Efficient Multicomponent Active Template Synthesis of Catenanes

Advancements and strategic approaches in catenane synthesis

Catenanes, a distinctive category of mechanically interlocked molecules composed of intertwined macrocycles, have undergone significant advancements since their initial stages characterized by inefficient statistical synthesis methods. Through the aid of molecular recognition processes and principles of self-assembly, a diverse array of catenanes with intricate structures can now be readily accessed utilizing template-directed synthetic protocols. The rapid evolution and emergence of this field have catalyzed the design and construction of artificial molecular switches and machines, leading to the development of increasingly integrated functional systems and materials. This review endeavors to explore the pivotal advancements in catenane synthesis from its inception, offering a comprehensive discussion of the synthetic methodologies employed in recent years. By elucidating the progress made in synthetic approaches to catenanes, our aim is to provide a clearer understanding of the future challenges in further advancing catenane chemistry from a synthetic perspective.

Stepwise Dissipative Control of Multimodal Motion in a Silver(I) Catenate

Stepwise dissipative control of two distinct motions, i.e., shuttling and sliding, is demonstrated in a single multicomponent device. When [2]rotaxane 1, which acts as a biped, and deck 2 were treated with AgBF4/PhCH2Br+NEt3 as chemical fuel, the transient catenate [Ag(1)]+ ⋅ [Ag3(2)]3+ was instantly generated showing multimodal motion and autonomous return to 1 and 2. In the dissipative process, catenate [Ag(1)]+ ⋅ [Ag3(2)]3+ cleanly transformed into the follow-up transient device (1) ⋅ [Ag3(2)]3+ exhibiting only sliding motion. Two interference-free dissipative cycles proved the resilience and robustness of the process.

A High-Yielding Active Template Click Reaction (AT-CuAAC) for the Synthesis of Mechanically Interlocked Nanohoops

Mechanically interlocked molecules (MIMs) represent an exciting yet underexplored area of research in the context of carbon nanoscience. Recently, work from our group and others has shown that small carbon nanotube fragments-[n]cycloparaphenylenes ([n]CPPs) and related nanohoop macrocycles-may be integrated into mechanically interlocked architectures by leveraging supramolecular interactions, covalent tethers, or metal-ion templates. Still, available synthetic methods are typically difficult and low yielding, and general methods that allow for the creation of a wide variety of these structures are limited. Here we report an efficient route to interlocked nanohoop structures via the active template Cu-catalyzed azide-alkyne cycloaddition (AT-CuAAC) reaction. With the appropriate choice of substituents, a macrocyclic precursor to 2,2'-bipyridyl embedded [9]CPP (bipy[9]CPP) participates in the AT-CuAAC reaction to provide [2]rotaxanes in near-quantitative yield, which can then be converted into the fully π-conjugated catenane structures. Through this approach, two nanohoop[2]catenanes are synthesized which consist of a bipy[9]CPP catenated with either Tz[10]CPP or Tz[12]CPP (where Tz denotes a 1,2,3-triazole moiety replacing one phenylene ring in the [n]CPP backbone).

A Platform Approach to Cleavable Macrocycles for the Controlled Disassembly of Mechanically Caged Molecules

Inspired by interlocked oligonucleotides, peptides and knotted proteins, synthetic systems where a macrocycle cages a bioactive species that is "switched on" by breaking the mechanical bond have been reported. However, to date, each example uses a bespoke chemical design. Here we present a platform approach to mechanically caged structures wherein a single macrocycle precursor is diversified at a late stage to include a range of trigger units that control ring opening in response to enzymatic, chemical, or photochemical stimuli. We also demonstrate that our approach is applicable to other classes of macrocycles suitable for rotaxane and catenane formation.

Facial Selectivity in Mechanical Bond Formation: Axially Chiral Enantiomers and Geometric Isomers from a Simple Prochiral Macrocycle

In 1971, Schill recognized that a prochiral macrocycle encircling an oriented axle led to geometric isomerism in rotaxanes. More recently, we identified an overlooked chiral stereogenic unit in rotaxanes that arises when a prochiral macrocycle encircles a prochiral axle. Here, we show that both stereogenic units can be accessed using equivalent strategies, with a single weak stereodifferentiating interaction sufficient for moderate to excellent stereoselectivity. Using this understanding, we demonstrated the first direct enantioselective (70% ee) synthesis of a mechanically axially chiral rotaxane.

