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Rotaxanes are a type of mechanically interlocked molecules (MIMs), constituted by at least a thread surrounded by a wheel, which are widely employed in the research field of artificial molecular machines. Although applications retaining the integrity of the mechanical bond are usually reported, the dethreading of the components can be crucial to develop some advanced applications. Thus, different dethreading strategies have been reported, and advanced applications which require such a process have turned out to be suitable approaches towards machine‐like operation. This review article covers recent examples of applications of pseudorotaxanes and rotaxanes in which dethreading processes have a key role to accomplish the desired function. [ABSTRACT FROM AUTHOR]

Here is reported the synthesis and study of dibenzo‐24‐crown‐8 (DB24C8)‐based [2]‐ and [3]rotaxanes that contain an ammonium as the best molecular station and carbamate moieties as secondary molecular stations. The common post‐interlocking synthesis relies on the aminolysis of the N‐succinimidyl carbonate extremity of an activated though insulated pseudorotaxane. The N‐succinimidyl carbonate‐based thread's extremity proved to be small enough to allow the slow slippage of the DB24C8 around the thread and large enough to allow insulation of the kinetically stable pseudorotaxane. Due to its sensitivity towards amine compounds, the activated carbonate end of the encircled thread allowed post‐interlocking aminolysis‐based conversion into mechanically interlocked rotaxanes by providing the same way the carbamate secondary station for the DB24C8 in the thread backbone. Translation of the DB24C8 along the threaded axle between the two molecular stations was investigated. [ABSTRACT FROM AUTHOR]

Heteroditopic calix[6]arenes are widely employed macrocycles for the synthesis of pseudo‐oriented interlocked systems and stimuli‐induced molecular machines, among others. Herein, we introduce a new calix[6]arene receptor functionalised with three phenylselenoureido groups. These hydrogen‐bonding donor moieties are able to promote the threading of viologen‐based organic axles inside the macrocyclic cavity to form a stable pseudorotaxane species. Preliminary investigations using 1H‐NMR measurements and semi‐empirical and DFT studies suggested its further use as a platform for the synthesis of rotaxanes. [ABSTRACT FROM AUTHOR]

A homologous series of two‐station [2]rotaxanes incorporating amino‐acid units in the molecular thread has been developed. The degenerate [2]rotaxanes exhibit amino‐acid specific shuttling rates between two fumaric stations related to the steric factor associated to the amino‐acid side chain, as demonstrated by variable‐temperature 1H NMR spectroscopy and exchange spectroscopy (EXSY). This allows tuning of the macrocycle shuttling rate over 4 orders of magnitude, which has a relatively small solvent dependency. [ABSTRACT FROM AUTHOR]

This minireview overviews rotaxanes and pseudorotaxanes containing cyclodextrins (CDs) as the rotor component. Since CDs interact with molecules, e.g., hydrophobic compounds of the size and shape matching their cavity, to form inclusion complexes in aqueous media, CDs are an important class of macrocyclic compounds as rotor components of rotaxanes and pseudorotaxanes. Research on CD‐based rotaxanes has begun with the synthesis of rotaxanes from low molecular weight axis molecules and then developed into the construction of molecular machines and polyrotaxanes. Recently, the research has further developed into macroscopic functional materials, e.g., slide‐ring gels, macromolecular assemblies, self‐healing materials, and soft actuators. In the present article, typical examples of CD‐based rotaxanes and pseudorotaxanes are described; synthesis of rotaxane and construction of molecular machine are described in Section 2, synthesis of polyrotaxanes is dealt with in Section 3, and development of macroscopic functional materials based on rotaxane structure is discussed in Section 4. [ABSTRACT FROM AUTHOR]

The present study discusses the crystalline packing formation of several [2]rotaxanes with Leigh‐type tetralactam macrocycle bearing different threads. The presence of solvent molecules in some structures are also addressed to shed some light on this matter. Additionally, the degree of similarity between supramolecular structures of rotaxanes was discussed using similarity indices. For this, new descriptors and crystallization mechanisms, which were proposed in terms of contact area and stabilization energy, were carried out to evaluate the rotaxane molecules. It was possible to observe similar general stages of crystallization dominated by the formation of 1D‐blocks and, in fewer cases, by dimers in the first stage of nucleation. The preference for the formation of 1D nuclei resides in the large contact area and complementarity involved in the large set of interactions between the rotaxanes at the earliest stages of crystallization. In this context, it was possible to propose when solvent molecules are trapped between the rotaxanes during crystal formation. Therefore, a unique example of a rotaxane whose topology favored the entrapment of water molecules between rotaxanes during the first stage of the crystallization process is presented. Crystallization mechanisms showed to be a valuable asset in the supramolecular investigation of rotaxanes in the crystalline state. [ABSTRACT FROM AUTHOR]

