Iodophenylsulfonates and Iodobenzoates as Redox‐Active Supporting Electrolytes for Electrosynthesis

Electrocatalytic Desulfurizative Amination of Thioureas to Guanidines

Guanidine has been known as an important class of N-containing molecules with a wide range of applications. Described here is a selective and efficient electrochemical approach to the synthesis of guanidines from easily accessible thioureas and amines. The key to success for this reaction is the in situ generation of a hypervalent iodine reagent as a catalyst from iodoarene by anodic oxidation. This mild desulfurizative amination presents ample substrate scope and good functional group tolerance without the use of extra stoichiometric chemical oxidants. As only electrons serve as the oxidation reagents, this method offers a more straightforward and sustainable manner toward versatile guanidines, including late-stage functionalization of pharmaceutically relevant molecules.

Selective multi-electron aggregation at a hypervalent iodine center by sequential disproportionation

We demonstrate that sequential disproportionation reactions can enable selective aggregation of two- or four electron-holes at a hypervalent iodine center. Disproportionation of an anodically generated iodanyl radical affords an iodosylbenzene derivative. Subsequent iodosylbenzene disproportionation can be triggered to provide access to an iodoxybenzene. These results demonstrate multielectron oxidation at the one-electron potential by selective and sequential disproportionation chemistry.

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.

Toward a General Protocol for Catalytic Oxidative Transformations Using Electrochemically Generated Hypervalent Iodine Species

A simple catalytic electrosynthetic protocol for oxidative transformations mediated by hypervalent iodine reagents has been developed. In this protocol, electricity drives the iodine(I)/iodine(III) catalytic cycle enabling catalysis with in situ generated hypervalent iodine species, thereby eliminating chemical oxidants and the inevitable chemical waste associated with their mode of action. In addition, no added electrolytic salts are needed in this process. The developed method has been validated using two different hypervalent iodine-mediated transformations: (i) the oxidative cyclization of N-allylic and N-homoallylic amides to the corresponding dihydrooxazole and dihydro-1,3-oxazine derivatives, respectively, and (ii) the α-tosyloxylation of ketones. Both reactions proceeded smoothly under the developed catalytic electrosynthetic conditions without reoptimization, featuring a wide substrate scope and excellent functional group tolerance. In addition, scale-up to gram-scale and catalyst recovery were easily achieved maintaining the high efficiency of the process.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Electrochemistry and Reactivity of Chelation-stabilized Hypervalent Bromine(III) Compounds

Hypervalent bromine(III) reagents possess a higher electrophilicity and a stronger oxidizing power compared to their iodine(III) counterparts. Despite the superior reactivity, bromine(III) reagents have a reputation of hard‐to‐control and difficult‐to‐synthesize compounds. This is partly due to their low stability, and partly because their synthesis typically relies on the use of the toxic and highly reactive BrF₃ as a precursor. Recently, we proposed chelation‐stabilized hypervalent bromine(III) compounds as a possible solution to both problems. First, they can be conveniently prepared by electro‐oxidation of the corresponding bromoarenes. Second, the chelation endows bromine(III) species with increased stability while retaining sufficient reactivity, comparable to that of iodine(III) counterparts. Finally, their intrinsic reactivity can be unlocked in the presence of acids. Herein, an in‐depth mechanistic study of both the electrochemical generation and the reactivity of the bromine(III) compounds is disclosed, with implications for known applications and future developments in the field.

Recent Updates on Electrogenerated Hypervalent Iodine Derivatives and Their Applications as Mediators in Organic Electrosynthesis

With the renaissance of chemical electrosynthesis in the last decade, the electrochemistry of hypervalent iodine compounds has picked up the pace and achieved significant improvements. By employing traceless electrons instead of stoichiometric oxidants as the alternative clean “reagents”, many hypervalent iodine compounds were efficiently electro-synthesized via anodic oxidation methods and utilized as powerful redox mediators triggering valuable oxidative coupling reactions in a more sustainable way. This minireview gives an up-to-date overview of the recent advances during the past 3 years, encompassing enhanced electrosynthesis technologies, novel synthetic applications, and ideas for improving reaction sustainability.

