Electrochemically Enabled Total Syntheses of Natural Products

Electrochemical techniques have helped to enable the total synthesis of natural products since the pioneering work of Kolbe in the mid 1800's. The electrochemical toolset grows every day and these new possibilities change the way chemists look at and think about natural products. This review provides a perspective on total syntheses wherein electrochemical techniques enabled the carbon─carbon bond formations in the skeletal assembly of important natural products, discussion of mechanistic details, and representative examples of the bond formations enabled over the last several decades. These bond formations are often distinctly different from those possible with conventional chemistries and allow assemblies complementary to other techniques.

TEMPO-Modified Polymethacrylates as Mediators in Electrosynthesis: Influence of the Molecular Weight on Redox Properties and Electrocatalytic Activity

Homogeneous catalysts ("mediators") are frequently employed in organic electrosynthesis to control selectivity. Despite their advantages, they can have a negative influence on the overall energy and mass balance if used only once or recycled inefficiently. Polymediators are soluble redox-active polymers applicable as electrocatalysts, enabling recovery by dialysis or membrane filtration. Using anodic alcohol oxidation as an example, we have demonstrated that TEMPO-modified polymethacrylates (TPMA) can act as efficient and recyclable catalysts. In the present work, the influence of the molecular size on the redox properties and the catalytic activity was carefully elaborated using a series of TPMAs with well-defined molecular weight distributions. Cyclic voltammetry studies show that the polymer chain length has a pronounced impact on the key-properties. Together with preparative-scale electrolysis experiments, an optimum size range was identified for polymediator-guided sustainable reaction control.

Zinc-Free, Scalable Reductive Cross-Electrophile Coupling Driven by Electrochemistry in an Undivided Cell

Nickel-catalyzed reductive cross-electrophile coupling reactions are becoming increasingly important in organic synthesis, but application at scale is limited by three interconnected challenges: a reliance on amide solvents (complicated workup, regulated), the generation of stoichiometric Zn salts (complicated isolation, waste disposal issue), and mixing/activation challenges of zinc powder. We show here an electrochemical approach that addresses these three issues: the reaction works in acetonitrile with diisopropylethylamine as the terminal reductant in a simple undivided cell (graphite(+)/nickel foam(-)). The reaction utilizes a combination of two ligands, 4,4'-di-tert-butyl-2,2'-bipyridine and 4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine. Studies show that, alone, the bipyridine nickel catalyst predominantly forms protodehalogenated aryl and aryl dimer, whereas the terpyridine nickel catalyst predominantly forms bialkyl and product. By combining these two unselective catalysts, a tunable, general system results because excess radical formed by the terpyridine catalyst can be converted to product by the bipyridine catalyst. As the aryl bromide becomes more electron rich, the optimal ratio shifts to have more of the bipyridine nickel catalyst. Lastly, examination of a variety of flow-cell configurations establishes that batch recirculation can achieve higher productivity (mmol product/time/electrode area) than single-pass, that high flow rates are essential to maximizing current, and that two flow cells in parallel can nearly halve the reaction time. The resulting reaction is demonstrated on gram scale and should be scalable to kilogram scale.

A Transient Supercapacitor with a Water-Dissolvable Ionic Gel for Sustainable Electronics

We develop an environmentally benign supercapacitor, which decomposes by contact with water, incorporating an ionic liquid, carbon powder, a cellulose separator, and a molybdenum electrode. The ionic liquid is dispersed into a water-dissolvable polymer, poly(vinyl alcohol), to produce a solid electrolyte, so-called ionic gel. A carbon composite mixed with the ionic liquid maintains a gel form. The ionic gel and the carbon composite enable an all-solid-state supercapacitor, which can be charged at a voltage of 1.5 V. The supercapacitor shows areal and volumetric capacitances of 65 mF/cm2 and 2.2 F/cm3, respectively. A cycle test reveals that capacitance retention and Coulombic efficiency are 77 and 90%, respectively. As for the dissolution test, the ionic gel and carbon composite dissolves in phosphate buffer solution in 18 days, and the Mo electrode is able to fully dissolve in 500-588 days. Potential applications of the environmentally benign supercapacitor include smart agriculture by monitoring of soil and disaster prevention by a wireless sensor network without the need for retrieval of devices after use.

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis

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.

