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

Arynes in green solvent: employing o-silylaryl triflates with propylene carbonate

Arynes generated from o-silylaryl triflate precursors are shown to engage with a range of arynophiles in propylene carbonate solvent. Possessing similar physicochemical properties to acetonitrile, propylene carbonate is found to be an effective direct solvent replacement, affording similar yields and promoting increased reaction rates. This is the first time that arynes have been used in conjunction with green and sustainable cyclic carbonate solvents.

Diazomethyl-λ3-iodane meets aryne: dipolar cycloaddition and C-to-N iodane shift leading to indazolyl-λ3-iodanes

Diazomethyl-λ3-iodanes have recently emerged as carbyne equivalents in organic synthesis, enabling the construction of multi-substituted carbon centers through strategic sequential activation of the diazo and iodane functional groups. Distinct from such reaction modes, we report here on the reactivity of diazomethyl-λ3-iodanes as iodane-bound 1,3-dipoles toward arynes. Equipped with bis(trifluoromethyl)benzyl alcohol-based benziodoxole (BX) moiety, diazomethyl-λ3-iodanes undergo annulation with arynes generated from ortho-silylaryl triflates and cyclic diarylhalonium salts, resulting in indazolyl-λ3-iodanes through [3 + 2] cycloaddition and carbon-to-nitrogen iodane migration. DFT calculations reveal that diazomethyl-BX prefers [3 + 2] cycloaddition with aryne over aryne insertion into the carbon-iodine(iii) bond (carboiodanation) and that the subsequent iodane migration proceeds through two consecutive 1,5-iodane shifts. The utility of these indazolyl-BXs as indazole-transfer agents has been demonstrated by α-functionalization of N,N-dimethylaniline derivatives.

Cyclic Diphenylchloronium-Salt-Triggered Coupling of Sulfides with Nucleophiles: Modular Assembly of Pharmaceuticals

We report a novel coupling strategy enabled by cyclic diphenylchloronium salt that facilitates reactions between sulfides and diverse nucleophiles, including oxygen- and nitrogen-based species. The methodology efficiently produces structurally varied valuable compounds, including carbamates, carboxylic esters, aryl ethers, and alkylated amines, under mild, operationally simple conditions. The protocol's synthetic utility was highlighted through modular preparation of five important drugs and five structural analogues, demonstrating significant potential for drug discovery applications.

A general approach for activity-based protein profiling of oxidoreductases with redox-differentiated diarylhalonium warheads

Activity-based protein profiling (ABPP) is a unique proteomic tool for measuring the activity of enzymes in their cellular context, which has been well established for enzyme classes exhibiting a characteristic nucleophilic residue (e.g., hydrolases). In contrast, the enzyme class of oxidoreductases has received less attention, as its members rely mainly on cofactors instead of nucleophilic amino acid residues for catalysis. ABPP probes have been designed for specific oxidoreductase subclasses, which rely on the oxidative conversion of the probes into strong electrophiles. Here we describe the development of ABPP probes for the simultaneous labeling of various subclasses of oxidoreductases. The probe warheads are based on hypervalent diarylhalonium salts, which show unique reactivity as their activation proceeds via a reductive mechanism resulting in aryl radicals leading to covalent labeling of liver proteins at several different amino acids in close proximity to the active sites. The redox potential of the probes can be tuned by isosteric replacement varying the halonium central atom. ABPP experiments with liver using 16 probes differing in warhead, linker, and structure revealed distinct overlapping profiles and broad substrate specificities of several probes. With their capability of multi oxidoreductase subclass labeling - including rare examples for the class of reductases - and their unique design, the herein reported probes offer new opportunities for the investigation of the "oxidoreductome" of microorganisms, plants, animal and human tissues.

