Chemistry of polyvalent iodine

Rh(III)-catalyzed atroposelective C-H alkynylation of 1-aryl isoquinolines with hypervalent iodine-alkyne reagents

An efficient Rh(III)-catalyzed enantioselective C-H alkynylation of isoquinolines is disclosed. The C-H alkynylation of 1-aryl isoquinolines with hypervalent iodine-alkyne reagents proceeded in DMA at room temperature in the presence of 2.5 mol% chiral SCpRh(III) complex along with 20 mol% AgSbF6, providing axially chiral alkynylated 1-aryl isoquinolines in excellent yields (up to 93%) and enantioselectivity (up to 95% ee). The diverse transformations of the product further enhance the potential utility of this reaction.

Alkoxy Diazomethylation of Alkenes by Photoredox-Catalyzed Oxidative Radical-Polar Crossover

Herein, we present the discovery and development of the first photoredox-catalyzed alkoxy diazomethylation of alkenes with hypervalent iodine reagents and alcohols. This multicomponent process represents a new disconnection approach to diazo compounds and is featured by a broad scope, mild reaction conditions, and excellent selectivity. Key to the process was the generation of diazomethyl radicals, which engaged alkenes and alcohols in an inter- and intramolecular fashion by a photoredox-catalyzed oxidative radical-polar crossover leading to unexplored β-alkoxydiazo compounds. The synthetic utility of such diazo compounds was demonstrated with a series of transformations involving C-H, N-H, and O-H insertions as well as in the construction of complex sp3-rich heterocycles.

Organohypervalent heterocycles

This review summarizes the structural and synthetic aspects of heterocyclic molecules incorporating an atom of a hypervalent main-group element. The term "hypervalent" has been suggested for derivatives of main-group elements with more than eight valence electrons, and the concept of hypervalency is commonly used despite some criticism from theoretical chemists. The significantly higher thermal stability of hypervalent heterocycles compared to their acyclic analogs adds special features to their chemistry, particularly for bromine and iodine. Heterocyclic compounds of elements with double bonds are not categorized as hypervalent molecules owing to the zwitterionic nature of these bonds, resulting in the conventional 8-electron species. This review is focused on hypervalent heterocyclic derivatives of nonmetal main-group elements, such as boron, silicon, nitrogen, carbon, phosphorus, sulfur, selenium, bromine, chlorine, iodine(III) and iodine(V).

Visible-Light-Induced PhI(OAc)2-Mediated Alkylation of Heteroarenes with Simple Alkanes and Ethers

The direct alkylation of heteroarenes with alkanes has been successfully achieved through visible-light-induced hypervalent iodine-mediated C-H functionalization of both coupling partners at ambient temperatures. This reaction proceeds via the in situ generation of nucleophilic alkyl radicals from alkanes through hydrogen atom transfer (HAT), followed by a Minisci-type reaction with heteroarenes. These mild reaction conditions have demonstrated their suitability for the alkylation of a wide range of heterocycles, including azoles, pyridines, quinolines, isoquinolines, and quinoxalinones.

Hypervalent Iodine Compounds in Carbohydrate Chemistry: Glycosylation, Functionalization and Oxidation

This mini review article provides an overview on the use of hypervalent iodine compounds (HICs) in carbohydrate synthesis, focusing on their chemistry and recent applications. HICs are similar to transition metals in their reactivity but have the added benefit of being environmentally benign, and are therefore commonly used as selective oxidants and eco-friendly reagents in organic synthesis. Herein, we summarize various synthetic uses of hypervalent iodine reagents in reactions such as glycosylation, oxidations, functionalization, and C-C bond-forming reactions. The goal of this review is to illustrate the advantages and versatility of using HICs as an environmentally sustainable alternative to heavy metals in carbohydrate chemistry.

