Hypervalent iodine(III): selective and efficient single-electron-transfer (SET) oxidizing agent

Unusual Regio- and Chemoselectivity in Oxidation of Pyrroles and Indoles Enabled by a Thianthrenium Salt Intermediate

A dearomative oxidation of pyrroles to Δ3-pyrrol-2-ones is described, which employs a sulfoxide as oxidant, in conjunction with a carboxylic acid anhydride and a Brønsted acid additive. 3-substituted pyrroles undergo regioselective oxidation to give the product isomer in which oxygen has been introduced at the more hindered position. Regioselectivity is rationalized by a proposed mechanism that proceeds by initial thianthrenium introduction at the less-hindered pyrrole α-position, followed by distal attack of an oxygen nucleophile and subsequent elimination of thianthrene. The same reaction conditions are also able to effect a chemoselective oxidation of indoles to indolin-3-ones and additionally of indolin-3-ones to 2-hydroxyindolin-3-ones. Here again, the regio- and chemoselectivities are rationalized through the intermediacy of a thianthrenium salt.

Hydroxylation of Substituted Anilides with Metallaphotocatalysis

We report the combination of organo-photocatalysis with transition metal (TM) catalysis for directed ortho-hydroxylation of substituted anilides for the synthesis of α-aminophenol derivatives under mild conditions. The developed metallaphotocatalysis utilizes N-pivaloyl as a directing group and phenyl iodine(III) bis(trifluoroacetate) (PIFA) in the combination of the 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) photocatalyst and [RuCl2(p-cymene)]2 TM catalyst under visible-light irradiation at room temperature. The hydroxylation reaction works well for a wide range of substrates containing electron-withdrawing substituents and could be applied to late-stage functionalization and ortho-hydroxyl metabolite generation for drug compounds-containing anilides with electron-withdrawing substituents in a single mild reaction.

Synthesis of fluorinated aminium cations coupled with carborane anions for use as strong one-electron oxidants

Synthesis of a series of hydrocarbon-soluble triarylamines bearing F, CF3, and Br substituents showing quasi-reversible redox events in the 0.59-1.32 V range is reported. Chemical oxidation of the amines was carried out with 0.5PhI(OAc)2/Me3SiX/Na[RCB11Cl11] (X = Cl or OTf, R = H or Me), and a few aminium salts were isolated as pure solids.

Selective modification of tryptophan in polypeptides via C-N coupling with azoles using in situ-generated iodine-based oxidants in aqueous media

This study demonstrates the C-N coupling of tryptophan with azoles, promoted by an in situ-generated iodine-based oxidant. The protocol was successfully applied to the selective modification of tryptophan in nonprotected polypeptide bearing oxidatively sensitive residues in acidic aqueous media. The present method allows the attachment of reactive handles to polypeptides and the peptide stapling.

Electrochemical Synthesis of Dimeric λ3-Bromane: Platform for Hypervalent Bromine(III) Compounds

A straightforward and scalable approach to a previously unreported class of cyclic hypervalent Br(III) species capitalizes on the anodic oxidation of aryl bromide to dimeric benzbromoxole that serves as a versatile platform to access a range of structurally diverse Br(III) congeners such as acetoxy-, alkoxy-, and ethynyl-λ³-bromanes as well as diaryl-λ³-bromanes. The synthetic utility of dimeric λ³-bromane is exemplified by photoinduced Minisci-type heteroarylation reactions and benzylic oxidation.

Combining the best of both worlds: radical-based divergent total synthesis

A mature science, combining the art of the total synthesis of complex natural structures and the practicality of delivering highly diverged lead compounds for biological screening, is the constant aim of the organic chemistry community. Delivering natural lead compounds became easier during the last two decades, with the evolution of green chemistry and the concepts of atom economy and protecting-group-free synthesis dominating the field of total synthesis. In this new era, total synthesis is moving towards natural efficacy by utilizing both the biosynthetic knowledge of divergent synthesis and the latest developments in radical chemistry. This contemporary review highlights recent total syntheses that incorporate the best of both worlds.

