Importance of Nonclassical σ-Hole Interactions for the Reactivity of λ3-Iodane Complexes

Hypervalent Iodine (III)-Based Complexation for Chiroptical Supramolecular Glass, Deep Eutectic Solvent and Luminescent Switch

Hypervalent iodine(III) have widely been utilized for organic synthetic reagents. They are also recognized as positive charge-assisted, exceptionally robust biaxial halogen bond donors, while their potential in supramolecular materials is barely explored. This work reports a cyclic diaryliodonium ion as biaxial halogen bonding donor that displays remarkable binding affinity toward phenanthroline or acridine acceptors with chiral pendants. Biaxial halogen bonding enables chiroptical evolution in solution, allowing for rational control over supramolecular chirality. Leveraging their strong binding affinity, the halogen bonding complexes manifested amorphous properties and deep eutectic behavior in the solid state. Capitalizing on these attributes, this work achieves the successful preparation of supramolecular glasses and deep eutectic solvents. Additionally, halogen bonding appended light irradiation-triggered luminescence through a hydrogen atom transfer process, showing applications in anti-counterfeit and display.

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.

Stabilization of sp-Hybridized Nitrogen Cation by Lewis Acid-Base Complex Formation with Intramolecular Iodine

Here we show that the sp-hybridized nitrogen cation is strongly stabilized by a peri-iodine substituent in the tetralone system. The cation is captured by anionic species such as CF3 CO2 - , affording hypervalent iodine(III) compounds with a short nitrogen-iodine (N-I) bond, in which the cation serves as a Lewis acid. Notably, the O-I bond of the O-trifluoroacetate or O-acetate is intrinsically weaker than the N-I bond due to its more ionic character and is further weakened by protonation in trifluoroacetic acid. As a result, the oxygen ligand can dissociate in the presence of a Brønsted acid, affording a I+ cation intermediate that retains the N-I bond. We isolated the cation as the tetrafluoroborate, and characterized it experimentally by 1 H NMR spectroscopy and X-ray structure analysis, and theoretically by means of DFT calculation. The results suggest that the N-I bonded cation is intrinsically stable, and is weakly coordinated with water and the BF4 counter anion or trifluoroacetate anion. This cation can be employed as a reagent for α-oxidation of ketones.

Exploring the role of halogen bonding in iodonium ylides: insights into unexpected reactivity and reaction control

Halogen bonding is commonly found with iodine-containing molecules, and it arises when Lewis bases interact with iodine's σ-holes. Halogen bonding and σ-holes have been encountered in numerous monovalent and hypervalent iodine-containing compounds, and in 2022 σ-holes were computationally confirmed and quantified in the iodonium ylide subset of hypervalent iodine compounds. In light of this new discovery, this article provides an overview of the reactions of iodonium ylides in which halogen bonding has been invoked. Herein, we summarize key discoveries and mechanistic proposals from the early iodonium ylide literature that invoked halogen bonding-type mechanisms, as well as recent reports of reactions between iodonium ylides and Lewis basic nucleophiles in which halogen bonding has been specifically invoked. The reactions discussed herein are organized to enable the reader to build an understanding of how halogen bonding might impact yield and chemoselectivity outcomes in reactions of iodonium ylides. Areas of focus include nucleophile σ-hole selectivity, and how ylide structural modifications and intramolecular halogen bonding (e.g., the ortho-effect) can improve ylide stability or solubility, and alter reaction outcomes.

Catalyst-Substrate Helical Character Matching Determines the Enantioselectivity in the Ishihara-Type Iodoarenes Catalyzed Asymmetric Kita-Dearomative Spirolactonization

