Selective Functionalization of Styrenes with Oxygen Using Different Electrode Materials: Olefin Cleavage and Synthesis of Tetrahydrofuran Derivatives

Electrochemically Enabled Hydroxyphosphorylation of 1,3-Enynes to Access Phosphinyl-Substituted Propargyl Alcohols

Catalytic difunctionalization with the direct activation of (O)P-H bonds has been recently established as a potentially robust platform to generate valuable organophosphorus compounds. In terms of 1,3-enynes, despite of the various catalytic methods developed for hydrophosphorylation, the radical-mediated hetero-functionalization of two different atoms has been less explored. In this study, we disclosed an electrochemically induced hydroxyphosphorylation of 1,3-enynes for the construction of phosphinyl-substituted propargyl alcohols. The system involves the direct activation of both arylphosphine oxides and oxygen in ambient air with no external metal or additive needed. The use of electrochemistry ensures the regioselective, atom-economic and eco-friendly for the difunctionalization process. This strategy highlights the advantages of mild reaction conditions, readily available starting materials and broad substrate scope, showing its practical synthetic value in organic synthesis.

Triaryl Carbenium Ion Pair Mediated Electrocatalytic Benzylic C-H Oxygenation in Air

The selective oxidation of benzylic C-H bonds is a pivotal transformation in organic synthesis. Undoubtedly, achieving efficient and highly selective aerobic oxidation of methylarenes to benzaldehydes has been highly challenging due to the propensity of benzaldehyde to undergo overoxidation under typical aerobic conditions. Herein, we propose an innovative approach to address this issue by leveraging electrocatalytic processes, facilitated by ion-pair mediators [Ph3C]+[B(C6F5)4]-. By harnessing the power of electrochemistry, we successfully demonstrated the effectiveness of our strategy, which enables the selective oxidation of benzylic C-H bonds in benzylic molecules and toluene derivatives. Notably, our approach exhibited high efficiency, excellent selectivity, and compatibility with various functional groups, underscoring the broad applicability of our methodology.

Electrochemical aerobic Wacker-type oxygenation of triaryl substituted alkenes to 1,2,2-triarylethanones

An effective synthetic approach for various 1,2,2-triarylethanones from triaryl substituted alkenes has been developed via an electrochemical Wacker-type oxygenation with O2 as the sole oxygen source. It presents the first instance of the Wacker-type oxidation expanding its substrate scope to trisubstituted alkenes. The approach is transition-metal-free, compatible with various functional groups, and can be carried out under mild conditions resulting in satisfactory yields. Mechanistic experiments suggest the CO bond formation occurs through reactions between cationic carbon species and the superoxide radical, which involves the 1,2-shift of the electron-rich substituent.

Electrochemical C-H/C-C Bond Oxygenation: A Potential Technology for Plastic Depolymerization

Herein, we provide eco-friendly and safely operated electrocatalytic methods for the selective oxidation directly or with water, air, light, metal catalyst or other mediators serving as the only oxygen supply. Heavy metals, stoichiometric chemical oxidants, or harsh conditions were drawbacks of earlier oxidative cleavage techniques. It has recently come to light that a crucial stage in the deconstruction of plastic waste and the utilization of biomass is the selective activation of inert C(sp3 )-C/H(sp3 ) bonds, which continues to be a significant obstacle in the chemical upcycling of resistant polyolefin waste. An appealing alternative to chemical oxidations using oxygen and catalysts is direct or indirect electrochemical conversion. An essential transition in the chemical and pharmaceutical industries is the electrochemical oxidation of C-H/C-C bonds. In this review, we discuss cutting-edge approaches to chemically recycle commercial plastics and feasible C-C/C-H bonds oxygenation routes for industrial scale-up.

Electrochemical Trifluoromethylation/Bicyclization of N-Cyanamide Alkenes: Synthesis of Bicyclic Amidine Derivatives

An anodically oxidizing trifluoromethylation cascade of N-cyanamide alkene bearing two electronically differentiated olefin moieties was reported, in which various N-unsaturated acyl cyanamide alkenes and CF3SO2Na acting as readily available starting materials furnished nonaromatic fused azaheterobicyclic compounds in a highly efficient and sustainable manner. The broad substrate scope, facile scalability, and sustainability enabled this electrochemical process to be an appealing complement for trifluoromethylated cyclic amidines.

