Recent developments in the difunctionalization of alkenes with C–N bond formation

Various alkene difunctionalization reactions involving nitridization, diamination, azidation, oxyamination, carboamination, aminohalogenation, and nitration are introduced in this review.

Mangana(iii/iv)electro-catalyzed C(sp3)-H azidation

Manganaelectro-catalyzed azidation of otherwise inert C(sp³)-H bonds was accomplished using most user-friendly sodium azide as the nitrogen-source. The operationally simple, resource-economic C-H azidation strategy was characterized by mild reaction conditions, no directing group, traceless electrons as the sole redox-reagent, Earth-abundant manganese as the catalyst, high functional-group compatibility and high chemoselectivity, setting the stage for late-stage azidation of bioactive compounds. Detailed mechanistic studies by experiment, spectrophotometry and cyclic voltammetry provided strong support for metal-catalyzed aliphatic radical formation, along with subsequent azidyl radical transfer within a manganese(iii/iv) manifold.

Harnessing Radical Chemistry via Electrochemical Transition Metal Catalysis

The merger of transition metal catalysis and electroorganic synthesis has recently emerged as a versatile platform for the development of highly enabling radical reactions in a sustainable fashion. Electrochemistry provides access to highly reactive radical species under extremely mild reaction conditions from abundant native functionalities. Transition metal catalysts can be used as redox-active electrocatalysts to shuttle electrons, chiral information to organic substrates, and the reactive intermediates in the electrolytic systems. The combination of these strategies in this mechanistic paradigm thus makes the generation and utilization of radical species in a chemoselective manner and allows further application to more synthetically attractive enantioselective radical transformations. This perspective discusses key advances over the past few years in the field of electrochemical transition metal catalysis and demonstrates how the unique features of this strategy permit challenging or previously elusive transformations via radical pathways to be successfully achieved.

Designing a Nanoscale Three-phase Electrochemical Pathway to Promote Pt-catalyzed Formaldehyde Oxidation

Gas-phase heterogeneous catalysis is a process spatially constrained on the two-dimensional surface of a solid catalyst. Here, we introduce a new toolkit to open up the third dimension. We discovered that the activity of a solid catalyst can be dramatically promoted by covering its surface with a nanoscale-thin layer of liquid electrolyte while maintaining efficient delivery of gas reactants, a strategy we call three-phase catalysis. Introducing the liquid electrolyte converts the original surface catalytic reaction into an electrochemical pathway with mass transfer facilitated by free ions in a three-dimensional space. We chose the oxidation of formaldehyde as a model reaction and observed a 25000-times enhancement in the turnover frequency of Pt in three-phase catalysis as compared to conventional heterogeneous catalysis. We envision three-phase catalysis as a new dimension for catalyst design and anticipate its applications in more chemical reactions from pollution control to the petrochemical industry.

Electroreductive Carbofunctionalization of Alkenes with Alkyl Bromides via a Radical-Polar Crossover Mechanism

Electrochemistry grants direct access to reactive intermediates (radicals and ions) in a controlled fashion toward selective organic transformations. This feature has been demonstrated in a variety of alkene functionalization reactions, most of which proceed via an anodic oxidation pathway. In this report, we further expand the scope of electrochemistry to the reductive functionalization of alkenes. In particular, the strategic choice of reagents and reaction conditions enabled a radical-polar crossover pathway wherein two distinct electrophiles can be added across an alkene in a highly chemo- and regioselective fashion. Specifically, we used this strategy in the intermolecular carboformylation, anti-Markovnikov hydroalkylation, and carbocarboxylation of alkenes-reactions with rare precedents in the literature-by means of the electroreductive generation of alkyl radical and carbanion intermediates. These reactions employ readily available starting materials (alkyl halides, alkenes, etc.) and simple, transition-metal-free conditions and display broad substrate scope and good tolerance of functional groups. A uniform protocol can be used to achieve all three transformations by simply altering the reaction medium. This development provides a new avenue for constructing Csp3-Csp3 bonds.

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.

Vicinal difunctionalization of carbon-carbon double bond for the platform synthesis of trifluoroalkyl amines

Regioselective vicinal diamination of carbon-carbon double bonds with two different amines is a synthetic challenge under transition metal-free conditions, especially for the synthesis of trifluoromethylated amines. However, the synthesis of ethylene diamines and fluorinated amine compounds is demanded, especially in the pharmaceutical sector. Herein, we demonstrate that the controllable double nucleophilic functionalization of an activated alkene synthon, originated from a trifluoropropenyliodonium salt with two distinct nucleophiles, enables the selective synthesis of trifluoromethylated ethylene amines and diamines on broad scale with high efficiency under mild reaction conditions. Considering the chemical nature of the reactants, our synthetic approach brings forth an efficient methodology and provides versatile access to highly fluorinated amines.

