Hydrogen-bonded aromatic amide macrocycles: synthesis, properties and functions

As a classic example of nearly planar cyclic compounds, hydrogen-bonded aromatic amide (H-bonded aramide) macrocycles, consisting of consecutive intramolecular hydrogen bonds and aromatic residues, receive considerable research attention due to their rich host-guest chemistry. This review provides a detailed summary of the synthesis, properties and functions of H-bonded aramide macrocycles and their derivatives. Herein, the constitutional patterns of these macrocycles are divided into two subcategories: interior hydrogen bonding motifs and exterior hydrogen bonding motifs. Based on these two motifs, we summarize the facile synthesis, self-assembly, host-guest interaction complexation of H-bonded aramide macrocycles and the resulting applications such as molecular recognition, artificial ion channels, soft materials, supramolecular catalysis, and artificial molecular machines. The development of H-bonded aramide macrocycles is still in its infancy, although a considerable number of examples have been reported. We hope that this review will provide useful information and unlock new opportunities in this field.

Cavity-containing aromatic oligoamide foldamers and macrocycles: progress and future perspectives

As a major class of foldamers, aromatic oligoamide foldamers have attracted intense interest. The rigidity of aromatic residues and amide linkages allows the development of foldamers with readily predictable, stable conformations. Aromatic oligoamide foldamers having backbones fully constrained by intramolecular hydrogen bonds have attracted wide attention. Depending on their lengths, such foldamers adopt crescent or helical conformations with highly negative inner cavities. Cyclizing the backbone of the aromatic oligoamides affords the corresponding macrocycles which are characterised by persistent shapes and non-deformable inner cavities. With their defined, inner cavities, such aromatic oligoamide foldamers and macrocycles have served as hosts for cationic and polar guests, and as transmembrane channels for transporting ions and molecules. Recent synthetic progress resulted in the construction of multi-turn hollow helices that offer three-dimensional inner pores with adjustable depth. Reducing the number of backbone-constraining hydrogen bonds leads to oligoamides which, with their partially constrained backbones, undergo either solvent- or guest-dependent folding. One class of such aromatic olgioamide foldamders, which offer multiple backbone amide NH groups as hydrogen-bond donors, are designed to bind anions with adjustable affinities.

Polypyridine-Based Helical Amide Foldamer Channels: Rapid Transport of Water and Protons with High Ion Rejection

Synthetic strategies that enable rapid construction of covalent organic nanotubes with an angstrom-scale tubular pore remain scarcely reported. Reported here is a remarkably simple and mild one-pot polymerization protocol, employing POCl₃ as the polymerization agent. This protocol efficiently generates polypyridine amide foldamer-based covalent organic nanotubes with a 2.8 nm length at a yield of 50 %. Trapping single-file water chains in the 2.8 Å tubular cavity, rich in hydrogen-bond donors and acceptors, these tubular polypyridine ensembles rapidly and selectively transport water at a rate of 1.6×10⁹  H₂ O⋅S^-1 ⋅channel^-1 and protons at a speed as fast as gramicidin A, with a high rejection of ions.

Sequence-Defined Macrocycles for Understanding and Controlling the Build-up of Hierarchical Order in Self-Assembled 2D Arrays