A Modular Strategy for the Synthesis of Macrocycles and Medium-Sized Rings via Cyclization/Ring Expansion Cascade Reactions

Macrocycles and medium-sized rings are important in many scientific fields and technologies but are hard to make using current methods, especially on a large scale. Outlined herein is a strategy by which functionalized macrocycles and medium-sized rings can be prepared using cyclization/ring expansion (CRE) cascade reactions, without resorting to high dilution conditions. CRE cascade reactions are designed to operate exclusively via kinetically favorable 5-7-membered ring cyclization steps; this means that the problems typically associated with classical end-to-end macrocyclization reactions are avoided. A modular synthetic approach has been developed to facilitate the simple assembly of the requisite linear precursors, which can then be converted into an extremely broad range of functionalized macrocycles and medium-sized rings using one of nine CRE protocols.

A thermally driven rotaxane-catenane interconversion with a dynamic bis(hindered amino) disulfide

We have developed a versatile and simple synthetic method to produce a [3]catenane. Heating a rotaxane with bis(hindered amino) disulfide groups at both ends spontaneously and selectively produces the [3]catenane. The successful polymerization of the obtained [3]catenane provides a platform for the synthesis of various interlocking polymers.

Noncovalent Modification of Cycloparaphenylene by Catenane Formation Using an Active Metal Template Strategy

The active metal template (AMT) strategy is a powerful tool for the formation of mechanically interlocked molecules (MIMs) such as rotaxanes and catenanes, allowing the synthesis of a variety of MIMs, including π-conjugated and multicomponent macrocycles. Cycloparaphenylene (CPP) is an emerging molecule characterized by its cyclic π-conjugated structure and unique properties. Therefore, diverse modifications of CPPs are necessary for its wide application. However, most CPP modifications require early stage functionalization and the direct modification of CPPs is very limited. Herein, we report the synthesis of a catenane consisting of [9]CPP and a 2,2'-bipyridine macrocycle as a new CPP analogue that contains a reliable synthetic scaffold enabling diverse and concise post-modification. Following the AMT strategy, the [9]CPP-bipyridine catenane was successfully synthesized through Ni-mediated aryl-aryl coupling. Catalytic C-H borylation/cross-coupling and metal complexation of the bipyridine macrocycle moiety, an effective post-functionalization method, were also demonstrated with the [9]CPP-bipyridine catenane. Single-crystal X-ray structural analysis revealed that the [9]CPP-bipyridine catenane forms a tridentated complex with an Ag ion inside the CPP ring. This interaction significantly enhances the phosphorescence lifetime through improved intermolecular interactions.

Generating water/MeOH-soluble and luminescent polymers by grafting 2,6-bis(1,2,3-triazol-4-yl)pyridine (btp) ligands onto a poly(ethylene-alt-maleic anhydride) polymer and cross-linking with terbium(III)

The synthesis of two new polymers made from P(E-alt-MA) (poly(ethylene-alt-maleic anhydride) and possessing 2,6-bis(1,2,3-triazol-4-yl)pyridine (btp) ligand side chains in 3 and 6 mol%, respectively (P1 and P2, respectively) is described. These polymers were shown to be soluble in MeOH solution and, in the case of P1, also in water, while P2 needed prolonged heating to enable water dissolution. Btp ligands are known for coordinating both d- and f-metal ions and so, herein, we demonstrate by using both UV-Vis absorption, fluorescence emission, as well as time-gated phosphorescence spectroscopies, that both P1 and P2 can bind to Tb(III) ions to give rise to luminescent polymers. From the analysis of the titration data, which demonstrated large changes in the emission intensity properties of the polymer upon Tb(III) binding (ground state changes were also clearly observed, with the absorption being red-shifted at lower energy), we show that the dominant stoichiometry in solution is 1 : 2 (M : L; Tb(III) : btp ratio) which implies that two btp ligands from the polymer background are able to crosslink through lanthanide coordination and that the backbone of the polymer is very likely to aid in coordinating the ions.