Phenyliodine bis(trifluoroacetate) as a sustainable reagent: exploring its significance in organic synthesis

Iodine-containing molecules, especially hypervalent iodine compounds, have gained significant attention in organic synthesis. They are valuable and sustainable reagents, leading to a remarkable surge in their use for chemical transformations. One such hypervalent iodine compound, phenyliodine bis(trifluoroacetate)/bis(trifluoroacetoxy)iodobenzene, commonly referred to as PIFA, has emerged as a prominent candidate due to its attributes of facile manipulation, moderate reactivity, low toxicity, and ready availability. PIFA presents an auspicious prospect as a substitute for costly organometallic catalysts and environmentally hazardous oxidants containing heavy metals. PIFA exhibits remarkable catalytic activity, facilitating an array of consequential organic reactions, including sulfenylation, alkylarylation, oxidative coupling, cascade reactions, amination, amidation, ring-rearrangement, carboxylation, and numerous others. Over the past decade, the application of PIFA in synthetic chemistry has witnessed substantial growth, necessitating an updated exploration of this field. In this discourse, we present a concise overview of PIFA's applications as a 'green' reagent in the domain of synthetic organic chemistry. A primary objective of this article is to bring to the forefront the scientific community's awareness of the merits associated with adopting PIFA as an environmentally conscientious alternative to heavy metals.

A decade of lessons in the activation of ArIL2 species

Hypervalent iodine(iii) compounds of the general structure ArIL2 are widely used as oxidizing agents for a variety of applications across both organic and inorganic chemistry. Considerable work has been done on the activation of these compounds by tuning the ligands at the iodine centre. This perspective summarises the work of our and other groups on rectification of historically misidentified iodine(iii) reagents of this class, and the syntheses of activated species. Recent advances focusing on increasing the oxidative capacity of I(iii) moieties using Lewis and Brønsted acids and Lewis bases as well as the activation of halogens with I(iii) are discussed.

Bond Strength and Interaction Energies in Togni Reagents: Insights from Molecular Electrostatic Potential-Based Parameters

Togni reagents and their analogs, classified as hypervalent iodine(III) complexes, serve as potent trifluoromethylation agents. The interplay of cis and trans factors plays a pivotal role in shaping their performance, affecting aspects such as bond strength, interaction energies, stability, and subsequent nucleophilic reactions. In this context, we propose the utilization of the molecular electrostatic potential (MESP) at the carbon atom (VC) of the I-CF3 moiety as a sensitive parameter to quantify the cis and trans influences in Togni-type reagents. Our study has shown that VC serves as a convenient probe for determining the heterolytic bond dissociation energy (BDE) and, consequently, assessing the reactivity of these reagents. Moreover, these parameters have been successfully applied to evaluate the strength of the σ-hole interactions with nucleophiles (Cl- and NMe3). Additionally, we provide insights into interactions of Togni reagents with Brønsted acids such as HCl and HSO3F, elucidating them in terms of MESP topological parameters. These findings yield valuable information about the electronic properties of hypervalent iodine reagents, particularly Togni-type reagents, offering the potential for optimizing structurally modified reagents with enhanced activity and stability.

Diaryl hypervalent bromines and chlorines: synthesis, structures and reactivities

In the field of modern organic chemistry, hypervalent compounds have become indispensable tools for synthetic chemists, finding widespread applications in both academic research and industrial settings. While iodine-based reagents have historically dominated this research field, recent focus has shifted to the potent yet relatively unexplored chemistry of diaryl λ3-bromanes and -chloranes. Despite their unique reactivities, the progress in their development and application within organic synthesis has been hampered by the absence of straightforward, reliable, and widely applicable preparative methods. However, recent investigations have uncovered innovative approaches and novel reactivity patterns associated with these specialized compounds. These discoveries suggest that we have only begun to tap into their potential, implying that there is much more to be explored in this captivating area of chemistry.

Sequential regioselective arylation of pyrazolones with diaryliodonium salts

The introduction of aromatic substituents into organic compounds significantly alters their physical and chemical characteristics. Yet, achieving precise control over the site-selectivity of arylation continues to pose a considerable challenge. We present here a controllable method for the site-selective mono-, di-, and triarylation of pyrazolone with diaryliodonium salts. The method showcases robustness, flexibility, and excellent compatibility with a broad range of functional groups. It enables control over both the site of arylation and the number of aryl additions. Specifically, three of the four substitutable positions in pyrazolone can be selectively arylated, effectively producing four products under controlled conditions. Additionally, the method supports one-pot sequential arylation, leading to an array of products with diverse aromatic substituents. Control experiments revealed the specific conditions of each reaction step.

