4,4'-Azobis(halopyridinium) derivatives: strong multidentate halogen-bond donors with a redox-active core

A decade of lessons in the activation of ArIL2 species

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

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.

Balancing Activity and Stability in Halogen-Bonding Catalysis: Iodopyridinium-Catalyzed One-Pot Synthesis of 2,3-Dihydropyridinones

A one-pot cascade reaction for 2,3-dihydropyridinone synthesis was accomplished with 3-fluoro-2-iodo-1-methylpyridinium triflate as the halogen bond catalyst. The desired [4+2] cycloaddition products, bearing aryl, heteroaryl, alkyl, and alicyclic substituents, were successfully furnished in 28-99% yields. Mechanistic investigations proved that a strong halogen-bonding interaction forged between the iodopyridinium catalyst and imine intermediate was essential to dynamically masking the vulnerable C-I bond on the catalyst and accelerating the following aza-Diels-Alder reaction.

Mechanism and Impact of Mono/bis(iodoimidazolium) Halogen-Bond Donor Catalysts on Michael Addition of Indole with Trans-crotonophenone: DFT Investigations

The bidentate halogen bond donor catalysts in organic reactions have attracted greatly attention in recent years. In this work, the catalytic mechanism of mono/bis(iodoimidazolium) halogen bond donor catalysts on the...

Lewis acid activation of Weiss' reagents ([PhI(Pyr)2]2+) with boranes and isolation of [PhI(4-DMAP)]2

Abstraction of a pyridine ligand from Weiss' reagent ([PhI(Pyr)₂]²⁺) using BF₃-Et₂O was found to activate Weiss' reagent towards electrophilic aromatic substitution reactions. The activated species can be isolated when 4-DMAP is used as the pyridine ligand and was determined to be [PhI(4-DMAP)]²⁺ in solution. The isolated cation was reactive in electrophilic aromatic substitution reactions towards mesitylene, xylene and toluene that Weiss' reagent itself does not react with.

KOtBu-Promoted Halogen-Bond-Assisted Intramolecular C-S Cross-Coupling of o-Iodothioanilides for the Synthesis of 2-Substituted Benzothiazoles

An efficacious and mild KO^tBu-promoted intramolecular C-S cross-coupling of ortho-iodothioanilides in conjunction with a catalytic quantity of phenanthroline as an additive has been described for the convenient synthesis of 2-substituted benzothiazoles. The methodology is suitable for attaining a wide variety of 2-alkyl- and 2-aryl-substituted benzothiazoles. Single-crystal XRD, DFT calculations, NMR, and UV studies suggest that halogen bonds between the units of ortho-iodothioanilides may assist in the electron transfer process.

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 .

The Chloroazaphosphatrane Motif for Halogen Bonding in Solution

Chloroazaphosphatranes, the corresponding halogenophosphonium cations of the Verkade superbases, were evaluated as a new motif for halogen bonding (XB). Their modulable synthesis allowed for synthetizing chloroazaphosphatranes with various substituents on the nitrogen atoms. The binding constants determined from NMR titration experiments for Cl^-, Br^-, I^-, AcO^-, and CN^- anions are comparable to those obtained with conventional iodine-based monodentate XB receptors. Remarkably, the protonated azaphosphatrane counterparts display no affinity for anions under the same conditions. The strength of the XB interaction is, to some extent, related to the basicity of the corresponding Verkade superbase. The halogen bonding abilities of this new class of halogen donor motif were also revealed by the Δδ(³¹P) NMR shift observed in CD₂Cl₂ solution in the presence of triethylphosphine oxide (TEPO). Thus, chloroazaphosphatranes constitute a new class of halogen bond donors, expanding the repertory of XB motifs mainly based on C_Ar-I bonds.

Correlations between Substituent Effects and Catalytic Activities: A Quantitative Approach for the Development of Halogen-Bonding-Driven Anion-Binding Catalysts

A quantitative approach for the development of halogen-bonding-driven anion-binding catalysts was studied using 4-substituted perfluorinated iodobenzene. ¹⁹ F NMR titrations were used to determine the binding constants K for chloride, and their catalytic activities were evaluated in the allylation reaction of a N-activated pyridine. We discovered that the log K and product yields were linearly correlated, and that they were dependent on the Hammett substituent parameter, σ_meta (r² =0.99). This linear correlation provided a quantitative predictive model for both the binding constant and the reaction yield. Concomitantly, this efficiently permitted the development of a highly active anion-binding catalyst, namely 4-CNC₆ F₄ I (K=489±5 M^-1 ). Additionally, the catalytic activity of 4-CNC₆ F₄ I was established in the allylation and crotylation of N-activated isoquinolines (7 examples). Overall, this approach highlights the value of quantitative analysis by exploring experimentally informed correlations in the development of halogen bond donor catalysts.

