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Formation of a Hetero[3]Rotaxane by a Dynamic Component-Swapping Strategy
Abstract
Acid-catalysed scrambling of the mechanically interlocked components between two different homo[3]rotaxanes, constituted of dumbbells containing two secondary dialkylammonium ion recognition sites encircled by two [24]crown-8 rings, each containing a couple of imine bonds, affords a statistical mixture of a hetero[3]rotaxane along with the two homo[3]rotaxanes, indicating that neither selectivity nor cooperativity is operating during the assembly process.
... As being part of the dynamic covalent chemistry (DCC) [21,26,27], the dynamic clipping approach allows self-error checking and self-sorting to favor the most thermodynamically stable rotaxane as the major product based on equilibrium reactions. Besides [2]rotaxanes, efficient methods for constructing dendritic [n]rotaxanes [28][29][30][31][32] homocircuit [n]rotaxanes [33][34][35][36][37][38] and heterocircuit [n]rotaxanes [39][40][41][42][43][44][45] have recently been investigated. ...
... As the stopper of rotaxanes, the anthracene unit was adopted due to its fluorescence property and bulkiness. Although the crystal structures of [3]rotaxane [36,42] nickel(II)-salen [2]rotaxane [48,49] and triimine [2]rotaxanes [50,51] had been previously reported, single crystals of pure organic diimine [2]rotaxane were not discovered yet. ...
The synthesis of mechanically interlocked molecules is valuable due to their unique topologies. With π-stacking intercomponent interaction, e.g., phenanthroline and anthracene, novel [2]rotaxanes have been synthesized by dynamic imine clipping reaction. Their X-ray crystal structures indicate the π-stackings between the anthracene moiety (stopper) on the thread and the (hetero)aromatic rings at the macrocycle of the rotaxanes. Moreover, the length of glycol chains affects the extra π-stacking intercomponent interactions between the phenyl groups and the dimethoxy phenyl groups on the thread. Dynamic combinatorial library has shown at best 84% distribution of anthracene-threaded phenanthroline-based rotaxane, coinciding with the crystallography in that the additional π-stacking intercomponent interactions could increase the thermodynamic stability and selectivity of the rotaxanes.
... Whilst the use of non-covalent interactions to direct the self-assembly of oligo[n]pseudorotaxanes prior to formation of a permanent mechanical bond is the most operationally simple method towards forming multi-component interlocked species [46][47][48][49][50][51], this generally offers little in the way of control over structural or sequence specificity. A limited number of examples of self-sorting systems have been reported [49][50][51][52][53][54]. However, iterative approaches [55][56][57][58][59][60][61] tend to offer the best opportunity for rapid and economical formation of precision designed oligo[n]rotaxanes. ...
Despite significant advances in the last three decades towards high yielding syntheses of rotaxanes, the preparation of systems constructed from more than two components remains a challenge. Herein we build upon our previous report of an active template copper-catalyzed azide-alkyne cycloaddition (CuAAC) rotaxane synthesis with a diyne in which, following the formation of the first mechanical bond, the steric bulk of the macrocycle tempers the reactivity of the second alkyne unit. We have now extended this approach to the use of 1,3,5-triethynylbenzene in order to successively prepare [2]-, [3]- and [4]rotaxanes without the need for protecting group chemistry. Whilst the first two iterations proceeded in good yield, the steric shielding that affords this selectivity also significantly reduces the efficacy of the active template (AT)-CuAAC reaction of the third alkyne towards the preparation of [4]rotaxanes, resulting in severely diminished yields.
... Aryl-substituted 2,6-diformylpyridines and bis(2-aminophenyl)ethers are essentially condensed into rings organized along oligoammonium ion templates under thermodynamic control. The high fidelity and versatility of this thermodynamic approach to MIM synthesis promised access to more complex, energetically demanding assemblies of concise and precise [3][4][5] lengths-a standing challenge in supramolecular assembly. ...
Dr. Alyssa-Jennifer Avestro is a native of the San Francisco Bay Area, California, and has recently relocated to the United Kingdom to assume a 3-year Royal Commission for the Exhibition of 1851 Research Fellowship in Science in the Department of Chemistry at Durham University. Prior to this, Alyssa received her university training in functional supramolecules and redox/photo-active organic materials first at the University of California, Berkeley (BS, 2010), with Prof. Jean Fréchet and then at Northwestern University (PhD, 2015) with Prof. Sir Fraser Stoddart, FRS. Alyssa currently sits on Chem’s Next-Generation Advisory Board and is eager to promote early-career scientists, elevate the presence of women and minorities in the STEM (science, technology, engineering and mathematics) academe, and help generate broad research impact for the chemical sciences at the international level.
Despite their chemistry being known for several decades, the development of mechanically interlocked molecules as molecular machines is still in its infancy. Focusing on rotaxanes, controlling the position where subcomponents self-assemble to from a macrocycle on a given thread is imperative to develop new materials. Particularly, intercomponent interactions govern the mechanical bonds formed between these components. Making use of π-stacking intercomponent interactions, four novel [n]rotaxanes namely 4O-[2], 5O-[2], 4O-[3] and 5O-[3] have been synthesized by dynamic imine clipping reactions. 2D NMR spectra indicate the phenanthroline containing macrocycles are located on ammonium stations closest to the anthracene stoppers. Whilst the X-ray structures of 4O-[2], 5O-[2] and 4O-[3] reveal π-stacking interactions between the anthracene and phenanthroline units, indicating the thermodynamically stable structures are the major products. Furthermore, unforeseen Michael Addition reactions of the [2]rotaxanes with dimethyl acetylenedicarboxylate afforded two novel rotaxane adducts 4O-[2]-DMAD and 5O-[2]-DMAD, demonstrating these structures may be conjugated for cargo-carrying applications. This work provides an elegant strategy to control site recognition in the template directed synthesis of [n]rotaxanes.
From a library of five crown ether macrocycles with different ring sizes and redox-active moieties, such as tetrathiafulvalene (TTF) and naphthalene dimiide (NDI), directional heterocircuit[3]rotaxanes were constructed. Using an axle...
Selective synthesis of [2]- and [3]rotaxanes is realized using two macrocycles with little disparity of side chains in steric hindrance.
Mg2+/Li+ hybrid ion batteries (MLIBs) are regarded as a promising candidate for electrical energy devices due to the combination of fast lithium intercalation cathode and safe magnesium anode. Herein, the two-dimensional Nb2O5 holey nanosheets are synthesized by a graphene oxide sacrificial template method and used as the cathode of MLIBs for the first time. It exhibits a high discharge capacity (195.9 mAh g−1 at 100 mA g−1), excellent rate performance (72.1 mAh g−1 at 2000 mA g−1) and long cycling life (capacity fading rate of 0.032 % per cycle at 2000 mA g−1 over 1000 cycles). The remarkable electrochemical performance could be attributed to the unique two-dimensional holey nanosheets structure, which is beneficial for improving electronic conductivity and lithium ion conductivity.