Iodoxybenzoic Acid (IBX) in Organic Synthesis: A Septennial Review

This study reviews the oxidative applications of 2-iodoxybenzoic acid (IBX) in organic synthesis, focusing on C-H functionalization, hetero-hetero bond formations, ring cleavage reactions, dehydrogenation, heterocyclic ring formations, and some miscellaneous reactions in a comprehensive and critical way. It compiles the literature starting from mid-2015 to date.

Directing group assisted para-selective C-H alkynylation of unbiased arenes enabled by rhodium catalysis

Regioselective C-H alkynylation of arenes via C-H activation is challenging yet a highly desirable transformation. In this regard, directing group assisted C(sp2)-H alkynylation of arenes offers a unique opportunity to ensure precise regioselectivity. While the existing methods are mainly centered around ortho-C-H alkynylation and a few for meta-C-H alkynylation, the DG-assisted para-selective C-H alkynylation is yet to be reported. Herein we disclose the first report on Rh-catalyzed para-C-H alkynylation of sterically and electronically unbiased arenes. The para-selectivity is achieved with the assistance of a cyano-based directing template and the selectivity remained unaltered irrespective of the steric and electronic influence of the substituents. The post-synthetic modification of synthesized para-alkynylated arenes is also demonstrated. The mechanistic intricacies of the developed protocol are elucidated through experimental and computational studies.

The Tetrel Bonds of Hypervalent Halogen Compounds

The tetrel bond between PhXF2Y(TF3) (T = C and Si; X = Cl, Br, and I; Y = F and Cl) and the electron donor MCN (M = Li and Na) was investigated at the M06-2X/aug-cc-pVDZ level of theory. As the electronegativity of the halogen atom X increases, the strength of the tetrel bond also increases, but as the electronegativity of the halogen atom Y increases, the strength of the tetrel bond decreases. The magnitude of the interaction energy in most -CF3 complexes was found to be less than 10 kcal/mol, but to exceed 11 kcal/mol for PhClF2Cl(CF3)⋯NCNa. The tetrel bond is greatly enhanced when the -SiF3 group interacts with LiCN or NaCN, with the largest interaction energy approaching 100 kcal/mol and displaying a covalent Si⋯N interaction. Along with this enhancement, the Si⋯N distance was found to be less than the X-Si bond length, the -SiF3 group to be closer to the N atom, and in most -SiF3 systems, the X-Si-F angle to be less than 90°; the -SiF3 group therefore undergoes inversion and complete transfer in some systems.

Diaryliodonium Salts Enabled Arylation, Arylocyclization, and Aryl-Migration

Our research interest focusing on synthetic methodology with diaryliodonium salts, is summarized in this account. Besides employing a dual activation strategy of C-I and ortho C-H bonds, we have introduced vicinal functional groups at ortho-positions of diaryliodonium salts, in which their unique reactivities have been explored in various processes, including arylation, diarylation, cascade annulation, benzocyclization, arylocyclization, and intramolecular aryl migration. The variety of mechanisms of these reactions that involves either transition metals, especially palladium in organometallic catalysis, or transition-metal free conditions, were discussed in the context.

Iodine(III)-Mediated Migratory gem-Difluorinations: Synthesis of β Transformable Functionality Substituted gem-Difluoroalkanes

Geminal-difluoroalkanes featuring intriguing steric and electronic properties are of great significance in medicinal chemistry, and great progresses have been achieved for their synthesis. In recent years, iodine(III) reagent-mediated migratory gem-difluorination of alkenes has proved to be an efficient and powerful strategy to access to diverse gem-difluoroalkanes, especially those bearing a readily transformable functionality (TF), which are important for rapid assembly of complex gem-difluorinated molecules in a modular and diverse manner. In this review, we systematically summarize the recent development of iodine(III)-mediated migratory gem-difluorination reactions for the synthesis of gem-difluoroalkanes bearing a synthetically versatile TF at the β position. The reaction mechanism and the utilities of the products are also discussed. This review is presented and grouped basically according to the types of transformable functionalities within the products.

