Silver(I) as a widely applicable, homogeneous catalyst for aerobic oxidation of aldehydes toward carboxylic acids in water-"silver mirror": From stoichiometric to catalytic

Ultrafast Solid-Phase Oxidation of Aldehydes to Carboxylic Acids by Atmosphseric Plasma Treatment

Although atmospheric plasma treatment is an industrially widespread, scalable, and environmentally friendly method, it has been generally used for surface modification, decontamination, or sterilization. In this paper, a novel, sustainable, green, and ultrafast oxidation method is described for aldehydes on a preparative thin-layer chromatographic plate as a solid support. The plasma treatment has proven to be suitable for producing the corresponding carboxylic acids by using only air as a reactant source under mild reaction conditions, while the isolation of the products is also directly integrated into the oxidation process. Extensibility to other reaction types is not explored yet, but we are sure that this novel synthesis conception carries a lot of possibilities.

Chemoselective oxidation of aromatic aldehydes to carboxylic acids: potassium tert-butoxide as an anomalous source of oxygen

Chemoselective oxidation of aromatic and heteroaromatic aldehydes (>45 examples) to their corresponding carboxylic acids has been developed. Potassium tert-butoxide acts as an oxygen source during this transformation that delivers the corresponding acids without chromatographic purifications. The use of bench-top reagents, operational simplicity, and high level of chemo-selectivity with respect to oxidation of the least preferred aldehyde functionality, in the presence of more susceptible functional groups, are some of the highlights of this strategy.

Oxidation of Aldehydes into Carboxylic Acids by a Mononuclear Manganese(III) Iodosylbenzene Complex through Electrophilic C-H Bond Activation

The oxidation of aldehyde is one of the fundamental reactions in the biological system. Various synthetic procedures and catalysts have been developed to convert aldehydes into corresponding carboxylic acids efficiently under ambient conditions. In this work, we report the oxidation of aldehydes by a mononuclear manganese(III) iodosylbenzene complex, [Mn^III(TBDAP)(OIPh)(OH)]²⁺ (1), with kinetic and mechanistic studies in detail. The reaction of 1 with aldehydes resulted in the formation of corresponding carboxylic acids via a pre-equilibrium state. Hammett plot and reaction rates of 1 with 1°-, 2°-, and 3°-aldehydes revealed the electrophilicity of 1 in the aldehyde oxidation. A kinetic isotope effect experiment and reactivity of 1 toward cyclohexanecarboxaldehyde (CCA) analogues indicate that the reaction of 1 with aldehyde occurs through the rate-determining C-H bond activation at the formyl group. The reaction rate of 1 with CCA is correlated to the bond dissociation energy of the formyl group plotting a linear correlation with other aliphatic C-H bonds. Density functional theory calculations found that 1 electrostatically interacts with CCA at the pre-equilibrium state in which the C-H bond activation of the formyl group is performed as the most feasible pathway. Surprisingly, the rate-determining step is characterized as hydride transfer from CCA to 1, affording an (oxo)methylium intermediate. At the fundamental level, it is revealed that the hydride transfer is composed of H atom abstraction followed by a fast electron transfer. Catalytic reactions of aldehydes by 1 are also presented with a broad substrate scope. This novel mechanistic study gives better insights into the metal oxygen chemistry and would be prominently valuable for development of transition metal catalysts.

Research Progress of Highly Efficient Noble Metal Catalysts for the Oxidation of 5-Hydroxymethylfurfural

5-hydroxymethylfurfural (HMF) is considered to be one of the most pivotal multifunctional biomass platform chemicals. This Review discusses recent advances in catalytic oxidation of HMF towards high-value products. The reaction mechanism of different noble metals and the path of HMF oxidation to high-value products have been deeply investigated in the noble metal catalytic system. The reaction mechanisms of different noble metals and HMF conversion paths were compared in detail. Moreover, the factors affecting the performance of different noble metal catalysts were summarized. Finally, effective strategies were put forward to improve the catalytic performance of noble metal catalysts. The purpose is to provide a valuable reference for the academic research on the preparation of oxidation products from biomass-based HMF and the industrial application of noble metal catalysts.

