S-Alkylation of Thiacalixarenes: How the Regio- and Stereoselectivities Depend on the Starting Conformation

Synthesis of Thiapillar[6]arenes Bearing Redox-Active (Hydro)quinone Groups. Electrochemical and XRD Study

Pillar[n]arenes are among the newest members of the macrocyclic family. Nevertheless, their conformational behavior and binding properties as well as redox properties of dealkylated pillar[n]arenes are well-studied. At the same time, introducing a heteroatom into a cyclophane macrocycle is already known to alter all the above properties drastically. This study presents a simple synthetic approach based on thia-Michael addition cyclization that readily resulted into hexathiapillar[6]arene with four phenylene units alternated by two redox-active hydroquinone moieties. The straightforward synthesis of the macrocycle enabled a systematic study of its conformation and redox behavior. The modification of hexathiapillar[6]arene afforded five functionalized derivatives, which were studied structurally in detail. The findings revealed interesting redox and structural properties of the macrocycle and its derivatives including the formation of crystal lattices with continuous channels and empty voids.

Synthesis, Structure, and Properties of Unsaturated Thiacrown Ethers Possessing Sulfonium Groups

Thiacrown ethers have attracted much attention in host-guest chemistry. Herein, we present the polymethylation reactions of unsaturated thiacrown ethers. Unsaturated thiacrown ethers having sulfonium groups were synthesized by the reaction of corresponding unsaturated thiacrown ethers with methyl triflate. The obtained methylated unsaturated thiacrown ethers were characterized by single-crystal X-ray diffraction analysis, UV absorption measurement, and cyclic voltammetry. The occupied and unoccupied molecular orbitals of the obtained methylated unsaturated thiacrown ethers are essentially localized respectively around the sulfide groups and the sulfonium groups. Furthermore, the results of electrostatic potential maps indicated that positive charges were distributed throughout the entire unsaturated thiacrown ether moieties.

Chemistry of 2,14-Dithiacalix[4]arene: Searching for the Missing Fifth Conformer

Contrary to theoretical predictions, direct alkylation of 2,14-dithiacalix[4]arene provides only four out of five basic conformers (atropisomers). To prepare the missing 1,2 (C)-alternate conformer, the indirect alkylation strategy was applied using 1,3-dichloro-1,1,3,3-tetraisopropyldisiloxane as a protective agent. As proved by the combination of NMR and X-ray crystallography, the position of the disiloxane bridge on the macrocycle is not fixed and can be changed under basic conditions, representing thus so far unknown rearrangement of the siloxane moiety. The subsequent dialkylation/deprotection and dialkylation enabled the synthesis of the last missing conformer. As demonstrated by several examples, the mixed-bridge macrocycle (with both CH₂ and S bridging units) enables preparation of unusual conformers or substitution patterns, which are difficult to obtain, if at all, in classical calixarene chemistry. This feature makes 2,14-dithiacalix[4]arene a very promising candidate for the role of molecular scaffold or platform in various supramolecular applications.

Allosteric binding properties of a 1,3-alternate thiacalix[4]arene-based receptor having phenylthiourea and 2-pyridylmethyl moieties on opposite faces

Thiacalix[4]arene receptors with two thiourea moieties and two 2-pyridyl moieties exhibited a heteroditopic dinuclear receptor with F− and Ag+ ions by a positive allosteric effect.

Regioselective SNAr reaction of the phenoxathiin-based thiacalixarene as a route to a novel macrocyclic skeleton

The formation of a novel type of macrocyclic skeleton possessing meta- and para-bridging units can be achieved by S_NAr cleavage of an easily accessible phenoxathiin-based thiacalix[4]arene.

Inherent chirality through a simple dialkylation of 2,14-dithiacalix[4]arene

Dialkylation of 2,14-dithiacalix[4]arene offers the most straightforward way to inherently chiral calixarene-based macrocycles.

Chemoselective oxidation of phenoxathiin-based thiacalix[4]arene and the stereoselective alkylation of products

Phenoxathiin-based thiacalix[4]arene was subjected to exhaustive oxidation. Depending on the reaction conditions, the macrocyclic systems bearing four sulfonyl groups or three sulfonyl and one sulfoxide group are available in high yields.

Selective O-Alkylation of the Crown Conformer of Tetra(4-hydroxyphenyl)calix[4]resorcinarene to the Corresponding Tetraalkyl Ether

Reactions of glycidyl methacrylate with the crown and chair conformers of tetra(4-hydroxyphenyl)calix[4]resorcinarene were studied. The reactions were done over epoxide groups present in the ester, which can easily undergo an opening reaction with hydroxyl groups in the macrocyclic system. Initially, epoxidation reactions were carried out with pure conformers, and it was observed that the reaction between tetra(4-hydroxyphenyl)calix[4]resorcinarene fixed in the chair conformation does not occur, while for the molecule fixed in the crown conformation only one tetraalkylated derivative was obtained. The obtained product was characterized using IR, ¹H-NMR, ¹³C-NMR, COSY, HMQC and HMBC techniques. An exhaustive NMR study showed that the reaction is selective at the hydroxyl groups in the lower rim, without affecting the hydroxyl groups in the upper rim. In addition, the RP-HPLC analysis of the epoxidation reaction mixture, using both crown and chair conformers, showed that only the crown conformer reacted under tested conditions. Finally, a comparative study of the reactivity of tetranonylcalix[4]resorcinarene with glycidyl methacrylate showed that the reaction does not take place. Instead, the formation of the tetranonylcalix[4]resorcinarene tetrasodium salt was observed, which confirms that the hydroxyl groups in the upper rim are unreactive under these conditions.

Unexpected cleavage of thiacalix[4]arene sulfoxides

Thiacalix[4]arene monosulfoxides possess an unprecedented reactivity towards n-BuLi to form unusual linear structures.

A study of anion binding behaviour of 1,3-alternate thiacalix[4]arene-based receptors bearing urea moieties

Thiacalix[4]arene receptors with two urea moieties exhibited a high affinity towards all of the selected anions.

Arylation of thiacalix[4]arenes using organomercurial intermediates

Direct mercuration of thiacalix[4]arenes immobilized in the cone or 1,3-alternate conformations followed by Pd-catalyzed arylation was applied to achieve either meta- or para-substitution of basic skeletons.

‘Honeycomb’ nanotube assembly based on thiacalix[4]arene derivatives by weak interactions

Crystallisation of six thiacalix[4]arene derivatives from hexane–chloroform leads to ‘honeycomb’ nanotube architectures and each tubular stack is surrounded by six close tubular neighbours via weak interactions, such as S⋯π interactions, C–H⋯π interactions, and so on.

Positive and negative allosteric effects of thiacalix[4]arene-based receptors having urea and crown-ether moieties

Allosteric binding behaviors of heteroditopic receptors bearing the 1,3-alternate conformation based thiacalix[4]arene have been evaluated by absorption and ¹H NMR titration experiments.

Application of RDC enhanced NMR spectroscopy in structural analysis of thiacalix[4]arene derivatives

The conformational behaviour of novel phenoxathiin-based skeletons was studied using the Residual Dipolar Coupling constant (RDC) method.