Silver-Catalyzed Decarboxylative Nitrooxylation of Aliphatic Carboxylic Acids

Here, we present a silver-catalyzed decarboxylative nitrooxylation via a radical-based approach. The substrate scope of this reaction prototype extends to nonactivated primary and secondary carboxylic acids. This protocol provides a practical method for the synthesis of an unprecedented family of organic nitrates and exhibits wide functional group compatibility. Preliminary mechanistic studies reveal that a high-valent silver(II) nitrate complex is a versatile NO3 resource pool, allowing for facile C-O bond formation.

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.

FeIII -Based Eutectic Mixtures as Multi-task and Reusable Reaction Media for Efficient and Selective Conversion of Alkynes into Carbonyl Compounds

An efficient, simple and general protocol for the selective hydration of terminal alkynes into the corresponding methyl ketones has been developed by using a cheap, easy-to-synthesise and sustainable FeIII -based eutectic mixture [FeCl3  ⋅ 6H2 O/Gly (3 : 1)] as both promoter and solvent for the hydration reaction, working: i) under mild (45 °C) and bench-type reaction conditions (air); and ii) in the absence of ligands, co-catalysts, co-solvents or toxic, non-abundant and expensive noble transition metals (Au, Ru, Pd). When the final methyl ketones are solid/insoluble in the eutectic mixture, the hydration reaction takes place in 30 min, and the obtained methyl ketones can be isolated by simply decanting the liquid FeIII -DES, allowing the direct isolation of the desired ketones without VOC solvents. By using this straightforward and simple isolation protocol, we have been able to recycle the FeIII -based eutectic mixture system up to eight consecutive times. Furthermore, the FeIII -eutectic mixture is able to promote the selective and efficient formal oxidation of internal alkynes into 1,2-diketones, with the possibility of recycling this system up to three consecutive times. Preliminary investigations into a possible mechanism for the oxidation of the internal alkynes seem to indicate that it proceeds through the formation of the corresponding methyl ketones and α-chloroketones.

Recent Developments on Photochemical Synthesis of 1,n-Dicarbonyls

1,n-dicarbonyls are one of the most fascinating chemical feedstocks finding abundant usage in the field of pharmaceuticals. Besides, they are utilized in a plethora of synthesis in general synthetic organic chemistry. A number of 'conventional' methods are available for their synthesis, such as the Stetter reaction, Baker-Venkatraman rearrangement, oxidation of vicinal diols, and oxidation of deoxybenzoins, synonymous with unfriendly reagents and conditions. In the last 15 years or so, photocatalysis has taken the world of synthetic organic chemistry by a remarkable renaissance. It is fair to say now that everybody loves the light and photoredox chemistry has opened a new gateway to organic chemists towards milder, more simpler alternatives to the previously available methods, allowing access to many sensitive reactions and products. In this review, we present the readers with the photochemical synthesis of a variety of 1,n-dicarbonyls. Diverse photocatalytic pathways to these fascinating molecules have been discussed, placing special emphasis on the mechanisms, giving the reader an opportunity to find all these significant developments in one place.

Organo-photocatalytic C-H bond oxidation: an operationally simple and scalable method to prepare ketones with ambient air

Oxidative C-H functionalization with O₂ is a sustainable strategy to convert feedstock-like chemicals into valuable products. Nevertheless, eco-friendly O₂-utilizing chemical processes, which are scalable yet operationally simple, are challenging to develop. Here, we report our efforts, via organo-photocatalysis, in devising such protocols for catalytic C-H bond oxidation of alcohols and alkylbenzenes to ketones using ambient air as the oxidant. The protocols employed tetrabutylammonium anthraquinone-2-sulfonate as the organic photocatalyst which is readily available from a scalable ion exchange of inexpensive salts and is easy to separate from neutral organic products. Cobalt(ii) acetylacetonate was found to be greatly instrumental to oxidation of alcohols and therefore was included as an additive in evaluating the alcohol scope. The protocols employed a nontoxic solvent, could accommodate a variety of functional groups, and were readily scaled to 500 mmol scale in a simple batch setting using round-bottom flasks and ambient air. A preliminary mechanistic study of C-H bond oxidation of alcohols supported the validity of one possible mechanistic pathway, nested in a more complex network of potential pathways, in which the anthraquinone form - the oxidized form - of the photocatalyst activates alcohols and the anthrahydroquinone form - the relevant reduced form of the photocatalyst - activates O₂. A detailed mechanism, which reflected such a pathway and was consistent with previously accepted mechanisms, was proposed to account for formation of ketones from aerobic C-H bond oxidation of both alcohols and alkylbenzenes.

