Size-selective Catalytic Polymer Acylation with a Molecular Tetrahedron

Artificial Processive Catalytic Systems

Processive catalysts remain attached to a substrate and perform multiple rounds of catalysis. They are abundant in nature. This review highlights artificial processive catalytic systems, which can be divided into (A) catalytic rings that move along a polymer chain, (B) catalytic pores that hold polymer chains and decompose them, (C) catalysts that remain attached to and move around a cyclic substrate via supramolecular interactions, and (D) anchored catalysts that remain in contact with a substrate via multiple catalytic interactions (see frontispiece).

Kinetically controlled synthesis of rotaxane geometric isomers

Geometric isomerism in mechanically interlocked systems-which arises when the axle of a mechanically interlocked molecule is oriented, and the macrocyclic component is facially dissymmetric-can provide enhanced functionality for directional transport and polymerization catalysis. We now introduce a kinetically controlled strategy to control geometric isomerism in [2]rotaxanes. Our synthesis provides the major geometric isomer with high selectivity, broadening synthetic access to such interlocked structures. Starting from a readily accessible [2]rotaxane with a symmetrical axle, one of the two stoppers is activated selectively for stopper exchange by the substituents on the ring component. High selectivities are achieved in these reactions, based on coupling the selective formation reactions leading to the major products with inversely selective depletion reactions for the minor products. Specifically, in our reaction system, the desired (major) product forms faster in the first step, while the undesired (minor) product subsequently reacts away faster in the second step. Quantitative 1H NMR data, fit to a detailed kinetic model, demonstrates that this effect (which is conceptually closely related to minor enantiomer recycling and related processes) can significantly improve the intrinsic selectivity of the reactions. Our results serve as proof of principle for how multiple selective reaction steps can work together to enhance the stereoselectivity of synthetic processes forming complex mechanically interlocked molecules.

Substrate Selectivity Imparted by Self-Assembled Molecular Containers and Catalysts

Recent trends in catalysis are devoted to mimicking some peculiar features of enzymes like site selectivity, through functional group recognition, and substrate selectivity, through recognition of the entire surface of the substrate. The latter is a specific feature of enzymes that is seldomly present in homogeneous catalysis. Supramolecular catalysis, thanks to the self-assembly of simple subunits, enables the creation of cavities and surfaces whose confinement effects drive the preferential binding of a substrate among others with consequent substrate selectivity. The topic is an emerging field that exploits recognition phenomena to discriminate the reagents based on their size and shape. This review deals this cutting-edge field of research covering examples of supramolecular self-assembled molecular containers and catalysts operating in organic as well as aqueous media, with special emphasis for catalytic systems dealing with direct competitive experiments involving two or more substrates.

Selective Synthesis and Functionalization of an Acyclic Methylene-Bridged-Arene Trimer in a Cage

Arene-formaldehyde condensation is a versatile reaction for producing various oligomeric/polymeric materials. However, the precise control of oligomerization degree is still challenging because the starting materials and intermediates have similar reactivities. Here, we demonstrate the selective synthesis of a methylene-bridged arene trimer using the confined cavity of a coordination cage. The limited space of the cavity prevents unregulated polymerization. The confinement effect for the kinetic protection is also demonstrated by the subsequent site-selective iodination of the trimer product within the cage.

Picking the lock of coordination cage catalysis

The design principles of metallo-organic assembly reactions have facilitated access to hundreds of coordination cages of varying size and shape. Many of these assemblies possess a well-defined cavity capable of hosting a guest, pictorially mimicking the action of a substrate binding to the active site of an enzyme. While there are now a growing collection of coordination cages that show highly proficient catalysis, exhibiting both excellent activity and efficient turnover, this number is still small compared to the vast library of metal-organic structures that are known. In this review, we will attempt to unpick and discuss the key features that make an effective coordination cage catalyst, linking structure to activity (and selectivity) using lessons learnt from both experimental and computational analysis of the most notable exemplars. We will also provide an outlook for this area, reasoning why coordination cages have the potential to become the gold-standard in (synthetic) non-covalent catalysis.

Efficient Multigram Procedure for the Synthesis of Large Hydrazone-linked Molecular Cages

Covalently linked molecular cages can provide significant advantages (including, but not limited to enhanced thermal and chemical stability) over metal-linked coordination cages. Yet, while large coordination cages can now be created routinely, it is still challenging to create chemically robust, covalently linked molecular cages with large internal cavities. This fundamental challenge has made it difficult, for example, to introduce endohedral functional groups into covalent cages to enhance their practical utility (e.g., for selective guest recognition or catalysis), since the cavities would have simply been filled up with such endohedral functional groups in most cases. Here we now report the synthesis of some of the largest known covalently linked molecular tetrahedra. Our new covalent cages all contain 12 peripheral functional groups, which keep them soluble. They are formed from a common vertex, which aligns the hydrazide functions required for the hydrazone linkages with atropisomerism. While we previously reported this vertex as a building block for the smallest member of our hydrazone-linked tetrahedra, our original synthesis was not feasible to be carried out on the larger scales required to successfully access the larger tetrahedra. To overcome this synthetic challenge, we now present a greatly improved synthesis of our vertex, which only requires a single chromatographic step (compared to 3 chromatographic purification steps, which were needed for the initial synthesis). Our new synthetic route enabled us to create a whole family of molecular cages with increasing size (all linked with hydrolytically stable hydrazone bonds), with our largest covalent cage featuring p-quarterphenyl linkers and the ability to encapsulate a hypothetical sphere of approximately 3 nm in diameter. These results now open up the possibility to introduce functional groups required for selective recognition and catalysis into chemically robust covalent cages (without blocking the cavities of the covalent cages).

