Electrochemical synthesis of 3a-bromofuranoindolines and 3a-bromopyrroloindolines mediated by MgBr2

We report an efficient and environmentally friendly electrochemical approach to perform the bromo cyclization of tryptophol, tryptamine and tryptophan derivatives.

Transition metal-catalyzed electrochemical processes for C–C bond formation

A comprehensive electro-organometallic review has been carried out on C–C bond formation via variety of metals between 1984 and 2019.

Tailoring radicals by asymmetric electrochemical catalysis

We highlight asymmetric electrochemical catalysis as a powerful tool in facilitating radical transformations. The tunable features of electrochemical methods enable mild generations of radicals and provide control of chemo- and stereoselectivity.

Electrochemical decarboxylative C3 alkylation of quinoxalin-2(1H)-ones with N-hydroxyphthalimide esters

Electrochemical decarboxylative C3 alkylation of a wide range of quinoxalin-2(1H)-ones under metal- and additive-free conditions was reported.

Selenophosphoramide-catalyzed diamination and oxyamination of alkenes

Scavenging fluoride from a selenophosphoramide-catalyzed alkene oxidation reaction suppresses the known syn-elimination pathway, enabling alkene diamination/oxyamination reactions via substitution.

A new selenophosphoramide-catalyzed diamination of terminal- and trans-1,2-disubstituted olefins is presented. Key to the success of this transformation was the introduction of a fluoride scavenger, trimethylsilyl trifluoromethanesulfonate (TMSOTf), to prevent a competitive syn-elimination pathway, as was the use of a phosphoramide ligand on selenium to promote the desired substitution reaction. A screen of catalysts revealed that more electron-rich phosphine ligands resulted in higher yields of the desired product, with selenophosphoramides giving the optimal results. A broad range of substrates and functional groups were tolerated and yields were generally good to excellent. For (E)-1,2-disubstituted olefins, diastereoselectivities were always high, giving exclusively anti products. The conditions were also applied to substrates bearing internal nucleophiles such as esters and carbonates, giving rise to 1,2-aminoesters and cyclic carbonates, respectively.

Iron-Electrocatalyzed C-H Arylations: Mechanistic Insights into Oxidation-Induced Reductive Elimination for Ferraelectrocatalysis

Despite major advances, organometallic C-H transformations are dominated by precious 5d and 4d transition metals, such as iridium, palladium and rhodium. In contrast, the unique potential of less toxic Earth-abundant 3d metals has been underexplored. While iron is the most naturally abundant transition metal, its use in oxidative, organometallic C-H activation has faced major limitations due to the need for superstoichiometric amounts of corrosive, cost-intensive DCIB as the sacrificial oxidant. To fully address these restrictions, we describe herein the unprecedented merger of electrosynthesis with iron-catalyzed C-H activation through oxidation-induced reductive elimination. Thus, ferra- and manganaelectro-catalyzed C-H arylations were accomplished at mild reaction temperatures with ample scope by the action of sustainable iron catalysts, employing electricity as a benign oxidant.

