Arene Core Research Articles

Circularly polarized luminescence from a common alkoxy pillar[5]arene and its co‐aggregates with π‐conjugated rods

AbstractCylinder‐shaped macrocycles composed of π‐panels have attracted special attention as one of the best platforms for the development of organic molecule‐based chiroptical materials. Pillar[n]arenes are a class of macrocycles with the advantage of easy preparation but have not been extensively investigated from the perspective of luminescent molecules. However, common alkoxy pillar[n]arenes are fluorescent in non‐haloalkane solvents, showing potential to be used for molecule‐based chiroptical materials. In this work, circularly polarized luminescence (CPL) spectra are reported for a pillar[5]arene with stable planar chirality using tetrahydrofuran (THF) and cyclohexane as solvents, which has been missing for many years. The pillar[5]arene also forms co‐aggregates with 1,4‐bis(phenylethynyl)benzene and 1,4‐bis[(pentafluorophenyl)ethynyl]benzene in THF/H2O mixtures, owing to a hydrophobic effect. The co‐aggregates with the fluorinated π‐rod display a new low‐energy absorption peak and broad emission band as well as intense circular dichroism and CPL signals. Chiral information from the enantiopure pillar[5]arene core is efficiently transmitted to the co‐aggregates with the π‐conjugated rod, leading to the highest dissymmetry factor for CPL (2.9 × 10−2 at 472 nm) among pillar[n]arene‐based CPL materials.

Epichlorohydrin-based CuAAC dendrimers with a calix[4]arene core and polar hydroxyl/oxyethyl terminal groups: synthesis, aggregation and use in catalysis

1st and 2nd generation amphiphilic dendrimers were obtained. Spherical palladium nanodendrites stabilized by amphiphilic dendrimers show excellent catalytic activities in coupling and reduction of aromatics in water.

Homotrinuclear ruthenium(ii) and rhodium(i) complexes of redox-active tris(ferrocenyl)arene-based tris-phosphanes.

Homotrinuclear complexes of the C 3-symmetric tris(ferrocenyl)arene-based tris-phosphanes 1a-d with ruthenium(ii) ([1a-d(Ru)3]) and rhodium(i) ([1a-d(Rh)3]) were prepared and fully characterised. Complexes [1a-d(Ru)3] and [1a-d(Rh)3] are electrochemically active. The nature of the arene core in 1a-d ranging from benzene, 1,3,5-trifluorobenzene and mesitylene to s-triazine allows to fine-tune the exact oxidation potentials for tailoring the electrochemical response. With a BArF 4 --based supporting electrolyte, a distinct separation of the three iron-centred oxidations of the ligand backbone is observable. Under these conditions, these oxidations are mostly reversible but, especially for the third oxidation, already show signs of irreversibility. In general, while the coordinated metal complex fragment does not strongly alter the electrochemical response of the arene-trisferrocenyl core 1a-d, there are observable differences. Rhodium(i) complexes are oxidised at slightly higher potentials than ruthenium(ii) complexes. In both cases, individual oxidation states for the C6H3(CH2)3-based ligand (1d) are difficult to address and the C3N3-based ligand (1c) shows the most complicated and least reversible electrochemistry with severely broadened third oxidations and reduced reversibility in cyclic voltammetry. The most well-suited system for potential applications in redox-switchable catalysis, in all cases, is the C6H3-based ligand (1a), showing entirely reversible and well-separated redox events.

Sterically Selective [3 + 3] Cycloaromatization in the On-Surface Synthesis of Nanographenes.