Transition Metal Catalyzed Azidation Reactions

A wide range of methodologies for the preparation of organic azides has been reported in the literature for many decades, due to their interest as building blocks for different transformations and their applications in biology as well as in materials science. More recently, with the spread of the use of transition metal-catalyzed reactions, new perspectives have also materialized in azidation processes, especially concerning the azidation of C–H bonds and direct difunctionalization of multiple carbon-carbon bonds. In this review, special emphasis will be placed on reactions involving substrates bearing a leaving group, hydroazidation reactions and azidation reactions that proceed with the formation of more than one bond. Further reactions for the preparation of allyl and vinyl azides as well as for azidations involving the opening of a ring complete the classification of the material.

Manganese-Catalyzed Oxidative Azidation of C(sp3)-H Bonds under Electrophotocatalytic Conditions

The selective installation of azide groups into C(sp³)-H bonds is a priority research topic in organic synthesis, particularly in pharmaceutical discovery and late-stage diversification. Herein, we demonstrate a generalized manganese-catalyzed oxidative azidation methodology of C(sp³)-H bonds using nucleophilic NaN₃ as an azide source under electrophotocatalytic conditions. This approach allows us to perform the reaction without the necessity of adding an excess of the substrate and successfully avoiding the use of stoichiometric chemical oxidants such as iodine(III) reagent or NFSI. A series of tertiary and secondary benzylic C(sp³)-H, aliphatic C(sp³)-H, and drug-molecule-based C(sp³)-H bonds in substrates are well tolerated under our protocol. The simultaneous gram-scale synthesis and the ease of transformation of azide to amine collectively advocate for the potential application in the preparative synthesis. Good reactivity of the tertiary benzylic C(sp³)-H bond and selectivity of the tertiary aliphatic C(sp³)-H bond in substrates to incorporate nitrogen-based functionality at the tertiary alkyl group also provide opportunities to manipulate numerous potential medicinal candidates. We anticipate our synthetic protocol, consisting of metal catalysis, electrochemistry, and photochemistry, would provide a new sustainable option to execute challenging organic synthetic transformations.

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.

Electrochemical oxidative decarboxylation and 1,2-aryl migration towards the synthesis of 1,2-diaryl ethers

Carboxylic acid compounds are important chemicals and are widely present in various natural products. They are not only nucleophiles, but also radical precursors. Classic transition-metal-catalyzed and photochemical decarboxylation have shown their excellent site selectivity in radical chemistry. However, electrochemical decarboxylation with a long history hasn't got enough attention in recent years. In this work, the electrochemical oxidative decarboxylation and 1,2-aryl migration of 3,3-diarylpropionic acids have been introduced to construct C–O bonds with alcohols. Remarkably, this transformation can proceed smoothly without metal catalysts and external oxidants.

Carboxylic acid compounds are important chemicals and are widely present in various natural products. The conversion of carboxylic acids into valuable compounds is a promising field.

New Redox Strategies in Organic Synthesis by Means of Electrochemistry and Photochemistry

As the breadth of radical chemistry grows, new means to promote and regulate single-electron redox activities play increasingly important roles in driving modern synthetic innovation. In this regard, photochemistry and electrochemistry-both considered as niche fields for decades-have seen an explosive renewal of interest in recent years and gradually have become a cornerstone of organic chemistry. In this Outlook article, we examine the current state-of-the-art in the areas of electrochemistry and photochemistry, as well as the nascent area of electrophotochemistry. These techniques employ external stimuli to activate organic molecules and imbue privileged control of reaction progress and selectivity that is challenging to traditional chemical methods. Thus, they provide alternative entries to known and new reactive intermediates and enable distinct synthetic strategies that were previously unimaginable. Of the many hallmarks, electro- and photochemistry are often classified as "green" technologies, promoting organic reactions under mild conditions without the necessity for potent and wasteful oxidants and reductants. This Outlook reviews the most recent growth of these fields with special emphasis on conceptual advances that have given rise to enhanced accessibility to the tools of the modern chemical trade.

