Functional Mechanically Interlocked Molecules: Asymmetric Organocatalysis with a Catenated Bifunctional Brønsted Acid

Functionalization of a [2]Catenane with Donor-Acceptor Chromophores Using a Metal Template and Click Reactions

Synthesizing molecules with significant topological features, such as catenanes, tailored with specific groups to confer desired functionality, is essential for investigating various properties arising from the entanglement due to mechanical bonds. This investigation can pave the way for uncovering novel functional materials employing mechanically interlocked molecules (MIMs). In this direction, we have synthesized a π-donor (D) and π-acceptor (A) functionalized [2]catenane using a non-labile Co(III) metal ion as a template with pyridine-diamide templating center and utilizing click reaction for ring-closing. The donor group is a fluorene derivative, and the acceptor is a benzophenazine derivative, commonly employed in synthesizing conjugated polymers for various optoelectronic devices. Synthetically, the acceptor group was introduced into a macrocycle with a pyridine diamide unit. It was then threaded with a ligand having alkyne terminals to obtain the desired [2]pseudorotaxane utilizing cobalt ion as a template. Ring-closing was then performed with a di-azide functionalized molecule with the donor chromophore. The desired D-A functionalized [2]catenane was obtained after demetalation. All the starting materials, macrocycle, and entangled structures have been characterized by 1H-NMR, 13C-NMR, and mass spectroscopy. Some of these materials were also characterized by single-crystal X-ray analysis. The photophysical properties are studied by UV-visible and fluorescence spectroscopy.

Template Assisted Synthesis of Linear [5]Catenane by Post-Functionalization of Templated [2]Catenane and Using Click Reaction

Polymers with all mechanically interlocked rings, such as linear [n]catenanes, have great potential as functional materials due to possible higher degrees of freedom that may contribute to their flexibility but remain elusive. All the synthetic methods used to prepare such a polymer yield mixtures of products. In the absence of higher molecular weight linear [n]catenanes, emphasis on synthesizing low molecular weight oligomers is being pursued. Here, we have described the synthesis of a linear [5]catenane by post-functionalizing a Co(III) templated [2]catenane having a pyridine-diamide unit free for further metal ion coordination. Two molecules were synthesized with suitable threading groups: one, two terminal azide groups, and two, with two terminal alkyne groups to form two [3]pseudorotaxane utilizing Co(III) coordination. These units were then joined, forming a macrocycle, using click reaction, giving the desired metalated linear [5]catenane in 40 % yield. Removal of metal ions leads to linear [5]catenane. In addition, the formation of linear [3] and [2]catenane are also observed. All synthesized structures have been isolated by column chromatographic technique and characterized by 1H-NMR, 13C-NMR, and mass spectroscopy.

Highly acidic N-triflylphosphoramides as chiral Brønsted acid catalysts: the effect of weak hydrogen bonds and multiple acceptors on complex structures and aggregation

N-Triflylphosphoramides (NTPAs) represent an important catalyst class in asymmetric catalysis due to their multiple hydrogen bond acceptor sites and acidity, which is increased by several orders of magnitude compared to conventional chiral phosphoric acids (CPAs). Thus, NTPAs allow for several challenging transformations, which are not accessible with CPAs. However, detailed evidence on their hydrogen bonding situation, complex structures and aggregation is still lacking. Therefore, this study covers the hydrogen bonding behavior and structural features of binary NTPA/imine complexes compared to their CPA counterparts. Deviating from the single-well potential hydrogen bonds commonly observed in CPA/imine complexes, the NTPA/imine complexes exhibit a tautomeric equilibrium between two proton positions. Low-temperature NMR at 180 K supported by computer simulations indicates a OHN hydrogen bond between the phosphoramide oxygen and the imine, instead of the mostly proposed NHN H-bond. Furthermore, this study finds no evidence for the existence of dimeric NTPA/NTPA/imine complexes as previously suggested for CPA systems, both synthetically and through NMR studies.

