Electrochemical oxidative selective halogenation of pyrazolones for the synthesis of 4-halopyrazolones

An efficient and environmentally friendly electrochemical oxidative selective halogenation of pyrazolones has been developed under conditions free of metals, external oxidants, and external supporting electrolytes. The reaction demonstrates good functional group tolerance and maintains high efficiency in large-scale synthesis, yielding moderate to excellent yields of the desired 4-halopyrazolones. This method provides a green and convenient route for the direct installation of a halogen moiety into bioactive pyrazolone derivatives, which can be utilized in a myriad of applications.

Synthesis of 3,4,5-Trisubstituted 1,2,4-Triazoles via I2-Catalyzed Cycloaddition of Amidines with Hydrazones

A general and practical method for the construction of various 3,4,5-trisubstituted 1,2,4-triazoles via I2-catalyzed cycloaddition of N-functionalized amidines with hydrazones is reported. This strategy features cheap and readily available catalyst and starting materials, broader substrate scope, and moderate-to-good yields. The mechanism study shows that the existence of hydrogen on the nitrogen of hydrazones is crucial for this transformation.

Enantioselective C-H bond functionalization under Co(III)-catalysis

While progress in enantioselective C-H functionalization has been accomplished by employing 4d and 5d transition metal-based catalysts, the rapid depletion of these metals in the earth's crust poses a serious threat to making these protocols sustainable. On the other hand, because of their unique reactivity, low toxicity, and high earth abundance, newer strategies utilizing affordable 3d transition metals have come to the forefront. Among the first-row transition metals, high-valent cobalt has recently attracted a lot of attention for catalytic C-H functionalization with mono and bidentate directing groups. This approach was extended for asymmetric catalysis due to a fairly thorough knowledge of its catalytic cycles. Four major themes have been investigated as a result of this insight: (1) rational design of a chiral Cp#Co(III)-catalyst, (2) chiral carboxylic acid with achiral Cp*Co(III)-catalysts using monodentate directing groups, (3) cobalt/salox-based systems, and (4) cobalt/chiral phosphoric acid-based hybrid systems with bidentate directing groups. Herein, we highlight the recent developments in high-valent cobalt-catalyzed enantioselective C-H functionalization up to October 2023, with the strong belief that the current state-of-the-art can attract considerable interest in the synthetic community, encouraging discoveries in the evolving landscape of asymmetric catalysis.

B(C6 F5 )3 -Catalyzed [2+3]-Cyclative o,m-diC-H Functionalization of Phenols

Metal-free catalytic C-H functionalization is highly desired for the construction of C-C bonds. We herein report a highly chemoselective consecutive C-H [2+3]-cyclative functionalization for the simultaneous formation of two C-C bonds with construction of polycyclic phenols catalyzed by commercially available and low-cost B(C6 F5 )3 . This catalytic system tolerates a wide range of substrate scope, providing a series of 2,6,7,8-tetrahydroacenaphthylen-3-ol-type polycyclic compounds efficiently. Several derivatizations of the catalytic products have also been conducted to show the potential application of this method in synthesis of polycyclic compounds.

K2S2O8-Induced [4+2] Annulation of Tertiary Anilines and Alkenes toward Tetrahydroquinolines

Due to the unique physicochemical properties of heterocyclic compounds, their construction is one of the central issues in synthetic chemistry. Here, we report a K₂S₂O₈-induced protocol for constructing tetrahydroquinolines from bulk chemicals (alkenes and anilines). The merit of this method has been demonstrated by its operational simplicity, wide scope, mild conditions, and transition-metal-free system.

Light-Driven Carbon-Carbon Coupling of α-sp3-CH of Aliphatic Alcohols with sp2-CH Bond of 1,4-Naphthoquinones

Here, an α-selective C_sp3-H bond functionalization of primary aliphatic alcohols with 1,4-naphthoquinones yielded C_sp²-C_sp² coupled products driven by blue-LED light under catalyst, metal, base, and reagent-free conditions. In this transformation, cleavage of three C-H bonds (two sp³-C-H, one sp²-C-H, and one O-H) and four new bonds formed, leading to fluorescent 2-acylated-1,4-naphthohydroquinones.

