are achieved. The process uses a dual-function chiral Lewis acid/photoredox catalyst at loadings of 2-4 mol% and constitutes a redox-neutral, electron-catalyzed reaction that proceeds via intermediate perfluoroalkyl radicals. [ABSTRACT FROM AUTHOR]

Starting from a readily available, enantiomerically pure 2,6-disubstituted piperidine the synthesis of pyrido[1,2-a]azepines was accomplished. Key reactions for the ring closure were a photochemically induced acyl radical addition or a SmI2-promoted ketyl radical addition to an α,β-unsaturated ester. En route to the cyclization precursor an epoxidiation/ring opening sequence led to an undesired oxazolidinone which turned out to be useful for the configuration assignment. The compound was successfully converted into (+)-methyl dihydropalustramate. [ABSTRACT FROM AUTHOR]

A DFT investigation on the conditions required for the photochemical generation of triplet aryl cations from differently para-substituted aryl halides has been carried out. The results showed a correlation between the energy change associated with the Ar–X bond heterolytic cleavage and the substituent Hammett–Brown constant (σp+). The method provides a means to predict the viability of the ArSN1 photosubstitution reaction. [ABSTRACT FROM AUTHOR]

Quinone organocatalysis is an emerging area, and this report highlights some recently developed thermal and photocatalytic reactions, with particular emphasis on photooxygenation reactions. Further, it is discussed how the orthogonal ground- and excited-state reactivities of quinones can be utilized for the development of tandem catalytic processes. [ABSTRACT FROM AUTHOR]

A new and efficient approach for the Beckmann rearrangement is reported. The protocol involves eosin Y catalyzed, visible-light-mediated in situ formation of the Vilsmeier-Haack reagent from CBr4 and a catalytic amount of DMF for activation of ketoximes at room temperature. The method is operationally simple and avoids the need for any corrosive, water-sensitive reagents and elevated temperatures. [ABSTRACT FROM AUTHOR]

Photoredox catalysis by well-known ruthenium(II) polypyridine complexes and appropriate cyclometalated iridium(III) derivatives is a useful tool for redox reactions in synthetic chemistry, because these materials can effectively catalyze singleelectron- transfer (SET) processes by irradiation with visible light. This provides a new strategy for efficient and selective radical reactions. We have developed several photocatalytic reactions in which organic radicals are generated by photoredox catalysis and which have remarkable merits in terms of green and sustainable chemistry. 1 Introduction 2 Photoredox Cycles of [Ru(bpy)3]2+ and Cyclometalated Iridium Complexes 3 Radical Reactions that Occur Through a Reductive Quenching Cycle 3.1 Oxyamination of Enamines 3.2 Tin-Free Giese-Type Reactions Through Oxidation of Organoborates 4 Radical Reactions Through an Oxidative Quenching Cycle 4.1 Two-Electron Transfer Process Mediated by Duroquinone: Oxidative Coupling of an Enamine with a Silyl Enol Ether 4.2 Radical Trifluoromethylation of Alkenes 5 Conclusions and Outlook. [ABSTRACT FROM AUTHOR]

1-(N,N-Dimethylamino)pyrene was applied as chemical photocatalyst for two different organic reactions: Both the photocatalytically driven nucleophilic addition of methanol to 1,1-diphenylethylene and the photocatalytic deprotection of N-phenylsulfonylindole gave the corresponding products in good yields and in a highly sustainable way after short irradiation with high-power LED. This concept can potentially be transferred to other photochemical reactions. [ABSTRACT FROM AUTHOR]

The development of transition-metal-free arylation procedures represents a key goal in modern organic synthesis. A green arylation can be obtained by having recourse to light as a 'clean' reagent in place of toxic metals. In particular, a new method based upon the photoheterolytic fragmentation of substituted aryl halides or esters, with the formation of aryl cations has recently emerged. The peculiar reactivity of these cations with carbon nucleophiles allows several straightforward photo-arylations, which compare favorably with other thermal or photochemical metal-free analogues. 1 Introduction 2 Aryl Cation Chemistry 3 Metal-Free Formation of Aryl-Carbon Bonds 3.1 Formation of Aryl-Alkyl Bonds 3.1.1 Arylation of Carbonyl Compounds 3.1.2 Formation of Aryl-Allyl Bonds 3.1.3 Formation of Other Aryl-Alkyl Bonds 3.2 Formation of Aryl-Vinyl Bonds 3.3 Formation of Aryl-Alkynyl Bonds 3.4 Formation of Aryl-Aryl Bonds 3.5 Cyanation 4 Conclusions [ABSTRACT FROM AUTHOR]

A series of enamines were photocatalytically cleaved to produce amide products under simple visible-light irradiation from a 45 W household light bulb. Mechanistically, the reactions appear to involve photosensitized formation of a singlet oxygen intermediate and a subsequent [2+2] cycloaddition event. [ABSTRACT FROM AUTHOR]

The conversion of allyl esters into carboxylic acids using radical initiators has been achieved in high yields in aqueous media. [ABSTRACT FROM AUTHOR]