Bidentate Nitrogen-Ligated I(V) Reagents, Bi(N)-HVIs: Preparation, Stability, Structure, and Reactivity

Hypervalent iodine(V) reagents are a powerful class of organic oxidants. While the use of I(V) compounds Dess-Martin periodinane and IBX is widespread, this reagent class has long been plagued by issues of solubility and stability. Extensive effort has been made for derivatizing these scaffolds to modulate reactivity and physical properties but considerable room for innovation still exists. Herein, we describe the preparation, thermal stability, optimized geometries, and synthetic utility of an emerging class of I(V) reagents, Bi(N)-HVIs, possessing datively bound bidentate nitrogen ligands on the iodine center. Bi(N)-HVIs display favorable safety profiles, improved solubility, and comparable to superior oxidative reactivity relative to common I(V) reagents. The highly modular synthesis and in situ generation of Bi(N)-HVIs provides a novel and convenient screening platform for I(V) reagent and reaction development.

Heterocyclic group transfer reactions with I(iii) N-HVI reagents: access to N-alkyl(heteroaryl)onium salts via olefin aminolactonization

Pyridinium and related N-alkyl(heteroaryl)onium salts are versatile synthetic intermediates in organic chemistry, with applications ranging from ring functionalizations to provide diverse piperidine scaffolds to their recent emergence as radical precursors in deaminative cross couplings. Despite their ever-expanding applications, methods for their synthesis have seen little innovation, continuing to rely on a limited set of decades old transformations and a limited subset of coupling partners. Herein, we leverage (bis)cationic nitrogen-ligated I(iii) hypervalent iodine reagents, or N-HVIs, as "heterocyclic group transfer reagents" to provide access to a broad scope of N-alkyl(heteroaryl)onium salts via the aminolactonization of alkenoic acids, the first example of engaging an olefin to directly generate these salts. The reactions proceed in excellent yields, under mild conditions, and are capable of incorporating a broad scope of sterically and electronically diverse aromatic heterocycles. The N-HVI reagents can be generated in situ, the products isolated via simple trituration, and subsequent derivatizations demonstrate the power of this platform for diversity-oriented synthesis of 6-membered nitrogen heterocycles.

Towards redox-switchable organocatalysts based on bidentate halogen bond donors

Redox-active bidentate halogen bond donors based on halopyridinium groups as halogen-bond donating units were synthesized and their structures were elucidated by X-ray diffraction analyses and DFT calculations. Via reversible twofold reduction, these dicationic species can be transformed to neutral compounds which should be much weaker Lewis acids. The corresponding electrochemical data were obtained, and CV as well as UV-vis and NMR techniques were also used to determine binding constants of these halogen bond donors to halides. While all titrations agree on the relative order of binding strengths (with chloride being bound strongest), there are marked deviations in the overall affinity constants which are discussed. In contrast to earlier azo-bridge analogues, the ethylene-linked variants presented herein do not oxidize halides, and thus the novel halogen bond donors could also be used as Lewis acidic organocatalysts in a halide abstraction benchmark reaction, yielding a performance similar to bis(haloimidazolium)-derived catalysts.

Fixation of CO2 into Cyclic Carbonates by Halogen-Bonding Catalysis

Halogen bonding, parallel to hydrogen bonding, was introduced into the catalytic cycloaddition of carbon dioxide into epoxide (CCE) reactions. A series of halogen-bond donor (XBD) catalysts of N-iodopyridinium halide featured with N-I bond were synthesized and evaluated in CCE reactions. The optimal XBD catalyst, 4-(dimethylamino)-N-iodopyridinium bromide ([DMAPI]Br), under screened conditions at 100 °C, ambient pressure, and 1 mol % catalyst loading, realized 93 % conversion of styrene oxide into cyclic carbonate in 6 h. The substrate scope was successfully extended with excellent yields (mostly ≥93 %) and quantitative selectivity (more than 99 %). ¹ H NMR spectroscopy of the catalyst [DMAPI]Br on substrate epoxide certified that the N-I bond directly coordinated with the epoxide oxygen. A plausible mechanism of halogen-bonding catalysis was proposed, in which the DMAPI cation functioned as halogen-bond donor to activate the epoxide, and the counter anion bromide attacked the methylene carbon to initiate the ring-opening of the epoxide. CCE reactions promoted by N-iodopyridinium halide, exemplify a first case of halogen-bonding catalysis in epoxide activation and CO₂ transformation.