Organic Synthesis Using Nitroxides

Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R1R2N-O•. The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R1 and R2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R1R2N═O+) or reduced to anions (R1R2N-O-), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C-O bonds, allowing for the thermal generation of C radicals through reversible C-O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

Optimizing the Synthetic Potential of O2: Implications of Overpotential in Homogeneous Aerobic Oxidation Catalysis

Molecular oxygen is the quintessential oxidant for organic chemical synthesis, but many challenges continue to limit its utility and breadth of applications. Extensive historical research has focused on overcoming kinetic challenges presented by the ground-state triplet electronic structure of O2 and the various reactivity and selectivity challenges associated with reactive oxygen species derived from O2 reduction. This Perspective will analyze thermodynamic principles underlying catalytic aerobic oxidation reactions, borrowing concepts from the study of the oxygen reduction reaction (ORR) in fuel cells. This analysis is especially important for "oxidase"-type liquid-phase catalytic aerobic oxidation reactions, which proceed by a mechanism that couples two sequential redox half-reactions: (1) substrate oxidation and (2) oxygen reduction, typically affording H2O2 or H2O. The catalysts for these reactions feature redox potentials that lie between the potentials associated with the substrate oxidation and oxygen reduction reactions, and changes in the catalyst potential lead to variations in effective overpotentials for the two half reactions. Catalysts that operate at low ORR overpotential retain a more thermodynamic driving force for the substrate oxidation step, enabling O2 to be used in more challenging oxidations. While catalysts that operate at high ORR overpotential have less driving force available for substrate oxidation, they often exhibit different or improved chemoselectivity relative to the high-potential catalysts. The concepts are elaborated in a series of case studies to highlight their implications for chemical synthesis. Examples include comparisons of (a) NOx/oxoammonium and Cu/nitroxyl catalysts, (b) high-potential quinones and amine oxidase biomimetic quinones, and (c) Pd aerobic oxidation catalysts with or without NOx cocatalysts. In addition, we show how the reductive activation of O2 provides a means to access potentials not accessible with conventional oxidase-type mechanisms. Overall, this analysis highlights the central role of catalyst overpotential in guiding the development of aerobic oxidation reactions.

Dibenzoylperoxide as a Versatile Oxidant for Oxidative Azidations of Phenols Using Quaternary Ammonium Iodides or Bromides

Dibenzoylperoxide emerged as a versatile oxidant for quaternary ammonium iodide or bromide‐catalyzed reactions of phenol derivatives with NaN3. While the use of iodides allowed for efficient benzylic azidations under these oxidative conditions, the use of bromides allowed for dearomative azidations instead. Both approaches have been successfully applied to different phenol derivatives and a first proof‐of‐concept for an enantioselective variant using chiral quat. ammonium bromides has been obtained as well.

Synthesis of Oxazolidines and Dihydroxazines via Cyclization of α-Aminated Ketones

A new approach to oxazolidines and dihydroxazines was developed by regioselective cyclization of α-aminated ketones under transition metal-free conditions. Oxazolidine derivatives were generated in the presence of chloro benziodoxole and TFA, while dihydroxazines were formed without a hypervalent iodine reagent. The reaction was performed under room temperature and gave the products in good to excellent yields.

Visible-Light-Induced Metal- and Photocatalyst-Free Radical Cascade Cyclization of Cinnamamides for Synthesis of Functionalized Dihydroquinolinones

Visible-light-promoted metal- and photocatalyst-free radical cascade cyclization of cinnamamides with α-oxocarboxylic acids is described for sustainable synthesis of diverse pharmaceutically important dihydroquinolinone scaffolds in one pot under mild conditions. The decarboxylative cascade cyclization proceeded efficiently at room temperature without the need for expensive photocatalysts such as Ir or Ru complexes, which indicates the practicability and environmentally benign nature of this protocol. Preliminary mechanistic studies reveal that the blue LED irradiation efficiently cleaves the I-O bond of the hypervalent iodine reagent PhI(O₂CCOAr)₂ formed through ligand exchange between iodobenzene diacetate and arylglyoxylic acid to initiate the cascade reaction. The synthetic value of this operationally simple and energy-efficient method is further demonstrated by late-stage functionalization of drug molecules in excellent yields.

Chemoselective cycloisomerization of O-alkenylbenzamides via concomitant 1,2-aryl migration/elimination mediated by hypervalent iodine reagents

As an ambident nucleophile, controlling the reaction selectivities of nitrogen and oxygen atoms in amide moiety is a challenging issue in organic synthesis. Herein, we present a chemodivergent cycloisomerization approach to construct isoquinolinone and iminoisocoumarin skeletons from o-alkenylbenzamide derivatives. The chemo-controllable strategy employed an exclusive 1,2-aryl migration/elimination cascade, enabled by different hypervalent iodine species generated in situ from the reaction of iodosobenzene (PhIO) with MeOH or 2,4,6-tris-isopropylbenzene sulfonic acid. DFT studies revealed that the nitrogen and oxygen atoms of the intermediates in the two reaction systems have different nucleophilicities and thus produce the selectivity of N or O-attack modes.