Alkali halides as nucleophilic reagent sources for N-directed palladium-catalysed ortho-C-H halogenation of s-tetrazines and other heteroaromatics

A general palladium-catalysed selective C-H halogenation reaction is reported, which was successfully achieved for a large variety of functionalized aromatic rings incorporating diverse N-directing groups. By using simple alkali halides of MX type as the nucleophilic reagent source (M = Li, Na, K, Cs and X = I, Br and Cl), and phenyliodanediacetate oxidant, clean C-H-iodination, bromination and chlorination reactions were performed. This general protocol of selective ortho-monohalogenation, which complements but contrasts with the classical methods using electrophilic reagents, is achievable in a short time (30 min) with microwave irradiation assistance. The reaction was extended to substrates bearing N-directing pyridine, pyrimidine, pyrazole, oxazoline, naphtho[1,2-d]thiazole, and azobenzene groups. Notably, the topical and selectivity-challenging s-tetrazine, as a nitrogen-rich heteroaromatic, was successfully halogenated by this protocol.

Mechanistic Insight into Phenol Dearomatization by Hypervalent Iodine: Direct Detection of a Phenoxenium Cation

Phenol dearomatization is one of several oxidation reactions enabled by hypervalent iodine reagents. However, the presence of a proposed free phenoxenium intermediate in phenol dearomatization is a matter of debate in the literature. Here, we report the unambiguous detection of a free phenoxenium intermediate in the reaction of an electron-rich phenol, 2,4,6-trimethoxyphenol, and (diacetoxyiodo)benzene using UV-vis and resonance Raman spectroscopies. In contrast, we predominantly detect single electron oxidation products of less electron-rich phenols or alkoxy-substituted aromatics in their reaction with (diacetoxyiodo)benzene using UV-vis and electron paramagnetic resonance (EPR) spectroscopies. We conclude that the often-postulated free phenoxenium intermediate, while possible with highly stabilizing substituents, is unlikely to be a general mechanistic pathway in the reaction of typical phenols with hypervalent iodine reagents. The polar solvent 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) or the use of more strongly oxidizing hypervalent iodine reagents, such as [bis(trifluoroacetoxy)iodo]benzene (PIFA) or [hydroxy(tosyloxy)iodo]benzene (HTIB), can help reduce the formation of radical byproducts and favors the formation of phenoxenium intermediates.

Hypervalent Iodine(III)-Mediated Umpolung Dialkoxylation of N-Substituted Indoles

Herein, we report dialkoxylation of N-substituted indoles through a hypervalent iodine-mediated umpolung strategy, affording trans-2,3-dimethoxyindolines with up to 95% yield. In addition, C5-selective bromination of 2,3-dialkoxyindoline via NBS-mediated rearomatization was achieved. DFT calculation of the sequence of electrophilic addition and nucleophilic substitution pathway of N-substituted indoles has also been investigated.

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.

Iodine-Iodine Cooperation Enables Metal-Free C-N Bond-Forming Electrocatalysis via Isolable Iodanyl Radicals

Small molecule redox mediators convey interfacial electron transfer events into bulk solution and can enable diverse substrate activation mechanisms in synthetic electrocatalysis. Here, we report that 1,2-diiodo-4,5-dimethoxybenzene is an efficient electrocatalyst for C-H/E-H coupling that operates at as low as 0.5 mol % catalyst loading. Spectroscopic, crystallographic, and computational results indicate a critical role for a three-electron I-I bonding interaction in stabilizing an iodanyl radical intermediate (i.e., formally I(II) species). As a result, the optimized catalyst operates at more than 100 mV lower potential than the related monoiodide catalyst 4-iodoanisole, which results in improved product yield, higher Faradaic efficiency, and expanded substrate scope. The isolated iodanyl radical is chemically competent in C-N bond formation. These results represent the first examples of substrate functionalization at a well-defined I(II) derivative and bona fide iodanyl radical catalysis and demonstrate one-electron pathways as a mechanistic alternative to canonical two-electron hypervalent iodine mechanisms. The observation establishes I-I redox cooperation as a new design concept for the development of metal-free redox mediators.

Hypoiodite-Catalyzed Oxidative Umpolung of Indoles for Enantioselective Dearomatization

Here we report the oxidative umpolung of 2,3-disubstituted indoles toward enantioselective dearomative aza-spirocyclization to give the corresponding spiroindolenines using chiral quaternary ammonium hypoiodite catalysis. Mechanistic studies revealed the umpolung reactivity of C3 of indoles by iodination of the indole nitrogen atom. Moreover, the introduction of pyrazole as an electron-withdrawing auxiliary group at C2 suppressed a competitive dissociative racemic pathway, and enantioselective spirocyclization proceeded to give not only spiropyrrolidines but also four-membered spiroazetidines that are otherwise difficult to access.