Direct Detection of Hydrogen Bonds in Supramolecular Systems Using 1H-15N Heteronuclear Multiple Quantum Coherence Spectroscopy

Hydrogen-bonded supramolecular systems are usually characterized in solution through analysis of NMR data such as complexation-induced shifts and nuclear Overhauser effects (nOe). Routine direct detection of hydrogen bonding particularly in multicomponent mixtures, even with the aid of 2D NMR experiments for full assignment, is more challenging. We describe an elementary rapid ¹H-¹⁵N HMQC NMR experiment which addresses these challenges without the need for complex pulse sequences. Under readily accessible conditions (243/263 K, 50 mM solutions) and natural ¹⁵N abundance, unambiguous assignment of ¹⁵N resonances facilitates direct detection of intra- and intermolecular hydrogen bonds in mechanically interlocked structures and quadruply hydrogen-bonded dimers─of dialkylaminoureidopyrimidinones, ureidopyrimidinones, and diamidonaphthyridines─in single or multicomponent mixtures to establish tautomeric configuration, conformation, and, to resolve self-sorted speciation.

Mechanically axially chiral catenanes and noncanonical mechanically axially chiral rotaxanes

Chirality typically arises in molecules because of a rigidly chiral arrangement of covalently bonded atoms. Less generally appreciated is that chirality can arise when molecules are threaded through one another to create a mechanical bond. For example, when two macrocycles with chemically distinct faces are joined to form a catenane, the structure is chiral, although the rings themselves are not. However, enantiopure mechanically axially chiral catenanes in which the mechanical bond provides the sole source of stereochemistry have not been reported. Here we re-examine the symmetry properties of these molecules and in doing so identify a straightforward route to access them from simple chiral building blocks. Our analysis also led us to identify an analogous but previously unremarked upon rotaxane stereogenic unit, which also yielded to our co-conformational auxiliary approach. With methods to access mechanically axially chiral molecules in hand, their properties and applications can now be explored.

A Co-conformationally "Topologically" Chiral Catenane

Catenanes composed of two achiral rings that are oriented (C_nh symmetry) because of the sequence of atoms they contain are referred to as topologically chiral. Here, we present the synthesis of a highly enantioenriched catenane containing a related but overlooked "co-conformationally 'topologically' chiral" stereogenic unit, which arises when a bilaterally symmetric C_nv ring is desymmetrized by the position of an oriented macrocycle.

Distinctive features and challenges in catenane chemistry

From being an aesthetic molecular object to a building block for the construction of molecular machines, catenanes and related mechanically interlocked molecules (MIMs) continue to attract immense interest in many research areas. Catenane chemistry is closely tied to that of rotaxanes and knots, and involves concepts like mechanical bonds, chemical topology and co-conformation that are unique to these molecules. Yet, because of their different topological structures and mechanical bond properties, there are some fundamental differences between the chemistry of catenanes and that of rotaxanes and knots although the boundary is sometimes blurred. Clearly distinguishing these differences, in aspects of bonding, structure, synthesis and properties, between catenanes and other MIMs is therefore of fundamental importance to understand their chemistry and explore the new opportunities from mechanical bonds.

Macrocyclic vs. [2]catenane btp structures: influence of (aryl) substitution on the self templation of btp ligands in macrocyclic synthesis

The synthesis of four 2,6-bis(1,2,3-triazol-4-yl)pyridine (btp) olefin based ligands 3, 4, 11 and 12 is described and their attempted use to form mechanically interlocked molecules using ring closing metatheses (RCM) reactions. The btp ligands were modified in two ways, in 3 and 4 the aryl substitution pattern was changed from 4^th position to 3^rd position and in the case of 11 and 12, the arms were replaced with aliphatic chains. Our study demonstrates that for all four ligands, the RCM reactions only result in the formation of macrocyclic structures, which in three of the cases, were structurally characterised in both solution (using NMR and HRMS) and in the solid-state using X-ray crystallography. NMR studies were also carried out to investigate if these ligands could preorganise in solution via hydrogen bonding interactions. This study provides a handle of how such precursor substitution can be used to direct the formation of macrocycles or mechanically interlocked structures.

A chiral interlocking auxiliary strategy for the synthesis of mechanically planar chiral rotaxanes

Rotaxanes can display molecular chirality solely due to the mechanical bond between the axle and encircling macrocycle without the presence of covalent stereogenic units. However, the synthesis of such molecules remains challenging. We have discovered a combination of reaction partners that function as a chiral interlocking auxiliary to both orientate a macrocycle and, effectively, load it onto a new axle. Here we use these substrates to demonstrate the potential of a chiral interlocking auxiliary strategy for the synthesis of mechanically planar chiral rotaxanes by producing a range of examples in high enantiopurity (93–99% e.e.), including so-called ‘impossible’ rotaxanes whose axles lack any functional groups that would allow their direct synthesis by other means. Intriguingly, by varying the order of bond-forming steps, we can effectively choose which end of an axle the macrocycle is loaded onto, enabling the synthesis of both hands of a single target using the same reactions and building blocks.