Synthesis and Reactivity of Six-Membered Cyclic Diaryl λ3-Bromanes and λ3-Chloranes

Despite the remarkable advancements in hypervalent iodine chemistry, exploration of bromine and chlorine analogues remains in its infancy due to their difficult synthesis. Herein, we introduce six-membered cyclic λ3-bromanes and λ3-chloranes. Through single-crystal X-ray structural analyses and conformational studies, we delineate the crucial bonding patterns pivotal for the thermodynamic stability of these compounds. Notably, these investigations reveal pronounced π-π stacking phenomena within the crystal lattice of hypercoordinated bromine(III) and chlorine(III) species. Their reactivity profile is explored as they are radical precursors or electrophilic reagents in metal-free intermolecular biaryl couplings, O- and S-arylations, and Cu(I)-promoted intramolecular biaryl couplings which is complementary to the known reactivity of five-membered bromanes and chloranes. Mechanistic insights are provided, elucidating the pathways governing their reactivity and underscoring the potential in organic synthesis.

Access to arynes from arenes via net dehydrogenation: scope, synthetic applications and mechanistic analysis

Arynes undergo a wide range of chemical transformations making them versatile reactive intermediates for organic synthesis. Access to arynes has long been dominated by pre-functionalised reagents, e.g., the venerable o-trimethylsilylaryl triflates. However, a move toward developing methods to access arynes that are both mild and efficient has prompted research into aryl "onium" aryne precursors. Here, we leverage aryl "onium" species as in situ or isolated intermediates in a net dehydrogenation of simple arenes as a novel and efficient way to access arynes. We describe a unified strategy in which two different tactics are employed to access diversely substituted arynes from simple arenes. (1) We developed a one-pot method that converts simple arenes into aryl thianthrenium salts and uses them in situ to generate arynes. (2) We developed a two-step process to convert arenes into aryl(Mes)iodonium salts and ultimately trapped arynes to expand the scope of compatible arenes. The net transformations from arenes to trapped arynes are complete with 2-4 hours. Mechanistic analysis through competition experiments, deuterium kinetic isotope effects (DKIE) and Density Functional Theory (DFT) provide key comparisons of the two approaches described in this work and yield a user's guide for selecting the appropriate "onium" leaving group based on the arene.

Pentafluorosulfanoxylation of Hypervalent Chlorines and Bromines for Access to Pentafluoro(biaryloxy)-λ6-sulfanes

The SF5O group has been less explored as a highly fluorinated substituent on an organic framework. In fact, only a few SF5O-containing compounds have been reported, and the preparation of SF5O-containing compounds relied on hazardous reagents and special apparatuses. Herein we describe safe and efficient access to various pentafluoro(aryloxy)-λ6-sulfanes (ArOSF5) through the reaction of hypervalent λ3-chlor(brom)anes with [Et3MeN]+[OSF5]-. The synthetic and application potentials of ArOSF5 moieties were demonstrated by the inertness of OSF5 moieties in the presence of palladium catalysts, alkalis, and nucleophiles.

Asymmetric synthesis of β-amino cyanoesters with contiguous tetrasubstituted carbon centers by halogen-bonding catalysis with chiral halonium salt

β-Amino cyanoesters are important scaffolds because they can be transformed into useful chiral amines, amino acids, and amino alcohols. Halogen bonding, which can be formed between halogen atoms and electron-rich chemical species, is attractive because of its unique interaction in organic synthesis. Chiral halonium salts have been found to have strong halogen-bonding-donor abilities and work as powerful asymmetric catalysts. Recently, we have developed binaphthyl-based chiral halonium salts and applied them in several enantioselective reactions, which formed the corresponding products in high to excellent enantioselectivities. In this paper, the asymmetric synthesis of β-amino cyanoesters with contiguous tetrasubstituted carbon stereogenic centers by the Mannich reaction through chiral halonium salt catalysis is presented, which provided the corresponding products in excellent yields with up to 86% ee. To the best of our knowledge, the present paper is the first to report the asymmetric construction of β-amino cyanoesters with contiguous tetrasubstituted carbon stereogenic centers by the catalytic Mannich reaction.