Surface-catalyzed reactions have been used to synthesize carbon nanomaterials with atomically predefined structures. The recent discovery of a gold surface-catalyzed [3 + 3] cycloaromatization of isopropyl substituted arenes has enabled the on-surface synthesis of arylene-phenylene copolymers, where the surface activates the isopropyl substituents to form phenylene rings by intermolecular coupling. However, the resulting polymers suffered from undesired cross-linking when more than two molecules reacted at a single site. Here we show that such cross-links can be prevented through steric protection by attaching the isopropyl groups to larger arene cores. Upon thermal activation of isopropyl-substituted 8,9-dioxa-8a-borabenzo[fg]tetracene on Au(111), cycloaromatization is observed to occur exclusively between the two molecules. The cycloaromatization intermediate formed by the covalent linking of two molecules is prevented from reacting with further molecules by the wide benzotetracene core, resulting in highly selective one-to-one coupling. Our findings extend the versatility of the [3 + 3] cycloaromatization of isopropyl substituents and point toward steric protection as a powerful concept for suppressing competing reaction pathways in on-surface synthesis.

Building Triazolated Macrocycles from Bis-Propargylated Calix[4]arenes and Bis-Azidomethylated Azobenzene or Stilbene

Copper(I)-catalyzed azide-alkyne cycloaddition was employed to construct biscalixarene assemblies from the calix[4]arene dipropargyl ethers and 4,4′-bis-azidomethylated azobenzene or stilbene. Three bis(calixarenes) having the calix[4]arene cores linked to each other by pairs of (E)-azobenzene/stilbene units through four triazole groups were obtained as confirmed by NMR, HRMS and X-ray diffraction data. Nevertheless, the formation of larger macrocycles and polymeric/oligomeric products was found to be the major competing process that seriously limited the applicability of the one-step macrocyclization approach for the construction of photoresponsive biscalixarene assemblies linked by pairs of azobenzene/stilbene units.

Unimolecular Chiral Stepping Inversion Machine.

Intelligent molecular machines that are driven by light, electricity, and temperature have attracted considerable interest in the fields of chemistry, materials, and biology. Herein, a unimolecular chiral stepping inversion molecular machine (SIMM) was constructed by a coupling reaction between dibromo pillar[5]arene and a tetrathiafulvalene (TTF) derivative (PT3 and PT5). Compared with the longer aliphatic linker PT5, PT3 with a shorter aliphatic linker shows chiral stepping inversion, achieving chiral inversion under a two-electron redox potential. Benefiting from the successive reversible two-electron redox potential of TTF, the self-exclusion and self-inclusion conformational transformations of SIMM can proceed in two steps under redox, leading to the chirality step inversion in the pillar[5]arene core. Electrochemical experiments and circular dichroism (CD) spectra show that the redox processes can cause SIMM CD signaling to reversibly switch. More importantly, as the oxidant Fe(ClO4)3 was increased from 0.1 to 1 equiv, the CD spectral signal of SIMM disappeared at 1 equiv, and further addition of Fe(ClO4)3 resulted in the CD signal reversed from positive to negative at 309 nm, indicating that the chirality was reversed after chemical oxidation and reached a negative maximum with the addition of 2 equiv Fe(ClO4)3; thus, redox-triggered chiral stepping inversion was achieved. Furthermore, the chiral inversion can be restored to its original state after the addition of 2 equiv of reducing agent, sodium ascorbate. This work demonstrates unimolecular chiral stepping inversion, providing a new perspective on stimulus-responsive chirality in molecular machines.

Ionic Liquid Crystalline Calixarene with Photo‐Switchable Proton Conduction

AbstractWe have developed a new strategy for the preparation of a light‐responsive ionic liquid crystal (LC) that shows photo‐switchable proton conduction. The ionic LC consists of a bowl‐shaped calix[4]arene core ionically functionalized with azobenzene moieties. The non‐covalent architectures were obtained by the formation of ionic salts between the carboxylic acid group of an azo‐derivative and the terminal amine groups of a calixarene core. The presence of ionic salts results in a hierarchical self‐assembly process that extends to the formation of a nanostructured lamellar LC arrangement (smectic A phase). In this LC phase, the ionic LC calixarene is able to display proton conductive properties, since the ionic nanosegregated areas (formed by the ionic pairs) generate the continuous channels that favor proton transport. The optical and photo‐responsive properties were studied by UV‐Vis spectroscopy, demonstrating that the azobenzene moieties of the ionic LC undergo reversible (E)‐to‐(Z) isomerization by irradiation with UV light. Interestingly, this (E)‐to‐(Z) photoisomerization results in a decrease of the proton conductivity values since the bent‐shaped (Z)‐isomer disrupts the lamellar LC phase. This isomerization process is totally reversible and leads to an ionic LC material with unique photo‐switchable proton conductive properties.