An azine-containing bispillar[5]arene was designed and synthesized by the reaction of aldehyde functionalized-pillar[5]arene and hydrazine. Then, a novel bispillar[5]arene-based supramolecular pseudopolyrotaxane has been successfully prepared via host-guest interaction. Interestingly, by taking advantage of the host-guest interactions, π-π stacking interactions and hydrogen bonding interactions, the mult-stimuli responsive gel-sol phase transitions of such a supramolecular pseudopolyrotaxane gels were successfully realized under different stimuli, such as acid, temperature, concentration, and competitive guests. Moreover, this supramolecular system could effectively adsorb dye molecule rhodamine B. It is worth noting that this supramolecular pseudopolyrotaxane gels prepared in cyclohexanol solution (BP5·G·C) could be used as adsorbent material for adsorbing rhodamine B with adsorption efficiency of 98.4%. Meanwhile, the adsorption efficiency was 97.6% for supramolecular pseudopolyrotaxane gels prepared in DMSO-H2O (v:v, 8:2) binary solution (BP5·G·D), also indicating the superior adsorption effect of the BP5·G·D to dye molecule rhodamine B.
Heterorotaxanes, in which at least two types of macrocycles were introduced as the wheel components in rotaxanes, have attracted more and more attentions during the past decades owing to their unique structural features and intriguing properties. The coexistence of varied macrocycles endows the resultant heterorotaxanes not only versatile shuttling and switching behaviors but also great potential for the construction of functional rotaxane systems for applications. In this feature article, classified by the various strategies towards the synthesis of heterorotaxanes, i.e. orthogonal binding approach, self-sorting approach, cooperative capture approach, and active metal template approach etc, a survey of the successful synthesis of heterorotaxanes will be provided.
A general and efficient way for enlarging the absorption wavelength of optical materials with high molar absorptivity was explored by incorporating a hydroxyl group ortho-position to a bridging group between the electron-withdrawing and the electron-donating groups. Both theoretical deduction and experimental data indicated that the molecular design strategy was rational and efficacious. Comparing to its analogy, the target materials, 3-hydroxyl-4-(N- ethyl-N-ethoxyl phenylazo) naphthalene sulfonic acid (HPNSA), possessed the improved optical property by extending the intrinsic D-π-A conjugated structure and selective recognition to Fe²⁺ by forming stable 5-member rings. Importantly, the proposed material was effectively applied for colorimetric determination of Fe²⁺ in four real environmental water samples with an accurate deviation of 4.0% and a low detection limit of 38 nmol L⁻¹. The action mechanism between HPNSA and Fe²⁺ was investigated in detail.
In this article, a six-component self-sorting process that involves three types of crown ether macrocycles and three sorts of cation guest molecules, was carefully and thoroughly investigated. The six components include three kinds of crown ethers, namely bis(p-phenylene-34-crown-10) (BPP34C10), dibenzo-24-crown-8 (DB24C8), benzo-21-crown-7 (B21C7), and their corresponding cation guest molecules, namely, 4,4’-bipyridine dications (BPY2+), dibenzylammonium (DBA) and benzylalkylammonium (BAA) ions, respectively. Based on this well-established highly selective six-component self-sorting process, a hetero[6]rotaxane bearing three different kinds of crown ether macrocycles was designed and successfully synthesized through a facile and efficient one-pot “click” stoppering strategy. Such a work is supposed to be a significant advancement in the construction of mechanically interlocked molecules with high structural complexity as well as a good supplement in the areas of multi-component self-sorting and noncovalent self-assembly.
Crown ether usually plays the role of macrocyclic host in supramolecular chemistry, but here the crown ether is also utilized as the stoppers in rotaxanes. In this work, we designed and synthesized two [3]rotaxanes containing four crown ether components by using an approach of template-directed clipping reaction, of which, two crown ether components were employed as the macrocyclic hosts to assemble the mechanically interlocked framework while another two crown ether units located on the two ends of ammonium template acting as the stoppering groups of rotaxanes. Their self-assembling process was monitored by the 1H NMR and one of the single crystal structures of [3]rotaxane was obtained.
Examples of using two-dimensional shape-persistent macrocycles, i.e. those having noncollapsible and geometrically well-defined skeletons, for constructing mechanically interlocked molecules are scarce, which contrasts the many applications of these macrocycles in molecular recognition and functional self-assembly. Herein, we report the crucial role played by macrocyclic shape-persistency in enhancing multipoint recognition for the highly efficient template-directed synthesis of rotaxanes. Cyclo[6]aramides, with a near-planar conformation, are found to act as powerful hosts that bind bipyridinium salts with high affinities. This unique recognition module, composed of two macrocyclic molecules with one bipyridinium ion thread through the cavity, is observed both in the solid state and in solution, with unusually high binding constants ranging from ∼10¹³ M⁻² to ∼10¹⁵ M⁻² in acetone. The high efficacy of this recognition motif is embodied by the formation of compact [3]rotaxanes in excellent yields based on either a “click-capping” (91%) or “facile one-pot” (85%) approach, underscoring the great advantage of using H-bonded aromatic amide macrocycles for the highly efficient template-directed synthesis of mechanically interlocked structures. Furthermore, three cyclo[6]aramides bearing different peripheral chains 1-3 demonstrate high specificity in the synthesis of a [3]rotaxane from 1 and 2, and a [2]rotaxane from 3via a “facile one-pot” approach, in each case as the only isolated product. Analysis of the crystal structure of the [3]rotaxane reveals a highly compact binding mode that would be difficult to access using other macrocycles with a flexible backbone. Leveraging this unique recognition motif, resulting from the shape-persistency of these oligoamide macrocycles, in the template-directed synthesis of compact rotaxanes may open up new opportunities for the development of higher order interlocked molecules and artificial molecular machines.
This chapter highlights the mechanically interlocked molecules (MIMs) to illuminate the diverse strategies, recognition chemistry, and reversible bonding motifs that have been invoked in order to create mechanical bonds. The synthesis of rotaxanes by slippage is unique among these approaches because it does not involve the making and breaking of any chemical bonds. Reversible covalent bonds have also enriched the field of mechanostereochemistry by allowing MIMs to be synthesized in up to near-quantitative yields from relatively simple and even commercially available building blocks. The chapter focuses on discrete mechanically interlocked assemblies, metallosupramolecular polymers based on the self-assembly of interpenetrated ligands and metals can also be assembled under thermodynamic control. Reversible condensation reactions, especially those involving the loss of water, such as the formation of imines, hydrazones, and boronic esters or boroxines, are powerful tools for establishing mechanical bonds in interpenetrated assemblies under thermodynamic control.
We present an operationally simple iterative coupling strategy for the synthesis of oligomeric homo- and hetero[n]rotaxanes with precise control over the position of each macrocycle. The exceptional yield of the AT-CuAAC reaction, combined with optimized conditions that allow the rapid synthesis of the target oligomers, opens the door to the study of precision-engineered oligomeric interlocked molecules.