Template Synthesis of Cyclometalated Macrocycle Iridium(III) Complexes Based on Photoinduced C-N Cross-Coupling Reactions In Situ

The synthesis of metal macrocycle complexes holds paramount importance in coordination and supramolecular chemistry. Toward this end, we report a new, mild, and efficient protocol for the synthesis of cyclometalated macrocycle Ir(III) complexes: [Ir(L1)](PF6) (1), [Ir(L2)](PF6) (2), and [Ir(L3)](PF6) (3), where L1 presents 10,17-dioxa-3,6-diaza-2(2,8),7(8,2)-diquinolina-1,8(1,4)-dibenzenacyclooctadecaphane, L2 is 10,13,16,19,22,25-hexaoxa-3,6-diaza-2(2,8),7(8,2)-diquinolina-1,8(1,4)-dibenzenacyclohexacosaphane, and L3 is 4-methyl-10,13,16,19,22,25-hexaoxa-3,6-diaza-2(2,8),7(8,2)-diquinolina-1,8(1,4)-dibenzenacyclohexacosaphane. This synthesis involves the preassembly of two symmetric 2-phenylquinoline arms into C-shape complexes, followed by cyclization with diamine via in situ interligand C-N cross-coupling, employing a metal ion as a template. Moreover, the synthetic yield of these cyclometalated Ir(III) complexes, tethered by an 18-crown-6 ether-like chain, is significantly enhanced in the presence of K+ ion as a template. The resultant cyclometalated macrocycle Ir(III) complexes exhibit high stability, efficient singlet oxygen generation, and superior catalytic activity for the aerobic selective oxidation of sulfides into sulfoxides under visible light irradiation in aqueous media at room temperature. The photocatalyst 2 demonstrates recyclability and can be reused at least 10 times without a significant loss of catalytic activity. These results unveil a new and complementary approach to the design and in situ synthesis of cyclometalated macrocycle Ir(III) complexes via a mild interligand-coupling strategy.

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.

Chiral Bis-phosphate Macrocycles for Catalytic, Efficient, and Enantioselective Electrophilic Fluorination

Incorporation of privileged catalytic scaffolds into a macrocyclic skeleton represents an attractive strategy to furnish supramolecular catalysis systems with enzyme-mimetic cavity and multi-site cooperation. Herein we reported the synthesis, structure, binding properties and catalytic application of a series of chiral bis-phosphate macrocycles toward the challenging asymmetric electrophilic fluorination. With a large, integrated chiral cavity and two cooperative phosphate sites, these macrocycles exhibited good inclusion toward 1,4-diazabicyclo[2.2.2]octane (DABCO) dicationic ammoniums through complementary ion-pair and C-H⋅⋅⋅O interactions, as confirmed by crystallographic and solution binding studies. In fluorocyclization of tryptamines with Selectfluor reagent which has a similar DABCO-based dicationic structure, only 2 mol% macrocycle catalyst afforded the desired pyrroloindoline products in moderate yields and up to 91 % ee. For comparison, the acyclic mono-phosphate analogue gave obviously lower reactivity and enantioselectivity (<20 % ee), suggesting a remarkable macrocyclic effect. The high catalytic efficiency and superior stereocontrol were ascribed to the tight ion-pair binding and cavity-directed noncovalent interaction cooperation.

Template Assisted One-Pot Synthesis of [2], Linear [3], and Radial [4]Catenane via Click Reaction

Design and synthesis of higher order catenane are unexpectedly complex and involve precise cooperation among the precursors overcoming competing and opposing interactions. We achieved synthesis of [2], linear [3], radial [4] in a one-pot reaction by consecutive ring closing through click reactions. This synthesis gave three isolable products due to two, four, and six-click reactions between suitable coupling partners. Yields of the isolate templated-catenane decrease from lower to higher-ordered catenane (40 %, 12 %, and 4 %), probably due to the bite angle as well as the flexibility of the reacting partners. Removal of templating cobalt(III) ion leads to the formation of fully organic [2], linear [3], and radial [4]catenane. These synthesized catenanes were purified by column chromatography and characterized by 1H-NMR, 13C-NMR, and ESI-MS spectroscopy. The synthesized catenanes have free binding sites suitable for post-functionalization and may be used for the synthesis of higher-ordered catenane.

Heterobifunctional rotaxanes featuring two chiral subunits - synthesis and application in asymmetric organocatalysis

Rotaxanes can serve as scaffolds for the generation of bifunctional catalysts. We have now generated acid-base functionalized rotaxanes featuring two chiral subunits. The mechanical bond leads to increased reaction rates and also to strongly altered enantioselectivites in comparison to the non-interlocked control catalysts.