"Anti-Electrostatic" Halogen Bonding

Halogen bonding is often described as being driven predominantly by electrostatics, and thus adducts between anionic halogen bond (XB) donors (halogen-based Lewis acids) and anions seem counterintuitive. Such "anti-electrostatic" XBs have been predicted theoretically but for organic XB donors, there are currently no experimental examples except for a few cases of self-association. Reported herein is the synthesis of two negatively charged organoiodine derivatives that form anti-electrostatic XBs with anions. Even though the electrostatic potential is universally negative across the surface of both compounds, DFT calculations indicate kinetic stabilization of their halide complexes in the gas phase and particularly in solution. Experimentally, self-association of the anionic XB donors was observed in solid-state structures, resulting in dimers, trimers, and infinite chains. In addition, co-crystals with halides were obtained, representing the first cases of halogen bonding between an organic anionic XB donor and a different anion. The bond lengths of all observed interactions are 14-21 % shorter than the sum of the van der Waals radii.

Bidentate Chiral Bis(imidazolium)-Based Halogen-Bond Donors: Synthesis and Applications in Enantioselective Recognition and Catalysis

Even though halogen bonding-the noncovalent interaction between electrophilic halogen substituents and Lewis bases-has now been established in molecular recognition and catalysis, its use in enantioselective processes is still very rarely explored. Herein, we present the synthesis of chiral bidentate halogen-bond donors based on two iodoimidazolium units with rigidly attached chiral sidearms. With these Lewis acids, chiral recognition of a racemic diamine is achieved in NMR studies. DFT calculations support a 1:1 interaction of the halogen-bond donor with both enantiomers and indicate that the chiral recognition is based on a different spatial orientation of the Lewis bases in the halogen-bonded complexes. In addition, moderate enantioselectivity is achieved in a Mukaiyama aldol reaction with a preorganized variant of the chiral halogen-bond donor. This represents the first case in which asymmetric induction was realized with a pure halogen-bond donor lacking any additional active functional groups.

Preorganization: A Powerful Tool in Intermolecular Halogen Bonding in Solution

Preorganization is a powerful tool in supramolecular chemistry which has been utilized successfully in intra- and intermolecular halogen bonding. In previous work, we had developed a bidentate bis(iodobenzimidazolium)-based halogen bond donor which featured a central trifluoromethyl substituent. This compound showed a markedly increased catalytic activity compared to unsubstituted bis(iodoimidazolium)-based Lewis acids, which could be explained either by electronic effects (the electron withdrawal by the fluorinated substituent) or by preorganization (the hindered rotation of the halogen bonding moieties). Herein, we systematically investigate the origin of this increased Lewis acidity via a comparison of the two types of compounds and their respective derivatives with or without the central trifluoromethyl group. Calorimetric measurements of halide complexations indicated that preorganization is the main reason for the higher halogen bonding strength. The performance of the catalysts in a series of benchmark reactions corroborates this finding.

Cationic polymerization of vinyl monomers using halogen bonding organocatalysts with varied activity

Cationic polymerization of vinyl monomers was investigated using non-ionic and ionic halogen bonding organocatalysts.

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.

Chemoselective Oxidation of Equatorial Alcohols with N-Ligated λ3-Iodanes

The site-selective and chemoselective functionalization of alcohols in complex polyols remains a formidable synthetic challenge. Whereas significant advancements have been made in selective derivatization at the oxygen center, chemoselective oxidation to the corresponding carbonyls is less developed. In cyclic systems, whereas the selective oxidation of axial alcohols is well known, a complementary equatorial selective process has not yet been reported. Herein we report the utility of nitrogen-ligated (bis)cationic λ3-iodanes (N-HVIs) for alcohol oxidation and their unprecedented levels of selectivity for the oxidation of equatorial over axial alcohols. The conditions are mild, and the simple pyridine-ligated reagent (Py-HVI) is readily synthesized from commercial PhI(OAc)2 and can be either isolated or generated in situ. Conformational selectivity is demonstrated in both flexible 1,2-substituted cyclohexanols and rigid polyol scaffolds, providing chemists with a novel tool for chemoselective oxidation.

Organocatalysis by a multidentate halogen-bond donor: an alternative to hydrogen-bond based catalysis

Stoichiometric organocatalysis using a charge-neutral bis-iodoethynyl halogen-bond decorated scaffold.

An efficient synthesis of benzothiazole using tetrabromomethane as a halogen bond donor catalyst

A CBr₄ catalyzed simple, mild, and efficient protocol has been developed for the synthesis of 2-substituted benzothiazole from 2-aminothiophenols and N-methylthioamides under solvent free conditions.

Chalcogen Bonding: An Overview

In the last few decades, "unusual" noncovalent interactions like anion-π and halogen bonding have emerged as interesting alternatives to the ubiquitous hydrogen bonding in many research areas. This is also true, to a somewhat lesser extent, for chalcogen bonding, the noncovalent interaction involving Lewis acidic chalcogen centers. Herein, we aim to provide an overview on the use of chalcogen bonding in crystal engineering and in solution, with a focus on the recent developments concerning intermolecular chalcogen bonding in solution-phase applications. In the solid phase, chalcogen bonding has been used for the construction of nano-sized structures and the self-assembly of sophisticated self-complementary arrays. In solution, until very recently applications mostly focused on intramolecular interactions which stabilized the conformation of intermediates or reagents. In the last few years, intermolecular chalcogen bonding has increasingly also been exploited in solution, most notably in anion recognition and transport as well as in organic synthesis and organocatalysis.

