超分子凝胶在催化有机反应中的应用

超分子凝胶是有机小分子通过分子间非共价作用形成的使溶剂固定的三维网络结构胶体。 综述关注了近年来超分子凝胶在催化有机反应中一个新的应用方向,依据凝胶剂结构特征和催化反应的类型对迄今报道的小分子凝胶催化剂进行了深入的总结与归类,揭示凝胶催化的独特性,并对其存在的问题和发展趋势进行了讨论。     

Recent Advances in Supramolecular Gels and Catalysis

Over the past two decades, supramolecular gels have attracted significant attention from scientists in diverse research fields and have been extensively developed. This review mainly focuses on the significant achievements in supramolecular gels and catalysis. First, by incorporating diverse catalytic sites and active organic functional groups into gelator molecules, supramolecular gels have been considered as a novel matrix for catalysis. In addition, these rationally designed supramolecular gels also provide a variety of templates to access metal nanocomposites, which may function as catalysts and exhibit high activity in diverse catalytic transformations. Finally, as a new kind of biomaterial, supramolecular gels formed in situ by self‐assembly triggered by catalytic transformations are also covered herein.     

Emergent Catalytic Behavior of Self‐Assembled Low Molecular Weight Peptide‐Based Aggregates and Hydrogels

We report a series of short peptides possessing the sequence (FE)_n or (EF)_n and bearing l ‐proline at their N‐terminus that self‐assemble into high aspect ratio aggregates and hydrogels. We show that these aggregates are able to catalyze the aldol reaction, whereas non‐aggregated analogues are catalytically inactive. We have undertaken an analysis of the results, considering the accessibility of catalytic sites, pK_a value shifts, and the presence of hydrophobic pockets. We conclude that the presence of hydrophobic regions is indeed relevant for substrate solubilization, but that the active site accessibility is the key factor for the observed differences in reaction rates. The results presented here provide an example of the emergence of a new chemical property caused by self‐assembly, and support the relevant role played by self‐assembled peptides in prebiotic scenarios. In this sense, the reported systems can be seen as primitive aldolase I mimics, and have been successfully tested for the synthesis of simple carbohydrate precursors.     

2‐(1 H‐1,2,3‐Triazol‐4‐yl)‐Pyridine Ligands as Alternatives to 2,2′‐Bipyridines in Ruthenium(II) Complexes

The synthesis of a variety of 2‐(1H‐1,2,3‐triazol‐4‐yl)‐pyridines by click chemistry is demonstrated to provide straightforward access to mono‐functionalized ligands. The ring‐opening polymerization of ε‐caprolactone initiated by such a mono‐functionalized ligand highlights the synthetic potential of this class of bidentate ligands with respect to polymer chemistry or the attachment onto surfaces and nanoparticles. The coordination to Ru^II ions results in homoleptic and heteroleptic complexes with the resultant photophysical and electrochemical properties strongly dependent on the number of these ligands attached to the Ru^II core.     

A Carborane‐Derivative “Click” Reaction under Heterogeneous Conditions for the Synthesis of a Promising Lipophilic MRI/GdBNCT Agent

In this study, the Huisgen reaction has been used to functionalise a carborane cage with a lipophilic moiety and a 1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid (DOTA) ligand to obtain a new Gd boron neutron‐capture therapy (BNCT)/magnetic resonance imaging (MRI) agent. The introduction of the triazole units has been accomplished under both heterogeneous conditions, by the use of a Cu‐supported ionic‐liquid catalyst, and homogeneous conditions. The ability of the Gd complex of the synthesised ligand to form stable adducts with low‐density lipoproteins (LDLs) has been evaluated and then MRI has been performed on tumour melanoma cells incubated in the presence of a Gd‐complex/LDL imaging probe. It has been concluded that the high amount of intracellular boron necessary to perform BNCT can be reached even in the presence of a relatively low‐boron‐containing LDL concentration.     

Construction of Alkynylplatinum(II) Bzimpy‐Functionalized Metallacycles and Their Hierarchical Self‐Assembly Behavior in Solution Promoted by Pt⋅⋅⋅Pt and π–π Interactions

A family of new alkynylplatinum(II) 2,6‐bis(benzimidazol‐2′‐yl)pyridine (bzimpy)‐functionalized supramolecular metallacycles with different shapes and sizes have been successfully prepared by coordination‐driven self‐assembly. The obtained metallacycles showed switchable emission and a strong tendency to form intermolecular Pt???Pt and π–π stacking interactions in solution that were not displayed by their individual precursors. Further investigation revealed that the existence of the metallacyclic scaffold at the core could facilitate the formation of intermolecular Pt???Pt and π–π stacking interactions of peripheral alkynylplatinum(II) bzimpy units. Moreover, the shapes and sizes of the metallacyclic scaffold have a significant influence on the hierarchical self‐assembly behavior. Among the three metallacycles, hexagonal metallacycle A , with a relatively small size, could spontaneously self‐assemble into an aromatic guest stimuli‐responsive metallogel at room temperature without a heating–cooling process.     