Quantifying Intermolecular Interactions in Asymmetric Peptide Organocatalysis as a Key toward Understanding Selectivity

The kinetic resolution of trans-cyclohexane-1,2-diol with a lipophilic oligopeptide catalyst shows extraordinary selectivities. To improve our understanding of the factors governing selectivity, we quantified the Gibbs free energies of interactions of the peptide with both enantiomers of trans-cyclohexane-1,2-diol using nuclear magnetic resonance (NMR) spectroscopy. For this, we use advanced methods such as transverse relaxation (R2), diffusion measurements, saturation transfer difference (STD), and chemical shift (δ) analysis of peptide-diol mixtures upon varying their composition (NMR titrations). The methods employed give comparable and consistent results. The molecular recognition by the catalyst is approximately 3 kJ mol-1 in favor of the preferentially acetylated (R,R)-enantiomer in the temperature range studied. Interestingly, the difference of 3 kJ mol-1 is also confirmed by results from reaction monitoring of the acylation step under catalytic conditions, indicating that this finding is true regardless of whether the investigation is performed on the acetylated species or on the free catalyst. To arrive at these conclusions, the self-association of both the catalyst and the substrate in toluene was found to play an important role and thus needs to be taken into account in reaction screening.

Dimerization of confined Brønsted acids in enantioselective organocatalytic reactions

The formation of Brønsted acid aggregates in the course of asymmetric organocatalytic reactions is often overlooked in mechanistic studies, even though it might have a deep impact on the stereo-controlling factors of the transformations. In this work, we shed light on the influence of the catalyst structure and reaction conditions on the spontaneity of the aggregation process for popular chiral organocatalysts derived from phosphoric acids using high-level quantum mechanical calculations. Our study encompasses small and sterically unhindered chiral phosphoric acids as well as large and "confined" imidodiphosphates and imidodiphosphorimidates. These systems have recently proven particularly effective in promoting a large number of highly relevant asymmetric transformations. While cooperative catalytic effects of sterically less hindered chiral phosphoric acid catalysts are well appreciated in literature, it is found that the formation of catalyst dimers in solution is possible for both standard and confined catalysts. The spontaneity of the aggregation process depends on reaction conditions like solvent polarity, polarizability, temperature, the nature of the interaction with the substrate, as well as the catalyst architecture. Finally, it is shown that, at low temperatures (153 K), the aggregation process can profoundly influence the reaction kinetics and selectivity.

Catenane and Rotaxane Synthesis from Cucurbit[6]uril-Mediated Azide-Alkyne Cycloaddition

The chemistry of mechanically interlocked molecules (MIMs) such as catenane and rotaxane is full of new opportunities for the presence of a mechanical bond, and the efficient synthesis of these molecules is therefore of fundamental importance in realizing their unique properties and functions. While many different types of preorganizing interactions and covalent bond formation strategies have been exploited in MIMs synthesis, the use of cucurbit[6]uril (CB[6]) in simultaneously templating macrocycle interlocking and catalyzing the covalent formation of the interlocked components is particularly advantageous in accessing high-order catenanes and rotaxanes. In this review, catenane and rotaxane obtained from CB[6]-catalyzed azide-alkyne cycloaddition will be discussed, with special emphasis on the synthetic strategies employed for obtaining complex [n]rotaxanes and [n]catenanes, as well as their properties and functions.

Mechanical Interlocking Enhances the Electrocatalytic Oxygen Reduction Activity and Selectivity of Molecular Copper Complexes

Efficient O₂ reduction reaction (ORR) for selective H₂O generation enables advanced fuel cell technology. Nonprecious metal catalysts are viable and attractive alternatives to state-of-the-art Pt-based materials that are expensive. Cu complexes inspired by Cu-containing O₂ reduction enzymes in nature are yet to reach their desired ORR catalytic performance. Here, the concept of mechanical interlocking is introduced to the ligand architecture to enforce dynamic spatial restriction on the Cu coordination site. Interlocked catenane ligands could govern O₂ binding mode, promote electron transfer, and facilitate product elimination. Our results show that ligand interlocking as a catenane steers the ORR selectivity to H₂O as the major product via the 4e^- pathway, rivaling the selectivity of Pt, and boosts the onset potential by 130 mV, the mass activity by 1.8 times, and the turnover frequency by 1.5 fold as compared to the noninterlocked counterpart. Our Cu catenane complex represents one of the first examples to take advantage of mechanical interlocking to afford electrocatalysts with enhanced activity and selectivity. The mechanistic insights gained through this integrated experimental and theoretical study are envisioned to be valuable not just to the area of ORR energy catalysis but also with broad implications on interlocked metal complexes that are of critical importance to the general fields in redox reactions involving proton-coupled electron transfer steps.