Halonium and chalconium salt-catalyzed Schiff condensation: kinetics and DFT insights into organocatalyst activity parameters

The Schiff condensation between 4-methylbenzaldehyde and 2-aminopyridine, catalyzed by chloronium, bromonium, and iodonium triflates, as well as sulfonium, selenonium, and telluronium triflates, was investigated. 1H NMR monitoring revealed that the catalytic activity increased significantly with heavier σ-hole-bearing heteroatoms. DFT calculations showed that the maximum electrostatic potential at the σ-hole was a more reliable predictor of the catalytic activity of these organoelement species. In contrast, the equilibrium concentrations of the activated form of the electrophile and the electrophile-to-onium cation charge transfer values did not accurately reflect the catalytic activity of the onium salts.

Metal-free four-component coupling of cyclic diarylchloronium salts, tetrahydrothiophene, amines and carbon dioxide

A four-component coupling reaction of cyclic diarylchloronium salts, tetrahydrothiophene, amines and CO2 has been reported for the first time under transition metal-free conditions, giving rise to a range of structurally diverse and useful sulfur-containing carbamates in high yields with excellent regioselectivities.

Aryne Chemistry: Generation Methods and Reactions Incorporating Multiple Arynes

Arynes hold significance for the efficient fusion of (hetero) arenes with diverse substrates, advancing the construction of complex molecular frameworks. Employing multiple equivalents of arynes is particularly effective in the rapid formation of polycyclic cores found in optoelectronic materials and bioactive compounds. However, the inherent reactivity of arynes often leads to side reactions, yielding unanticipated products and underlining the importance of a detailed investigation into the use of multiple arynes to fine-tune their reactivity. This review centers on methodologies and syntheses in organic reactions involving multiple arynes, categorizing based on mechanisms like cycloadditions, σ-bond insertions, nucleophilic additions, and ene reactions, and discusses aryne polymerization. The categorization based on these mechanisms includes two primary approaches: the first entails multiple aryne engagement within a single step while the second approach involves using a single equivalent of aryne sequentially across multiple steps, with both requiring strict reactivity control to ensure precise aryne participation in each respective step. Additionally, the review provides an in-depth analysis of the selection of aryne precursors, organized chronologically and by activation strategy, offering a comprehensive background that supports the main theme of multiple aryne utilization. The expectation remains that this comprehensive review will be invaluable in designing advanced syntheses engaging multiple arynes.

syn-1,2-Diaminobenzocyclobutenes from [2+2] cycloaddition of 2-imidazolones with arynes

Formal [2+2] cycloaddition of arynes with 2-imidazolones affords syn-1,2-diaminobenzocyclobutenes. The transformation can also be conducted as a one-pot, three-stage process direct from simple propargyl amines and isocyanates to afford the new stereochemically defined benzocyclobutene frameworks.

Metal-free site-selective functionalization with cyclic diaryl λ3-chloranes: suppression of benzyne formation for ligand-coupling reactions

While hypervalent halogens are versatile reagents enabling diverse reactions in organic synthesis, the utility of hypervalent chlorine compounds, particularly cyclic λ3-chloranes, remains underdeveloped despite their unique electronic properties and innate enhanced reactivity. Herein, we illustrate the elusive ligand coupling reaction of cyclic λ3-chloranes that suppresses the more facile competing reaction modality involving benzyne intermediates. The methodology can be performed in three-component as well as two-component fashions, offering direct access to a wide range of unsymmetrically substituted biaryl molecules in very high yields and excellent ortho-regioselectivity. The reactions were scalable, and the versatility was demonstrated by constructing different types of C-S and C-N bonds under mild conditions. The reaction outcomes were also compared with those of corresponding λ3-iodanes and λ3-bromanes, demonstrating the superiority of cyclic λ3-chloranes in ligand-coupling reactions under metal-free conditions.

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.

