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

Combined crystallographic and computational investigation of the solvent disorder present in a new tipiracil hydrochloride methanol solvate–hydrate

We report a computational and crystallographic analysis of tipiracil hydrochloride (methanol solvate–hydrate). It shows a complex network of H-bonds with static discrete disorder in two out of the three symmetry-independent solvent molecules.

Static discrete disorder in the crystal structure of iododiflunisal: on the importance of hydrogen bond, halogen bond and π-stacking interactions

We report a combined computational/crystallographic analysis focused on the static discrete disorder shown by the drug iododiflunisal.

Exploring halogen⋯halogen interactions in supramolecular self-assemblies of BODIPY networks

In this study, the efficiency of halogen⋯halogen interactions to control supramolecular assemblies of boron dipyrromethene (BODIPY) (B1–B5) derivatives was explored.

On the Importance of Halogen Bonding Interactions in Two X-ray Structures Containing All Four (F, Cl, Br, I) Halogen Atoms

This manuscript reports the synthesis and X-ray characterization of two octahydro-1H-4,6-epoxycyclopenta[c]pyridin-1-one derivatives that contain the four most abundant halogen atoms (Ha) in the structure with the aim of studying the formation of Ha···Ha halogen bonding interactions. The anisotropy of electron density at the heavier halogen atoms provokes the formation of multiple Ha···Ha contacts in the solid state. That is, the heavier Ha-atoms exhibit a region of positive electrostatic potential (σ-hole) along the C–Ha bond and a belt of negative electrostatic potential (σ-lumps) around the atoms. The halogen bonding assemblies in both compounds were analyzed using density functional theory (DFT) calculations, molecular electrostatic potential (MEP) surfaces, the quantum theory of “atom-in-molecules” (QTAIM), the noncovalent interaction plot (NCIplot), and the electron localization function (ELF).

Metal Centers as Nucleophiles: Oxymoron of Halogen Bond-Involving Crystal Engineering

This review highlights recent studies discovering unconventional halogen bonding (HaB) that involves positively charged metal centers. These centers provide their filled d‐orbitals for HaB, and thus behave as nucleophilic components toward the noncovalent interaction. This role of some electron‐rich transition metal centers can be considered an oxymoron in the sense that the metal is, in most cases, formally cationic; consequently, its electron donor function is unexpected. The importance of Ha⋅⋅⋅d‐[M] (Ha=halogen; M is Group 9 (Rh, Ir), 10 (Ni, Pd, Pt), or 11 (Cu, Au)) interactions in crystal engineering is emphasized by showing remarkable examples (reported and uncovered by our processing of the Cambridge Structural Database), where this Ha⋅⋅⋅d‐[M] directional interaction guides the formation of solid supramolecular assemblies of different dimensionalities.

Modulating Chalcogen Bonding and Halogen Bonding Sigma-Hole Donor Atom Potency and Selectivity for Halide Anion Recognition

A series of acyclic anion receptors containing chalcogen bond (ChB) and halogen bond (XB) donors integrated into a neutral 3,5‐bis‐triazole pyridine scaffold are described, in which systematic variation of the electronic‐withdrawing nature of the aryl substituents reveal a dramatic modulation in sigma‐hole donor atom potency for anion recognition. Incorporation of strongly electron‐withdrawing perfluorophenyl units appended to the triazole heterocycle telluro‐ or iodo‐ donor atoms, or directly linked to the tellurium donor atom dramatically enhances the anion binding potency of the sigma‐hole receptors, most notably for the ChB and XB receptors displaying over thirty‐fold and eight‐fold increase in chloride anion affinity, respectively, relative to unfluorinated analogues. Linear free energy relationships for a series of ChB based receptors reveal the halide anion recognition behaviour of the tellurium donor is highly sensitive to local electronic environments. This is especially the case for those directly appended to the Te centre (3⋅ChB), where a remarkable enhancement of strength of binding and selectivity for the lighter halides is observed as the electron‐withdrawing ability of the Te‐bonded aryl group increases, highlighting the exciting opportunity to fine‐tune anion affinity and selectivity in ChB‐based receptor systems.

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 .

