Predicting bond dissociation energies of cyclic hypervalent halogen reagents using DFT calculations and graph attention network model

Although hypervalent iodine(III) reagents have become staples in organic chemistry, the exploration of their isoelectronic counterparts, namely hypervalent bromine(III) and chlorine(III) reagents, has been relatively limited, partly due to challenges in synthesizing and stabilizing these compounds. In this study, we conduct a thorough examination of both homolytic and heterolytic bond dissociation energies (BDEs) critical for assessing the chemical stability and functional group transfer capability of cyclic hypervalent halogen compounds using density functional theory (DFT) analysis. A moderate linear correlation was observed between the homolytic BDEs across different halogen centers, while a strong linear correlation was noted among the heterolytic BDEs across these centers. Furthermore, we developed a predictive model for both homolytic and heterolytic BDEs of cyclic hypervalent halogen compounds using machine learning algorithms. The results of this study could aid in estimating the chemical stability and functional group transfer capabilities of hypervalent bromine(III) and chlorine(III) reagents, thereby facilitating their development.

A comparison of structure, bonding and non-covalent interactions of aryl halide and diarylhalonium halogen-bond donors

Halogen bonding permeates many areas of chemistry. A wide range of halogen-bond donors including neutral, cationic, monovalent, and hypervalent have been developed and studied. In this work we used density functional theory (DFT), natural bond orbital (NBO) theory, and quantum theory of atoms in molecules (QTAIM) to analyze aryl halogen-bond donors that are neutral, cationic, monovalent and hypervalent and in each series we include the halogens Cl, Br, I, and At. Within this diverse set of halogen-bond donors, we have found trends that relate halogen bond length with the van der Waals radii of the halogen and the non-covalent or partial covalency of the halogen bond. We have also developed a model to calculate ΔG of halogen-bond formation by the linear combination of the % p-orbital character on the halogen and energy of the σ-hole on the halogen-bond donor.

Halogen, Chalcogen, Pnictogen, and Tetrel Bonding in Non-Covalent Organocatalysis: An Update

The use of noncovalent interactions based on electrophilic halogen, chalcogen, pnictogen, or tetrel centers in organocatalysis has gained noticeable attention. Herein, we provide an overview on the most important developments in the last years with a clear focus on experimental studies and on catalysts which act via such non-transient interactions.

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).

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.

Halogen Bond Catalysis: A Physical Chemistry Perspective

As important noncovalent interactions, halogen bonds have been widely used in material science, supramolecular chemistry, medicinal chemistry, organocatalysis, and other fields. In the past 15 years, halogen bond catalysis has become a developed field in organocatalysis for the catalysts' advantages of being environmentally friendly, inexpensive, and recyclable. Halogen bonds can induce various organic reactions, and halogen bond catalysis has become a powerful alternative to the fully explored hydrogen bond catalysis. From a physical chemistry view, this perspective provides an overview of the latest progress and key examples of halogen bond catalysis via activation of the lone pair systems of organic functional group, π systems, and metal complexes. The research progresses in halogen bond catalysis by our group were also introduced.

Preparation and X-ray Structural Study of Dibenzobromolium and Dibenzochlorolium Derivatives

Various five-membered cyclic dibenzobromolium salts (dibenzo[b,d]bromol-5-ium chloride, nitrate, hydrogen sulfate, dihydrogen phosphate, trifluoroacetate, and tetrafluoroborate) were prepared by diazotization-cyclization of 2'-bromo-[1,1'-biphenyl]-2-amine in solution of appropriate acids. The chlorolium analogues (iodide, trifluoroacetate, and tetrafluoroborate) were obtained by a similar procedure. Additional dibenzohalolium derivatives (dibenzo[b,d]bromol-5-ium and dibenzo[b,d]chlorol-5-ium azides, bis(trifluoromethanesulfonyl)imidates, thiocyanates, and trifluoromethanesulfonates) were prepared by anion exchange. Structures of ten of these dibenzohalolium derivatives were established by X-ray analysis. Bond distances and angles for the halogen atoms in different dibenzohalolium derivatives were summarized and discussed.

