Catalytic Photooxidation of 4-Methoxybenzyl Alcohol with a Flavin-Zinc(II)-Cyclen Complex

Aerobic oxidative synthesis of benzimidazoles by flavin photocatalysis

Flavin photocatalysis were utilised for an aerobic oxidative reaction between arylamines and o-phenylenediamine. This metal-free reaction proceeded in methanol under visible light irradiation and consumed only atmospheric molecular oxygen, providing a novel eco-friendly method for the synthesis of benzimidazoles.

Chiral Supramolecular Organogel Constructed Using Riboflavin and Melamine: Its Application in Photo-Catalyzed Colorimetric Chiral Sensing and Enantioselective Adsorption

The synthesis of a chiral supramolecular organogel via the hierarchical helical self-assembly of optically active riboflavin and melamine derivatives is described herein. Owing to the photocatalysis of riboflavin and the supramolecular chirality induced in the helically stacked riboflavin/melamine complex, the gel is observed to act as a light-stimulated chiral sensor of optically active alcohols by detecting the change in color from yellow to green. The gel also served as an efficient chiral adsorbent, enabling optical resolution of a racemic compound with high chiral recognition ability.

Riboflavin Photocatalyzed Dearomative Spiro-Etherification of Naphthols

The dearomative spiro-etherification of naphthols is achieved using catalytic amounts of riboflavin tetracetate (RFTA) as a photosensitizer and molecular oxygen as a terminal oxidizing agent under blue light (440 nm) irradiation in the presence of acid. The presence of acid increases the photooxidation power of RFTA and facilitates the dearomatization reaction.

Atom-Economical Syntheses of Dihydropyrroles Using Flavin-Iodine-Catalyzed Aerobic Multistep and Multicomponent Reactions

Herein, we report facile, atom-economical syntheses of multisubstituted 2,3-dihydropyrroles using flavin-iodine-catalyzed aerobic oxidative multistep transformations of chalcones with β-enamine ketones or 1,3-dicarbonyl compounds and amines. Exploiting coupled flavin-iodine catalysis, the multistep reaction, including C-C and C-N bond formation, is promoted only by the consumption of O₂ (1 atm), thus allowing aerobic oxidative synthesis that generates green H₂O as the only waste.

Aerobic cross-dehydrogenative coupling of toluenes and o-phenylenediamines by flavin photocatalysis for the facile synthesis of benzimidazoles

Herein, we demonstrate a green atom-economical synthesis of benzimidazoles via the flavin-photocatalysed aerobic oxidative cross-dehydrogenative coupling of toluenes and o-phenylenediamines. The proposed metal-free reaction proceeds in methanol/H₂O under visible light irradiation by consuming only molecular oxygen from atmospheric air and produces only water as waste.

Boosting Sulfides Photooxidation by Fusing Naphthalimide and Flavin together

Efficient and selective photocatalytic conversion of chemicals with visible light and naturally abundant resources has long been desired, but this requires finely designed sensitizers that are capable to convert light...

Chemical catalyst-promoted photooxygenation of amyloid proteins

Misfolded proteins produce aberrant fibrillar aggregates, called amyloids, which contain cross-β-sheet higher order structures. The species generated in the aggregation process (i.e., oligomers, protofibrils, and fibrils) are cytotoxic and can cause various diseases. Interfering with the amyloid formation of proteins could be a drug development target for treating diseases caused by aberrant protein aggregation. In this review, we introduce a variety of chemical catalysts that oxygenate amyloid proteins under light irradiation using molecular oxygen as the oxygen atom donor (i.e., photooxygenation catalysts). Catalytic photooxygenation strongly inhibits the aggregation of amyloid proteins due to covalent installation of hydrophilic oxygen atoms and attenuates the neurotoxicity of the amyloid proteins. Recent in vivo studies in disease model animals using photooxygenation catalysts showed promising therapeutic effects, such as memory improvement and lifespan extension. Moreover, photooxygenation catalysts with new modes of action, including interference with the propagation of amyloid core seeds and enhancement in the metabolic clearance of amyloids in the brain, have begun to be identified. Manipulation of catalytic photooxygenation with secured amyloid selectivity is indispensable for minimizing the side effects in clinical application. Here we describe several strategies for designing catalysts that selectively photooxygenate amyloids without reacting with other non-amyloid biomolecules.