The conceptual basis for mechanostereochemistry, both static and dynamic, can be appreciated most readily by drawing analogies with the stereochemistry of compounds whose molecules possess only covalent bonds, for example, cyclohexane and its derivatives. Since compounds such as catenanes and rotaxanes contain mechanically interlocked entities, it is convenient to call these entities component parts. This chapter reviews many dynamic mechanostereoisomers that interconvert between different co-conformations by intramolecular translation. The static mechanostereoisomers arising from differences in orientation, ring sequence, mechanical chirality or topology can sometimes be interconverted by dynamic processes, e.g., the inversion of mechanical helicity by dynamic rocking processes, the inversion of mechanically planar chirality by bowl-to-bowl inversion of cyanostar macrocycles and calixarenes, and the inversion of conditional topological chirality as a result of the dynamic rearrangement of coordinative bonds in Zn2C5524+. The promise of robust dynamics in solid-state materials and at interfaces may support the development of novel functions for these mechanically bonded systems.
In this proof-of-concept study, an active-template coupling is used to demonstrate a novel kinetic self-sorting process. This process iteratively increases the yield of the target heterocircuit [3]rotaxane product at the expense of other threaded species.
In this proof-of-concept study, an active-template coupling is used to demonstrate a novel kinetic self-sorting process. This process iteratively increases the yield of the target heterocircuit [3]rotaxane product at the expense of other threaded species.
Supramolecular chemistry and self-assembly provide a valuable chance to understand the complicated topological structures on a molecular level. Two types of classical mechanically interlocked molecules, rotaxanes and catenanes, possess non-covalent mechanical bonds and have attracted more attention not only in supramolecular chemistry but also in the fields of materials science, nanotechnology and bioscience. In the past decades, the template-directed clipping reaction based on imine chemistry has become one of the most efficient methods for the construction of functionalized rotaxanes and catenanes. In this review, we outlined the main progress of rotaxanes and catenanes using the template-directed clipping approach of imine chemistry. The review contains the novel topological structures of rotaxanes and catenanes, functions and applications.
Rotacatenane is an interlocked compound composed of two mechanically interlocked macrocyclic components, i. e., a [2]catenane, and one axle component. In this paper we describe the selective synthesis of isomeric rotacatenanes. Two [2]rotaxanes with different phenanthroline moieties were synthesized by the oxidative coupling of alkyne with bulky blocking group, which proceeded in the cavity of the macrocyclic phenanthroline-Cu complex. The metal template method was used to install another cyclic component: the tetrahedral Cu(I) complex, which was composed of a [2]rotaxane and an acyclic phenanthroline derivative, was synthesized and the cyclization of the phenanthroline derivative gave the rotacatenane. The sequential isomers of rotacatenanes were distinguished by 1H and 13C NMR spectroscopy.
With the rapid development of supramolecular chemistry, the mechanically interlocked molecules based on N-hetero crown ethers (NHCE) have attracted increasing interest in recent years. Compared with the traditional synthetic methods, the templated clipping approach has unique advantages in constructing mechanically interlocked molecules based on NHCE, and many important advances have been obtained. Scientists have not only gradually expanded the topological structures of mechanically interlocked molecules based on NHCE, but also enriched their application in various regions. Herein, we summarize the progress of templated clipping approach in the structure construction of the mechanically interlocked molecules (such as rotaxanes, catenanes, heterorotaxanes, rotacatenanes, oligomers, daisy chains and dendritic molecules) and their potential applications.
[3]Rotaxanes with two axle components and one linear component were synthesized by the combination of a coupling reaction using transition metal catalyst and metal-template approach. Thus, [2]rotaxanes were prepared by the oxidative dimerization of alkyne promoted by macrocyclic phenanthroline-CuI complexes. The [2]rotaxane was reacted with a CuI salt and an acyclic ligand to generate a tetrahedral CuI complex. Metal-free [3]rotaxane was isolated by the end-capping reaction of the acyclic ligand followed by the removal of CuI ion. The stability of the tetrahedral CuI complexes depended on the size of both the ring component and the acyclic ligand, which was correlated with the yield of the corresponding [3]rotaxane.
We have now identified competitive formation of doubly interlocked [3]rotaxanes as the origin of the non-linear variation in yield of [2]rotaxane with macrocycle size in the bipyridine-mediated AT-CuAAC reaction. Selection of reaction conditions lead to [2]rotaxanes in essentially quantitative yield in all cases and otherwise hard to access doubly threaded [3]rotaxanes in up to 50% isolated yield in a single step. Based on the effect of macrocycle structure on the reaction outcome we propose a detailed mechanism of [3]rotaxane formation.
Three flexible chelating and bridging ligands, (E)-N-(pyridin-2-ylmethylene)-1-(pyridin-3-yl)methanamine (L1), 1-(pyridine-2-yl)-N-(pyridin-3-ylmethyl)methanamine (L1′), (E)-N-(pyridin-2-ylmethylene)-1-(pyridin-4-yl)methanamine (L2), have been synthesized. The reaction of these ligands with various metal salts yielded a series of compounds, namely [Cu2(L1)(PPh3)2I2]2 (1), [Cu2(L2)(PPh3)2I2]n (2), [CuL1(NO3)2]2 (3), [ZnL1(NO3)2]2 (4), [CdL1(NO3)2]2 [(5), [CdL1′ (NO3)2]2 (6) and [HgL1Cl2]n (7). The compounds were characterized by elemental analysis, spectroscopic methods (IR, UV–Vis) and X-ray crystallography. X-ray structural analysis of compound 1 reveals the formation of a centrosymmetric tetranuclear compound. The repeat unit of coordination polymer 2 comprises a Cu2I2 unit with a triphenyl phosphane substituent on Cu1 linked to an adjacent Cu2I2 unit by the ligand L2 that acts as a bridge. Compounds 3, 4, 5 and 6 exhibit a common metallo-cyclophane [M2(L)2(NO3)4] skeleton. Two M2+ cations are linked by two L1 ligands and these ligands chelate to each metal atom through the imine nitrogen (or amine nitrogen) atoms and the nitrogen atom of the 2-pyridyl ring, and bind to the other metal atom of the pair via the nitrogen atoms of the 3-pyridyl rings. Structural studies of 7 revealed the formation of a coordination polymer in which the polymer chain propagates in helical spirals along a twofold screw axis parallel to b. The metal coordination geometries and coordination of the nitrato anions to the metals are noted and discussed.
High yields syntheses of Cu (II) and Ni(II) templated [2]pseudorotaxane precursors, CuPRT and NiPRT are achieved by threading bis-azide-bis-amide-2,2´-bipyridine axle into bis-amide-tris-amine macrocycle. Single crystal X-ray structural analysis of CuPRT reveals complete threading of the axle fragment into the wheel cavity where strong aromatic pi-pi stacking interactions (ranges from 3.474 to 3.494 Angstrom) between two parallel arene moieties of the wheel and the pyridyl unit of axle are operative besides metal ion templation. These azide terminated [2]pseudorotaxane precursors are further explored towards synthesis of metal free [2]rotaxane upon attaching a newly developed bulky stopper molecule with alkyne terminal via Cu(I)-catalyzed azide-alkyne cycloaddition reaction. Attachment of the stopper to CuPRT precursor via click chemistry, fails due to de-threading of azide terminated axle in the reaction conditions. However, synthesis of metal free [2]rotaxane containing triazole with other functionalities in the axle is achieved upon coupling between azide terminated NiPRT and the alkyne terminated stopper with ~45% yield. The [2]rotaxane is characterized by mass spectrometry, 1D and 2D NMR (COSY, DOSY, ROESY) experiments. Comparative solution state NMR studies of [2]rotaxane and in its protonated state are carried out to locate the position of the wheel on the axle of metal free [2]rotaxane. Furthermore, variable temperature 1H- NMR study in DMSO-d6 of [2]rotaxane supports the kinetic-inertness of the interlocked structure, where newly developed stopper prevents dethreading of 30-membered wheel from the axle.
