Synthesis of a new calix[n]pyrrole: meso-pentaspirocyclohexyl calix[5]pyrrole

We describe the first synthesis of the novel meso-pentaspirocyclohexyl calix[5]pyrrole 2b. Anion-guest properties of the new compound were evaluated with respect to fluoride, chloride, and bromide tetrabutylammonium salts by ¹H NMR titration techniques in deuterated dichloromethane at 22 °C by following the induced shifts in the NH resonances upon complexation.     

Pentapyrrolic calix[4]pyrrole

A new calix[4]pyrrole has been synthesised that contains a 3,4,5-trisbromopyrrole appended to a meso-position which shows enhanced anion affinity as compared to the parent meso-octamethylcalix[4]pyrrole macrocycle.     

Cyclo[2]carbazole[2]pyrrole: a preorganized calix[4]pyrrole analogue

A cyclo[2]carbazole[2]pyrrole (2) consisting of two carbazoles and two pyrroles has been synthesized by directly linking the carbazole 1- and 8-carbon atoms to the pyrrole α-carbon atoms. Macrocycle 2 is an extensively conjugated 16-membered macrocyclic ring that is fixed in a pseudo-1,3-alternate conformation. This provides a preorganized anion binding site consisting of two pyrrole subunits. ¹H NMR spectroscopic analysis revealed that only the two diagonally opposed pyrrole NH protons, as opposed to the carbazole protons, take part in anion binding. Nevertheless, cyclo[2]carbazole[2]pyrrole 2 binds representative anions with higher affinity in CD₂Cl₂ than calix[4]pyrrole (1), a well-studied non-conjugated tetrapyrrole macrocycle that binds anions via four pyrrolic NH hydrogen bond interactions. On the basis of computational studies, the higher chloride anion affinity of receptor 2 relative to 1 is rationalized in terms of a larger binding energy and a lower host strain energy associated with anion complexation. In the presence of excess fluoride or bicarbonate anions, compound 2 loses two pyrrolic NH protons to produce a stable dianionic macrocycle [2–2H]²⁻ displaying a quenched fluorescence.

Less is more: two NH hydrogen bond donors in a preorganized receptor provide greater anion affinity than the four NH moieties present in the classic anion receptor, calix[4]pyrrole.     

Self-assembled calix[6]pyrrole capsules: solid-state encapsulation of different guests in preorganized calix[6]pyrrole capsules

meso-hexamethyl-meso-hexaphenyl calix[6]pyrrole assembles into well-defined dimeric capsules in the crystalline state; the preorganized capsule serves as an efficient host for different organic guests as well as for solvent molecules.     

meso-Indanyl calix[4]pyrrole receptors

Two new meso-indanyl-substituted calix[4]pyrrole receptors, 2 and 3, have been synthesized. A range of calix[4]pyrrole host-neutral molecule complexes crystallise from solutions of 2 in a variety of solvents and the structures of four have been elucidated by X-ray crystallography. The F⁻ and Cl⁻ anion affinities of 2 have been measured in acetonitrile, and are significantly different from the corresponding affinities of the prototypical calix[4]pyrrole, the octamethyl-derivative, 1. ESI-FTICR-MS has been used to determine the relative F⁻ and Cl⁻ anion affinities of receptors 1 and 2 in methanol-acetonitrile solution. Deprotonation of 1 and 2 by fluoride is observed (under the conditions of the MS experiment).     

Efficient organocatalysis with a calix[4]pyrrole derivative

The 10α,20β-bis(4-nitrophenyl)-calix[4]pyrrole was found to act as an effective organocatalyst for the hetero Diels-Alder reaction of Danishefsky’s diene with aromatic aldehydes. This discovery is the first reported case of a calixpyrrole that exhibits organocatalytic activity.     

Asymmetrically substituted calix[4]pyrrole with chiral substituents

We have prepared first calix[4]pyrrole containing unprotected carbohydrate moiety directly linked to meso-position of oligopyrrole by stable "C-glycosidic" bond. Basic examination of its superassembly capability is presented.     

A calix[2]phenol[2]pyrrole and a fused pyrrolidine-containing derivative

The hybrid calix[2]phenol[2]pyrrole 4 and the fused pyrrolidine-containing macrocycle 9 were synthesized from two different isomeric starting materials, namely dimethyl 2-hydroxyisophthalate and 5-hydroxyisophthalate, respectively. The fused species 9 is devoid of obvious substrate binding properties. In contrast, the heterocalix system 4 displays the fluoride-induced conformational changes characteristic of the parent system.     