Carbon-Carbon Bond Cleavage for Late-Stage Functionalization

Late-stage functionalization (LSF) introduces functional group or structural modification at the final stage of the synthesis of natural products, drugs, and complex compounds. It is anticipated that late-stage functionalization would improve drug discovery's effectiveness and efficiency and hasten the creation of various chemical libraries. Consequently, late-stage functionalization of natural products is a productive technique to produce natural product derivatives, which significantly impacts chemical biology and drug development. Carbon-carbon bonds make up the fundamental framework of organic molecules. Compared with the carbon-carbon bond construction, the carbon-carbon bond activation can directly enable molecular editing (deletion, insertion, or modification of atoms or groups of atoms) and provide a more efficient and accurate synthetic strategy. However, the efficient and selective activation of unstrained carbon-carbon bonds is still one of the most challenging projects in organic synthesis. This review encompasses the strategies employed in recent years for carbon-carbon bond cleavage by explicitly focusing on their applicability in late-stage functionalization. This review expands the current discourse on carbon-carbon bond cleavage in late-stage functionalization reactions by providing a comprehensive overview of the selective cleavage of various types of carbon-carbon bonds. This includes C-C(sp), C-C(sp2), and C-C(sp3) single bonds; carbon-carbon double bonds; and carbon-carbon triple bonds, with a focus on catalysis by transition metals or organocatalysts. Additionally, specific topics, such as ring-opening processes involving carbon-carbon bond cleavage in three-, four-, five-, and six-membered rings, are discussed, and exemplar applications of these techniques are showcased in the context of complex bioactive molecules or drug discovery. This review aims to shed light on recent advancements in the field and propose potential avenues for future research in the realm of late-stage carbon-carbon bond functionalization.

Alkene reactions with superoxide radical anions in flow electrochemistry

Alkenes were cleaved to ketones by using dioxygen in an electrochemical flow set-up. The pressurised system allowed efficient gas-liquid mixing with a stabilised flow. This mild and straightforward approach avoids the use of transition metals and harsh oxidants.

Electrochemical oxo-functionalization of cyclic alkanes and alkenes using nitrate and oxygen

Direct functionalization of C(sp³)-H bonds allows rapid access to valuable products, starting from simple petrochemicals. However, the chemical transformation of non-activated methylene groups remains challenging for organic synthesis. Here, we report a general electrochemical method for the oxidation of C(sp³)-H and C(sp²)-H bonds, in which cyclic alkanes and (cyclic) olefins are converted into cycloaliphatic ketones as well as aliphatic (di)carboxylic acids. This resource-friendly method is based on nitrate salts in a dual role as anodic mediator and supporting electrolyte, which can be recovered and recycled. Reducing molecular oxygen as a cathodic counter reaction leads to efficient convergent use of both electrode reactions. By avoiding transition metals and chemical oxidizers, this protocol represents a sustainable oxo-functionalization method, leading to a valuable contribution for the sustainable conversion of petrochemical feedstocks into synthetically usable fine chemicals and commodities.

Electro-/photocatalytic alkene-derived radical cation chemistry: recent advances in synthetic applications

Alkene-derived radical cations are versatile reactive intermediates and have been widely applied in the construction of complex functionalized molecules and cyclic systems for chemical synthesis. Therefore, the synthetic application of these alkene-derived radical cations represents a powerful and green tool that can be used to achieve the functionalization of alkenes partially because the necessity of stoichiometric external chemical oxidants and/or hazardous reaction conditions is eliminated. This review summarizes the recent advances in the synthetic applications of the electro-/photochemical alkene-derived radical cations, emphasizing the key single-electron oxidation steps of the alkenes, the scope and limitations of the substrates, and the related reaction mechanisms. Using electrocatalysis and/or photocatalysis, single electron transfer (SET) oxidation of the CC bonds in the alkenes occurs, generating the alkene-derived radical cations, which sequentially enables the functionalization of translocated radical cations to occur in two ways: the first involves direct reaction with a nucleophile/radical or two molecules of nucleophiles to realize hydrofunctionalization, difunctionalization and cyclization; and the second involves the transformation of the alkene-derived radical cations into carbon-centered radicals using a base followed by radical coupling or oxidative nucleophilic coupling.

