Electrochemical oxidation-induced etherification via C(sp3)─H/O─H cross-coupling
TL;DR: This protocol not only offers a practical strategy for the construction of C─O bonds using nonsolvent amounts of alcohols but also allows direct electrochemical benzylic and allylic C(sp3)─H functionalization in the absence of transition metal catalysis.
Abstract: Direct electrochemical construction of C─O bonds through C(sp3)─H functionalization still remains fundamentally challenging. Here, electrochemical oxidation-induced benzylic and allylic C(sp3)─H etherification has been developed. This protocol not only offers a practical strategy for the construction of C─O bonds using nonsolvent amounts of alcohols but also allows direct electrochemical benzylic and allylic C(sp3)─H functionalization in the absence of transition metal catalysis. A series of alcohols and benzylic and allylic C(sp3)─H compounds were compatible with this transformation. Mechanistically, the generation of aryl radical cation intermediates is the key to this C(sp3)─H etherification, as evidenced by radical probe substrate (cyclopropane ring opening) and electron paramagnetic resonance experiments.
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TL;DR: In this paper, the authors highlight the differences and similarities between electrochemistry and photoredox catalysis by comparing their underlying physical chemistry principles and describing their impact on electrochemical and photochemical methods.
Abstract: Redox processes are at the heart of synthetic methods that rely on either electrochemistry or photoredox catalysis, but how do electrochemistry and photoredox catalysis compare? Both approaches provide access to high energy intermediates (e.g., radicals) that enable bond formations not constrained by the rules of ionic or 2 electron (e) mechanisms. Instead, they enable 1e mechanisms capable of bypassing electronic or steric limitations and protecting group requirements, thus enabling synthetic chemists to disconnect molecules in new and different ways. However, while providing access to similar intermediates, electrochemistry and photoredox catalysis differ in several physical chemistry principles. Understanding those differences can be key to designing new transformations and forging new bond disconnections. This review aims to highlight these differences and similarities between electrochemistry and photoredox catalysis by comparing their underlying physical chemistry principles and describing their impact on electrochemical and photochemical methods.
132 citations
TL;DR: An overview of the most recent developments in electrochemical oxidative cross-coupling with hydrogen evolution involving radicals is provided in this paper, where the focus is mainly placed on synthetic and mechanistic aspects.
Abstract: Oxidative cross-coupling has developed into a robust method for carbon–carbon (C–C), carbon–heteroatom (C–X), and heteroatom–heteroatom (X–Y) bond formation. Despite considerable advances in this field, the traditional oxidative cross-coupling reactions usually employ stoichiometric amounts of chemical oxidants to clean up surplus electrons from substrates to form new chemical bonds. Organic electrosynthesis is recognized as an environmentally benign and particularly powerful synthetic platform. Recent advancements have revealed that radical-involved electrochemical oxidative cross-coupling reactions can be achieved under exogenous-oxidant-free conditions. This tutorial review provides an overview of the most recent developments in electrochemical oxidative cross-coupling with hydrogen evolution involving radicals. Emphasis is mainly placed on synthetic and mechanistic aspects. We hope that this tutorial review can promote the development of radical chemistry, electrochemistry, and oxidative cross-coupling reactions.
125 citations
TL;DR: This work presents a new strategy for silyl radical generation via electroreduction of readily available chlorosilanes through energetically uphill reductive cleavage of strong Si-Cl bonds, which proved to be general in various alkene silylation reactions including disilylation, hydrosilylated, and allylic silylated under simple and transition-metal-free conditions.
Abstract: The construction of C(sp3)-Si bonds is important in synthetic, medicinal, and materials chemistry. In this context, reactions mediated by silyl radicals have become increasingly attractive but methods for accessing these intermediates remain limited. We present a new strategy for silyl radical generation via electroreduction of readily available chlorosilanes. At highly biased potentials, electrochemistry grants access to silyl radicals through energetically uphill reductive cleavage of strong Si-Cl bonds. This strategy proved to be general in various alkene silylation reactions including disilylation, hydrosilylation, and allylic silylation under simple and transition-metal-free conditions.
78 citations
TL;DR: A site-selective electrochemical amination reaction that can convert benzylic C-H bonds to C-N linkages via H 2 evolution without need for external oxidants or metal catalysts is reported.
Abstract: C-H/N-H cross-coupling represents an ideal strategy to synthesize various amines but remains challenging due to the requirement for sacrificial chemical oxidants and the difficulty in controlling the regio- and chemo-selectivity. Herein we report a site-selective electrochemical amination reaction that can convert benzylic C-H bonds to C-N linkages via H 2 evolution without need for external oxidants or metal catalysts. The synthetic strategy involves anodic cleavage of benzylic C-H to form a carbocation intermediate, which is then trapped with an amine nucleophile leading to C-N bond formation. Key to the success is to include HFIP as a cosolvent to modulate the oxidation potentials of the alkylbenzene substrate and the aminated product to avoid overoxidation of the latter.
