Kinetic and Thermodynamic Control in Dynamic Covalent Synthesis

Modular Strategy for Constructing para-Cage[n]arenes, meta-Cage[n]arenes, and meta-Bimacrocyclic-Arenes

Here, we present a versatile modular strategy for crafting novel covalent organic cages (para-cage[n]arenes and meta-cage[n]arenes, n = 3,4) and bimacrocycles (meta-bimacrocyclic-arenes) with stable backbones and modifiable rims. These structures can be synthesized from commercially available aromatic multialdehydes in a three-step process: quantitative bromination, Suzuki-Miyaura reaction (yielding over 60%), and a rapid one-pot Friedel-Crafts reaction with paraformaldehyde. Notably, the cage[n]arenes exhibit a well-defined prismatic shape, and the bimacrocyclic-arenes display both dimeric and monomeric configurations.

Proton Affinity and Conformational Integrity of a 24-Atom Triazine Macrocycle across Physiologically Relevant pH

For 24-atom triazine macrocycles, protonation of the heterocycle leads to a rigid, folded structure presenting a network of hydrogen bonds. These molecules derive from dynamic covalent chemistry wherein triazine monomers bearing a protected hydrazine group and acetal tethered by the amino acid dimerize quantitatively in an acidic solution. Here, lysine is used, and the product is a tetracation. The primary amines of the lysine side chains do not interfere with quantitative yields of the desired bis(hydrazone) at concentrations of 5-125 mg/mL. Mathematical modeling of data derived from titration experiments of the macrocycle reveals that the pKa values of the protonated triazines are 5.6 and 6.7. Changes in chemical shifts of resonances in the 1H NMR spectra corroborate these values and further support assignment of the protonation sites. The pKa values of the lysine side chains are consistent with expectation. Upon deprotonation, the macrocycle enjoys greater conformational freedom as evident from the broadening of resonances in the 1H and 13C NMR spectra indicative of dynamic motion on the NMR time scale and the appearance of additional conformations at room temperature. While well-tempered metadynamics suggests only a modest difference in accessible conformational footprints of the protonated and deprotonated macrocycles, the shift in conformation(s) supports the stabilizing role that the protons adopt in the hydrogen-bonded network.

Dynamic covalent synthesis

Dynamic covalent synthesis aims to precisely control the assembly of simple building blocks linked by reversible covalent bonds to generate a single, structurally complex, product. In recent years, considerable progress in the programmability of dynamic covalent systems has enabled easy access to a broad range of assemblies, including macrocycles, shape-persistent cages, unconventional foldamers and mechanically-interlocked species (catenanes, knots, etc.). The reversibility of the covalent linkages can be either switched off to yield stable, isolable products or activated by specific physico-chemical stimuli, allowing the assemblies to adapt and respond to environmental changes in a controlled manner. This activatable dynamic property makes dynamic covalent assemblies particularly attractive for the design of complex matter, smart chemical systems, out-of-equilibrium systems, and molecular devices.

Spontaneous and Selective Macrocyclization in Nitroaldol Reaction Systems

Through a dynamic polymerization and self-sorting process, a range of lowellane macrocycles have been efficiently generated in nitroaldol systems composed of aromatic dialdehydes and aliphatic or aromatic dinitroalkanes. All identified macrocycles show a composition of two repeating units, resulting in tetra-β-nitroalcohols of different structures. The effects of the building block structure on the macrocyclization process have been demonstrated, and the influence from the solvent has been explored. In general, the formation of the lowellanes was amplified in response to phase-change effects, although solution-phase structures were, in some cases, favored.