Gallic acid-based dendrimers with a thiacalix[4]arene core: synthesis, aggregation and use for stabilization of Pd nanoparticles

First-generation gallic acid-based “click” dendrimers with a thiacalix[4]arene core, including a symmetric dendrimer with four dendrons and amphiphilic dendrimers with alkyl lipoiphilic groups, were synthesized using convergent approach.

A review of nonfullerene solar cells: Insight into the correlation among molecular structure, morphology, and device performance

AbstractNonfullerene acceptors (NFAs) lead the continuous development of organic solar cells (OSCs) with competitive efficiency over 19%. Design and synthesis of novel photovoltaic materials are effective methods to improve the OSCs performance, which can regulate the optoelectric properties, such as energy level, absorption spectra, charge transport, and so on. So far, hundreds of NFAs have been reported. Meanwhile, it has been demonstrated that intrinsic morphology of active layer is partially determined by the chemical structures of NFAs. Hence, only in‐depth understanding of the relationship between different structures of NFAs and morphology can guide the molecular design of NFAs for highly efficient OSCs. Herein, we review some state‐of‐the‐art NFAs according to their functional moieties, that is, arene core, end group and side chain, and discuss the relationship between molecular structure, morphology and device parameter. Additionally, the challenges and prospects for further development of OSCs based on NFAs are briefly considered. This review brings a unique insight into structure–function correlation in this field, which may help to rapidly develop efficient OSCs.

PAMAM-Calix-Dendrimers: Second Generation Synthesis, Fluorescent Properties and Catecholamines Binding.

A convenient method for the synthesis of the second generation of PAMAM dendrimers based on a p-tert-butylthiacalix[4]arene core in cone, partial cone and 1,3-alternate conformations was developed. Unusual fluorescence of the obtained PAMAM-calix-dendrimers has been found and explained. The binding ability of the synthesized dendrimers toward catecholamines (dopamine, L-adrenaline and L-noradrenaline) was shown by UV-Vis, fluorescence, 1D and 2D NMR spectroscopy and the binding constants (logKa 3.85-4.74) calculated. As was shown, the PAMAM-calix-dendrimers bind catecholamines by the internal cavities. All the studied hormones were most efficiently bound by the dendrimers bearing a macrocyclic core in 1,3-alternate conformation. The size of the formed supramolecular systems of dendrimer/catecholamine was established by the DLS method. A decrease in hemolytic activity of the PAMAM-calix-dendrimers with an increase in the generation number of a dendrimer was shown for the dendrimers with a core in 1,3-alternate conformation. The prospects for the use of the synthesized dendrimers with the macrocyclic core as drug delivery agents were discussed.

Arene Activation through Iminium Ions: Product Diversity from Intramolecular Photocycloaddition Reactions

AbstractWhile 2‐alk‐ω‐enyloxy‐sustituted benzaldehydes do not display any photochemical reactivity at the arene core, the respective iminium perchlorates were found to undergo efficient reactions either upon direct irradiation (λ=366 nm) or under sensitizing conditions (λ=420 nm, 2.5 mol% thioxanthen‐9‐one). Three pathways were found: (a) Most commonly, the reaction led to benzoxacyclic products in which the olefin in the tether underwent a formal, yet unprecedented carboformylation (13 examples, 44–99 % yield). The cascade process occurred with high diastereoselectivity and was found to be stereoconvergent. (b) If a substituent resides in the 3‐position of the benzene ring, a meta photocycloaddition was observed which produced tetracyclic skeletons with five stereogenic centers in excellent regio‐ and diastereoselectivity (2 examples, 58–79 % yield). (c) If the tether was internally substituted at the alkene, an arene photocycloaddition was avoided and an azetidine was formed in an aza Paternò–Büchi reaction (2 examples, 95–98 % yield).