Reactive oxygen species formation driven by acidophiles mediated pyrite oxidation and its potential role on 2,4-dichlorophenol transformation

Researches of reactive oxygen species (ROS) generation from pyrite oxidation and its impact on contaminants transformation has been constrained to abiotic conditions. However, pyrite oxidation by acidophiles is widespread in acidic environments. The potential role of these microorganisms on pyrite-induced ROS formation and pollutants processing is not understood well. Here, ROS production and 2,4-DCP transformation during pyrite oxidation under oxic and anoxic atmospheres by Acidithiobacillus ferrooxidans (A. ferrooxidans) were explored. 2,4-DCP removal was enhanced in biosystem. Under oxic and anoxic conditions, microbially mediated pyrite oxidation resulted in removing 93.66% and 43.77% 2,4-DCP, which were 1.14- and 1.51-fold greater than that without cells. Based on intermediates identified by LC-MS, the transformation pathway of 2,4-DCP was proposed. The trapping experiments demonstrated ROS contributed during 2,4-DCP transformation. The improving effect of A. ferrooxidans on 2,4-DCP degradation was mainly due to ROS increase. A. ferrooxidans was to promote pyrite surface renew, exposing more Fe(II) and Fe(III) sites that facilitated O₂ reduction and H₂O dissociation for ROS generation. Biogenic ROS and sulfite bio-oxidation with the free radical mechanism provided other ROS sources. ESR revealed A. ferrooxidans-pyrite interaction led to sustainable ROS production, indicating it could be a significant pathway in driving geochemical cycles of elements.

A porous organic polymer supported Pd/Cu bimetallic catalyst for heterogeneous oxidation of alkynes to 1,2-diketones

A reusable Pd/Cu bimetal-loaded porous organic polymer (Pd/Cu@POP–POPh3) has been developed for heterogeneous oxidation of various alkynes to afford the corresponding 1,2-diketones in high to excellent yields.

[Ru(NꓥNꓥN)2-Ce]-based Framework for Photocatalytic Sulfide Oxidation and Hydrogen Production

Using photosensitized Ru(NꓥNꓥN)2-metalloligand, a series of Ce-frameworks were synthesized. The incorporation of visible-light-responsive Ru(NꓥNꓥN)2-unit endows the Ce-frameworks with photocatalytic activities in both sulfide oxidation and hydrogen production. The doping of...

Efficient Syntheses of Biobased Terephthalic Acid, p-Toluic Acid, and p-Methylacetophenone via One-Pot Catalytic Aerobic Oxidation of Monoterpene Derived Bio-p-cymene

An efficient elevated-pressure catalytic oxidative process (2.5 mol % Co(NO₃)₂, 2.5 mol % MnBr₂, air (30 bar), 125 °C, acetic acid, 6 h) has been developed to oxidize p-cymene into crystalline white terephthalic acid (TA) in ∼70% yield. Use of this mixed Co²⁺/Mn²⁺ catalytic system is key to obtaining high 70% yields of TA at relatively low reaction temperatures (125 °C) in short reaction times (6 h), which is likely to be due to the synergistic action of bromine and nitrate radicals in the oxidative process. Recycling studies have demonstrated that the mixed metal catalysts present in recovered mother liquors could be recycled three times in successive p-cymene oxidation reactions with no loss in catalytic activity or TA yield. Partial oxidation of p-cymene to give p-methylacetophenone (p-MA) in 55-60% yield can be achieved using a mixed CoBr₂/Mn(OAc)₂ catalytic system under 1 atm air for 24 h, while use of Co(NO₃)₂/MnBr₂ under 1 atm O₂ for 24 h gave p-toluic acid in 55-60% yield. Therefore, access to these simple catalytic aerobic conditions enables multiple biorenewable bulk terpene feedstocks (e.g., crude sulfate turpentine, turpentine, cineole, and limonene) to be converted into synthetically useful bio-p-MA, bio-p-toluic acid, and bio-TA (and hence bio-polyethylene terephthalate) as part of a terpene based biorefinery.