Catalyst design has traditionally focused on rigid structural elements to prevent conformational flexibility. Ishihara's elegant design of conformationally flexible C₂-symmetric iodoarenes, a new class of privileged organocatalysts, for the catalytic asymmetric dearomatization (CADA) of naphthols is a notable exception. Despite the widespread use of the Ishihara catalysts for CADAs, the reaction mechanism remains the subject of debate, and the mode of asymmetric induction has not been well established. Here, we report an in-depth computational investigation of three possible mechanisms in the literature. Our results, however, reveal that this reaction is best rationalized by a fourth mechanism called "proton-transfer-coupled-dearomatization (PTCD)", which is predicted to be strongly favored over other competing pathways. The PTCD mechanism is consistent with a control experiment and further validated by applying it to rationalize the enantioselectivities. Oxidation of the flexible I(I) catalyst to catalytic active I(III) species induces a defined C₂-symmetric helical chiral environment with a delicate balance between flexibility and rigidity. A match/mismatch effect between the active catalyst and the substrate's helical shape in the dearomatization transition states was observed. The helical shape match allows the active catalyst to adapt its conformation to maximize attractive noncovalent interactions, including I(III)···O halogen bond, N-H···O hydrogen bond, and π···π stacking, to stabilize the favored transition state. A stereochemical model capable of rationalizing the effect of catalyst structural variation on the enantioselectivities is developed. The present study enriches our understanding of how flexible catalysts achieve high stereoinduction and may serve as an inspiration for the future exploration of conformational flexibility for new catalyst designs.

N-Heterocyclic Iod(az)olium Salts - Potent Halogen-Bond Donors in Organocatalysis

This article describes the application of N-heterocyclic iod(az)olium salts (NHISs) as highly reactive organocatalysts. A variety of mono- and dicationic NHISs are described and utilized as potent XB-donors in halogen-bond catalysis. They were benchmarked in seven diverse test reactions in which the activation of carbon- and metal-chloride bonds as well as carbonyl and nitro groups was achieved. N-methylated dicationic NHISs rendered the highest reactivity in all investigated catalytic applications with reactivities even higher than all previously described monodentate XB-donors based on iodine(I) and (III) and the strong Lewis acid BF3 .

Structure and Reactivity of N-Heterocyclic Alkynyl Hypervalent Iodine Reagents

Ethynylbenziodoxol(on)e (EBX) cyclic hypervalent iodine reagents have become popular reagents for the alkynylation of radicals and nucleophiles, but only offer limited possibilities for further structure and reactivity fine-tuning. Herein, the synthesis of new N-heterocyclic hypervalent iodine reagents with increased structural flexibility based on amide, amidine and sulfoximine scaffolds is reported. Solid-state structures of the reagents are reported and the analysis of the I-Calkyne bond lengths allowed assessing the trans-effect of the different substituents. Molecular electrostatic potential (MEP) maps of the reagents, derived from DFT computations, revealed less pronounced σ-hole regions for sulfonamide-based compounds. Most reagents reacted well in the alkynylation of β-ketoesters. The alkynylation of thiols afforded more variable yields, with compounds with a stronger σ-hole reacting better. In metal-mediated transformations, the N-heterocyclic hypervalent iodine reagents gave inferior results when compared to the O-based EBX reagents.

Iodine(III)-Based Halogen Bond Donors: Properties and Applications

Halogen bonding, the non-covalent interaction of Lewis bases with an electron-deficient region of halogen substituents, received increased attention recently. Consequently, the design and evaluation of numerous halogen-containing species as halogen bond donors have been subject to intense research. More recently, organoiodine compounds at the iodine(III) state have been receiving growing attention in the field. Due to their electronic and structural properties, they provide access to unique binding modes. For this reason, our groups have been involved in the development of such compounds, in the quantification of their halogen bonding strength (through the evaluation of their Lewis acidities), as well as in the evaluation of their activities as catalysts in several model reactions. This account will describe these contributions.

A Bidentate Iodine(III)-Based Halogen-Bond Donor as a Powerful Organocatalyst*

In contrast to iodine(I)-based halogen bond donors, iodine(III)-derived ones have only been used as Lewis acidic organocatalysts in a handful of examples, and in all cases they acted in a monodentate fashion. Herein, we report the first application of a bidentate bis(iodolium) salt as organocatalyst in a Michael and a nitro-Michael addition reaction as well as in a Diels-Alder reaction that had not been activated by noncovalent organocatalysts before. In all cases, the performance of this bidentate XB donor distinctly surpassed the one of arguably the currently strongest iodine(I)-based organocatalyst. Bidentate coordination to the substrate was corroborated by a structural analysis and by DFT calculations of the transition states. Overall, the catalytic activity of the bis(iodolium) system approaches that of strong Lewis acids like BF3 .

