Photocatalytic Oxygenation Reactions Using Water and Dioxygen

Photocatalytic Asymmetric Acyl Radical Truce-Smiles Rearrangement for the Synthesis of Enantioenriched α-Aryl Amides

The radical Truce-Smiles rearrangement is a straightforward strategy for incorporating aryl groups into organic molecules for which asymmetric processes remains rare. By employing a readily available and non-expensive chiral auxiliary, we developed a highly efficient asymmetric photocatalytic acyl and alkyl radical Truce-Smiles rearrangement of α-substituted acrylamides using tetrabutylammonium decatungstate (TBADT) as a hydrogen atom-transfer photocatalyst, along with aldehydes or C-H containing precursors. The rearranged products exhibited excellent diastereoselectivities (7 : 1 to >98 : 2 d.r.) and chiral auxiliary was easily removed. Mechanistic studies allowed understanding the transformation in which density functional theory (DFT) calculations provided insights into the stereochemistry-determining step.

Oxygenation of Phenols with Water as the Oxygen Source and Oxoammonium Salt as the Oxidant

Aromatic C-H oxygenation is important in both industrial production and organic synthesis. Here we report a metal-free approach for phenol oxygenation with water as the oxygen source using oxoammonium salts as the renewable oxidant. Employing this protocol, various alkyl-substituted phenols were converted into benzoquinones in yields of 59-98%. On the basis of 18O-labeling and kinetic studies, the hydroxy-oxoammonium adduct was proposed to attack the aromatic ring similarly to electrophilic aromatic substitution. We suppose that the findings described here not only provide an efficient and highly selective protocol for aromatic C-H oxygenation but also may encourage further developments of possible transition-metal-free catalytic methods.

Visible-Light-Induced DDQ-Catalyzed Fluorocarbamoylation Using CF3SO2Na and Oxygen

The synthesis of carbamoyl fluorides via visible-light induced DDQ catalysis of secondary amines is described. This protocol employs sodium trifluorosulfinate and molecular oxygen for the in situ generation of carbonyl difluoride, which is reacted with amines to afford the corresponding carbamoyl fluorides efficiently. Moreover, carbamoyl fluorides are easily transformed to synthetically useful carbonyl compounds under mild reaction conditions.

Visible light mediated synthesis of 1,3-diarylated imidazo[1,5-a]pyridines via oxidative amination of C-H catalyzed by graphitic carbon nitride

Graphitic carbon nitride (g-C3N4) as a novel heterogeneous catalyst is employed for the visible light-mediated synthesis of the imidazo[1,5-a]pyridines via the oxidative amination of C-H bond at room temperature without the need for any additional solvent. Extensive characterization of the catalyst was performed using techniques such as FT-IR, PXRD, TGA, SEM and EDX analysis. The optimized conditions enabled the successful and expeditious conversion of a wide range of substrates to imidazo[1,5-a]pyridines in good yields; a notable advantage of this catalyst being recyclability, as it can be reused for up to five cycles without significant loss of activity. This feature makes it suitable for gram-scale synthesis of imidazo[1,5-a]pyridines. Additionally, this approach offers several benefits from a green chemistry perspective as affirmed by its favorable green chemistry metrics (GCM), including low process mass intensity (PMI), low E-factor, high atom economy (AE), and good reaction mass efficiency (RME) relative to existing protocols. In addition, chemical yield (CY), mass intensity (MI), mass productivity (MP) and optimum efficiency were also calculated. This environmentally friendly method offers multiple advantages and represents a significant advancement in the synthesis of imidazo[1,5-a]pyridines.

A breath of sunshine: oxygenic photosynthesis by functional molecular architectures

The conversion of light into chemical energy is the game-changer enabling technology for the energetic transition to renewable and clean solar fuels. The photochemistry of interest includes the overall reductive/oxidative splitting of water into hydrogen and oxygen and alternatives based on the reductive conversion of carbon dioxide or nitrogen, as primary sources of energy-rich products. Devices capable of performing such transformations are based on the integration of three sequential core functions: light absorption, photo-induced charge separation, and the photo-activated breaking/making of molecular bonds via specific catalytic routes. The key to success does not rely simply on the individual components' performance, but on their optimized integration in terms of type, number, geometry, spacing, and linkers dictating the photosynthetic architecture. Natural photosynthesis has evolved along this concept, by integrating each functional component in one specialized "body" (from the Greek word "soma") to enable the conversion of light quanta with high efficiency. Therefore, the natural "quantasome" represents the key paradigm to inspire man-made constructs for artificial photosynthesis. The case study presented in this perspective article deals with the design of artificial photosynthetic systems for water oxidation and oxygen production, engineered as molecular architectures then rendered on electrodic surfaces. Water oxidation to oxygen is indeed the pervasive oxidative reaction used by photosynthetic organisms, as the source of reducing equivalents (electrons and protons) to be delivered for the processing of high-energy products. Considering the vast and abundant supply of water (including seawater) as a renewable source on our planet, this is also a very appealing option for photosynthetic energy devices. We will showcase the progress in the last 15 years (2009-2023) in the strategies for integrating functional building blocks as molecular photosensitizers, multi-redox water oxidation catalysts and semiconductor materials, highlighting how additional components such as redox mediators, hydrophilic/hydrophobic pendants, and protective layers can impact on the overall photosynthetic performance. Emerging directions consider the modular tuning of the multi-component device, in order to target a diversity of photocatalytic oxidations, expanding the scope of the primary electron and proton sources while enhancing the added-value of the oxidation product beyond oxygen: the selective photooxidation of organics combines the green chemistry vision with renewable energy schemes and is expected to explode in coming years.

