Photoredox Catalysis Research Articles

Homologation of Ketones: Direct Transformation of Alkyl Ketones to Aryl Ketones via Photoredox Catalyzed Deacylation-Aroylation Sequence.

Ketones, as essential functional group skeletons, have garnered significant interest due to their diverse transformations. Herein, we describe a versatile photoredox catalyzed deacylation-aroylation strategy that enables the direct transformation of alkyl ketones to aryl ketones. This process involves photoredox deacylation of dihydroquinazolinones derived from alkyl ketones to generate alkyl radicals, followed by subsequent NHC-catalyzed or NHC-mediated radical aroylation.

Substituent Effects in the Photophysical and Electrochemical Properties of Meso-Tetraphenylporphyrin Derivatives.

Porphyrins were identified some years ago as a promising, easily accessible, and tunable class of organic photoredox catalysts, but a systematic study on the effect of the electronic nature and of the position of the substituents on both the ground-state and the excited-state redox potentials of these compounds is still lacking. We prepared a set of known functionalized porphyrin derivatives containing different substituents either in one of the meso positions or at a β-pyrrole carbon, and we determined their ground- and (singlet) excited-state redox potentials. We found that while the estimated singlet excited-state energies are essentially unaffected by the introduction of substituents, the redox potentials (both in the ground- and in the singlet excited-state) depend on the electron-withdrawing or electron-donating nature of the substituents. Thus, the presence of groups with electron-withdrawing resonance effects results in an enhancement of the reduction facility of the photocatalyst, both in the ground and in the excited state. We next prepared a second set of four previously unknown meso-substituted porphyrins, having a benzoyl group at different positions. The reduction facility of the porphyrin increases with the proximity of the substituent to the porphine core, reaching a maximum when the benzoyl substituent is introduced at a meso position.

Efficient photoredox catalysis in C–C cross-coupling reactions by two-coordinated Au(I) complex

Photocatalysis provides a versatile approach to redox activation of various organic substrates for synthetic applications. To broaden the scope of photoredox catalysis, developing catalysts with strong oxidizing or reducing power in the excited state is imperative. Catalysts that feature highly cathodic oxidation potentials and long lifetimes in their excited states are particularly in demand. In this research, we demonstrate the catalytic utility of two-coordinate Au(I) complex photocatalysts that exhibit an exclusive ligand-to-ligand charge-transfer (LLCT) transition in C–C cross-coupling reactions between N-heterocycles and (hetero)aryl halides, including redox-resistant (hetero)aryl chlorides. Our photocatalysis system can steer reactions under visible-light irradiation at a catalyst loading as low as 0.1 mol% and exhibits a broad substrate scope with high chemo- and regioselectivity. Our mechanistic investigations provide direct spectroscopic evidence for each step in the catalysis cycle and demonstrate that the LLCT-active Au(I) complex catalysts offer several benefits, including strong visible-light absorption, a 210 ns-long excited-state lifetime without short-lived components, and a 91% yield in the production of free-radical intermediates. Given the wide structural versatility of the proposed catalysts, we envision that our research will provide useful insights into the future utilization of the LLCT-active Au(I) complex for organic transformations.

Direct Synthesis of 2-Hydroxytrifluoroethylacetophenones via Organophotoredox-Mediated Net-Neutral Radical/Polar Crossover.

Alkene difunctionalization is a very attractive tool in synthetic organic chemistry. Herein, we disclose an operationally and practically simple method to access 2-hydroxytrifluoroethylacetophenones from styrene derivatives via photoredox catalysis. This light-mediated transformation promotes the generation of the 1-hydroxy-2,2,2-trifluoroethyl carbon-centered radical as key synthon, which undergoes Giese addition with styrenes followed by a Kornblum oxidation process. The presented method is not only mild and cost-effective, but also utilizes an organic photocatalyst and DMSO as oxidant. Experimental investigations support the operative mechanism via net-neutral radical/polar crossover.