Asymmetric Reduction of Quinolines: A Competition between Enantioselective Transfer Hydrogenation and Racemic Borane Catalysis

A chiral phosphoric acid catalyzed asymmetric transfer hydrogenation of quinolines with regenerable dihydrophenanthridine derived by a borane-catalyzed hydrogenation of phenanthridine under H₂ has been successfully realized. Despite the competition of a racemic hydrogenation pathway, a variety of tetrahydroquinolines were furnished in high yields with up to 91% ee.

Multifunctional Organocatalysts - Singly-Linked and Macrocyclic Bisphosphoric Acids for Asymmetric Phase-Transfer and Brønsted-Acid Catalysis

The linking of phosphoric acids via covalent or mechanical bonds has proven to be a successful strategy for the design of novel organocatalysts. Here, we present the first systematic investigation of singly-linked and macrocyclic bisphosphoric acids, including their synthesis and their application in phase-transfer and Brønsted acid catalysis. We found that the novel bisphosphoric acids show dramatically increased enantioselectivities in comparison to their monophosphoric acid analogues. However, the nature, length and number of linkers has a profound influence on the enantioselectivities. In the asymmetric dearomative fluorination via phase-transfer catalysis, bisphosphoric acids with a single, rigid bisalkyne-linker give the best results with moderate to good enantiomeric excesses. In contrast, bisphosphoric acids with flexible linkers give excellent enantioselectivities in the transfer-hydrogenation of quinolines via cooperative Brønsted acid catalysis. In the latter case, sufficiently long linkers are needed for high stereoselectivities, as found experimentally and supported by DFT calculations.

The Story of the Little Blue Box: A Tribute to Siegfried Hünig

The tetracationic cyclophane, cyclobis(paraquat-p-phenylene), also known as the little blue box, constitutes a modular receptor that has facilitated the discovery of many host-guest complexes and mechanically interlocked molecules during the past 35 years. Its versatility in binding small π-donors in its tetracationic state, as well as forming trisradical tricationic complexes with viologen radical cations in its doubly reduced bisradical dicationic state, renders it valuable for the construction of various stimuli-responsive materials. Since the first reports in 1988, the little blue box has been featured in over 500 publications in the literature. All this research activity would not have been possible without the seminal contributions carried out by Siegfried Hünig, who not only pioneered the syntheses of viologen-containing cyclophanes, but also revealed their rich redox chemistry in addition to their ability to undergo intramolecular π-dimerization. This Review describes how his pioneering research led to the design and synthesis of the little blue box, and how this redox-active host evolved into the key component of molecular shuttles, switches, and machines.

Theoretical Perspectives in Organocatalysis

It is clear that the field of organocatalysis is continuously expanding during the last decades. With increasing computational capacity and new techniques, computational methods have provided a more economic approach to explore different chemical systems. This review offers a broad yet concise overview of current state-of-the-art studies that have employed novel strategies for catalyst design. The evolution of the all different theoretical approaches most commonly used within organocatalysis is discussed, from the traditional approach, manual-driven, to the most recent one, machine-driven.

Emerging properties from mechanical tethering within a post-synthetically functionalised catenane scaffold

Maintaining close spatial proximity of functional moieties within molecular systems can result in fascinating emergent properties. Whilst much work has been done on covalent tethering of functional units for myriad applications, investigations into mechanically linked systems are relatively rare. Formation of the mechanical bond is usually the final step in the synthesis of interlocked molecules, placing limits on the throughput of functionalised architectures. Herein we present the synthesis of a bis-azide [2]catenane scaffold that can be post-synthetically modified using CuAAC 'click' chemistry. In this manner we have been able to access functionalised catenanes from a common precursor and study the properties of electrochemically active, emissive and photodimerisable units within the mechanically interlocked system in comparison to non-interlocked analogues. Our data demonstrates that the greater (co-)conformational flexibility that can be obtained with mechanically interlocked systems compared to traditional covalent tethers paves the way for developing new functional molecules with exciting properties.

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.

BINOL as a chiral element in mechanically interlocked molecules

In this minireview we present the use of the axially chiral 1,1'-binaphthyl-2,2'-diol (BINOL) unit as a stereogenic element in mechanically interlocked molecules (MIMs). We describe the synthesis and properties of such BINOL-based chiral MIMs, together with their use in further diastereoselective modifications, their application in asymmetric catalysis, and their use in stereoselective chemosensing. Given the growing importance of mechanically interlocked molecules and the key advantages of the privileged chiral BINOL backbone, we believe that this research area will continue to grow and deliver many useful applications in the future.