Solvent Effect on Product Distribution in the Aerobic Autoxidation of 2-Ethylhexanal: Critical Role of Polarity

In the aerobic oxidation of aldehydes to acids, how the solvent affect the reaction remains unclear. Herein, the solvent effect in the oxidation of 2-ethylhexanal (2-ETH) to 2-ethylhexanoic acid (2-ETA) was systematically investigated. The vastly different product distributions were observed which could be ascribed to the dominant intermolecular forces. Though strong intermolecular forces in protic solvents limit the oxidation, the optimal 2-ETA yield (96%) was obtained in ipropanol via gradually evaporating the solvent to remove the interactions. Theoretical calculations further revealed that the hydrogen bonds between reactant and protic solvent increase the C-H bond energy (-CHO in 2-ETH). Meanwhile, the hydrogen bonds may improve 2-ETA selectivity by promoting H transfer in the oxidation rearrangement step. Our work discloses the critical role of polarity in determining the reactivity and selectivity of 2-ETH oxidation, and could guide the rational design of more desirable reaction processes with solvent effect.

CO2 as oxidant: an unusual light-assisted catalyst free oxidation of aldehydes to acids under mild conditions

A novel visible light-driven catalyst-free oxidation of aldehydes using CO2 both in batch and flow photoreactors to get corresponding acids along with the formation of CO in the effluent gas is described.

Anaerobic oxidation of aldehydes to carboxylic acids under hydrothermal conditions

Examples of anaerobic oxidation of aldehydes in hydrothermal solutions are reported. The reaction using iron(iii) nitrate as the oxidant occurs under mild hydrothermal conditions and generates carboxylic acids in good yields. This method differs from previous studies which use atmospheric oxygen as the oxidant.

Selective oxidation of aldehydes by oxygen over macroporous alkaline resin

In this reaction, the oxidation of aldehydes is selectively catalyzed by basic resin D201-OH for the first time. Moreover, mild and high-yield oxidation of aldehydes can be achieved without any cocatalyst.

Aerobic oxidation of aldehydes to acids in water with cyclic (alkyl)(amino)carbene copper under mild conditions

In this work, cyclic (alkyl)(amino)carbene copper ((CAAC)Cu) catalyzed aerobic oxidation of aldehydes in water at room temperature has been reported. Good to excellent yields were obtained using different substrates. A possible reaction mechanism was proposed, in which (CAAC)Cu dioxygen activates the C-H bond of aldehyde with a low barrier of 10.6 kcal mol^-1.

Modular synthesis of 3-substituted isocoumarins via silver-catalyzed aerobic oxidation/6-endo heterocyclization of ortho-alkynylbenzaldehydes

A method involving silver-catalyzed aerobic oxidation/6-endo heterocyclization of ortho-alkynylbenzaldehydes to yield 3-substituted isocoumarins is described. The developed protocol allows convenient access to a range of synthetically useful 3-substituted isocoumarins and related fused heterocyclolactones in good to high yields, using silver tetrafluoroborate as the catalyst, and atmospheric oxygen as the terminal oxidant and the source of endocyclic oxygen. Mechanistic studies suggest the involvement of a free-radical pathway.

Green Synthesized of Ag/Ag2O Nanoparticles Using Aqueous Leaves Extracts of Phoenix dactylifera L. and Their Azo Dye Photodegradation

