The IX (X=O,N,C) Double Bond in Hypervalent Iodine Compounds: Is it Real?

Recent Progress in Synthetic Applications of Hypervalent Iodine(III) Reagents

Hypervalent iodine(III) compounds have found wide application in modern organic chemistry as environmentally friendly reagents and catalysts. Hypervalent iodine reagents are commonly used in synthetically important halogenations, oxidations, aminations, heterocyclizations, and various oxidative functionalizations of organic substrates. Iodonium salts are important arylating reagents, while iodonium ylides and imides are excellent carbene and nitrene precursors. Various derivatives of benziodoxoles, such as azidobenziodoxoles, trifluoromethylbenziodoxoles, alkynylbenziodoxoles, and alkenylbenziodoxoles have found wide application as group transfer reagents in the presence of transition metal catalysts, under metal-free conditions, or using photocatalysts under photoirradiation conditions. Development of hypervalent iodine catalytic systems and discovery of highly enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important recent achievement in the field of hypervalent iodine chemistry. Chemical transformations promoted by hypervalent iodine in many cases are unique and cannot be performed by using any other common, non-iodine-based reagent. This review covers literature published mainly in the last 7-8 years, between 2016 and 2024.

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).

Polysulfide Regulation by Hypervalent Iodine Compounds for Durable and Sustainable Lithium-Sulfur Battery

Herein, a type of hypervalent iodine compound-iodosobenzene (PhIO)-is proposed to regulate the LiPSs electrochemistry and enhance the performance of Li-S battery. PhIO owns the practical advantages of low-cost, commercial availability, environmental friendliness and chemical stability. The lone pair electrons of oxygen atoms in PhIO play a critical role in forming a strong Lewis acid-base interaction with terminal Li in LiPSs. Moreover, the commercial PhIO can be easily converted to nanoparticles (≈20 nm) and uniformly loaded on a carbon nanotube (CNT) scaffold, ensuring sufficient chemisorption for LiPSs. The integrated functional PhIO@CNT interlayer affords a LiPSs-concentrated shield that not only strongly obstructs the LiPSs penetration but also significantly enhances the electrolyte wettability and Li⁺ conduction. The PhIO@CNT interlayer also serves as a "vice current collector" to accommodate various LiPSs and render smooth LiPSs transformation, which suppresses insulating Li₂ S₂ /Li₂ S layer formation and facilitates Li⁺ diffusion. The Li-S battery based on PhIO@CNT interlayer (6 wt% PhIO) exhibits stable cycling over 1000 cycles (0.033% capacity decay per cycle) and excellent rate performance (686.6 mAh g^-1 at 3 C). This work demonstrates the great potential of PhIO in regulating LiPSs and provides a new avenue towards the low-cost and sustainable application of Li-S batteries.

Synthesis and Transformations of NH-Sulfoximines

Recent years have seen a marked increase in the occurrence of sulfoximines in the chemical sciences, often presented as valuable motifs for medicinal chemistry. This has been prompted by both pioneering works taking sulfoximine containing compounds into clinical trials and the concurrent development of powerful synthetic methods. This review covers recent developments in the synthesis of sulfoximines concentrating on developments since 2015. This includes extensive developments in both S-N and S-C bond formations. Flow chemistry processes for sulfoximine synthesis are also covered. Finally, subsequent transformations of sulfoximines, particularly in N-functionalization are reviewed, including N-S, N-P, N-C bond forming processes and cyclization reactions.