Cyclic diaryliodonium salts: applications and overview

Owing to the recent renewed interest and groundbreaking advances in hypervalent chemistry, cyclic diaryliodonium salts have had a myriad of unique applications in the past decade. Their numerous properties, such as an efficient dual arylation mechanism, straightforward one-pot synthesis compatibility, wide substrate scope, and functionalization tolerance, have made them appropriate starting materials for many bioactive compounds. Fluorenes, thiophenes, carbazoles, phenanthrenes, and many other useful cyclic bioactive molecules that are essential for pharmaceutical synthesis can be readily accessed from cyclic diaryliodonium salts. Particular focus has been given to the high optical activity and good enantiomeric excess of the products that facilitate the easy formation of many difficult-to-obtain optical isomers, such as atropisomers. This review aims to compile and summarize all the recent advances in synthesizing methodologies to prepare the important compounds where cyclic diaryliodonium salt is an integral part of the methodologies and would hopefully provide a good foundation for further research on this topic.

Ions Speciation at the Water-Air Interface

In typical aqueous systems, including naturally occurring sweet and salt water and tap water, multiple ion species are co-solvated. At the water-air interface, these ions are known to affect the chemical reactivity, aerosol formation, climate, and water odor. Yet, the composition of ions at the water interface has remained enigmatic. Here, using surface-specific heterodyne-detected sum-frequency generation spectroscopy, we quantify the relative surface activity of two co-solvated ions in solution. We find that more hydrophobic ions are speciated to the interface due to the hydrophilic ions. Quantitative analysis shows that the interfacial hydrophobic ion population increases with decreasing interfacial hydrophilic ion population at the interface. Simulations show that the solvation energy difference between the ions and the intrinsic surface propensity of ions determine the extent of an ion's speciation by other ions. This mechanism provides a unified view of the speciation of monatomic and polyatomic ions at electrolyte solution interfaces.

Generating Fischer-Type Rh-Carbenes with Rh-Carbynoids

We describe the first catalytic generation of Fischer-type acyloxy Rh(II)-carbenes from carboxylic acids and Rh(II)-carbynoids. This novel class of transient donor/acceptor Rh(II)-carbenes evolved through a cyclopropanation process providing access to densely functionalized cyclopropyl-fused lactones with excellent diastereoselectivity. DFT calculations allowed the analysis of the properties of Rh(II)-carbynoids and acyloxy Rh(II)-carbenes as well as the characterization of the mechanism.

Enantioselective N-Heterocyclic Carbene Catalyzed α-Oxidative Coupling of Enals with Carboxylic Acids Using an Iodine(III) Reagent

The enantioselective α-oxidative coupling of enals with carboxylic acids was developed via the umpolung of an NHC-bound enolate with an iodine(III) reagent. The corresponding α-acyloxyl-β,γ-unsaturated esters were afforded in good yields, with high regio- and enantioselectivities. The key step of the reaction involves the formation of enol iodine(III) intermediate from the enolate with iodosobenzene, which changes the polarity of α-carbon of the enal from nucleophilic to electrophilic, and thus facilitates the subsequent addition of carboxylate.

Metal -free PhI(OAc)2-oxidized decarboxylation of propiolic acids towards synthesis of α-acetoxy ketones and insights into general decarboxylation with DFT calculations

A metal-free oxidative decarboxylation reaction of propiolic acids mediated by hypervalent iodine(III) reagents is described. This decarboxylative C-O bond-forming reaction used a combination of (diacetoxyiodo)benzene and aromatic, heteroaromatic or aliphatic propiolic acids to give the corresponding α-acetoxy ketones. Preliminary mechanistic studies based on both DFT calculations and high-resolution mass spectroscopy (HRMS) suggested that the reaction proceeded through decarboxylation to form a propargyl iodide intermediate. This reaction provides an attractive alternative to existing methods for the exclusive synthesis of α-acyloxy ketones.