Biomimetic catalytic aerobic oxidation of C-sp(3)-H bonds under mild conditions using galactose oxidase model compound CuIIL

Developing highly efficient catalytic protocols for C-sp(3)-H bond aerobic oxidation under mild conditions is a long-desired goal of chemists. Inspired by nature, a biomimetic approach for the aerobic oxidation of C-sp(3)-H by galactose oxidase model compound CuIIL and NHPI (N-hydroxyphthalimide) was developed. The CuIIL-NHPI system exhibited excellent performance in the oxidation of C-sp(3)-H bonds to ketones, especially for light alkanes. The biomimetic catalytic protocol had a broad substrate scope. Mechanistic studies revealed that the CuI-radical intermediate species generated from the intramolecular redox process of CuIILH2 was critical for O2 activation. Kinetic experiments showed that the activation of NHPI was the rate-determining step. Furthermore, activation of NHPI in the CuIIL-NHPI system was demonstrated by time-resolved EPR results. The persistent PINO (phthalimide-N-oxyl) radical mechanism for the aerobic oxidation of C-sp(3)-H bond was demonstrated.

Non-directed Pd-catalysed electrooxidative olefination of arenes

The Fujiwara-Moritani reaction is a powerful tool for the olefination of arenes by Pd-catalysed C-H activation. However, the need for superstoichiometric amounts of toxic chemical oxidants makes the reaction unattractive from an environmental and atom-economical view. Herein, we report the first non-directed and regioselective olefination of simple arenes via an electrooxidative Fujiwara-Moritani reaction. The versatility of this operator-friendly approach was demonstrated by a broad substrate scope which includes arenes, heteroarenes and a variety of olefins. Electroanalytical studies suggest the involvement of a Pd(ii)/Pd(iv) catalytic cycle via a Pd(iii) intermediate.

Silver-catalyzed decarboxylative cyclization for the synthesis of substituted pyrazoles from 1,2-diaza-1,3-dienes and α-keto acids

A silver-catalyzed decarboxylative cyclization process has been developed for the synthesis of substituted pyrazoles from the readily available 1,2-diaza-1,3-dienes and α-keto acids. Under the optimized conditions, a series of multisubstituted pyrazoles were well prepared in moderate to good yields. In addition, the synthetic utility of this protocol has been demonstrated by synthesizing analogs of FDA approved drugs such as anti-inflammatory drug, lonazolac and antiobesity drug, rimonabant.

Expedient cobalt-catalyzed stereospecific cascade C-N and C-O bond formation of styrene oxides with hydrazones

Cobalt-catalyzed cascade C-N and C-O bond formation of epoxides with hydrazones is described to furnish oxadiazines using air as an oxidant. The catalyst plays a dual role as a Lewis acid followed by a redox catalyst to accomplish the C-H/O-H cyclization. Optically active styrene oxide can be reacted enantiospecifically (>99% ee).

Electrochemically driven regioselective C-H phosphorylation of group 8 metallocenes

Metallocenes are privileged backbones for synthesis and catalysis. However, the direct dehydrogenative C−H functionalization of unsymmetric metallocenes suffers from reactivity and selectivity issues. Herein, we report an electrochemically driven regioselective C−H phosphorylation of group 8 metallocenes. Mechanistic investigations indicate this dehydrogenative cross coupling occurs through an electrophilic radical substitution of the metallocene with a phosphoryl radical, facilitated by the metallocene itself. This work not only offers an efficient and divergent synthesis of phosphorylated metallocenes, but also provides a guide to interpret the reactivity and regioselectivity for the C−H functionalization of unsymmetric metallocenes.

Metallocene-based phosphines are compounds with potential use in catalysis. Here, the authors report the electrochemical regioselective functionalization of group 8 metallocenes with phosphine oxides; over 60 examples of phosphorylated (benzo)ferrocenes and ruthenocenes can be accessed via this method without the need for a preinstalled directing group.