Easy access to polyhalogenated biaryls: regioselective (di)halogenation of hypervalent bromines and chlorines

Polyhalogenated biaryls are unique motifs offering untapped potential as versatile building blocks for the expedient synthesis of complex biaryl compounds. Overcoming the limitations of conventional syntheses, we introduce a novel, metal-free, operationally simple and one-pot approach to regioselectively (di)halogenate biaryl compounds under mild conditions using cyclic biaryl hypervalent bromine and chlorine substrates as masked arynes. Through chemoselective post-functionalizations, these valuable products can expand the toolbox for synthesizing biaryl-containing scaffolds, addressing a critical gap in the field.

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).

Switchable chemoselective aryne reactions between nucleophiles and pericyclic reaction partners using either 3-methoxybenzyne or 3-silylbenzyne

Arynes are known to serve as highly reactive benzene-based synthons, which have gained numerous successes in preparing functionalized arenes. Due to the superb electrophilic nature of these fleeting species, however, it is challenging to modulate the designated aryne transformation chemoselectively, when substrates possess multiple competing reaction sites. Here, we showcase our effort to manipulate chemoselective control between two major types of aryne transformations using either 3-methoxybenzyne or 3-silylbenzyne, where nucleophilic addition-triggered reactions and non-polar pericyclic reactions could be differentiated. This orthogonal chemoselective protocol is found to be applicable between various nucleophiles, i.e., imidazole, N-tosylated/N-alkyl aniline, phenol, and alcohol, and an array of pericyclic reaction partners, i.e., furan, cyclopentadiene, pyrrole, cycloheptatrienone, and cyclohexene. Beyond arylation reactions, C-N bond insertion, Truce-Smiles rearrangement, and nucleophilic annulation are appropriate reaction modes as well. Moreover, this chemoselective protocol can find potential synthetic application.

Cyclic Diaryl λ3-Bromanes as a Precursor for Regiodivergent Alkynylation Reactions

Regiodivergent reactions are a fascinating tool to rapidly access molecular diversity while using identical coupling partners. We have developed a new approach for regiodivergent synthesis using the dual character of hypervalent bromines. In addition to the recently reported reactivity of hypervalent bromines as aryne precursors, the first transition metal-catalyzed reaction is reported. Accordingly, the development of these two complementary transformations allows for the alteration of regioselectivity to furnish both ortho- and meta-substituted alkynylation products. Mechanistic and computational studies show how these selectivities are controlled.

Csp2-H functionalization of phenols: an effective access route to valuable materials via Csp2-C bond formation

In the vast majority of top-selling pharmaceutical and industrial products, phenolic structural motifs are highly prevalent. Non-functionalized simple phenols serve as building blocks in the synthesis of value-added chemicals. It is worth mentioning that lignin, being the largest renewable biomass source of aromatic building blocks in nature, mainly consists of phenolic units, which enable the production of structurally diverse phenols. Given their remarkable applicability in the chemical value chain, many efforts have been devoted to increasing the molecular complexity of the phenolic scaffold. Among the key techniques, direct functionalization of Csp2-H is a powerful tool, enabling the construction of new Csp2-C bonds in an economical and atomic manner. Herein we present and summarize the large plethora of direct Csp2-H functionalization methods that enables scaffold diversification of simple, unprotected phenols, leading to the formation of new Csp2-C bonds. In this review article, we intend to summarize the contributions that appeared in the literature mainly in the last 5 years dealing with the functionalization of unprotected phenols, both catalytic and non-catalytic. Our goal is to highlight the key findings and the ongoing challenges in the stimulating and growing research dedicated to the development of new protocols for the valorization of phenols.

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.

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.