Arene Activation through Iminium Ions: Product Diversity from Intramolecular Photocycloaddition Reactions.

While 2-alk-ω-enyloxy-sustituted benzaldehydes do not display any photochemical reactivity at the arene core, the respective iminium perchlorates were found to undergo efficient reactions either upon direct irradiation (λ=366 nm) or under sensitizing conditions (λ=420 nm, 2.5 mol% thioxanthen-9-one). Three pathways were found: (a) Most commonly, the reaction led to benzoxacyclic products in which the olefin in the tether underwent a formal, yet unprecedented carboformylation (13 examples, 44-99 % yield). The cascade process occurred with high diastereoselectivity and was found to be stereoconvergent. (b) If a substituent resides in the 3-position of the benzene ring, a meta photocycloaddition was observed which produced tetracyclic skeletons with five stereogenic centers in excellent regio- and diastereoselectivity (2 examples, 58-79 % yield). (c) If the tether was internally substituted at the alkene, an arene photocycloaddition was avoided and an azetidine was formed in an aza Paternò-Büchi reaction (2 examples, 95-98 % yield).

Cu-Catalyzed Cyclization/Coupling of Alkenyl Aldimines with Arylzinc Reagents: Access to Indole-3-diarylmethanes.

We report a Cu(II)-catalyzed cyclization/coupling of alkenyl aldimines with arylzinc reagents to create indole-3-diarylmethane derivatives (Sapkota et al. ChemRxiv 2022, DOI: 10.26434/chemrxiv-2022-d6qn). The current reaction provides a unified modular route from readily available starting materials to indole-3-diarylmethanes in which all three arene cores can be decorated with differential functional substitutions on demand. Since the cyclization/coupling of alkenyl aldimines is unknown to date, the current method widens the scope with regard to both the substrate and product diversity for this class of reaction.

Recent Progress in the Synthesis of Drugs and Bioactive Molecules Incorporating Nitro(het)arene Core.

Aromatic nitro compounds play a unique role in the synthesis of drugs and pharmaceutically oriented molecules. This field of organic chemistry continues to be in demand and relevant. A significant number of papers are published annually on new general methods for the synthesis of nitrodrugs and related biomolecules. This review is an analysis of the literature on methods for the synthesis of both new and already-known aromatic and heteroaromatic nitrodrugs covering the period from 2010 to the present.

A Novel Grafting-Onto Approach to the Facile Synthesis of Star-Shaped Poly(2-Oxazoline)S with Calix[4]Arene Core as Curcumin Nanocarriers

A Novel Grafting-Onto Approach to the Facile Synthesis of Star-Shaped Poly(2-Oxazoline)S with Calix[4]Arene Core as Curcumin Nanocarriers

Synthesis and Conformational Characteristics of Calix[4]arene Oligomers

Abstract Lower- and/or upper-rim functionalization of calix[4]arene realized a variety of calix[4]arene systems. Compared to these monomeric calix[4]arenes, covalently linked calix[4]arene oligomers have not been studied well. Calix[4]arene oligomers can be utilized as building blocks of supramolecular complexes as well as for the synthesis of calix[4]arene polymers. This background motivated us to develop synthetic procedures for calix[4]arene oligomers and to conduct conformational analysis of these oligomers. We produce oligomers ranging from the monomer to the pentamer. The coupling of the pentamer with 2,3-dibenzyloxy-1,4-benzenedicarboxylic acid can access a decamer and oligomers. NMR measurements, X-ray crystal structure analysis, and computational studies demonstrate that calix[4]arene oligomers can regulate their length by changing the conformations of the calix[4]arene cores.

Reversible "On/Off" Chiral Amplification of Pillar[5]arene Assemblies by Dual External Stimuli.