Electrochemical Oxidative Cross-Coupling with Hydrogen Evolution Reactions

Oxidative cross-coupling has proved to be one of the most straightforward strategies for forming carbon-carbon and carbon-heteroatom bonds from easily available precursors. Over the past two decades, tremendous efforts have been devoted in this field and significant advances have been achieved. However, in order to remove the surplus electrons from substrates for chemical bonds formation, stoichiometric oxidants are usually needed. Along with the development of modern sustainable chemistry, considerable efforts have been devoted to perform the oxidative cross-coupling reactions under external-oxidant-free conditions. Electrochemical synthesis is a powerful and environmentally benign approach, which can not only achieve the oxidative cross-couplings under external-oxidant-free conditions, but also release valuable hydrogen gas during the chemical bond formation. Recently, the electrochemical oxidative cross-coupling with hydrogen evolution reactions has been significantly explored. This Account presents our recent efforts toward the development of electrochemical oxidative cross-coupling with hydrogen evolution reactions. (1) We explored the oxidative cross-coupling of thiols/thiophenols with arenes, heteroarenes, and alkenes for C-S bond formation. (2) Using the strategy of electrochemical oxidative C-H/N-H cross-coupling with hydrogen evolution, we successfully realized the C-H amination of phenols, anilines, imidazopyridines, and even ethers. (3) Employing halide salts as the green halogenating reagents, we developed a clean C-H halogenation protocol under electrochemical oxidation conditions. To address the limitation that this reaction had to carry out in aqueous solvent, we also developed an alternative method that uses CBr₄, CHBr₃, CH₂Br₂, CCl₃Br, and CCl₄ as halogenating reagents and the mixture of acetonitrile and methanol as cosolvent. (4) We also developed an approach for constructing C-O bonds in a well-developed electrochemical oxidative cross-coupling with hydrogen evolution manner. (5) Under mild external-oxidant-free electrochemical conditions, we realized the C(sp²)-H and C(sp³)-H phosphonylation with modest to high yields. (6) We successfully achieved the S-H/S-H cross-coupling with hydrogen evolution under electrochemical oxidation conditions. By anodic oxidation instead of chemical oxidants, the overoxidation of thiols and thiophenols was well avoided. (7) The methods for constructing structurally diverse heterocyclic compounds were also developed via the electrochemical oxidative annulations. (8) We have also applied the electrochemical oxidative cross-coupling with hydrogen evolution strategy to the alkenes difunctionalization for constructing multiple bonds in one step, such as C-S/C-O bonds, C-S/C-N bonds, C-Se/C-O bonds, and C-Se/C-N bonds. We hope our studies will stimulate the research interest of chemists and pave the way for the discovery of more electrochemical oxidative cross-coupling with hydrogen evolution reactions.

Electrochemical oxidation induced intermolecular aromatic C-H imidation

The dehydrogenative aryl C-H/N-H cross-coupling is a powerful synthetic methodology to install nitrogen functionalities into aromatic compounds. Herein, we report an electrochemical oxidation induced intermolecular cross-coupling between aromatics and sulfonimides with high regioselectivity through N-radical addition pathway under external-oxidant-free and catalyst-free conditions. A wide variety of arenes, heteroarenes, alkenes and sulfonimides are applicable scaffolds in this transformation. In addition, aryl sulfonamides or amines (aniline derivatives) can be obtained through different deprotection process. The cyclic voltammetry mechanistic study indicates that the N-centered imidyl radicals are generated via proton-coupled electron transfer event jointly mediated by tetrabutylammonium acetate and anode oxidation process.

The dehydrogenative C-H/N-H cross-coupling serves to install nitrogen functionalities into arenes with the highest atom economy. Here, the authors report an electrochemical cross-coupling between aromatics and sulfonimides through an N-radical addition pathway under oxidant- and catalyst-free conditions.

Synthesis of Isoxazolines and Oxazines by Electrochemical Intermolecular [2 + 1 + n] Annulation: Diazo Compounds Act as Radical Acceptors

Reported herein is an unprecedented synthesis of isoxazolines and oxazines through electrochemical intermolecular annulation of alkenes with tert-butyl nitrite, in which diazo compounds serve as radical acceptors. Notably, [2 + 1 + 2] and [2 + 1 + 3] annulations occur when styrenes and allylbenzenes are used as substrates, respectively. The latter reaction undergoes group migration to form more stable radical, manifesting radical route instead of conventional 1,3-dipolar cycloaddition occurs. Moreover, scale-up experiments suggest the potential application value of these transformations in industry.

Manganese-Catalyzed Electrochemical Deconstructive Chlorination of Cycloalkanols via Alkoxy Radicals

A manganese-catalyzed electrochemical deconstructive chlorination of cycloalkanols has been developed. This electrochemical method provides access to alkoxy radicals from alcohols and exhibits a broad substrate scope, with various cyclopropanols and cyclobutanols converted into synthetically useful β- and γ-chlorinated ketones (40 examples). Furthermore, the combination of recirculating flow electrochemistry and continuous inline purification was employed to access products on a gram scale.