Anfinsen's dogma that sequence dictates structure is fundamental to understanding the activity and assembly of proteins. This idea has been applied to all manner of oligomers but not to the behavior of cyclic oligomers, aka macrocycles. We do this here by providing the first proofs that sequence controls the hierarchical assembly of nonbiological macrocycles, in this case, at graphite surfaces. To design macrocycles with one (AAA), two (AAB), or three (ABC) different carbazole units, we needed to subvert the synthetic preferences for one-pot macrocyclizations. We developed a new stepwise synthesis with sequence-defined targets made in 11, 17, and 22 steps with 25, 10, and 5% yields, respectively. The linear build up of primary sequence (1°) also enabled a thermal Huisgen cycloaddition to proceed regioselectively for the first time using geometric control. The resulting macrocycles are planar (2° structure) and form H-bonded dimers (3°) at surfaces. Primary sequences encoded into the suite of tricarb macrocycles were shown by scanning-tunneling microscopy (STM) to impact the next levels of supramolecular ordering (4°) and 2D crystalline polymorphs (5°) at solution-graphite interfaces. STM imaging of an AAB macrocycle revealed the formation of a new gap phase that was inaccessible using only C3-symmetric macrocycles. STM imaging of two additional sequence-controlled macrocycles (AAD, ABE) allowed us to identify the factors driving the formation of this new polymorph. This demonstration of how sequence controls the hierarchical patterning of macrocycles raises the importance of stepwise syntheses relative to one-pot macrocyclizations to offer new approaches for greater understanding and control of hierarchical assembly.

Macrocyclic pentamers functionalised around their periphery as potential building blocks

The elaboration of a five-fold symmetric macrocyclic aromatic pentamer bearing peripheral benzyloxy and hydroxyl groups is described. These could be used to explore further functionalisation for use as pentagonal building blocks. The internal fluorine-substituted macrocycle has been prepared via a one-pot procedure which is an improvement on the stepwise chain growth approach reported in the literature.

The elaboration of a five-fold symmetric macrocyclic aromatic pentamer bearing peripheral benzyloxy and hydroxyl groups is described. The macrocycle bearing internal fluorine substituents has been prepared via a one-pot procedure.

Computation-guided improved one-pot synthesis of macrocyclic cation-binding aromatic pyridone pentamers

By using polar DMF to relax the H-bonded rigid backbone and to lower the energetic penalty associated with the sterically-crowded environment, the yields for BOP-mediated one-pot synthesis of pentameric macrocycles can be improved from 10-25% as obtained in CH₂Cl₂ to 13-47% when 15% DMF in CH₂Cl₂ was used as the reaction medium.

Tuning cation-binding selectivity and capacity via side chain-dependent molecular packing in the solid state

Cavity-containing macrocycles assemble, via side chain-dependent molecular packing, into various nanostructures able to bind cations in varying selectivities and capacities.

One-Pot Synthesis of Strained Macrocyclic Pyridone Hexamers and Their High Selectivity toward Cu(2+) Recognition

The removal of Cu(2+) ions is relevant to environmental pollution control and neurodegenerative disease treatment. A novel family of strained macrocyclic pyridone hexamers, which exhibit highly selective recognition of Cu(2+) ions and reduce copper content in artificial seawater by 97% at a very low [host]:[CuCl2] molar ratio of 2:1, is documented.

Size-dependent patterned recognition and extraction of metal ions by a macrocyclic aromatic pyridone pentamer

A macrocyclic aromatic pyridine pentamer was found to exhibit patterned recognition of metal ions and efficiently extract larger ions, such as Cs⁺, Ba²⁺, Tl⁺, Au⁺, K⁺ and Rb⁺ preferentially over the other 18 smaller metal ions from the aqueous phase into the chloroform layer.

An unusual macrocyclization reagent for highly selective one-pot synthesis of strained macrocyclic aromatic hexamers

One-pot, multi-molecular macrocyclization allows the highly selective preparation of strained macrocyclic hexamers stabilized by an inward-pointing continuous hydrogen-bonding network.

Nonaggregational shape-persistent cyclo[6]aramide and its macrocyclic effect toward binding secondary ammonium salts in moderately polar media

Simply by introducing steric side chains, the shape-persistent cyclo[6]aramides were found to exhibit nonaggregational behavior and strong association (3 × 10(4) M(-1)) ability in acetone for binding secondary ammonium salt. The complexation can be switched in an on-and-off fashion using AgPF6 and TBACl, contrasting sharply with their corresponding acyclic pentamer and demonstrating the macrocyclic effect.