Recent progress in alkynylation with hypervalent iodine reagents

Although alkynes are one of the smallest functional groups, they are among the most versatile building blocks for organic chemistry, with applications ranging from biochemistry to material sciences. Alkynylation reactions have traditionally relied on the use of acetylenes as nucleophiles. The discovery and development of ethynyl hypervalent iodine reagents have allowed to greatly expand the transfer of alkynes as electrophilic synthons. In this feature article the progress in the field since 2018 will be presented. After a short introduction on alkynylation reactions and hypervalent iodine reagents, the developments in the synthesis of alkynyl hypervalent iodine reagents will be discussed. Their recent use in base-mediated and transition-metal catalyzed alkynylations will be described. Progress in radical-based alkynylations and atom-economical transformations will then be presented.

The Suzuki-Miyaura Cross-Coupling-Claisen Rearrangement-Cross-Metathesis Approach to Prenylated Isoflavones

Isoflavones were synthesized via Suzuki-Miyaura coupling of 3-iodochromones and para-methoxybenzene- and para-phenolboronic acid. In our hands, conditions commonly used for similar cross couplings turned out to be unsuccessful or difficult to reproduce, for example, due to the unplanned partial cleavage of MOM-protecting groups. Using Pd(dba)₂ as a precatalyst and tricyclohexylphosphine as an activating ligand, reliable cross-coupling conditions were identified. In all cases, notably higher yields of isoflavones were obtained with para-phenolboronic acid than with para-methoxybenzene boronic acid. This observation and the commercial availability of para-phenolboronic acid suggest that for the synthesis of the important 3'-prenyl- or 3',5'-diprenylisoflavone substitution pattern a synthetic route that introduces the prenyl substituents after the Pd-catalyzed cross-coupling step, thereby avoiding laborious and protecting-group-intensive multistep syntheses of C-prenylated arene boronic acids, is advantageous.

In Situ Hypoiodite-Catalyzed Oxidative Rearrangement of Chalcones: Scope and Mechanistic Investigation

It is highly desirable to avoid using rare or toxic metals for oxidative reactions in the synthesis of pharmaceuticals and fine chemicals. Hypervalent iodine compounds are environmentally benign alternatives, but their catalytic use has been quite limited. Herein, the protocol for in situ hypoiodite-catalyzed oxidative rearrangement of chalcones is first realized under mild and metal-free conditions, which provided a nontoxic, environmental-benign, and catalytic alternative to the thallium-based protocol. Also, the applicability and effectiveness of this catalytic protocol got well demonstrated via gram-scale synthesis and product derivatization. What is more, control and NMR tracking experiments were performed to figure out the possible catalytic species and intermediates.

Metal-free synthesis of dihydrofuran derivatives as anti-vicinal amino alcohol isosteres

Dihydrofuran cores are commonly incorporated into synthetically and pharmacologically significant scaffolds in natural product and drug discovery chemistry. Herein, we report a concise and practical strategy to construct spiro-dihydrofuran and amino dihydrofuran scaffolds as anti-vicinal amino alcohol isosteres. Hypervalent iodine (PhI(OAc)(NTs₂))-mediated C-H activation of alkynes resulted in two-bond formations with one pi bond cleavage: (i) C(sp²)-N(sp³) and O(sp³)-C(sp²); (ii) C(sp²)-N(sp³) and C(sp³)-C(sp²). The metal-free 5-endo-dig oxidative cyclization provided versatile amino 2,3- and 2,5-dihydrofurans bearing the C₅ quaternary carbon. The non-toxicity of all synthesised dihydrofurans was verified via in vitro cell viability assay.

Sustainable Photoinduced Decarboxylative Chlorination Mediated by Halogen Atom Transfer

Chlorinated organic backbones constitute important components in existing biologically active chemicals, and they are extraordinary useful intermediates in organic synthesis. Herein, an operationally simple and sustainable halodecarboxylation protocol via halogen-atom transfer (XAT) as a key step is presented. The method merges a metal-free photoredox system with (diacetoxyiodo)benzene (PIDA) as a hypervalent iodine reagent using 1,2-dihaloethanes as halogen sources to afford haloalkanes in an efficient manner. The sustainability of this protocol is highlighted by an important waste recovery protocol as well as by atom economy and carbon efficiency parameters.