A flavin-inspired covalent organic framework for photocatalytic alcohol oxidation

Covalent organic frameworks (COFs) offer a number of key properties that predestine them to be used as heterogeneous photocatalysts, including intrinsic porosity, long-range order, and light absorption. Since COFs can be constructed from a practically unlimited library of organic building blocks, these properties can be precisely tuned by choosing suitable linkers. Herein, we report the construction and use of a novel COF (FEAx-COF) photocatalyst, inspired by natural flavin cofactors. We show that the functionality of the alloxazine chromophore incorporated into the COF backbone is retained and study the effects of this heterogenization approach by comparison with similar molecular photocatalysts. We find that the integration of alloxazine chromophores into the framework significantly extends the absorption spectrum into the visible range, allowing for photocatalytic oxidation of benzylic alcohols to aldehydes even with low-energy visible light. In addition, the activity of the heterogeneous COF photocatalyst is less dependent on the chosen solvent, making it more versatile compared to molecular alloxazines. Finally, the use of oxygen as the terminal oxidant renders FEAx-COF a promising and “green” heterogeneous photocatalyst.

In this manuscript, we report the development of a novel alloxazine COF inspired by naturally occurring flavin cofactors for photoredox catalysis.

Molecular Editing of Flavins for Catalysis

The diverse activity of flavoenzymes in organic transformations has fascinated researchers for a long time. However, when applied outside an enzyme environment, the isolated flavin cofactor only shows largely reduced activity. This highlights the importance of embedding the reactive isoalloxazine core of flavins in defined surroundings. The latter include crucial non-covalent interactions with amino acid side chains or backbone as well as controlled access to reactants such as molecular oxygen. Nevertheless, molecular flavins are increasingly applied in the organic laboratory as valuable organocatalysts. Chemical modification of the parent isoalloxazine structure is of particular interest in this context in order to achieve reactivity and selectivity in transformations, which are so far only known with flavoenzymes or even unprecedented. This review aims to give a systematic overview of the reported designed flavin catalysts and highlights the impact of each structural alteration. It is intended to serve as a source of information when comparing the performance of known catalysts, but also when designing new flavins. Over the last few decades, molecular flavin catalysis has emerged from proof-of-concept reactions to increasingly sophisticated transformations. This stimulates anticipating new flavin catalyst designs for solving contemporary challenges in organic synthesis.

1 Introduction

2 N1-Modification

3 N3-Modification

4 N5-Modification

5 C6–C9-Modification

6 N10-Modification

7 Conclusion

Aerobic Oxidative C-H Azolation of Indoles and One-Pot Synthesis of Azolyl Thioindoles by Flavin-Iodine-Coupled Organocatalysis

The aerobic oxidative cross-coupling of indoles with azoles driven by flavin-iodine-coupled organocatalysis has been developed for the green synthesis of 2-(azol-1-yl)indoles. The coupled organocatalytic system enabled the one-pot three-component synthesis of 2-azolyl-3-thioindoles from indoles, azoles, and thiols in an atom-economical manner by utilizing molecular oxygen as the only sacrificial reagent.