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.

Electrochemical oxidation induced selective tyrosine bioconjugation for the modification of biomolecules

Directly introducing a beneficial functional group into biomolecules under mild, clean and easy-to-handle conditions is of great importance in the field of chemical biology and pharmacology. Herein, we described an electrochemical strategy to perform the bioconjugation of tyrosine residues with phenothiazine derivatives in a rapid and simple manner. In this electrochemical system, various polypeptides and proteins were successfully labelled with excellent site- and chemo-selectivity, and metals, oxidants or additives were also avoided.

New Bisoxazoline Ligands Enable Enantioselective Electrocatalytic Cyanofunctionalization of Vinylarenes

In contrast to the rapid growth of synthetic electrochemistry in recent years, enantioselective catalytic methods powered by electricity remain rare. In this work, we report the development of a highly enantioselective method for the electrochemical cyanophosphinoylation of vinylarenes. A new family of serine-derived chiral bisoxazolines with ancillary coordination sites were identified as optimal ligands.

Electrochemical Difluoromethylation of Electron-Deficient Alkenes

Electrochemical 1,2-hydroxydifluoromethylation and C-H difluoromethylation of acrylamides were developed by using CF₂ HSO₂ NHNHBoc as the source of the CF₂ H group. These electricity-powered oxidative alkene functionalization reactions do not need transition-metal catalysts or chemical oxidants. The reaction outcome, 1,2-difuntionalization or C-H functionalization, is determined by the substituents on the amide nitrogen atom of the acrylamides instead of by the reaction conditions.

Hypercoordinate iodine for catalytic asymmetric diamination of styrene: insights into the mechanism, role of solvent, and stereoinduction

Stereoselectivity in the asymmetric diamination of styrene catalyzed by chiral hypercoordinate iodine originates from the prochiral face recognition when the substrate binds to the catalyst.

Hypercoordinate iodine has evolved as an impressive class of catalysts for various organic transformations. Extension of this idea to asymmetric applications, such as in the asymmetric difunctionalization of styrene or its derivatives, constitutes an important reaction. In this study, the mechanism and origin of stereoinduction in styrene diamination, with a sulfonimide (HNMs₂) as the diaminating agent and iodoresorcinol (((iPr)₂N(CO)-CH(Me)-O)₂Ar–I) based chiral hypercoordinate iodine as the catalyst, are investigated using density functional theory calculations. The energetically preferred catalytic pathway has been found to involve, among other steps, two very important mechanistic events: (a) the formation of a catalyst–substrate complex by the action of styrene on the catalyst ArI(NMs₂)₂, resulting in the displacement of one of the imidates (NMs₂^–); and (b) a rebound of the departed imidate on the iodine-bound styrene to form an iodonium ion intermediate with a N–C bond. Explicit interaction of the imidate ion with hexafluoroisopropanol (HFIP), used as a solvent additive, lowers the barrier for the formation of the iodonium ion. The P helical fold of the chiral arms of the iodoresorcinol catalyst is found to offer a chiral environment for the reactants. Coordination of the iodine catalyst to the styrene double bond is found to make the benzylic carbon more electrophilic and hence makes it the preferred site for the nucleophilic addition. In the chiral environment of the catalyst, an enhanced polarization of the styrene double bond is noticed when the double bond coordinates through the si prochiral face than the re face. Nucleophilic addition on the re face of the catalyst–substrate complex is associated with a lower activation barrier leading to the experimentally observed S enantiomeric product. The stereoselective model developed in this study can be employed to related asymmetric styrene difunctionalizations using similar hypercoordinate iodine catalysts.

