Detection of Fleeting Amine Radical Cations and Elucidation of Chain Processes in Visible-Light-Mediated [3 + 2] Annulation by Online Mass Spectrometric Techniques

Visible-light-mediated photoredox reactions have recently emerged as a powerful means for organic synthesis and thus have generated significant interest from the organic chemistry community. Although the mechanisms of these reactions have been probed by a number of techniques such as NMR, fluorescence quenching, and laser flash photolysis and various degrees of success has been achieved, mechanistic ambiguity still exists (for instance, the involvement of the chain mechanism is still under debate) because of the lack of structural information about the proposed and short-lived intermediates. Herein, we present the detection of transient amine radical cations involved in the intermolecular [3 + 2] annulation reaction of N-cyclopropylaniline (CPA, 1) and styrene 2 by electrospray ionization mass spectrometry (ESI-MS) in combination with online laser irradiation of the reaction mixture. In particular, the reactive CPA radical cation 1^+•, the reduced photocatalyst Ru(I)(bpz)₃⁺, and the [3 + 2] annulation product radical cation 3^+• are all successfully detected and confirmed by high-resolution MS. More importantly, the post-irradiation reaction with an additional substrate, isotope-labeled CPA, following photolysis of 1, 2, and Ru catalyst provides strong evidence to support the chain mechanism in the [3 + 2] annulation reaction. Furthermore, the key step of the proposed chain reaction, the oxidation of CPA 1 to amine radical cation 1^+• by product radical cation 3^+• (generated using online electrochemical oxidation of 3), is successfully established. Additionally, the coupling of ESI-MS with online laser irradiation has been successfully applied to probe the photostability of photocatalysts.

Bodipy-Anthracene Dyads as Triplet Photosensitizers: Effect of Chromophore Orientation on Triplet-State Formation Efficiency and Application in Triplet-Triplet Annihilation Upconversion

Bodipy-anthracene dyads with two chromophores assuming orthogonal geometry to enhance the spin-orbital charge-transfer intersystem crossing (SOCT-ISC) were prepared. The photosensitizers show strong absorption of visible light, efficient triplet-state formation (quantum yield 90%), and a long-lived triplet state (85 μs). The dipole moment orientation exerts significant effect on the ISC efficiency. It is also the first time that photosensitizers based on SOCT-ISC were used for triplet-triplet annihilation upconversion. The upconversion quantum yield is up to 15.8%.

A Conjugated Microporous Polymer for Palladium-Free, Visible Light-Promoted Photocatalytic Stille-Type Coupling Reactions

The Stille coupling reaction is a versatile method to mainly form aromatic C-C bonds. However, up to now, the use of palladium catalysts is necessary. Here, a palladium-free and photocatalytic Stille-type coupling reaction of aryl iodides and aryl stannanes catalyzing a conjugated microporous polymer-based phototcatalyst under visible light irradiation at room temperature is reported. The novel coupling reaction mechanism occurs between the photogenerated aryl radical under oxidative destannylation of the aryl stannane, and the electron-activated aryl iodide, resulting into the aromatic C-C bond formation reaction. The visible light-promoted Stille-type coupling reaction using the polymer-based pure organic photocatalyst offers a simple, sustainable, and more economic synthetic pathway toward palladium-free aromatic C-C bond formation.

Metal-free di- and tri-fluoromethylation of alkenes realized by visible-light-induced perylene photoredox catalysis

Regioselective amino-difluoromethylation of aromatic alkenes via C(sp³)-CF₂H and C(sp³)-N bond formation with the C 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 C moiety has been achieved in a single operation by visible-light photoredox catalysis. The combination of a shelf-stable and easy-to-handle sulfonium salt, S-difluoromethyl-S-di(p-xylyl)sulfonium tetrafluoroborate, and perylene catalysis is the key to the successful transformation. Furthermore, this noble metal-free protocol allows for the photocatalytic trifluoromethylation of alkenes.