Graph-based Automated Macro-Molecule Assembly

We present a general molecular framework assembly algorithm that takes a largely arbitrary molecular fragment database and a user-supplied target template graph as input. Automatic assembly of molecular fragments from the database, following a prescribed, user-supplied set of connection rules, then turns the template graph into an actual, chemically reasonable molecular framework. Assembly capabilities of our algorithm are tested by producing several abstract, closed-loop shapes. To indicate a few of many possible application areas we demonstrate a host-guest complex and a road toward catalysis. Postassembly substituent exchange can be used to produce electric fields of desired values at desired points inside the framework or at its surface as a stepping stone toward rationally designed, artificial heterogeneous catalysts.

Synthesis of a large-cavity carbazole macrocycle for size-dependent recognition

A large-cavity carbazole macrocycle (1) is reported through condensation of a long and rigid monomer and paraformaldehyde. 1 exhibits highly selective binding of large-sized tetra(n-propyl) ammonium cation 3⁺. The complexation of 3⁺ by 1 is counter anion-dependent, where Cl^- gives the highest association constant of 3010 ± 230 M^-1.

FeII 4L4 tetrahedron binds and aggregates DNA G-quadruplexes

Since the discovery of the G-quadruplex (G4) structure in telomeres in 1980s, studies have established the role it plays in various biological processes. Here we report binding between DNA G4 and a self-assembled tetrahedral metal-organic cage 1 and consequent formation of aggregates, whereby the cage protects the DNA G4 from cleavage by S1 nuclease. We monitor DNA–cage interaction using fluorescence spectroscopy, firstly by quenching of a fluorescent label appended to the 5′ end of G4. Secondly, we detect the decrease in fluorescence of the G4-selective dyes thioflavin-T and Zn-PPIX bound to various DNA G4 sequences following the addition of cage 1. Our results demonstrate that 1 interacts with a wide range of G4s. Moreover, gel electrophoresis, circular dichroism and dynamic light scattering measurements establish the binding of 1 to G4 and indicate the formation of aggregate structures. Finally, we find that DNA G4 contained in an aggregate of cage 1 is protected from cleavage by S1 nuclease.

We find Fe^II₄L₄ binds to G-quadruplex and forms aggregates. G-quadruplex in the aggregates is protected from digestion by S₁ nuclease.

New horizons for catalysis disclosed by supramolecular chemistry

The adoption of a supramolecular approach in catalysis promises to address a number of unmet challenges, ranging from activity (unlocking of novel reaction pathways) to selectivity (alteration of the innate selectivity of a reaction, e.g. selective functionalization of C-H bonds) and regulation (switch ON/OFF, sequential catalysis, etc.). Supramolecular tools such as reversible association and recognition, pre-organization of reactants and stabilization of transition states upon binding offer a unique chance to achieve the above goals disclosing new horizons whose potential is being increasingly recognized and used, sometimes reaching the degree of ripeness for practical use. This review summarizes the main developments that have opened such new frontiers, with the aim of providing a guide to researchers approaching the field. We focus on artificial supramolecular catalysts of defined stoichiometry which, under homogeneous conditions, unlock outcomes that are highly difficult if not impossible to attain otherwise, namely unnatural reactivity or selectivity and catalysis regulation. The different strategies recently explored in supramolecular catalysis are concisely presented, and, for each one, a single or very few examples is/are described (mainly last 10 years, with only milestone older works discussed). The subject is divided into four sections in light of the key design principle: (i) nanoconfinement of reactants, (ii) recognition-driven catalysis, (iii) catalysis regulation by molecular machines and (iv) processive catalysis.

Iterative Exponential Growth of Oxygen-Linked Aromatic Polymers Driven by Nucleophilic Aromatic Substitution Reactions

This work presents the first transition metal-free synthesis of oxygen-linked aromatic polymers by integrating iterative exponential polymer growth (IEG) with nucleophilic aromatic substitution (SNAr) reactions. Our approach applies methyl sulfones as the leaving groups, which eliminate the need for a transition metal catalyst, while also providing flexibility in functionality and configuration of the building blocks used. As indicated by 1) 1H-1H NOESY NMR spectroscopy, 2) single-crystal X-ray crystallography, and 3) density functional theory (DFT) calculations, the unimolecular polymers obtained are folded by nonclassical hydrogen bonds formed between the oxygens of the electron-rich aromatic rings and the positively polarized C-H bonds of the electron-poor pyrimidine functions. Our results not only introduce a transition metal-free synthetic methodology to access precision polymers but also demonstrate how interactions between relatively small, neutral aromatic units in the polymers can be utilized as new supramolecular interaction pairs to control the folding of precision macromolecules.

Selective Monofunctionalization Enabled by Reaction-History-Dependent Communication in Catalytic Rotaxanes

Selective monofunctionalization of substrates with distant, yet equally reactive functional groups is difficult to achieve, as it requires the second functional group to selectively modulate its reactivity once the first functional group has reacted. We now show that mechanically interlocked catalytic rings can effectively regulate the reactivity of stoppering groups in rotaxanes over a distance of about 2 nm. Our mechanism of communication is enabled by a unique interlocked design, which effectively removes the catalytic rings from the substrates by fast dethreading as soon as the first reaction has taken place. Our method not only led to a rare example of selective monofunctionalization, but also to a "molecular if function". Overall, the study presents a way to get distant functional groups to communicate with each other in a reaction-history-dependent manner by creating linkers that can ultimately perform logical operations at the molecular level.