Highly selective electrochemical hydrogenation of alkynes: Rapid construction of mechanochromic materials

Electrochemical hydrogenation has emerged as an environmentally benign and operationally simple alternative to traditional catalytic reduction of organic compounds. Here, we have disclosed for the first time the electrochemical hydrogenation of alkynes to a library of synthetically important Z-alkenes under mild conditions with great selectivity and efficiency. The deuterium and control experiments of electrochemical hydrogenation suggest that the hydrogen source comes from the solvent, supporting electrolyte, and base. The scanning electron microscopy and x-ray diffraction experiments demonstrate that palladium nanoparticles generated in the electrochemical reaction act as a chemisorbed hydrogen carrier. Moreover, complete reduction of alkynes to saturated alkanes can be achieved through slightly modified conditions. Furthermore, a series of novel mechanofluorochromic materials have been efficiently constructed with this protocol that showed blue-shifted mechanochromism. This discovery represents the first example of cis-olefins-based organic mechanochromic materials.

Hydrodehalogenation of Aryl Halides through Direct Electrolysis

A catalyst- and metal-free electrochemical hydrodehalogenation of aryl halides is disclosed. Our reaction by a flexible protocol is operated in an undivided cell equipped with an inexpensive graphite rod anode and cathode. Trialkylamines nBu₃ N/Et₃ N behave as effective reductants and hydrogen atom donors for this electrochemical reductive reaction. Various aryl and heteroaryl bromides worked effectively. The typically less reactive aryl chlorides and fluorides can also be smoothly converted. The utility of our method is demonstrated by detoxification of harmful pesticides and hydrodebromination of a dibrominated biphenyl (analogues of flame-retardants) in gram scale.

A general, electrocatalytic approach to the synthesis of vicinal diamines

This protocol describes an electrochemical synthesis of 1,2-diazides from alkenes. Organic azides are highly versatile intermediates for synthetic chemistry, materials, and biological applications. 1,2-Diazides are commonly reduced to form 1,2-diamines, which are prevalent structural motifs in bioactive natural products, therapeutic agents, and molecular catalysts. The electrochemical formation of 1,2-diazides involves the anodic generation of an azidyl radical from sodium azide, followed by two successive additions of this N-centered radical to the alkene, and is assisted by a Mn catalyst. The electrosynthesis of 1,2-diazides can be carried out using various experimental setups comprising custom-made or commercially available reaction vessels and a direct-current power supply. Readily accessible electrode materials can be used, including carbon (made from reticulated vitreous carbon and pencil lead), nickel foam, and platinum foil. This protocol is also demonstrated using ElectraSyn, a standardized electrochemistry kit. Compared with conventional synthetic approaches, electrochemistry allows for the precise control of the anodic potential input, eliminates the need for stoichiometric and often indiscriminate oxidants, and minimizes the generation of wasteful byproducts. As such, our electrocatalytic synthesis exhibits various advantages over existing methods for alkene diamination, including sustainability, operational simplicity, substrate generality, and exceptional functional-group compatibility. The resultant 1,2-diazides can be smoothly reduced to 1,2-diamines in a single step with high chemoselectivity. To exemplify this, we include a procedure for catalytic hydrogenation using palladium on carbon. This protocol, therefore, constitutes a general approach to accessing 1,2-diazides and 1,2-diamines from alkenes.

Electrochemical Semipinacol Rearrangements of Allylic Alcohols: Construction of All-Carbon Quaternary Stereocenters

The first examples of electrochemical trifluoromethylation and sulfonylation/semipinacol rearrangements of allylic alcohols were developed using cheap and stable RSO₂Na (R = CF₃, Ph) as reagents. Various β-trifluoromethyl and sulfonated ketones were obtained in moderate to excellent yields. This strategy provides a facile, direct, and complementary approach to construct all-carbon quaternary stereocenters. In addition, the reaction has the advantages of being chemical oxidant-free and metal-free and has safe and mild reaction conditions.

Electrochemical fluoromethylation triggered lactonizations of alkenes under semi-aqueous conditions

An electrochemical difluoromethylation triggered lactonization of alkenes was developed for the first time. This protocol employs readily prepared CF2HSO2Na as the difluoromethylating reagent, affording unprecedented CF2H-containing lactones in moderate yields. Moreover, with CF3SO2Na as the trifluoromethylating reagent, a wide array of CF3-containing lactones were obtained under additional supporting electrolyte- and catalyst-free conditions.