Photoinduced Olefin Diamination with Alkylamines

Vicinal diamines are ubiquitous materials in organic and medicinal chemistry. The direct coupling of olefins and amines would be an ideal approach to construct these motifs. However, alkene diamination remains a long-standing challenge in organic synthesis, especially when using two different amine components. We report a general strategy for the direct and selective assembly of vicinal 1,2-diamines using readily available olefin and amine building blocks. This mild and straightforward approach involves in situ formation and photoinduced activation of N-chloroamines to give aminium radicals that enable efficient alkene aminochlorination. Owing to the ambiphilic nature of the β-chloroamines produced, conversion into tetra-alkyl aziridinium ions was possible, thus enabling diamination by regioselective ring-opening with primary or secondary amines. This strategy streamlines the preparation of vicinal diamines from multistep sequences to a single chemical transformation.

Visible light photoredox alkylazidation of alkenes with sodium azide and heteroarenium salts: entry to azido-containing 1,4-dihydropyridines

A three-component alkene alkylazidation using sodium azide as the azido resource and heteroarenium salts as functionalized alkyl reagents for producing highly valuable 2-azido-1-(1,4-dihydropyridin-4-yl)-ethanes is described. This reaction allows the incorporation of both an azido group and a 1,4-dihydropyridin-4-yl group across C[double bond, length as m-dash]C bonds to construct two new bonds in a single reaction step, and represents a practical and mechanistically distinct alternative that harnesses an electrophilic heteroarenium ion to accomplish the alkene difunctionalization reaction.

Electrocatalytic Deuteration of Halides with D2 O as the Deuterium Source over a Copper Nanowire Arrays Cathode

Precise deuterium incorporation with controllable deuterated sites is extremely desirable. Here, a facile and efficient electrocatalytic deuterodehalogenation of halides using D₂ O as the deuteration reagent and copper nanowire arrays (Cu NWAs) electrochemically formed in situ as the cathode was demonstrated. A cross-coupling of carbon and deuterium free radicals might be involved for this ipso-selective deuteration. This method exhibited excellent chemoselectivity and high compatibility with the easily reducible functional groups (C=C, C≡C, C=O, C=N, C≡N). The C-H to C-D transformations were achieved with high yields and deuterium ratios through a one-pot halogenation-deuterodehalogenation process. Efficient deuteration of less-active bromide substrates, specific deuterium incorporation into top-selling pharmaceuticals, and oxidant-free paired anodic synthesis of high-value chemicals with low energy input highlighted the potential practicality.

A light- and heat-driven glycal diazidation approach to nitrogenous carbohydrate derivatives with antiviral activity

The aminated mimetics of 2-keto-3-deoxy-sugar acids such as the anti-influenza clinical drugs oseltamivir (Tamiflu) and zanamivir (Relenza) are important bioactive molecules. Development of synthetic methodologies for accessing such compound collections is highly desirable. Herein, we describe a simple, catalyst-free glycal diazidation protocol enabled by visible light-driven conditions. This new method requires neither acid promoters nor transition-metal catalysts and takes place at ambient temperature within 1-2 hours. Notably, the desired transformations could be promoted by thermal conditions as well, albeit with lower efficacy compared to the light-induced conditions. Different sugar acid-derived glycal templates have been converted into a range of 2,3-diazido carbohydrate analogs by harnessing this mild and scalable approach, leading to the discovery of new antiviral agents.

Electroreductive Cobalt-Catalyzed Carboxylation: Cross-Electrophile Electrocoupling with Atmospheric CO2

The chemical use of CO2 as an inexpensive, nontoxic C1 synthon is of utmost topical interest in the context of carbon capture and utilization (CCU). We present the merger of cobalt catalysis and electrochemical synthesis for mild catalytic carboxylations of allylic chlorides with CO2 . Styrylacetic acid derivatives were obtained with moderate to good yields and good functional group tolerance. The thus-obtained products are useful as versatile synthons of γ-arylbutyrolactones. Cyclic voltammetry and in operando kinetic analysis were performed to provide mechanistic insights into the electrocatalytic carboxylation with CO2 .

Enantiospecific electrochemical rearrangement for the synthesis of hindered triazolopyridinone derivatives

Triazolopyridinone derivatives are of high value in both medicinal and material chemistry. However, the chiral or hindered triazolopyridinone derivatives remain an underexplored area of chemical space because they are difficult to prepare via conventional methods. Here we report an electrochemical rearrangement for the efficient synthesis of otherwise inaccessible triazolopyridinones with diverse alkyl carboxylic acids as starting materials. This enables the efficient preparation of more than 60 functionalized triazolopyridinones under mild conditions in a sustainable manner. This method is evaluated for the late stage modification of bioactive natural products, amino acids and pharmaceuticals, and it is further applied to the decagram scale preparation of enantiopure triazolopyridinones. The control experiments support a mechanism involving an oxidative cyclization and 1,2-carbon migration. This facile and scalable rearrangement demonstrates the power of electrochemical synthesis to access otherwise-inaccessible triazolopyridinones and may find wide application in organic, material and medicinal chemistry.