Catalyst-Free and Transition-Metal-Free Approach to 1,2-Diketones via Aerobic Alkyne Oxidation

A catalyst-free and transition-metal-free method for the synthesis of 1,2-diketones from aerobic alkyne oxidation was reported. The oxidation of various internal alkynes, especially more challenging aryl-alkyl acetylenes, proceeded smoothly with inexpensive, easily handled, and commercially available potassium persulfate and an ambient air balloon, achieving the corresponding 1,2-diketones with up to 85% yields. Meanwhile, mechanistic studies indicated a radical process, and the two oxygen atoms in the 1,2-diketons were most likely from persulfate salts and molecular oxygen, respectively, rather than water.

Excited State Anions in Organic Transformations

Utilizing light is a smart way to fuel chemical transformations as it allows the energy to be selectively focused on certain molecules. Many reactions involving electronically excited species proceed via open-shell intermediates, which offer novel and unique routes to expand the hitherto used synthetic toolbox in organic chemistry. The direct conversion of non-prefunctionalized, less activated compounds is a highly desirable goal to pave the way towards more sustainable and atom-economic chemical processes. Photoexcited closed-shell anions have been shown to reach extreme potentials in single electron transfer reactions and reveal unusual excited-state reactivity. It is, therefore, surprising that their use as a reagent or photocatalyst is limited to a few examples. In this Review, we briefly discuss the characteristics of anionic photochemistry, highlight pioneering work, and show recent progress which has been made by utilizing photoexcited anionic species in organic synthesis.

Csp–Csp bond cleavage and fragment coupling: a transition metal-free “extrusion and recombination” approach towards synthesis of 1,2-diketones

A metal-free strategy for C–C bond activation of 1,3-diynes has been established via an “extrusion and recombination” approach to derive structurally important 1,2-diketones in good yields with excellent selectivity.

Recent advancements in the development of molecular organic photocatalysts

Research in the development of molecular organic photocatalysts for applications in chemical syntheses has burgeoned in recent years. While organic photosensitizers have been known for over a century, tuning the properties of these molecules to increase photocatalytic efficiencies is now of growing importance. The properties that help improve the performance of organic photocatalysts include: a wider range of redox potentials, increased molar absorptivity (ε) in the visible spectrum, increased quantum yields (Φ), long-lived excited-state lifetimes (ns to μs), and increased chemical stability. This review examines some of the recent advancements in the development of molecular organic photocatalysts, specifically cyanoarenes, acridinium dyes, phenazines, thiazines, oxazines, and xanthenes, with respect to these properties and examines the chemical synthesis routes now achieved by organic photocatalysts.

Homobenzylic Oxygenation Enabled by Dual Organic Photoredox and Cobalt Catalysis

Activation of aliphatic C(sp3)-H bonds in the presence of more activated benzylic C(sp3)-H bonds is often a nontrivial, if not impossible task. Herein we show that leveraging the reactivity of benzylic C(sp3)-H bonds to achieve reactivity at the homobenzylic position can be accomplished using dual organic photoredox/cobalt catalysis. Through a two-part catalytic system, alkyl arenes undergo dehydrogenation followed by an anti-Markovnikov Wacker-type oxidation to grant benzyl ketone products. This formal homobenzylic oxidation is accomplished with high atom economy without the use of directing groups, achieving valuable reactivity that traditionally would require multiple chemical transformations.