We report dual-stimuli, thermo- and photostimuli, responsive chiral assemblies, of planar-chiral pillar[5]arenes with azobenzene groups on their rims. The azobenzene-substituted planar-chiral pillar[5]arenes were synthesized by copper(I)-catalyzed alkyne-azide cycloaddition "click" reaction of azide-substituted planar-chiral pillar[5]arenes containing S or R stereogenic carbon atoms with an alkyne-substituted azobenzene. These decaazides with stereogenic carbons could act as starting points for a large library of planar-chiral pillar[5]arenes. Homeotropic alignment of azobenzenes, caused by the mesogenic property of the azobenzene groups, was induced by annealing a film of the azobenzene-substituted planar-chiral pillar[5]arenes. The alignment resulted in chiral propagation from the planar-chiral pillar[5]arene cores to the azobenzene area and caused significant chiral amplification consequently. These aligned chiral assemblies were collapsed by trans to cis photoisomerization of the azobenzene groups, resulting in chiral amplification off, and reconstructed by cis to trans thermo-isomerization, again turning on the chiral amplification.

Novel Amphiphilic Star-Shaped Poly(2-Oxazoline)s with Calix[4]Arene Branching Center

Novel amphiphlic four-arm star-shaped poly (2-alkyl-2-oxazoline) s with calix [4] arene core were synthesized using the “grafting from” approach. The chlorosulfonated calix [4] arene derivative was synthesized and successfully applied as a multifunctional initiator for the cationic ring-opening polymerization of 2-alkyl-2-oxazolines. Obtained star-shaped poly (2-alkyl-2-oxazoline) s were characterized by means of NMR, UV-Vis spectroscopy and gel-permeation chromatography. It was shown that star-shaped poly (2-isopropyl-2-oxazoline) perform thermosensitivity in aqueous solutions.

Conversion of 5-methyl-4H-benzo[d][1,3]dioxin-4-one derivatives into functionalized 8-hydroxyisochroman-1-one under basic conditions

Syntheses of 8-hydroxyisochroman-1-one derivatives are described, promoted via a base-induced anionic accelerated rearrangement reaction from dimethyl dioxinone protected resorcylates. A biomimetic polyketide aromatization strategy via selective C-acylation of a β-keto ester, Pd(0)-catalyzed allylic migration and base-promoted cyclization and aromatization furnished the arene core of the resorcylate precursors.

Closing a Calix[6]arene-Based Funnel Zn2+ Complex at Its Large Rim Entrance: Consequences on Metal Ion Affinity and Host-Guest Properties.

A molecular capsule based on a calix[6]arene core closed at the small rim by a three-point coordinated metal ion and at the large rim by a three-point covalent capping is described. It is derived from a trisimidazole funnel complex capped by a trenamide unit that prevents in/out exchange of guest molecules through the large rim. A detailed comparative study with three different calixarenes provides a unique opportunity for (i) comparing the binding ability of two different coordination sites in well three-dimensional (3D)-structured macrocyclic receptors and (ii) evaluating the impact of a covalent closing of one rim of a funnel receptor while the other rim is closed by weaker coordination bonds. Indeed, this study allowed for highlighting various interesting new features. It is first shown that the trenamide site can bind a metal ion such as Zn2+ by itself. This involves a 1:1 coordination of the metal ion to the three carbonyl groups of the amide functions, which undergo trans-to-cis isomerization and are partially embedded in the calix core. When the trisimidazole core is present, the Zn2+ ion preferentially binds at the small rim, thus closing the cavity. Guest ligand exchange must then occur through a decoordination/recoordination process of the metal ion. The modification and rigidification of the calixarene conformation induced by the large rim capping strengthen the metal ion coordination at the small rim. This also leads to a selective metallo-receptor that readily binds EtNH2 under conditions where PrNH2 is not recognized at all. The increased rigidity of the receptor, however, weakens the host-guest interactions, precluding important induced-fit behaviors that are at work in the parent, large rim opened, funnel complex.