Alkyne Oxidation by a Vitamin B2-Based Photocatalytic System with Both H2O and O2 as the Oxygen Source

The employment of readily available photocatalysts and green oxygen atom sources is recognized as a promising strategy to develop sustainable catalysis for oxidation reactions. We herein reported a sacrificial reagent-free system consisting of riboflavin tetraacetate (RFT), an ester of natural vitamin B2 as the photocatalyst, and Sc(OTf)3 and NaCl as the cocatalysts for alkyne oxidation under blue light or even sunlight irradiation to produce 1,2-diketone in which the oxygen atoms were from both water and molecular oxygen, respectively. A major Cl-/Cl• cycle was proposed to be involved and achieved by the excited [RFT-2Sc3+]* complex via single electron transfer for the first time, distinguished from the OCl- active species by a two-electron process in previous flavin-halide photo-oxidation systems.

Arene C-H Amination with N-Heteroarenes by Catalytic DDQ Photocatalysis

Arene C-H aminations using catalytic amounts of a 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) photocatalyst are described. Benzene, which has an oxidation potential of 2.48 V (vs SCE), was functionalized by pyrazoles, triazoles, tetrazoles, purines, and tert-butoxycarbonyl amine. Arenes underwent amination via a combination of ultraviolet (UV) light and a DDQ photocatalyst without a typical co-oxidant. Although the mechanism remains an open question, DDQH₂, which is generated from DDQ after oxidation, is reactivated to DDQ under UV light irradiation conditions, possibly with the assistance of adventitious O₂ and/or a solvent as the terminal oxidant(s) in this system.

Scalable Electrochemical Aerobic Oxygenation of Indoles to Isatins without Electron Transfer Mediators by Merging with an Oxygen Reduction Reaction

An approach to electrochemical oxygenation of indoles leading to isatins was developed by merging with a complementary cathode oxygen reduction reaction. The features of this green protocol include the use of molecular oxygen as the sole oxidant, it being free of an electron transfer mediator, and gram-scale preparation. Mechanistic studies suggested a radical process, and the two oxygen atoms in the isatins were both most likely from molecular oxygen. A detailed mechanism of the reaction utilizing density functional theory calculations was elucidated.

Beyond Water Oxidation: Hybrid, Molecular-Based Photoanodes for the Production of Value-Added Organics

The political and environmental problems related to the massive use of fossil fuels prompted researchers to develop alternative strategies to obtain green and renewable fuels such as hydrogen. The light-driven water splitting process (i.e., the photochemical decomposition of water into hydrogen and oxygen) is one of the most investigated strategies to achieve this goal. However, the water oxidation reaction still constitutes a formidable challenge because of its kinetic and thermodynamic requirements. Recent research efforts have been focused on the exploration of alternative and more favorable oxidation processes, such as the oxidation of organic substrates, to obtain value-added products in addition to solar fuels. In this mini-review, some of the most intriguing and recent results are presented. In particular, attention is directed on hybrid photoanodes comprising molecular light-absorbing moieties (sensitizers) and catalysts grafted onto either mesoporous semiconductors or conductors. Such systems have been exploited so far for the photoelectrochemical oxidation of alcohols to aldehydes in the presence of suitable co-catalysts. Challenges and future perspectives are also briefly discussed, with special focus on the application of such hybrid molecular-based systems to more challenging reactions, such as the activation of C–H bonds.

Light-mediated aerobic oxidation of C(sp3)–H bonds by a Ce(iv) hexachloride complex

A photochemical C(sp3)–H oxygenation of arene and alkane substrates (including methane) catalyzed by [NEt4]2[CeIVCl6] under mild conditions (1 atm, 25 °C) is described.

Bioinspired Heterobimetallic Photocatalyst () for Visible-Light-Driven C-H Oxidation of Organic Substrates via Dioxygen Activation

We report a bioinspired heterobimetallic photocatalyst RuIIchrom-FeIIIcat and its relevant applications toward visible-light-driven C-H bond oxidation of a series of hydrocarbons using O2 as the O-atom source. The RuII center absorbs visible light near 460 nm and triggers a cascade of electrons to FeIII to afford a catalytically active high-valent FeIV═O species. The in situ formed FeIV═O has been employed for several high-impact oxidation reactions in the presence of triethanolamine (TEOA) as the sacrificial electron donor.