Access to β‐Lactam Sulfonamides from Sulfamoyl Chlorides via Photoredox Catalyzed C−S Bond Formation

AbstractSulfonyl chlorides are a synthetically attractive source of sulfonyl radicals in photoredox catalysis and are useful precursors in the synthesis of sulfones. Sulfamoyl chlorides, on the other hand, remain poorly represented despite their similar potential. In this study, N‐chlorosulfonylated β‐lactams were prepared from readily available precursors and utilized in an atom transfer radical addition (ATRA) reaction with a variety of olefins, producing β‐lactam sulfonamides in 49–95% yields. β‐Lactams fused to a dihydro‐1,2‐thiazine ring which closely resemble carbacephems, a widely used class of antibiotics, were also synthesized by an intra‐molecular ATRA reaction. This methodology enables the preparation of β‐lactam sulfonamides, a class of compounds that are of great interest in pharmacology.

Sunlight induced C-H arylation of aromatic heterocycles: First example of A3 and A2B type corroles as photoredox catalysts

The visible light mediated metal-free approach has been developed for C–H arylation and borylation employing anilines as starting material. A series of meso-aryl corroles (A3 and A2B type) were synthesized and their catalytic potential was tested in blue light and sunlight. The method includes substituted anilines as source of expensive aryl diazonium salts and utilizes only 0.5 to 1 mol% of corrole catalyst at room temperature. Corrole catalyst worked efficiently under the blue light/sunlight to produce heterobiaryls and aryl boronic esters in decent to good yields (15–80 %). Mechanistic insight was obtained from TD-DFT studies, which suggested single electron transfer (SET) from the singlet state of corrole catalyst to the aryl diazonium salt. The paper demonstrates corrole based photo-redox catalysis as a mild, green and eco-sustainable alternative for the C–H arylation reactions.

Exploring Porphyrins, Phthalocyanines and Corroles as Photocatalysts for Organic Transformations

AbstractIn recent years, macrocycles have emerged as efficient and sustainable photosensitizers for the catalysis of organic transformations. This graphical review provides a concise overview of photocatalysis and photoredox catalysis utilizing three common macrocycles: porphyrins, phthalocyanines and corroles. They exhibit strong absorption in the visible region and can be easily oxidized or reduced, making them good candidates for photocatalysis.

Identification of origin of insulating polymer maneuvered photoredox catalysis

Identification of origin of insulating polymer maneuvered photoredox catalysis

Rational Design of Fluorophores Using MO Theory: Our Journey from BODIPYs to BOIMPYs

This short review demonstrates how MO-theoretical considerations can support the tailor-made design of new dye scaffolds, specifically the recently introduced BOIMPY class of fluorophores. Starting with historical and structural foundations, the influence of canonical streptocyanines on the electronic features of diarylmethenes and rhodamines is examined and the BODIPY scaffold is introduced as the primary structural inspiration for our work. The attachment of five-membered ring heterocycles at the meso position of the BODIPY core enables a relaxation into a co-planar and twofold chelating triarylmethene system. After introduction of two electron-withdrawing BF2 units, efficient rigidity is achieved since hindered rotation prevents non-radiative dissipation of energy via excited state relaxation. Hence, a lowered LUMO level allows the combination of a large red shift with high quantum efficiencies. The synthetic approach to BOIMPYs is straightforward and analogous to BODIPY syntheses starting from benzimidazole or tetrazole carbaldehydes. Cyclic voltammetric measurements prove that BOIMPYs are able to easily accept two electrons and might act as efficient photoredox catalysts.

1,3-Hydro-di/monofluoromethylation of N,N'-Cyclic Azomethine Imines with HCF2SO2Na/H2CFSO2Na via Photocatalytic Radical Addition.

The radical 1,3-hydro-di/monofluoromethylation of N,N'-cyclic azomethine imines with HCF2SO2Na/H2CFSO2Na via photoredox catalysis is described. This reaction exhibits broad functional group compatibility, providing the desired products in good yields. However, CF3SO2Na failed to produce the trifluoromethyl product. DFT calculations revealed that the transition state activation energy for radical trifluoromethylation was significantly higher and the isotropic charge repulsion makes it difficult for the CF3 radical to transfer electrons.