One‐pot synthesis of various α‐cyclodextrin‐(α‐CD‐) based [3]rotaxanes in water and their structural modifications are discussed in detail. Pseudo[3]rotaxane was prepared from α‐CD and α,ω‐diaminododecane in water (above 100 g, quantitative yield in a 1 L flask). Successive urea‐forming end‐capping reactions with isocyanate in water selectively afforded [3]rotaxane with head‐to‐head structured α‐CDs. The yields were varied by bulkiness and functionality of end‐capping agents. Significant modifications of the axle end and wheel OH group of the [3]rotaxanes were performed by the Suzuki coupling with arylboronic acids and by the perfect acylation with acid anhydrides, respectively. [ABSTRACT FROM AUTHOR]

Research at the University of Freiburg is summarized which aimed at realizing catenanes, rotaxanes, and a molecular knot from 61–62 until 41 years ago. Taking a fresh view at ansa‐compounds – distinct from those in which he had been interested for other reasons before – Arthur Lüttringhaus began to tackle syntheses of catenanes in 1957. The first isolation of a pure catenane (37) succeeded in 1964 jointly with Gottfried Schill. Most of the progress made in the sequel was due to Schill alone or achieved under his direction; in addition, a few follow‐up studies were published without Schill or Lüttringhaus being their authors. The major feats of these endeavors were the syntheses of the following molecules: (1) the first catenanes (38 – de‐acetyl‐41, 74) without a ring in any constituent (this feature makes them unique till today; the catenane 74 holds the additional record of being the only all‐hydrocarbon catenane till now); the first [3]catenanes at all (obtained as a mixture of constitutional isomers C2v‐97 and C2h‐97); the first [3]catenanes exhibiting topological isomerism (89, iso‐89, and neo‐iso‐89); the rotaxanes 70 [which had the design peculiarity of being convertible into a catenane (74)], 113 (Schill's #1‐rotaxane), and 115; the pre‐knotane rac‐dia‐iso‐123 [provided it was obtained and not (only) its topological isomer dia‐iso‐123]. – For due perspectives, pertinent studies of third parties are included, too: 1) Wasserman's reports on the isolation of the catenane 8 are analyzed in great detail; the conclusion is that the identity of 8 was never established. 2) Schill's oscillating rotaxane 70 from 1988, which is subject to a reversible and non‐degenerate ring migration (→ iso‐70), is supplemented by Stoddart's much faster oscillating rotaxane 71 from 1991, which undergoes a reversible and degenerate ring migration (→ 71 instead of → iso‐71) and was dubbed "molecular shuttle". 3) The first [3]catenanes from the groups of Schill (97; 1969), Sauvage (98 and 99; 1985), and Stoddart (100–102; 1991) are juxtaposed. 4) The Harrisons' synthesis of the rotaxane 106 by a most noteworthy solid‐phase approach (1967) is recalled in the context of Schill's synthesis of the rotaxane 113 (1967 or 1969). 5) Attempts at threading α‐cyclodextrin by long‐chain 1,ω‐dithiols (→ 130) and ring‐closing the latter oxidatively for obtaining catenated disulfides failed in Lüttringhaus' 1957/1958 studies; in contrast, Stoddart's threading permethyl‐β‐cyclodextrin (133) by the long‐chain 1,ω‐diamine 132 and ring‐closing the latter by a double condensation with terephthaloyl dichloride was successful by providing 3 % of the catenated bislactame 134 in 1993. The concluding section characterizes the synthetic strategies towards catenanes and rotaxanes developed in Freiburg as "template‐based". This differs from the Nobel Prize Committee's assessment. [ABSTRACT FROM AUTHOR]

This study investigated the proper classification and quantification of intercomponent trifurcated hydrogen bonds, in the crystalline state, of rotaxanes bearing a tetralactam Leigh‐type macrocycle. Quantum mechanical calculations were carried out to obtain the interaction paths and their stabilization energies. In general, the use and necessity of fragmentation in types of interaction have been reported in the literature, although they are commonly performed only using a geometric approach. This results in N–H···O interactions that are indicated as the main interactions between components, neglecting other possible interactions. The use of energetic fragmentation was demonstrated here under an appropriate demarcation considering all interactions and showing the existence and role of the C–H···O interaction. The C–H···O interactions showed similar energy to N–H···O interactions with average values around –4.75 kcal mol–1 and –4.60 kcal mol–1, respectively. This revealed that the three hydrogen bonds are part of a cooperative set of interactions with a similar contribution. The trifurcated hydrogen bonds presented high contribution in the total intercomponent stabilization energy of the rotaxanes, with an average value of 51 % in the studied series. [ABSTRACT FROM AUTHOR]