Single-crystal polymers (SCPs): from 1D to 3D architectures

Single-crystal polymers (SCPs) with unambiguous chemical structures at atomic-level resolutions have attracted great attention. Obtaining precise structural information of these materials is critical as it enables a deeper understanding of the potential driving forces for specific packing and long-range order, secondary interactions, and kinetic and thermodynamic factors. Such information can ultimately lead to success in controlling the synthesis or engineering of their crystal structures for targeted applications, which could have far-reaching impact. Successful synthesis of SCPs with atomic level control of the structures, especially for those with 2D and 3D architectures, is rare. In this review, we summarize the recent progress in the synthesis of SCPs, including 1D, 2D, and 3D architectures. Solution synthesis, topochemical synthesis, and extreme condition synthesis are summarized and compared. Around 70 examples of SCPs with unambiguous structure information are presented, and their synthesis methods and structural analysis are discussed. This review offers critical insights into the structure-property relationships, providing guidance for the future rational design and bottom-up synthesis of a variety of highly ordered polymers with unprecedented functions and properties.

A Model for the Rapid Assessment of Solution Structures for 24-Atom Macrocycles: The Impact of β-Branched Amino Acids on Conformation

Experiment and computation are used to develop a model to rapidly predict solution structures of macrocycles sharing the same Murcko framework. These 24-atom triazine macrocycles result from the quantitative dimerization of identical monomers presenting a hydrazine group and an acetal tethered to an amino acid linker. Monomers comprising glycine and the β-branched amino acids threonine, valine, and isoleucine yield macrocycles G-G, T-T, V-V, and I-I, respectively. Elements common to all members of the framework include the efficiency of macrocyclization (quantitative), the solution- and solid-state structures (folded), the site of protonation (opposite the auxiliary dimethylamine group), the geometry of the hydrazone (E), the C2 symmetry of the subunits (conserved), and the rotamer state adopted. In aggregate, the data reveal metrics predictive of the three-dimensional solution structure that derive from the fingerprint region of the 1D 1H spectrum and a network of rOes from a single resonance. The metrics also afford delineation of more nuanced structural features that allow subpopulations to be identified among the members of the framework. Well-tempered metadynamics provides free energy surfaces and population distributions of these macrocycles. The areas of the free energy surface decrease with increasing steric bulk (G-G > V-V ∼ T-T > I-I). In addition, the surfaces are increasingly isoenergetic with decreasing steric bulk (G-G > V-V ∼ T-T > I-I).

Polymerization, Stimuli-induced Depolymerization, and Precipitation-driven Macrocyclization in a Nitroaldol Reaction System

Dynamic covalent polymers of different topology have been synthesized from an aromatic dialdehyde and α,ω-dinitroalkanes via the nitroaldol reaction. All dinitroalkanes yielded dynamers with the dialdehyde, where the length of the dinitroalkane chain played a vital role in determining the structure of the final products. For longer dinitroalkanes, linear dynamers were produced, where the degree of polymerization reached a plateau at higher feed concentrations. In the reactions involving 1,4-dinitrobutane and 1,5-dinitropentane, specific macrocycles were formed through depolymerization of the linear chains, further driven by precipitation. At lower temperature, the same systemic self-sorting effect was also observed for the 1,6-dinitrohexane-based dynamers. Moreover, the dynamers showed a clear adaptive behavior, displaying depolymerization and rearrangement of the dynamer chains in response to alternative building blocks as external stimuli.

Ultramacrocyclization in water via external templation

Condensing a dihydrazide and each of a series of cationic bisaldehyde compounds bearing polymethylene chains in weakly acidic water produces either a macrocycle in a [1 + 1] manner or its dimer namely a [2]catenane, or their mixture. The product distribution is determined by the length of the bisaldehydes. Addition of cucurbit[8]uril (CB[8]) drives the catenane/macrocycle equilibria to the side of macrocycles, by forming ring-in-ring complexes with the latter. When the polymethylene unit of the bisaldehyde is replaced with a more rigid p-xylene linker, its self-assembly with the dihydrazide leads to quantitative formation of a [2]catenane. Upon addition of CB[8], the [2]catenane is transformed into an ultra-large macrocycle condensed in a [2 + 2] manner, which is encircled by two CB[8] rings. The framework of this macrocycle contains one hundred and two atoms, whose synthesis would be a formidable task without the external template CB[8]. Removal of CB[8] with a competitive guest leads to recovery of the [2]catenane.