α-Cyclodextrin (CD)-based size-complementary [3]rotaxanes with alkylene axles were prepared in one-pot by end-capping reactions with aryl isocyanates in water. The selective formation of [3]rotaxane with a head-to-head regularity was indicated by the X-ray structural analyses. Thermal degradation of the [3]rotaxanes bearing appropriate end groups proceeded by stepwise dissociation to yield not only the original components but also [2]rotaxanes. From the kinetic profiles of the deslippage, it turned out that the maximum yield of [2]rotaxane was estimated to be 94 %. Thermodynamic studies and NOESY analyses of such rotaxanes revealed that [2]rotaxanes are specially stabilized, and that the dissociation capability of the [3]rotaxanes to the components can be adjusted by controlling the structure of the end groups, direction of the CD groups, and length of the alkylene axle.
A dumbbell-shaped dialkylammonium ion (see scheme) templates the formation, about its NH2⁺ center, of a crown ether like macrocycle under thermodynamic control from dialdehyde and diamine precursors. The imine-containing [2]rotaxane that ensues was converted by reduction into a kinetically stable interlocked molecule, which has been characterized in the solid state by X-ray crystallography in both free base and salt forms.
Mechanically interlocked molecular compounds can be synthesized in high yields by using template-directed assistance to covalent synthesis. Catenanes and rotaxanes are two classes of mechanically interlocked molecules that have been prepared using a variety of methods such as “clipping”,“slipping”, and “threading-followed-by-stoppering”, under both kinetic and thermodynamic regimes. These different methods have utilized a range of templates such as transistion metals, p-donor/p-acceptors, and hydrogen-bonding motifs. Multivalency has emerged as another tool to aid and abet the supramolecularly assisted synthesis of mechanically interlocked molecules. Recent advances in our understanding of the nature of the mechanical bond has led to the construction of molecular machines with controllable motions that have, in one instance, been introduced into molecular electronic devices.
Research on mechanically interlocked molecules has advanced substantially over the last five decades. A large proportion of the published work focusses on the synthesis of these challenging targets, and the subsequent control of the relative position of the covalent sub-components, to generate novel molecular devices and machines. In this Feature Article we instead review some of the less discussed consequences of mechanical bonding for the chemical behaviour of catenanes and rotaxanes, and their application in synthesis, including striking recent examples of molecular machines which carry out complex synthetic tasks.
It has become common to reference “pi-stacking” forces or “pi–pi interactions” when describing the interactions between neighbouring aromatic rings. Here, we review experimental and theoretical literature across several fields and conclude that the terms “pi-stacking” and “pi–pi interactions” do not accurately describe the forces that drive association between aromatic molecules of the types most commonly studied in chemistry or biology laboratories. We therefore propose that these terms are misleading and should no longer be used. Even without these terms, electrostatic considerations relating to polarized pi systems, as described by Hunter and Sanders, have provided a good qualitative starting place for predicting and understanding the interactions between aromatics for almost two decades. More recent work, however, is revealing that direct electrostatic interactions between polarized atoms of substituents as well as solvation/desolvation effects in strongly interacting solvents must also be considered and even dominate in many circumstances.
Two different approaches are described for the creation of supramolecular systems potentially capable of recognition and catalysis. Using the design approach, we have been able to accelerate and influence two different Diels-Alder reactions within the cavities of porphyrin dimers and trimers; this is templating from the outside inwards. The selection approach is a synthetic chemical attempt to capture some of the key evolutionary features of biological systems: dynamic combinatorial chemistry is used to create equilibrating mixtures of potential receptors, and then a template is used to select and amplify the desired system. Five potential reactions for such dynamic chemistry are discussed: base-catalyzed transes- terification, hydrazone exchange, disulfide exchange, alkene metathesis, and Pd-catalyzed allyl exchange, and preliminary templating results (inside outwards) are presented.
A comprehensive experimental study was carried out measuring the relative strengths of parallel π-stacking interactions of N-heterocycles with non-heterocycles. A versatile and rigid model system was developed, which was in equilibrium between a "closed" conformation that forms an intramolecular π-stacking interaction and an "open" conformation that cannot form the interaction. First, the formation and geometries of the intramolecular N-heterocyclic π-stacking interactions were verified by X-ray crystallography. Next, the closed/open ratios were measured in solution via the integration of the 1H NMR spectra, providing an accurate comparison of the N-heterocyclic π-stacking interactions. The synthetic versatility of this model system enabled the systematic and comprehensive comparison of the influences of position, charge, and substituent effects of the nitrogen atom of the N-heterocycles within a single model system. The π-stacking interactions of the neutral N-heterocyclic rings were slightly stronger than that of non-heterocyclic rings. Cationic N-heterocycles formed significantly stronger π-stacking interactions than neutral N-heterocycles. The position of the nitrogen atom also had a strong influence on the stability of N-heterocyclic π-stacking complexes. Interestingly, opposite stability trends were observed for neutral and cationic N-heterocycles. For neural N-heterocycles, geometries with the nitrogen away from the π-face of the opposing ring were the more stable. For cationic N-heterocycles, geometries with the nitrogen close to the π-face of the opposing ring were more stable. Finally, N-methylated heterocycles were consistently observed to form stronger π-stacking interactions than N-protonated heterocycles.
A couple of [c2]daisy chains have been assembled in each case from four components in quantitative yields at room temperature in acetonitrile as a result of the self-templated clippings of their [24]crown-8 rings by reversible imine bond formation around secondary dialkylammonium recognition sites in their stalks.
Molecular interactions underlie the whole of chemistry and biology. This tutorial review illustrates the use of rotameric folding molecules, topoisomers, atropoisomers, and tautomers as molecular balances for quantifying non-covalent interactions. This intramolecular approach enables a wide variety of interactions to be examined with a degree of geometric control that is difficult to achieve in supramolecular complexes. Synthetic variation of molecular balances allows the fundamental physicochemical origins of molecular recognition to be systematically examined by providing insights into the interplay of geometry and solvation on non-covalent interactions.
Whereas combinatorial chemistry is based on extensive libraries of prefabricated molecules, dynamic combinatorial chemistry (DCC) implements the reversible connection of sets of basic components to give access to virtual combinatorial libraries (VCLs), whose constituents comprise all possible combinations that may potentially be generated. The constituent(s) actually expressed among all those accessible is(are) expected to be that(those) presenting the strongest interaction with the target, that is, the highest receptor/substrate molecular recognition. The overall process is thus instructed (target-driven), combinatorial, and dynamic. It bypasses the need to actually synthesize the constituents of a combinatorial library by letting the target perform the assembly of the optimal partner. It comprizes both molecular and supramolecular events. The basic features of the DCC/VCL approach are presented together with its implementation in different fields and the perspectives it offers in a variety of areas of science and technology, such as the discovery of biologically active substances, of novel materials, of efficient catalysts, and so forth. Finally, it participates in the progressive development of an adaptive chemistry.