Electrocatalysis as an enabling technology for organic synthesis

Electrochemistry has recently gained increased attention as a versatile strategy for achieving challenging transformations at the forefront of synthetic organic chemistry. Electrochemistry's unique ability to generate highly reactive radical and radical ion intermediates in a controlled fashion under mild conditions has inspired the development of a number of new electrochemical methodologies for the preparation of valuable chemical motifs. Particularly, recent developments in electrosynthesis have featured an increased use of redox-active electrocatalysts to further enhance control over the selective formation and downstream reactivity of these reactive intermediates. Furthermore, electrocatalytic mediators enable synthetic transformations to proceed in a manner that is mechanistically distinct from purely chemical methods, allowing for the subversion of kinetic and thermodynamic obstacles encountered in conventional organic synthesis. This review highlights key innovations within the past decade in the area of synthetic electrocatalysis, with emphasis on the mechanisms and catalyst design principles underpinning these advancements. A host of oxidative and reductive electrocatalytic methodologies are discussed and are grouped according to the classification of the synthetic transformation and the nature of the electrocatalyst.

Electrochemical access to benzimidazolone and quinazolinone derivatives via in situ generation of isocyanates

Isocyanates are the key intermediates for several organic transformations towards the synthesis of diverse pharmaceutical targets. Herein, we report the development of an oxidant-free protocol for electrochemical in situ generation of isocyanates. This strategy highlights expedient access to benzimidazolones and quinazolinones and eliminates the need for exogenous oxidants. Furthermore, detailed mechanistic studies provide strong support towards our hypothesis of in situ isocyanate generation.

Isolation and synthesis of cryptosanguinolentine (isocryptolepine), a naturally-occurring bioactive indoloquinoline alkaloid

Cryptosanguinolentine (isocryptolepine) is one of the minor naturally-occurring monomeric indoloquinoline alkaloids, isolated from the West African climbing shrub Cryptolepis sanguinolenta. The natural product displays such a simple and unique skeleton, which chemists became interested in well before it was found in Nature. Because of its structure and biological activity, the natural product has been targeted for synthesis on numerous occasions, employing a wide range of different strategies. Hence, discussed here are aspects related to the isolation of isocryptolepine, as well as the various approaches toward its total synthesis.

Cobalta-Electrocatalyzed C-H Activation in Biomass-Derived Glycerol: Powered by Renewable Wind and Solar Energy

Aqueous glycerol was identified as a renewable reaction medium for metalla-electrocatalyzed C-H activation powered by sustainable energy sources. The renewable solvent was employed for cobalt-catalyzed C-H/N-H functionalizations under mild conditions. The cobalta-electrocatalysis manifold occurred with high levels of chemo- and positional selectivity and allowed for electrochemical C-H activations with broad substrate scope. The resource economy of this strategy was considerably substantiated by the direct use of renewable solar and wind energy.

A Decade of Electrochemical Dehydrogenative C,C-Coupling of Aryls

The importance of sustainable and green synthetic protocols for the synthesis of fine chemicals has rapidly increased during the last decades in an effort to reduce the use of fossil fuels and other finite resources. The replacement of common reagents by electricity provides a cost- and atom-efficient, environmentally friendly, and inherently safe access to novel synthetic routes. The selective formation of carbon-carbon bonds between two distinct substrates is a crucial tool in organic chemistry. This fundamental transformation enables access to a broad variety of complex molecular architectures. In particular, the aryl-aryl bond formation has high significance for the preparation of organic materials, drugs, and natural products. Besides well-known and well-established reductive- and oxidative-reagent-mediated or transition-metal-catalyzed coupling reactions, novel synthetic protocols have arisen, which require fewer steps than conventional synthetic approaches. Electroorganic conversions can be categorized according to the nature of the electron transfer processes occurring. Direct transformations at inert electrode materials are environmentally benign and cost-effective, whereas catalytic processes at active electrodes and mediated electrosynthesis using an additional soluble reagent can have beneficial properties in terms of selectivity and reactivity. In general, these conversions require challenging optimization of the reaction parameters and the appropriate cell design. Galvanostatic reactions enable fast conversions with a rather simple setup, whereas potentiostatic electrolysis may enhance selectivity. This Account discusses the development of seminal carbon-carbon bond formations over the past two decades, focusing on phenols leading to precursors for ligands in, e.g., hydroformylation reaction. A key element in the success of these electrochemical transformations is the application of electrochemically inert, non-nucleophilic, highly fluorinated alcohols such as 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), which exhibit a large potential window for transformations and enable selective cross-coupling reactions. This selectivity is based on the capability of HFIP to stabilize organic radicals. Inert, carbon-based and metal-free electrode materials like graphite or boron-doped diamond (BDD) open up novel electroorganic pathways. Furthermore, novel active electrode materials have been developed to enable intra- and intermolecular dehydrogenative coupling reactions of electron-rich aryls. The application of 2,2'-biphenol derivatives as ligand components for catalysts requires reactions to be carried out on larger scale. In order to achieve this, continuous flow transformations have been established to overcome the drawbacks of heat transfer, overconversion, and conductivity. Modular cell designs enable the transfer of a broad variety of electroorganic conversions into continuous processes. Recent results demonstrate the application of organic electrochemistry to natural product synthesis of the pharmaceutically relevant opiate alkaloids (-)-thebaine or (-)-oxycodone.