Oxidation Potential Gap (ΔEox ): The Hidden Parameter in Redox Chemistry

Oxidative biaryl coupling of aryls with different electronic features generally fails. However, this has not been systematically studied via theoretical analysis, and thus, the crucial factor governing coupling efficiency remains unclear. Herein, we propose that the "oxidation potential gap (ΔE_ox )" is a key parameter in predicting the efficiency of an intramolecular oxidative coupling reaction, with ΔE_ox defined as a difference in the oxidation potentials of the relevant aromatic rings. Our experimental and computational analyses revealed that the efficiency of an aromatic intramolecular coupling reaction correlates with the activation energy (ΔE^≠ ) of C-C bond formation of the radical cation intermediates. Furthermore, ΔE^≠ correlates with ΔE_ox . Therefore, we demonstrate the tuning of ΔE_ox by attaching cleavable extra electron-donating/-withdrawing groups, enabling the rational synthesis of a phenanthridone skeleton using aromatic rings with an electronic gap.

Electrochemical two-electron oxygen reduction reaction (ORR) induced aerobic oxidation of α-diazoesters

Electrochemical oxygen reduction reaction (ORR) is a powerful tool for introducing oxygen functional groups in synthetic chemistry. However, compared with the well-developed one-electron oxygen reduction process, the applications of two-electron oxygen reduction in electrochemical synthesis have been seldom studied. We present herein our recent progress in the oxidation of α-diazoesters to α-ketoesters by in situ generated hydrogen peroxide via a two-electron oxygen reduction approach. A diverse collection of valuable α-ketoester products was obtained with moderate to high yields under an exogenous-oxidant-free and metal catalyst-free electrochemical conditions.

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.

Electrochemical Difunctionalization of Styrenes via Chemoselective Oxo-Azidation or Oxo-Hydroxyphthalimidation

Atom- and step-economic oxo-azidation and oxo-hydroxyphthalimidation of styrenes have been developed under mild electrolytic conditions, respectively. Various valuable alpha-azido or hydroxyphthalimide aromatic ketones were synthesized efficiently from readily available styrenes, azides, and N-hydroxyphthalimides. Mechanism studies show that two different pathways involved in these two transformations.

1,2-Dibutoxyethane-Promoted Oxidative Cleavage of Olefins into Carboxylic Acids Using O2 Under Clean Conditions

Herein, we report the first example of an effective and green approach for the oxidative cleavage of olefins to carboxylic acids using a 1,2-dibutoxyethane/O₂ system under clean conditions. This novel oxidation system also has excellent functional-group tolerance and is applicable for large-scale synthesis. The target products were prepared in good to excellent yields by a one-pot sequential transformation without an external initiator, catalyst, and additive.

Directing transition metal-based oxygen-functionalization catalysis

This review presents the recent progress of oxygen functionalization reactions based on non-electrochemical (conventional organic synthesis) and electrochemical methods. Although both methods have their advantages and limitations, the former approach has been used to synthesize a broader range of organic substances as the latter is limited by several factors, such as poor selectivity and high energy cost. However, because electrochemical methods can replace harmful terminal oxidizers with external voltage, organic electrosynthesis has emerged as greener and more eco-friendly compared to conventional organic synthesis. The progress of electrochemical methods toward oxygen functionalization is presented by an in-depth discussion of different types of electrically driven-chemical organic synthesis, with particular attention to recently developed electrochemical systems and catalyst designs. We hope to direct the attention of readers to the latest breakthroughs of traditional oxygen functionalization reactions and to the potential of electrochemistry for the transformation of organic substrates to useful end products.

Electrochemical synthesis of quinazolinone via I2-catalyzed tandem oxidation in aqueous solution

The development of protocols for synthesizing quinazolinones using biocompatible catalysts in aqueous medium will help to resolve the difficulties of using green and sustainable chemistry for their synthesis. Herein, using I₂ in coordination with electrochemical synthesis induced a C-H oxidation reaction which is reported when using water as the environmentally friendly solvent to access a broad range of quinazolinones at room temperature. The reaction mechanism strongly showed that I₂ cooperates electrochemically promoted the oxidation of alcohols, then effectively cyclizing amides to various quinazolinones.