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.

Anion-π Catalysis Enabled by the Mechanical Bond

We report a series of rotaxane-based anion-π catalysts in which the mechanical bond between a bipyridine macrocycle and an axle containing an NDI unit is intrinsic to the activity observed, including a [3]rotaxane that catalyses an otherwise disfavoured Michael addition in >60 fold selectivity over a competing decarboxylation pathway that dominates under Brønsted base conditions. The results are rationalized by detailed experimental investigations, electrochemical and computational analysis.

Controlling catalyst activity, chemoselectivity and stereoselectivity with the mechanical bond

Mechanically interlocked molecules, such as rotaxanes and catenanes, are receiving increased attention as scaffolds for the development of new catalysts, driven by both their increasing accessibility and high-profile examples of the mechanical bond delivering desirable behaviours and properties. In this Review, we survey recent advances in the catalytic applications of mechanically interlocked molecules organized by the effect of the mechanical bond on key catalytic properties, namely, activity, chemoselectivity and stereoselectivity, and focus on how the mechanically bonded structure leads to the observed behaviour. Our aim is to inspire future investigations of mechanically interlocked catalysts, including those outside of the supramolecular community.

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.

Mechanical Bond Approach to Introducing Self-Adaptive Active Sites in Covalent Organic Frameworks for Zinc-Catalyzed Organophosphorus Degradation

Mechanically interlocked molecules (MIMs) with discrete molecular components linked through a mechanical bond in space can be harnessed for the operation of molecular switches and machines, which shows huge potential to imitate the dynamic response of natural enzymes. In this work, rotaxane compounds were adopted as building monomers for the synthesis of a crown-ether ring mechanically intercalated covalence organic framework (COF). This incorporation of MIMs into open architecture implemented large amplitude motions, whose wheel slid along the axle in response to external stimulation. After impregnation with Zn²⁺ ions, the relative locations of two zinc active sites (crown-ether coordinated Zn(II) and bipyridine coordinated Zn(II)) are endowed with great flexibility to fit the conformational transformation of an organophosphorus agent during the hydrolytic process. Notably, the resulting self-adaptive binuclear zinc center in a crown-ether-threaded COF network is endowed with a record catalytic ability, with a rate over 85.5 μM min^-1 for organophosphorus degradation. The strategy of synthesis for porous artificial enzymes through the introduction of mechanically bound crown ether will enable significant breakthroughs and new synthetic concepts for the development of advanced biomimetic catalysts.

Chiral Phosphoric Acids as Versatile Tools for Organocatalytic Asymmetric Transfer Hydrogenations

Herein, recent developments in the field of organocatalytic asymmetric transfer hydrogenation (ATH) of C=N, C=O and C=C double bonds using chiral phosphoric acid catalysis are reviewed. This still rapidly growing area of asymmetric catalysis relies on metal-free catalysts in combination with biomimetic hydrogen sources. Chiral phosphoric acids have proven to be extremely versatile tools in this area, providing highly active and enantioselective alternatives for the asymmetric reduction of α,β-unsaturated carbonyl compounds, imines and various heterocycles. Eventually, such transformations are more and more often used in multicomponent/cascade reactions, which undoubtedly shows their great synthetic potential and the bright future of organocatalytic asymmetric transfer hydrogenations.

Chirality Transfer in a Calixarene-Based Directional Pseudorotaxane Complex

Hexamethoxycalix[6]arene 3 forms a directional pseudorotaxane complex with the chiral axle (S)-(α-methyl-benzyl)benzylammonium 2+. Between the two (endo-chiral)-2+@3 and (exo-chiral)-2+@3 pseudorotaxane stereoisomers, the former is preferentially formed. This result confirms the validity of the “endo-α-methyl-benzyl rule”, previously reported by us. DFT calculations suggest that C-H … π interactions between the methyl group of 2+ and the calixarene aromatic rings, determine the stereoselectivity of the threading process toward the “endo-α-methyl-benzyl preference”. An amplification of optical rotation is observed upon formation of the pseudorotaxane complex (endo-chiral)-2+@3 with respect to free axle 2+. Thus, the specifical rotation of the 1:1 mixture of chiral 2+·B(ArF)4− salt and achiral 3 was augmented upon formation of the pseudorotaxane and DFT calculations were used to rationalize this result.