In this study, silver/silver oxide nanoparticles (Ag/Ag2O NPs) were successfully biosynthesized using Phoenix dactylifera L. aqueous leaves extract. The effect of different plant extract/precursor contractions (volume ratio, v/v%) on Ag/Ag2O NP formation, their optical properties, and photocatalytic activity towards azo dye degradation, i.e., Congo red (CR) and methylene blue (MB), were investigated. X-ray diffraction confirmed the crystalline nature of Ag/Ag2O NPs with a crystallite size range from 28 to 39 nm. Scanning electron microscope images showed that the Ag/Ag2O NPs have an oval and spherical shape. UV-vis spectroscopy showed that Ag/Ag2O NPs have a direct bandgap of 2.07-2.86 eV and an indirect bandgap of 1.60-1.76 eV. Fourier transform infrared analysis suggests that the synthesized Ag/Ag2O NPs might be stabilized through the interactions of -OH and C=O groups in the carbohydrates, flavonoids, tannins, and phenolic acids present in Phoenix dactylifera L. Interestingly, the prepared Ag/Ag2O NPs showed high catalytic degradation activity for CR dye. The photocatalytic degradation of the azo dye was monitored spectrophotometrically in a wavelength range of 250-900 nm, and a high decolorization efficiency (84.50%) was obtained after 50 min of reaction. As a result, the use of Phoenix dactylifera L. aqueous leaves extract offers a cost-effective and eco-friendly method.

Practical scale up synthesis of carboxylic acids and their bioisosteres 5-substituted-1H-tetrazoles catalyzed by a graphene oxide-based solid acid carbocatalyst

Herein, catalytic application of a metal-free sulfonic acid functionalized reduced graphene oxide (SA-rGO) material is reported for the synthesis of both carboxylic acids and their bioisosteres, 5-substituted-1H-tetrazoles. SA-rGO as a catalytic material incorporates the intriguing properties of graphene oxide material with additional benefits of highly acidic sites due to sulfonic acid groups. The oxidation of aldehydes to carboxylic acids could be efficiently achieved using H₂O₂ as a green oxidant with high TOF values (9.06-9.89 h^-1). The 5-substituted-1H-tetrazoles could also be effectively synthesized with high TOF values (12.08-16.96 h^-1). The synthesis of 5-substituted-1H-tetrazoles was corroborated by single crystal X-ray analysis and computational calculations of the proposed reaction mechanism which correlated well with experimental findings. Both of the reactions could be performed efficiently at gram scale (10 g) using the SA-rGO catalyst. SA-rGO displays eminent reusability up to eight runs without significant decrease in its productivity. Thus, these features make SA-rGO riveting from an industrial perspective.

Single-Atom Catalysts across the Periodic Table

Isolated atoms featuring unique reactivity are at the heart of enzymatic and homogeneous catalysts. In contrast, although the concept has long existed, single-atom heterogeneous catalysts (SACs) have only recently gained prominence. Host materials have similar functions to ligands in homogeneous catalysts, determining the stability, local environment, and electronic properties of isolated atoms and thus providing a platform for tailoring heterogeneous catalysts for targeted applications. Within just a decade, we have witnessed many examples of SACs both disrupting diverse fields of heterogeneous catalysis with their distinctive reactivity and substantially enriching our understanding of molecular processes on surfaces. To date, the term SAC mostly refers to late transition metal-based systems, but numerous examples exist in which isolated atoms of other elements play key catalytic roles. This review provides a compositional encyclopedia of SACs, celebrating the 10th anniversary of the introduction of this term. By defining single-atom catalysis in the broadest sense, we explore the full elemental diversity, joining different areas across the whole periodic table, and discussing historical milestones and recent developments. In particular, we examine the coordination structures and associated properties accessed through distinct single-atom-host combinations and relate them to their main applications in thermo-, electro-, and photocatalysis, revealing trends in element-specific evolution, host design, and uses. Finally, we highlight frontiers in the field, including multimetallic SACs, atom proximity control, and possible applications for multistep and cascade reactions, identifying challenges, and propose directions for future development in this flourishing field.

A new Ni-diaminoglyoxime-g-C3N4 complex towards efficient photocatalytic ethanol splitting via a ligand-to-metal charge transfer (LMCT) mechanism

We report a novel Ni-diaminoglyoxime-g-C₃N₄ (Ni-DAG-CN) complex for H₂ evolution through photocatalytic ethanol splitting. Compared to that of pristine g-C₃N₄, Ni-DAG-CN exhibits a 21-fold enhancement of photocatalytic activity (296.1 μmol h^-1 g^-1) under irradiation with excellent stability. The enhanced photocatalytic activity can be attributed to a proposed ligand-to-metal charge transfer (LMCT) mechanism, which is illustrated both experimentally and theoretically. This work provides great potential in the future design of low-cost, high-performance photocatalysts for H₂ evolution from alcohol splitting.