Poly(ethylene glycol) dimethyl ether mediated oxidative scission of aromatic olefins to carbonyl compounds by molecular oxygen

A simple, and practical oxidative scission of aromatic olefins to carbonyl compounds using O₂ as the sole oxidant with poly(ethylene glycol) dimethyl ether as a benign solvent has been developed. A wide range of monosubstituted, gem-disubstituted, 1,2-disubstituted, trisubstituted and tetrasubstituted aromatic olefins was successfully converted into the corresponding aldehydes and ketones in excellent yields even with gram–scale reaction. Some control experiments were also conducted to support a possible reaction pathway.

A simple and practical O₂ oxidized scission of monosubstituted, gem- and 1,2-disubstituted, trisubstituted and tetrasubstituted aromatic olefins to aldehydes and ketones in PEGDME has been developed.

Electrochemical Oxidative Carbon-Atom Difunctionalization: Towards Multisubstituted Imino Sulfide Ethers

Ethers (C-O/S) are ubiquitously found in a wide array of functional molecules and natural products. Nonetheless, the synthesis of imino sulfide ethers, containing an N(sp² )=C(sp² )-O/S fragment, still remains a challenge because of its sensitivity to acid. Developed here in is an unprecedented electrochemical oxidative carbon-atom difunctionalization of isocyanides, providing a series of novel multisubstituted imino sulfide ethers. Under metal-free and external oxidant-free conditions, isocyanides react smoothly with simple and readily available mercaptans and alcohols. Importantly, the procedure exhibited high stereoselectivities, excellent functional-group tolerance, and good efficiency on large-scale synthesis, as well as further derivatization of the products.

Highly efficient oxidative cleavage of olefins with O2 under catalyst-, initiator- and additive-free conditions

Without employing any external catalyst, initiator and additives, an efficient and eco-friendly protocol has been developed for the synthesis of carbonyl compound via 1,4-dioxane- promoted oxidation of olefins with atmospheric O₂ as the sole oxidant.

Selective Dealkenylative Functionalization of Styrenes via C-C Bond Cleavage

As a readily available feedstock, styrene with about 25 million tons of global annual production serves as an important building block and organic synthon for the synthesis of fine chemicals, polystyrene plastics, and elastomers. Thus, in the past decades, many direct transformations of this costless styrene feedstock were disclosed for the preparation of high-value chemicals, which to date, generally performed on the functionalization of styrenes through the allylic C-H bond, C(sp²)-H bond, or the C=C double bond cleavage. However, the dealkenylative functionalization of styrenes via the direct C-C single bond cleavage is so far challenging and still unknown. Herein, we report the novel and efficient C-C amination and hydroxylation reactions of styrenes for the synthesis of valuable aryl amines and phenols via the site-selective C(Ar)-C(alkenyl) single bond cleavage. This chemistry unlocks the new transformation and application of the styrene feedstock and provides an efficient protocol for the late-stage modification of substituted styrenes with the site-directed dealkenylative amination and hydroxylation.

Manganese/Copper Co-catalyzed Electrochemical Wacker-Tsuji-Type Oxidation of Aryl-Substituted Alkenes

A manganese/copper co-catalyzed electrochemical Wacker-Tsuji-type oxidation of aryl-substituted alkenes has been developed. The process involves the use of 5 mol % MnBr₂ and 7.5 mol % CuCl₂, in 4:1 acetonitrile/water in an undivided cell at 60 °C, with 2.8 V constant applied potential. α-Aryl ketones are formed in moderate to excellent yields, with the advantages of avoidance of palladium as a catalyst and any external chemical oxidant in an easily operated, cost-effective procedure.

Amphiphilic Carbon Quantum Dots as a Bridge to a Pseudohomogeneous Catalyst for Selective Oxidative Cracking of Alkenes to Aldehydes: A Nonmetallic Oxidation System