Polyhalogenation-Facilitated Spirolactonization at the meta-Position of Phenols

A novel dearomative spirolactonization/polyhalogenation of phenols that employs hypervalent iodine PhICl₂ (iodobenzene dichloride) as both an oxidant and chlorine source with an indispensable base, or only using NBS (N-bromosuccinimide) without any additives, is presented. Halide participations are a vital factor in the cascade reaction of 3'-hydroxy-[1,1'-biphenyl]-2-carboxylic acids with good selectivities and reactivities and induced the rapid constructions of multiple C-halogen bonds and directional C═O bonds in a one-step operation under mild conditions. In gaining a good understanding of the mechanism, the increase in number of bromine atoms was inferred rationally from the spirolactonization process, assisted by DFT calculations and high-resolution mass spectrometry. Mechanistic experiments suggest that the formation of a stable carbocation intermediate plays a great role in the migration of oxygen to spirolactonization.

Recent Progress in Cyclic Aryliodonium Chemistry: Syntheses and Applications

Hypervalent aryliodoumiums are intensively investigated as arylating agents. They are excellent surrogates to aryl halides, and moreover they exhibit better reactivity, which allows the corresponding arylation reactions to be performed under mild conditions. In the past decades, acyclic aryliodoniums are widely explored as arylation agents. However, the unmet need for acyclic aryliodoniums is the improvement of their notoriously low reaction economy because the coproduced aryl iodides during the arylation are often wasted. Cyclic aryliodoniums have their intrinsic advantage in terms of reaction economy, and they have started to receive considerable attention due to their valuable synthetic applications to initiate cascade reactions, which can enable the construction of complex structures, including polycycles with potential pharmaceutical and functional properties. Here, we are summarizing the recent advances made in the research field of cyclic aryliodoniums, including the nascent design of aryliodonium species and their synthetic applications. First, the general preparation of typical diphenyl iodoniums is described, followed by the construction of heterocyclic iodoniums and monoaryl iodoniums. Then, the initiated arylations coupled with subsequent domino reactions are summarized to construct polycycles. Meanwhile, the advances in cyclic aryliodoniums for building biaryls including axial atropisomers are discussed in a systematic manner. Finally, a very recent advance of cyclic aryliodoniums employed as halogen-bonding organocatalysts is described.

Iodonium ylides: an emerging and alternative carbene precursor for C-H functionalizations

The metal-catalyzed successive activation and functionalization of arene/heteroarene is one of the most fundamental transformations in organic synthesis and leads to privileged scaffolds in natural products, pharmaceuticals, agrochemicals, and fine chemicals. Particularly, transition-metal-catalyzed C-H functionalization of arenes with carbene precursors via metal carbene migratory insertion has been well studied. As a result, diverse carbene precursors have been evaluated, such as diazo compounds, sulfoxonium ylides, triazoles, etc. In addition, there have been significant developments with the use of iodonium ylides as carbene precursors in recent years, and these reactions proceed with high efficiencies and selectivities. This review provides a comprehensive overview of iodonium ylides in C-H functionalizations, including the scope, limitations, and their potential synthetic applications.

Cyclic Diaryl λ3-Chloranes: Reagents and Their C-C and C-O Couplings with Phenols via Aryne Intermediates

Hypervalent chloranes are a class of rare and poorly explored reagents. Their unique electronic properties confer reactivity that is complementary to that of the common iodanes and emerging bromanes. Highly chemo- and regioselective, metal-free, and mild C-C and C-O couplings are reported here. Experimental and computational mechanistic studies elucidate the unprecedented reactivities and selectivities of these systems and the intermediacy of aryne intermediates. The synthetic potential of these transformations is further demonstrated via the post-functionalization of C-C and C-O coupling products obtained from reactions of chloranes with phenols under different conditions.

Vinyl cation-mediated intramolecular hydroarylation of alkynes using pyridinium reagents

Once considered to be exotic species of limited synthetic utility, vinyl cations have recently been shown to be highly versatile intermediates in a variety of processes. Here, we report a method for the synthesis of aryl-substituted benzocycloheptenes and -hexenes using the hydrotriflate salt of an electron-poor pyridine as a uniquely efficient proton source for a vinyl cation mediated Friedel-Crafts cyclization. The mild conditions made possible by this reagent allowed a range of simple and functionalized alkynes bearing pendant aryl groups to serve as suitable substrates for this scalable and convenient protocol.