Visible light initiated amino group ortho-directed copper(i)-catalysed aerobic oxidative C(sp)-S coupling reaction: synthesis of substituted 2-phenylbenzothiazoles via thia-Wolff rearrangement

A facile amino group ortho-directed visible-light-driven copper-catalysed aerobic oxidative C(sp)-S coupling reaction of a dimer of 2-aminothiophenol with terminal alkynes was achieved. This photochemical reaction shows an excellent conversion and chemoselectivity towards the formation of C(sp)-S coupling and has been employed for a wide range of thiol dimers, and alkynes. Furthermore, the synthetic utility of the synthesized alkynyl sulfides was demonstrated as a direct method for the construction of 2-phenylbenzothiazoles from the corresponding alkynyl sulfides via "thia-Wolff rearrangement" using AgNO3 and visible light using 9-mesityl-10-methylacridinium ions (Acr+-Mes) as photoredox catalyst system.

ICl/AgNO3 Co-Catalyzed Radical Oxidation of Diaryl- and Alkylarylalkynes into 1,2-Diketones

A novel ICl/AgNO3 co-catalyzed radical oxidation of diaryl- and alkylarylalkynes into 1,2-diketones is reported. The reaction proceeded smoothly under mild conditions and generated 1,2-diketones in moderate to good yields with a good tolerance of functional groups. Furthermore, the obtained C4-(1,2-diketoaryl)isoxazoles could react smoothly with 1,2-diaminobenzene to form C4-(3-arylquinoxalin-2-yl)isoxazoles. At last, a new one-pot strategy for the synthesis of quinoxalines from 1,2-diphenylethynes and 1,2-diaminobenzene is also reported.

Electrochemistry and Photoredox Catalysis: A Comparative Evaluation in Organic Synthesis

This review provides an overview of synthetic transformations that have been performed by both electro- and photoredox catalysis. Both toolboxes are evaluated and compared in their ability to enable said transformations. Analogies and distinctions are formulated to obtain a better understanding in both research areas. This knowledge can be used to conceptualize new methodological strategies for either of both approaches starting from the other. It was attempted to extract key components that can be used as guidelines to refine, complement and innovate these two disciplines of organic synthesis.

Electrochemical synthesis of 1,2-diketones from alkynes under transition-metal-catalyst-free conditions

A novel electrochemical protocol for the direct oxidation of internal alkynes in air to provide 1,2-diketones was developed.

The Existence of Nitrate Radicals in Irradiated TiO2 Aqueous Suspensions in the Presence of Nitrate Ions

Evidence of the existence of nitrate radical in irradiated aqueous TiO₂ suspensions in the presence of nitrate ions are reported for the first time. The joint use of UV/Vis and EPR spectroscopy showed that nitrate radicals are formed by hole induced oxidation of nitrate ions. Photocatalytic degradation of a model alkene compound allowed to highlight the presence of an intermediate organic nitrate deriving from nitrate radical attack to the double bond of the substrate. These results not only allow deeper understanding of photocatalytic processes, but open the route to new green photocatalytic syntheses initiated by nitrate radicals and to new insights in the field of atmospheric chemistry.

Eosin Y photoredox catalyzed net redox neutral reaction for regiospecific annulation to 3-sulfonylindoles via anion oxidation of sodium sulfinate salts

An eosin Y photoredox catalyzed net redox neutral process for 3-sulfonylindoles via the anionic oxidation of sodium sulfinate salts and its radical cascade cyclization with 2-alkynyl-azidoarenes was developed with visible light as a mediator.

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.

Nitrosonium-Mediated Phenol–Arene Cross-Coupling Involving Direct C–H Activation

The nitrosonium ion (NO+) is a highly versatile nitration and nitrosation reagent, as well as a strong one-electron oxidant. Herein, we describe an environmentally benign and mild method for the in situ formation of NO+ from readily available inorganic nitrate salts, i.e. lithium nitrate, through a finely tuned interplay between formic acid and MeOH, which are used as the solvent system. This methodology was applied to the NO+-induced oxidative C–H activation of methoxy-substituted phenols, which are versatile lignin-derived aromatic feedstocks, to achieve C–C cross-coupling reactions with arenes. The regeneration of NO+ by atmospheric molecular oxygen enables substoichiometric use of the nitrate.

Mechanochemically-assisted solid-state photocatalysis (MASSPC)

Simultaneous milling and visible light irradiation have been utilised to perform the photocatalytic oxidation of an alkyne in the solid state.