The oxidative cleavage of alkenes to the corresponding aldehydes using new amphiphilic carbon quantum dots (A-CQDs) as a pseudohomogeneous carbocatalyst is achieved for the first time through green and sustainable chemical processes. In this work, we successfully design a recyclable pseudohomogeneous catalyst based on A-CQDs, which is decorated with 1-aminopropyl-3-methyl-imidazolium chloride and stearic acid. The functionalization is conducted to introduce a hydrophilic/hydrophobic functionality on the surface of the catalyst to achieve high catalyst availability in polar and nonpolar media with the green goal of eliminating organic (co)solvents and additives. This amphiphilic carbocatalyst provides high mass transferability to the biphasic system, which is beneficial to promoting the oxidative cracking of a variety of olefins into corresponding aldehydes with a substrate/A-CQD ratio of 150. Around 87% of the substrates are converted to the related aldehydes using the carbocatalyst in the presence of H₂O₂, in pure water, without using a phase-transfer catalyst or any additives and organic solvents, which is comparable with the current metal-based cleavage systems. Surprisingly, A-CQDs exhibit high catalytic activity for the scission of electron-deficient C═C bond of coumarin derivatives, accompanied by the cleavage of C-O bonds to produce the corresponding salicylaldehyde derivatives without overoxidation to acid. As a brief conclusion, A-CQDs exhibit high conversion efficiency without significant loss of activity even after six catalytic cycles. The conversion of alkenes into aldehydes is fast and high-throughput without overoxidation to acids and is accompanied by excellent solubility and stability in various solvents. Moreover, the product and the catalyst are recoverable from the reaction medium by simple extraction. So, this pseudohomogeneous carbocatalyst promises new horizons in imminent "catalytic age". All in all, this paper provides a significant and novel advancement in carbocatalyst chemistry.

Electrochemical Total Synthesis of Pyrrolophenanthridone Alkaloids: Controlling the Anodically Initiated Electron Transfer Process

Electrochemical intramolecular C(sp²)-H cross-coupling and dehydrogenative indole synthesis were developed. Both reactions were initiated by anodic oxidation of the same electron-rich indoline moiety, but the product selectivity was controlled by different electron-transfer processes. Intramolecular cross-coupling was achieved by the generation of a strong electrophilic radical cation intermediate in the MeNO₂-HFIP-LiClO₄ system. Indole formation was accomplished through benzylic oxidation and continuous deprotonation. We applied these reactions to the total synthesis of natural pyrrolophenanthridone alkaloids.

Selenium-catalyzed oxidation of alkenes: insight into the mechanisms and developing trend

Recent progresses of the selenium-catalyzed oxidation of alkenes are summarized at the mechanism level. It may be beneficial for designing novel selenium-containing catalysts and alkene oxidation protocols for the next phase of studies.

Vanadium-Catalyzed Selective Oxidative Homocoupling of Alkenyl Phenols to Synthesize Lignan Analogs

The oxidative homocoupling of para-alkenyl phenols and subsequent trapping of the resulting quinone methide with a variety of oxygen and nitrogen nucleophiles was achieved. Both β-β and β-O coupling isomers can be synthesized via either C-C coupling and two nucleophilic additions of one water molecule (β-β isomer) or C-O coupling followed by one nucleophilic addition of a water molecule (β-O isomer), respectively. Selectivity between these outcomes was achieved by leveraging understanding of the mechanism. Specifically, a qualitative predictive model for the selectivity of the coupling was formulated based on catalyst electronics, solvent polarity, and concentration.

Practical and stereoselective electrocatalytic 1,2-diamination of alkenes

The 1,2-diamine motif is widely present in natural products, pharmaceutical compounds, and catalysts used in asymmetric synthesis. The simultaneous introduction of two amino groups across an alkene feedstock is an appealing yet challenging approach for the synthesis of 1,2-diamines, primarily due to the inhibitory effect of the diamine products to transition metal catalysts and the difficulty in controlling reaction diastereoselectivity and regioselectivity. Herein we report a scalable electrocatalytic 1,2-diamination reaction that can be used to convert stable, easily available aryl alkenes and sulfamides to 1,2-diamines with excellent diastereoselectivity. Monosubstituted sulfamides react in a regioselective manner to afford 1,2-diamines bearing different substituents on the two amino groups. The combination of an organic redox catalyst and electricity not only obviates the use of any transition metal catalyst and oxidizing reagent, but also ensures broad reaction compatibility with a variety of electronically and sterically diverse substrates.

Methods to prepare 1,2-diamines are desirable due the importance of these compounds as drug scaffolds and organic ligands for metals. Here, the authors report an electrochemical metal-free 1,2- diamination of aryl alkenes with sulfamides to 1,2-diamines with excellent diastereoselectivity.

The Mn-catalyzed paired electrochemical facile oxychlorination of styrenes via the oxygen reduction reaction

Reported herein is the electrochemical engendering of chlorine radicals by a manganese catalyst with a controllable pattern, and inexpensive MgCl₂ as the chlorine source.