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.

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⁻.

Merging hypervalent iodine and sulfoximine chemistry: a new electrophilic trifluoromethylation reagent

Electrophilic trifluoromethylation is at the forefront of methodologies available for the installation of the CF3 moiety to organic molecules; research in this field is largely spurred by the availability of stable and accessible trifluoromethylation reagents, of which hypervalent iodine and sulfoximine based compounds have emerged as two prominent reagent classes. Herein, we describe the facile synthesis of an electrophilic trifluoromethylation reagent which merges these two scaffolds in a novel hypervalent iodosulfoximine compound. This presents the first analogue of the well-known Togni reagents which neither compromises stability or reactivity. The electronic and physical properties of this new compound were fully explored by X-ray crystallography, cyclic voltammetry, TGA/DSC and DFT analysis. This solution stable, crystalline reagent was found to be competent in the electrophilic trifluoromethylation of a variety of nucleophiles as well as a source of the trifluoromethyl radical. Furthermore, the possibility of enantioinductive transformations could be probed with the isolation of the first enantiopure hypervalent iodine compound bearing a CF3 group, thus this new reagent scaffold offers the opportunity of structurally diversifying the reagent towards asymmetric synthesis.

Chiral Hypervalent Iodines: Active Players in Asymmetric Synthesis

Asymmetric organocatalytic oxidations have been witnessed to an impressive development in the last years thanks to the establishment of important chiral hypervalent iodines(III/V). Many different approaches involving both stoichiometric and catalytic versions have provided a fundamental advance in this area within asymmetric synthesis. The easily handing, nontoxic, mild, environmentally friendly (green oxidants), and high stability that are features of these reagents have been applied to many reactions and also have allowed exploring further unprecedented enantioselective transformations. The intention of the present review is thus to highlight as a whole the many approaches utilized up to date to prepare chiral iodines(III/V), as well as their reactivity in a comprehensive manner.

Frontiers in Halogen and Chalcogen-Bond Donor Organocatalysis

Non-covalent molecular interactions on the basis of halogen and chalcogen bonding represent a promising, powerful catalytic activation mode. However, these "unusual" non-covalent interactions are typically employed in the solid state and scarcely exploited in catalysis. In recent years, an increased interest in halogen and chalcogen bonding has been awaken, as they provide profound characteristics that make them an appealing alternative to the well-explored hydrogen bonding. Being particularly relevant in the binding of "soft" substrates, the similar strength to hydrogen bonding interactions and its higher directionality allows for solution-phase applications with halogen and chalcogen bonding as the key interaction. In this mini-review, the special features, state-of-the-art and key examples of these so-called σ-hole interactions in the field of organocatalysis are presented.

Hypercoordinate iodine for catalytic asymmetric diamination of styrene: insights into the mechanism, role of solvent, and stereoinduction

Stereoselectivity in the asymmetric diamination of styrene catalyzed by chiral hypercoordinate iodine originates from the prochiral face recognition when the substrate binds to the catalyst.

Hypercoordinate iodine has evolved as an impressive class of catalysts for various organic transformations. Extension of this idea to asymmetric applications, such as in the asymmetric difunctionalization of styrene or its derivatives, constitutes an important reaction. In this study, the mechanism and origin of stereoinduction in styrene diamination, with a sulfonimide (HNMs₂) as the diaminating agent and iodoresorcinol (((iPr)₂N(CO)-CH(Me)-O)₂Ar–I) based chiral hypercoordinate iodine as the catalyst, are investigated using density functional theory calculations. The energetically preferred catalytic pathway has been found to involve, among other steps, two very important mechanistic events: (a) the formation of a catalyst–substrate complex by the action of styrene on the catalyst ArI(NMs₂)₂, resulting in the displacement of one of the imidates (NMs₂^–); and (b) a rebound of the departed imidate on the iodine-bound styrene to form an iodonium ion intermediate with a N–C bond. Explicit interaction of the imidate ion with hexafluoroisopropanol (HFIP), used as a solvent additive, lowers the barrier for the formation of the iodonium ion. The P helical fold of the chiral arms of the iodoresorcinol catalyst is found to offer a chiral environment for the reactants. Coordination of the iodine catalyst to the styrene double bond is found to make the benzylic carbon more electrophilic and hence makes it the preferred site for the nucleophilic addition. In the chiral environment of the catalyst, an enhanced polarization of the styrene double bond is noticed when the double bond coordinates through the si prochiral face than the re face. Nucleophilic addition on the re face of the catalyst–substrate complex is associated with a lower activation barrier leading to the experimentally observed S enantiomeric product. The stereoselective model developed in this study can be employed to related asymmetric styrene difunctionalizations using similar hypercoordinate iodine catalysts.