Exclusive self-assembly of a polar dimeric capsule between tetraurea calix[4]pyrrole and tetraurea calix[4]arene

The exclusive self-sorting observed in the self-assembly process between tetraurea benzyl calix[4]pyrroles and tetraurea tolyl calix[4]arene to afford unprecedented hybrid dimeric capsules with polar interiors is described. The coencapsulation of a solvent molecule with an organic guest yields four particle aggregates in which the guests are confined and restricted into single hemispheres due to the polar requirements of the internal cavity.     

Anion complexation by calix[3]thieno[1]pyrrole: the medium effect

The interaction of calix[3]thieno[1]pyrrole, 1, and halide and dihydrogen phosphate anions in a variety of solvents (acetonitrile, propylene carbonate, N,N-dimethylformamide, and dimethyl sulfoxide) has been investigated through 1H NMR, conductance measurements, and titration calorimetry. 1H NMR measurements reveal the sites of interaction of the ligand with the anions in CD3CN while the composition of the complex was determined through conductance measurements. A quantitative assessment of anion-ligand interactions is provided. Thus the thermodynamics of complexation of 1 with halide and dihydrogen phosphate anions in dipolar aprotic media at 298.15 K is reported. These data are interpreted in terms of the thermodynamics of transfer of reactants and product from a reference solvent (acetonitrile) to other solvents. The crucial role played by the solvent on the ability of the ligand to interact with anions and on the composition of the complex is demonstrated.     

N-Tosylpyrrolidine calix[4]pyrrole: synthesis and ion binding studies

The synthesis and preliminary solution phase ion binding properties of the N-tosylpyrrolidine calix[4]pyrrole 2 are reported. This β-octaalkyl-substituted calix[4]pyrrole, the first to be prepared via a direct condensation reaction, was obtained by reacting the 3,4-alkyl-functionalized pyrrole 8 with acetone in the presence of an acid catalyst. On the basis of (1)H NMR spectroscopic analyses and isothermal titration calorimetry, it was concluded that, compared with the parent, β-unsubstituted calix[4]pyrrole (1), compound 2 possesses significantly enhanced binding ability for halide anions in chloroform. Furthermore, 2 proved capable of solubilizing in chloroform solution the otherwise insoluble salts, CsF and CsCl. These effects are ascribed to the interactions between the four tosyl groups present in 2 and the counter cations of the halide anion salts.     

Squaramide-substituted calix[4]pyrrole: Synthesis and ion binding studies

Squaramide-substituted calix[4]pyrrole 2 was synthesized and its anion-binding properties were investigated. On the basis of UV spectroscopy, ¹H NMR spectroscopic and computational DFT model analyses, compared with the parent calix[4]pyrrole 1, compound 2 shows significantly enhanced binding affinities for anions in CH₂Cl₂, and it was concluded that this increase in affinity is ascribed to participation of the squaramide unit in anion binding.     

4,4,9,9-Tetraphenyl pyrroloindolizine: a structural analogue of calix[2]pyrrole

Synthesis, spectral and structural characterization of a pyrroloindolizine derivative having structural similarity with calix[2]pyrrole is described. Here, two pyrrole rings are connected with two meso-carbon atoms having an N,α-linkage and an α,β-linkage to afford the smallest analogue in the calixpyrrole family. Detailed NMR spectroscopic studies along with single crystal X-ray analysis confirm the assigned structure of the molecule.     

Synthesis of new calix[4]pyrrole derivatives via 1,3-dipolar cycloadditions

Octamethylcalix[4]pyrrole-2-carbaldehyde 1 and 3-(octamethylcalix[4]pyrrol-2-yl)propenal 5 were used as precursors of azomethine ylides, which were trapped in situ with a range of dipolarophiles, such as 1,4-benzoquinone, 1,4-naphthoquinone, and fumaronitrile. Aldehyde 1 showed very low reactivity but the azomethine ylide generated from the reaction of aldehyde 5 with N-methylglycine could be trapped with those dipolarophiles to afford new β-substituted octamethylcalix[4]pyrrole derivatives in moderate yields. The resulting cycloadducts show high affinity constants for fluoride and acetate anions; compounds 7 and 8 display sharp changes in color in the presence of these anions.     