Photoinitiated Charge Transfer in a Triangular Silver(I) Hydride Complex and Its Oxophilicity

The photoexcitation of a triangular silver(I) hydride complex, [Ag₃ (μ₃ -H)(μ₂ -dcpm)₃ ](PF₆ )₂ ([P](PF₆ )₂ , dcpm=bis(dicyclohexylphosphino)methane), designed with "UV-silent" bis-phosphine ligands, provokes hydride-to-Ag₃ single and double electron transfer. The nature of the electronic transitions has been authenticated by absorption and photodissociation spectroscopy in parallel with high-level quantum-chemical computations utilizing the GW method and Bethe-Salpeter equation (GW-BSE). Specific photofragments of mass-selected [P]²⁺ ions testify to charge transfer and competing pathways resulting from the unique [Ag₃ (μ₃ -H)]²⁺ scaffold. This structural motif of [P](PF₆ )₂ has been unequivocally verified by ¹ H NMR spectroscopy in concert with DFT and X-ray diffraction structural analysis, which revealed short equilateral Ag-Ag distances (d_AgAg =3.08 Å) within the range of argentophilic interactions. The reduced radical cation [P]^{. +} exhibits strong oxophilicity, forming [P+O₂ ]^.+ ,which is a model intermediate for silver oxidation catalysis.

Silver-Catalyzed Reduction of Quinolines in Water

A ligand- and base-free silver-catalyzed reduction of quinolines and electron-deficient aromatic N-heteroarenes in water has been described. Mechanistic studies revealed that the effective reducing species was Ag-H. This versatile catalytic protocol provided facile, environmentally friendly, and practical access to a variety of 1,2,3,4-tetrahydroquinoline derivatives at room temperature.

Organocatalyzed Aerobic Oxidation of Aldehydes to Acids

The first example organocatalyzed aerobic oxidation of aldehydes to carboxylic acids in both organic solvent and water under mild conditions is developed. As low as 5 mol % N-hydroxyphthalimide was used as the organocatalyst, and molecular O₂ was used as the sole oxidant. No transition metals or hazardous oxidants or cocatalysts were involved. A wide range of carboxylic acids bearing diverse functional groups were obtained from aldehydes, even from alcohols, in high yields.

A Supramolecular Catalyst Self-Assembled From Polyoxometalates and Cationic Pillar[5]arenes for the Room Temperature Oxidation of Aldehydes

Oxidizing aldehydes to generate carboxylic acids is a crucial reaction in nature and in chemical industry. The aldehyde oxidation, an easily achieved process in liver cells, is inert toward autoxidation in industrial production and difficultly achieved under enzymatic condition (in water, at pH 7, at room temperature). Herein, we prepared a supramolecular catalyst which are nanospheres assembled in aqueous media by chromium centered Anderson polyoxometalates Na₃[CrMo₆O₁₈(OH)₃] (namely, CrMo₆) and cationic pillar[5]arenes (namely, P5A) with 10 positive charges which can be used as the phase transfer catalysts (PTCs). This supramolecular catalyst was exploited on aldehydes oxidation under enzymatic condition with relatively good conversion. Through DLS monitoring, the diameters of nanospheres were variable while changing the charge ratios of the ionic complexes (P5A-CrMo₆), and it is probably because of the closer charge ratios causing the more compact assemblies. Also, the nano-morphologies were monitored by TEM and SEM, and the nanostructures were characterized by zeta potential, the X-ray energy-dispersive spectroscopy (EDS), elemental analysis.