Exploring the Reducing Ability of Organic Dye (Acr+-Mes) for Fluorination and Oxidation of Benzylic C(sp3)-H Bonds under Visible Light Irradiation

The excellent oxidizing capability of acridinium-based organic dye (Acr⁺-Mes) is fully studied in photoredox catalysis. However, its reducing ability is always considered weak for organic transformation. The reducing ability of Acr⁺-Mes is developed by Selectfluor to achieve effective fluorination and oxidation of benzylic C(sp³)-H bonds under visible light irradiation, which is not available for the direct use of oxidizing ability of excited Acr⁺-Mes. Mechanistic insights provided strong evidence for the oxidative quenching of Acr⁺-Mes.

Mild, visible light-mediated decarboxylation of aryl carboxylic acids to access aryl radicals

Herein we present the first example of aryl radical formation via the visible light-mediated decarboxylation of aryl carboxylic acids using photoredox catalysis.

Herein we present the first example of aryl radical formation via the visible light-mediated decarboxylation of aryl carboxylic acids using photoredox catalysis. This method constitutes a mild protocol for the decarboxylation of cheap and abundant aryl carboxylic acids and tolerates both electron-rich substrates and those lacking ortho-substitution. The in situ formation of an acyl hypobromite is proposed to prevent unproductive hydrogen atom abstraction and trapping of the intermediate aroyloxy radical, enabling mild decarboxylation.

Preparation of visible-light-activated metal complexes and their use in photoredox/nickel dual catalysis

Visible-light-activated photoredox catalysts provide synthetic chemists with the unprecedented capability to harness reactive radicals through discrete, single-electron transfer (SET) events. This protocol describes the synthesis of two transition metal complexes, [Ir{dF(CF₃)₂ppy}₂(bpy)]PF₆ (1a) and [Ru(bpy)₃](PF₆)₂ (2a), that are activated by visible light. These photoredox catalysts are SET agents that can be used to facilitate transformations ranging from proton-coupled electron-transfer-mediated cyclizations to C-C bond constructions, dehalogenations, and H-atom abstractions. These photocatalysts have been used in the synthesis of medicinally relevant compounds for drug discovery, as well as the degradation of biological polymers to access fine chemicals. These catalysts are prepared from IrCl₃ and RuCl₃, respectively, in three chemical steps. These steps can be described as a series of two ligand modifications followed by an anion metathesis. Using the cost-effective, scalable procedures described here, the ruthenium-based photocatalyst 2a can be synthesized in a 78% overall yield (∼8.1 g), and the iridium-based photocatalyst 1a can be prepared in a 56% overall yield (∼4.4 g). The total time necessary for the complete protocols ranges from ∼2 d for 2a to 5-7 d for 1a. Procedures for applying each catalyst in representative photoredox/Ni cross-coupling to form C_sp3-C_sp2 bonds using the appropriate radical precursor-organotrifluoroborates with 1a and bis(catecholato)alkylsilicates with 2a-are described. In addition, more traditional photoredox-mediated transformations are included as diagnostic tests for catalytic activity.

Asymmetric radical alkylation of N-sulfinimines under visible light photocatalytic conditions

In this communication, a new photocatalytic strategy for the addition of alkyl-radical derivatives to N-sulfinimines with complete diastereoselectivity and moderate to good yields is presented.

Photoredox-catalysed chloro-, bromo- and trifluoromethylthio-trifluoromethylation of unactivated alkenes with sodium triflinate

A convenient and practical protocol is developed for radical vicinal difunctionalization of unactivated alkenes under transition-metal-free conditions.

Enzyme and photoredox sequential catalysis for the synthesis of 1,3-oxazine derivatives in one pot

A novel strategy combining visible-light and enzyme catalysis for the synthesis of 1,3-oxazine derivatives is described for the first time.

Transition-metal-free radical fluoroalkylation of isocyanides for the synthesis of tri-/di-/monofluoromethylated phenanthridines

A convenient and scalable protocol is developed for the radical fluoroalkylation of 2-isocyanobiphenyls under transition-metal-free conditions.

Visible-light-mediated radical cascade reaction: synthesis of 3-bromocoumarins from alkynoates

The development of a visible-light mediated bromo radical addition/spirocyclization/ester migration cascade reaction to generate 3-bromocoumarins from alkynoates is reported.