Can 2-Pyridyl-1,2,3-triazole "Click" Ligands be Used to Develop Cu(I)/Cu(II) Molecular Switches?

Molecular switching processes are important in a range of areas including the development of molecular machines. While there are numerous organic switching systems available, there are far less examples that exploit inorganic materials. The most common inorganic switching system remains the copper(I)/copper(II) switch developed by Sauvage and co-workers over 20 years ago. Herein, we examine if bidentate 2-(1-benzyl-1H-1,2,3-triazol-4-yl)pyridine (pytri) and tridentate 2,6-bis[(4-phenyl-1H-1,2,3-triazol-1-yl)methyl]pyridine (tripy) moieties can be used to replace the more commonly exploited polypyridyl ligands 2,2'-bypyridine (bpy)/1,10-phenanthroline (phen) and 2,2';6',2″-terpyridine (terpy) in a copper(I)/(II) switching system. Two new ditopic ligands that feature bidentate (pytri, L1 or bpytri, L2) and tridentate tripy metal binding pockets were synthesized and used to generate a family of heteroleptic copper(I) and copper(II) 6,6'-dimesityl-2,2'-bipyridine (diMesbpy) complexes. Additionally, we synthesized a series of model copper(I) and copper(II) diMesbpy complexes. A combination of techniques including nuclear magnetic resonance (NMR) and UV-vis spectroscopies, high-resolution electrospray ionization mass spectrometry, and X-ray crystallography was used to examine the behavior of the compounds. It was found that L1 and L2 formed [(diMesbpy)Cu(L1 or L2)]²⁺ complexes where the copper(II) diMesbpy unit was coordinated exclusively in the tridenate tripy binding site. However, when the ligands (L1 and L2) were complexed with copper(I) diMesbpy units, a complex mixture was obtained. NMR and MS data indicated that a 1:1 stoichiometry of [Cu(diMesbpy)]⁺ and either L1 or L2 generated three complexes in solution, the dimetallic [(diMesbpy)₂Cu₂(L1 or L2)]²⁺ and the monometallic [(diMesbpy)Cu(L1 or L2)]⁺ isomers where the [Cu(diMesbpy)]⁺ unit is coordinated to either the bidentate or tridentate tripy binding sites of the ditopic ligands. The dimetallic [(diMesbpy)₂Cu₂(L1 or L2)](PF₆)₂ complexes were structurally characterized using X-ray crystallography. Both complexes feature a [Cu(diMesbpy)]⁺ coordinated to the bidentate (pytri or bpytri) pocket of the ditopic ligands (L1 or L2), as expected. They also feature a second [Cu(diMesbpy)]⁺ coordinated to the nominally tridentate tripy binding site in a four-coordinate hypodentate κ²-fashion. Competition experiments with model complexes showed that the binding strength of the bidentate pytri is similar to that of the κ²-tripy ligand, leading to the lack of selectivity. The results suggest that the pytri/tripy and bpytri/tripy ligand pairs cannot be used as replacements for the more common bpy/phen-terpy partners due to the lack of selectivity in the copper(I) state.

High-component reactions (HCRs): An overview of MCRs containing seven or more components as versatile tools in organic synthesis

Recently, multi-component reactions (MCRs) have gained special attention due to their versatility for the synthesis of polycyclic heterocycles. Moreover, their applicability can become more widespread as they can be combined together as a union of MCRs. In this overview, the authors have tried to collect the MCRs containing more than seven components that can lead to effectual heterocycles in organic and/or pharmaceutical chemistry. The review contains papers published up to the end of 2020. The subject is classified based on the number of substrates, such as seven-, eight-, nine-, ten-, and more components. The authors expect their report to be helpful for researchers to clarify their route to significant MCRs.

Rotaxane CoII Complexes as Field-Induced Single-Ion Magnets

Mechanically chelating ligands have untapped potential for the engineering of metal ion properties. Here we demonstrate this principle in the context of CoII -based single-ion magnets. Using multi-frequency EPR, susceptibility and magnetization measurements we found that these complexes show some of the highest zero field splittings reported for five-coordinate CoII complexes to date. The predictable coordination behaviour of the interlocked ligands allowed the magnetic properties of their CoII complexes to be evaluated computationally a priori and our combined experimental and theoretical approach enabled us to rationalize the observed trends. The predictable magnetic behaviour of the rotaxane CoII complexes demonstrates that interlocked ligands offer a new strategy to design metal complexes with interesting functionality.