Lewis Acid-Catalyzed [3 + 2]-Cyclization of Iodonium Ylides with Azadienes: Access to Spiro[benzofuran-2,2'-furan]-3-ones

A highly regioselective synthesis of spiro[benzofuran-2,2'-furan]-3-ones has been explored via Lewis acid-catalyzed [3 + 2] cyclization of iodonium ylides with azadienes. The acidity of the Lewis acid was significantly strengthened with strong hydrogen bond donors, thereby promoting the enolization isomerization of iodonium ylides for the subsequent cycloaddition. This reaction was compatible with a broad range of substrates under the mild reaction conditions, and efficiently delivered spiro-heterocycles with excellent stereoselectivity.

Efficient Oxidative Dearomatisations of Substituted Phenols Using Hypervalent Iodine (III) Reagents and Antiprotozoal Evaluation of the Resulting Cyclohexadienones against T. b. rhodesiense and P. falciparum Strain NF54

Quinones and quinols are secondary metabolites of higher plants that are associated with many biological activities. The oxidative dearomatization of phenols induced by hypervalent iodine(III) reagents has proven to be a very useful synthetic approach for the preparation of these compounds, which are also widely used in organic synthesis and medicinal chemistry. Starting from several substituted phenols and naphthols, a series of cyclohexadienone and naphthoquinone derivatives were synthesized using different hypervalent iodine(III) reagents and evaluated for their in vitro antiprotozoal activity. Antiprotozoal activity was assessed against Plasmodium falciparum NF54 and Trypanosoma brucei rhodesiense STIB900. Cytotoxicity of all compounds towards L6 cells was evaluated and the respective selectivity indices (SI) were calculated. We found that benzyl naphthoquinone 5c was the most active and selective molecule against T. brucei rhodesiense (IC₅₀ = 0.08 μM, SI = 275). Furthermore, the antiprotozoal assays revealed no specific effects. In addition, some key physicochemical parameters of the synthesised compounds were calculated.

Oxidation of p-[125I]Iodobenzoic Acid and p-[211At]Astatobenzoic Acid Derivatives and Evaluation In Vivo

The alpha particle-emitting radionuclide astatine-211 (²¹¹At) is of interest for targeted radiotherapy; however, low in vivo stability of many ²¹¹At-labeled cancer-targeting molecules has limited its potential. As an alternative labeling method, we evaluated whether a specific type of astatinated aryl compound that has the At atom in a higher oxidation state might be stable to in vivo deastatination. In the research effort, para-iodobenzoic acid methyl ester and dPEG₄-amino acid methyl ester derivatives were prepared as HPLC standards. The corresponding para-stannylbenzoic acid derivatives were also prepared and labeled with ¹²⁵I and ²¹¹At. Oxidization of the [¹²⁵I]iodo- and [²¹¹At]astato-benzamidyl-dPEG₄-acid methyl ester derivatives provided materials for in vivo evaluation. A biodistribution was conducted in mice with coinjected oxidized ¹²⁵I- and ²¹¹At-labeled compounds. The oxidized radioiodinated derivative was stable to in vivo deiodination, but unfortunately the oxidized [²¹¹At]astatinated benzamide derivative was found to be unstable under the conditions of isolation by radio-HPLC (post animal injection). Another biodistribution study in mice evaluated the tissue concentrations of coinjected [²¹¹At]NaAtO₃ and [¹²⁵I]NaIO₃. Comparison of the tissue concentrations of the isolated material from the oxidized [²¹¹At]benzamide derivative with those of [²¹¹At]astatate indicated the species obtained after isolation was likely [²¹¹At]astatate.

Catalytic Synthesis of Cyclopropenium Cations with Rh-Carbynoids

Herein, we report the first catalytic one-step synthesis of cyclopropenium cations (CPCs) with readily available alkynes and hypervalent iodine reagents as carbyne sources. Key to the process is the catalytic generation of a novel Rh-carbynoid that formally transfers monovalent cationic carbynes (:⁺C-R) to alkynes via an oxidative [2+1] cycloaddition. Our process is able to synthesize a new type of CPC substituted with an ester group that underpins the regioselective attack of a broad range of carbon and heteroatomic nucleophiles, thus providing a new platform for the synthesis of valuable cyclopropenes difficult or not possible to make by current methodologies.