Synthesis and Thermoreversible Gelation Properties of Main‐Chain Poly(pyridine‐2,6‐dicarboxamide‐triazole)s

A series of main‐chain poly(amide‐triazole)s were prepared by copper(I)‐catalyzed alkyne–azide AABB‐type copolymerizatons between five structurally similar diacetylenes 1 – 5 with the same diazide 6 . The acetylene units in monomers 1 – 5 possessed different degrees of conformational flexibility due to the different number of intramolecular hydrogen bonds built inside the monomer architecture. Our study showed that the conformational freedom of the monomer had a profound effect on the polymerization efficiency and the thermoreversible gelation properties of the resulting copolymers. Among all five diacetylene monomers, only the one, that is, 1 ‐Py(NH)₂ which possesses the pyridine‐2,6‐dicarboxamide unit with two built‐in intramolecular H bonds could produce the corresponding poly(amide‐triazole) Poly‐(PyNH)₂ with a significantly higher degree of polymerization (DP) than other monomers with a lesser number of intramolecular H bonds. In addition, it was found that only this polymer exhibited excellent thermoreversible gelation ability in aromatic solvents. A self‐assembling model of the organogelating polymer Poly‐(PyNH)₂ was proposed based on FTIR spectroscopy, XRD, and SEM analyses, in which H bonding, π–π aromatic stacking, hydrophobic interactions, and the structural rigidity of the polymer backbone were identified as the main driving forces for the polymer self‐assembly process.     

Water‐Regulated Self‐Assembly Structure Transformation and Gelation Behavior Prediction Based on a Hydrazide Derivative

Herein, we report the water‐regulated supramolecular self‐assembly structure transformation and the predictability of the gelation ability based on an azobenzene derivative bearing a hydrazide group, namely, N‐(3,4,5‐tributoxyphenyl)‐N′‐4‐[(4‐hydroxyphenyl)azophenyl] benzohydrazide (BNB‐t4). The regulation effects are demonstrated in the morphological transformation from spherical to lamellar particles then back to spherical in different solvent ratios of n‐propanol/water. The self‐assembly behavior of BNB‐t4 was characterized by minimum gelation concentration, microstructure, thermal, and mechanical stabilities. From the spectroscopy studies, it is suggested that gel formation of BNB‐t4 is mainly driven by intermolecular hydrogen bonding, accompanied with the contribution from π–π stacking as well as hydrophobic interactions. The successfully established correlation between the self‐assembly behavior and solubility parameters yields a facile way to predict the gelation performance of other molecules in other single or mixed solvents.     

An Easy Access to Organic Salt‐Based Stimuli‐Responsive and Multifunctional Supramolecular Hydrogels

By exploiting orthogonal hydrogen bonding involving supramolecular synthons and hydrophobic/hydrophilic interactions, a new series of simple organic salt based hydrogelators derived from pyrene butyric acid and its β‐alanine amide derivative, and various primary amines has been achieved. The hydrogels were characterised by microscopy, table‐top rheology and dynamic rheology. FTIR, variable‐temperature ¹H NMR and emission spectroscopy established the role of various supramolecular interactions such as hydrogen bonding and π–π stacking in hydrogelation. Single‐crystal X‐ray diffraction (SXRD) studies supported the conclusion that orthogonal hydrogen bonding involving amide–amide and primary ammonium monocarboxylate (PAM) synthons indeed played a crucial role in hydrogelation. The hydrogels were found to be stimuli‐responsive and were capable of sensing ammonia and adsorbing water‐soluble dye (methylene blue). All the hydrogelators were biocompatible (MTT assay in RAW 264.7 cells), indicating their suitability for use in drug delivery.     

Heterogeneous ‘Organoclick’ Derivatization of Polysaccharides

Summary: A simple method for the direct catalytic heterogeneous modification of polysaccharides is presented. The novel method is exemplified by the combination of organic acid‐catalyzed esterification and copper‐catalyzed Huisgen reaction (click chemistry) to attach a fluorescent probe to solid cellulose. The heterogeneous ‘organoclick’ derivatization of cellulose allows for a mild, highly modular surface modification of cellulose under environmentally benign reaction conditions.