Efficient Catalytic Synthesis of Condensed Isoxazole Derivatives via Intramolecular Oxidative Cycloaddition of Aldoximes

The intramolecular oxidative cycloaddition reaction of alkyne- or alkene-tethered aldoximes was catalyzed efficiently by hypervalent iodine(III) species to afford the corresponding polycyclic isoxazole derivatives in up to a 94% yield. The structure of the prepared products was confirmed by various methods, including X-ray crystallography. Mechanistic study demonstrated the crucial role of hydroxy(aryl)iodonium tosylate as a precatalyst, which is generated from 2-iodobenzoic acid and m-chloroperoxybenzoic acid in the presence of a catalytic amount of p-toluenesulfonic acid.

Visible Light-Promoted Synthesis of β-Keto Sulfoximines from N-Tosyl-Protected Sulfoximidoyl Chlorides

Under visible light, N-tosyl-protected sulfoximidoyl chlorides react with aryl alkynes to give β-keto sulfoximines. The reaction is characterized by a high functional group tolerance and good yields. It can be improved by the presence of a ruthenium photocatalyst. Air is the source of the ketonic oxygen in the products.

Photochemical and electrochemical regioselective cross-dehydrogenative C(sp2)–H sulfenylation and selenylation of substituted benzo[a]phenazin-5-ols

The essence of photo- and electrochemistry: sulfenylation and selenylation of substituted benzo[a]phenazin-5-ols through cross-dehydrogenative C(sp2)–H functionalization.

Electrochemical oxidative selenocyclization of olefinic amides towards the synthesis of iminoisobenzofurans

We introduced an electrochemical oxidative radical cascade cyclization of olefinic amides and diselenides without a transition-metal catalyst and external oxidant. This selenocyclization reaction provided a facile method to construct C–Se and C–O bonds in one step.

Electrochemical Difunctionalization of Terminal Alkynes: Access to 1,4-Dicarbonyl Compounds

1,4-Dicarbonyl compounds are versatile scaffolds for the heterocycle synthesis, including the Paal-Knorr reaction. Herein, a feasible electrosynthesis method to access 1,4-dicarbonyl compounds has been developed from simple alkynes and 1,3-dicarbonyl compounds. When the undivided cell is combined with the constant current mode, aryl alkynes containing numerous medicinal motifs with 1,3-dicarbonyl esters or ketones react smoothly. External oxidant and catalyst-free conditions conform to the requirements of green synthesis.

Time-Resolved EPR Revealed the Formation, Structure, and Reactivity of N-Centered Radicals in an Electrochemical C(sp3)-H Arylation Reaction

Electrochemical synthesis has been rapidly developed over the past few years, while a vast majority of the reactions proceed through a radical pathway. Understanding the properties of radical intermediates is crucial in the mechanistic study of electrochemical transformations and will be beneficial for developing new reactions. Nevertheless, it is rather difficult to determine the "live" radical intermediates due to their high reactivity. In this work, the formation and structure of sulfonamide N-centered radicals have been researched directly by using the time-resolved electron paramagnetic resonance (EPR) technique under electrochemical conditions. Supported by the EPR results, the reactivity of N-centered radicals as a mediator in the hydrogen atom transfer (HAT) approach has been discussed. Subsequently, these mechanistic study results have been successfully utilized in the discovery of an unactivated C(sp³)-H arylation reaction. The kinetic experiments have revealed the rate-determined step is the anodic oxidation of sulfonamides.

Facile and scalable synthesis of baphicacanthin A by a two-pot procedure

Facile two-pot total synthesis of baphicacanthin A, a natural phenoxazinone alkaloid isolated from the roots of Baphicacanthus cusia which has been utilized as a traditional chinese medicine to effectively treat disease caused by coronavirus, has been developed from simple and commercially available starting materials. Catalytic aerobic oxidative cross-cyclocondensation of equimolar 2-aminophenol and 3-methoxy-2-hydroxylphenol in water was used to construct the key molecular skeleton 2-hydroxy-3H-phenoxazin-3-one. Gram scale synthesis was realized in 80% overall yield with practical convenience.