Electrochemical Generation of Hypervalent Bromine(III) Compounds

In sharp contrast to hypervalent iodine(III) compounds, the isoelectronic bromine(III) counterparts have been little studied to date. This knowledge gap is mainly attributed to the difficult-to-control reactivity of λ³ -bromanes as well as to their challenging preparation from the highly toxic and corrosive BrF₃ precursor. In this context, we present a straightforward and scalable approach to chelation-stabilized λ³ -bromanes by anodic oxidation of parent aryl bromides possessing two coordinating hexafluoro-2-hydroxypropanyl substituents. A series of para-substituted λ³ -bromanes with remarkably high redox potentials spanning a range from 1.86 V to 2.60 V vs. Ag/AgNO₃ was synthesized by the electrochemical method. We demonstrate that the intrinsic reactivity of the bench-stable bromine(III) species can be unlocked by addition of a Lewis or a Brønsted acid. The synthetic utility of the λ³ -bromane activation is exemplified by oxidative C-C, C-N, and C-O bond forming reactions.

Electrifying Sustainability on Transition Metal-Free Modes: An Eco-Friendly Approach for the Formation of C-N Bonds

Embracing sustainable green methodologies and techniques in chemical transformations has always been in the limelight to the synthetic community. Electrosynthesis has emerged as a powerful, sustainable synthetic tool for molecular synthesis exploiting inexpensive electricity in place of sacrificial chemical oxidizing/reducing reagents. Herein, recent advances in the incorporation of transition metal-free redox mediators in electrosynthesis for the construction of C-N bonds are outlined. Furthermore, conjugation of this strategy with flow catalysis allows easy scale up of the synthesis of molecular assembly. This comprehensive Review provides an overview of metal-free mediated electro-construction of C-N bonds, focusing on the reaction mechanisms involved and its synthetic applications.

Photoinduced Aerobic Iodoarene-Catalyzed Spirocyclization of N-Oxy-amides to N-Fused Spirolactams*

Iodoarene catalysis is a powerful methodology that usually requires an excess of oxidant, or of redox mediator if the terminal oxidant is dioxygen, to generate the key hypervalent iodine intermediate to proceed efficiently. We report that, using the spiro-cyclization of amides as a benchmark reaction, aerobic iodoarene catalysis can be enabled by relying on a pyrylium photocatalyst under blue light irradiation. This unprecedented dual organocatalytic system allows the use of low catalytic loading of both catalysts under very mild operating conditions.

Scalable electrochemical synthesis of diaryliodonium salts

Cyclic and acyclic diaryliodonium are synthesised by anodic oxidation of iodobiaryls and iodoarene/arene mixtures, respectively, in a simple undivided electrolysis cell in MeCN-HFIP-TfOH without any added electrolyte salts. This atom efficient process does not require chemical oxidants and generates no chemical waste. More than 30 cyclic and acyclic diaryliodonium salts with different substitution patterns were prepared in very good to excellent yields. The reaction was scaled-up to 10 mmol scale giving more than four grams of dibenzo[b,d]iodol-5-ium trifluoromethanesulfonate (>95%) in less than three hours. The solvent mixture of the large-scale experiment was recovered (>97%) and recycled several times without significant reduction in yield.

Efficient electrochemical synthesis of robust, densely functionalized water soluble quinones

Conjugate addition of thiols to benzoquinones has been coupled to in situ electrochemical oxidation of the resulting hydroquinone to enable full substitution of quinone C-H bonds. The sulfonated thioether-substituted quinones exhibit high solublity and stability in aqueous solution and have redox potentials ranging from 440-750 mV vs. SHE. The electrosynthetic protocol is effective on >100 g scale.