AT-CuAAC Synthesis of Mechanically Interlocked Oligonucleotides

We present a simple strategy for the synthesis of main chain oligonucleotide rotaxanes with precise control over the position of the macrocycle. The novel DNA-based rotaxanes were analyzed to assess the effect of the mechanical bond on their properties.

Weak functional group interactions revealed through metal-free active template rotaxane synthesis

Modest functional group interactions can play important roles in molecular recognition, catalysis and self-assembly. However, weakly associated binding motifs are often difficult to characterize. Here, we report on the metal-free active template synthesis of [2]rotaxanes in one step, up to 95% yield and >100:1 rotaxane:axle selectivity, from primary amines, crown ethers and a range of C=O, C=S, S(=O)2 and P=O electrophiles. In addition to being a simple and effective route to a broad range of rotaxanes, the strategy enables 1:1 interactions of crown ethers with various functional groups to be characterized in solution and the solid state, several of which are too weak - or are disfavored compared to other binding modes - to be observed in typical host-guest complexes. The approach may be broadly applicable to the kinetic stabilization and characterization of other weak functional group interactions.

Rotaxane PtII-complexes: mechanical bonding for chemically robust luminophores and stimuli responsive behaviour

We report an approach to rotaxanes in which the metal ion of a cyclometallated Pt^II luminophore is embedded in the space created by the mechanical bond. Our results show that the interlocked ligand environment stabilises a normally labile Pt^II–triazole bond against displacement by competing ligands and that the crowded environment of the mechanical bond retards oxidation of the Pt^II centre, without perturbing the photophysical properties of the complex. When an additional pyridyl binding site is included in the axle, the luminescence of the Pt^II centre is quenched, an effect that can be selectively reversed by the binding of Ag^I. Our results suggest that readily available interlocked metal-based phosphors can be designed to be stimuli responsive and have advantages as stabilised triplet harvesting dopants for device applications.

We report an approach to interlocked Pt^II luminophores in which the mechanical bond stabilises the coordination environment of the embedded metal ion.

A Potent Halogen-Bonding Donor Motif for Anion Recognition and Anion Template Mechanical Bond Synthesis

The covalent attachment of electron deficient perfluoroaryl substituents to a bis-iodotriazole pyridinium group produces a remarkably potent halogen bonding donor motif for anion recognition in aqueous media. Such a motif also establishes halogen bonding anion templation as a highly efficient method for constructing a mechanically interlocked molecule in unprecedented near quantitative yield. The resulting bis-perfluoroaryl substituted iodotriazole pyridinium axle containing halogen bonding [2]rotaxane host exhibits exceptionally strong halide binding affinities in competitive 50 % water containing aqueous media, by a factor of at least three orders of magnitude greater in comparison to a hydrogen bonding rotaxane host analogue. These observations further champion and advance halogen bonding as a powerful tool for recognizing anions in aqueous media.

Active template rotaxane synthesis through the Ni-catalyzed cross-coupling of alkylzinc reagents with redox-active esters

The synthesis of unsymmetrical axle [2]rotaxanes through a recently developed Ni-catalyzed C(sp3)-C(sp3) cross-coupling of redox-active esters (formed directly from carboxylic acids) and organozinc reagents (derived from alkyl bromides) is reported. The method also furnishes, as a minor product, the symmetrical axle [2]rotaxanes resulting from the homo-coupling of the organozinc half-thread. The rotaxanes are formed in up to 56% yield with the ratio of unsymmetrical rotaxane increasing with the cavity size of the macrocycle. In the absence of the redox-active ester neither rotaxane is formed, even though the homo-coupling rotaxane product does not incorporate the redox-active ester building block. A Ni(iii) intermediate is consistent with these observations, providing support for the previously postulated mechanism of the Ni-catalyzed cross-coupling reaction.

An Auxiliary Approach for the Stereoselective Synthesis of Topologically Chiral Catenanes

Catenanes, molecules in which two rings are threaded through one another like links in a chain, can form as two structures related like an object and its mirror image but otherwise identical if the individual rings lack bilateral symmetry. These structures are described as “topologically chiral” because, unlike most chiral molecules, it is not possible to convert one mirror-image form to the other under the rules of mathematical topology. Although intriguing and discussed as early as 1961, to date all methods of accessing molecules containing only this topological stereogenic element require the separation of the mirror-image forms via chiral stationary phase high-performance liquid chromatography, which has limited their investigation to date. Here, we present a simple method that uses a readily available source of chiral information to allow the stereoselective synthesis of topologically chiral catenanes.

