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

Enantiodiscrimination of Inherently Chiral Thiacalixarenes by Residual Dipolar Couplings

Inherently chiral compounds, such as calixarenes, are chiral due to a nonplanar three-dimensional (3D) structure. Determining their conformation is essential to understand their properties, with nuclear magnetic resonance (NMR) spectroscopy being one applicable method. Using alignment media to measure residual dipolar couplings (RDCs) to obtain structural information is advantageous when classical NMR parameters like the nuclear Overhauser effect (NOE) or J-couplings fail. Besides providing more accurate structural information, the alignment media can induce different orientations of enantiomers. In this study, we examined the ability of polyglutamates with different side-chain moieties─poly-γ-benzyl-l-glutamate (PBLG) and poly-γ-p-biphenylmethyl-l-glutamate (PBPMLG) ─to enantiodifferentiate the inherently chiral phenoxathiin-based thiacalix[4]arenes. Both media, in combination with two solvents, allowed for enantiodiscrimination, which was, to the best of our knowledge, proved for the first time on inherently chiral compounds. Moreover, using the experimental RDCs, we investigated the calix[4]arenes conformational preferences in solution, quantitatively analyzed the differences in the alignment of the enantiomers, and discussed the pitfalls of the use of the RDC analysis.

Reactivity of phenoxathiin-based thiacalixarenes towards C-nucleophiles

A starting thiacalix[4]arene can be easily transformed into oxidized phenoxathiin-based macrocycles 9 and 9', representing an unusual structural motif in calixarene chemistry. The presence of electron-withdrawing groups (SO2, SO) and the considerable internal strain caused by the condensed heterocyclic moiety render these molecules susceptible to nucleophilic attack. The reaction with various organolithium reagents provides a number of different products resulting from the cleavage of either the calixarene skeleton or the phenoxathiin group or both ways simultaneously. This enables the preparation of thiacalixarene analogues with unusual structural features, including systems containing a biphenyl fragment as a part of the macrocyclic skeleton. The above-described transformations, unparalleled in classical calixarene chemistry, clearly demonstrate the synthetic potential of this thiacalixarene subgroup.

Thiacalix[4]arene-based metal-organic framework/reduced graphene oxide composite for electrochemical detection of chlorogenic acid

A novel metal-organic framework [Co2LCl4]·2DMF (Co-L) based on thiacalix[4]arene derivative was synthesized using the solvothermal method. Then Co-L was respectively mixed with reduced graphene oxide (RGO), multi-walled carbon nanotubes (MWCNT) and mesoporous carbon (MC) to prepare corresponding composite materials. PXRD, SEM and N2 adsorption-desorption illustrated that composite materials have been successfully prepared. After optimizing experimental conditions for detecting chlorogenic acid (CGA), the Co-L@RGO(1:1) composite material showed the optimal electrocatalytic activity for CGA, which may be because RGO possessed large specific surface area and hydroxyl and carboxyl groups that could form hydrogen-bonding with the oxide of CGA. Benefiting from the synergetic effect of Co-L and RGO, the glassy carbon electrode modified with Co-L@RGO(1:1) (Co-L@RGO(1:1)/GCE) exhibited a low limit of detection (LOD) of 7.24 nM for CGA within the concentration of 0.1-2 μM and 2-20 μM. Co-L@RGO(1:1)/GCE also showed excellent selectivity, stability, and reproducibility for the CGA detection. Co-L@RGO(1:1)/GCE could detect the CGA in honeysuckle with satisfactory results. This work provided a great example for the thiacalix[4]arene-based MOF in the application of electrochemical sensors.

Nucleophile-induced transformation of phenoxathiin-based thiacalixarenes

Oxidized phenoxathiin-based macrocycles, easily accessible thiacalix[4]arene derivatives, consist of a unique set of structural elements representing a key prerequisite for the unexpected reactivity described in this paper. As proposed, the internal strain, imposed by the presence of a heterocyclic moiety, together with a number of electron-withdrawing groups (SO₂) opens the way to the cleavage of the macrocyclic skeleton through a cascade of three S_NAr reactions triggered by the nucleophilic attack of an SH^- anion. The whole transformation, which is unparalleled in classical calixarene chemistry, leads to unique linear sulfinic acid derivatives with a rearranged phenoxathiin moiety that can serve as building blocks for macrocyclic systems of a new type.

Iterative Exponential Growth of Oxygen-Linked Aromatic Polymers Driven by Nucleophilic Aromatic Substitution Reactions

This work presents the first transition metal-free synthesis of oxygen-linked aromatic polymers by integrating iterative exponential polymer growth (IEG) with nucleophilic aromatic substitution (SNAr) reactions. Our approach applies methyl sulfones as the leaving groups, which eliminate the need for a transition metal catalyst, while also providing flexibility in functionality and configuration of the building blocks used. As indicated by 1) 1H-1H NOESY NMR spectroscopy, 2) single-crystal X-ray crystallography, and 3) density functional theory (DFT) calculations, the unimolecular polymers obtained are folded by nonclassical hydrogen bonds formed between the oxygens of the electron-rich aromatic rings and the positively polarized C-H bonds of the electron-poor pyrimidine functions. Our results not only introduce a transition metal-free synthetic methodology to access precision polymers but also demonstrate how interactions between relatively small, neutral aromatic units in the polymers can be utilized as new supramolecular interaction pairs to control the folding of precision macromolecules.

Heteroatom-bridged molecular belts as containers

Hoop-shaped or belt-like molecules have been fascinating not only due to their challenging synthesis, but also unique physical and chemical properties. The incorporation of heteroatoms (N, O, S, etc.) into these belts could alter both molecular structures and electronic properties which will lead to versatile applications, from advanced host-guest systems to functional materials. Despite numerous computational studies, the synthesis and characterization of heteroatom-bridged double-stranded molecular belts remains scarce. Here we report the synthesis, crystal structure, and host-guest chemistry of two novel heteroatom-bridged belt-like macrocycles composed of phenoxathiin. The bowl-shaped belt demonstrates a strong binding affinity (K_a = 3.6 × 10⁹ M^‒2) towards fullerene C₆₀ and forms a 2:1 capsule-like complex with the aid of C‒H···S hydrogen bonds. The column-like belt can bind the cyclic guest [2,2]paracyclophane to form a ring-in-ring complex. The modular synthesis, structural specificity, and diverse host-guest chemistry of cyclophenoxathiins markedly expands the known chemistry of molecular belts.