Entropy directs the self-assembly of supramolecular palladium coordination macrocycles and cages

The self-assembly of palladium-based cages is frequently rationalized via the cumulative enthalpy (ΔH) of bonds between coordination nodes (M, i.e., Pd) and ligand (L) components. This focus on enthalpic rationale limits the complete understanding of the Gibbs free energy (ΔG) for self-assembly, as entropic (ΔS) contributions are overlooked. Here, we present a study of the M₂^linL₃ intermediate species (M = dinitrato(N,N,N′,N′-tetramethylethylenediamine)palladium(ii), ^linL = 4,4′-bipyridine), formed during the synthesis of triangle-shaped (M₃^linL₃) and square-shaped (M₄^linL₄) coordination macrocycles. Thermochemical analyses by variable temperature (VT) ¹H-NMR revealed that the M₂^linL₃ intermediate exhibited an unfavorable (relative) ΔS compared to M₃^linL₃ (triangle, ΔTΔS = +5.22 kcal mol⁻¹) or M₄^linL₄ (square, ΔTΔS = +2.37 kcal mol⁻¹) macrocycles. Further analysis of these constructs with molecular dynamics (MD) identified that the self-assembly process is driven by ΔG losses facilitated by increases in solvation entropy (ΔS_solv, i.e., depletion of solvent accessible surface area) that drives the self-assembly from “open” intermediates toward “closed” macrocyclic products. Expansion of our computational approach to the analysis of self-assembly in Pd_n^benL_2n cages (^benL = 4,4'-(5-ethoxy-1,3-phenylene)dipyridine), demonstrated that ΔS_solv contributions drive the self-assembly of both thermodynamic cage products (i.e., Pd₁₂^benL₂₄) and kinetically-trapped intermediates (i.e., Pd₈^cL₁₆).

These studies demonstrate that ΔS drives the self-assembly of supramolecular palladium-based coordination macrocycles and cages. As this ΔS contribution arises from solvation, these findings broadly reflect the thermodynamic drive of self-assembly to form compact structures.

Design of Stimuli-Responsive Dynamic Covalent Delivery Systems for Volatile Compounds (Part 2): Fragrance-Releasing Cleavable Surfactants in Functional Perfumery Applications

Amphiphilic imines prepared by condensation of a hydrophobic fragrance aldehyde with a hydrophilic amine derived from a poly(propylene oxide) and poly(ethylene oxide) diblock copolymer were investigated as cleavable surfactant profragrances in applications of functional perfumery. In water, the cleavable surfactants assemble into micelles that allow solubilization of perfume molecules that are not covalently attached to the surfactant. Dynamic headspace analysis on a glass surface showed that solubilized perfume molecules evaporated in a similar manner in the presence of the cleavable surfactant as compared with a non-cleavable reference surfactant. Under application conditions, the cleavable surfactant imine hydrolysed to release the covalently linked fragrance aldehyde. The profragrances were stable during storage in aqueous media, and upon dilution showed a blooming effect for the hydrolytical fragrance release and a more balanced performance of a solubilized perfume by retaining the more volatile fragrances and boosting the evaporation of the less volatile fragrances.

Quantification and Prediction of Imine Formation Kinetics in Aqueous Solution by Microfluidic NMR Spectroscopy

Quantitatively predicting the reactivity of dynamic covalent reaction is essential to understand and rationally design complex structures and reaction networks. Herein, the reactivity of aldehydes and amines in various rapid imine formation in aqueous solution by microfluidic NMR spectroscopy was quantified. Investigation of reaction kinetics allowed to quantify the forward rate constants k₊ by an empirical equation, of which three independent parameters were introduced as reactivity parameters of aldehydes (S_E , E) and amines (N). Furthermore, these reactivity parameters were successfully used to predict the unknown forward rate constants of imine formation. Finally, two competitive reaction networks were rationally designed based on the proposed reactivity parameters. Our work has demonstrated the capability of microfluidic NMR spectroscopy in quantifying the kinetics of label-free chemical reactions, especially rapid reactions that are complete in minutes.