The role of charge in 1,2,3-triazol(ium)-based halogen bonding activators

Monocationic halogen-bonding-based activators are equally potent as structurally related dicationic ones.

The influence of charge on the performance of monocationic and dicationic triazol(ium)-based halogen bond donors was investigated. Next to the activity in a halide abstraction benchmark reaction, halogen bonding was also evaluated via X-ray structural analyses and isothermal titration calorimetry.

(Poly)cationic λ3-Iodane Mediated Oxidative Ring Expansion of Secondary Alcohols

Herein, we report a simplified approach to the synthesis of medium-ring ethers through the electrophilic activation of secondary alcohols with (poly)cationic λ3-iodanes (N-HVI). Excellent levels of selectivity are achieved for C-O bond migration over established α-elimination pathways, enabled by the unique reactivity of a novel 2-OMe-pyridine-ligated N-HVI. The resulting HFIP-acetals are readily derivatized with a range of nucleophiles, providing a versatile functional handle for subsequent manipulations. The utility of this methodology for late-stage natural product derivatization was also demonstrated, providing a new tool for diversity-oriented synthesis and complexity-to-diversity (CTD) efforts. Preliminary mechanistic investigations reveal a strong effect of alcohol conformation on reactive pathway, thus providing a predictive power in the application of this approach to complex molecule synthesis.

Cation-cation and anion-anion complexes stabilized by halogen bonds

Stable minima showing halogen bonds between charged molecules with the same sign have been explored by means of theoretical calculations. The dissociation transition states and their corresponding barriers have also been characterized. In all cases, the results indicate that the complexes are thermodynamically unstable but kinetically stable with respect to the isolated monomers in gas phase. A corrected binding energy profile by removing the charge-charge repulsion of the monomers shows a profile similar to the one observed for the dissociation of analogous neutral systems. The nature of the interaction in the minima and TSs has been analyzed using the symmetry adapted perturbation theory (SAPT) method. The results indicate the presence of local favorable electrostatic interactions in the minima that vanish in the TSs. Natural bond orbital (NBO) and "atoms-in-molecules" (AIM) theories were used to analyze the complexes, obtaining good correlations between Laplacian and electron density values with both bond distances and charge-transfer energy contributions E(2). The largest E(2) orbital interaction energies for cation-cation and anion-anion complexes are 561.2 and 197.9 kJ mol^-1, respectively.

A chiral organic base catalyst with halogen-bonding-donor functionality: asymmetric Mannich reactions of malononitrile with N-Boc aldimines and ketimines

Asymmetric Mannich reactions of malononitrile with various N-Boc imines were developed using a chiral organic base catalyst with halogen-bonding-donor functionality.

Bulky iodotriazolium tetrafluoroborates as highly active halogen-bonding-donor catalysts

Bulky halogen-bonding-donor catalysts have been synthesized, and their catalytic activities for aza-Diels–Alder reaction were evaluated.

A halogen-bonding-catalyzed Michael addition reaction

Only a few studies on the use of halogen bonding in catalysis have been published so far. Herein, (benz)imidazolium-based halogen bond donors are used as catalysts in a Michael addition reaction. The most potent catalyst, a rigid atropisomer featuring two iodobenzimidazolium moieties, provided a rate acceleration versus a reference compound of ca. 50.

Comparative study of non-covalent interactions between cationic N-phenylviologens and halides by electrochemistry and NMR: the halogen bonding effect

Five N-phenylviologen (PV²⁺) derivatives have been synthesized and their electrochemical behavior in the presence of halide anions has been studied. Further investigations were carried out by ¹H and ¹⁹F NMR spectroscopy at different chloride concentrations. This is the first time a systematic study combines cyclic voltammetry and NMR spectroscopy in order to analyse the contribution of halogen bonding among the various non-covalent interactions between iodinated N-phenylviologens. The results show strong evidence for a significant "halogen bonding effect" in the interaction between halides and the iodo-tetrafluoro-phenylviologen PV2+-C6F4I. A significant influence of halogen bonding on reduction potentials of the novel halogen bond donor PV2+-C6F4I has been evidenced resulting in the first example of "inverse redox switching" of an XB-donor being partially deactivated upon reduction. Furthermore the particular binding properties of the perfluorinated derivative PV2+-C6F5 towards chloride are discussed considering a possible contribution of π-anion interaction in solution.