Coal-Tar Dye-based Coordination Cages and Helicates

A strategy to implement four members of the classic coal-tar dye family, Michler's ketone, methylene blue, rhodamine B, and crystal violet, into [Pd2 L4 ] self-assemblies is introduced. Chromophores were incorporated into bis-monodentate ligands using piperazine linkers that allow to retain the auxochromic dialkyl amine functionalities required for intense colors deep in the visible spectrum. Upon palladium coordination, ligands with pyridine donors form lantern-shaped dinuclear cages while quinoline donors lead to strongly twisted [Pd2 L4 ] helicates in solution. In one case, single crystal X-ray diffraction revealed rearrangement to a [Pd3 L6 ] ring structure in the solid state. For nine examined derivatives, showing colors from yellow to deep violet, CD spectroscopy discloses different degrees of chiral induction by an enantiomerically pure guest. Ion mobility mass spectrometry allows to distinguish two binding modes. Self-assemblies based on this new ligand class promise application in chiroptical recognition, photo-redox catalysis and optical materials.

Insight into functionalized-macrocycles-guided supramolecular photocatalysis

Due to the unique characteristics of macrocycles (e.g., the ease of modification, hydrophobic cavities, and specific guest recognition), they can provide a suitable environment to realize photocatalysis via noncovalent interactions with different substrates. In this minireview, we emphasized the photochemical transformation and catalytic reactivity of different guests based on the binding with various macrocyclic hosts as well as on the role of macrocyclic-hosts-assisted hybrid materials in energy transfer. To keep the clarity of this review, the macrocycles are categorized into the most commonly used supramolecular hosts, including crown ethers, cyclodextrins, cucurbiturils, calixarenes, and pillararenes. This minireview not only summarizes the role that macrocycles play in photocatalytic reactions but also clarifies the photocatalytic mechanisms. Finally, the future research efforts and new pathways to apply macrocycles and supramolecular hybrid materials in photocatalysis are also discussed.

Phototropin-Inspired Chemoselective Synthesis of Unsymmetrical Disulfides: Aerobic Oxidative Heterocoupling of Thiols Using Flavin Photocatalysis

Inspired by the photochemical mechanism of a plant blue-light receptor, a unique flavin-based photocatalytic system was developed for the chemoselective heterocoupling of two different thiols, which enabled the facile synthesis of unsymmetrical disulfides. Owing to the redox- and photo-organocatalysis of flavin, the coupling reaction took place under mild metal-free conditions and visible light irradiation with the use of air, which is recognized as the ideal green oxidant.

Photocatalytic Oxidative Dearomatization of Orcinaldehyde Derivatives

For decades, oxidative dearomatization has been employed as a key step in the synthesis of complex molecules. Challenges in controlling the chemo- and site-selectivity of this transformation have sparked the development of a variety of specialized oxidants; however, these result in stoichiometric amounts of organic byproducts. Herein, we describe a photocatalytic method for oxidative dearomatization using molecular oxygen as the stoichiometric oxidant. This provides environmentally benign entry to highly substituted o-quinols, reactive intermediates which can be elaborated to a number of natural product families.

Encapsulation of Flavin Cofactor within a Manganese Porphyrin-Based Metal-Organic Polyhedron for Reductive Dioxygen Activation

Encapsulation of flavin mononucleotide (FMN) in a porphyrinatomanganese(III)-based cubic cage allowed the fast reduction of manganese(III) porphyrin in the presence of nicotinamide adenine dinucleotide (NADH). This supramolecular system was capable of efficiently activating dioxygen and catalyzing the oxidation of benzyl alcohol. Control experiments suggested that the close proximity between FMN and manganese(III) porphyrins forced by the host-guest interaction might benefit the electron-transfer process from the FMN cofactor to the metal centers.

Electrochemistry Broadens the Scope of Flavin Photocatalysis: Photoelectrocatalytic Oxidation of Unactivated Alcohols

Riboflavin-derived photocatalysts have been extensively studied in the context of alcohol oxidation. However, to date, the scope of this catalytic methodology has been limited to benzyl alcohols. In this work, mechanistic understanding of flavin-catalyzed oxidation reactions, in either the absence or presence of thiourea as a cocatalyst, was obtained. The mechanistic insights enabled development of an electrochemically driven photochemical oxidation of primary and secondary aliphatic alcohols using a pair of flavin and dialkylthiourea catalysts. Electrochemistry makes it possible to avoid using O2 and an oxidant and generating H2 O2 as a byproduct, both of which oxidatively degrade thiourea under the reaction conditions. This modification unlocks a new mechanistic pathway in which the oxidation of unactivated alcohols is achieved by thiyl radical mediated hydrogen-atom abstraction.