Visible-light-mediated radical insertion/cyclization cascade reaction: synthesis of phenanthridines and isoquinolines from isocyanides

The development of a visible-light promoted radical insertion/cyclization cascade reaction to generate phenanthridines and isoquinolines from isocyanides under mild reaction conditions is reported.

Photoredox-catalysed redox-neutral trifluoromethylation of vinyl azides for the synthesis of α-trifluoromethylated ketones

A redox-neutral, mild, and simple protocol is developed for the synthesis of α-trifluoromethylated ketones from vinyl azides upon visible-light irradiation.

Visible-light-mediated C2-amination of thiophenes by using DDQ as an organophotocatalyst

A direct C–H amination of thiophenes was presented via an oxidation pathway under visible-light irradiation, in which the thiophene radical cation serves as the key intermediate. Various thiophenes and azoles could be transformed into the corresponding amination products well, and H₂O was the only byproduct, which is environmentally benign.

Au-Pd alloy nanoparticles supported on layered double hydroxide for heterogeneously catalyzed aerobic oxidative dehydrogenation of cyclohexanols and cyclohexanones to phenols

Phenol, an important industrial chemical, is widely produced using the well-developed cumene process. However, demand for the development of a novel alternative method for synthesizing phenol from benzene has been increasing. Herein, we report a novel system for the synthesis of phenols through aerobic oxidative dehydrogenation of cyclohexanols and cyclohexanones, including ketone-alcohol (KA) oil, catalyzed by Mg-Al-layered double hydroxide (LDH)-supported Au-Pd alloy nanoparticles (Au-Pd/LDH). Alloying of Au and Pd and basicity of LDH are key factors in achieving the present transformation. Although monometallic Au/LDH, Pd/LDH, and their physical mixture showed almost no catalytic activity, Au-Pd/LDH exhibited markedly high catalytic activity for the dehydrogenative phenol production. Mechanistic studies showed that β-H elimination from Pd-enolate species is accelerated by Au species, likely via electronic ligand effects. Moreover, the effect of supports was critical; despite the high catalytic performance of Au-Pd/LDH, Au-Pd bimetallic nanoparticles supported on Al2O3, TiO2, MgO, and CeO2 were ineffective. Thus, the basicity of LDH plays a deterministic role in the present dehydrogenation possibly through its assistance in the deprotonation steps. The synthetic scope of the Au-Pd/LDH-catalyzed system was very broad; various substituted cyclohexanols and cyclohexanones were efficiently converted into the corresponding phenols, and N-substituted anilines were synthesized from cyclohexanones and amines. In addition, the observed catalysis was truly heterogeneous, and Au-Pd/LDH could be reused without substantial loss of its high performance. The present transformation is scalable, utilizes O2 in air as the terminal oxidant, and generates water as the only by-product, highlighting the potential practical utility and environmentally benign nature of the present transformation. Dehydrogenative aromatization of cyclohexanols proceeds through (1) oxidation of cyclohexanols to cyclohexanones; (2) dehydrogenation of cyclohexanones to cyclohexenones; and (3) disproportionation of cyclohexenones to afford the desired phenols. In the present Au-Pd/LDH-catalyzed transformation, the oxidation of the Pd-H species is included in the rate-determining step.

Transition-Metal-Free Decarboxylative Photoredox Coupling of Carboxylic Acids and Alcohols with Aromatic Nitriles

A transition-metal-free protocol for the redox-neutral light-induced decarboxylative coupling of carboxylic acids with (hetero)aromatic nitriles at ambient temperature is presented. A broad scope of acids and nitriles is accepted, and alcohols can be coupled in a similar fashion through their oxalate half esters. Various inexpensive sources of UV light and even sunlight can be used to achieve this C-C bond formation proceeding through a free radical mechanism.

Photoinduced Charge-Transfer State of 4-Carbazolyl-3-(trifluoromethyl)benzoic Acid: Photophysical Property and Application to Reduction of Carbon-Halogen Bonds as a Sensitizer

The photoinduced persistent intramolecular charge-transfer state of 4-carbazolyl-3-(trifluoromethyl)benzoic acid was confirmed. It showed a higher catalytic activity in terms of yield and selectivity in the photochemical reduction of alkyl halides compared to the parent carbazole. Even unactivated primary alkyl bromides could be reduced by this photocatalyst. The high catalytic activity is rationalized by considering the slower backward single-electron transfer owing to the spatial separation of the donor and acceptor subunits.