While a single pillar[6]arene ring, nestling between two cucurbit[6]uril rings in a series of three hetero[4]rotaxanes, is conformationally mobile in solution, it adopts the energetically most favourable conformation with local C3V symmetry in the solid state.
The controlled self-assembly of well-defined and spatially ordered π-systems has attracted considerable interest because of their potential applications in organic electronics. An important contemporary pursuit relates to the investigation of charge transport across noncovalently coupled components in a stepwise fashion. Dynamic oligorotaxanes, prepared by template-directed methods, provide a scaffold for directing the construction of monodisperse one-dimensional assemblies in which the functional units communicate electronically through-space by way of π-orbital interactions. Reported herein is a series of oligorotaxanes containing one, two, three and four naphthalene diimide (NDI) redox-active units, which have been shown by cyclic voltammetry, and by EPR and ENDOR spectroscopies, to share electrons across the NDI stacks. Thermally driven motions between the neighboring NDI units in the oligorotaxanes influence the passage of electrons through the NDI stacks in a manner reminiscent of the conformationally gated charge transfer observed in DNA.
Reversible covalent bond formation under thermodynamic control adds reactivity to self-assembled supramolecular systems, and is therefore an ideal tool to assess complexity of chemical and biological systems. Dynamic combinatorial/covalent chemistry (DCC) has been used to read structural information by selectively assembling receptors with the optimum molecular fit around a given template from a mixture of reversibly reacting building blocks. This technique allows access to efficient sensing devices and the generation of new biomolecules, such as small molecule receptor binders for drug discovery, but also larger biomimetic polymers and macromolecules with particular three-dimensional structural architectures. Adding a kinetic factor to a thermodynamically controlled equilibrium results in dynamic resolution and in self-sorting and self-replicating systems, all of which are of major importance in biological systems. Furthermore, the temporary modification of bioactive compounds by reversible combinatorial/covalent derivatisation allows control of their release and facilitates their transport across amphiphilic self-assembled systems such as artificial membranes or cell walls. The goal of this review is to give a conceptual overview of how the impact of DCC on supramolecular assemblies at different levels can allow us to understand, predict and modulate the complexity of biological systems.
We present a novel strategy to synthesize multi-molecular arrays in a programmable way by stepwise elongation based on repetition of two-fold rotaxane formation and construction of threads. A cofacially triply stacked porphyrin array was obtained via the repetitive two-fold rotaxane formation.
Heterorotaxanes, one class of topological organic structures, have attracted increasing interesting during the past two decades. In general, two types of heterorotaxane structures exist, one in which two or more different macrocycles are threaded onto one dumbbell-shaped molecule and the other where one macrocycle is threaded onto two or more different dumbbell-shaped molecules. In comparison to these traditional types, another family of topologically interesting heterorotaxanes can be envisaged as arising from polyfunctional molecules that possess both host (crown ether) and guest (ammonium templates). In the current investigation, we have explored the construction of selected members of this new heterorotaxane family, which possess crown ether moieties that are wrapped dumbbell-shaped molecule. These structures are prepared by routes in which "stitching" processes, involving template-directed clipping reaction or olefin metathesis reactions, are used to install crown ether ring systems encircling ammonium cation centers. This is then followed by implementation of a threading-followed-by-stoppering sequence to install a second encircling crown ether ring. The results show that the polyfunctional building blocks assemble with high efficiencies. Finally, this investigation provides a foundation of future studies aimed at constructing more complicated heterorotaxane architectures, such as switchable systems, self-assembling polymers and functional molecular machines.
After the manner in which co-enzymes often participate in the binding of substrates in the active sites of enzymes, pillar[5]arene - a macrocycle containing five hydroquinone rings linked through their para positions by methylene bridges - modifies the binding properties of cucurbit[6]uril, such that the latter templates azide-alkyne cycloadditions that do not occur in the presence of only the cucurbit[6]uril - a macrocycle comprised of six glycoluril residues doubly linked through their nitrogen atoms to each other by methylene groups. Here, we describe how a combination of pillar[5]arene and cucurbit[6]uril interacts cooperatively with bipyridinium dications substituted on their nitrogen atoms with 2-azidoethyl- to 5-azidopentyl moieties to afford, as a result of orthogonal templation, two [4]rotaxanes and one [5]rotaxane in > 90% yields inside two hours at 55 oC in acetonitrile. Since the hydroxyl groups on pillar[5]arene and the carbonyl groups on cucurbit[6]uril form hydrogen bonds readily, these two macrocycles work together in a cooperative fashion to the extent that the four conformational isomers of pillar[5]arene can be trapped on the dumbbell components of the [4]rotaxanes. In the case of the [5]rotaxane, it is possible to isolate a compound containing two pillar[5]arene rings with local C5 symmetries. In addition to fixing the stereochemistries of the pillar[5]arene rings, the regiochemistries associated with the 1,3-dipolar cycloadditions have been extended in their constitutional scope. Under mild conditions, orthogonal recognition motifs have been shown to lead to templation with positive cooperativity that is fast and all but quantitative, as well as being green and efficient.
A simple model of the charge distribution in a π-system is used to explain the strong geometrical requirements for interactions between aromatic molecules. The key feature of the model is that it considers the σ-framework and the π-electrons separately and demonstrates that net favorable π-π interactions are actually the result of π-σ attractions that overcome π-π repulsions. The calculations correlate with observations made on porphyrin π-π interactions both in solution and in the crystalline state. By using an idealized π-atom, some general rules for predicting the geometry of favorable π-π interactions are derived. In particular a favorable offset or slipped geometry is predicted. These rules successfully predict the geometry of intermolecular interactions in the crystal structures of aromatic molecules and rationalize a range of host-guest phenomena. The theory demonstrates that the electron donor-acceptor (EDA) concept can be misleading: it is the properties of the atoms at the points of intermolecular contact rather than the overall molecular properties which are important.
Efficient templates play an important role in the construction of mechanically interlocked molecules such as catenanes and rotaxanes. This minireview presents a retrospective introduction on the traditional templates and highlights recent significant accomplishments in developing novel and efficient templates, especially active metal templates and radical templates, employed in the construction of rotaxanes and catenanes. The current status of this field is summarized and the scope and future prospects are also discussed in this minireview.
Since the discovery of crown ethers, macrocycles have been recognized as powerful platforms for supramolecular chemistry. Although their numbers and variations are now legion, macrocycles that are simple to make using high-yielding reactions in one pot and on the multigram scale are rare. Here we present such a discovery obtained during the creation of a C5-symmetric cyanostilbene 'campestarene' macrocycle, cyanostar, that employs Knoevenagel condensations in the preparation of its cyanostilbene repeat unit. In the solid state, cyanostars form π-stacked dimers constituted of chiral P and M enantiomers. The electropositive central cavity stabilizes anions with CH hydrogen-bonding units that are activated by electron-withdrawing cyano groups. In solution, the cyanostar shows high-affinity binding as 2:1 sandwich complexes, log β2 ≈ 12 and ΔG ≈ -70 kJ mol(-1), of large anions (BF4(-), ClO4(-) and PF6(-)) usually considered weakly coordinating. The cyanostar's size preference allowed formation of an unprecedented [3]rotaxane templated around a dialkylphosphate.