Amorphous Fe(OH)3 Passivating CeO2 Nanorods: A Noble-Metal-Free Photocatalyst for Water Oxidation

Noble-metal-free composites with good photocatalytic property are of great interest. Here, CeO₂ nanorods composites loaded with amorphous Fe(OH)₃ cocatalyst were designed and prepared via a secondary water bath at 100 °C. The as-synthesized CeO₂ /amorphous Fe(OH)₃ composites exhibited superior light photocatalytic activities compared to pure CeO₂ , especially the sample with a loading time of 60 min. The photocatalytic oxygen generation rate could reach to 357.2 μmol h^-1  g^-1 , and the average apparent quantum yield (AQY) was 24.67 %, which was a 5.5-fold increase compared to the CeO₂ sample. The improvement of photocatalytic performance could be ascribed to three main reasons: First, loading the amorphous Fe(OH)₃ enlarged the specific surface area and passivated the surface of the pristine CeO₂ . Second, the amorphous Fe(OH)₃ ,which acted as a cocatalyst, provided many active sites, and reduced the reaction activation energy. Thirdly, the maximum interface with intimate contact between CeO₂ and amorphous Fe(OH)₃ cocatalyst accelerated the photogenerated charge separation efficiency and thus improved the photocatalytic performance of CeO₂ in photocatalytic water oxidation.

Harnessing In Situ Radical Oxygenation: Copper-Catalyzed Interrupted Azirine-Alkyne Ring-Expansion Reaction for the Synthesis of Pyrrolones

Here we report a novel interrupted azirine-alkyne ring-expansion reaction with molecular oxygen for the direct synthesis of highly functionalized pyrrolones enabled by copper catalysis. Mechanistic investigations indicate that the present three-component reaction proceeds via two copper-catalyzed sequential reactions, an azirine-ring-opening alkynylation and an amine-directed radical oxygenation, leading to the formation of interesting pyrrolone structures under mild conditions.

Photooxidation of 2-(tert-Butyl)-3-Methyl-2,3,5,6,7,8-Hexahydroquinazolin-4(1H)-one, an Example of Singlet Oxygen ene Reaction

Singlet oxygen ene reactions produce 2-(tert-butyl)-4a-hydroperoxy-3-methyl-2,4a, 5,6,7,8-hexahydroquinazolin-4(3H)-one quantitatively during diffusion crystallization of 2-(tert-butyl)-3-methyl-2,3,5,6,7,8-hexahydroquinazolin-4(1H)-one in n-hexane/CH₂Cl₂ solvent mixture. To confirm this photo-oxidation, a ¹H-NMR study in CDCl₃ was performed with exposure to ambient conditions (light and oxygen), with neither additional reactants nor catalysts. A theoretical study at the B3LyP/6311++G** level using the QST2 method of locating transition states suggests a two-step mechanism where the intermediate, which unexpectedly did not come from the peroxide intermediate, has a low activation energy.

Antimony-Oxo Porphyrins as Photocatalysts for Redox-Neutral C-H to C-C Bond Conversion

The use of high-valent antimony-oxo porphyrins as visible-light photocatalysts operating via direct hydrogen atom transfer has been demonstrated. Computational analysis indicates that the triplet excited state of these complexes shows an oxyl radical behavior, while the Sb^V center remains in a high-valent oxidation state, serving uniquely to carry the oxo moiety and activate the coordinated ligands. This porphyrin-based system has been exploited upon irradiation to catalyze C-H to C-C bond conversion via the addition of hydrogen donors (ethers and aldehydes) onto Michael acceptors in a redox-neutral fashion without the need of any external oxidant. Laser flash photolysis experiments confirmed that the triplet excited state of the photocatalyst triggers the desired C-H cleavage.

Mechanistic Insights into Visible Light-Induced Direct Hydroxylation of Benzene to Phenol with Air and Water over Pt-Modified WO3 Photocatalyst

Activation of C(sp2)-H in aromatic molecules such as benzene is one of the challenging reactions. The tungsten trioxide supported Pt nanoparticles (Pt-WO3) exhibited hydroxylation of benzene in the presence of air and H2O under visible-light (420 < λ < 540 nm) irradiation. The photocatalytic activities (yields and selectivity of phenol) were studied under several experimental conditions. Furthermore, investigations of mechanistic insight into hydroxylation of benzene have been carried out by analyses with apparent quantum yields (AQY), an H218O isotope-labeling experiment, kinetic isotope effects (KIE), electrochemical measurements and density functional theory (DFT) calculations. It was proposed that dissociation of the O–H bond in H2O is the rate-determining step. Furthermore, the substitution of the OH derived from H2O with H abstracted from benzene by photo-formed H2O2 indicated a mechanism involving a push-pull process for the hydroxylation of benzene into phenol.