Flavinium Catalysed Photooxidation: Detection and Characterization of Elusive Peroxyflavinium Intermediates

Flavin-based catalysts are photoactive in the visible range which makes them useful in biology and chemistry. Herein, we present electrospray-ionization mass-spectrometry detection of short-lived intermediates in photooxidation of toluene catalysed by flavinium ions (Fl+ ). Previous studies have shown that photoexcited flavins react with aromates by proton-coupled electron transfer (PCET) on the microsecond time scale. For Fl+ , PCET leads to FlH.+ with the H-atom bound to the N5 position. We show that the reaction continues by coupling between FlH.+ and hydroperoxy or benzylperoxy radicals at the C4a position of FlH.+ . These results demonstrate that the N5-blocking effect reported for alkylated flavins is also active after PCET in these photocatalytic reactions. Structures of all intermediates were fully characterised by isotopic labelling and by photodissociation spectroscopy. These tools provide a new way to study reaction intermediates in the sub-second time range.

Coupling photoredox and biomimetic catalysis for the visible-light-driven oxygenation of organic compounds

Recent advances in the development of coupled photoredox systems for the oxygenation of organic compounds are reviewed.

Flavin photocatalysis

Thanks to rapid development in the last decades, flavins have been recognized as promising photoactive compounds to design new valuable synthetic methodologies based on photoredox catalysis. The review summarizes general photochemical properties of flavins as well as their early applications in transformations mediated by visible light. Special attention has been paid to the catalyst design for benzylic oxidations as well as to recent flavin applications, for example in E/Z-isomerization, [2+2] cycloaddition, cycloelimination, electrophilic chlorination and sulfide oxidation.

[Amyloid-selective Photooxygenation toward Treatment for Amyloid Diseases]

 Amyloid proteins and peptides form aggregates which lead to amyloid diseases. For example, Alzheimer's disease-related amyloid β (Aβ) forms oligomers, protofibrils, and amyloid fibrils, which exhibit neurotoxicity. Controlling the aggregation and toxicity of Aβ would be a therapeutic strategy for the treatment of Alzheimer's disease. Recently, we have investigated an artificial oxygenative modification (chemical introduction of oxygen atoms) of amyloid proteins using a photocatalyst, which attenuated the aggregation potency and toxicity of these proteins. The oxygenation of Aβ1-42 was efficiently induced using a riboflavin catalyst (1). The oxygenated Aβ was less aggregative and cytotoxic than native Aβ. The oxygenated Aβ also showed inhibitory activity against aggregation and the onset of toxicity of native Aβ. Flavin catalyst 2, bearing an Aβ-binding peptide, allowed the selective oxygenation of Aβ even in the presence of living cells, due to its Aβ-affinity. Furthermore, "On/Off" switchable photooxygenation catalysts 3 and 4, which can sense a higher-order amyloid structure (i.e., cross-β-sheet structure), were developed based on the amyloid fluorescence probe thioflavin-T. The photo-excited catalysts generated singlet oxygens to induce oxygenation when binding to the amyloid structure ("On"). In contrast, the free catalysts, without binding to the amyloid structure, produced no singlet oxygen, even if photo-excited ("Off"). This "On/Off" switchable function enabled highly Aβ-selective oxygenation. Catalyst 3 was successfully used for the selective oxygenation of other amyloid proteins and peptides. These findings suggest that amyloid-selective oxygenation could provide a versatile system in developing effective new treatments for amyloid diseases.

Solvent-free, visible-light photocatalytic alcohol oxidations applying an organic photocatalyst

A method for the solvent-free photocatalytic conversion of solid and liquid substrates was developed, using a novel rod mill apparatus. In this setup, thin liquid films are realized which is crucial for an effective photocatalytic conversion due to the low penetration depth of light in heterogeneous systems. Several benzylic alcohols were oxidized with riboflavin tetraacetate as photocatalyst under blue light irradiation of the reaction mixture. The corresponding carbonyl compounds were obtained in moderate to good yields.