Catenanes are molecules comprising at least two mechanically interlocked rings: they cannot be separated, yet do not possess covalent links between ring constituents. Over the last two decades several efficient templating mechanisms for the assembly of catenanes, and other topologically complex molecules, have been developed and exploited in the synthesis of numerous systems, often with impressive efficiency. Kinetically controlled assembly routes, employing transition metal complexation or amide hydrogen bonding interactions, have proved tremendously successful. A third arena of investigation, and perhaps the most thoroughly explored and exploited, is the utilisation of π-complementary components, principally bipyridinium dications and aromatic ethers. In our work electron deficient bipyridinium dications were replaced with uncharged, yet electron accepting, aromatic diimides. This replacement permitted the use of a variety of ring closing reactions for catenane formation by allowing us to step away from the structurally powerful but ultimately limited chemistry demanded by bipyridinium systems. A total of four ring-closing reactions were employed: acetylenic coupling, Mitsunobu alkylation, Grubbs' alkene metathesis, and zinc(II)-bipyridyl ligation. The first three methods yielded fully covalent interlocked systems, the fourth a catenane containing a metallomacrocycle. The first two methods employed irreversible bond forming reactions in catenane formation, the latter two thermodynamically controlled processes. This flexible system of interacting components, the synthetic chemistry used in their preparation, and the structural flexibility offered by the combination of these factors, is discussed in terms of a series of model systems leading to the proposition of a method for the synthesis of a polycatenane. Such systems, polymeric chains of interlocked rings, are unrealised yet coveted goals of chemists working in the area of supramolecular topology and are predicted to exhibit valuable and unusual material properties.
It has become clear over the past two decades that, in order to create functional synthetic nanoscale structures, the chemist must exploit a fundamental understanding of the self-assembly of large-scale biological structures, which exist and function at and beyond the nanoscale. This mode of construction of nanoscale structures and nanosystems represents the so-called ‘bottom-up’ or ‘engineering-up’ approach to fabrication. Significant progress has been made in the development of nanoscience by transferring concepts found in the biological world into the chemical arena. The development of simple chemical systems that are capable of instructing their own organisation into large aggregates of molecules through their mutual recognition properties has been central to this success. By utilising a diverse array of intermolecular interactions as the information source for assembly processes, chemists have successfully applied biological concepts in the construction
of complex nanoscale structures and superstructures with a variety of forms and functions. More recently, the utility of assembly processes has been extended through the realisation that recognition processes can be used to select a single structure from a library of equilibrating structures. These developments open the way for the design and implementation of artificial assembly processes that are capable of adapting themselves to the local environment in which they are conducted.
The key feature of enzymic catalysis is recognition of the transition state. Synthesis of designed systems rarely leads to successful catalysts as the rules for conformation and intermolecular interactions are to imperfectly understood. This review describes several current ‘selection’ approaches to the generation of systems that can recognise transitionstate analogues. Examples covered include catalytic antibodies, ribozymes, imprinted polymers. Combinatorial chemistry, and thermodynamic templating. All have the potential to yeild effective catalysts without prior design of every detail.
Cyclocholates are efficiently and rapidly synthesised from suitable monomers by transesterification under thermodynamic, equilibrating conditions using a potassium methoxide–crown ether complex.
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Three recognition sites—a bipyridinium unit and two secondary dibenzylammonium centers—within the tetracation 1 can be differentiated by two simple macrocyclic receptors, bis-para-phenylene[34]crown-10 (BPP34C10) and dibenzo[24]crown-8 (DB24C8). Strict self-assembly processes under thermodynamic control yielded either [(DB24C8)2 ·BPP34C10·1] in a selective manner or the pseudopolyrotaxane [(BPP34C10)2 ·1]n.
Although the principle of template synthesis has been known since the sixties, surprising discoveries and new applications in the field of supramolecular chemistry over the last decade have provoked a boom in the subject. The synthesis of supramolecular species has been made much more efficient, or even in some cases possible, by the introduction of template ions or molecules. It is not just metal ions that can act as templates. Neutral molecules, electrostatic interactions, and hydrogen bonds also support the formation of binary and tertiary complexes. Energetically favorable conformations then lead to the formation of a specific desired product in high yield. In addition to the discussion of metal ions and neutral molecules as templates, covalent, positive, and negative templates are differentiated. Kinetic and thermodynamic aspects will also be considered in this review, together with the influence of templates on the phenomenon of self-organization. Further developments and applications include the synthesis of oligonucleotides, peptide blocks capable of forming secondary structure, and template polymers. Template synthesis of defined molecular cavities ultimately leads to “inclusion chemistry on a nanometer scale.”
It is the purpose of this review article to show that interaction between polyether ligands and metal ions is an effective force for catalysis. Rates of methyl transfer and acetyl transfer reactions from functionalized crown ethers to anionic nucleophiles are enhanced - even to surprisingly large extents - by group 1 and group 2 metal ions. Discussion is focused on transition state stabilization by metal ions, and on the role played by the additional binding energy rendered available by a polyether chain proximal to the reaction zone. A barium (II)-based prototype catalyst with transacylase activity is described.
Partner preferences in pseudorotaxane formation were exploited to establish an integrative self-sorting system able to discriminate simultaneously at the sequence and stereochemical level. It was found that calix[6]arenes were threaded selectively with a preferred orientation onto bisammonium axles, even when the structural differences between the possible building blocks were small and located remote from the binding sites.
Chemistry is progressively unraveling the processes that underlie the evolution of matter towards states of higher complexity and the generation of novel features along the way by self-organization under the pressure of information. Chemistry has evolved from molecular to supramolecular to become adaptive chemistry by way of constitutional dynamics, which allow for adaptation, through component selection in an equilibrating set. Dynamic systems can be represented by weighted dynamic networks that define the agonistic and antagonistic relationships between the different constituents linked through component exchange. Such networks can be switched through amplification/up-regulation of the best adapted/fittest constituent(s) in a dynamic set. Accessing higher level functions such as training, learning, and decision making represent future lines of development for adaptive chemical systems.
This review article concerns with the syntheses of polyrotaxanes and polyrotaxane networks that can be constructed mainly on the basis of the concept of dynamic covalent bond chemistry. At the beginning of the review, synthetic methods of rotaxanes are briefly summarized along with several high yielding preparations. Synthesis of poly[3]rotaxane by the reaction of homoditopic monomers utilizing the thiol-disulfide interchange reaction was discussed in detail, among polyrotaxanes possessing topological bonds for the monomer linking in the main chain. Polyrotaxane network was prepared by a similar protocol from a polyfunctional crown ether and a dumbbell-shaped homoditopic axle containing two sec-ammonium salt moieties and centrally located disulfide bond. Some characteristics of the polyrotaxane network were demonstrated, including the recyclable property as the crosslinked polymer. The meaning and applicability of the reversible crosslinking/decrosslinking system developed in the polyrotaxane network are specially emphasized for unique and potential application.Keywords: Rotaxane, Polyrotaxane, Polyrotaxane Network, Gel, Interlocked Polymer
This tutorial review summarizes the progress made towards mechanically interlocked daisy chains. Such materials can be seen as a further development in polymer science, where the conventional covalent interlinking bonds are replaced by supramolecular binding concepts. Materials in which the mechanical bond is an integral part of the polymeric backbone are expected to possess unique macroscopic properties and are therefore the synthetic aim in an ever growing research community. After introducing general considerations about daisy chains, the most common analytic methods to get insight into the aggregation behaviour of such self-complementary monomers are presented. Cyclodextrins/aromatic rods, crown ethers/cationic rods and pillararenes/alkyl chains are systems used to achieve daisy chain-like molecular arrays. By comparison of the reported systems, conclusions about an improved structural design are drawn.