Charge frustration in ligand design and functional group transfer

Molecules with different resonance structures of similar importance, such as heterocumulenes and mesoionics, are prominent in many applications of chemistry, including 'click chemistry', photochemistry, switching and sensing. In coordination chemistry, similar chameleonic/schizophrenic entities are referred to as ambidentate/ambiphilic or cooperative ligands. Examples of these had remained, for a long time, limited to a handful of archetypal compounds that were mere curiosities. In this Review, we describe ambiphilicity - or, rather, 'charge frustration' - as a general guiding principle for ligand design and functional group transfer. We first give a historical account of organic zwitterions and discuss their electronic structures and applications. Our discussion then focuses on zwitterionic ligands and their metal complexes, such as those of ylidic and redox-active ligands. Finally, we present new approaches to single-atom transfer using cumulated small molecules and outline emerging areas, such as bond activation and stable donor-acceptor ligand systems for reversible 1e^- chemistry or switching.

Recent discoveries on the structure of iodine(iii) reagents and their use in cross-nucleophile coupling

This perspective article discusses structural features of iodine(iii) compounds as a prelude to presenting their use as umpolung reagents, in particular as pertains to their ability to promote the selective coupling of two nucleophilic species via 2e^- oxidation.

Hypervalent Iodine-Mediated Diastereoselective α-Acetoxylation of Cyclic Ketones

A binary hybrid system comprising a hypervalent iodine(III) reagent and BF₃•OEt₂ Lewis acid was found to be effective for the diastereoselective α-acetoxylation of cyclic ketones. In this hybrid system, BF₃•OEt₂ Lewis acid allowed the activation of the hypervalent iodine(III) reagent and cyclic ketones for smooth α-acetoxylation reaction, achieving high diastereoselectivity. This hypervalent iodine-mediated α-acetoxylation of the cyclic ketone reaction plausibly undergoes an S_N2 substitution mechanism via an α-C-bound hypervalent iodine intermediate. The diastereoselectivity of the reaction mainly originates from thermodynamic control.

Highly Reactive Cyclic Monoaryl Iodoniums Tuned as Carbene Generators Couple with Nucleophiles under Metal-Free Conditions

Cross-coupling reactions between aryl iodide and nucleophiles have been well developed. Iodoniums equipped with a reactive C-I(III) bond accelerate cross-coupling reactions of aryl iodide. Among them, cyclic diaryliodoniums are more atom economical; however; they are often in the trap of metal reliance and encounter regioselectivity issues. Now, we have developed a series of highly reactive cyclic monoaryl-vinyl iodoniums that can be tuned to construct C-N, C-O, and C-C bonds without metal catalysis. Under promotion of triethylamine, coupling reactions with aniline, phenol, aromatic acid, and indole proceed rapidly and regioselectively at room temperature. The carbene species is conceptualized as a key intermediate in our mechanism model. Furthermore, the coupling products enable diversity-oriented synthesis strategy to further build up a chemical library of diverse heterocyclic fragments that are in demand in the drug discovery field. Our current work provides a deep insight into the synthetic application of these highly reactive cyclic iodoniums.

Diaryliodonium as a double σ-hole donor: the dichotomy of thiocyanate halogen bonding provides divergent solid state arylation by diaryliodonium cations

The reactivity of [Ar¹Ar²I](SCN) toward the solid-state arylation depends on the preorganization of halogen bond (XB)-bound SCN⁻: N-XB-bound thiocyanates, which, in contrast to N,S-XB-bound, undergoes the extremely rare N-arylation of SCN⁻.

Blue LED Irradiation of Iodonium Ylides Gives Diradical Intermediates for Efficient Metal-free Cyclopropanation with Alkenes

A facile and highly chemoselective synthesis of doubly activated cyclopropanes is reported where mixtures of alkenes and β-dicarbonyl-derived iodonium ylides are irradiated with light from blue LEDs. This metal-free synthesis gives cyclopropanes in yields up to 96 %, is operative with cyclic and acyclic ylides, and proceeds with a variety of electronically-diverse alkenes. Computational analysis explains the high selectivity observed, which derives from exclusive HOMO to LUMO excitation, instead of free carbene generation. The procedure is operationally simple, uses no photocatalyst, and provides access in one step to important building blocks for complex molecule synthesis.