Intramolecular Azo Coupling Reaction of Binaphthyl Compounds: Synthesis of Pyrazole-Containing Helicene-Like Molecules

A method for accessing pyrazole-containing helicene-like molecules from easily accessible NOBIN derivatives was developed. The reaction proceeded efficiently via diazonium salt intermediates, which provided a series of helicene-like molecular products in yields of 77%-89% regardless of their steric and electronic natures. The photophysical properties of the products were investigated. The 3,3'-disubstituted molecules showed a characteristic blue shift in their emission spectra. Product derivatizations were conducted, and interesting reactivities toward nucleophiles were observed.

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.

Xenon Derivatives as Aerogen Bond-Donating Catalysts for Organic Transformations: A Theoretical Study on the Metaphorical "Spherical Cow in a Vacuum" Provides Insights into Noncovalent Organocatalysis

Computations indicate that cationic and noncharged xenon derivatives should exhibit higher catalytic activity than their iodine-based noncovalent organocatalytic congeners. Perfluorophenyl xenonium(II) is expected to demonstrate the best balance between catalytic activity and chemical stability for use in organocatalysis. Comparing its catalytic activity with that of isoelectronic perfluoroiodobenzene indicates that the high catalytic activity of cationic noncovalent organocatalysts is predominantly attributed to the electrostatic interactions with the reaction substrates, which cause the polarization of ligated species during the reaction progress. In contrast, the electron transfer and covalent contributions to the bonding between the catalyst and substrate have negligible effects. The dominant effect of electrostatic interactions results in a strong negative correlation between the calculated Gibbs free energies of activation for the modeled reactions and the highest potentials of the σ-holes on the central atoms of the catalysts. No such correlation is observed for noncharged catalysts.

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.

A Quantum-chemical Analysis on the Lewis Acidity of Diarylhalonium Ions

Cyclic diaryliodonium compounds like iodolium derivatives have increasingly found use as noncovalent Lewis acids in the last years. They are more stable toward nucleophilic substitution than acyclic systems and are markedly more Lewis acidic. Herein, this higher Lewis acidity is analyzed and explained via quantum-chemical calculations and energy decomposition analyses. Its key origin is the change in energy levels and hybridization of iodine's orbitals, leading to both more favorable electrostatic interaction and better charge transfer. Both of the latter seem to contribute in similar fashion, while hydrogen bonding as well as steric repulsion with the phenyl rings play at best a minor role. In comparison to iodolium, bromolium and chlorolium are less Lewis acidic the lighter the halogen, which is predominantly based on less favorable charge-transfer interactions.

Chiral Bromonium Salt (Hypervalent Bromine(III)) with N-Nitrosamine as a Halogen-Bonding Bifunctional Catalyst

There has been a great focus on halogen-bonding as a unique interaction between electron-deficient halogen atoms with Lewis basic moieties. Although the application of halogen-bonded atoms in organic chemistry has been eagerly researched in these decades, the development of chiral molecules with halogen-bonding functionalities and their utilization in asymmetric catalysis are still in the\ir infancy. We have previously developed chiral halonium salts with amide functionalities, which behaved as excellent catalysts albeit in only two reactions due to the lack of substrate activation abilities. In this manuscript, we have developed chiral halonium salts with an N-nitrosamine moiety and applied them to the Mannich reaction of isatin-derived ketimines with malonic esters. The study focused on our novel bromonium salt catalyst which provided the corresponding products in high yields with up to 80% ee. DFT calculations of the chiral catalyst structure suggested that the high asymmetric induction abilities of this catalyst are due to the Lewis basic role of the N-nitrosamine part. To the best of our knowledge, this is the first catalytic application of N-nitrosamines.

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.

