Mild Friedel-Crafts Reactions inside a Hexameric Resorcinarene Capsule: C−Cl Bond Activation through Hydrogen Bonding to Bridging Water Molecules

Computational Designing and Prediction of ADMET Properties of Four Novel Imidazole-Based Drug Candidates Inhibiting Heme Oxygenase-1 Causing Cancers

The overexpression of heme oxygenase-1 (HO-1) contributes to the development of several types of cancers. The inhibition of HO-1 through imidazole-based drugs, which is non-competitive with heme, is a focus of anticancer drug research. We designed the four following novel HO-1 inhibiting compounds: 2-(1-cyclopentyl-4-(1H-imidazol-4-yl)butan-2-yl)pyrazine (M11), 2-[(2-chloro-3-methylcyclohexyl)methyl]-1H-imidazole (M26), 2-(2-phenethyl-1H-imidazol-4-yl)ethanesulfonamide (M28), and 5-chloro-2-[2-(2,5-dihydro-1H-imidazol-2-yl)propan-2-yl]-1H-imidazole (M31). All compounds showed a strong binding affinity with HO-1 in molecular docking studies. The in silico absorption, distribution, metabolism, excretion and toxicity (ADMET) data showed that the compounds would be available orally in an acceptable manner. The bioactivity scores revealed that they were moderately active substances. They were found as non-mutagen, non-tumorigenic, non-irritant, and non-detrimental to the reproductive system. Finally, the drug-likeness values of the compounds were obtained as -0.71, -1.64, -2.04, and 0.4 respectively, with the final drug-score of 0.60, 0.54, 0.51, and 0.77 respectively.

Dodecameric Anion-Sealed Capsules based on Pyrogallol[5]arenes and Resorcin[5]arenes

The paper reports formation of exceptionally large capsular species (diameter of c. a. 30 Å) by interactions of polyphenolic macrocycles with 5-fold symmetry with anions. Pyrogallol[5]arenes and resorcin[5]arenes interact with anions via hydrogen bonds involving phenolic OH groups or aromatic CH groups. Based on NMR titration experiments, diffusion coefficients, and geometric requirements, it is postulated that the capsules have (P5H)₁₂ (X^- )₆₀ or (R5H)₁₂ (X^- )₆₀ stoichiometry and a unique geometry of one of the Platonic solids-a dodecahedron. The capsules exist in THF and in benzene, but not in chloroform, reflecting competitive effects in the solvation of anions. It is also demonstrated that mechanochemical pre-treatment (dry-milling) of solid samples is indispensable to initialize self-assembly in benzene.

A Promising Approach for Controlling the Second Coordination Sphere of Biomimetic Metal Complexes: Encapsulation in a Dynamic Hydrogen-Bonded Capsule

The design of biomimetic models of metalloenzymes needs to take into account many factors and is therefore a challenging task. We propose in this work an original strategy to control the second coordination sphere of a metal centre and its distal environment. A biomimetic complex, reproducing the first coordination sphere, is encapsulated in a self-assembled hydrogen-bonded capsule. The cationic complex is co-encapsulated with its counter-anion or with solvent molecules. The capsule is dynamic, allowing a fast in/out exchange of the co-encapsulated species. It also provides both a hydrogen-bonding site in the second coordination sphere and a source of proton as it can be deprotonated in the presence of the complex, providing a globally neutral host-guest assembly. This simple and broad scope strategy is unprecedented in biomimetic studies. The approach appears to be a very promising method for the stabilisation of reactive species and for the study of their reactivity.

B(C6 F5 )3 -Catalyzed Tandem Friedel-Crafts and C-H/C-O Coupling Reactions of Dialkylanilines

Tandem Friedel-Crafts (FC) and C-H/C-O coupling reactions catalyzed by tris(pentafluorophenyl) borane (B(C₆ F₅ )₃ ) were achieved without using any other additive in the absence of solvent. This process can be used for the reactions between a series of dialkylanilines and vinyl ethers with good isolated yields of bis(4-dialkylaminophenyl) compounds. Based on combined theoretical and experimental studies, the possible reaction mechanism was proposed. B(C₆ F₅ )₃ can activate the C=C and C-O bond for FC and C-H/C-O coupling reactions respectively. The FC reaction is slow, which is followed by a fast C-H/C-O coupling.