In one fell swoop, polyrotaxanes comprising up to 64 rings can be synthesized as a result of cucurbit[6]uril-templated 1,3-dipolar azide-alkyne cycloadditions accelerated in the presence of cyclodextrins as a consequence of self-sorting and positive cooperativity, brought about by hydrogen bonding. Mixing six components in one pot affords a hetero[4]rotaxane in one minute in quantitative yield.
Multivalent interactions can be applied universally for a targeted strengthening of an interaction between different interfaces or molecules. The binding partners form cooperative, multiple receptor-ligand interactions that are based on individually weak, noncovalent bonds and are thus generally reversible. Hence, multi- and polyvalent interactions play a decisive role in biological systems for recognition, adhesion, and signal processes. The scientific and practical realization of this principle will be demonstrated by the development of simple artificial and theoretical models, from natural systems to functional, application-oriented systems. In a systematic review of scaffold architectures, the underlying effects and control options will be demonstrated, and suggestions will be given for designing effective multivalent binding systems, as well as for polyvalent therapeutics.
Imine formation between 25 aldehydes and 13 amines in aqueous solution in the pH range 7–11 was studied by 1H NMR spectroscopy. A three-parameter linear equation correlating logarithms of imine formation constants with pKa and HOMO energies of amines and LUMO energies of aldehydes is proposed. In view of the widespread occurrence of imine-forming processes in both chemistry and biology, the data presented are of significance for physical organic chemistry and of particular interest for dynamic combinatorial chemistry. Copyright © 2005 John Wiley & Sons, Ltd.
A dumbbell-shaped dialkylammonium ion (see scheme) templates the formation, about its NH2+ center, of a crown ether like macrocycle under thermodynamic control from dialdehyde and diamine precursors. The imine-containing [2]rotaxane that ensues was converted by reduction into a kinetically stable interlocked molecule, which has been characterized in the solid state by X-ray crystallography in both free base and salt forms.
In this chapter we are mainly concerned with the transition-metal-templated synthesis of rotaxanes. General features of these compounds, including stereochemical issues, are addressed first. We present an analytical and concise approach, that helps rationalize this chemistry by way of selected examples.
The quest to construct mechanically interlocked polymers, which present precise monodisperse primary structures that are produced both consistently and with high efficiencies, has been a daunting goal for synthetic chemists for many years. Our ability to realise this goal has been limited, until recently, by the need to develop synthetic strategies that can direct the formation of the desired covalent bonds in a precise and concise fashion while avoiding the formation of unwanted kinetic by-products. The challenge, however, is a timely and welcome one, as a consequence of, primarily, the potential for mechanically interlocked polymers to act as dynamic (noncovalent) yet robust (covalent) new materials for a wide array of applications. One such strategy which has been employed widely in recent years to address this issue, known as Dynamic Covalent Chemistry (DCC), is a strategy in which reactions operate under equilibrium and so offer elements of "proof-reading" and "error-checking" to the bond forming and breaking processes such that the final product distribution always reflects the thermodynamically most favourable compound. By coupling DCC with template-directed protocols, which utilise multiple weak noncovalent interactions to pre-organise and self-assemble simpler small molecular precursors into their desired geometries prior to covalent bond formation, we are able to produce compounds with highly symmetric, robust and complex topologies that are otherwise simply unobtainable by more traditional methods. Harnessing these strategies in an iterative, step-wise fashion brings us ever so much closer towards perfecting the controlled synthesis of high order main-chain mechanically interlocked polymers. This tutorial review focuses (i) on the development of DCC-namely, the formation of dynamic imine bonds-used in conjunction with template-directed protocols to afford a variety of mechanically interlocked molecules (MIMs) and ultimately (ii) on the synthesis of highly ordered poly[n]rotaxanes with high conversion efficiencies.
Dynamic covalent chemistry relates to chemical reactions carried out reversibly under conditions of equilibrium control. The reversible nature of the reactions introduces the prospects of “error checking” and “proof-reading” into synthetic processes where dynamic covalent chemistry operates. Since the formation of products occurs under thermodynamic control, product distributions depend only on the relative stabilities of the final products. In kinetically controlled reactions, however, it is the free energy differences between the transition states leading to the products that determines their relative proportions. Supramolecular chemistry has had a huge impact on synthesis at two levels: one is noncovalent synthesis, or strict self-assembly, and the other is supramolecular assistance to molecular synthesis, also referred to as self-assembly followed by covalent modification. Noncovalent synthesis has given us access to finite supermolecules and infinite supramolecular arrays. Supramolecular assistance to covalent synthesis has been exploited in the construction of more-complex systems, such as interlocked molecular compounds (for example, catenanes and rotaxanes) as well as container molecules (molecular capsules). The appealing prospect of also synthesizing these types of compounds with complex molecular architectures using reversible covalent bond forming chemistry has led to the development of dynamic covalent chemistry. Historically, dynamic covalent chemistry has played a central role in the development of conformational analysis by opening up the possibility to be able to equilibrate configurational isomers, sometimes with base (for example, esters) and sometimes with acid (for example, acetals). These stereochemical “balancing acts” revealed another major advantage that dynamic covalent chemistry offers the chemist, which is not so easily accessible in the kinetically controlled regime: the ability to re-adjust the product distribution of a reaction, even once the initial products have been formed, by changing the reaction's environment (for example, concentration, temperature, presence or absence of a template). This highly transparent, yet tremendously subtle, characteristic of dynamic covalent chemistry has led to key discoveries in polymer chemistry. In this review, some recent examples where dynamic covalent chemistry has been demonstrated are shown to emphasise the basic concepts of this area of science.
This review discusses the synthesis of mechanically interlocked molecules where templates orient the reactants to produce permanent structures as the result of new linkages. An introduction outlines the concepts and opportunities of the field, paying special attention to the components of chemical templates. Next, the chemical template types most successfully applied to the synthesis of new interlocked molecular structures, metal ion templates, hydrogen bonded templates, cyclodextrin templates, and π-donor π-acceptor templates, are described. The progress each template type has made towards the goal of true polymeric interlocked structures is noted. The conclusion summarizes the current state of the field and points out new directions that appear ripe for future exploration.