Rh-Catalyzed Oxidation and Trifluoroethoxylation of N-Aryl-pyrrolidin-2-ones : A Domino Approach for the Synthesis of N-Aryl-5-(2,2,2-trifluoroethoxy)-1,5- dihydro-2H-pyrrol-2-ones

The synthesis of trifluoroethoxylated dihydropyrrolidones via rhodium-catalyzed oxidation and trifluoroethoxylation of pyrrolidones is presented in this paper. This process realized trifluoroethoxylation of non-activated sp3 C-H by domino approach for the...

Multicomponent Synthesis of Biologically Relevant S-Aryl Dithiocarbamates Using Diaryliodonium Salts

A transition-metal-free one-pot three-component annulation between diaryliodonium triflates, cyclic and acyclic aliphatic amines, and carbon disulfide providing a convenient and efficient access to biologically relevant S-aryl dithiocarbamates is developed. The reaction does not require metal, base, or any other additive and operates under mild and ambient conditions. This methodology is robust, scalable, and exhibits a broad substrate scope. The in silico analysis revealed that the majority of the compounds have a drug-likeness and good ADMET characteristics.

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.

Hypervalent Iodine Reagent-Promoted Hofmann-Type Rearrangement/Carboxylation of Primary Amides

A novel transformation of primary amides to secondary amides promoted by hypervalent iodine reagents was developed. The hypervalent iodine reagent-mediated Hofmann-type rearrangement generated an isocyanate intermediate, which was subsequently trapped by an in situ generated carboxylic acid from the hypervalent iodine reagent to provide the corresponding secondary amides. This method provided a facile and efficient route for the synthesis of secondary amides from primary amides and also revealed novel reactivities of hypervalent iodine reagents.

Conformation-Changeable π-Electronic Systems with Metastable Bent-Core Conformations and Liquid-Crystalline-State Electric-Field-Responsive Properties

This study focused on the use of nonconventional bent-core π-electronic systems, 2,2'-bipyrroles substituted with modified benzoyl units, as building units of stimuli-responsive assemblies. Electric-field-responsive mesophase behaviors were observed in homochiral synclinic ferroelectric smectic C structures comprising the syn conformations. Electric-field application induced changes in the polarized optical microscopy textures with dynamic behaviors derived from the conversion from twisted to untwisted states.

Utilization of Unsymmetric Diaryliodonium Salts in α-Arylation of α-Fluoroacetoacetamides

The use of unsymmetric diaryliodonium salts as a versatile class of arylating agents has been demonstrated by developing a novel strategy to quickly access α-arylated α-fluoroacetoacetamides. The protocol provides a convenient metal-free method for the α-arylation of a diverse class of fluorinated acetoacetamides, and the products are obtained in good yields. The strategy, upon use of electron-deficient diaryliodonium salts as an arylating agent, provides α-fluoroacetamides through a spontaneous arylation/deacylation cascade.

The C3-H Bond Functionalization of Quinoxalin-2(1H)-Ones With Hypervalent Iodine(III) Reagents

The modification of quinoxalin-2(1H)-ones via direct C-H bond functionalization has begun to receive widespread attention, due to quinoxalin-2(1H)-one derivatives' various biological activities and pharmaceutical properties. This mini review concentrates on the accomplishments of arylation, trifluoromethylation, alkylation, and alkoxylation of quinoxalin-2(1H)-ones with hypervalent iodine(III) reagents as reaction partners or oxidants. The reaction conditions and mechanisms are compared and discussed in detail.

Reactions between Diazo Compounds and Hypervalent Iodine(III) Reagents

Site-selective "cut and sew" transformations employing diazo compounds and hypervalent iodine(III) compounds involve the departure of leaving groups, a "cut" process, followed by a reorganization of the fragments by bond formation, a "sew" process. Bearing controllable cleavage sites, diazo compounds and hypervalent iodine(III) compounds play a critical role as versatile reagents in a wide range of organic transformations because their excellent nucleofugality allows for a large number of unusual reactions to occur. In recent years, the combination of diazo compounds and hypervalent iodine(III) reagents has emerged as a promising tool for developing new and valuable approaches, and has met considerable success. In this Minireview, this combination is systematically illustrated with recent advances in the field, with the aim of elaborating the synthetic utility and potential of this concept as a powerful strategy in organic synthesis.

