Single-electron transfer oxidation-induced C–H bond functionalization via photo-/electrochemistry

Electrochemical Decarboxylation Coupling Reactions

Carboxylic acids are attractive synthetic feedstocks with stable, non-toxic, and inexpensive properties that can be easily obtained from natural sources or through synthesis. Carboxylic acids have long been considered environmentally friendly coupling agents in various organic transformations. In recent years, electrochemically mediated decarboxylation reactions of decarboxylic acids and their derivatives (NHPI) have emerged as effective new methods for constructing carbon-carbon or carbon-heterocarbon chemical bonds. Compared with transition metal and photochemistry-mediated catalytic reactions, which do not require the addition of oxidants and strong bases, electrochemically-mediated decarboxylative transformations are considered a sustainable strategy. In addition, various functional groups tolerate the electrochemical decarboxylation conversion strategy well. Here, we summarize the recent electrochemical decarboxylation reactions to better elucidate the advantages of electrochemical decarboxylation reactions.

Electrochemical and photochemical reaction of isatins: a decade update

This review presents the latest progress in photochemical and electrochemical reactions involving isatins. Isatin and its functionalized scaffolds e.g., oxindoles, spirooxindoles, and quinolines are privileged heterocycles as they are largely present in several agrochemical, natural products, and pharmaceuticals. Thus, the functionalization of isatins using sustainable approaches, i.e., electro- and photochemical methods is of recent research interest worldwide. In this review, we have discussed most of the important reactions of isatins based on types of bond formation involved under electro- and photochemical conditions over the last decade. The reaction mechanism for each reaction has been discussed in detail to offer an inclusive guide to readers. Lastly, a summary of current challenges and the future outlook toward the development of effective electrochemical and photochemical methods for the reaction of isatins is also presented.

Electrochemical Radical Retro-Allylation of Homoallylic Alcohols with Sulfonyl Hydrazides

We report herein the first examples of electrochemical radical retro-allylation of homoallylic alcohols via the cleavage of the C(sp3)-C(sp3) bond. In this reaction, a variety of sulfonyl hydrazides were employed as the environmentally friendly radical sources via an electrochemical dehydrazination with the release of N2 and H2 as the byproducts, leading to sulfonyl allylic compounds in moderate to good yields. The reaction features metal- and base-free reaction conditions, broad functional group tolerance, and a broad substrate scope.

Photocatalytic C-N bond construction toward high-value nitrogenous chemicals

The construction of carbon-nitrogen bonds is vital for producing versatile nitrogenous compounds for the chemical and pharmaceutical industries. Among developed synthetic approaches to nitrogenous chemicals, photocatalysis is particularly prominent and has become one of the emerging fields due to its unique advantages of eco-sustainable characteristics, efficient process integration, no need for high-pressure H2, and tunable synthesis methods for developing advanced photocatalytic materials. Here, the review focuses on potential photocatalytic protocols developed for the construction of robust carbon-nitrogen bonds in discrepant activation environments to produce high-value nitrogenous chemicals. The photocatalytic C-N bond construction strategies and involved reaction mechanisms are elucidated.

Divergent Photosensitizer Controlled Reactions of 4-Hydroxycoumarins and Unactivated Olefins: Hydroarylation and Subsequent [2+2] Cycloaddition

Herein, we report a photoinduced approach for hydroarylation of unactivated olefins using 4-hydroxycoumarins as the arylating reagent. Key to the success of this reaction is the conversion of nucleophilic 4-hydroxycoumarins into electrophilic carbon radicals via photocatalytic arene oxidation, which not only circumvents the polarity-mismatch issue encountered under ionic conditions but also accommodates a broad substrate scope and inhibits side reactions that were previously observed. Moreover, divergent reactivity was achieved by changing the photocatalyst, enabling a subsequent [2+2] cycloaddition to deliver cyclobutane-fused pentacyclic products that are otherwise challenging to access in high yields and with high diastereoselectivity. Mechanistic studies have elucidated the mechanism of the reactions and the origin of the divergent reactivity.

Unlocking the Nucleophilicity of Strong Alkyl C-H Bonds via Cu/Cr Catalysis

Direct functionalization of inert C-H bonds is one of the most attractive yet challenging strategies for constructing molecules in organic chemistry. Herein, we disclose an unprecedented and Earth abundant Cu/Cr catalytic system in which unreactive alkyl C-H bonds are transformed into nucleophilic alkyl-Cr(III) species at room temperature, enabling carbonyl addition reactions with strong alkyl C-H bonds. Various aryl alkyl alcohols are furnished under mild reaction conditions even on a gram scale. Moreover, this new radical-to-polar crossover approach is further applied to the 1,1-difunctionalization of aldehydes with alkanes and different nucleophiles. Mechanistic investigations reveal that the aldehyde not only acts as a reactant but also serves as a photosensitizer to recycle the Cu and Cr catalysts.

OH radical-initiated single-electron transfer for accelerated degradation via carbocation intermediates

Single electron transfer (SET) has made great contributions to a broad range of chemical processes, whose radical cation and carbocation intermediates are important for mechanism studies. Herein, hydroxyl radical (˙OH)-initiated SET was revealed in accelerated degradations, via the online examination of radical cations and carbocations by electrosonic spray ionization mass spectrometry (ESSI-MS). In the green and efficient non-thermal plasma catalysis system (MnO₂-plasma), hydroxychloroquine was efficiently degraded upon SET via carbocations. In the plasma field full of active oxygen species, ˙OH was generated on the MnO₂ surface to initiate SET-based degradations. Furthermore, theoretical calculations revealed that ˙OH preferred to withdraw the electron from the N atom that was conjugated to the benzene ring. This facilitated the generation of radical cations through SET, which was followed by the sequential formation of two carbocations for accelerated degradations. Transition states and energy barriers were calculated to study the formation of radical cations and subsequent carbocation intermediates. This work demonstrates an ˙OH-initiated SET for accelerated degradation via carbocations, providing a deeper understanding and the potential for the wider application of SET in green degradations.

Electrochemically Enabled Direct C3-Formylation of Imidazopyridines with Me3 N as a Carbonyl Source

A metal-free and oxidant-free electrochemically enabled strategy for C-3 formylation of imidazopyridines using trimethylamine as a one-carbon source has been established. This conversion has high functional group compatibility under mild conditions and ensures late-stage functionalization of pharmaceutical molecules. Furthermore, unexpected hexafluoroisopropoxylation products have been observed in some cases. Mechanistic studies using cyclic voltammetry and control experiments reveal the key intermediate of the formylation process.