Schematic of the combined organic acid‐catalyzed esterification and copper‐catalyzed Huisgen reaction (click chemistry) to modify a polysaccharide with a fluorescent probe.     

Singly Hydrogen Bonded Supramolecular Ligands for Highly Selective Rhodium‐Catalyzed Hydrogenation Reactions

H bonds make the catalysts! A single hydrogen bond between ligands coordinated to a rhodium center is critical for the formation of pure supramolecular catalysts for asymmetric hydrogenation reactions. The ester group of the amidite ligand (see scheme) also forms a hydrogen bond with the coordinated substrate. Use of the heterocomplex afforded the highest enantioselectivity reported to date for the hydrogenation of several ester substrates.

     

Programmable Self‐Assembly of Heterometallic Palladium(II)–Copper(II) 1D Grid‐Chain using Dinuclear Palladium(II) Corners with Pyrazole–Carboxylic Acid Ligands

A novel heterometallic diPd^II–diCu^II grid‐chain, {[(bpy)₄Pd₄Cu₂L₄](NO₃)₄}_n ( 2 ; bpy=2,2′‐bipyridine), was synthesized through a programmable self‐assembly approach from the molecular corners [(bpy)₂Pd₂(HL)(L)](NO₃) ( 1 ) as linkers with Cu^II nitrate by using the bifunctional H₂L ligand featuring primary (pyrazole) and secondary (benzoic acid) groups. Structural analysis revealed that 1D structure 2 consists of one [Cu₂(O₂CPh)₄]_n unit as a bridge and two [(bpy)₂Pd₂L₂]_n corners. Additionally, the catalytic effect of the heterometallic synergy on the Suzuki coupling reaction by using 2 was further explored.     

Self-Propelling Hybrid Gels Incorporating an Active Self-Assembled,Low-Molecular-Weight Gelator

Hybrid gel beads based on combining a low-molecular-weight gelator (LMWG) with a polymer gelator (PG) demonstrate an enhanced ability to self-propel in water, with the LMWG playing an active role. Hybrid gel beads were loaded with ethanol and shown to move in water owing to the Marangoni effect changes in surface tension caused by the expulsion of ethanol – smaller beads move farther and faster than larger beads. Flat shapes of the hybrid gel were cut using a “stamp” – circles moved the furthest, whereas stars showed more rotation on their own axes. Comparing hybrid LMWG/PG gel beads with PG-only beads demonstrated that the LMWG speeds up the beads, enhancing the rate of self-propulsion. Self-assembly of the LMWG into a “solid-like” network prevents its leaching from the gel. The LMWG also retains its own unique function – specifically, remediating methylene blue pollutant dye from basic water as a result of noncovalent interactions. The mobile hybrid beads accumulate this dye more effectively than PG-only beads. Self-propelling gel beads have potential applications in removal/delivery of active agents in environmental or biological settings. The ability of self-assembling LMWGs to enhance mobility and control removal/delivery suggests that adding them to self-propelling systems can add significant value.     

Gels Based on Anion Recognition Between Triurea Receptor and Phosphate Anion

Anion recognition between the triurea receptor and phosphate anion is demonstrated as the cross‐linkage to build supramolecular polymer gels for the first time. A novel multi‐block copolymer ( 3) is designed to have functional triurea groups as cross‐linking units along the polymer main chain. By virtue of anion coordination between the triurea receptor and phosphate anion with a binding mode of 2:1, supramolecular polymer gels are then prepared based on anion recognition using 3 as the building block.

     

Copper immobilized onto polymer‐coated magnetic nanoparticles as recoverable catalyst for ‘click’ reaction

Copper supported on polymer‐coated magnetic nanoparticles was designed and synthesized as a separable heterogeneous catalyst. The catalyst was fully characterized using several techniques such as Fourier transform infrared and energy‐dispersive X‐ray spectroscopies, scanning and transmission electron microscopies, X‐ray diffraction, vibrating sample magnetometry, thermogravimetric analysis and inductively coupled plasma atomic emission spectrometry. All results showed that copper was successfully supported on the polymer‐coated magnetic nanoparticles. Also, results showed that the synthesized material can be used as an efficient catalyst for the preparation of a series of 1,4‐disubstituted 1,2,3‐triazoles from corresponding halides, alkynes and sodium azide. The catalyst can be easily isolated from the reaction solution by applying an external magnet and reused for nine runs without any significant loss of catalytic activity.     