Mechanically Interlocked Chiral Self-Templated [2]Catenanes from 2,6-Bis(1,2,3-triazol-4-yl)pyridine (btp) Ligands

We report the efficient self-templated formation of optically active 2,6-bis(1,2,3-triazol-4-yl)pyridine (btp) derived homocircuit [2]catenane enantiomers. This represents the first example of the enantiopure formation of chiral btp homocircuit [2]catenanes from starting materials consisting of a classical chiral element; X-ray diffraction crystallography enabled the structural characterization of the [2]catenane. The self-assembly reaction was monitored closely in solution facilitating the characterization of the pseudo-rotaxane reaction intermediate prior to mechanically interlocking the pre-organised system via ring-closing metathesis.

Using the Mechanical Bond to Tune the Performance of a Thermally Activated Delayed Fluorescence Emitter*

We report the characterization of rotaxanes based on a carbazole-benzophenone thermally activated delayed fluorescence luminophore. We find that the mechanical bond leads to an improvement in key photophysical properties of the emitter, notably an increase in photoluminescence quantum yield and a decrease in the energy difference between singlet and triplet states, as well as fine tuning of the emission wavelength, a feat that is difficult to achieve when using covalently bound substituents. Computational simulations, supported by X-ray crystallography, suggest that this tuning of properties occurs due to weak interactions between the axle and the macrocycle that are enforced by the mechanical bond. This work highlights the benefits of using the mechanical bond to refine existing luminophores, providing a new avenue for emitter optimization that can ultimately increase the performance of these molecules.

Spin-labelled mechanically interlocked molecules as models for the interpretation of biradical EPR spectra

Biradical spin probes can provide detailed information about the distances between molecules/regions of molecules because the through-space coupling of radical centres, characterised by J, is strongly distance dependent. However, if the system can adopt multiple configurations, as is common in supramolecular complexes, the shape of the EPR spectrum is influenced not only by J but also the rate of exchange between different states. In practice, it is often hard to separate these variables and as a result, the effect of the latter is sometimes overlooked. To demonstrate this challenge unequivocally we synthesised rotaxane biradicals containing nitronyl nitroxide units at the termini of their axles. The rotaxanes exchange between the available biradical conformations more slowly than the corresponding non-interlocked axles but, despite this, in some cases, the EPR spectra of the axle and rotaxane remain remarkably similar. Detailed analysis allowed us to demonstrate that the similar EPR spectral shapes result from different combinations of J and rates of conformational interconversion, a phenomenon suggested theoretically more than 50 years ago. This work reinforces the idea that thorough analysis must be performed when interpreting the spectra of biradicals employed as spin probes in solution.

Heteroternary cucurbit[8]uril complexes as supramolecular scaffolds for self-assembled bifunctional photoredoxcatalysts

The self-assembly of bifunctional photoredoxcatalysts is reported. A series of photosensitizers and water-reducing catalysts were functionalized with viologen- and naphthol-units, respectively. Subsequent formation of the heteroternary cucurbit[8]uril-viologen-naphthol complexes was used for the constitution of bifunctional photoredoxcatalysts for hydrogen generation.

Functional Rotaxanes in Catalysis

Mechanically interlocked molecules (MIMs) have gained attention in the field of catalysis due to their unique molecular properties. Central to MIMs, rotaxanes are highly promising and attractive supramolecular catalysts due to their unique three-dimensional structures and the flexibility of their subcomponents. This Minireview discusses the use of rotaxanes in organocatalysis and transition-metal catalysis.

Mechanical bonding activation in rotaxane-based organocatalysts

Interlocked organocatalysts show enhanced catalytic performance when compared with their non-interlocked threads.The ring cooperatively activates the substrates, facilitating the formation and stabilization of catalytically active intermediates.

Suit[3]ane

Suitanes are a class of mechanically interlocked molecules (MIMs) that consist of two components: a body with limbs protruding outward and a suit that fits appropriately around it, so that there is no easy way for the suit to be removed from the body. Herein, we report the synthesis and characterization of a suit[3]ane, which contains a benzotrithiophene derivative (THBTT) with three protruding hexyl chains as the body and a 3-fold symmetric, extended pyridinium-based cage, namely, HexaCage⁶⁺, as the suit. Central to its realization is effective templation, provided by THBTT during cage formation, an observation that has been supported by the strong binding constant between benzotrithiophene (BTT) and the empty cage. The solid-state structure of the suit[3]ane reveals that the body is confined within the suit's cavity with its alkyl chains protruding outward through the orifices in the cage. Notably, such a seemingly unstable molecule, having three flexible alkyl chains as its only protruding limbs, does not dissociate after prolonged heating in CD₃CN at 100 °C under pressure for 7 days. No evidence for guest exchange with the host was observed at this temperature in a 2:1 mixture of THBTT and HexaCage⁶⁺ in CD₃CN. The results indicate that flexible protruding limbs are sufficient for a suit[3]ane to remain mechanically stable even at high temperatures in solution.