Reversible Ion-Conducting Switch in a Novel Single-Ion Supramolecular Hydrogel Enabled by Photoresponsive Host-Guest Molecular Recognition

A novel ion-conducting supramolecular hydrogel with reversible photoconductive properties in which the azobenzene motif, α-cyclodextrin (α-CD), and ionic liquid are grafted onto the gel matrix is reported. Host-guest interactions with different association constants between α-CD and azobenzene or the anionic part of the ionic liquid can be readily tuned by photoinduced trans-cis isomerization of the azobenzene unit. When irradiated by 365 nm light, α-CD prefers to form a complex with the anionic part of the ionic liquid, resulting in decreased ionic mobility and thus high resistance of the hydrogel. However, under 420 nm light irradiation, a more stable complex is again formed between α-CD and trans-azobenzene, thereby releasing the bound anions to regenerate the low-resistive hydrogel. As such, remote control of the ionic conductivity of the hydrogel is realized by simple host-guest chemistry. With the incorporation of a logic gate, this hydrogel is able to reversibly switch an electric circuit on and off by light irradiation with certain wavelengths. The concept of photoswitchable ionic conductivity of a hydrogel mediated by competitive molecular recognition is potentially promising toward the fabrication of optoelectronic devices and applications in bioelectronic technology.

Charge transport and glassy dynamics in polymeric ionic liquids as reflected by their inter- and intramolecular interactions

Polymeric ionic liquids (PILs) form a novel class of materials in which the extraordinary properties of ionic liquids (ILs) are combined with the mechanical stability of polymeric systems qualifying them for multifold applications. In the present study broadband dielectric spectroscopy (BDS), Fourier transform infrared spectroscopy (FTIR), AC-chip calorimetry (ACC) and differential scanning calorimetry (DSC) are combined in order to unravel the interplay between charge transport and glassy dynamics. Three low molecular weight ILs and their polymeric correspondents are studied with systematic variations of anions and cations. For all examined samples charge transport takes place by glassy dynamics assisted hopping conduction. In contrast to low molecular weight ILs the thermal activation of DC conductivity for the polymeric systems changes from a Vogel-Fulcher-Tammann- to an Arrhenius-dependence at a (sample specific) temperature Tσ0. This temperature has been widely discussed to coincide with the glass transition temperature Tg, a refined analysis, instead, reveals Tσ0 of all PILs under study at up to 80 K higher values. In effect, below the Tσ0 charge transport in PILs becomes more efficient - albeit on a much lower level compared to the low molecular weight pendants - indicating conduction paths along the polymer chain. This is corroborated by analysing the temperature dependence of specific IR-active vibrations showing at Tσ0 distinct changes in the spectral position and the oscillator strength, whereas other molecular units are not affected. This leads to the identification of charge transport responsive (CTR) as well as charge transport irresponsive (CTI) moieties and paves the way to a refined molecular understanding of electrical conduction in PILs.

Electrochemical strategies for C-H functionalization and C-N bond formation

Conventional methods for carrying out carbon-hydrogen functionalization and carbon-nitrogen bond formation are typically conducted at elevated temperatures, and rely on expensive catalysts as well as the use of stoichiometric, and perhaps toxic, oxidants. In this regard, electrochemical synthesis has recently been recognized as a sustainable and scalable strategy for the construction of challenging carbon-carbon and carbon-heteroatom bonds. Here, electrosynthesis has proven to be an environmentally benign, highly effective and versatile platform for achieving a wide range of nonclassical bond disconnections via generation of radical intermediates under mild reaction conditions. This review provides an overview on the use of anodic electrochemical methods for expediting the development of carbon-hydrogen functionalization and carbon-nitrogen bond formation strategies. Emphasis is placed on methodology development and mechanistic insight and aims to provide inspiration for future synthetic applications in the field of electrosynthesis.