A facile three-component route to powerful 5-aryldeazaalloxazine photocatalysts.

Functionalized 5-aryldeazaalloxazines have been successfully synthesised through a one-pot, three-component reaction involving N,N-dimethylbarbituric acid, an aromatic aldehyde and aniline. By utilizing readily available reagents, this approach opens up the opportunity for the efficient formation of a variety of 5-aryldeazaalloxazines bearing electron-donating or halogen groups. This practical method is characterised by atom economy and offers a direct route to the introduction of an aryl moiety into the C(5)-position of deazaalloxazines, thereby generating novel catalysts for photoredox catalysis without the need for subsequent purification. Thus, it significantly improves existing approaches.

Factors Controlling Cage Escape Yields of Closed- and Open-Shell Metal Complexes in Bimolecular Photoinduced Electron Transfer.

The cage escape yield, i.e., the separation of the geminate radical pair formed immediately after bimolecular excited-state electron transfer, was studied in 11 solvents using six Fe(III), Ru(II), and Ir(III) photosensitizers and tri-p-tolylamine as the electron donor. Among all complexes, the largest cage escape yields (0.67-1) were recorded for the Ir(III) photosensitizer, showing the highest potential as a photocatalyst in photoredox catalysis. These yields dropped to values around 0.65 for both Ru(II) photosensitizers and to values around 0.38 for the Os(II) photosensitizer. Interestingly, for both open-shell Fe(III) complexes, the yields were small (<0.1) in solvents with dielectric constant greater than 20 but were shown to reach values up to 0.58 in solvents with low dielectric constants. The results presented herein on closed-shell photosensitizers suggest that the low rate of triplet-singlet intersystem crossing within the manifold of states of the geminate radical pair implies that charge recombination toward the ground state is a spin-forbidden process, favoring large cage escape yields that are not influenced by dielectric effects. Geminate charge recombination in open-shell metal complexes, such as the two Fe(III) photosensitizers studied herein, is no longer a spin-forbidden process and becomes highly sensitive to solvent effects. Altogether, this study provides general guidelines for factors influencing bimolecular excited-state reactivity using prototypical photosensitizers but also allows one to foresee a great development of Fe(III) photosensitizers with the 2LMCT excited state in photoredox catalysis, providing that solvents with low dielectric constants are used.

Manganese Complexes with Consecutive Mn(IV) → Mn(III) Excitation for Versatile Photoredox Catalysis.

Manganese complexes stand out as promising candidates for photocatalyst design, attributed to their eco- and biocompatibility, versatile valence states, and capability for facilitating multiple electronic excitations. However, several intrinsic constraints, such as inadequate visible light response and short excited-state lifetimes, hinder effective photoinduced electron transfer and impede photoredox activation of substrates. To overcome this obstacle, we have developed a class of manganese complexes featuring boron-incorporated N-heterocyclic carbene ligands. These complexes enable prolonged excited-state durations encapsulating both Mn(IV) and Mn(III) oxidation stages, with lifetimes reaching microseconds for Mn(IV) and nanoseconds for Mn(III), concurrently exhibiting robust redox capabilities. They efficiently catalyze direct, site-selective cross-couplings between diverse arenes and aryl bromides, at a low catalyst loading of 0.5 mol %. Their proficiency spans an extensive array of substrates including both highly electron-rich and electron-deficient molecules, which underscore the superior performance of these manganese complexes in tackling intricate transformations. Furthermore, the versatility of these complexes is further highlighted by their successful applications in various photochemical transformations, encompassing reductive cross-couplings for the formation of C-P, C-B, C-S and C-Se bonds, alongside oxidative couplings for creating C-N bonds. This study sheds light on the distinctive photoredox properties and the remarkable catalytic flexibility of manganese complexes, highlighting their immense potential to drive progress in photochemical synthesis and green chemistry applications.

Stereoselective Three-Component Carbocyclization of 1,7-Enynes with Sulfoxonium Ylides and Water under Photoredox Catalysis.