Alkyliodines in High Oxidation State: Enhanced Synthetic Possibilities and Accelerated Catalyst Turn-Over

In contrast to aryliodine(III) compounds, which have matured into a particularly attractive class of oxidants in modern synthesis, the synthetic potential of related alkyliodine(III) derivatives has remained widely underestimated. This is surprising since several unique synthetic possibilities arise directly from the low stability of their central carbon-iodine bond. In this respect, these high-oxidation-state iodine compounds resemble environmentally benign variants of the prominent metal counterparts such as those derived from palladium, nickel and copper. This Concept article summarizes the general reactivity trends in alkyliodine(III) chemistry and discusses selected examples of their strategic use as highly reactive, transient species in organic synthesis and homogeneous catalysis.

Iodosylbenzene Coordination Chemistry Relevant to Metal-Organic Framework Catalysis

Hypervalent iodine compounds formally feature expanded valence shells at iodine. These reagents are broadly used in synthetic chemistry due to the ability to participate in well-defined oxidation-reduction processes and because the ligand-exchange chemistry intrinsic to the hypervalent center allows hypervalent iodine compounds to be applied to a broad array of oxidative substrate functionalization reactions. We recently developed methods to generate these compounds from O₂ that are predicated on diverting reactive intermediates of aldehyde autoxidation toward the oxidation of aryl iodides. Coupling the aerobic oxidation of aryl iodides with catalysts that effect C-H bond oxidation would provide a strategy to achieve aerobic C-H oxidation chemistry. In this Forum Article, we discuss the aspects of hypervalent iodine chemistry and bonding that render this class of reagents attractive lynchpins for aerobic oxidation chemistry. We then discuss the oxidation processes relevant to the aerobic preparation of 2-(tert-butylsulfonyl)iodosylbenzene, which is a popular hypervalent iodine reagent for use with porous metal-organic framework (MOF)-based catalysts because it displays significantly enhanced solubility as compared with unsubstituted iodosylbenzene. We demonstrate that popular synthetic methods to this reagent often provide material that displays unpredictable disproportionation behavior due to the presence of trace impurities. We provide a revised synthetic route that avoids impurities common in the reported methods and provides access to material that displays predictable stability. Finally, we describe the coordination chemistry of hypervalent iodine compounds with metal clusters relevant to MOF chemistry and discuss the potential implications of this coordination chemistry to catalysis in MOF scaffolds.

Step-by-step reaction-powered mechanical motion triggered by a chemical fuel pulse

Natural molecular machines perform important tasks in organisms under a reasonable energy supply strategy: a series of step-by-step biochemical reactions after the intake of a fuel such as glucose. As analogues of biomolecular machines, most artificial molecular machines and shuttles are still powered by two opposite stimuli, such as acid and base or oxidation and reduction. This inconvenient stimulus method for artificial molecular machines and shuttles creates great obstacles for realizing more functions of artificial molecular machines and shuttles owing to low energy utilization efficiency and troublesome operation. In this work, we report a new step-by-step reaction system combining two known reactions: (1) the formation of [bis(trifluoroacetoxy)iodo]benzene from the reaction of iodosylbenzene and trifluoroacetic acid; and (2) the catalytic oxidation of alcohols by [bis(trifluoroacetoxy)iodo]benzene in the presence of TEMPO. Owing to the acid absorption features of the first reaction and the acid releasing characteristics of the second reaction, the new step-by-step reaction obtains a pH oscillation feature. Taking advantage of the pH oscillation feature, the new reaction cycle triggered by iodosylbenzene was coupled with an acid-base switchable helicarene-based molecular shuttle. Benefitting from the reaction rate difference of the two known reactions (the reaction rate of the first reaction is faster than that of the second reaction), the movement of the helicarene on the axle could be controlled automatically by the reaction system through adding iodosylbenzene to a solution of alcohol, TEMPO and protonated rotaxane, and the manual addition of another opposite stimulus could be avoided.