Metal-Free Synthesis of Indolopyrans and 2,3-Dihydrofurans Based on Tandem Oxidative Cycloaddition

The synthesis of versatile scaffold indolopyrans based on C-C radical-radical cross-coupling under metal-free conditions is described. The reaction involving single electron transfer between coupling partners followed by cage collapse allows highly selective cross-coupling while employing only equimolar amounts of coupling partners. Moreover, the mechanistic manifold was expanded for the functionalization of enamines to give the stereoselective synthesis of 2,3-dihydrofurans. This iodine-mediated oxidative coupling features mild conditions and fast reaction kinetics.

C-H Oxygenation Reactions Enabled by Dual Catalysis with Electrogenerated Hypervalent Iodine Species and Ruthenium Complexes

The catalytic generation of hypervalent iodine(III) reagents by anodic electrooxidation was orchestrated towards an unprecedented electrocatalytic C-H oxygenation of weakly coordinating aromatic amides and ketones. Thus, catalytic quantities of iodoarenes in concert with catalytic amounts of ruthenium(II) complexes set the stage for versatile C-H activations with ample scope and high functional group tolerance. Detailed mechanistic studies by experiment and computation substantiate the role of the iodoarene as the electrochemically relevant species towards C-H oxygenations with electricity as a sustainable oxidant and molecular hydrogen as the sole by-product. para-Selective C-H oxygenations likewise proved viable in the absence of directing groups.

Metal-free late-stage C(sp2)-H functionalization of N-aryl amines with various sodium salts

Metal-free consecutive C(sp2)-X (X = Cl, Br, S, N) bond formations of N-aryl amines (cyclic, fused, carbamate, and aminium radicals) were achieved under mild conditions using [bis(trifluoroacetoxy)iodo]benzene (PIFA) and simple nonharmful sodium salts. This direct and selective C(sp2)-H functionalization showed excellent functional group compatibility, cost effectiveness, and late-stage applicability for the synthesis of biologically active natural products. Two mechanisms were proposed to explain the ortho- or para-preference, as well as the accelerating effect of CH3NO2.

Hypervalent iodine reactions utilized in carbon–carbon bond formations

This review covers recent developments of hypervalent iodine chemistry in dearomatizations, radicals, hypervalent iodine-guided electrophilic substitution, arylations, photoredox, and more.

Experimental evidence for the formation of cationic intermediates during iodine(iii)-mediated oxidative dearomatization of phenols

Iodine(iii)-based oxidants are commonly used reagents for the oxidative dearomatization of phenols. Having a better understanding of the mechanism through which these reactions proceed is important for designing new iodine(iii)-based reagents, catalysts, and reactions. We have performed a Hammett analysis of the oxidative dearomatization of substituted 4-phenylphenols. This study confirms that iodine(iii)-mediated oxidative dearomatizations likely proceed through cationic phenoxenium ions and not the direct addition of a nucleophile to an iodine-bound phenol intermediate.

Metal-Free Oxidative Cross Coupling of Indoles with Electron-Rich (Hetero)arenes

A new method for the synthesis of bi-heteroaryls is reported, based on the umpolung of indoles with benziodoxol(on)e hypervalent iodine reagents (IndoleBX). The oxidative coupling of IndoleBX with an equimolar amount of electron-rich benzenes, indoles, pyrroles, and thiophenes proceeded under mild transition-metal-free conditions. Functionalized non-symmetrical bi-indolyl heterocycles were accessed efficiently. Introduction of a new type of C2-substituted indole benziodoxole reagents further allowed extending the scope of the reaction to NH unprotected and C3-alkylated indoles. The obtained bi-heterocycles are important building blocks in synthetic and medicinal chemistry, and could be easily transformed into more complex heterocyclic systems.

Perfluorocyclopentadienyl Radical Derivative as an Organocatalyst for Oxidative Coupling of Aryl- and Thienylmagnesium Compounds under Atmospheric Oxygen

The oxidative homocoupling reaction of Grignard reagents in the presence of atmospheric oxygen molecules proceeded in the presence of a heptafluorotolyl-substituted perfluorocyclopentadienyl radical. The turnover number (TON) was over 30 for the coupling reactions of PhMgBr to give biphenyl. The organocatalyst could couple thienylmagnesium compounds to give bithiophene derivatives in up to 94% yield. Furthermore, a gram-scale synthesis of 6,6'-dimethoxybiphenyl-2,2'-diyl-bis(phosphonic acid diethyl ester) was demonstrated. Stabilization of the phenyl radical for the inhibition of the side reaction was also considered using DFT calculations.