On the activation of hypercoordinate iodine(iii) compounds for reactions of current interest

Central to the reactivity of hypercoordinate iodine as a continually emerging compound for organic transformation is its activation by various additives. We wish to present the current understanding on bonding and activation modes under different reaction conditions involving hypercoordinate iodine(iii) compounds.

Mechanism and Origins of Chemo- and Stereoselectivities of Aryl Iodide-Catalyzed Asymmetric Difluorinations of β-Substituted Styrenes

The mechanism of the aryl iodide-catalyzed asymmetric migratory geminal difluorination of β-substituted styrenes ( Banik et al. Science 2016, 353, 51 ) has been explored with density functional theory computations. The computed mechanism consists of (a) activation of iodoarene difluoride (ArIF2), (b) enantiodetermining 1,2-fluoroiodination, (c) bridging phenonium ion formation via SN2 reductive displacement, and (d) regioselective fluoride addition. According to the computational model, the ArIF2 intermediate is stabilized through halogen-π interactions between the electron-deficient iodine(III) center and the benzylic substituents at the catalyst stereogenic centers. Interactions with the catalyst ester carbonyl groups (I(III)+···O) are not observed in the unactivated complex, but do occur upon activation of ArIF2 through hydrogen-bonding interactions with external Brønsted acid (HF). The 1,2-fluoroiodination occurs via alkene complexation to the electrophilic, cationic I(III) center followed by C-F bond formation anti to the forming C-I bond. The bound olefin and the C-I bond of catalyst adopt a spiro arrangement in the favored transition structures but a nearly periplanar arrangement in the disfavored transition structures. Multiple attractive non-covalent interactions, including slipped π···π stacking, C-H···O, and C-H···π interactions, are found to underlie the high asymmetric induction. The chemoselectivity for 1,1-difluorination versus 1,2-difluorination is controlled mainly by (1) the steric effect of the substituent on the olefinic double bond and (2) the nucleophilicity of the carbonyl oxygen of substrate.

Dual function of the boron center of BH(CO)2/BH(N2)2 in halogen- and triel-bonded complexes with hypervalent halogens

The complexes between BH(CO)₂/BH(N₂)₂ and XF₃/XF₅ are stabilized by a halogen bond and a triel bond. The MEP analyses of BH(CO)₂/BH(N₂)₂ indicate that there are both a region with negative MEPs on the B atom in the vertical direction of the molecular plane and a σ-hole at the B-H bond end. Therefore, the boron atom in BH(CO)₂/BH(N₂)₂ plays a dual role of a Lewis base and an acid in the halogen bond and triel bond, respectively. The halogen and triel bonds are stronger in order of IF₃< BrF₃< ClF₃, IF₅< BrF₅< ClF₅, and BH(CO)₂< BH(N₂)₂ in most complexes. These complexes have large stability since the interaction energy varies from -5 to -115 kcal/mol. The halogen bond belongs to a covalent interaction or a partially covalent interaction in most complexes. The subsystems in these complexes have prominent deformation, accompanied with big charge transfer and large polarization.

Insight into trifluoromethylation – experimental electron density for Togni reagent I

The experimentally observed electron density for the “Togni reagent” explains the interaction of the hypervalent iodine atom with a nucleophile.