Modern Electrochemical Aspects for the Synthesis of Value-Added Organic Products

The use of electricity instead of stoichiometric amounts of oxidizers or reducing agents in synthesis is very appealing for economic and ecological reasons, and represents a major driving force for research efforts in this area. To use electron transfer at the electrode for a successful transformation in organic synthesis, the intermediate radical (cation/anion) has to be stabilized. Its combination with other approaches in organic chemistry or concepts of contemporary synthesis allows the establishment of powerful synthetic methods. The aim in the 21st Century will be to use as little fossil carbon as possible and, for this reason, the use of renewable sources is becoming increasingly important. The direct conversion of renewables, which have previously mainly been incinerated, is of increasing interest. This Review surveys many of the recent seminal important developments which will determine the future of this dynamic emerging field.

Frontiers in poly(ionic liquid)s: syntheses and applications

We review recent works on the synthesis and application of poly(ionic liquid)s (PILs). Novel chemical structures, different synthetic strategies and controllable morphologies are introduced as a supplement to PIL systems already reported. The primary properties determining applications, such as ionic conductivity, aqueous solubility, thermodynamic stability and electrochemical/chemical durability, are discussed. Furthermore, the near-term applications of PILs in multiple fields, such as their use in electrochemical energy materials, stimuli-responsive materials, carbon materials, and antimicrobial materials, in catalysis, in sensors, in absorption and in separation materials, as well as several special-interest applications, are described in detail. We also discuss the limitations of PIL applications, efforts to improve PIL physics, and likely future developments.

On the Use of Polyelectrolytes and Polymediators in Organic Electrosynthesis

Although organic electrosynthesis is generally considered to be a green method, the necessity for excess amounts of supporting electrolyte constitutes a severe drawback. Furthermore, the employment of redox mediators results in an additional separation problem. In this context, we have explored the applicability of soluble polyelectrolytes and polymediators with the TEMPO-mediated transformation of alcohols into carbonyl compounds as a test reaction. Catalyst benchmarking based on cyclic voltammetry studies indicated that the redox-active polymer can compete with molecularly defined TEMPO species. Alcohol oxidation was also highly efficient on a preparative scale, and our polymer-based approach allowed for the separation of both mediator and supporting electrolyte in a single membrane filtration step. Moreover, we have shown that both components can be reused multiple times.

An Easy-to-Machine Electrochemical Flow Microreactor: Efficient Synthesis of Isoindolinone and Flow Functionalization

Flow electrochemistry is an efficient methodology to generate radical intermediates. An electrochemical flow microreactor has been designed and manufactured to improve the efficiency of electrochemical flow reactions. With this device only little or no supporting electrolytes are needed, making processes less costly and enabling easier purification. This is demonstrated by the facile synthesis of amidyl radicals used in intramolecular hydroaminations to produce isoindolinones. The combination with inline mass spectrometry facilitates a much easier combination of chemical steps in a single flow process.

Synthetic Organic Electrochemical Methods Since 2000: On the Verge of a Renaissance

Electrochemistry represents one of the most intimate ways of interacting with molecules. This review discusses advances in synthetic organic electrochemistry since 2000. Enabling methods and synthetic applications are analyzed alongside innate advantages as well as future challenges of electroorganic chemistry.

Metal- and Reagent-Free Intramolecular Oxidative Amination of Tri- and Tetrasubstituted Alkenes

A metal- and reagent-free, electrochemical intramolecular oxidative amination reaction of tri- and tetrasubstituted alkenes has been developed. The electrosynthetic method proceeds through radical cyclization to form the key C-N bond, allowing a variety of hindered tri- and tetrasubstituted olefins to participate in the amination reaction. The result is the efficient synthesis of a host of alkene-bearing cyclic carbamates and ureas and lactams.

Electrochemical C-H/N-H Functionalization for the Synthesis of Highly Functionalized (Aza)indoles

Indoles and azaindoles are among the most important heterocycles because of their prevalence in nature and their broad utility in pharmaceutical industry. Reported herein is an unprecedented noble-metal- and oxidant-free electrochemical method for the coupling of (hetero)arylamines with tethered alkynes to synthesize highly functionalized indoles, as well as the more challenging azaindoles.

Aqueous RAFT Polymerization of Imidazolium-Type Ionic Liquid Monomers: En Route to Poly(ionic liquid)-Based Nanoparticles through RAFT Polymerization-Induced Self-Assembly

The synthesis by aqueous RAFT polymerization of hydrophilic narrowly dispersed imidazolium-based poly(ionic liquid)s (Đ typically below 1.20) is reported. Full monomer conversion is achieved within hours and high end-group fidelity of the living end groups affords the preparation of well-defined block copolymers. The resulting poly(ionic liquid) macroRAFT agents are finally exploited to polymerize 2-vinylpyridine in water and generate PIL-based nanoparticles of various morphologies (spheres, vesicles, or worms) in a one-pot surfactant-free process.