Dual Catalysis Strategies in Photochemical Synthesis

The interaction between an electronically excited photocatalyst and an organic molecule can result in the genertion of a diverse array of reactive intermediates that can be manipulated in a variety of ways to result in synthetically useful bond constructions. This Review summarizes dual-catalyst strategies that have been applied to synthetic photochemistry. Mechanistically distinct modes of photocatalysis are discussed, including photoinduced electron transfer, hydrogen atom transfer, and energy transfer. We focus upon the cooperative interactions of photocatalysts with redox mediators, Lewis and Brønsted acids, organocatalysts, enzymes, and transition metal complexes.

Organized Aggregation Makes Insoluble Perylene Diimide Efficient for the Reduction of Aryl Halides via Consecutive Visible Light-Induced Electron-Transfer Processes

The consecutive photo-induced electron-transfer (conPET) process found with perylene diimide (PDI) overcomes the limitation of visible-light photocatalysis and sheds light on effective solar energy conversion. By the incorporation of PDI into a metal-organic polymer Zn-PDI, a heterogeneous approach was achieved to tackle the poor solubility and strong tendency to aggregate of PDIs that restricted the exploitation of this outstanding homogeneous process. The interplay between metal-PDI coordination and π···π stacking of the organized PDI arrays in Zn-PDI facilitates the conPET process for the visible light-driven reduction of aryl halides by stabilizing the radical-anion intermediate and catalyst-substrate interacted moiety. These synergistic effects between the PDI arrays and Zn sites further render Zn-PDI photoactivity for fundamental oxidation of benzyl alcohols and amines. The tunable and modular nature of the two-dimensional metal-organic polymers makes the catalyst-embedding strategy promising for the development of ideal photocatalysts toward the better utilization of solar energy.

Supramolecular photocatalysis: combining confinement and non-covalent interactions to control light initiated reactions

Using non-bonding interactions to control photochemical reactions requires an understanding of not only thermodynamics and kinetics of ground state and excited state processes but also the intricate interactions that dictate the dynamics within the system of interest. This review is geared towards a conceptual understanding of how one can control the reactivity and selectivity in the excited state by employing confinement and non-covalent interactions. Photochemical reactivity of organic molecules within confined containers and organized assemblies as well as organic templates that interact through H-bonding and/or cation-carbonyl/cation-π interactions is reviewed with an eye towards understanding supramolecular effects and photocatalysis.

Photocatalytic benzylic C–H bond oxidation with a flavin scandium complex

Riboflavin tetraacetate–scandium triflate (RFTH⁺–2Sc³⁺) is an efficient photocatalyst for the aerobic oxidation of alkylbenzenes and benzyl alcohols to the corresponding aldehydes or ketones.

Attenuation of the aggregation and neurotoxicity of amyloid-β peptides by catalytic photooxygenation

Alzheimer's disease (AD), a progressive severe neurodegenerative disorder, is currently incurable, despite intensive efforts worldwide. Herein, we demonstrate that catalytic oxygenation of amyloid-β peptides (Aβ) might be an effective approach to treat AD. Aβ1-42 was oxygenated under physiologically-relevant conditions (pH 7.4, 37 °C) using a riboflavin catalyst and visible light irradiation, with modifications at the Tyr(10) , His(13) , His(14) , and Met(35) residues. The oxygenated Aβ1-42 exhibited considerably lower aggregation potency and neurotoxicity compared with native Aβ. Photooxygenation of Aβ can be performed even in the presence of cells, by using a selective flavin catalyst attached to an Aβ-binding peptide; the Aβ cytotoxicity was attenuated in this case as well. Furthermore, oxygenated Aβ1-42 inhibited the aggregation and cytotoxicity of native Aβ.