Cyclization of interlocked fumaramides into β-lactams: experimental and computational mechanistic assessment of the key intercomponent proton transfer and the stereocontrolling active pocket

A detailed mechanistic study of the diastereoselective CsOH-promoted cyclization of interlocked fumaramides to give β-lactams is described. The mechanistic analysis comprises the experimental evaluation of the structure-reactivity relationship for a wide range of fumaramides [2]rotaxanes (Hammet-plots), KIE studies with deuterium-labelled interlocked fumaramides and computational analysis of two alternative mechanistic pathways for the cyclization process. The obtained results confirm that: (a) the rate-determining step is the deprotonation of the N-benzyl group of the thread by the amidate group of the macrocycle generated by the external base, (b) the polyamide macrocycle plays an important role not only as activating element but also as the stereodifferenciating factor responsible for the observed diastereoselection and (c) the higher flexibility of the adamantyl core speeds up the cyclization process in diadamantyl-derived rotaxanes.

A mechanistic study of the diastereoselective cyclization of interlocked fumaramides to give β-lactams unveils the key factors for successfully taming the process.

Cross dehydrogenative C-O coupling catalysed by a catenane-coordinated copper(i)

Catalytic activity of copper(i) complexes supported by phenanthroline-containing catenane ligands towards a new C(sp³)–O dehydrogenative cross-coupling of phenols and bromodicarbonyls is reported. As the phenanthrolines are interlocked by the strong and flexible mechanical bond in the catenane, the active catalyst with an open copper coordination site can be revealed only transiently and the stable, coordinatively saturated Cu(i) pre-catalyst is quickly regenerated after substrate transformation. Compared with a control Cu(i) complex supported by non-interlocked phenanthrolines, the catenane-supported Cu(i) is highly efficient with a broad substrate scope, and can be applied in gram-scale transformations without a significant loss of the catalytic activity. This work demonstrates the advantages of the catenane ligands that provide a dynamic and responsive copper coordination sphere, highlighting the potential of the mechanical bond as a design element in transition metal catalyst development.

The use of a catenane-supported copper(i) complex for the cross dehydrogenative C–O coupling of phenols and bromodicarbonyls is described.

Rotaxane-Branched Dendrimers with Enhanced Photosensitization

During the past few decades, fabrication of functional rotaxane-branched dendrimers has become one of the most attractive yet challenging topics within supramolecular chemistry and materials science. Herein, we present the successful fabrication of a family of new rotaxane-branched dendrimers containing up to 21 platinum atoms and 42 photosensitizer moieties through an efficient and controllable divergent approach. Notably, the photosensitization efficiencies of these rotaxane-branched dendrimers gradually increased with the increase of dendrimer generation. For example, third-generation rotaxane-branched dendrimer PG3 revealed 13.3-fold higher ¹O₂ generation efficiency than its corresponding monomer AN. The enhanced ¹O₂ generation efficiency was attributed to the enhancement of intersystem crossing (ISC) through the simple and efficient incorporation of multiple heavy atoms and photosensitizer moieties on the axles and wheels of the rotaxane units, respectively, which has been validated by UV-visible and fluorescence techniques, time-dependent density functional theory calculations, photolysis model reactions, and apparent activation energy calculations. Therefore, we develop a new promising platform of rotaxane-branched dendrimers for the preparation of effective photosensitizers.

Mechanism and origins of selectivity in the enantioselective oxa-Pictet-Spengler reaction: a cooperative catalytic complex from a hydrogen bond donor and chiral phosphoric acid

Enantioselective additions to oxocarbenium ions are high-value synthetic transformations but have proven challenging to achieve. In particular, the oxa-Pictet-Spengler reaction has only recently been rendered enantioselective. We report experimental and computational studies on the mechanism of this unusual transformation. Herein we reveal that this reaction is hypothesized to proceed through a self-assembled ternary hydrogen bonding complex involving the substrate, chiral phosphate ion, and a urea hydrogen-bond donor. The computed transition state reveals C2-symmetric grooves in the chiral phosphate that are occupied by the urea and substrate. Occupation of one of these grooves by the urea co-catalyst tunes the available reactive volume and enhances the stereoselectivity of the chiral phosphate catalyst.