Tailoring synthetic strategies toward complex, but well‐defined, molecular architectures is an important aspect of Today's organic chemistry. Herein, we present designs to build up [3]rotaxanes of Type 2.1 and Type 3 that feature both a polyyne and a cumulene axle in a single mechanically interlocked molecule. The combination of two sp‐hybridized carbon chains was achieved by functionalization of a phenanthroline‐based macrocycle, which allowed the coupling of two [2]rotaxanes toward Type 3 [3]rotaxanes with two polyynes, two cumulenes, or one polyyne and one cumulene axle. The structures have been established by NMR and UV/Vis spectroscopies, as well as high‐resolution mass spectrometry. A [3]rotaxane of Type 3, featuring two [9]cumulene axles, has been assembled and spectroscopically characterized despite expectations that it would be unstable. This represents only the second cumulene rotaxane reported to date. All designs and attempts to form [3]rotaxanes of Type 2.1, which combine a polyyne and cumulene threaded through a single macrocycle, have been unsuccessful. [ABSTRACT FROM AUTHOR]

Mechanically interlocked molecules (MIMs) have emerged as intriguing building blocks for the construction of stimuli‐responsive devices and materials. A particularly interesting and well‐implemented subclass of MIMs is composed of symmetric bistable [c2]daisy chain rotaxanes. Topologically, they consist in the double thread of two symmetric macrocycles that are covalently linked to an axle bearing two switchable stations and a bulky stopper to avoid unthreading. Herein we report the synthesis and characterization of a series of unsymmetric bistable [c2]daisy chain rotaxanes that present two different electroactive units as stoppers (an electron donor triarylamine and an electron acceptor perylene bisimide unit). Using a combination of 1D and 2D NMR along with cyclic voltammetry, we demonstrate that the pH actuation of the mechanical bond can be used to modulate the electrochemical properties of the bistable [c2]daisy chain rotaxanes when switching between the contracted and extended forms. [ABSTRACT FROM AUTHOR]

Pillar[5]arene‐containing [2]rotaxanes have been efficiently prepared under solvent‐free conditions. Specifically, solid‐to‐solid ball‐milling of primary or secondary amine reagents with the inclusion complex resulting from the association of dodecanedioyl dichloride and a pillar[5]arene derivative provided the corresponding diamide [2]rotaxanes in good to excellent yields. Importantly, [2]rotaxanes difficult or even impossible to prepare under classical reaction conditions in solution, are now easily obtained. [ABSTRACT FROM AUTHOR]

The thermally and photochemically induced dethreading of a series of pseudo[2]rotaxanes bearing tetraalkyl‐substituted fumaramide axles and polyamide macrocycles is reported herein. The length of the alkyl chains at the ends of the thread, the void size, and flexibility of the macrocycle and the intercomponent interactions are critical for a satisfactory dethreading process. 1H NMR kinetic experiments were carried out in order to calculate the rate constants of the thermal dethreading processes and stabilities of the rotaxanes, by comparing with the similar data for the deslipping of the succinamide surrogates. The deslipping reaction occurs faster with the fumaramide systems than succinamide ones. The incorporation of adamantane moieties into the polyamide macrocycle drastically slowed down the disconnecting process. Furthermore, interlocked systems with dipropylamino groups as stoppers experienced a rapid dethreading when irradiated under UV‐light, a process triggered by the fumaramide–maleamide isomerization. In this case, the n‐propyl groups are not bulky enough to keep together both components as the number of hydrogen bonds between thread and macrocycle is halved in the maleamide systems. [ABSTRACT FROM AUTHOR]

The activation energies of the shuttling of [2]rotaxanes with various N‐arylpyrrole moieties were determined by variable temperature (VT) 1H NMR experiments. The value provided a new quantitative scale for the estimation of the steric bulk of the substituents. The rigidity and the electronic effect of the substituents, which could inadequately influence the estimation of the steric bulk, were minimized in the method we presented. [ABSTRACT FROM AUTHOR]

Understanding the role played by the nature, number, and arrangement of binding sites anchored to a macrocycle remains a topic of current interest for the synthesis of new interwoven species. We report the synthesis and detailed structural characterization of a new calix[6]arene derivative decorated with two phenylureido groups anchored at the diametrical position of the calix upper rim that adopts a 1,2,3‐alternate conformation in solution and in the solid state. Preliminary data on the ability of this host to form redox‐active pseudorotaxanes and rotaxanes are reported. [ABSTRACT FROM AUTHOR]

Two [2]rotaxanes have been assembled in water from modular subunits through CuI-catalyzed azide--alkyne "click" chemistry. For this purpose, 2,6-disubstituted naphthalene axles with solubilizing oligo(ethylene glycol) (OEG) chains (n = 1-5) and propargyl terminal groups were synthesized and examined for their propensity to form inclusion complexes with a dicationic Diederich-type cyclophane host. The dependence of pseudorotaxane formation on the linkers between the naphthalene core and OEG chains, and in the case of ester linkers on different spacer lengths, was analyzed by titration experiments. In addition, the inclusion complexes of two [2]rotaxanes were trapped by using a water-soluble azide-functionalized stopper. Repetitive chromatography finally enabled the isolation of both mechanically interlocked [2]rotaxanes. [ABSTRACT FROM AUTHOR]