Halonium, chalconium, and pnictonium salts as noncovalent organocatalysts: a computational study on relative catalytic activity

This theoretical study sheds light on the relative catalytic activity of pnictonium, chalconium, and halonium salts in reactions involving elimination of chloride and electrophilic activation of a carbonyl group. DFT calculations indicate that for cationic aromatic onium salts, values of the electrostatic potential on heteroatom σ-holes gradually increase from pnictogen- to halogen-containing species. The higher values of the potential on the halogen atoms of halonium salts result in the overall higher catalytic activity of these species, but in the case of pnictonium and chalconium cations, weak interactions from the side groups provide an additional stabilization effect on the reaction transition states. Based upon quantum-chemical calculations, the catalytic activity of phosphonium(V) and arsenonium(V) salts is expected to be too low to obtain effective noncovalent organocatalytic compounds, whereas stibonium(V), telluronium(IV) and iodonium(III) salts exhibit higher potential in application as noncovalent organocatalysts.

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.

Diaryliodoniums as Hybrid Hydrogen- and Halogen-Bond-Donating Organocatalysts for the Groebke-Blackburn-Bienaymé Reaction

Dibenziodolium and diphenyliodonium triflates display high catalytic activity for the multicomponent reaction that leads to a series of imidazopyridines. Density functional theory (DFT) calculations indicate that both the salts can play the role of hybrid hydrogen- and halogen-bond-donating organocatalysts, which electrophilically activate the carbonyl and imine groups during the reaction process. The ortho-H atoms in the vicinal position to the I atom play a dual role: forming additional noncovalent bonds with the ligated substrate and increasing the maximum electrostatic potential on the σ-hole at the iodine atom owing to the effects of polarization. Dibenziodolium triflate exhibits higher catalytic activity, and the results obtained from ¹H nuclear magnetic resonance (NMR) titrations, in conjunction with those from DFT calculations, indicate that this could be explained in terms of the additional energy required for the rotation of the phenyl ring in the diphenyliodonium cation during ligation of the substrate.

Orbital analysis of bonding in diarylhalonium salts and relevance to periodic trends in structure and reactivity

Diarylhalonium compounds provide new opportunities as reagents and catalysts in the field of organic synthesis. The three center, four electron (3c–4e) bond is a center piece of their reactivity, but structural variation among the diarylhaloniums, and in comparison with other λ³-iodanes, indicates that the model needs refinement for broader applicability. We use a combination of Density Functional Theory (DFT), Natural Bond Orbital (NBO) Theory, and X-ray structure data to correlate bonding and structure for a λ³-iodane and a series of diarylchloronium, bromonium, and iodonium salts, and their isoelectronic diarylchalcogen counterparts. This analysis reveals that the s-orbital on the central halogen atom plays a greater role in the 3c–4e bond than previously considered. Finally, we show that our revised bonding model and associated structures account for both kinetic and thermodynamic reactivity for both acyclic phenyl(mesityl)halonium and cyclic dibenzohalolium salts.

A revised bonding model for diarylhalonium salts, that involves partial s-orbital contribution, provides new insight into periodic trends in structure and reactivity.

Cyclic Diaryl λ3-Bromanes: A Rapid Access to Molecular Complexity via Cycloaddition Reactions

Biaryls have widespread applications in organic synthesis. However, sequentially polysubstituted biaryls are underdeveloped due to their challenging preparation. Herein, we report the synthesis of dissymetric 2,3,2',3',4-substituted biaryls via pericyclic reactions of cyclic diaryl λ³-bromanes. The functional groups tolerance and atom economy allow access to molecular complexity in a single reaction step. Continuous flow protocol has been designed for the scale-up of the reaction, while postfunctionalizations have been developed taking advantage of the residual Br-atom.

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 .

Cyclic Diaryl λ3 -Bromanes as Original Aryne Precursors

Despite the widespread application of hypervalent iodines, the corresponding λ³ -bromanes are less explored. Herein we report a general, safe, and high-yielding strategy to access cyclic diaryl λ³ -bromanes. These unique compounds feature reactivity that is appealing and complementary to that of λ³ -iodanes, generating arynes under mild reaction conditions and in the presence of a weak base. Accordingly, formal meta-selective transition-metal-free C-O and C-N couplings may be achieved. Mechanistic studies unambiguously support the aryne generation mechanism.

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.