While electrosynthesis represents a green and advantageous alternative to traditional synthetic methods, electrochemical reactions still suffer from some drawbacks that require further efforts in ...order to fully express the potential of electricity-driven transformations. In this Comment, we will briefly discuss both the advantages and limitations of electrosynthesis, especially when compared with the other traditional synthetic organic methods, and share some forward-looking thoughts on the future developments of electrochemical reactions.
In recent decades, iodine-catalyzed oxidative coupling reactions utilizing C - H and X - H as nucleophiles have received considerable attention because they represent more efficient, greener, more ...atom-economical, and milder bond-formation strategies over transition-metal-catalyzed oxidative coupling reactions. This Focus Review gives a brief summary of recent development on iodine-catalyzed oxidative coupling reactions utilizing C - H and X - H as nucleophiles.
Conspectus Oxidative cross-coupling reactions between two nucleophiles are a powerful synthetic strategy to synthesize various kinds of functional molecules. Along with the development of ...transition-metal-catalyzed oxidative cross-coupling reactions, chemists are applying more and more first-row transition metal salts (Fe, Co, etc.) as catalysts. Since first-row transition metals often can go through multiple chemical valence changes, those oxidative cross-couplings can involve single electron transfer processes. In the meantime, chemists have developed diverse mechanistic hypotheses of these types of reactions. However, none of these hypotheses have led to conclusive reaction pathways until now. From studying both our own work and that of others in this field, we believe that radical oxidative cross-coupling reactions can be classified into four models based on the final bond formations. In this Account, we categorize and summarize these models. In model I, one of the starting nucleophiles initially loses one electron to generate its corresponding radical under oxidative conditions. Then, bond formations between this radical and another nucleophile create a new radical, Nu1–Nu2•, followed by a further radical oxidation step to generate the cross-coupling product. The radical oxidative alkenylation with olefin, radical oxidative arylative-annulation, and radical oxidative amidation are examples of this model. In model II, one of the starting nucleophiles loses its two electrons via two steps of single-electron-transfer to generate an electrophilic intermediate, followed by a direct bond formation with the other nucleophile. For example, the oxidative C–O coupling of benzylic sp3 C–H bonds with carboxylic acids and oxidative C–N coupling of aldehydes with amides are members of this model group. For model III, both nucleophiles are oxidized to their corresponding radicals. Then, the radicals combine to form the final coupling product. The dioxygen-involved radical oxidative cross-couplings between sulfinic acids and olefins or alkynes belong to this bond formation model. Lastly, in model IV, one nucleophile loses two electrons to generate an electrophilic intermediate, while the other nucleophile loses one electron to generate a radical. Then, a bond forms between the cation and the radical to generate a cationic radical, followed by a one-electron reduction to afford the final coupling product. The oxidative coupling between arylboronic acids and simple ethers was classified in this model. At the current stage, there are only a few examples presented for models III and IV, but they represent two types of potentially important transformations. More and more examples of these two models will be developed in the future.
Bond formations between two nucleophiles are examined. Chemical reactions during transition metal matalyzed oxidative cross-coupling reactions are also discussed.
Direct olefinic C-H functionalization represents the ideal way of introducing an alkenyl group into organic molecules. A well-known process is the Heck reaction, which involves alkene insertion and ...β-hydride elimination in the presence of a transition metal. However, the traditional Heck reaction mainly deals with the alkenylation of aryl or vinyl electrophiles. Recent developments have revealed that alkenylation can also be achieved through radical addition to alkenes and following single-electron-transfer (SET) oxidation/elimination. The radical alkenylation pathway allows alkenylation with a variety of carbon-centered radicals and even heteroatom-centered radicals. This tutorial review gives an overview of recent advances in this emerging field.
Considering the synthetic value of introducing active alcoholic hydroxyl group, developing C-H functionalization of alcohols is of significance. Herein, we present a photochemical method that under ...visible light irradiation, selectfluor can effectively promote the oxidative cross-coupling between alcohols and heteroarenes without the external photocatalysis, achieving the selective α sp
C-H arylation of alcohol, even in the presence of ether. The N-F activation of selectfluor under blue LEDs irradiation is evidenced by electron paramagnetic resonance (EPR) study, which is the key process for the oxidative activation of α sp
C-H alcohols. The observed reactivity may have significant implications for chemical transformations.
Intermolecular 3 + 2 annulation is one of the most straightforward approaches to construct five membered heterocycles. However, it generally requires the use of functionalized substrates. An ideal ...reaction approach is to achieve dehydrogenative 3 + 2 annulation under oxidant-free conditions. Here we show an electrooxidative 3 + 2 annulation between phenols and N-acetylindoles under undivided electrolytic conditions. Neither external chemical oxidants nor metal catalysts are required to facilitate the dehydrogenation processes. This reaction protocol provides an environmentally friendly way for the selective synthesis of benzofuroindolines. Various N-acetylindoles bearing different C-3 and C-2 substituents are suitable in this electrochemical transformation, furnishing corresponding benzofuroindolines in up to 99% yield.Electrochemical oxidation provides a green alternative to the use of hazardous chemical oxidants and forcing conditions. Here, the authors show the electrocatalytic cross-coupling of phenols and indoles to generate biologically relevant benzofuroindolines in high yields.
Oxidative C-H/N-H cross-coupling is one of the most atom-economical methods for the construction of C-N bonds. However, traditional oxidative C-H/N-H cross-coupling either required the use of strong ...oxidants or high reaction temperature, which makes it difficult to tolerate redox active functional groups. Herein we describe an external chemical oxidant-free electrooxidative C-H/N-H cross-coupling between electron-rich arenes and diarylamine derivatives. Under undivided electrolytic conditions, a series of triarylamine derivatives are produced from electron-rich arenes and diarylamine derivatives with high functional group tolerance. Both of the coupling partners are redox active in oxidative C-H/N-H cross-coupling, which enables high regioselectivity in C-N bond formation. Exclusive para-selectivity is observed for the coupling with anilines.
Ethylene and ethyne are among the simplest two-carbon building blocks. However, quite limited methods can be applied to incorporate ethylene or ethyne into fine chemicals. Here we demonstrate a ...cobalt-catalyzed dehydrogenative C-H/N-H 4+2 annulation of aryl/vinyl amides with ethylene or ethyne by using an electrochemical reaction protocol. Significantly, this work shows an example of electrochemical recycling of cobalt catalyst in oxidative C-H functionalization reactions, avoiding the use of external chemical oxidants and co-oxidants. The electrochemical method provides a reliable and safe way for incorporating gas-phase ethylene or ethyne into fine chemicals. High reaction efficiency and good functional group tolerance are observed under divided electrolytic conditions.
Oxidative carbonylation reactions have attracted broad interest from both academia and industry in recent years. Enormous efforts have gone into the syntheses of carbonate and urea derivatives ...through the oxidative carbonylation of alcohols and amines. Very recently, organometallic reagents (RM) and hydrocarbons(RH) were directly employed as nucleophiles to construct a CC bond in oxidative carbonylation reactions. This Minireview summarizes this novel type of oxidative carbonylation reaction.
A new option: Classical carbonylation reactions employ organohalides as electrophiles to produce carbonyl compounds. Now a series of carbonylative derivatives can also efficiently synthesized by oxidative carbonylation reactions that have an enriched substrate scope and milder reaction conditions. This Minireview summarizes newly developed oxidative carbonylation reactions employing either RM or RH as the nucleophiles.
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