A visible-light-induced photoredox three-component carbocyclization reaction of 1,7-enynes with sulfoxonium ylides and water is reported. The protocol provides a facile entry to structurally valuable highly functionalized cyclopenta[c]quinoline scaffolds in a highly chemoselective and stereoselective manner. Salient features of this method include facile redox-neutral conditions, no requirement of base or other additive, and good functional-group tolerance.

Water mediated redox-neutral cleavage of arylalkenes via photoredox catalysis

Cleavage of carbon-carbon bonds remains a challenging task in organic synthesis. Traditional methods for splitting Csp2=Csp2 bonds into two halves typically involve non-redox (metathesis) or oxidative (ozonolysis) mechanisms, limiting their synthetic potential. Disproportionative deconstruction of alkenes, which yields one reduced and one oxidized fragment, remains an unexplored area. In this study, we introduce a redox-neutral approach for deleting a Csp2 carbon unit from substituted arylalkenes, resulting in the formation of an arene (reduction) and a carbonyl product (oxidation). This transformation is believed to proceed through a mechanistic sequence involving visible-light-promoted anti-Markovnikov hydration, followed by photoredox cleavage of Csp3-Csp3 bond in the alcohol intermediate. A crucial consideration in this design is addressing the compatibility between the highly reactive oxy radical species in the latter step and the required hydrogen-atom-transfer (HAT) reagent for both steps. We found that ethyl thioglycolate serves as the optimal hydrogen-atom shuttle, offering remarkable chemoselectivity among multiple potential HAT events in this transformation. By using D2O, we successfully prepared dideuteromethylated (-CD2H) arenes with good heavy atom enrichment. This work presents a redox-neutral alternative for alkene deconstruction, with considerable potential in late-stage modification of complex molecules.

Bimetallic Single Atom/Nanoparticle Ensemble for Efficient Photochemical Cascade Synthesis of Ethylene from Methane

AbstractLight‐driven photoredox catalysis presents a promising approach for the activation and conversion of methane (CH4) into high value‐added chemicals under ambient conditions. However, the high C−H bond dissociation energy of CH4 and the absence of well‐defined C−H activation sites on catalysts significantly limit the highly efficient conversion of CH4 toward multicarbon (C2+) hydrocarbons, particularly ethylene (C2H4). Herein, we demonstrate a bimetallic design of Ag nanoparticles (NPs) and Pd single atoms (SAs) on ZnO for the cascade conversion of CH4 into C2H4 with the highest production rate compared with previous works. Mechanistic studies reveal that the synergistic effect of Ag NPs and Pd SAs, upon effecting key bond‐breaking and ‐forming events, lowers the overall energy barrier of the activation process of both CH4 and the resulting C2H6, constituting a truly synergistic catalytic system to facilitate the C2H4 generation. This work offers a novel perspective on the advancement of photocatalytic directional CH4 conversion toward high value‐added C2+ hydrocarbons through the subtle design of bimetallic cascade catalyst strategy.

Bimetallic Single Atom/Nanoparticle Ensemble for Efficient Photochemical Cascade Synthesis of Ethylene from Methane.

Light-driven photoredox catalysis presents a promising approach for the activation and conversion of methane (CH4) into high value-added chemicals under ambient conditions. However, the high C-H bond dissociation energy of CH4 and the absence of well-defined C-H activation sites on catalysts significantly limit the highly efficient conversion of CH4 toward multicarbon (C2+) hydrocarbons, particularly ethylene (C2H4). Herein, we demonstrate a bimetallic design of Ag nanoparticles (NPs) and Pd single atoms (SAs) on ZnO for the cascade conversion of CH4 into C2H4 with the highest production rate compared with previous works. Mechanistic studies reveal that the synergistic effect of Ag NPs and Pd SAs, upon effecting key bond-breaking and -forming events, lowers the overall energy barrier of the activation process of both CH4 and the resulting C2H6, constituting a truly synergistic catalytic system to facilitate the C2H4 generation. This work offers a novel perspective on the advancement of photocatalytic directional CH4 conversion toward high value-added C2+ hydrocarbons through the subtle design of bimetallic cascade catalyst strategy.