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.

Iodine(III) Reagents in Radical Chemistry

The chemistry of hypervalent iodine(III) compounds has gained great interest over the past 30 years. Hypervalent iodine(III) compounds show valuable ionic reactivity due to their high electrophilicity but also express radical reactivity as single electron oxidants for carbon and heteroatom radical generation. Looking at ionic chemistry, these iodine(III) reagents can act as electrophiles to efficiently construct C–CF₃, X–CF₃ (X = heteroatom), C–R_f (R_f = perfluoroalkyl), X–R_f, C–N₃, C–CN, S–CN, and C–X bonds. In some cases, a Lewis or a Bronsted acid is necessary to increase their electrophilicity. In these transformations, the iodine(III) compounds react as formal “CF₃⁺”, “R_f⁺”, “N₃⁺”, “Ar⁺”, “CN⁺”, and “X⁺” equivalents. On the other hand, one electron reduction of the I(III) reagents opens the door to the radical world, which is the topic of this Account that focuses on radical reactivity of hypervalent iodine(III) compounds such as the Togni reagent, Zhdankin reagent, diaryliodonium salts, aryliodonium ylides, aryl(cyano)iodonium triflates, and aryl(perfluoroalkyl)iodonium triflates. Radical generation starting with I(III) reagents can also occur via thermal or light mediated homolysis of the weak hypervalent bond in such reagents. This reactivity can be used for alkane C–H functionalization. We will address important pioneering work in the area but will mainly focus on studies that have been conducted by our group over the last 5 years.

We entered the field by investigating transition metal free single electron reduction of Togni type reagents using the readily available sodium 2,2,6,6-tetramethylpiperidine-1-oxyl salt (TEMPONa) as an organic one electron reductant for clean generation of the trifluoromethyl radical and perfluoroalkyl radicals. That valuable approach was later successfully also applied to the generation of azidyl and aryl radicals starting with the corresponding benziodoxole (Zhdankin reagent) and iodonium salts. In the presence of alkenes as radical acceptors, vicinal trifluoromethyl-, azido-, and arylaminoxylation products result via a sequence comprising radical addition to the alkene and subsequent TEMPO trapping. Electron-rich arenes also react with I(III) reagents via single electron transfer (SET) to give arene radical cations, which can then engage in arylation reactions. We also recognized that the isonitrile functionality in aryl isonitriles is a highly efficient perfluoroalkyl radical acceptor, and reaction of R_f-benziodoxoles (Togni type reagents) in the presence of a radical initiator provides various perfluoroalkylated N-heterocycles (indoles, phenanthridines, quinolines, etc.). We further found that aryliodonium ylides, previously used as carbene precursors in metal-mediated cyclopropanation reactions, react via SET reduction with TEMPONa to the corresponding aryl radicals. As a drawback of all these transformations, we realized that only one ligand of the iodine(III) reagent gets transferred to the substrate. To further increase atom-economy of such conversions, we identified cyano or perfluoroalkyl iodonium triflate salts as valuable reagents for stereoselective vicinal alkyne difunctionalization, where two ligands from the I(III) reagent are sequentially transferred to an alkyne acceptor.

Finally, we will discuss alkynyl-benziodoxoles as radical acceptors for alkynylation reactions. Similar reactivity was found for the Zhdankin reagent that has been successfully applied to azidation of C-radicals, and also cyanation is possible with a cyano I(III) reagent. To summarize, this Account focuses on the design, development, mechanistic understanding, and synthetic application of hypervalent iodine(III) reagents in radical chemistry.

The nature of the chemical bond in BeO0,-, BeOBe+,0,-, and in their hydrogenated products HBeO0,-, BeOH, HBeOH, BeOBeH+,0,-, and HBeOBeH

The nature of the chemical bond in BeO^0,-, BeOBe^+,0,-, and in their hydrogenated products HBeO^0,-, BeOH, HBeOH, BeOBeH^+,0,-, and HBeOBeH has been studied through single and multi reference correlation methods. In all these species, excited and ionized atomic states participate in a resonant way making chemically possible molecules that have been termed hypervalent and explain also the "incompatible" geometrical structure of some species.