Dissection of the Origin of π-Holes and the Noncovalent Bonds in Which They Engage

Accompanying the rapidly growing list of σ-hole bonds has come the acknowledgment of parallel sorts of noncovalent bonds which owe their stability in large part to a deficiency of electron density in the area above the molecular plane, known as a π-hole. The origins of these π-holes are probed for a wide series of molecules, comprising halogen, chalcogen, pnicogen, tetrel, aerogen, and spodium bonds. Much like in the case of their σ-hole counterparts, formation of the internal covalent π-bond in the Lewis acid molecule pulls density toward the bond midpoint and away from its extremities. This depletion of density above the central atom is amplified by an electron-withdrawing substituent. At the same time, the amplitude of the π*-orbital is enhanced in the region of the density-depleted π-hole, facilitating a better overlap with the nucleophile's lone pair orbital and a stabilizing n → π* charge transfer. The presence of lone pairs on the central atom acts to attenuate the π-hole and shift its position somewhat, resulting in an overall weakening of the π-hole bond. There is a tendency for π-hole bonds to include a higher fraction of induction energy than σ-bonds with proportionately smaller electrostatic and dispersion components, but this distinction is less a product of the σ- or π-character and more a function of the overall bond strength.

Enantio- and diastereoselective double Mannich reaction of malononitrile with N-Boc imines using quinine-derived bifunctional organoiodine catalyst

A chiral quinine-derived organic base catalyst with halogen bond donor functionality was used to catalyze the asymmetric double Mannich reaction of malononitrile with N-Boc and N-Cbz imines to afford 1,3-diamines in excellent yields with high enantio- and diastereoselectivities. With 2.2 equiv. of a single imine electrophile, symmetrical 1,3-diamines were obtained, whereas, with two different imine partners, unsymmetrically substituted 1,3-diamine was obtained. The monohydration of the double Mannich product was also achieved.

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.

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.

Preparation and Synthetic Applicability of Imidazole-Containing Cyclic Iodonium Salts

A novel approach to the preparation of imidazole-substituted cyclic iodonium salts has been developed via the oxidative cyclization of 1-phenyl-5-iodoimidazole using a cheap and available Oxone/H₂SO₄ oxidative system. The structure of the new polycyclic heteroarenes has been confirmed by single-crystal X-ray diffractometry, revealing the characteristic structure features for cyclic iodonium salts. The newly produced imidazole-flanked cyclic iodonium compounds were found to readily engage in a heterocyclization reaction with elemental sulfur, affording benzo[5,1-b]imidazothiazoles in good yields.

A Hypervalent Cyclic Dibenzoiodolium Salt as a Halogen-Bond-Donor Catalyst for the [4+2] Cycloaddition of 2-Alkenylindoles

A stable, hypervalent cyclic dibenzoiodolium salt acted as a strong halogen bonding (XB)-donor catalyst for [4+2] cycloaddition of 2-alkenylindoles, and not as an oxidizing agent. The cross-[4+2] cycloaddition of 2-vinylindoles with 2-alkenylindoles was catalyzed smoothly by the hypervalent cyclic dibenzoiodolium triflate catalyst to give the tetrahydrocarbazoles in up to 99 % yield with 17 : 1 diastereoselectivity. The hypervalent cyclic dibenzoiodolium salt was also applicable to the Povarov reaction of 2-vinylindole with N-p-methoxyphenyl (PMP) imine to give the indolyl-tetrahydroquinoline in 83 % yield.

Catalysis by Bidentate Iodine(III)-Based Halogen Donors: Surpassing the Activity of Strong Lewis Acids

The poorly understood mode of activation and catalysis of bidentate iodine(III)-based halogen donors have been quantitatively explored in detail by means of state-of-the-art computational methods. To this end, the uncatalyzed Diels–Alder cycloaddition reaction between cyclohexadiene and methyl vinyl ketone is compared to the analogous process mediated by a bidentate iodine(III)-organocatalyst and by related, highly active iodine(I) species. It is found that the bidentate iodine(III)-catalyst accelerates the cycloaddition by lowering the reaction barrier up to 10 kcal mol^–1 compared to the parent uncatalyzed reaction. Our quantitative analyses reveal that the origin of the catalysis is found in a significant reduction of the steric (Pauli) repulsion between the diene and dienophile, which originates from both a more asynchronous reaction mode and a significant polarization of the π-system of the dienophile away from the incoming diene. Notably, the activity of the iodine(III)-catalyst can be further enhanced by increasing the electrophilic nature of the system. Thus, novel systems are designed whose activity actually surpasses that of strong Lewis acids such as BF₃.

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 .

Modulation of supramolecular self-assembly of BODIPY tectons via halogen bonding

In this study, the efficiency of halogen bonding strategies to control over the supramolecular assemblies of boron dipyrromethene (BODIPY) (B-1, B-2, and B-3) derivatives was explored.