An efficient and chemoselective method to generate arynes

Arynes hold immense potential as reactive intermediates in organic synthesis as they engage in a diverse range of mechanistically distinct chemical reactions. However, the poor functional group compatibility of generating arynes or their precursors has stymied their widespread use. Here, we show that generating arynes by deprotonation of an arene and elimination of an "onium" leaving group is mild, efficient and broad in scope. This is achieved by using aryl(TMP)iodonium salts (TMP = 2,4,6-trimethoxyphenyl) as the aryne precursor and potassium phosphate as the base, and a range of arynophiles are compatible. Additionally, we have performed the first quantitative analysis of functional group compatibility for several methods to generate arynes, including the method developed here and the current state of the art. Finally, we show that a range of "sensitive" functional groups such as Lewis and Brønsted acids and electrophiles are compatible under our conditions.

Aryne-Enabled C-N Arylation of Anilines

Anilines are potentially high-value arylating agents, but are limited by the low reactivity of the strong C-N bond. We show that the reactive intermediate benzyne can be used to both activate anilines, and set-up an aryl transfer reaction in a single step. The reaction does not require any transition metal catalysts or stoichiometric organometallics, and establishes a metal-free route to valuable biaryl products by functionalizing the aniline C-N bond.

Cyclic Homo- and Heterohalogen Di-λ3-diarylhalonium Structures

In the context of the ever-growing interest in the cyclic diaryliodonium salts, this work presents synthetic design principles for a new family of structures with two hypervalent halogens in the ring. The smallest bis-phenylene derivative, [(C₆H₄)₂I₂]²⁺, was prepared through oxidative dimerization of a precursor bearing the ortho-disposed iodine and trifluoroborate groups. We also report, for the first time, the formation of cycles containing two different halogen atoms. These present two phenylenes linked by hetero-(I/Br) or -(I/Cl) halogen pairs. This approach was also extended to the cyclic bis-naphthylene derivative [(C₁₀H₆)₂I₂]²⁺. The structures of these bis-halogen(III) rings were further assessed through X-ray analysis. The simplest cyclic phenylene bis-iodine(III) derivative features the interplanar angle of ∼120°, while a smaller angle of ∼103° was found for the analogous naphthylene-based salt. All dications form dimeric pairs through a combination of π-π and C-H/π interactions. As the largest member of the family, a bis-I(III)-macrocycle was also assembled using the quasi-planar xanthene backbone. Its geometry enables the two iodine(III) centers to be bridged intramolecularly by two bidentate triflate anions. In a preliminary manner, the interaction of the phenylene- and naphthalene-based bis-iodine(III) dications with a new family of rigid bidentate bis-pyridine ligands was studied in solution and the solid state, with an X-ray structure showing the chelating donor bonding to just one of the two iodine centers.

3-sulfonyloxyaryl(mesityl)iodonium triflates as 1,2-benzdiyne precursors with activation via ortho-deprotonative elimination strategy

Benzyne has long captivated the attention of chemists and has gained numerous synthetic achievements. Among typical benzyne generation methods, removal of two vicinal substituents from 1,2-difunctionalized benzenes, i.e., Kobayashi's protocol, are prevailing, while ortho-deprotonative elimination from mono-substituted benzene lags far behind. Despite the advantages of atom economy and ready achievability of precursors, a bottle neck for ortho-deprotonative elimination strategy resides in the weak acidity of the ortho-hydrogen, which normally demands strong bases as the activating reagents. Here, an efficient aryne generation protocol is developed, where ortho-deprotonative elimination on 3-sulfonyloxyaryl(mesityl)iodonium triflates occurs under mild conditions and the generated 3-sulfonyloxyarynes can serve as efficient 1,2-benzdiyne synthons. This array of 1,2-benzdiyne precursors can be conveniently prepared with high functional group tolerance, and densely substituted scaffolds can be accessed as well. Carbonate and fluoride salts are found to serve as efficient activating reagents, which are the weakest bases used in ortho-deprotonative elimination strategies. Particularly, this scaffold has predictable chemoselective generation of the designated aryne intermediates. The success of this ortho-deprotonative elimination protocol sets up a unique platform with a broad spectrum of synthetic applications.