A biocatalytic method for the chemoselective aerobic oxidation of aldehydes to carboxylic acids

Herein, we present a study on the oxidation of aldehydes to carboxylic acids using three recombinant aldehyde dehydrogenases (ALDHs). The ALDHs were used in purified form with a nicotinamide oxidase (NOx), which recycles the catalytic NAD+ at the expense of dioxygen (air at atmospheric pressure). The reaction was studied also with lyophilised whole cell as well as resting cell biocatalysts for more convenient practical application. The optimised biocatalytic oxidation runs in phosphate buffer at pH 8.5 and at 40 °C. From a set of sixty-one aliphatic, aryl-aliphatic, benzylic, hetero-aromatic and bicyclic aldehydes, fifty were converted with elevated yield (up to >99%). The exceptions were a few ortho-substituted benzaldehydes, bicyclic heteroaromatic aldehydes and 2-phenylpropanal. In all cases, the expected carboxylic acid was shown to be the only product (>99% chemoselectivity). Other oxidisable functionalities within the same molecule (e.g. hydroxyl, alkene, and heteroaromatic nitrogen or sulphur atoms) remained untouched. The reaction was scaled for the oxidation of 5-(hydroxymethyl)furfural (2 g), a bio-based starting material, to afford 5-(hydroxymethyl)furoic acid in 61% isolated yield. The new biocatalytic method avoids the use of toxic or unsafe oxidants, strong acids or bases, or undesired solvents. It shows applicability across a wide range of substrates, and retains perfect chemoselectivity. Alternative oxidisable groups were not converted, and other classical side-reactions (e.g. halogenation of unsaturated functionalities, Dakin-type oxidation) did not occur. In comparison to other established enzymatic methods such as the use of oxidases (where the concomitant oxidation of alcohols and aldehydes is common), ALDHs offer greatly improved selectivity.

Metal-Free Aerobic Oxidation of Nitro-Substituted Alkylarenes to Carboxylic Acids or Benzyl Alcohols Promoted by NaOH

Efficient and selective aerobic oxidation of nitro-substituted alkylarenes to functional compounds is a fundamental process that remains a challenge. Here, we report a metal-free, efficient, and practical approach for the direct and selective aerobic oxidation of nitro-substituted alkylarenes to carboxylic acids or benzyl alcohols. This sustainable system uses O₂ as clean oxidant in a cheap and green NaOH/EtOH mixture. The position and type of substituent critically affect the products. In addition, this sustainable protocol enabled gram-scale preparation of carboxylic acid and benzyl alcohol derivatives with high chemoselectivities. Finally, the reactions can be conducted in a pressure reactor, which can conserve oxygen and prevent solvent loss. The approach was conducive to environmental protection and potential industrial application.

Regioselective oxidation and metalation of meso-unsubstituted azuliporphyrins

Azuliporphyrins are intriguing porphyrin analogues that incorporate an azulene ring in place of a pyrrolic unit. This system undergoes regioselective oxidation reactions and favors the formation of stable organometallic derivatives. Reaction of meso-unsubstituted azuliporphyrins with Co₂(CO)₈ or CoCl₂·6H₂O gave 21-oxyazuliporphyrins, while Cu(OAc)₂ produced the corresponding copper(ii) complexes. Treatment of an oxyazuliporphyrin with Ni(OAc)₂ or Pd(OAc)₂ afforded analogous nickel(ii) and palladium(ii) derivatives. Silver(i) acetate in pyridine reacted with azuliporphyrins to give moderate yields of silver(iii) benzocarbaporphyrins, and the prevalence of structures with a formyl moiety at the sterically crowded 2¹-position suggested that the ring contraction reactions were triggered in part by intramolecular attack from an axial peroxide ligand. Related thiaazuliporphyrins reacted with palladium(ii) acetate to give palladium(ii) benzothiacarbaporphyrins but this chemistry did not give rise to structures with 2¹-formyl groups, suggesting that the ring contraction reactions occurred by a different mechanistic pathway. These results demonstrate the existence of a rich tapestry of oxidation and metalation reactions for azuliporphyrin systems.