Electrochemical Palladium-Catalyzed Oxidative Sonogashira Carbonylation of Arylhydrazines and Alkynes to Ynones

Oxidative carbonylation using carbon monoxide has evolved as an attractive tool to valuable carbonyl-containing compounds, while mixing CO with a stoichiometric amount of a chemical oxidant especially oxygen is hazardous and limits its application in scale-up synthesis. By employing anodic oxidation, we developed an electrochemical palladium-catalyzed oxidative carbonylation of arylhydrazines with alkynes, which is regarded as an alternative supplement of the carbonylative Sonogashira reaction. Combining an undivided cell with constant current mode, oxygen-free conditions avoids the explosion hazard of CO. A diversity of ynones are efficiently obtained using accessible arylhydrazines and alkynes under copper-free conditions. A possible mechanism of the electrochemical Pd(0)/Pd(II) cycle is rationalized based upon cyclic voltammetry, kinetic studies, and intermediates experiments.

Oxidant-Induced Azolation of Electron-Rich Phenol Derivatives

Since N-arylazoles are widely present in natural products, pharmaceuticals, and functional materials, it is important to develop a simple and efficient synthetic method for the synthesis of N-arylazoles. Herein, an oxidant-induced intermolecular azolation of phenol derivatives was demonstrated under catalyst-free condition. Both monoazolation and diazolation of phenols can be successfully achieved via this practical and powerful method.

Transition metal-catalyzed coupling of heterocyclic alkenes via C–H functionalization: recent trends and applications

Heterocyclic alkenes and their derivatives are an important class of reactive feedstock and valuable synthons. This review highlights the transition-metal-catalyzed coupling of heterocyclic alkenes via a C–H functionalization strategy.

CO2 = CO + [O]: recent advances in carbonylation of C–H bonds with CO2

Carbon dioxide (CO₂) is an ideal one-carbon source owing to its nontoxicity, abundance, availability, and recyclability.

Radical-mediated oxidative annulations of 1,n-enynes involving C–H functionalization

Recent progress in oxidative annulations of 1,n-enynes involving C–H functionalization is summarized.

Ultrasmall Silver Clusters Stabilized on MgO for Robust Oxygen-Promoted Hydrogen Production from Formaldehyde Reforming

Efficient molecular hydrogen generation from renewable biomass-derived resources and water is of great importance to the sustainable development of the future society. Herein, ultrasmall Ag nanoclusters supported on a defect-rich MgO matrix (AgUCs/MgO) are synthesized by a facile impregnation/calcination method and are applied to robust oxygen-promoted formaldehyde reforming into H₂ at room temperature. Density functional theory calculations and experimental observations show that the catalyst spatially builds up a channel for directional electron transfer from electron-rich Ag sites to the anti-bonding π orbital of chemisorbed bridged O₂ molecules, leading to the implementation of low-temperature O₂ adsorption and activation. The catalytically active species, ^•OOH, is thus selectively generated via a preferential two-electron reduction of O₂ with a low energy barrier on Ag sites, involving an unusual long-range proton-coupled electron transfer process. The ^•OOH-AgUCs/MgO active center is efficient for the subsequent C-H activation and H₂ generation, leading to a 3-fold improvement of the turnover frequency as compared with its analogous AgNPs/MgO catalyst. Our atomic-level design and synthetic strategy provide a platform that facilitates the construction of an electron-proton transfer channel for catalysis, altered adsorption configurations of activated reactants, and enhancement of catalytic hydrogen generation activity, extending a promising direction for the development of next-generation energy catalysts.

tBuOK-triggered bond formation reactions

Recently, inexpensive and readily available tBuOK has seen widespread use in transition-metal-free reactions. Herein, we report the use of tBuOK for S-S, S-Se, N[double bond, length as m-dash]N and C[double bond, length as m-dash]N bond formations, which significantly extends the scope of tBuOK in chemical synthesis. Compared with traditional methods, we have realized mild and general methods for disulfide, azobenzenes imine etc. synthesis.