A Photoredox Thiol-yne Reaction for the Synthesis of Vinyl Sulfide-Based Coumarins and its Effect on Fluorescence Properties.

Coumarins still remain one of the most widely explored fluorescent dyes, with a broad spectrum of applications spanning various fields, such as molecular imaging, bioorganic chemistry, materials chemistry, or medical sciences. Their fluorescence is strongly based on a push-pull mechanism involving an electron-donating group (EDG), mainly located at the C7 or C8 positions of the dye core. Unfortunately, up to now, these positions have been very limited to hydroxyl or amino groups. In this study, we present in detail the synthesis of the first series of coumarins bearing a vinyl sulfide as the EDG at the C7 position. These derivatives were prepared by thiol-yne reaction, promoted by ruthenium- or porphyrin-based photoredox catalysis, enabling rapid late-stage diversification. We also functionalized coumarins with short peptides, and BSA protein as a proof-of-concept study, in a single-step process. This strategy, capable of proceeding under aqueous conditions, overcomes the protection/deprotection steps usually required by traditional methods, which also use strong bases and organic solvents. Moreover, the photophysical properties such as absorption and emission of obtained coumarins (for 3-CF3, 3-benzothiazole, 6-8-difluoro derivatives), predominantly exhibited large Stokes shifts (up to 204 nm) and maintained intramolecular charge transfer (ICT) characteristics.

Ultrasensitive electrochemical microRNA-21 detection based on MXene and ATRP photocatalytic strategy.

ATi3C2TxMXene-based biosensor has been developedand the photocatalytic atom transfer radical polymerization (photo ATRP) amplification strategy appliedto detect target miRNA-21 (tRNA). Initially, Ti3C2TxMXene nanosheets were synthesized from the Ti3AlC2 MAX precursor via selective aluminum etching. Then, functionalization of Ti3C2TxMXene nanosheets with 3-aminopropyl triethoxysilane (APTES) via silylation reactions to facilitate covalent bonding with hairpin DNA biomolecules specifically designed for tRNA detection. Upon binding with the tRNA, the hairpin DNA liberated the azide (N₃) group, initiating a click reaction to affix to the photo ATRP initiator. Through the ATRP photoreaction, facilitated by an organic photoredox catalyst and light, a significant amount of ferrocenyl methyl methacrylate (FMMA) monomer was immobilized on the electrode. Therefore, the electrochemical signal is amplified. The electrochemical efficacy of the biosensor was assessed using square wave voltammetry (SWV). Under optimized conditions, the biosensor demonstrated remarkable sensitivity in detecting tRNA, with a linear detection range from 0.01 fM to 10pM and a detection limit of 2.81 aM. The findings elucidate that thedeveloped biosensor, in conjunction with the photo ATRP strategy, offers reproducibility, stability, and increasedsensitivity, underscoring its potential applications within the experimental medical sector of the biomolecular industry.

Coupling photocatalytic CO2 reduction and CH3OH oxidation for selective dimethoxymethane production

Currently, conventional dimethoxymethane synthesis methods are environmentally unfriendly. Here, we report a photo-redox catalysis system to generate dimethoxymethane using a silver and tungsten co-modified blue titanium dioxide catalyst (Ag.W-BTO) by coupling CO2 reduction and CH3OH oxidation under mild conditions. The Ag.W-BTO structure and its electron and hole transfer are comprehensively investigated by combining advanced characterizations and theoretical studies. Strikingly, Ag.W-BTO achieve a record photocatalytic activity of 5702.49 µmol g−1 with 92.08% dimethoxymethane selectivity in 9 h of ultraviolet-visible irradiation without sacrificial agents. Systematic isotope labeling experiments, in-situ diffuse reflectance infrared Fourier-transform analysis, and theoretical calculations reveal that the Ag and W species respectively catalyze CO2 conversion to *CH2O and CH3OH oxidation to *CH3O. Subsequently, an asymmetric carbon-oxygen coupling process between these two crucial intermediates produces dimethoxymethane. This work presents a CO2 photocatalytic reduction system for multi-carbon production to meet the objectives of sustainable economic development and carbon neutrality.