Combination of organotrifluoroborates with photoredox catalysis marking a new phase in organic radical chemistry

Combination of organotrifluoroborates and visible-light-driven photoredox catalysis, both of which have attracted the attention of synthetic chemists, marks a new phase in the field of organic radical chemistry. We have developed photoredox-catalyzed radical reactions with organotrifluoroborates, which turn out to serve not only as a source of organic radicals but also as radical acceptors. The first part of this Perspective deals with the generation of organic radicals from organotrifluoroborates, and the latter part describes addition of the CF3 radical to alkenyltrifluoroborates. The good chemistry between organoborates and photoredox catalysis and its future will be discussed.

Catalytic Access to Alkyl Bromides, Chlorides and Iodides via Visible Light-Promoted Decarboxylative Halogenation

Herein is reported the catalytic, visible light-promoted, decarboxylative halogenation (bromination, chlorination, and iodination) of aliphatic carboxylic acids. This operationally-simple reaction tolerates a range of functional groups, proceeds at room temperature, and is redox neutral. By employing an iridium photocatalyst in concert with a halogen atom source, the use of stoichiometric metals such as silver, mercury, thallium, and lead can be circumvented. This reaction grants access to valuable synthetic building blocks from the large pool of cheap, readily available carboxylic acids.

Intramolecular Interaction, Photoisomerization, and Mechanical C-C Bond Dissociation of 1,2-Di(9-anthryl)benzene and Its Photoisomer: A Fundamental Moiety of Anthracene-Based π-Cluster Molecules

We report variable and unique properties of 1,2-di(9-anthryl)benzene 1 as a fundamental moiety of anthracene-based π-cluster molecules. Due to a through-space π-conjugation between anthracene units, excimer emission at room temperature and charge delocalized state in radical cation state of 1 could be observed. Photoirradiation to 1 afforded an intramolecular [4 + 4] cyclized anthracene dimer 1' having a high strain energy with long C-C bond that exceeded 1.68 Å, resulting in C-C bond dissociation by simple mechanical grinding.

Applications of Continuous-Flow Photochemistry in Organic Synthesis, Material Science, and Water Treatment

Continuous-flow photochemistry in microreactors receives a lot of attention from researchers in academia and industry as this technology provides reduced reaction times, higher selectivities, straightforward scalability, and the possibility to safely use hazardous intermediates and gaseous reactants. In this review, an up-to-date overview is given of photochemical transformations in continuous-flow reactors, including applications in organic synthesis, material science, and water treatment. In addition, the advantages of continuous-flow photochemistry are pointed out and a thorough comparison with batch processing is presented.

Visible Light-Promoted Decarboxylative Di- and Trifluoromethylthiolation of Alkyl Carboxylic Acids

Described herein is a new and straightforward decarboxylative di- and trifluoromethylthiolation of alkyl carboxylic acids promoted by visible light. This approach enables the synthesis of biologically relevant alkyl SCF2H and SCF3 compounds from cheap and abundant carboxylic acids. The method is operationally simple, using irradiation from household light sources, and its mild reaction conditions make it tolerant of a range of functional groups. The strategy employs electrophilic phthalimide-derived di- and trifluoromethylthiolation reagents and exploits the ability of the imidyl radical to carry a radical chain.

Visible-Light-Promoted Trifluoromethylthiolation of Styrenes by Dual Photoredox/Halide Catalysis

Herein, we report a new visible-light-promoted strategy to access radical trifluoromethylthiolation reactions by combining halide and photoredox catalysis. This approach allows for the synthesis of vinyl-SCF3 compounds of relevance in pharmaceutical chemistry directly from alkenes under mild conditions with irradiation from household light sources. Furthermore, alkyl-SCF3-containing cyclic ketone and oxindole derivatives can be accessed by radical-polar crossover semi-pinacol and cyclization processes. Inexpensive halide salts play a crucial role in activating the trifluoromethylthiolating reagent towards photoredox catalysis and aid the formation of the SCF3 radical.