Oriented rotaxanes composed of tris( N-phenylureido)calix[6]arene wheel 1 and N, N′-dialkyl viologen-based axles were synthesized in which the span between the diphenylacetyl stoppers at the wheel upper and lower rim and the bis-pyridinium cation portion of the axial component is different in length. A sequential synthetic pathway was followed to achieve the selective formation of orientational isomers, wherein the short (or long) chain of the axle is positioned either on the upper or lower rim of the wheel. The synthetic strategy adopted involves the upper rim threading of axles 2a- c to yield oriented pseudorotaxanes, which were then stoppered in situ. The structure of the oriented rotaxanes was unravelled by detailed NMR experiments, and their spectroscopic, electrochemical and dynamic properties were investigated by UV/Vis, cyclic voltammetry, and EPR measurements. The results were compared with those of their symmetric counterparts. [ABSTRACT FROM AUTHOR]

Rotaxanes are designated as molecular machines due their different movements. Systematic studies regarding the different conformations adopted by these systems and the factors that lead to the distribution of the conformations, in both solution and the solid state, have not been widely explored, especially for rotaxanes with nonsymmetric stoppers. Therefore, in this study we have investigated three novel [2]rotaxanes containing threads derived from nonsymmetric succinamides [R1R2NC(O)‐CH2CH2‐C(O)NR2R1, with R1/R2 = Bu/Bn, Bu/2‐furylmethyl, and 5‐methylisoxazol‐3‐yl/2‐furylmethyl]. The proportions of rotamers were investigated for threads and rotaxanes by solution and solid‐state NMR spectroscopy as well as by single‐crystal and powder X‐ray diffraction. In solution, the threads present different proportions of conformer, with the E,Z conformation prevailing, whereas only one conformer is observed in the solid state. For the rotaxanes, only one conformer prevails in the single crystal, whereas the solution and solid (bulk) states present more than one rotamer. These proportions are modified when the threads are incorporated into the macrocycle during rotaxane formation. The intramolecular interactions in each rotamer were investigated by QTAIM and variable‐temperature 1H NMR experiments. The changes in conformational population between the threads and respective rotaxanes can be explained by a set of different intramolecular interactions, with trifurcated hydrogen bonds responsible for most of the stabilization energy. [ABSTRACT FROM AUTHOR]

Alongside the use of metal cations and π–π stacking, hydrogen bonding is one of the major templating interactions used to prepare rotaxanes and catenanes. In this review, a brief summary of key historical milestones will be followed by discussion of developments from over the last decade in both synthetic methodology and application of hydrogen bond templated interlocked molecules. Hydrogen bond templation can allow for rapid access to interlocked molecules in high yields, with select examples having been put to useful purpose, in applications such as organocatalysis and cellular imaging. [ABSTRACT FROM AUTHOR]

Inherent chirality represents a distinct category of molecular chirality that does not fall within the traditional classification of four chiral elements: central, axial, planar, and helical chirality. While extensive research has been conducted on the catalytic enantioselective construction of these conventional chiralities, the corresponding synthesis of inherently chiral molecules has remained largely unexplored. This minireview provides a comprehensive summary of recent advancements in this field, focusing on the catalytic asymmetric synthesis of inherently chiral calixarenes, saddle‐shaped tetraphenylenes and their heterocycle derivatives, mechanically planar chiral rotaxanes and chiral multilayer 3D frameworks, as well as our perspective in this field. [ABSTRACT FROM AUTHOR]

Mechanically interlocked molecules have found extensive applications in areas all across the physical sciences, from materials to catalysis and sensing. However, introducing mechanical bonds and entanglements at the molecular level is still a significant challenge due to the inherent restriction in entropy needed to preorganize strands before interlocking. Over the last decade, dynamic covalent chemistry has emerged as one of the most efficient methods of forming rotaxanes, catenanes and molecular knots. By using reversible bonds such as imines, disulfides and boronate esters, one can use the inherent error‐correction in these linkages to form interlocked architectures with high fidelity and often in excellent yields. This review reports on recent advances in the use of dynamic covalent chemistry to make mechanically interlocked molecules, systematically surveying clipping, capping and templating approaches with dynamic bonds. Furthermore, it is also discussed how dynamic bonds can be used to control motion, co‐conformational expression and catalytic activity in mechanically interlocked molecular machinery. [ABSTRACT FROM AUTHOR]

Guest editor Jean‐Pierre Sauvage introduces the Special Issue dedicated to 50 Years of Rotaxanes. [ABSTRACT FROM AUTHOR]