[2]Catenane Synthesis via Covalent Templating

Pretzelanes, [1]rotaxanes and molecular figures-of-eight - crossing the bridge from fundamentals to functional applications

There are myriad [2]catenanes and [2]rotaxanes that consist of two interlocked molecular components. On occasion, supramolecular chemists prepare interlocked molecules where there are covalent linkages between the interlocked molecular components. In this review, progress on pretzelanes ([1]catenanes), [1]rotaxanes and molecular figures-of-eight is surveyed. Particular attention is paid to the application of such molecules, especially where the interlocked structure and/or the covalent linkage(s) play a key functional role.

Covalent Template-Directed Synthesis: A Powerful Tool for the Construction of Complex Molecules

Template-directed synthesis has become a powerful methodology to access complex molecules. Noncovalent templating has been widely used in the last few decades, but less attention has been paid to covalent template-directed synthesis, despite the fact that this methodology was used for the first reported synthesis of a catenane. This review highlights the evolution of covalent templating over the last 60 years, thereby providing a toolbox for the design of efficient covalent templating processes. Covalent templating represents a useful synthetic tool for accessing complex molecules, and the examples described here include the synthesis of macrocycles, mechanically interlocked molecules, linear oligomers, polydisperse linear polymers, and cross-linked polymer networks.

Boronic ester-templated pre-rotaxanes as versatile intermediates for rotaxane endo-functionalisation

We report on the synthesis of [2]rotaxanes from vicinal diols through dynamic covalent boronic ester templates, as well as the use of the boronic ester for rotaxane post-functionalisation. A boronic acid pincer ligand with two alkene-appended arms was condensed with a linear diol-containing thread, and ring-closing metathesis established a pre-rotaxane architecture along with a non-entangled isomer. Advanced NMR spectroscopy and mass spectrometry unambiguously assigned the isomers and revealed that the pre-rotaxane was in equilibrium with its hydrolyzed free [2]rotaxane form. The boronic ester handle in the pre-rotaxane could be synthetically addressed in a multitude of ways to obtain different endo-functionalised [2]rotaxanes, including with direct oxidation reactions, protodeboronation, functional group interconversions and Pd-catalysed cross-couplings.

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.

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.

Covalent [2]Catenane and [2]Rotaxane Synthesis via a δ-Amino Acid Template

Despite the advances in the synthesis of mechanically interlocked molecules, a generally applicable approach to interlocked natural products, such as lasso peptides, is yet to be formulated. While amino acid sequences have been introduced into several rotaxanes, the key structural components have always been dictated by the method used for supramolecular preorganization. In this work, we report the use of an ester-functionalized, aromatic δ-amino acid as the central covalent templating unit in the synthesis of both a [2]catenane and a [2]rotaxane from the same multimacrocyclic intermediate. This represents a key step toward future synthetic peptide-based interlocked products.

Covalently Templated Syntheses of Mechanically Interlocked Molecules

Mechanically interlocked molecules (MiMs), such as catenanes and rotaxanes, exhibit unique properties due to the mechanical bond which unites their components. The translational and rotational freedom present in these compounds may be harnessed to create stimuli-responsive MiMs, which find potential application as artificial molecular machines. Mechanically interlocked structures such as lasso peptides have also been found in nature, making MiMs promising albeit elusive targets for drug discovery. Although the first syntheses of MiMs were based on covalent strategies, approaches based on non-covalent interactions rose to prominence thereafter and have remained dominant. Non-covalent strategies are generally short and efficient, but do require particular structural motifs which are difficult to alter. In a covalent approach, MiMs can be more easily modified while the components may have increased rotational and translational freedom. Both approaches have complementary merits and combining the unmatched efficiency of non-covalent approaches with the scope of covalent syntheses may open up vast opportunities. In this review, recent covalently templated syntheses of MiMs are discussed to show their complementarity and anticipate future developments in this field.

1 Introduction

2 Tetrahedral Templates

2.1 A Carbonate Template for Non-Rusty Catenanes

2.2 All-Benzene Catenanes on a Silicon Template

2.3 Backfolding from Quaternary Carbon

3 Planar Templates

3.1 Rotaxanes Constructed in a Ring

3.2 Hydrindacene as a Dynamic Covalent Template

3.3 Templating on Tri- and Tetrasubstituted Benzenes

4 Conclusion