How to Twist, Split and Warp a σ-Hole with Hypervalent Halogens

Halogen bonds (XB) are no longer newcomers in the chemistry family. However, XB in hypervalent halogens has not been thoroughly studied. We provide a molecular orbital explanation of the shape and strength of XBs in hypervalent halogens and other species, focusing on the charge transfer and electrostatic aspects of these bonds. Our results show that σ-holes (and subsequently the XBs associated with them) can be easily divided and bent by the influence of equatorial substituents. The inductive effect of both the equatorial and axial groups can affect these distortions, but also the angle between the equatorial ligands has a large influence on the shape of the σ-holes and the molecular orbitals acting as electron acceptor. Although the observation of these warped XB can be hindered by other noncovalent interactions, they may be ubiquitous in crystal structures of hypervalent species, where multiple XB can appear as secondary interactions on each halogen. We propose what can be considered the archetypal hypervalent halogen donor (a pincer type iodosodilactone) and a Lewis dot structure that includes the σ-holes.

Transfer of Electrophilic NH Using Convenient Sources of Ammonia: Direct Synthesis of NH Sulfoximines from Sulfoxides

A new system for NH transfer is developed for the preparation of sulfoximines, which are emerging as valuable motifs for drug discovery. The protocol employs readily available sources of nitrogen without the requirement for either preactivation or for metal catalysts. Mixing ammonium salts with diacetoxyiodobenzene directly converts sulfoxides into sulfoximines. This report describes the first example of using of ammonia sources with diacetoxyiodobenzene to generate an electrophilic nitrogen center. Control and mechanistic studies suggest a short-lived electrophilic intermediate, which is likely to be PhINH or PhIN(+) .

Alkynylation of heterocyclic compounds using hypervalent iodine reagent

The alkynylation of various nitrogen- and/or sulphur-containing heterocyclic compounds using hypervalent iodine TMS-EBX by utilization of tertiary amines under mild conditions is described. The developed metal-free methodology furnishes the corresponding alkynylated heterocycles bearing quaternary carbon in high yields.

Advances in Synthetic Applications of Hypervalent Iodine Compounds

The preparation, structure, and chemistry of hypervalent iodine compounds are reviewed with emphasis on their synthetic application. Compounds of iodine possess reactivity similar to that of transition metals, but have the advantage of environmental sustainability and efficient utilization of natural resources. These compounds are widely used in organic synthesis as selective oxidants and environmentally friendly reagents. Synthetic uses of hypervalent iodine reagents in halogenation reactions, various oxidations, rearrangements, aminations, C-C bond-forming reactions, and transition metal-catalyzed reactions are summarized and discussed. Recent discovery of hypervalent catalytic systems and recyclable reagents, and the development of new enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important achievement in the field of hypervalent iodine chemistry. One of the goals of this Review is to attract the attention of the scientific community as to the benefits of using hypervalent iodine compounds as an environmentally sustainable alternative to heavy metals.

Iodosobenzene-mediated direct and efficient oxidation of β-dicarbonyls to vicinal tricarbonyls catalyzed by iron(iii) salts

The direct and efficient oxidation of β-dicarbonyls to vicinal tricarbonyl compounds has been achieved, which was mediated by PhIO and catalyzed by Fe(NO₃)₃·9H₂O under mild and environmentally friendly conditions.