Formation of an imine--from an amine and an aldehyde--is a reversible reaction which operates under thermodynamic control such that the formation of kinetically competitive intermediates are, in the fullness of time, replaced by the thermodynamically most stable product(s). For this fundamental reason, the imine bond has emerged as an extraordinarily diverse and useful one in the hands of synthetic chemists. Imine bond formation is one of a handful of reactions which define a discipline known as dynamic covalent chemistry (DCC), which is now employed widely in the construction of exotic molecules and extended structures on account of the inherent 'proof-reading' and 'error-checking' associated with these reversible reactions. While both supramolecular chemistry and DCC operate under the regime of reversibility, DCC has the added advantage of constructing robust molecules on account of the formation of covalent bonds rather than fragile supermolecules resulting from noncovalent bonding interactions. On the other hand, these products tend to require more time to form--sometimes days or even months--but their formation can often be catalysed. In this manner, highly symmetrical molecules and extended structures can be prepared from relatively simple precursors. When DCC is utilised in conjunction with template-directed protocols--which rely on the use of noncovalent bonding interactions between molecular building blocks in order to preorganise them into certain relative geometries as a prelude to the formation of covalent bonds under equilibrium control--an additional level of control of structure and topology arises which offers a disarmingly simple way of constructing mechanically-interlocked molecules, such as rotaxanes, catenanes, Borromean rings, and Solomon knots. This tutorial review focuses on the use of dynamic imine bonds in the construction of compounds and products formed with and without the aid of additional templates. While synthesis under thermodynamic control is giving the field of chemical topology a new lease of life, it is also providing access to an endless array of new materials that are, in many circumstances, simply not accessible using more traditional synthetic methodologies where kinetic control rules the roost. One of the most endearing qualities of chemistry is its ability to reinvent itself in order to create its own object, as Berthelot first pointed out a century and a half ago.
Two series of oligorotaxanes R and R' that contain -CH(2)NH(2)(+)CH(2)- recognition sites in their dumbbell components have been synthesized employing template-directed protocols. [24]Crown-8 rings self-assemble by a clipping strategy around each and every recognition site using equimolar amounts of 2,6-pyridinedicarboxaldehyde and tetraethyleneglycol bis(2-aminophenyl) ether to efficiently provide up to a [20]rotaxane. In the R series, the -NH(2)(+)- recognition sites are separated by trismethylene bridges, whereas in the R' series the spacers are p-phenylene linkers. The underpinning idea here is that in the former series, the recognition sites are strategically positioned 3.5 Å apart from one another so as to facilitate efficient [π···π] stacking between the aromatic residues in contiguous rings in the rotaxanes and consequently, a discrete rigid and rod-like conformation is realized; these noncovalent interactions are absent in the latter series rendering them conformationally flexible/nondiscrete. Although in the R' series, the [3]-, [4]-, [8]-, and [12]rotaxanes were isolated after reaction times of <5-30 min in yields of 72-85%, in the R series, the [3]-, [4]-, [5]-, [8]-, [12]-, [16]-, and [20]rotaxanes were isolated in <5 min to 14 h in 88-98% yields. It follows that while in the R' series the higher order oligorotaxanes are formed in lower yields more rapidly, in the R series, the higher order oligorotaxanes are formed in higher yields more slowly. In the R series, the high percentage yields are sustained throughout, despite the fact that up to 39 components are participating in the template-directed self-assembly process. Simple arithmetic reveals that the conversion efficiency for each imine bond formation peaks at 99.9% in the R series and 99.3% in the R' series. This maintenance of reaction efficiency in the R series can be ascribed to positive cooperativity, that is, when one ring is formed it aids and abets the formation of subsequent rings presumably because of stabilizing extended [π···π] stacking interactions between the arene units. Experiments have been performed wherein the dumbbell is starved of the macrocyclic components, and up to five times more of the fully saturated rotaxane is formed than is predicted based on a purely statistical outcome, providing a clear indication that positive cooperativity is operative. Moreover, it would appear that as the R series is traversed from the [3]- to the [4]- to the [5]rotaxane, the cooperativity becomes increasingly positive. This kind of cooperative behavior is not observed for the analogous oligorotaxanes in the R' series. The conventional bevy of analytical techniques (e.g., HR-MS (ESI) and both (1)H and (13)C NMR spectroscopy) help establish the fact that all the oligorotaxanes are pure and monodisperse. Evidence of efficient [π···π] stacking between contiguous arene units in the rings in the R series is revealed by (1)H NMR spectroscopy. Ion-mobility mass spectrometry performed on the R and R' series yielded the collisional cross sections (CCSs), confirming the rigidity of the R oligorotaxanes and the flexibility of the R' ones. The extended [π···π] stacking interactions are found to be present in the solid-state structures of the [3]- and [4]rotaxanes in the R series and also on the basis of molecular mechanics calculations performed on the entire series of oligomers. The collective data presented herein supports our original design in that the extended [π···π] stacking between contiguous arene units in the rings of the R series of oligorotaxanes facilitate an essentially rigid rod-like conformation with evidence that positive cooperativity improves the efficiency of their formation. This situation stands in sharp contrast to the conformationally flexible R' series where the oligorotaxanes form with no cooperativity.
Two in one: Two pseudorotaxanes can be combined to form a twin-axial hetero[7]rotaxane (see picture) by using the copper-catalyzed alkyne-azide "click" reaction. The synthetic route, in which twin-axial and single-axial rotaxanes are formed, combines self-assembly and the formation of covalent bonds to ensure the correct positioning of the two types of rings in the final product.
More than a quarter of a century after the first metal template synthesis of a [2]catenane in Strasbourg, there now exists a plethora of strategies available for the construction of mechanically bonded and entwined molecular level structures. Catenanes, rotaxanes, knots and Borromean rings have all been successfully accessed by methods in which metal ions play a pivotal role. Originally metal ions were used solely for their coordination chemistry; acting either to gather and position the building blocks such that subsequent reactions generated the interlocked products or by being an integral part of the rings or "stoppers" of the interlocked assembly. Recently the role of the metal has evolved to encompass catalysis: the metal ions not only organize the building blocks in an entwined or threaded arrangement but also actively promote the reaction that covalently captures the interlocked structure. This Review outlines the diverse strategies that currently exist for forming mechanically bonded molecular structures with metal ions and details the tactics that the chemist can utilize for creating cross-over points, maximizing the yield of interlocked over non-interlocked products, and the reactions-of-choice for the covalent capture of threaded and entwined intermediates.
Using chemical intuition often allows one to predict what might
transpire on throwing a batch of chemicals into a beaker, but sometimes
the unexpected can occur. Bruce C. Gibb discusses how you define an
'emergent phenomenon', recognizing that it's not a simple exercise and
can actually be different for each of us.
The combination of fullerenes and mechanically interlocked molecular architectures has opened up a new field of research. A selection of representative examples of mechanically interlocked molecules functionalised with fullerenes are discussed here.
Durch strategisches Platzieren von Erkennungsstellen für sekundäre Dialkylammoniumionen im magischen Abstand von 3.5 Å in der Hantelkomponente einer Serie von Oligorotaxanen (siehe Bild) werden andernfalls konformativ flexible [n]Rotaxane starr und stabförmig gemacht. Die Oligorotaxane entstehen mit nahezu quantitativem Umsatz unter thermodynamischer Kontrolle mittels dynamischer kovalenter Chemie.