Synthesis and Properties of Functionalized Schiff Bases of 5α,10α-Di（4-hydroxylphenyl）calix[4]pyrrole and 5α,15β-Di（4-hydroxylphenyl）calix｜4] pyrrole

A series of calix[4]pyrrole meso-substituted Schiff bases was synthesized with 5α,10α-di（4- hydroxylphenyl）calix[4]pyrrole or 5α,15β-di（4-hydroxylphenyl）calix[4]pyrrole as starting materials. The synthetic routes included alkylation with methyl a-chlroroaceate, ammonolysis with alkylene diamine, and condensation with salieylladehyde or 2-hydroxynaphthaldehyde. The crystal structures of the new calix[4]pyrroles and their Schiff bases were determined by X-ray diffraction. The coordination properties of the representative ealix[4]pyrrole Sehiff bases to transition metal ions were also investigated by UV-Vis spectra.     

Fullerene encapsulation with calix[5]arenes

This paper presents the synthesis of the fullerene hosts based on the calix[5]arenes and their binding properties. Calix[5]arenes 1a, 2, 3a bind C₆₀ or C₇₀ in organic solvents. The solvent effect of the fullerene complexation was clearly observed; the association constant decreases in a solvent with high solubility for C₆₀. Covalently linked double-calix[5]arenes 4-6 were also investigated on their binding properties for fullerenes in organic solvents. Their binding abilities for both C₆₀ and C₇₀ are extremely high in toluene solution. Higher binding selectivity toward C₇₀ is observed by all the double-calix[5]arenes. The selectivity of 5a toward C₇₀/C₆₀ is highest in toluene with a value of 10. The structures of the supramolecular complexes of the calix[5]arene hosts and C₆₀ or C₇₀ were investigated by using ¹H and ¹³C NMR studies. The molecular mechanics calculation and X-ray structure reveal that the interior of the calix[5]arene is complementary to the exterior of C₆₀ molecule. In contrast, the host-guest complexes of C₇₀ with the simple calix[5]arenes take many conformational options due to its less symmetric shape. The molecular mechanics calculation and our chemical shift simulation nicely worked to estimate the reliable structures; the calix[5]arene cavity takes up C₇₀ molecule, and the C₇₀ molecule tilts significantly from the C5 axis of the calix[5]arene. In the case of the host-guest complex of C₇₀ with the double-calix[5]arene, the molecular dynamics simulation of the host-guest complex represented the realistic movement of the bound C₇₀ inside the cavity. The combination of the molecular dynamics simulation and the chemical shift simulation of the host-guest complex suggested that the C₇₀ molecule rapidly moves inside the cavity.     

Double-cavity calix[4]pyrrole derivative with enhanced capacity for the fluoride anion

A double-cavity calix[4]pyrrole derivative, meso-tetramethyl-tetra[N-(2-phenoxyethyl)-N'-phenylurea]calix[4]pyrrole, 1, with enhanced hosting ability for the fluoride anion has been designed and characterized. Its interaction with anions (fluoride, chloride, bromide, iodide, dihydrogen phosphate, hydrogen sulfate, perchlorate, nitrate, and trifluoromethane sulfonate) was qualitatively and quantitatively assessed through 1H NMR, conductance, and calorimetric studies. The outcome of these investigations demonstrates that 1 interacts only with fluoride and dihydrogen phosphate anions in dipolar aprotic media. However, the composition of these complexes differs in that two units of fluoride are taken per unit of 1, while a 1:1 anion/ligand complex is formed with the dihydrogen phosphate anion. Results from the 1H NMR studies are striking in that these not only provide information about the active sites of the ligand-anion interaction but also allow the establishment of the sequence of events taking place during fluoride complexation. Thus, hydrogen-bond formation between the pyrrolic hydrogen and the fluoride anion is followed by the uptake of a second anion through the same type of interaction, but with the phenyl urea. It is also the latter group that is responsible for the interaction of 1 with the dihydrogen phosphate anion. Finally, this paper illustrates the importance of structural information for the interpretation of the thermodynamics associated with these systems.     

Lithiation of meso-octamethylcalix[4]pyrrole: a general route to C-Rim monosubstituted calix[4]pyrroles

Lithiation and subsequent addition of an electrophile to meso-octamethylcalix[4]pyrrole provides a straightforward synthetic route to new, C-rim monosubstituted calix[4]pyrroles. A variety of electrophiles were used, resulting in calix[4]pyrroles with appended functional groups including carboxyl, ester, iodo, and formyl. This method was optimized to give maximum yields of the monosubstituted derivatives with lowest possible contamination by di- and trisubstituted congeners. Solid-state studies, performed for a number of these derivatives, showed unexpected supramolecular interactions involving both solvents and the monosubstituted calix[4]pyrrole derivatives themselves.     

Surprises in the energetics of host-guest anion binding to calix[4]pyrrole

Contrary to common expectation, calorimetric measurements do not corroborate the preference of calix[4]pyrrole for fluoride over chloride in acetonitrile solution.