Ba and Sr Binary Phosphides: Synthesis, Crystal Structures, and Bonding Analysis

Synthesis, crystal structures, and chemical bonding are reported for four binary phosphides with different degrees of phosphorus oligomerization, ranging from isolated P atoms to infinite phosphorus chains. Ba3P2 = Ba4P(2.67)□(0.33) (□ = vacancy) crystallizes in the anti-Th3P4 structure type with the cubic space group I4̅3d (no. 220), Z = 6, a = 9.7520(7) Å. In the Ba3P2 crystal structure, isolated P(3-) anions form distorted octahedra around the Ba(2+) cations. β-Ba5P4 crystallizes in the Eu5As4 structure type with the orthorhombic space group Cmce (no. 64), Z = 4, a = 16.521(2) Å, b = 8.3422(9) Å, c = 8.4216(9) Å. In the crystal structure of β-Ba5P4, one-half of the phosphorus atoms are condensed into P2(4-) dumbbells. SrP2 and BaP2 are isostructural and crystallize in the monoclinic space group P2₁/c (no. 14), Z = 6, a = 6.120(2)/6.368(1) Å, b = 11.818(3)/12.133(2) Å, c = 7.441(2)/7.687(2) Å, β = 126.681(4)/126.766(2)° for SrP2/BaP2. In the crystal structures of SrP2 and BaP2, all phosphorus atoms are condensed into ∞(1)P(1-) cis-trans helical chains. Electronic structure calculations, chemical bonding analysis via the recently developed solid-state adaptive natural density partitioning (SSAdNDP) method, and UV-vis spectroscopy reveal that SrP2 and BaP2 are electron-balanced semiconductors.

Six-coordinate Group 13 complexes: the role of d orbitals and electron-rich multi-center bonding

Bonding in six-coordinate complexes based on Group 13 elements (B, Al, Ga, In, Tl) is usually considered to be identical to that in transition-metal analogues. We herein demonstrate through sophisticated electronic-structure analyses that the bonding in these Group 13 element complexes is fundamentally different and better characterized as electron-rich hypervalent bonding with essentially no role for the d orbitals. This characteristic is carried through to the molecular properties of the complex.

Mechanistic insights on the iodine(III)-mediated α-oxidation of ketones

The synthesis of α-substituted carbonyl compounds is of great importance due to their ubiquity in both natural and man-made biologically active compounds. The field of hypervalent iodine chemistry has been a great contributor to access these molecules. For example, the α-oxidation of carbonyl compounds has been one of the most investigated iodine(III)-mediated stereoselective transformations. Yet, it is also the transformation that has met the most challenge in terms of achieving high stereoselectivities. The different mechanistic pathways of the iodine(III)-mediated α-tosyloxylation of ketones have been investigated. The calculations suggest an unprecedented iodine(III)-promoted enolization process. Indications that iodonium intermediates could serve as proficient Lewis acids are reported. This concept could have broad impact and foster new developments in the field of hypervalent iodine chemistry.

Revealing unusual chemical bonding in planar hyper-coordinate Ni2Ge and quasi-planar Ni2Si two-dimensional crystals

We predict two novel highly stable 2D planar hexacoordinate monolayer sheets Ni₂Si and Ni₂Ge with unusual chemical bonding.

Reactant cum solvent water: generation of transient λ3-hypervalent iodine, its reactivity, mechanism and broad application

We have demonstrated a benign strategy for amides, imides and their chiral analogues using water as a source of carbonyl oxygen and medium. DFT-MIR studies and controlled experiments established the mechanism involving PhI(OH)₂-bearing intermediates.

Aminations with Hypervalent Iodine

Recent progress in the area of hypervalent iodine-mediated and catalyzed amination reaction of hydrocarbons is reviewed. These reactions comprise processes under both intra and intermolecular control and include the functionalization of aromatic C-H bonds as well as conversion of sp-, sp(2)-, and sp(3)-hybridized carbon atoms. These developments demonstrate that hypervalent iodine(III) methodology has reached a high level in amination chemistry. The individual reactions are discussed with a focus on mechanistic details and emphasis is made to the underlying hypervalent iodine reagents, for which structural information is available.