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First stereoselective synthesis of a topologically chiral catenane

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First absolute stereochemical assignment of a topologically chiral catenane

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First example of an auxiliary approach to topologically chiral catenanes

Chiral molecules have occupied a special place in chemistry since Pasteur reported the painstaking separation of mirror-image crystals of tartaric acid salts in 1848. In the 21^st century, chiral molecules remain a major scientific focus because of their importance in biology and their emerging applications in materials science. However, topologically chiral molecules, such as the catenanes described here, have received little attention because they are hard to make; preparative chiral stationary phase high-performance liquid chromatography allows the separation of their mirror-image forms but only on a very small scale. Here, we demonstrate the synthesis of topologically chiral catenanes by using standard synthetic techniques, marking their transition from “inaccessible curiosities” to valid synthetic targets for investigation in catalysis, sensing, medicinal chemistry, and materials science. Furthermore, this work will inspire efforts to access other neglected classes of chiral interlocked molecules.

Strain effects determine the performance of artificial allosteric systems: calixarenes as models

It is shown that the performance of allosteric systems regarding the efficiency and the speed of response depends critically on the strain energy of the equilibrating conformers and of the corresponding interconversion transition state. The affinity of a substrate A can be large enough to overcome in the absence of an effector E by induced fit the strain involved in the formation of an optimal conformation for binding A. The efficiency as given by the ratio KAE/KA of binding constants in the presence or absence of an effector is, for many published synthetic allosteric systems, relatively low; in practice this means that these only function within rather limited concentration ranges. A small KAE/KA ratio means that the binding strength of A or the corresponding signal will increase only little by adding an effector, and may need higher concentration of E. Implementation of steric distortion in the minor conformer can lead to reduced binding of A in the absence of the effector E. Destabilization of conformers can also result from the inclusion of high energy water molecules within a cavity. Furthermore, until now it has been overlooked that strain in the transition state can lead to reaction times of up to days, and thus to the neglect of experimental observation. The role of conformational changes within an allosteric molecule is characterized with a variety of calixarenes and other compound classes, offering a clue for the design of more efficient synthetic systems with high cooperativity.

Chemical Consequences of the Mechanical Bond: A Tandem Active Template-Rearrangement Reaction

We report the unexpected discovery of a tandem active template CuAAC-rearrangement process, in which N2 is extruded on the way to the 1,2,3-triazole product to give instead acrylamide rotaxanes. Mechanistic investigations suggest this process is dictated by the mechanical bond, which stabilizes the CuI -triazolide intermediate of the CuAAC reaction and diverts it down the rearrangement pathway; when no mechanical bond is formed, the CuAAC product is isolated.

Self-templated synthesis of amide catenanes and formation of a catenane coordination polymer

A self-templation strategy was used to synthesise isophthalamide [2]catenanes of various sizes in up to 51% yield without the need for metal ions as templates or mediators of covalent bond formation. Using this strategy a bis-monodentate catenane was prepared incorporating exohedral pyridine units. Upon complexation of this ligand with AgOTf a one-dimensional coordination polymer was obtained in the solid state in which both macrocycles of the catenane are involved in binding to the metal nodes, resulting in a rare example of a coordination assembly in which mechanical bonds are incorporated into the structure backbone.

Stereoselective Synthesis of Mechanically Planar Chiral Rotaxanes

Chiral interlocked molecules in which the mechanical bond provides the sole stereogenic unit are typically produced with no control over the mechanical stereochemistry. Here we report a stereoselective approach to mechanically planar chiral rotaxanes in up to 98:2 d.r. using a readily available α-amino acid-derived azide. Symmetrization of the covalent stereocenter yields a rotaxane in which the mechanical bond provides the only stereogenic element.

Chirality in rotaxanes and catenanes

Although chiral mechanically interlocked molecules (MIMs) have been synthesised and studied, enantiopure examples are relatively under-represented in the pantheon of reported catenanes and rotaxanes and the underlying chirality of the system is often even overlooked. This is changing with the advent of new applications of MIMs in catalysis, sensing and materials and the appearance of new methods to access unusual stereogenic units unique to the mechanical bond. Here we discuss the different stereogenic units that have been investigated in catenanes and rotaxanes, examples of their application, methods for assigning absolute stereochemistry and provide a perspective on future developments.