Green, Mild, and Efficient Friedel-Crafts Benzylation of Scarcely Reactive Arenes and Heteroarenes under On-Water Conditions

Metal-free Friedel-Crafts benzylation (FCB) of scarcely reactive arenes and heteroarenes was performed under on-water conditions by an environmentally sustainable procedure. The catalytic strategy exploits the hydrophobicity of the resorcinarene macrocycle 1 a. The proposed mechanism is based on the activation of benzyl chloride by H-bonding interactions with catalyst 1 a. In fact, under on-water conditions the hydrophobic amplification of the strength of the H-bonding interactions between the OH groups of the resorcinarene catalyst and the chlorine atom of benzyl chloride leads to polarization of the C-Cl bond, which consequently promotes electrophilic attack of the π nucleophile. Thus, many arenes and heteroarenes were efficiently benzylated under mild on-water conditions by using resorcinarene 1 a as catalyst. The FCB of benzene is industrially relevant for the synthesis of diphenylmethane, and hence the on-water procedure was extended to the gram-scale synthesis of diphenylmethane, starting from benzene and benzyl chloride in the presence of resorcinarene catalyst 1 a.

pH-Responsive Pillar[6]arene-based Water-Soluble Supramolecular Hexagonal Boxes

We describe the preparation of the first water-soluble pH-responsive supramolecular hexagonal boxes (SHBs) based on multiple charge-assisted hydrogen bonds between peramino-pillar[6]arenes 2 with the molecular "lid" mellitic acid (1 a). The interaction between 2 and 1 a, as well as the other "lids" pyromellitic and trimesic acids (1 b and 1 c, respecively) were studied by a combination of experimental and computational methods. Interestingly, the addition of 1 a to the complexes of the protonated form of pillar[6]arene 2, that is, 3, with bis-sulfonate 4 a or 4 b, immediately led to guest escape along with the formation of closed 1 a₂ 2 supramolecular boxes. Moreover, the process of the openning and closing of the supramolecular boxes along with threading and escaping of the guests, respectively, was found to be reversible and pH-responsive. This study paves the way for the easy and modular preparation of different SHBs that may have myriad applications.

Catalysis inside the Hexameric Resorcinarene Capsule

In this Account, we outline our investigation into the supramolecular resorcinarene capsule as a catalyst. Molecular capsules not only are of interest due to the similarities of their binding pockets with those of natural enzymes but also feature potential advantages for catalysis. Due to the restricted internal volume of the binding pockets, substrate selectivities are commonly observed. Substrates that are encapsulated more efficiently will be converted selectively in the presence of less suitable substrates. This size selectivity cannot be obtained in a regular solution experiment. In addition, because of the distinct chemical environment inside the capsule, different product selectivities may be observed. Furthermore, the encapsulation of reactive catalysts inside confined environments may improve catalyst compatibility for multicatalyst tandem reactions. Although the potential advantages of performing catalysis inside closed microenvironments are generally recognized, the number of known catalytically active supramolecular host systems is still very limited. There are several reasons, the most important of which is that it is very difficult to predict the catalytic potential of known supramolecular host systems. In several cases, even the encapsulation behavior of host systems is not completely understood or explored. Therefore, it is evident that further research is required to explore the potential of catalysis inside supramolecular capsules. Our initial research mainly focused on understanding the puzzling encapsulation behavior of the self-assembled resorcinarene capsule I and the closely related pyrogallolarene capsule II. After the elucidation of the decisive differences between these two systems, we explored the catalytic potential of capsule I. A variety of different reactions were successfully performed inside its cavity. The most important examples highlighted in this Account are iminium catalysis, the tail-to-head terpene cyclization, and the carbonyl-olefin metathesis. In the case of proline-mediated iminium catalysis, we were able to demonstrate that the enantioselectivity for the product formation was increased when the reaction was performed inside the cavity of capsule I. This is remarkable since the capsule is formed from achiral building blocks and, therefore, does not add chiral information to the reaction mixture. The tail-to-head terpene cyclization is the most complex reaction performed so far inside capsule I. The cyclic monoterpenes eucalyptol and α-terpinene were formed in useful yields. Interestingly, these products have not yet been synthetically accessible in solution directly from acyclic terpene precursors. Furthermore, we demonstrated that the cocatalytic system of capsule I and HCl is suitable for carbonyl-olefin metathesis. HCl was shown to be an inefficient catalyst for this reaction in solution experiments. This demonstrates that the different chemical environment inside the supramolecular container can lead to altered product selectivity. In general, we hope to demonstrate in this Account that research on catalysis inside supramolecular capsules, although still in its infancy, is starting to produce the first synthetically relevant results.