Steric‐Structure‐Dependent Gel Formation,Hierarchical Structures,Rheological Behavior,and Surface Wettability

A series of bicholesteryl‐based gelators with different central linker atoms C, N, and O (abbreviated to GC , GN , and GO , respectively) have been designed and synthesized. The self‐assembly processes of these gelators were investigated by using gelation tests, field‐emission scanning electron microscopy, field‐emission transmission electron microscopy, UV/Vis absorption, IR spectroscopy, X‐ray diffraction, rheology, and contact‐angle experiments. The gelation ability, self‐assembly morphology, rheological, and surface‐wettability properties of these gelators strongly depend on the central linker atom of the gelator molecule. Specifically, GC and GN can form gels in three different solvents, whereas GO can only form a gel in N,N‐dimethylformamide (DMF). Morphologies from nanofibers and nanosheets to nanospheres and nanotubes can be obtained with different central atoms. Gels of GC , GN , and GO formed in the same solvent (DMF) have different tolerances to external forces. All xerogels gave a hydrophobic surface with contact angles that ranged from 121 to 152°. Quantum‐chemical calculations indicate that the GC , GN , and GO molecules have very different steric structures. The results demonstrate that the central linker atom can efficiently modulate the molecular steric structure and thus regulate the supramolecular self‐assembly process and properties of gelators.     

Site‐Directed Spin‐Labeling of DNA by the Azide–Alkyne ‘Click’ Reaction: Nanometer Distance Measurements on 7‐Deaza‐2′‐deoxyadenosine and 2′‐Deoxyuridine Nitroxide Conjugates Spatially Separated or Linked to a ‘dA‐dT’ Base Pair

Nucleobase‐directed spin‐labeling by the azide‐alkyne ‘click’ (CuAAC) reaction has been performed for the first time with oligonucleotides. 7‐Deaza‐7‐ethynyl‐2′‐deoxyadenosine ( 1 ) and 5‐ethynyl‐2′‐deoxyuridine ( 2 ) were chosen to incorporate terminal triple bonds into DNA. Oligonucleotides containing 1 or 2 were synthesized on a solid phase and spin labeling with 4‐azido‐2,2,6,6‐tetramethylpiperidine 1‐oxyl (4‐azido‐TEMPO, 3 ) was performed by post‐modification in solution. Two spin labels ( 3 ) were incorporated with high efficiency into the DNA duplex at spatially separated positions or into a ‘dA‐dT’ base pair. Modification at the 5‐position of the pyrimidine base or at the 7‐position of the 7‐deazapurine residue gave steric freedom to the spin label in the major groove of duplex DNA. By applying cw and pulse EPR spectroscopy, very accurate distances between spin labels, within the range of 1–2 nm, were measured. The spin–spin distance was 1.8±0.2 nm for DNA duplex 17 ( dA*^7,10 ) ?11 containing two spin labels that are separated by two nucleotides within one individual strand. A distance of 1.4±0.2 nm was found for the spin‐labeled ‘dA‐dT’ base pair 15 ( dA*⁷ ) ?16 ( dT*⁶ ). The ‘click’ approach has the potential to be applied to all four constituents of DNA, which indicates the universal applicability of the method. New insights into the structural changes of canonical or modified DNA are expected to provide additional information on novel DNA structures, protein interaction, DNA architecture, and synthetic biology.     

A Dual‐Response [2]Rotaxane Based on a 1,2,3‐Triazole Ring as a Novel Recognition Station

Two novel multilevel switchable [2]rotaxanes containing an ammonium and a triazole station have been constructed by a Cu^I‐catalyzed azide–alkyne cycloaddition reaction. The macrocycle of [2]rotaxane containing a C6‐chain bridge between the two hydrogen bonding stations exhibits high selectivity for the ammonium cation in the protonated form. Interestingly, the macrocycle is able to interact with the two recognition stations when the bridge between them is shortened. Upon deprotonation of both [2]rotaxanes, the macrocycle moves towards the triazole recognition site due to the hydrogen‐bond interaction between the triazole nitrogen atoms and the amide groups in the macrocycle. Upon addition of chloride anion, the conformation of [2]rotaxane is changed because of the cooperative recognition of the chloride anion by a favorable hydrogen‐bond donor from both the macrocycle isophthalamide and thread triazole CH proton.