Functionalized Macrocycles in Supramolecular Organocatalysis

Supramolecular organocatalysis has emerged as a novel research field in the context of homogeneous catalysis. In particular, the use of functionalized macrocycles as supramolecular catalysts is highly promising, as these systems are oftentimes easily accessible and offer distinct advantages in catalysis. Macrocyclic catalysts can provide defined binding pockets, such as hydrophobic cavities, and can thus create a reaction microenvironment for catalysis. In addition, macrocycles can offer a preorganized arrangement of functional groups, such as binding sites or catalytically active groups, thus enabling a defined and possibly multivalent binding and activation of substrates. The aim of this Minireview is to provide an overview of recent advances in the area of supramolecular organocatalysis based on functionalized macrocycles (including cyclodextrins, calixarenes, and resorcinarenes), with a focus on those examples where certain catalytically active groups (such as hydrogen bond donors/acceptors, Brønsted acid or base groups, or nucleophilic units) are present in or have been installed on the macrocycles.

Synthesis of a Mechanically Planar Chiral Rotaxane Ligand for Enantioselective Catalysis

Rotaxanes are interlocked molecules in which a molecular ring is trapped on a dumbbell-shaped axle because of its inability to escape over the bulky end groups, resulting in a so-called mechanical bond. Interlocked molecules have mainly been studied as components of molecular machines, but the crowded, flexible environment created by threading one molecule through another has also been explored in catalysis and sensing. However, so far, the applications of one of the most intriguing properties of interlocked molecules, their ability to display stereogenic units that do not rely on the stereochemistry of their covalent subunits, termed "mechanical chirality," have yet to be properly explored, and prototypical demonstration of the applications of mechanically chiral rotaxanes remain scarce. Here, we describe a mechanically planar chiral rotaxane-based Au complex that mediates a cyclopropanation reaction with stereoselectivities that are comparable with the best conventional covalent catalyst reported for this reaction.

What is the role of acid-acid interactions in asymmetric phosphoric acid organocatalysis? A detailed mechanistic study using interlocked and non-interlocked catalysts

Organocatalysis has revolutionized asymmetric synthesis. However, the supramolecular interactions of organocatalysts in solution are often neglected, although the formation of catalyst aggregates can have a strong impact on the catalytic reaction. For phosphoric acid based organocatalysts, we have now established that catalyst-catalyst interactions can be suppressed by using macrocyclic catalysts, which react predominantly in a monomeric fashion, while they can be favored by integration into a bifunctional catenane, which reacts mainly as phosphoric acid dimers. For acyclic phosphoric acids, we found a strongly concentration dependent behavior, involving both monomeric and dimeric catalytic pathways. Based on a detailed experimental analysis, DFT-calculations and direct NMR-based observation of the catalyst aggregates, we could demonstrate that intermolecular acid-acid interactions have a drastic influence on the reaction rate and stereoselectivity of asymmetric transfer-hydrogenation catalyzed by chiral phosphoric acids.

Heterobifunctional Rotaxanes for Asymmetric Catalysis

Heterobifunctional rotaxanes serve as efficient catalysts for the addition of malonates to Michael acceptors. We report a series of four different heterobifunctional rotaxanes, featuring an amine-based thread and a chiral 1,1'-binaphthyl-phosphoric-acid-based macrocycle. High-level DFT calculations provided mechanistic insights and enabled rational catalyst improvements, leading to interlocked catalysts that surpass their non-interlocked counterparts in terms of reaction rates and stereoselectivities.

Enhancing the selectivity of prolinamide organocatalysts using the mechanical bond in [2]rotaxanes

The synthesis of a pair of switchable interlocked prolinamides and their use as organocatalysts in three different enamine-activated processes are reported. A diacylaminopyridine moiety was incorporated into the thread for directing [2]rotaxane formation further allowing the association of complementary small molecules. The rotaxane-based systems were tested as organocatalysts in asymmetric enamine-mediated processes, revealing a significantly improved catalytic ability if compared with the non-interlocked thread. The presence of an electron-withdrawing nitro group at the macrocycle helps to achieve high conversions and enantioselectivities. These systems are able to interact with N-hexylthymine as a cofactor to form supramolecular catalysts displaying a divergent catalytic behaviour. The presence or absence of the cofactor controls the chemoselectivity in competitive reactions.