60 citations
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TL;DR: In this article, the authors highlight the recent developments in the copper-mediated (both stoichiometric and catalytic) reactions of aryl boronic acids as reaction partners in both O- and N-arylation.
Abstract: The copper-mediated C(aryl)N, C(aryl)O, and C(aryl)S bond formation is an important transformation and has been developed to include a wide range of substrates. This Review highlights the recent developments in the copper-mediated (both stoichiometric and catalytic) reactions of aryl boronic acids, aryl halides, iodonium salts, siloxanes, stannanes, plumbanes, bismuthates, and trifluoroborate salts as aryl donors. In particular, the recent introduction of boronic acids as reaction partners in both O- and N-arylation has been a significant discovery and will occupy centre-stage in this review. Clear improvements can be obtained by the correct choice of copper source, base, ligands, and other additives. Mechanistic investigations should provide insight into the catalytically active species, which would aid in the development of milder, more-efficient methods.
2,280 citations
TL;DR: Using R-Hydroxy Stannanes as a Model for a Methylenation Reaction and Conclusions and Future Prospects are presented.
Abstract: 6.4. Polyynes 3123 6.5. Using R-Hydroxy Stannanes 3124 6.6. Using the Hurtley Reaction 3124 6.7. Using a Methylenation Reaction 3125 7. Conclusions and Future Prospects 3125 8. Uncommon Abbreviations 3125 9. Acknowledgments 3125 10. Note Added in Proof 3125 11. References 3126 * Authorstowhomcorrespondenceshouldbeaddressed(evano@chimie.uvsq.fr, nicolas.blanchard@uha.fr). † Université de Versailles Saint Quentin en Yvelines. ‡ Université de Haute-Alsace. Chem. Rev. 2008, 108, 3054–3131 3054
1,789 citations
TL;DR: An increasing number of publications have appeared concerning Ullmann-type intermolecular reactions for the coupling of aryl and vinyl halides with N, O, and C nucleophiles, and this Minireview highlights recent and major developments in this topic since 2004.
Abstract: Copper-catalyzed Ullmann condensations are key reactions for the formation of carbon-heteroatom and carbon-carbon bonds in organic synthesis. These reactions can lead to structural moieties that are prevalent in building blocks of active molecules in the life sciences and in many material precursors. An increasing number of publications have appeared concerning Ullmann-type intermolecular reactions for the coupling of aryl and vinyl halides with N, O, and C nucleophiles, and this Minireview highlights recent and major developments in this topic since 2004.
1,458 citations
TL;DR: Electrochemical Properties of scCO2 2285 5.1.
Abstract: 4.2. Solid-Supported Electrolytes 2280 4.3. Solid-Supported Mediators 2282 4.4. Supported Substrate-Product Capture 2283 4.5. A Unique Electrolyte/Solvent System 2285 5. Reaction Conditions 2285 5.1. Supercritical Fluids 2285 5.1.1. Electrochemical Properties of scCO2 2285 5.1.2. Electroreductive Carboxylation in scCO2 2286 5.1.3. Electrochemical Polymerization in scCO2 2286 5.2. The Cation-Pool Method 2286 5.2.1. Generation of N-Acyliminium Ion Pools 2286 5.2.2. Generation of Alkoxycarbenium Ion Pools 2287 5.2.3. Generation of Diarylcarbenium Ion Pools 2288 5.2.4. Generation of Other Cation Pools 2289 6. Electrochemical Devices 2289 6.1. Electrode Materials 2289 6.2. Ultrasound and Centrifugal Fields 2290 6.3. Electrochemical Microflow Systems 2290 7. Combinatorial Electrochemical Synthesis 2292 7.1. Parallel Electrolysis Using a Macrosystem 2292 7.2. Parallel Electrolysis Using a Microsystem 2293 7.3. Serial Electrolysis Using a Microsystem 2294 8. Conclusions 2294 9. Acknowledgments 2294 10. References 2294
1,017 citations
TL;DR: It is demonstrated that an l-proline or N,N-dimethylglycine ligand can facilitate most typical Ullmann-type reactions, with reactions occurring under relatively mild conditions and using only 2-20 mol % copper catalysts.
Abstract: Copper-assisted Ullmann-type coupling reactions are valuable transformations for organic synthesis. Researchers have extensively applied these reactions in both academic and industrial settings. However, two important issues, the high reaction temperatures (normally above 150 °C) and the stoichiometric amounts of copper necessary, have greatly limited the reaction scope. To solve these problems, we and other groups have recently explored the use of special ligands to promote these coupling reactions. We first showed that the structure of α-amino acids can accelerate Cu-assisted Ullmann reactions, leading to the coupling reactions of aryl halides and α-amino acids at 80−90 °C. In response to these encouraging results, we also discovered that an l-proline ligand facilitated the following transformations: (1) coupling of aryl halides with primary amines, cyclic secondary amines, and N-containing heterocycles at 40−90 °C; (2) coupling of aryl halides with sulfinic acid salts at 80−95 °C; (3) azidation of aryl...
932 citations