Chiral and hindered triazolopyridinone derivatives are an underexplored area of chemical space mainly due to their challenging synthesis via classical methods. Here, the authors report an electrochemical rearrangement for the synthesis of triazolopyridinones using diverse, available alkyl carboxylic acids as starting materials.

3d metallaelectrocatalysis for resource economical syntheses

Resource economy constitutes one of the key challenges for researchers and practitioners in academia and industries, in terms of rising demand for sustainable and green synthetic methodology. To achieve ideal levels of resource economy in molecular syntheses, novel avenues are required, which include, but are not limited to the use of naturally abundant, renewable feedstocks, solvents, metal catalysts, energy, and redox reagents. In this context, electrosyntheses create the unique possibility to replace stoichiometric amounts of oxidizing or reducing reagents as well as electron transfer events by electric current. Particularly, the merger of Earth-abundant 3d metal catalysis and electrooxidation has recently been recognized as an increasingly viable strategy to forge challenging C-C and C-heteroatom bonds for complex organic molecules in a sustainable fashion under mild reaction conditions. In this review, we highlight the key developments in 3d metallaelectrocatalysis in the context of resource economy in molecular syntheses until February 2020.

Tunable Electrochemical C-N versus N-N Bond Formation of Nitrogen-Centered Radicals Enabled by Dehydrogenative Dearomatization: Biological Applications

Herein, an environmentally friendly electrochemical approach is reported that takes advantage of the captodative effect and delocalization effect to generate nitrogen-centered radicals (NCRs). By changing the reaction parameters of the electrode material and feedstock solubility, dearomatization enabled a selective dehydrogenative C-N versus N-N bond formation reaction. Hence, pyrido[1,2-a]benzimidazole and tetraarylhydrazine frameworks were prepared through a sustainable transition-metal- and exogenous oxidant-free strategy with broad generality. Bioactivity assays demonstrated that pyrido[1,2-a]benzimidazoles displayed antimicrobial activity and cytotoxicity against human cancer cells. Compound 21 exhibited good photochemical properties with a large Stokes shift (approximately 130 nm) and was successfully applied to subcellular imaging. A preliminary mechanism investigation and density functional theory (DFT) calculations revealed the possible reaction pathway.

Dual electrocatalysis enables enantioselective hydrocyanation of conjugated alkenes

Chiral nitriles and their derivatives are prevalent in pharmaceuticals and bioactive compounds. Enantioselective alkene hydrocyanation represents a convenient and efficient approach for synthesizing these molecules. However, a generally applicable method featuring a broad substrate scope and high functional group tolerance remains elusive. Here, we address this long-standing synthetic problem using dual electrocatalysis. Using this strategy, we leverage electrochemistry to seamlessly combine two canonical radical reactions—cobalt-mediated hydrogen-atom transfer and copper-promoted radical cyanation—to accomplish highly enantioselective hydrocyanation without the need for stoichiometric oxidants. We also harness electrochemistry’s unique feature of precise potential control to optimize the chemoselectivity of challenging substrates. Computational analysis uncovers the origin of enantio-induction, for which the chiral catalyst imparts a combination of attractive and repulsive non-covalent interactions to direct the enantio-determining C–CN bond formation. This work demonstrates the power of electrochemistry in accessing new chemical space and providing solutions to pertinent challenges in synthetic chemistry.

Dearomatization Reactions of Indoles to Access 3D Indoline Structures

This Account summarizes our involvement in the development of dearomatization reactions of indoles that has for origin a total synthesis problematic. We present the effort from our group to obtain 3D-indolines scaffold from the umpolung of N-acyl indoles via activation with FeCl3 to the oxidative spirocyclizations of N-EWG indoles and via the use of electrochemistry.

1 Introduction

2 Activation of N-Acyl Indoles with FeCl3

2.1 Hydroarylation of N-Acyl Indoles

2.2 Difunctionalization of N-Acyl Indoles

3 Radical-Mediated Dearomatization of Indoles for the Synthesis of Spirocyclic Indolines

4 Electrochemical Dearomatization of Indoles

4.1 Direct Electrochemical Oxidation of Indoles

4.2 Indirect Electrochemical Oxidation of Indoles

5 Conclusion